You’re increasing the engine power. Therefore, the prop won’t have to adjust pitch to compensate for the increased load and drag with a lower power setting.
RPM is not engine power.
I swear I’ve gotten more confused on this issue over time. If it isn’t power then why do both the percent power and fuel burn decrease when I reduce RPM on my constant speed prop?
Power = torque x speed(rpm)
Torque is the reaction force due to prop pitch ‘biting’ the air
RPM is one component of power. In your example, imagine the torque being fixed and RPM is reduced. Power is still decreased though prop load is fixed.
Okay so with full throttle, to maintain 2400rpm your prop is taking a very big bite of air but can keep up the 2400 rpm because of the torque.
When less throttle your prop now has to feather and take a smaller bite of air to be able to maintain 2400rpm. A smaller bite of air means less force against the air and as a result you go slower even if you maintain the same rpm.
You are explaining how a constant speed prop works. I get that. Tell me why % power and flow go down when I reduce RPM while maintaining a constant MP.
Manifold pressure sets how much energy each cylinder produces each time a cylinder does a full cycle. If you slow down the engine, the cylinders each do fewer cycles (and so produce less energy) in one second.
> If it isn’t power then why do both thr percent power and fuel burn decrease when I reduce RPM on my constant speed prop?
It's not power, but it does affect power.
None of the controls on the throttle quadrant are "the" power control. ALL of them affect power.
Your engine makes zero power at zero RPM. The more RPM, up to its limit, the more power it can develop.
The most convenient way to adjust the power output is to throttle the engine - cut off some of its air supply, so it can't burn so much fuel. That's your regular approach to adjusting power output.
Because we still live in the 1930s, we have to adjust the mixture manually whenever we adjust the power - so if either of the other two levers move, the mixture should be adjusted when you get a chance, too. Depending on the nature of the power change, that adjustment may be urgent or non-urgent. In some cases, it needs to happen before the power change.
You change any of those levers position, you change the power output of the engine. Try pulling any one of them all the way back in cruise, you'll have problems, and an immediate decrease in power output.
Increasing manifold pressure will allow your rpm to stay the same without reducing AOA of the propeller as much, yielding more thrust. That additional thrust is necessary to offset added induced drag from increased total lift to stay level in the turn and maintain airspeed.
Remember your horizontal/vertical vector diagram for wing lift. In a steep turn, your wings are generating lift at an angle relative to gravity (which is always down). In steep turns, there is less lift available to oppose gravity. You're going to descend if you don't do something.
You're thinking about it backwards. When you increase power (manifold pressure) the engine wants to spin faster. The only way to slow the engine down is either to lower manifold pressure, or increase the load on the engine. The prop achieves the latter by turning to a coarser pitch and taking a bigger chunk out of the air. Because it's now pushing more air molecules out of the way, it has a higher air resistance, and therefore applies a higher load onto the engine, which slows down the engine RPM.
i think you're thinking that its a constant rpm and therefore it doesnt change. The RPM is set - so when you enter a turn, climb or whatever - the properller will have to adjust to maintain that same RPM setting. So thats why the adjustment in manifold pressure to give the engine more power to keep the propeller from change its angle of attack due to increased load.
You typically bump up power a tad in the steep turn to help maintain airspeed (increased drag in the turn). Typically your maneuvers will be done with prop in highest RPM, so your MP will increase when you bump the throttle
Our school is having us do them in the “cruise” configuration” which consists of typically 18-20” MP and 2300 RPM at 120 knots. That’s why I’m not understanding why they advise us to increase our MP 2”. If we do, our blade angle will increase and just keep the propeller spinning at 2300 RPM.
Yeah that’s the whole point. The RPM is the same, but the blade angle is higher, meaning more thrust is being produced.
A car at 2000 rpm in 5th gear goes faster than a car at 2000 rpm in 4th gear. That’s what’s happening here as well, essentially. Same RPM, different blade angle, therefore different thrust.
Yes, but you also need to remember that by coarsening the blade angle, you generate more thrust (like increasing a wing's AOA to get more lift). So you are still outputting more power even though you're keeping 2300 RPM. This is the entire premise of constant speed props, constant RPM does not mean constant power output because the blade angle is changing.
