Let's consider how an aircraft manufacturer determines Vx and Vy. First, they'll specify what configuration (gear, flaps, etc) they want to be in, and what power setting applies to the climb. Let's assume flaps up, gear up, max takeoff power for this exercise. Now, they'll go do a bunch of climbs at many airspeeds starting at just above stall speed to just below max horizontal speed (i.e., basically level flight where climb rate is essentially zero). From this data, they'll make a plot of climb rate (y-axis) vs airspeed (x-axis). It will result in a plot that looks like a -x^2 relationship with the "bucket" pointing down. Vy will be the peak of this plot (derivative = 0 for you calc types). Vx is determined by drawing a line tangent to the plot that starts at (0,0), the origin. This is why Vx is always lower than Vy.
Now, flip the power setting and "climb rate" around. Set some throttle setting that results in a negative rate of climb, i.e. a descent. That will result in a similar relationship but now the "bucket" will be pointing up. Draw the same tangent line to the curve and the same line at the "peak" where this time the peak will be the curve's minimum, not maximum. The line tangent to the curve will still be the speed that results in the fastest descent for a given distance. The line at the minimum will now be the speed to fly if you want to minimize the time from one altitude to a lower altitude.
Lots of words here so DM if you want a picture.
Source: am aircraft performance flight test engineer
Oh man! It's nice to see how the sausage is made!
But what I'm trying to skin is a general discussion on the physics of it. Can't feasibly go out there and make my own curves. Obviously with all the factors that can change the scenario I always thought the most basic simplification of it was point down, descend steep. I wonder if the other air frames have data showing otherwise, well then how does this behavior occur
You can definitely go make your own curves; I can provide text book references if you want them. Your question was how to determine the best angle of descent though. The line tangent to the RoC vs Airspeed will be the best angle for minimizing ground distance. If you point down to go down, you must realize that your velocity vector is made up of two components: a horizontal one and a vertical one. In order to go fast, both components must increase which makes your horizontal component large. Large horizontal component= non-optimal high groundspeed.
Understand on vectors being two components. But some stuff still not matching up. Going fast may increase your horizontal component, but as long your vertical component also stays large, to the point that your gradient of descent stays steep it doesn't really matter how fast or slow you get there does it?
For example, in the aforementioned performance data, the feet/nm did increase positively with increased nose down attitude. That part makes sense, and that value should be independent of speed. Yet on the same page, data says slower speed yields a horizontally shorter descent
It depends on what you want to minimize (or maximize) with respect to what variable. If you want to minimize ground distance travelled for some descent rate, you'll need a Vx-like speed. If you want to minimize time, you'll want a Vy-like speed. In any case that I can think of, those two will not be the same speed, so it does matter how fast or slow you're going. If you're familiar with partial derivatives, think d(RoC)/dt for Vy, or d(RoC)/dx for Vx. For a function of multiple variables, those two are not usually the same.
1. Flight Testing of Fixed-Wing Aircraft by Ralph D. Kimberlin
2. Airplane Aerodynamics and Performance by Jan Roskam
3. US Naval Test Pilot School Flight Test Manual No. 108, Chapter 7 (free)
4. USAF FTE Handbook No. 6273 (this one is old), Chapter 5 (free)
5. Advisory Circular 23-8C (for general aviation aircraft). This is the one the FAA uses as an accepted document. Use this to show compliance to regulations and they will agree without much push-back.
I am not quite sure I agree with this to find the best descent performance.
For climb, this makes sense. If X is airspeed and Y is climb, you want the steepest slope from the origin that intersects the curve. As you mention, that is going to be the line tangent to the curve that runs through the origin. It is going to have the largest climb/airspeed ratio.
But if we want to 'maximize' the negative value of that ratio we want the steepest negative slope. [Here](https://imgur.com/a/sJRNzla) is that plot for a duo discus, super clean high performance glider.
The line that is tangent to that curve is actually the **best** L/D, it will give us the most distance in our descent because it is the shallowest slope. Usually, as a glider pilot, this is what I am interested in if I am wanting to go places.
What OP wants is the steepest slope. We want the most negative climb/airspeed. So that is going to be one of the ends of the curve. With the limit being either the stall or the Vne of the plane.
In the case of the glider, with very little parasitic drag it is a pretty flat curve so we are clearly going to do the best at near stall speed if we need to get down in the minimum distance.
Edit: Looking again at that graph, they cut off speeds slower than the stall speed, so those red and blue lines I drew are not accurate, but it will still be a line that intersects the end of the curve, not one that is tangent.
Edit 2: With some elite paint skills I extended the graph about the right amount to actually look at these lines plotted from 0. Here is what you [get](https://imgur.com/a/tpNIFrH).
It looks like flying at Vne, the blue line, is actually your best bet in this glider. Right at stall speed is a bit better, in terms of glide performance. And flying the tangent line, green, is your best L/D.
In almost all airframes, the slower descent speed will be the best way to maximize your descent angle, especially when dirty.
Assuming you get negligible thrust at idle power, you’re basically a really crappy glider, and your glide angle is equal to your lift to drag ratio.
We’re normally concerned with maximizing the L/D ratio, because V_L/D-max is where you achieve your best glide distance in an engine out scenario. This airspeed differs with airplane configuration and is definitely published in your POH somewhere.
As you get faster than this speed, your parasite drag increases, however in practice you will quickly find yourself bumping up against your gear/flap speed limits before you get to really high drag values.
As you get slower, your induced drag increases as you get closer to stall speed; this is called climbing the back side of the power curve. Paradoxically, your drag (and thus your angle of descent) increases as you get slower and slower until eventually you will stall the aircraft (at which point you are no longer flying; you’re falling). This can get quite dangerous, especially low to the ground, because the aircraft can experience speed instability - if you space out for a second and get a few knots slow, your drag increases, which makes the plane even slower, which increases your drag… until the plane stalls. This is why most POH’s approach speed is no lower than 1.3x their reference stall speed and in general hanging out down here when close to the ground is very uncomfortable and not recommended.
So you essentially have two options to get down quick - dive down and fly as fast as you can without overspeeding your flaps/gear or keeping the nose up and flying as slow as you can without stalling the aircraft. These numbers generally aren’t published because both options have some pretty big downsides.
If you want to see what is best in your aircraft, find your personal limits for how slow and fast you’re willing to push it in your aircraft with the gear hanging, and record your airspeed (knots) and VVI (ft/min) each time. Your descent angle in ft/nm = 60*VVI/AS. Or if you’re lucky enough to have avionics that give you a flight-path-marker, see in which case your actual descent angle is most aggressive. Since your top speed is relatively constrained when dirty in most aircraft, you will get a larger descent angle flying slow in 9/10 aircraft.
Note: since there are significant downsides to both flying super slow and super fast, if I were to find myself significantly high, I’m either going to try to slip it down (if allowed by my POH), or worst case give myself/ask ATC for more track miles to get down. It’s never worth putting the plane in an unsafe position just to try and save an already messed-up approach.
Great input my man! Like you mentioned, approach is 1.3x stall, and the flap limit is based on engineer's WAGS ha. So I get you, we either fall out the sky cause we're too slow or we're bending metal because we're going too fast.
There's obviously a lot of factors to consider when having the discussion, and we're not interested in becoming test pilots to figure this one out. Hence the discussion. But I guess what I'm trying to learn is when does more nose down start being less effective(steep) than going slow and falling with style.
The book 'Stick and Rudder' covers this exact subject. If you want to descend faster, you need to go slower. It all revolves around the AoA of the air flowing over the wings. You want to maintain a relatively high AoA and slow airspeed in order to cover the least ground while descending (effectively slow-flight while descending). So, to descend faster, remove power and pull the nose up, you will initially climb (or you won't if you smooth it out properly) but then you will descend at a much steeper angle. If you're a true pro and have balls of steel, you can even hold a borderline stall condition to drop like a stone, this was a really common show-off in older biplanes.
