# Constant Speed Propeller [closed]

Dear All I know that this topic is not new, but I really tried to grasp the concept using available sources (which are actually pretty much the same). So, if possible (really appreciated):

1. I'd ask for an practival example comparing two aircarfts (one with Constant Speed Propeller and another without) in the same conditions showing the advantage of the first one. Something like "you see with this one (without) our hands are tied and we can't do anything so we are buring fuel, wearing enging, etc, and with this one (with) we can just adjust and ... ".

2. I'd ask to clarify why fine pitch is considered more efficient (e.g. during the take - off). I can't understand why we increase AOA of the wing in order to produce more Lift and in regards of Propeller (which is basically the airfoil we consider low pitch as MAX performance)

3. last one... there are a lot of explanations where I see "and now propeller increase pitch and slow down..." Are we trying to limit engine output in a way of applying some resitance to the engine? If yes, why?

Sorry for potentially stupid questions, but I really can't get this one.

Thank you!

• I don't know if you've already checked our existing questions on constant speed props, some of them might be helpful, e.g. this one on blade pitch during takeoff. Also please note that we strongly prefer to have only one question at a time; the tour might be useful if you're new to StackExchange. Feb 14, 2022 at 17:04

You are correct that AoA is very important for propeller (airfoil) to be efficient.

The only reason to change pitch when going faster is to account for the change in relative wind because the aircraft moves forward faster and faster while propeller spins at a constant rate.

So, we have fine pitch for take-off run and climb (slower), then adjust pitch to coarse for cruise (faster).

We always try for best airfoil AoA to be most efficient based on prop rpm and airspeed as illustrated:

• Thank you. Any chance there is a picture which will visualize this? Somehow. I mean this relative wind component and it's direction. Feb 14, 2022 at 16:36
• @Sergey see added picture Feb 14, 2022 at 18:13
• Thank you, trying to get my head around it. Feb 14, 2022 at 22:05

Pushing a prop into fine pitch is like downshifting your car's transmission. In both cases, the engine is allowed to spin up to its maximum speed and thereby develop maximum power- for taking off or for passing a slow-moving truck.

Pulling the prop back into full deep pitch is like shifting your car into its top gear. In both cases, the engine speed falls which reduces the amount of power it can make but increases fuel mileage and decreases engine wear. This is best for constant-speed cruise under the most economical conditions.

And the prop pitch can be adjusted to any desired value between the engine's redline and idle RPM, almost like a 5-speed car transmission. "Mixing up the gears" lets you choose the best combination of power and economy for any flight regime.

• I was going to offer the same comparison. Thing is though, depressingly few people can even relate to shifting gears any more. Maybe on a bicycle? Feb 15, 2022 at 4:48
• @MichaelHall, what an amazing world we live in. I've driven 3-on-the-tree, 4-on-the-tree, 3-on-the-floor, 4-on-the-floor and 5-on-the-floor... oh yes and I knowhow to use a rotary phone! Feb 15, 2022 at 4:51

This answer takes a different look at the constant speed propeller. I was confused about them too, many years ago.
I think the confusion started because everyone always describes the prop lever as controlling the pitch, and full forward as decreasing the pitch. Leading to the assumption that full forward must be least pitch or flat. And as we all know a "flat" blade (i.e., zero angle of attack) produces no thrust. So why do you want no thrust for takeoff?!? But this is not the way it works (or at least , not the way to think about it)
Let's start with the name: constant speed propeller. As the name implies a CS propeller tries to maintain a set RPM in the face of changes in engine power settings.
Think of the prop as always trying to absorb all the power the engine is putting out and thus reach equilibrium.
The prop lever sets the prop RPM or really, the upper speed limit of the prop. Full forward says, "Go as fast as possible".
Of course, for a given power setting, the prop can only go faster by decreasing the blade pitch.
At any given prop setting, the prop is basically modulating its pitch in order to maintain the selected RPM. If it's going too slow, it reduces pitch which allows it to speed up. If it's going too fast, it increases pitch which forces it to slow down.
Essentially, the constant speed prop always tries to absorb the engine's power at the speed selected by the pilot.

For takeoff one wants the most thrust you can get and since the blade is just an airfoil and the thrust for the propeller is really the lift generated by the blade's airfoil, thrust (1/2 ρ V2 CLα α A) goes up more quickly with the relative wind velocity (V) than it does with angle of attack (α) so we would like the highest V (RPM) that we can get. But here's where the confusion comes in. When you set the Prop to fine pitch it doesn't go to "flat" and stay there. What you are really doing is saying go to "full fast". Once the blade reaches its top speed allowed by the prop governor it will try to maintain that speed by increasing the blade pitch in order to absorb the rest of the power put out by the engine. Therefore, in this takeoff configuration you're developing maximum engine power and running it through the prop at it's maximum speed, thus developing maximum thrust.