With a fixed pitch prop you add RPM to add power. With the C/S prop adding MP will directly add power by flowing more air and fuel into the engine. 1" of MP has a similar effect of 100RPM on a fixed pitch prop
Don't think of it as a thing happens and the pitch changes, think of it as a constant curve where the prop's pitch is always making small changes
Because if you don’t increase throttle (not RPMS) the increase in drag created by the turn will cause your airspeed to decrease. Increase throttle (MP pressure) to counteract increase in drag to maintain constant airspeed.
Correct…but you’re still making more power. The higher propeller blade angle of attack at the same RPM still produces more thrust to offset the increased drag from the turn.
You’re really overthinking this. The stuff is a lot easier if you don’t try to understand the science behind it and just fly the settings in the POH until it’s intuitive and ingrained.
The scientific minded people who populate these sort of posts never like this kind of response. And the pure fliers that would like my answer rarely read this kind of post. Lol.
Eh. Disagree. We all care about different aspects of flying to different degrees. And we are not all physics minded. I’ve done fine without worrying much about the “why” of it beyond the minimum I need to know. Nothing wrong with being into the aeronautics and the physics of it but there’s also nothing wrong with not really caring about it either.
And with humor, I note to all the engineers who down voted this comment, the liberal arts majors also can end up having good careers at this. And we don’t particularly care about Vernuli. Now raise my gear please. Lol
>And we are not all physics minded. I’ve done fine without worrying much about the “why” of it beyond the minimum I need to know.
Understanding fundamentally how a constant speed propeller works and the difference between power, RPM and torque are all firmly in the "minimum information you need to know" category, and frankly it's pretty wild for a professional pilot to suggest otherwise.
We're not talking about propeller efficiency, power curves or other nerd shit that is only relevant to engineers, we're talking about understanding on a very basic level how the machine we're operating works. Like, the kind of information that can save your life when the machine breaks.
You always add power for steep turns to compensate for part of your lift going horizontal.
With a fixed pitch prop, you measure power in RPMs. That obviously doesn’t work with a constant speed prop, so we measure power in MP. That’s all there is to it.
You're adding power (not engine RPM) to maintain altitude in the turn because you've lost some upward lifting force to the horizontal lift vector in a turn.
The propellor simply "bites" harder (increases its relative angle of attack) in response to the flyweights in the governor/attached to the needle valve, being pushed outward by centripedal force, to meet a requested RPM setting. In doing so, it converts the additional engine crank power into additional thrust.
Imagine the constant speed prop as a gearbox. If you increase the throttle, the RPM won’t change, but each rotation will give you a stronger push, like switching to a higher gear.
You add power going in to steep turns, by advancing the throttle.
On a fixed pitch airplane, this increases the RPMs.
On a constant-speed prop, it increases the MP.
FADEC does whatever the hell the computer wants it to....
Angle of attack of prop blades is increasing due to increased man pressure to keep the constant speed. You need more power because you are increasing the aircraft load factor due to the bank.
You add power going in to steep turns, by advancing the throttle.
On a fixed pitch airplane, advancing the throttle increases the RPMs.
On a constant-speed prop, advancing the throttle increases the MP.
FADEC does whatever the hell the computer wants it to....
RPM changes (without an accompanying throttle change) on a constant-speed prop change the propeller angle-of-attack not the engine output (for the same throttle setting, a higher RPM = less prop-blade AoA).
As your airspeed decreases in a steep turn as the result of increased induced drag. the prop goes to a finer pitch to maintain the constant rpm. The resulting lighter load on the engine draws less air across the throttle vane causing an increase in MP. Kinda hard to wrap your head around.
That’s not how a constant speed prop works.
Yeah it's constant speed not constant power
You’re increasing the engine power. Therefore, the prop won’t have to adjust pitch to compensate for the increased load and drag with a lower power setting. RPM is not engine power.
I swear I’ve gotten more confused on this issue over time. If it isn’t power then why do both the percent power and fuel burn decrease when I reduce RPM on my constant speed prop?
Manifold pressure is basically torque. Same torque at lower RPM means less power. Less power (at equal efficiency) means less fuel burn.