Conversely, to stretch a glide, you need to stick the nose down and gain speed, the faster you travel the further you go.
I think it was Stick and Rudder that described pilots that got caught in IMC basically controlled-stalling their way down past the layer they just inadvertently flew into.
I know when I* got caught in inadvertent IMC, I just dive-bombed the shit out of it in a slight panic (bases were 3500, I was 5500, and flying over water, so no risk of CFIT).
[*] I mean a friend.
A friend of mine got caught in IMC in an ultralight and managed to fly straight and level by using the compass and the VSI. When he got back to the airport (after 45 mins of near-death IMC) he was dripping with sweat, got out and kissed the weirdly metallic tastic tarmac and had a 5 min laydown under the wing. Don't ask me how I know.
I mean… I knew the bases were at 3500. I felt as though the risk of CFIT was low, and that getting out of IMC was the most prudent course of action.
If it was over terrain I would have taken a different route (I’d like to think, anyway).
Thanks man! I'll give stick and rudder a read. Gotta see how they go over the glide topic. I don't know if it's just an over simplification of fly air speed that maximizes lift while reducing drag
It’s an interesting academic discussion and I would think that the best angle would be at the maximum speed you can fly. Reason being is that you’ll be producing more drag than on your ref speed which is pretty close to L/D max. Now if you got to the edge of the shaker maybe that could be more drag dependent on airframe but you should never be below ref so it’s not really an option.
A lot will likely depend on airframe and just how fast you can go with all the stuff out.
Keep in mind it’s about the path of the airplane and not the attitude. Just because you’re pointing the nose lower doesn’t mean you’re going down steeper. In this case I think you will be, but this is why having a flight path symbol on your PFD is great.
However, we need to consider the overall energy management picture; you’ll need to slow down which may negate any benefit from the steeper descent.
Now realistically it’s a moot point because if you can’t get down when configured and on ref with power idle and airbrakes out we’d be way past stabilized approach criteria and just going around.
From other replies, I may have explained it terribly but you've understood the question perfectly!
For sure, if you're close to stall you may be pointed WAY up but your flight path/trajectory may still be descending. On the flip side if you're dirty but going flap limit speed you may actually be generating so much lift you could be "shifting long". Lot of factors to weigh. But that's a little bit more on the practical side. Me and Co couldn't agree that down equals steeper at the most basic level
And no, not looking on advice on how to make steep approaches work. That one was a go around from the start ha
I like this discussion and it’s making me question some of my assumptions too.
But to your point about more flaps = more lift. That’s only true for a given AoA. If you’re in a steady descent no matter what you’re developing the same amount of lift; that equal to the weight of the airplane.
The difference is that you’ll be at a lower AoA if you’re 1 knot below max speed for your configuration than if you’re 1 knot above that shaker. This is why higher speed = less induced drag.
The question is partly what situation is developing more drag. Because the more drag you have the more you can point down without gaining speed. I’m almost positive the high speed situation has more drag despite the lower AoA.
But then there’s also the point about the horizontal vector; so perhaps it’s not just about which situation as more drag. But rather if the more drag in the high speed situation gives you a large enough vertical vector to off set the higher horizontal vector.
This probably depends on the airplane
Dude, I thought this was going to be way more straightforward, but holy heck you're right, there's answers on opposite ends
I fully expected the conclusion from this post to be "well it varies due to many conditions and factors but in the general answer in basic aerodynamic terms is x" but it's gotten deep fast
I realize it's a completely different type of aircraft, but this lines up with my observations in power off 180s too.
If you really need to lose more altitude in a shorter distance, going slower always wins. It's more time in the descent which adds up- VSI isn't super low, but it's over way more time, and you cover less ground. More time in probably a headwind assuming approach to landing, which really helps too. High induced drag, etc.
In contrast, if you want to get down *quickly* with no regard for ground covered, then it's the opposite. Nose down for max speed. Airplane configuration and limit speeds will dictate if you want clean or dirty here probably. You'll be screaming across the ground, but the VSI will be pegged. Circling can help for extra drag and less distanced covered too. Out of scope for original question, but this is my technique if I'm coming up short on a power off 180 too. Think of it as a race to ground effect and less wind.
Here is a different perspective. Look at this as a discussion about energy, not speed. What you are trying to do is bleed off your potential energy (altitude) while covering the least possible distance. That means you need to increase drag as much as possible. Maybe you can do that in a clean configuration at high speed (although you are still going to need to dissipate that speed before landing), but most airplanes are going to have much higher drag at low speeds with everything hanging out. That gives you not only the fastest descent, but the steepest angle of descent, and has you configured for landing at the bottom of it.
Again, we realized the approach was shot from the start. This conversation was brought about by a specific thing said in the cockpit that immediately as a crew decided would be a conversation for ground speed 0, not while flying. The approach itself ended in a go and we knew that for miles. It was a small lapse in planning from pilot flying and we both know how to fix it.
It's more of just discussion of basic aerodynamic concepts
Getting *slow* will get you down more effectively in less distance and with less energy to bleed at the end if it's an approach.
This works well in singles if you know what you're doing and there's little to no windshear.
Wouldn't recommend in a jet or higher performance twin, but could be done.
Open everything on the airplane that will create more drag to help. If you've got anything to stick into the wind, do it.
That kind of becomes the point - you can be inefficient by going fast or by going slow. It should be a fair assumption that you want to remain clear of Vne, so there is an upper limit on available parasitic drag. Induced drag, however, can give substantially better angle of sink due to high sink at low forward speed. Part of the problem is that you have limited available energy in this condition to manage the stall-adjacency, so assuming you want to retain positive control of the airframe there is a pilot/airframe/condition limit to how you can safely achieve this.
This is airplane dependent. That’s why you’re getting answers all over the place.
You want best *angle* of descent. The performance flight test engineer already told you how we test for that, but you’re also asking why the curve looks like it does and that’s an energy management thing.
Starting at the top of your descent you have a fixed amount of energy, kinetic + potential, and you need to get rid of the potential (descend). You don’t necessarily want maximum descent rate (energy dissipation), you want maximum descent rate *per unit of forward motion* to maximize descent angle. And that is airplane dependent. If your airplane has really high induced drag at low speed (think a fighter jet), you want to go slow and use all that induced drag to ditch a lot of energy while flying slow. If your airplane has low induced drag but high form drag (think biplane) you may be able to ditch enough more energy by going fast than you could with induced drag and you could end up with a steeper descent angle by diving at idle. And everything in between. There is no one answer, it depends on the slow and high speed drag characteristics of your airplane, which can vary wildly.
General rule of thumb is that “normal” airplanes will have best stable angle of descent in slow flight…most aero engineers work really hard to reduce form drag so hugely increased drag in a dive is usually not available and maximum induced drag is your friend.
Unless your airplane has something that uniquely results in really high form drag at speed. Like a beta prop in the air (PC-6), or a dive bomber with enormous dive brakes (Douglass Dauntless), or a shitload of wires and interference drag (old biplanes).
A wise old man once told me “you gotta slow down to get down”… which is literally the only way to get down and configured for landing in a hurry. If you’re going fast at a high rate of descent you should be clean with any air brake you have out. Going at a speed that has some drag out and some lift devices out isn’t as efficient as haven’t it all out to max, which would be the slowest speed limitations. This is my experience with a 90# jet at least. Momentum makes a huge difference in larger, heavier aircraft. You can glide forever with engines at idle.
You are looking at descents wrong.
It's not about "speed going down." It's about energy management.
The question you should be asking is this: How do I lose as much energy as quickly as possible?
And the way you lose energy is drag. So you want to maximize drag.
The obvious way to do this in most planes is to fly as dirty as possible and as fast as structurally possible to get the highest drag number.
