I've been doing some light reading on constant speed propellers. This is what little I know...

- When the control unit detects that the propeller is increasing speed, it will increase the pitch of the propellers to slow it down.
- When the control unit detects that the propeller is decreasing speed, it will decrease the pitch of the propellers to speed it up.

I know I'm missing a whole bunch of details, but let me ask my questions.

1) If the propeller is a constant speed, does that mean the engine is also running at a constant RPM?
2) What does the throttle do? Does it still control the fuel flow and air mixture in the carburator? I'm using RC lingo here.

Thanks in advance.
Juice

Many modern water craft use this principle. The engine is always turning at the same RPM. Starting from a stand still the propeller is very lightly pitch, but as the vessel increases speed the pitch becomes greater and greater till the maximum speed / pitch is reached. Uless there is a transmission or free turbine invloved the engine always turns the same speed. The Coast Guard has a 378" vessel(Cutter) that can go from 0 to 30 knots in 5 hull lengths, and stop from max speed in a little over two lengths. I have never heard of this being applied to flight though? The throttle increases useable horse power as it can be absorbed in the propeller without causing cavition, slipage.

Juice,

The prop is mechanically linked to the engine, so the engine is constant speed as well. The throttle is controlled by the pilot. The prop typically gets its signal from the engine oil pressure to vary the prop pitch in order for the prop/engine to maintain the same speed. The pilot monitors engine load (power) by the manifold gauge. Some new electronic equipment may be a bit different.

Bedford

My mind is stuck in the fixed pitch world of RC aircraft. []

In simple RC engines, the throttle input (Tx stick) is proportional to the amount of fuel/air flow (carburator), which is proportinal to the engine and prop RPM.

My trouble is imagining this... at low throttle the fuel/air flow is also low (producing a small bang at combustion), but the engine is still at the constant RPM (presumably high RPM).

This is possible because the props would have been adjusted to a very low pitch and the load on the engine is very small.

Small bang + light load = high RPM
Big bang + high load = same RPM

Am I thinking right?

Thanks,
Juice

I flew a CS prop airplane several hours before fully understanding what was going on. Once I got it sorted out it was easy. One pilot friend of mine has a CS prop Piper and he really doesn't understand what he is doing, or rather why...but anyway...


As was mentioned, the prop is always turning the same speed - RPM - as the engine. (for a fixed pitch prop and for a CS prop) Think of the prop as basically just an extension off the engine crankshaft. (there are some new diesel General Aviation engines which use a gear box between the engine and prop but it is not common to most GA planes)


In the airplane there is a throttle lever and a prop lever. I see your confusion on the engine power and fuel amounts etc. A concept to understand is that an engine can develop differing amounts of power at the same exact RPM. More or less fuel and air can be put into the engine at the same speed which will make it develop different power amounts. Normally we associate power with RPM but in a CS prop engine combo an engine can be set at differing amounts of power at the same exact RPM.

Power is set by the throttle which reads manifold pressure, that is the air/fuel going into the engine - some airplanes have fuel injection instead of a carb.

For a normally aspirated engine (not turbo charged) a cruise setting is commonly 22 inches of manifold pressure and 2400 RPM, but a lower cruise setting is 18 inches manifold pressure at the same 2400 RPM. The airplane will go faster at 22 inches than at 18 inches. At the 18 inch setting the engine is making less power but it is still turning the same RPM as at a higher power setting. The way the engine can turn at the same speed and have higher power is that the prop will actually be at a coarser pitch angle as compared to lower throttle setting. So the prop absorbs the higher power and converts that higher power into thrust by using a more coarse angle than at lower power setting.

Picture this: I am in cruise flight at 17 (or whatever) inches manifold pressure and cruising 110 knots. The prop is set at 2400 RPM (remember prop RPM = engine RPM) Now I increase the throttle to 22 inches and the speed increases to 130 knots. Now what happens is the engine with more fuel and air actually wants to increase RPM but the prop will prevent that by taking a larger bite of air (coarser pitch) to keep the RPM at the selected speed. So when the power is increased the prop will put a greater 'load' on the engine to keep the RPM at the selected amount. So the prop changes angle as necessary to convert more or less power which yields different airspeeds.


