ORIGINAL: 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, 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