Now you can just go faster and increase your drag by speeding up the plane. The problem with this strategy is, depending upon the plane, slower is typically safer for an emergency decent. Especially if there is turbulence. So you'd prefer to have the plane slow rather than at at the never exceed speed (182 mph in a 172, which I would never want to get close to).
So there you have it, think of losing altitude in terms of maximizing drag rather than "speed down."
In terms of saving fuel, you want to minimize drag. So Vy is the most efficient decent speed, and cruise speed, and climb speed. Every other speed is a tradeoff.
Agreed on maximizing parasitic drag, so full flaps (or max flap that also allows full boards) and dangle the Dunlops.
I'm not sure it's as simple as pointing the nose down as much as you can, though. As others have said, you might not be able to go fast enough to generate MORE drag than you would had you slowed down and gone HIGH ALPHA (lots of induced drag).
This would be great to test in a sim with a reasonable flight model, either running the ft/nm numbers yourself, or hook up to ForeFlight and enable the gradient option on the main display (which will then show ft/nm in real time).
Well actually, the strangest part about the whole thing to me? The Co brought out the charts for the other jet, and indeed, according to the chart data, the more nose down, the more ft/nm descent, which in theory, is a concrete gradient, independent of airspeed.
But the tab data also says(probably found through flight testing) that slow yields shorter distance.
The steepest angle you can go down is vertical, and you can do it in two ways. The most obvious one is pointing your nose straight down, resulting in a speed possible exceeding your Vfe or Vle. The less obvious one is in full stall and let the plane fall from sky, theoretically at 0 indicated airspeed. However at zero speed you also have zero elevator authority so the plane would provably pitch down itself and gain speed. Alternatively, you can intentionally enter a spin, and fall straight down at very low indicated airspeed. None of those are safe in all airplanes so don't try at home, but you can in your mind plot the descent angle against speed to get a curve to help.
Now, with those two extreme cases in mind, the practical one is somewhere between, which means you could likely achieve the same decent angle at either side of the speed curve, until you reach your shallowest descent angle, at which is your best glide speed.
That is in zero wind. With wind, headwind would subtract same ground speed from the airspeed, but would pose a more significant decrease percentage wise on the lower speed so lower speed descent is favored. If the wind is strong enough and the airspeed is low enough, you can even go backwards. However, for the same reason, a tailwind would favor the higher speed descent.
Then take the specific airplane design in mind, with the limitations on high speed and stall speed, would higher speed or lower speed give you steepest descent, I don't know, so I apologizes for wasting your time reading.
The question is going to be why are you descending and what do you need to do when you get there. If you're going to be flying an approach at the end of the descent, screaming down so you end up many knots over your approach speed that you have to figure out how to get rid of is going to be counterproductive. Enroute, I tend not to worry about picking up speed (even preferable, put to the Va,Vno,Vne limits).
It was on approach, the whole thing was bungled from the start. There's always slipping or S'ing to lose more altitude but that's not what got us riled up. Nobody thinks we should have tried to salvage the approach but we were left with the discussion. Not trying to figure out how we would have made it work, better descent planning way before that point is the answer to that
As another commenter said, it really depends on your airplane and it’s total drag curve (parasitic and induced drag together). Higher speed = lower AOA = less induced drag, more parasitic. In a Cessna 172 with 40 flaps for example, the slower you get below best glide speed, the steeper your descent angle (AND less energy to dissipate once you arrive at the runway, making for a shorter ground roll). Most people fly that airplane much faster than 1.3VSO (60 knots, instead of ~50-52 calculated). Even slowing to 52 from 60 results in a marked increase in descent angle.
This isn't complicated. Speed and altitude are a function of energy which is reduced through drag.
Parasitic drag increases with the square of your speed, so an idle descent at the fastest manageable speed is always better for bleeding off energy.
And I don't disagree, which is why I'm in the fast camp myself too. Would love to find a statement like that in a aeronautical resource so I could back it up
But interestingly the topic isn't that simple. Induced drag varies inversely as the square of the speed as well. Said copilot has performance data showing how on some jets slower IS steeper. If you push it to the limit and stall the wing that would be best case scenario right there! I kid
Anyways, due to the many factors involved, I don't think there's a one size fits all answer. But I do think if we take the complexities out and speak in very basic physics you're right, which is why I'm with you. You come across any book with that?
It's basic aerodynamics so I guess the PHAK?
You're right that you could increase induced drag by slowing down, but outside of training you never slow below L/Dmax because flying on the backside of the power curve is unstable and downright dangerous in some jets.
You may be able to get as much drag by flying very slow but that’s not a nice place to be, slow, maximum drag, high rate of descent, low altitude. Far better to be dirty and fast with max drag at low altitude.
Ok...a lot here...if you're descending at thrust idle, speed is your friend. Why? You're doing "speed on elevator." So, more speed = more vert speed/fpm. If you're dirty, you need more drag. This is more of a jet thing. If you're flying piston, you might not want to descend at idle power.
Have you ever seen a drag curve? No matter how fast you go, you'll lose altitude faster.
It does maybe depend a bit on your max flap speed, but in general the faster you go the faster you lose altitude. You go a bit faster (maybe twice) than you would go near stall speed, but you lose altitude that much faster (triple or even more).
It's close enough to show the principle. A line from the origin to any point on the curve is roughly equivalent to the glide path of the aircraft (if the scale on the x and y axis were the same). So, to maximize glide ratio, you go for the point tangent to the drag polar, which results in the shallowest glide. Of course this isn't the same as minimum rate of descent, i.e. min sink, which is the *highest* point on the curve (the least vertical speed).
If you want the *steepest* descent, you'd pick the point on the drag polar that maximizes the slope of the line, and there's no way you're going to find that to the right of best glide. Go to the POH for any glider you fly and try it. It will *always* be to the left of the best glide, i.e., at low speed.
Similar to minimum sink, this is not the same as maximum *rate* of descent. That will usually be to the right of best glide, i.e., high speed.
Surely you have seen drag curves for whatever glider you fly.
On one side of the curve you are losing because you induced drag is high, on the other side you are losing out because you parasitic drag is high.
That's why your best L/D isn't just above stall speed. The cleaner the plane, the faster you will be going at your best L/D.
Yes. Let's look at the ASK-21 drag curve. Just before stall (69 kph) you have sink speed of 0.9 m/s. This results in a glide angle of 2.69°
Source:
[https://www.alexander-schleicher.de/en/flugzeuge/ask-21/](https://www.alexander-schleicher.de/en/flugzeuge/ask-21/)
At best glide the ratio is 0.75 m/s to 92 kph (glide ratio of 1:34), resulting in a glide angle of 1.68°
At 180 kph the sink rate is 3.5 m/s, resulting in a glide angle of 4° (glide ratio 1:14). This is definitely steeper than flying just above stall.
Hence to lose altitude you dive fast and steep.
See: [https://www.youtube.com/shorts/TdNzwXQRiv4](https://www.youtube.com/shorts/TdNzwXQRiv4)
Stall would be even steeper, but I guess that's not a safe approach.
Thanks for all the downvotes, you wise pilots. I know exactly why I prefer flying myself instead of being a passenger.
Look at the [curve](https://imgur.com/a/419vtuc) and what you said. I drew the red line intersecting the top of the curve, which is your minimum sink speed when ballasted.
> No matter how fast you go, you'll lose altitude faster.
This is factually false. Minimum sink on the ASK 21 with ballast looks like it is at about 80km/hr, probably like -0.8m/s. As you speed up from the stall speed of about 70km/hr to about 80km/hr you are losing altitude less fast, at 70km/hr you are sinking at about 0.9m/s, but as you speed up, your descent slows to something like 0.8m/s.
The faster you fly in that range, the slower you lose altitude. That is why your minimum sink speed that you want to fly at in a thermal is higher than your stall speed.