The most power the engine can develop is at full throttle and highest engine/prop RPM; that will be fine pitch which lets the engine turn fastest, usually up to the 2700 RPM redline in most GA piston airplanes. The most power possible will be at the highest RPM (which will be the highest fuel flow the engine can accept). So for takeoff the prop control is full forward along with the throttle which will give the finest pitch, this lets the engine spin up to full RPM to make the most power. But in cruise the prop control is brought back to slow the engine/prop to say, 2300, 2400, 2500 RPM or whatever. A lower prop setting (coarser pitch or bigger 'bite') is more aerodynamically efficient.

Pilots say "set the prop" at whatever RPM, but that is just the way of saying it, it really means setting the engine speed too because the engine and prop speed are one and the same. But a pilot may say "what do you cruise your prop at?" because the prop control is the primary RPM control in cruise as opposed to the throttle. Of course on a fixed pitch prop airplane there is only a throttle lever so it is used to change power and RPM together but the two cannot be controlled seperately from each other. (after getting used to flying a CS prop airplane going back to a fixed pitch prop airplane feels sort of primative and restrictive in operation. once you get used to a CS prop and fully appreciate the benefits and flexibility of engine speed and power management fixed pitch seems very simple)

In a CS prop airplane the throttle lever is more like a power or thrust control, and the prop control becomes the RPM control. (of course at low throttle the RPM will decrease no matter how fine/flat the prop pitch goes but that is at low power settings like used in the pattern)

So in short, the engine can make differing amounts of power at the same RPM and the prop angle (which is controlled by oil pressure) will change to convert the differing power into the air.

One more quick example to see what goes on. I am flying in level cruise at 22 inches and 2400 RPM at 130 knots indicated. Now I pitch the nose up into a climb without touching the throttle or prop control. The airplane now slows to 100 knots in the climb. What the prop has now done is go to a finer pitch angle because there is a a lower airspeed and climb load as compared to cruise. If the RPM stays exactly the same, that means the prop HAS to change angle between the airplane going 130 knots to 100 knots. All the while the pilot will not notice anything because you can't see the prop change angle. Now if I level back off the airplane will accelerate back to 130 and the prop will increase the pitch to a coarser angle - actually the prop going to a coarser angle is what makes the airplane accelerate back to 130 knots. {normally the throttle and the prop/engine speed in increased by the pilot in a climb but by not adjusting the throttle or prop in the previous example one can understand what the prop does}

Here is a column from Flying Magazine. Most of it is about diesel engines for GA but there is some explanation of CS props; click "2" bottom left, it is 2 pages:

http://www.flyingmag.com/article.asp...&page_number=1

Way to go wantsaneagle, that was done very well.

I take off at 2,300 RPM and 37 inches, cruise at 1,850 and 24 inches.That is about 55%. It ain't a Lycoming or brand "C".

I know that you are talking about piston engines, but as another example, I flew in a P-3 Orion for several years, with Constant Speed props (I was a Tube Rat - the pilots just drove us to work...).

Anyway, the engines were Allison turboprops and had only two speed settings - high and low. That's because the Turboprop (at least this particular engine) was efficient only in a narrow RPM range. The "Low" setting was used for taxiing and other ground work, the "High" speed was used for normal flight. The principle of changing speed was the same as has been stated earlier, to increase speed the throttles were adjusted to provide more fuel to the engine and the props increased pitch.... and vice versa.

Bob

The Turbo commander uses the same system. 2 speeds, both are very loud. I don't remember exactly but I think it was 85% and 100%. You change power to go faster, as stated above, more fuel equals a bigger bite and more speed.