Now, what happens when you reduce the prop while maintaining full engine power? Well, the prop sees it has to slow down, so it increases blade pitch. This increases drag on the prop which is seen as torque at the engine. With that increased load on the engine the engine slows down. (Think about going up a hill in a car with out adding pressing a bit more on the accelerator) But note that load on the engine really transfers back all the way to the cylinders and pistons so that the pistons see an increase in resistance to motion. Inside the cylinders this "push back" from the pistons results in higher pressures being developed in the cylinders and higher torques in the drive shaft. Too much torque in the engine and it breaks. Too much compression in the cylinders and it starts acting like a diesel and detonates prematurely. So,clearly, pushing too much power through the engine/prop is bad!!

To prevent this you always need the prop set so as to be able to absorb the anticipated new engine power setting. So when moving to a higher power setting adjust the prop first so it is allowed to speed up before adding power. And when moving to a lower power setting, always reduce the power first before slowing the prop (i.e., increasing blade pitch/torque).

Also note that if the engine is not producing enough power, the prop attempt to go to its lowest pitch- trying to get the RPM up, but it won't be able to. This is what happens when you've got the prop lever full forward but the throttle at idle.

As far as why a CS prop is better than a fixed pitch prop, it comes down to being able to control the angle of attack of the propeller blades.
We know if you set too low of an RPM at a given power setting the prop will try to absorb that extra power by increasing pitch. It will do this without regard to how efficiently the prop is operating. That is, it would take the blade all the way to 90 degree angle of attack if it could to try to slow the prop down. Obviously this isn't desirable since it would not be generating any lift/thrust - it would all just be drag. You can get out of this inefficient condition by setting a higher prop speed, causing the prop to decrease blade angle of attack in order to reach the higher RPM, or by reducing power also causing the prop to decrease blade angle of attack in order to maintain the set RPM.
At some point in its travel from full blade pitch to no blade pitch the prop will cross a point of "optimal" blade pitch for the given flight conditions. Therefore for a given set of flight conditions there is an optimal power/speed combination that allows the prop to operate at its optimal angle of attack.

For best performance you always want to make sure you demand an RPM that "matches" a given power setting so that you keep the prop operating at its optimum efficiency. The more useful and important of these combinations are given in the aircraft's POH.

Other answers here discuss how the angle of relative wind as seen by the prop changes with RPM and forward airspeed so I won't repeat that. But the thing to remember is that unless you can change the blade angle, then for any RPM there is only one forward airspeed that delivers optimal blade efficiency. if you're not there then you are getting more drag than you need to for the given thrust generated. Some props are designed to give best performance in climb ( a slower airspeed), some at cruise ( a higher airspeed) and some a compromise somewhere in the middle. They still work, and are less expensive to maintain and are easier to operate but they aren't as efficient.

Bottom line: Don't think about setting a blade pitch, 'cuz that isn't what's happening. Think about setting the RPM at which you want the prop to absorb the engine's power.

Hope that helps.

• Thank you a lot for detailed answer. You got the root of confusion very precise (I mean "flat on take off" especially). I think I need to read it again to grasp all details, but for now could I clarify one thing - if the CSProp is able to find an optimum - why do we have the level to set a limit for it? Would it be enough just to ajust power (throttle) and to achieve equilibrium by govener work? Why do we need the possibility to say "don't go this fast" using the language above (it helped by the way). Also, relative wind here (just to check if I got it right) is what blade hit, not aircraft? Feb 14, 2022 at 22:36
• @Sergey - It can’t find an optimum by itself, it only provides the means by which the pilot can set it if he knows it.
– Jim
Feb 14, 2022 at 22:40
• Thank you, and one more question, if possible - one question from exam samples "Which statement regarding the "constant-speed propeller" is correct?" has the right answer "The pitch of the propeller rises with higher speeds." Is it aircraft speed or blade? Because if it's a blade - I don't understand the tendency "high RPM - lower pitch and vice versa". THanks again. Feb 14, 2022 at 22:50
• Sergey - That question (with the context you've provided here) is not a very good question IMO. But if they say that's the right answer then they must be talking about a/c speed. (See the drawings in @Roberts answer) Note that to achieve the same AoA the blade pitch must be higher when the prop RPM is held constant but the a/c speed is increased. But while it's possible to work backwards to justify this answer because we know what it's supposed to be. It wouldn't be possible to know whether they were talking about blade speed or a/c speed in the exam. Unless the other answers were false.
– Jim
Feb 15, 2022 at 0:04
• @Sergey note that the propeller governor on the engine has no idea what the propeller blade angle is. It just knows that when it sends oil this way, governor speed goes up, and that way, governor speed goes down, and it wants speed to stabilize at some value based on spring tension on internal fly weights. What blade angle results from what RPM, manifold pressure and airspeed condition is a function of the propeller's design and how it is all matched to the engine. Feb 15, 2022 at 5:25