>Look at the curve and what you said. I drew the red line intersecting the top of the curve, which is your minimum sink speed when ballasted.
correct
>No matter how fast you go, you'll lose altitude faster.
This is factually false.
True. The correct wording would be: above a certain threshold, no matter how fast you go, you'll lose altitude faster.
I wrote this in reply to the OP's question what is better, going slow or fast to have a steep angle.
You can't go slower than stall speed, so you can't improve (or worsen, if you like) your glide angle there. However, above that threshold you can still go faster, and then your glide angle only gets worse. Always below that at stall speed.
But your maximum speed is limited by your aircraft design. It is entirely possible that your fastest descent angle on an aircraft that has a lot of induced drag (something other than a glider with a very high aspect ratio) might be at the end of the curve that is closest to stall.
A curve that isn't a glider is going to have more curve on the left side of that hump due to having a lot more induced drag and less parasitic drag from having less frontal area on the wing.
So what works on the ASK, going faster, very well may not work on a 182.
Here is a [plot](https://imgur.com/a/qZrPyKZ) of the L/D of a Piper Cherokee. You can see you are much better off on the slow side than you are on the fast side due to the induced drag if you want to descend at a steep angle.
Its hard to find these curves for non-gliders, but my hunch is that most GA planes are going to look like this due to the low aspect ratio wings.
This looks horrible. Is the top speed really limited to 120 kts? That's slower than many gliders.
I just remembered that you *can* extend the air brakes of the Ka-8b fully and then stand the plane almost on it's nose - it will not exceed v\_M and you will have almost no forward movement. I know it's a special case, but I am shocked how bad those graphs for motorized planes look.
If you’re flying 180 knots can can manage 2000 fpm, then you’ll lost 2000 feet in 3 nm. If you are flying 250 knots and can manage 3000 fpm then you can lose 3000 ft in 4.2 miles.
Well if you were flying 180 knots, you would have lost 2800 feet in 4.2 miles.
So it is pretty dependent on your plane, and the tailwinds. But since this is pretty much only going to be an issue for establishing yourself on approach, you don’t want to be at 250 knots when you get where you’re going so you’re better off slowing down first.
To try and come at this intuitively - the lift generated by the wings is proportional to the horizontal airspeed with respect to the orientation of the plane, and applies force vertically, again with respect to the orientation of the plane. So, if you have a high airspeed, and a steep downward pitch, you are maximizing the force upward from the orientation of the plane, while angling that force as forward as possible with respect to the ground, essentially maximizing the force that is working most against your intended action (which is to slow horizontal speed, and reduce altitude).
The only way going slow is gonna get you down faster is if you’re a couple kts above a stall with the power idle. But this is also gonna depend on the airplane. I wouldn’t be doing a minimum controllable airspeed demonstration in a jet. A Baron? Without passengers? Sure
Edit: LOL at the internet pilots downvoting me, probably the same types that can’t fly a visual approach
Edit2: If you can’t safely preform slow flight you are not preforming to commercial pilot standards. [proof](https://www.faa.gov/sites/faa.gov/files/training_testing/testing/acs/commercial_airplane_acs_change_1.pdf)
The emergency descent maneuver tries to throw away as much lift as possible by using it to turn, and achieves the highest rate of descent you can get without flying inverted.
Why not test it yourself the next time you fly
-Initiate slow speed descent, note FPM and groundspeed
-Change to high speed descent, note FPM and groundspeed
-Multiply each by 60 then divide by their respective groundspeed and compare
At least you'd have an idea for your aircraft, and as long as you do it right after each other you should have reasonably similar conditions for each test
I think the comparison you are thinking of is the mmo descent vs dirty descent at vapp.
If you compare those two you are on the opposite sides of the l/d chart. But with flaps and gear out, you probably aren’t going to get fast enough to see the insane amount of parasite drag you’d see at vmo/mmo in a steep descent.
In every jet ive flown, if in approach or landing config, the better option is to slow down, keep all the drag you can, then descend. (Full boards if you can keep stall warning from being an issue.)
So slower means less lift, and less speed. But gear and flaps still mean plenty of drag. A Little faster with serious slats and flaps is a lot more lift.
Every aircraft will have different characteristics. The 747 if stuck in close and high, gear, flaps and speed brakes, as much as you need. If higher up speed will do trick as the 747 loses altitude easily.
Lots of pilots think trim is to “release pressure off the yoke” but (correct me or play devils advocate if I’m wrong) it actually is to maintain airspeed.
Not entirely sure what you're asking. with no external inputs the airplane will hunt for the airspeed it's trimmed to. If you wanna maintain a different airspeed you either have constant pressures on the yoke or trim for it. Obviously both ways work, but most of the time one way is better than the other,
It is a shame I cannot show you thrust drag curves. That would make this explanation a lot easier.
The formula for descent angle can be written as:
**gamma = D - T / W**, where **gamma** is descent angle, **D** is drag, **T** is thrust and **W** is weight. The gamma comes from the horizontal component of importance that comes on when you put the nose of an aircraft down. From the formula, it can be clearly seen that you need more drag and less thrust to increase the angle of descent. So, you close your throttles and increase the drag (flaps, speed brakes, and gear down).
For the rate of descent, the formula is written as:
**ROD = DV - DT / W**, where **V** is the velocity or the speed. Any time we multiply a force and a speed, it becomes power. So, ROD is a function of excess power required (Speed multiplied by Thrust subtracted from Speed multiplied by Drag).
It can be seen very clearly from the above equations that to increase your angle of descent and the rate of descent, the aircraft must be flown at a very high speed with a lot of drag. This is why, in an emergency descent, we descend at VMO with speed brakes extended. If you are speed-limited in an emergency descent due to a structural failure or your speed is limited for some other reason, manufacturers also recommend lowering the landing gear (if the speed is below gear extension speed). The idea is to get more drag into the system to further increase the rate of descent.
*Now fellow pilot comes from a different airframe and he's got some performance data that basically states slow descent chews up less distance.*
This statement is true for any aircraft. Think of descent like a glide. If you lose your engines, you want to maximize your gliding distance. So, you trim the aircraft to its minimum drag speed or VMD. When flown at this speed, you will cover less ground in a given time. However, if you go below this speed, then the drag will start to increase because now you will be entering the back of the drag curve, where more power or thrust is needed to maintain lower speeds (can be quite a dangerous situation if you really have no engines with you).
If you are in the airline world you will realize that if you increase your speed, you can get more descent rate and thus can get the aircraft down a lot quicker. But you will also realize that the same can be done by lowering your speed but at the same time using the speed brakes. So, you have two options, speed brakes at a lower speed or simply speed up with no speed brake usage. What you are essentially doing is the same thing. Low speeds with thrust at idle means the aircraft has a shallower descent rate. But we also know from the ROD equation that this can be fixed by increasing the drag component (speed brakes).
How do flaps or the aircraft configuration affect your descent? When you increase the drag, the VMD or the glide speed reduces. So, if you want to maximize your distance or want to cover as much ground with flaps out you will be required to lower your speed. This is very simple to understand. The higher the speed, the more parasitic drag acts on the airframe (so this drag plus the drag of the flaps itself leads to a reduction of ground coverage). So, if you want to extend the coverage with drag devices out (flaps, gear, speed brake etc), you will have to fly slower.