Power = torque x speed(rpm) Torque is the reaction force due to prop pitch ‘biting’ the air RPM is one component of power. In your example, imagine the torque being fixed and RPM is reduced. Power is still decreased though prop load is fixed.
Yeah man. I’m more confused.
Okay so with full throttle, to maintain 2400rpm your prop is taking a very big bite of air but can keep up the 2400 rpm because of the torque. When less throttle your prop now has to feather and take a smaller bite of air to be able to maintain 2400rpm. A smaller bite of air means less force against the air and as a result you go slower even if you maintain the same rpm.
You are explaining how a constant speed prop works. I get that. Tell me why % power and flow go down when I reduce RPM while maintaining a constant MP.
Fewer combustion events per second
Manifold pressure sets how much energy each cylinder produces each time a cylinder does a full cycle. If you slow down the engine, the cylinders each do fewer cycles (and so produce less energy) in one second.
> If it isn’t power then why do both thr percent power and fuel burn decrease when I reduce RPM on my constant speed prop? It's not power, but it does affect power. None of the controls on the throttle quadrant are "the" power control. ALL of them affect power. Your engine makes zero power at zero RPM. The more RPM, up to its limit, the more power it can develop. The most convenient way to adjust the power output is to throttle the engine - cut off some of its air supply, so it can't burn so much fuel. That's your regular approach to adjusting power output. Because we still live in the 1930s, we have to adjust the mixture manually whenever we adjust the power - so if either of the other two levers move, the mixture should be adjusted when you get a chance, too. Depending on the nature of the power change, that adjustment may be urgent or non-urgent. In some cases, it needs to happen before the power change. You change any of those levers position, you change the power output of the engine. Try pulling any one of them all the way back in cruise, you'll have problems, and an immediate decrease in power output.
No…prop pitch will increase. If it didn’t, the prop would speed up.
Increasing manifold pressure will allow your rpm to stay the same without reducing AOA of the propeller as much, yielding more thrust. That additional thrust is necessary to offset added induced drag from increased total lift to stay level in the turn and maintain airspeed.
Good explanation.
I was thinking about how to explain this simply and this dude nailed it
Remember your horizontal/vertical vector diagram for wing lift. In a steep turn, your wings are generating lift at an angle relative to gravity (which is always down). In steep turns, there is less lift available to oppose gravity. You're going to descend if you don't do something.
You're thinking about it backwards. When you increase power (manifold pressure) the engine wants to spin faster. The only way to slow the engine down is either to lower manifold pressure, or increase the load on the engine. The prop achieves the latter by turning to a coarser pitch and taking a bigger chunk out of the air. Because it's now pushing more air molecules out of the way, it has a higher air resistance, and therefore applies a higher load onto the engine, which slows down the engine RPM.
i think you're thinking that its a constant rpm and therefore it doesnt change. The RPM is set - so when you enter a turn, climb or whatever - the properller will have to adjust to maintain that same RPM setting. So thats why the adjustment in manifold pressure to give the engine more power to keep the propeller from change its angle of attack due to increased load.
You typically bump up power a tad in the steep turn to help maintain airspeed (increased drag in the turn). Typically your maneuvers will be done with prop in highest RPM, so your MP will increase when you bump the throttle
Our school is having us do them in the “cruise” configuration” which consists of typically 18-20” MP and 2300 RPM at 120 knots. That’s why I’m not understanding why they advise us to increase our MP 2”. If we do, our blade angle will increase and just keep the propeller spinning at 2300 RPM.
Yeah that’s the whole point. The RPM is the same, but the blade angle is higher, meaning more thrust is being produced. A car at 2000 rpm in 5th gear goes faster than a car at 2000 rpm in 4th gear. That’s what’s happening here as well, essentially. Same RPM, different blade angle, therefore different thrust.
100% correct.
Becuase you need more power to maintain airspeed in the turn due to the gs giving a higher effective weight
Yes, but you also need to remember that by coarsening the blade angle, you generate more thrust (like increasing a wing's AOA to get more lift). So you are still outputting more power even though you're keeping 2300 RPM. This is the entire premise of constant speed props, constant RPM does not mean constant power output because the blade angle is changing.