On a model engine Juice, at low RPM the prop may spin like 1000 RPM and at full throttle, you could get max RPM depending on the engine, 5000 to as much as 20,000 on some of the racing engines. The prop does not change pitch so you have to increase the RPM to increase speed or climb power.

Thanks wantsaneagle. That is a perfectly clear and concise explanation. I understood it the first time I read it, but I read it again just in case.

This sentence is the key to understanding how a constant speed prop works.

Thanks again,
Juice

I've been doing some more thinking about this subject.

I can see how a constant speed (CS) prop can improve the performance of take-off, climb and fuel efficiency when compared to a fixed pitch (FP) prop. Can a CS prop also increase the top speed when compared to the same airplane with a FP prop?

I think the answer is YES, but I'm not 100% convinced. The pitch of a FP prop is chosen so that it has acceptable performace at high and low speed. Which means that the pitch of a FP prop is not optimized for top speed. So if the CS prop can go beyond the pitch of the FP prop, the top speed will increase. Right?

But this is where I'm getting hung up... In the fixed pitch world of RC aircraft, when you want more pitch (speed) the diameter of the prop has to decrease to keep the same RPM. How does this rule of thumb translate to the constant speed world of full size aircraft? A CS prop doesn't shrink when the pitch is increased too far.

Thanks again,
Juice

Juice,
In the real world, when pitch is smaller, speed (airplane through the air) is smaller. If you want to draw a parallel to our model world, think of RC helicopters. The rotor of an RC helicopter is a constant speed prop. Helicopter pilots often refur to the helicopter's head speed, i.e. 1,800rpm. They then adjust the throttle curve to the pitch curve, so that when pitch increases, the throttle increases and the head speed stays constant. They also often utilize a governor, which reads the head speed and adjusts throttle accordingly.

I am not sure, but I would think that the reason why on real piston engine powered CS air craft, the engine load is determined by manifold pressure, is because that is a very good way of determining how hard the engine is working, and it is important to keep the engine alive.

Juice, I am the original poster "wantsaneagle" just with a different screen name.

That's an interesting question - which shows you have thought it out well -, though I am not sure I have 100% of the answer.

For one, once you get to GA planes with 200 hp or more, they all have CS prop engines. I guess there are a few exceptions (especially in the past but not new production) but nearly any piston engine with 200 horses or more will have a CS prop engine. I fly a 200 horse Piper with one but for example nearly every Cherokee 140 or 180 is fixed pitch. Though there are a few 140's I have heard about with a CS prop installation approval though I don't remember hearing that they picked up significant speed by having the CS prop. They pickup a handful of knots but not much more. You would think it is more but it isn't. (and Piper Cherokees aren't very fast as it is...it's not like the airframe has 30 or 40 knots to spare either, at least not in the green airspeed arc) What they do get is much better climb (because it's like shifting into low gear with a fine pitch with CS) and better efficiency.

There are some Reno race airplanes, things like Midget Mustangs and I'm pretty sure they have fixed pitch props. I saw a video of one taking off and it needed a real long roll because it was propped for speed so it took long to accelerate. With a plane propped like that a CS prop probably wouldnt make much difference in the top speed. Those little racers also have relatively low power engines because they are so small, like 100 hp or 150 hp or so.

I don't think you can compare the pitch and diameter much to the RC world. In theory yes because laws of physics don't change, but the application with respect to engine prop speed is quite different. (i.e. 2700 PRM Piper compared to 20K RPM or whatever nitro). Although propping those little Reno race planes is I'm sure a big topic among those pilots, just not that I am very familiar with.