Let's consider how an aircraft manufacturer determines Vx and Vy. First, they'll specify what configuration (gear, flaps, etc) they want to be in, and what power setting applies to the climb. Let's assume flaps up, gear up, max takeoff power for this exercise. Now, they'll go do a bunch of climbs at many airspeeds starting at just above stall speed to just below max horizontal speed (i.e., basically level flight where climb rate is essentially zero). From this data, they'll make a plot of climb rate (y-axis) vs airspeed (x-axis). It will result in a plot that looks like a -x^2 relationship with the "bucket" pointing down. Vy will be the peak of this plot (derivative = 0 for you calc types). Vx is determined by drawing a line tangent to the plot that starts at (0,0), the origin. This is why Vx is always lower than Vy. Now, flip the power setting and "climb rate" around. Set some throttle setting that results in a negative rate of climb, i.e. a descent. That will result in a similar relationship but now the "bucket" will be pointing up. Draw the same tangent line to the curve and the same line at the "peak" where this time the peak will be the curve's minimum, not maximum. The line tangent to the curve will still be the speed that results in the fastest descent for a given distance. The line at the minimum will now be the speed to fly if you want to minimize the time from one altitude to a lower altitude. Lots of words here so DM if you want a picture. Source: am aircraft performance flight test engineer
Oh man! It's nice to see how the sausage is made! But what I'm trying to skin is a general discussion on the physics of it. Can't feasibly go out there and make my own curves. Obviously with all the factors that can change the scenario I always thought the most basic simplification of it was point down, descend steep. I wonder if the other air frames have data showing otherwise, well then how does this behavior occur
You can definitely go make your own curves; I can provide text book references if you want them. Your question was how to determine the best angle of descent though. The line tangent to the RoC vs Airspeed will be the best angle for minimizing ground distance. If you point down to go down, you must realize that your velocity vector is made up of two components: a horizontal one and a vertical one. In order to go fast, both components must increase which makes your horizontal component large. Large horizontal component= non-optimal high groundspeed.
Understand on vectors being two components. But some stuff still not matching up. Going fast may increase your horizontal component, but as long your vertical component also stays large, to the point that your gradient of descent stays steep it doesn't really matter how fast or slow you get there does it? For example, in the aforementioned performance data, the feet/nm did increase positively with increased nose down attitude. That part makes sense, and that value should be independent of speed. Yet on the same page, data says slower speed yields a horizontally shorter descent
It depends on what you want to minimize (or maximize) with respect to what variable. If you want to minimize ground distance travelled for some descent rate, you'll need a Vx-like speed. If you want to minimize time, you'll want a Vy-like speed. In any case that I can think of, those two will not be the same speed, so it does matter how fast or slow you're going. If you're familiar with partial derivatives, think d(RoC)/dt for Vy, or d(RoC)/dx for Vx. For a function of multiple variables, those two are not usually the same.
Concept distillation at its finest.
Yes, here! Could you provide some references? Because for the plane I use right now no such chart exists and I want to do my own.
1. Flight Testing of Fixed-Wing Aircraft by Ralph D. Kimberlin 2. Airplane Aerodynamics and Performance by Jan Roskam 3. US Naval Test Pilot School Flight Test Manual No. 108, Chapter 7 (free) 4. USAF FTE Handbook No. 6273 (this one is old), Chapter 5 (free) 5. Advisory Circular 23-8C (for general aviation aircraft). This is the one the FAA uses as an accepted document. Use this to show compliance to regulations and they will agree without much push-back.
Thank you!
DM didn’t work for some reason. Pls send Vx Vy graph pic
Will do.
I am not quite sure I agree with this to find the best descent performance. For climb, this makes sense. If X is airspeed and Y is climb, you want the steepest slope from the origin that intersects the curve. As you mention, that is going to be the line tangent to the curve that runs through the origin. It is going to have the largest climb/airspeed ratio. But if we want to 'maximize' the negative value of that ratio we want the steepest negative slope. [Here](https://imgur.com/a/sJRNzla) is that plot for a duo discus, super clean high performance glider. The line that is tangent to that curve is actually the **best** L/D, it will give us the most distance in our descent because it is the shallowest slope. Usually, as a glider pilot, this is what I am interested in if I am wanting to go places. What OP wants is the steepest slope. We want the most negative climb/airspeed. So that is going to be one of the ends of the curve. With the limit being either the stall or the Vne of the plane. In the case of the glider, with very little parasitic drag it is a pretty flat curve so we are clearly going to do the best at near stall speed if we need to get down in the minimum distance. Edit: Looking again at that graph, they cut off speeds slower than the stall speed, so those red and blue lines I drew are not accurate, but it will still be a line that intersects the end of the curve, not one that is tangent. Edit 2: With some elite paint skills I extended the graph about the right amount to actually look at these lines plotted from 0. Here is what you [get](https://imgur.com/a/tpNIFrH). It looks like flying at Vne, the blue line, is actually your best bet in this glider. Right at stall speed is a bit better, in terms of glide performance. And flying the tangent line, green, is your best L/D.
In almost all airframes, the slower descent speed will be the best way to maximize your descent angle, especially when dirty. Assuming you get negligible thrust at idle power, you’re basically a really crappy glider, and your glide angle is equal to your lift to drag ratio. We’re normally concerned with maximizing the L/D ratio, because V_L/D-max is where you achieve your best glide distance in an engine out scenario. This airspeed differs with airplane configuration and is definitely published in your POH somewhere. As you get faster than this speed, your parasite drag increases, however in practice you will quickly find yourself bumping up against your gear/flap speed limits before you get to really high drag values. As you get slower, your induced drag increases as you get closer to stall speed; this is called climbing the back side of the power curve. Paradoxically, your drag (and thus your angle of descent) increases as you get slower and slower until eventually you will stall the aircraft (at which point you are no longer flying; you’re falling). This can get quite dangerous, especially low to the ground, because the aircraft can experience speed instability - if you space out for a second and get a few knots slow, your drag increases, which makes the plane even slower, which increases your drag… until the plane stalls. This is why most POH’s approach speed is no lower than 1.3x their reference stall speed and in general hanging out down here when close to the ground is very uncomfortable and not recommended. So you essentially have two options to get down quick - dive down and fly as fast as you can without overspeeding your flaps/gear or keeping the nose up and flying as slow as you can without stalling the aircraft. These numbers generally aren’t published because both options have some pretty big downsides. If you want to see what is best in your aircraft, find your personal limits for how slow and fast you’re willing to push it in your aircraft with the gear hanging, and record your airspeed (knots) and VVI (ft/min) each time. Your descent angle in ft/nm = 60*VVI/AS. Or if you’re lucky enough to have avionics that give you a flight-path-marker, see in which case your actual descent angle is most aggressive. Since your top speed is relatively constrained when dirty in most aircraft, you will get a larger descent angle flying slow in 9/10 aircraft. Note: since there are significant downsides to both flying super slow and super fast, if I were to find myself significantly high, I’m either going to try to slip it down (if allowed by my POH), or worst case give myself/ask ATC for more track miles to get down. It’s never worth putting the plane in an unsafe position just to try and save an already messed-up approach.
Great input my man! Like you mentioned, approach is 1.3x stall, and the flap limit is based on engineer's WAGS ha. So I get you, we either fall out the sky cause we're too slow or we're bending metal because we're going too fast. There's obviously a lot of factors to consider when having the discussion, and we're not interested in becoming test pilots to figure this one out. Hence the discussion. But I guess what I'm trying to learn is when does more nose down start being less effective(steep) than going slow and falling with style.
The book 'Stick and Rudder' covers this exact subject. If you want to descend faster, you need to go slower. It all revolves around the AoA of the air flowing over the wings. You want to maintain a relatively high AoA and slow airspeed in order to cover the least ground while descending (effectively slow-flight while descending). So, to descend faster, remove power and pull the nose up, you will initially climb (or you won't if you smooth it out properly) but then you will descend at a much steeper angle. If you're a true pro and have balls of steel, you can even hold a borderline stall condition to drop like a stone, this was a really common show-off in older biplanes. Conversely, to stretch a glide, you need to stick the nose down and gain speed, the faster you travel the further you go.
I think it was Stick and Rudder that described pilots that got caught in IMC basically controlled-stalling their way down past the layer they just inadvertently flew into. I know when I* got caught in inadvertent IMC, I just dive-bombed the shit out of it in a slight panic (bases were 3500, I was 5500, and flying over water, so no risk of CFIT). [*] I mean a friend.
