CVT-PRSU what do you think?
Continuously variable transmission propeller speed reduction units – what do you think?
In the Oct 2011 issue of EAA's Experimenter eNewsletter, EAA member Vince Homer submitted an article that asks the above question.
The article is merely a discussion on the potential of utilizing the technology found in the continuously variable transmission driving some all-terrain vehicles, snowmobiles, and a small number of automobiles. The idea is similar to a constant speed propeller except the engine speed (and horsepower) is increased, or otherwise variable, with the prop speed staying relatively constant. Some may already see a flaw in this concept. However, please read the article and share your thoughts.
Experimenter eNewsletter Editor
Please note that the article in the Oct issue has not been launched at this writing. Look for it in your email inbox tomorrow afternoon, (October 18th). If you are not a subscriber to Experimenter, click here: http://www.eaa.org/experimenter/
I have been considering this, but I haven't messed with one since we used one on our Formula SAE car back in the late 80s. My initial thought when I was pondering it was how would you go about deciding how to pitch the prop, as I believe the prop pitch is based on available HP at a certain rpm. (I would guess pitch it for cruise power & pick a prop RPM, say 2500? but how would it behave on takeoff?)
Taking my thoughts a bit further, i am planning on running a supercharged 302 Ford, which I would probably cruise somewhere between 3500 - 4000 engine rpm, with the prop at say 2500 rpm. On takeoff you could take the engine RPM quite a bit higher, but you would not want the prop RPM going more than say 2600 - 2800.
Using a fixed pitch prop with a variable transmission, if the prop was pitched for cruise you would not be getting peak efficiency at takeoff power settings.
(At least those were my thoughts at the time)
We've had discussions on this over on homebuiltairplanes.com. The usual conclusion is that a CVT doesn't make nearly as much sense as a controllable pitch prop (constant speed or otherwise).
This just seems all wrong to me. Constant speed propellers turn at a near constant speed, but the pitch changes with the speed of the aircraft. Take off=fine pitch due to low airspeed. cruise=high pitch due to higher airspeed. A fixed pitch propeller is at best a compromise. Most efficient at one airspeed at a given rpm. Varying the rpm and torque of the engine does nothing to increase the efficiency of a fixed pitch propeller turning at a constant speed. Am I missing something here?
I don't think so, I was thinking about this one night when I couldn't sleep & that was pretty much what I came up with.
Originally Posted by Chris
This is one of the challenges we have to deal with if we want to run a non-aviation engine (or any engine with a solid crankshaft). I haven't seen a good solution yet, the electric props aren't really true constant speed props, I liked some of the things about Bud Warren's Geared Drives design but after the latest crash that is probably dead (and from what I have heard the problems were with the GM engine management system, not the redrive)
As the author of the article I admit bias toward this concept. If you look at a plot of propeller efficiency vs advance ratio one thing pops out. At higher pitch angles the efficiency curve is rather flat for a wide advance ratio range. Conversly, the curves are very peaky for shallower prop angles. My thought was to test at high prop angles at speeds from fero to whatever I can get with my test stand and to do this you need to be do this you need a variable ration PSRU.
I am just starting to build a rolling engine/CVT/propeller test stand and will follow up this article when I have some data.
I am also still looking for a propeller calculation program that will accept RPM, Prop dia & pitch and HP as inputs to calculate thrust and speed.
The one advantage I can see, is whether you were running a constant speed prop or a fixed, using a variable transmission you could run an auto engine up to 6 - 8000 rpm on takeoff & get a lot more power, then back it down for cruise. I wouldn't want to run an auto engine over 4000 at cruise, but I have run a 289 at 8000 rpm peak for short durations (for road races basically "cruising" at 6000), and they will do it without flying apart
A CVT makes a lot more sense on a test stand, when you're not only designing a propeller but also trying to determine the best gear ratio for it.
Originally Posted by Vince
For a prop design program I suggest taking a look at Javaprop. But you're not going to get thrust and speed out of it; speed is an input and it tells you thrust. Also blade area is a factor.
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