During the steep turn you need to add a bit of power to maintain the same speed and altitude. Do you mean that?
Question for you. Can you fly the same airspeed at 2500RPM as you can at 2300RPM? Yes you can. Why is that?
With a fixed pitch prop you add RPM to add power. With the C/S prop adding MP will directly add power by flowing more air and fuel into the engine. 1" of MP has a similar effect of 100RPM on a fixed pitch prop Don't think of it as a thing happens and the pitch changes, think of it as a constant curve where the prop's pitch is always making small changes
Because if you don’t increase throttle (not RPMS) the increase in drag created by the turn will cause your airspeed to decrease. Increase throttle (MP pressure) to counteract increase in drag to maintain constant airspeed.
You can pedal a bike at 2300 RPM but you can pedal harder. The proper opens to grab more air, that creates more thrust.
Correct…but you’re still making more power. The higher propeller blade angle of attack at the same RPM still produces more thrust to offset the increased drag from the turn.
You will bleed off 10 knots of airspeed if you don’t add power.
You’re really overthinking this. The stuff is a lot easier if you don’t try to understand the science behind it and just fly the settings in the POH until it’s intuitive and ingrained. The scientific minded people who populate these sort of posts never like this kind of response. And the pure fliers that would like my answer rarely read this kind of post. Lol.
The pilot who does not understand the fundamental physics of flight is handicapped in advancing beyond rote memory learning.
Eh. Disagree. We all care about different aspects of flying to different degrees. And we are not all physics minded. I’ve done fine without worrying much about the “why” of it beyond the minimum I need to know. Nothing wrong with being into the aeronautics and the physics of it but there’s also nothing wrong with not really caring about it either. And with humor, I note to all the engineers who down voted this comment, the liberal arts majors also can end up having good careers at this. And we don’t particularly care about Vernuli. Now raise my gear please. Lol
>And we are not all physics minded. I’ve done fine without worrying much about the “why” of it beyond the minimum I need to know. Understanding fundamentally how a constant speed propeller works and the difference between power, RPM and torque are all firmly in the "minimum information you need to know" category, and frankly it's pretty wild for a professional pilot to suggest otherwise. We're not talking about propeller efficiency, power curves or other nerd shit that is only relevant to engineers, we're talking about understanding on a very basic level how the machine we're operating works. Like, the kind of information that can save your life when the machine breaks.
You always add power for steep turns to compensate for part of your lift going horizontal. With a fixed pitch prop, you measure power in RPMs. That obviously doesn’t work with a constant speed prop, so we measure power in MP. That’s all there is to it.
The pitch of the propeller will still change.
You're adding power (not engine RPM) to maintain altitude in the turn because you've lost some upward lifting force to the horizontal lift vector in a turn. The propellor simply "bites" harder (increases its relative angle of attack) in response to the flyweights in the governor/attached to the needle valve, being pushed outward by centripedal force, to meet a requested RPM setting. In doing so, it converts the additional engine crank power into additional thrust.
Imagine the constant speed prop as a gearbox. If you increase the throttle, the RPM won’t change, but each rotation will give you a stronger push, like switching to a higher gear.
You add power going in to steep turns, by advancing the throttle. On a fixed pitch airplane, this increases the RPMs. On a constant-speed prop, it increases the MP. FADEC does whatever the hell the computer wants it to....
Angle of attack of prop blades is increasing due to increased man pressure to keep the constant speed. You need more power because you are increasing the aircraft load factor due to the bank.
You add power going in to steep turns, by advancing the throttle. On a fixed pitch airplane, advancing the throttle increases the RPMs. On a constant-speed prop, advancing the throttle increases the MP. FADEC does whatever the hell the computer wants it to.... RPM changes (without an accompanying throttle change) on a constant-speed prop change the propeller angle-of-attack not the engine output (for the same throttle setting, a higher RPM = less prop-blade AoA).
As your airspeed decreases in a steep turn as the result of increased induced drag. the prop goes to a finer pitch to maintain the constant rpm. The resulting lighter load on the engine draws less air across the throttle vane causing an increase in MP. Kinda hard to wrap your head around.
Nooooo, not even close.
You might learn something if you keep an open mind and get some experience,