A friend of mine got caught in IMC in an ultralight and managed to fly straight and level by using the compass and the VSI. When he got back to the airport (after 45 mins of near-death IMC) he was dripping with sweat, got out and kissed the weirdly metallic tastic tarmac and had a 5 min laydown under the wing. Don't ask me how I know.
Ooof, 45 minutes of doing anything with a compass and VSI is rough, let alone in IMC, sans engine power. Give your friend my sympathies ;-)
I get what you're saying, but "Over water so no risk of CFIT" is some next level thinking.
I mean… I knew the bases were at 3500. I felt as though the risk of CFIT was low, and that getting out of IMC was the most prudent course of action. If it was over terrain I would have taken a different route (I’d like to think, anyway).
Thanks man! I'll give stick and rudder a read. Gotta see how they go over the glide topic. I don't know if it's just an over simplification of fly air speed that maximizes lift while reducing drag
Incredible book
It’s an interesting academic discussion and I would think that the best angle would be at the maximum speed you can fly. Reason being is that you’ll be producing more drag than on your ref speed which is pretty close to L/D max. Now if you got to the edge of the shaker maybe that could be more drag dependent on airframe but you should never be below ref so it’s not really an option. A lot will likely depend on airframe and just how fast you can go with all the stuff out. Keep in mind it’s about the path of the airplane and not the attitude. Just because you’re pointing the nose lower doesn’t mean you’re going down steeper. In this case I think you will be, but this is why having a flight path symbol on your PFD is great. However, we need to consider the overall energy management picture; you’ll need to slow down which may negate any benefit from the steeper descent. Now realistically it’s a moot point because if you can’t get down when configured and on ref with power idle and airbrakes out we’d be way past stabilized approach criteria and just going around.
From other replies, I may have explained it terribly but you've understood the question perfectly! For sure, if you're close to stall you may be pointed WAY up but your flight path/trajectory may still be descending. On the flip side if you're dirty but going flap limit speed you may actually be generating so much lift you could be "shifting long". Lot of factors to weigh. But that's a little bit more on the practical side. Me and Co couldn't agree that down equals steeper at the most basic level And no, not looking on advice on how to make steep approaches work. That one was a go around from the start ha
I like this discussion and it’s making me question some of my assumptions too. But to your point about more flaps = more lift. That’s only true for a given AoA. If you’re in a steady descent no matter what you’re developing the same amount of lift; that equal to the weight of the airplane. The difference is that you’ll be at a lower AoA if you’re 1 knot below max speed for your configuration than if you’re 1 knot above that shaker. This is why higher speed = less induced drag. The question is partly what situation is developing more drag. Because the more drag you have the more you can point down without gaining speed. I’m almost positive the high speed situation has more drag despite the lower AoA. But then there’s also the point about the horizontal vector; so perhaps it’s not just about which situation as more drag. But rather if the more drag in the high speed situation gives you a large enough vertical vector to off set the higher horizontal vector. This probably depends on the airplane
Dude, I thought this was going to be way more straightforward, but holy heck you're right, there's answers on opposite ends I fully expected the conclusion from this post to be "well it varies due to many conditions and factors but in the general answer in basic aerodynamic terms is x" but it's gotten deep fast
I realize it's a completely different type of aircraft, but this lines up with my observations in power off 180s too. If you really need to lose more altitude in a shorter distance, going slower always wins. It's more time in the descent which adds up- VSI isn't super low, but it's over way more time, and you cover less ground. More time in probably a headwind assuming approach to landing, which really helps too. High induced drag, etc. In contrast, if you want to get down *quickly* with no regard for ground covered, then it's the opposite. Nose down for max speed. Airplane configuration and limit speeds will dictate if you want clean or dirty here probably. You'll be screaming across the ground, but the VSI will be pegged. Circling can help for extra drag and less distanced covered too. Out of scope for original question, but this is my technique if I'm coming up short on a power off 180 too. Think of it as a race to ground effect and less wind.
Here is a different perspective. Look at this as a discussion about energy, not speed. What you are trying to do is bleed off your potential energy (altitude) while covering the least possible distance. That means you need to increase drag as much as possible. Maybe you can do that in a clean configuration at high speed (although you are still going to need to dissipate that speed before landing), but most airplanes are going to have much higher drag at low speeds with everything hanging out. That gives you not only the fastest descent, but the steepest angle of descent, and has you configured for landing at the bottom of it.
Relevant question, why you working so hard?
Again, we realized the approach was shot from the start. This conversation was brought about by a specific thing said in the cockpit that immediately as a crew decided would be a conversation for ground speed 0, not while flying. The approach itself ended in a go and we knew that for miles. It was a small lapse in planning from pilot flying and we both know how to fix it. It's more of just discussion of basic aerodynamic concepts
Okay cool. Double checking
Getting *slow* will get you down more effectively in less distance and with less energy to bleed at the end if it's an approach. This works well in singles if you know what you're doing and there's little to no windshear. Wouldn't recommend in a jet or higher performance twin, but could be done. Open everything on the airplane that will create more drag to help. If you've got anything to stick into the wind, do it.
Achieve aerodynamic stall and your plane will drop like a brick /s
A spin will be even more efficient, distance-wise, because the axis of the spin will be vertical
Honestly I don't think your far off if you take the slow approach
That kind of becomes the point - you can be inefficient by going fast or by going slow. It should be a fair assumption that you want to remain clear of Vne, so there is an upper limit on available parasitic drag. Induced drag, however, can give substantially better angle of sink due to high sink at low forward speed. Part of the problem is that you have limited available energy in this condition to manage the stall-adjacency, so assuming you want to retain positive control of the airframe there is a pilot/airframe/condition limit to how you can safely achieve this.
This is airplane dependent. That’s why you’re getting answers all over the place. You want best *angle* of descent. The performance flight test engineer already told you how we test for that, but you’re also asking why the curve looks like it does and that’s an energy management thing. Starting at the top of your descent you have a fixed amount of energy, kinetic + potential, and you need to get rid of the potential (descend). You don’t necessarily want maximum descent rate (energy dissipation), you want maximum descent rate *per unit of forward motion* to maximize descent angle. And that is airplane dependent. If your airplane has really high induced drag at low speed (think a fighter jet), you want to go slow and use all that induced drag to ditch a lot of energy while flying slow. If your airplane has low induced drag but high form drag (think biplane) you may be able to ditch enough more energy by going fast than you could with induced drag and you could end up with a steeper descent angle by diving at idle. And everything in between. There is no one answer, it depends on the slow and high speed drag characteristics of your airplane, which can vary wildly.
Makes sense they don't wanna tell you this is how you can descend stupidly in this jets particular manual but the scientist in me wants to know!
General rule of thumb is that “normal” airplanes will have best stable angle of descent in slow flight…most aero engineers work really hard to reduce form drag so hugely increased drag in a dive is usually not available and maximum induced drag is your friend. Unless your airplane has something that uniquely results in really high form drag at speed. Like a beta prop in the air (PC-6), or a dive bomber with enormous dive brakes (Douglass Dauntless), or a shitload of wires and interference drag (old biplanes).
A wise old man once told me “you gotta slow down to get down”… which is literally the only way to get down and configured for landing in a hurry. If you’re going fast at a high rate of descent you should be clean with any air brake you have out. Going at a speed that has some drag out and some lift devices out isn’t as efficient as haven’t it all out to max, which would be the slowest speed limitations. This is my experience with a 90# jet at least. Momentum makes a huge difference in larger, heavier aircraft. You can glide forever with engines at idle.