You know all those big powerful engine WWII aircraft? Well they all have CS props too. If one had a fixed pitch it would be much less efficient but more importantly much, much less restrictive in operation. The takeoff roll with a fixed prop on one of those would be huge, and it just wouldn't be able to perform in climb and cruise like the engine is capable of providing. Most of those are turbo or rather super charged so they really have to have the prop change angle because they can make such large power changes at different manifold power settings. With a fixed prop you could not take advantage of the large different power settings available with those big high HP engines. So it's just not feasable that those airplanes could even have a fixed pitch prop. Sure one could fly on one, but the performance envelope with regard to takeoff, climb, altitude, speed, would be a fraction of what it is with a CS prop. And with planes like those, the differen

The way I see it, the main idea/goal with the Constant Speed Prop is that you are increasing the efficiency of the engine/prop combination at various power settings and RPM settings.

In fixed pitch aircraft, there are two basic types of props selected, the "cruise" prop and the "climb" prop.

The cruise prop has a higher pitch to it. In RC terms think of it like a 10X10 prop, it takes a large bite of the air. This increases top speed, but is slower to accelerate. This allows the aircraft to increase its speed at the most efficient power setting ie the cruise power setting.

The climb prop has a lower pitch to it. In RC terms think of it like a 10X4 prop, it takes a small bite of the air. This increases acceleration and climbing ability, but has a lower top speed. This allows the aircraft to climb faster at the highest power setting because the prop puts out a higher amount of thrust at lower airspeeds.

The constant speed prop, covers all of these bases. It is a "climb" prop at take off (Low Pitch/High RPM) and a "cruise" prop at cruise (High Pitch/High RPM). The constant speed prop also covers the gap between these two basic types of fixed pitch prop. Increasing the overall efficiency of the engine/prop combination.

I have an excellent graph somewhere that shows what I am talking about, I will try to upload it later.

Juice, with respect to that above quote: in a CS prop airplane, when the blade pitch changes via moving the prop lever the RPM does change - it is supposed to change. That is the desired result, so you actually wouldn't want a CS prop to shrink in blade length (even if it were feasable).

Except at low power settings, any movement of the prop control is going to change the RPM of the engine. (that is actually how you know you are changing the pitch because the engine will change RPM, otherwise you wouldn't know the blade angle has changed because you certainly can't see the prop, and if you do clearly see the prop you are in trouble!) But when flying we don't really think of the prop control in prop pitch changes even though it does that, it is viewed as an engine RPM control.

So it is not like RC where you are striving to keep the same top/high engine RPM when changing pitch, which would be accomplished on an RC engine by decreasing the prop diameter when increasing the pitch to keep the same RPM. A CS prop will keep an exact engine RPM at whatever you set it at and that RPM setting is changed by moving the prop control to bring the engine to whatever RPM you want. By moving the prop control back it places a bigger load - greater bite of air - which is what slows the engine down a few hundred RPM, which is what you want. Does that make it any clearer or did I confuse you more?!

Juice,
Like racer said, in RC you decrease the diameter when increasing the pitch to keep the same RPM. In full scale, cruise RPM is lower than climb or takeoff RPM. So when you increase pitch the engine slows down. This is desirable and is the same thing that would happen in rc if you increased pitch while maintaining diamater.
In the King air (and other PT6 powered airplanes) we have a much larger range of engine operating options than in the direct drive turboprops mentioned. The prop, reduction gearing, and power turbines are not attached to the engine. Hence the "free turbine" designation. Engine operation is independant of prop operation. You can hold the prop during engine start and run the engine with the prop stationary. We have a full range of engine speeds available from 52% (idle) to 101.5%. If I remember correctly 101.5% is 38,100RPM. On takeoff, the prop reaches 2000rpm (red line) around 1000-1200 ft-lbs of torque. (Torque is just like MAP on a piston) So that is how much power it takes to turn the prop 2000rpm at its finest normal pitch. As power is increased to the 2230 ft-lb max the prop increases pitch to absorb the extra power to maintain rpm. I have a picture taken in flight of the prop pitch. This was probably around 20,000ft cruising around 270 TAS with the prop RPM at 1700rpm and engine power around 1800 ft-lbs. I was shocked how much pitch is on the prop, although I just worked it out to a pitch of 193". I don't know if that is is correct or not but it is impressive.