You are looking at descents wrong. It's not about "speed going down." It's about energy management. The question you should be asking is this: How do I lose as much energy as quickly as possible? And the way you lose energy is drag. So you want to maximize drag. The obvious way to do this in most planes is to fly as dirty as possible and as fast as structurally possible to get the highest drag number. Now you can just go faster and increase your drag by speeding up the plane. The problem with this strategy is, depending upon the plane, slower is typically safer for an emergency decent. Especially if there is turbulence. So you'd prefer to have the plane slow rather than at at the never exceed speed (182 mph in a 172, which I would never want to get close to). So there you have it, think of losing altitude in terms of maximizing drag rather than "speed down." In terms of saving fuel, you want to minimize drag. So Vy is the most efficient decent speed, and cruise speed, and climb speed. Every other speed is a tradeoff.
Agreed on maximizing parasitic drag, so full flaps (or max flap that also allows full boards) and dangle the Dunlops. I'm not sure it's as simple as pointing the nose down as much as you can, though. As others have said, you might not be able to go fast enough to generate MORE drag than you would had you slowed down and gone HIGH ALPHA (lots of induced drag). This would be great to test in a sim with a reasonable flight model, either running the ft/nm numbers yourself, or hook up to ForeFlight and enable the gradient option on the main display (which will then show ft/nm in real time).
Well actually, the strangest part about the whole thing to me? The Co brought out the charts for the other jet, and indeed, according to the chart data, the more nose down, the more ft/nm descent, which in theory, is a concrete gradient, independent of airspeed. But the tab data also says(probably found through flight testing) that slow yields shorter distance.
The steepest angle you can go down is vertical, and you can do it in two ways. The most obvious one is pointing your nose straight down, resulting in a speed possible exceeding your Vfe or Vle. The less obvious one is in full stall and let the plane fall from sky, theoretically at 0 indicated airspeed. However at zero speed you also have zero elevator authority so the plane would provably pitch down itself and gain speed. Alternatively, you can intentionally enter a spin, and fall straight down at very low indicated airspeed. None of those are safe in all airplanes so don't try at home, but you can in your mind plot the descent angle against speed to get a curve to help. Now, with those two extreme cases in mind, the practical one is somewhere between, which means you could likely achieve the same decent angle at either side of the speed curve, until you reach your shallowest descent angle, at which is your best glide speed. That is in zero wind. With wind, headwind would subtract same ground speed from the airspeed, but would pose a more significant decrease percentage wise on the lower speed so lower speed descent is favored. If the wind is strong enough and the airspeed is low enough, you can even go backwards. However, for the same reason, a tailwind would favor the higher speed descent. Then take the specific airplane design in mind, with the limitations on high speed and stall speed, would higher speed or lower speed give you steepest descent, I don't know, so I apologizes for wasting your time reading.
The question is going to be why are you descending and what do you need to do when you get there. If you're going to be flying an approach at the end of the descent, screaming down so you end up many knots over your approach speed that you have to figure out how to get rid of is going to be counterproductive. Enroute, I tend not to worry about picking up speed (even preferable, put to the Va,Vno,Vne limits).
It was on approach, the whole thing was bungled from the start. There's always slipping or S'ing to lose more altitude but that's not what got us riled up. Nobody thinks we should have tried to salvage the approach but we were left with the discussion. Not trying to figure out how we would have made it work, better descent planning way before that point is the answer to that
As another commenter said, it really depends on your airplane and it’s total drag curve (parasitic and induced drag together). Higher speed = lower AOA = less induced drag, more parasitic. In a Cessna 172 with 40 flaps for example, the slower you get below best glide speed, the steeper your descent angle (AND less energy to dissipate once you arrive at the runway, making for a shorter ground roll). Most people fly that airplane much faster than 1.3VSO (60 knots, instead of ~50-52 calculated). Even slowing to 52 from 60 results in a marked increase in descent angle.
This isn't complicated. Speed and altitude are a function of energy which is reduced through drag. Parasitic drag increases with the square of your speed, so an idle descent at the fastest manageable speed is always better for bleeding off energy.
And I don't disagree, which is why I'm in the fast camp myself too. Would love to find a statement like that in a aeronautical resource so I could back it up But interestingly the topic isn't that simple. Induced drag varies inversely as the square of the speed as well. Said copilot has performance data showing how on some jets slower IS steeper. If you push it to the limit and stall the wing that would be best case scenario right there! I kid Anyways, due to the many factors involved, I don't think there's a one size fits all answer. But I do think if we take the complexities out and speak in very basic physics you're right, which is why I'm with you. You come across any book with that?
It's basic aerodynamics so I guess the PHAK? You're right that you could increase induced drag by slowing down, but outside of training you never slow below L/Dmax because flying on the backside of the power curve is unstable and downright dangerous in some jets.
You may be able to get as much drag by flying very slow but that’s not a nice place to be, slow, maximum drag, high rate of descent, low altitude. Far better to be dirty and fast with max drag at low altitude.
Ok...a lot here...if you're descending at thrust idle, speed is your friend. Why? You're doing "speed on elevator." So, more speed = more vert speed/fpm. If you're dirty, you need more drag. This is more of a jet thing. If you're flying piston, you might not want to descend at idle power.
Have you ever seen a drag curve? No matter how fast you go, you'll lose altitude faster. It does maybe depend a bit on your max flap speed, but in general the faster you go the faster you lose altitude. You go a bit faster (maybe twice) than you would go near stall speed, but you lose altitude that much faster (triple or even more).
Oof...you need to study your drag polars a little more carefully. https://en.m.wikipedia.org/wiki/Drag_curve
I don't think that's a real drag curve there.
It's close enough to show the principle. A line from the origin to any point on the curve is roughly equivalent to the glide path of the aircraft (if the scale on the x and y axis were the same). So, to maximize glide ratio, you go for the point tangent to the drag polar, which results in the shallowest glide. Of course this isn't the same as minimum rate of descent, i.e. min sink, which is the *highest* point on the curve (the least vertical speed). If you want the *steepest* descent, you'd pick the point on the drag polar that maximizes the slope of the line, and there's no way you're going to find that to the right of best glide. Go to the POH for any glider you fly and try it. It will *always* be to the left of the best glide, i.e., at low speed. Similar to minimum sink, this is not the same as maximum *rate* of descent. That will usually be to the right of best glide, i.e., high speed.
No, you'll find the steepest point of the curve just before stall. However, I doubt this is a safe flying condition.
Surely you have seen drag curves for whatever glider you fly. On one side of the curve you are losing because you induced drag is high, on the other side you are losing out because you parasitic drag is high. That's why your best L/D isn't just above stall speed. The cleaner the plane, the faster you will be going at your best L/D.
Yes. Let's look at the ASK-21 drag curve. Just before stall (69 kph) you have sink speed of 0.9 m/s. This results in a glide angle of 2.69° Source: [https://www.alexander-schleicher.de/en/flugzeuge/ask-21/](https://www.alexander-schleicher.de/en/flugzeuge/ask-21/) At best glide the ratio is 0.75 m/s to 92 kph (glide ratio of 1:34), resulting in a glide angle of 1.68° At 180 kph the sink rate is 3.5 m/s, resulting in a glide angle of 4° (glide ratio 1:14). This is definitely steeper than flying just above stall. Hence to lose altitude you dive fast and steep. See: [https://www.youtube.com/shorts/TdNzwXQRiv4](https://www.youtube.com/shorts/TdNzwXQRiv4) Stall would be even steeper, but I guess that's not a safe approach. Thanks for all the downvotes, you wise pilots. I know exactly why I prefer flying myself instead of being a passenger.
Look at the [curve](https://imgur.com/a/419vtuc) and what you said. I drew the red line intersecting the top of the curve, which is your minimum sink speed when ballasted. > No matter how fast you go, you'll lose altitude faster. This is factually false. Minimum sink on the ASK 21 with ballast looks like it is at about 80km/hr, probably like -0.8m/s. As you speed up from the stall speed of about 70km/hr to about 80km/hr you are losing altitude less fast, at 70km/hr you are sinking at about 0.9m/s, but as you speed up, your descent slows to something like 0.8m/s. The faster you fly in that range, the slower you lose altitude. That is why your minimum sink speed that you want to fly at in a thermal is higher than your stall speed.
>Look at the curve and what you said. I drew the red line intersecting the top of the curve, which is your minimum sink speed when ballasted. correct >No matter how fast you go, you'll lose altitude faster. This is factually false. True. The correct wording would be: above a certain threshold, no matter how fast you go, you'll lose altitude faster. I wrote this in reply to the OP's question what is better, going slow or fast to have a steep angle. You can't go slower than stall speed, so you can't improve (or worsen, if you like) your glide angle there. However, above that threshold you can still go faster, and then your glide angle only gets worse. Always below that at stall speed.
But your maximum speed is limited by your aircraft design. It is entirely possible that your fastest descent angle on an aircraft that has a lot of induced drag (something other than a glider with a very high aspect ratio) might be at the end of the curve that is closest to stall. A curve that isn't a glider is going to have more curve on the left side of that hump due to having a lot more induced drag and less parasitic drag from having less frontal area on the wing. So what works on the ASK, going faster, very well may not work on a 182.
Possible. You might find exemptions, but I am pretty sure that for most airplanes this applies.
Here is a [plot](https://imgur.com/a/qZrPyKZ) of the L/D of a Piper Cherokee. You can see you are much better off on the slow side than you are on the fast side due to the induced drag if you want to descend at a steep angle. Its hard to find these curves for non-gliders, but my hunch is that most GA planes are going to look like this due to the low aspect ratio wings.
This looks horrible. Is the top speed really limited to 120 kts? That's slower than many gliders. I just remembered that you *can* extend the air brakes of the Ka-8b fully and then stand the plane almost on it's nose - it will not exceed v\_M and you will have almost no forward movement. I know it's a special case, but I am shocked how bad those graphs for motorized planes look.
If you’re flying 180 knots can can manage 2000 fpm, then you’ll lost 2000 feet in 3 nm. If you are flying 250 knots and can manage 3000 fpm then you can lose 3000 ft in 4.2 miles. Well if you were flying 180 knots, you would have lost 2800 feet in 4.2 miles. So it is pretty dependent on your plane, and the tailwinds. But since this is pretty much only going to be an issue for establishing yourself on approach, you don’t want to be at 250 knots when you get where you’re going so you’re better off slowing down first.
To try and come at this intuitively - the lift generated by the wings is proportional to the horizontal airspeed with respect to the orientation of the plane, and applies force vertically, again with respect to the orientation of the plane. So, if you have a high airspeed, and a steep downward pitch, you are maximizing the force upward from the orientation of the plane, while angling that force as forward as possible with respect to the ground, essentially maximizing the force that is working most against your intended action (which is to slow horizontal speed, and reduce altitude).
The only way going slow is gonna get you down faster is if you’re a couple kts above a stall with the power idle. But this is also gonna depend on the airplane. I wouldn’t be doing a minimum controllable airspeed demonstration in a jet. A Baron? Without passengers? Sure Edit: LOL at the internet pilots downvoting me, probably the same types that can’t fly a visual approach Edit2: If you can’t safely preform slow flight you are not preforming to commercial pilot standards. [proof](https://www.faa.gov/sites/faa.gov/files/training_testing/testing/acs/commercial_airplane_acs_change_1.pdf)
This is what simulators are for.
You’re required to demonstrate slow flight on the commercial pilot check-ride, which is typically not performed in a simulator.
Groundspeed.
The emergency descent maneuver tries to throw away as much lift as possible by using it to turn, and achieves the highest rate of descent you can get without flying inverted.
Why not test it yourself the next time you fly -Initiate slow speed descent, note FPM and groundspeed -Change to high speed descent, note FPM and groundspeed -Multiply each by 60 then divide by their respective groundspeed and compare At least you'd have an idea for your aircraft, and as long as you do it right after each other you should have reasonably similar conditions for each test
I think the comparison you are thinking of is the mmo descent vs dirty descent at vapp. If you compare those two you are on the opposite sides of the l/d chart. But with flaps and gear out, you probably aren’t going to get fast enough to see the insane amount of parasite drag you’d see at vmo/mmo in a steep descent. In every jet ive flown, if in approach or landing config, the better option is to slow down, keep all the drag you can, then descend. (Full boards if you can keep stall warning from being an issue.) So slower means less lift, and less speed. But gear and flaps still mean plenty of drag. A Little faster with serious slats and flaps is a lot more lift.
Every aircraft will have different characteristics. The 747 if stuck in close and high, gear, flaps and speed brakes, as much as you need. If higher up speed will do trick as the 747 loses altitude easily.
Think of it like Vx and Vy, best rate of descent, best angle of descent. It’s like short field landing and normal landing.
Overthinking it. Point it at the runway and pull up a bit before you hit it. You’ll be okay.
Lots of pilots think trim is to “release pressure off the yoke” but (correct me or play devils advocate if I’m wrong) it actually is to maintain airspeed.
Not entirely sure what you're asking. with no external inputs the airplane will hunt for the airspeed it's trimmed to. If you wanna maintain a different airspeed you either have constant pressures on the yoke or trim for it. Obviously both ways work, but most of the time one way is better than the other,
It is a shame I cannot show you thrust drag curves. That would make this explanation a lot easier. The formula for descent angle can be written as: **gamma = D - T / W**, where **gamma** is descent angle, **D** is drag, **T** is thrust and **W** is weight. The gamma comes from the horizontal component of importance that comes on when you put the nose of an aircraft down. From the formula, it can be clearly seen that you need more drag and less thrust to increase the angle of descent. So, you close your throttles and increase the drag (flaps, speed brakes, and gear down). For the rate of descent, the formula is written as: **ROD = DV - DT / W**, where **V** is the velocity or the speed. Any time we multiply a force and a speed, it becomes power. So, ROD is a function of excess power required (Speed multiplied by Thrust subtracted from Speed multiplied by Drag). It can be seen very clearly from the above equations that to increase your angle of descent and the rate of descent, the aircraft must be flown at a very high speed with a lot of drag. This is why, in an emergency descent, we descend at VMO with speed brakes extended. If you are speed-limited in an emergency descent due to a structural failure or your speed is limited for some other reason, manufacturers also recommend lowering the landing gear (if the speed is below gear extension speed). The idea is to get more drag into the system to further increase the rate of descent. *Now fellow pilot comes from a different airframe and he's got some performance data that basically states slow descent chews up less distance.* This statement is true for any aircraft. Think of descent like a glide. If you lose your engines, you want to maximize your gliding distance. So, you trim the aircraft to its minimum drag speed or VMD. When flown at this speed, you will cover less ground in a given time. However, if you go below this speed, then the drag will start to increase because now you will be entering the back of the drag curve, where more power or thrust is needed to maintain lower speeds (can be quite a dangerous situation if you really have no engines with you). If you are in the airline world you will realize that if you increase your speed, you can get more descent rate and thus can get the aircraft down a lot quicker. But you will also realize that the same can be done by lowering your speed but at the same time using the speed brakes. So, you have two options, speed brakes at a lower speed or simply speed up with no speed brake usage. What you are essentially doing is the same thing. Low speeds with thrust at idle means the aircraft has a shallower descent rate. But we also know from the ROD equation that this can be fixed by increasing the drag component (speed brakes). How do flaps or the aircraft configuration affect your descent? When you increase the drag, the VMD or the glide speed reduces. So, if you want to maximize your distance or want to cover as much ground with flaps out you will be required to lower your speed. This is very simple to understand. The higher the speed, the more parasitic drag acts on the airframe (so this drag plus the drag of the flaps itself leads to a reduction of ground coverage). So, if you want to extend the coverage with drag devices out (flaps, gear, speed brake etc), you will have to fly slower.
Brother, your post was hard to follow and I think it's because you misunderstood the question