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.
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.
Hello all,
I'm not sure I see the advantage either, just based on the fact that these are typically used to keep the engine right in the powerband for road use through the whole range of road speeds. Doing this reverse stuff is probably just going to take away some performance, right?
That said, I'll play along: how about Honda's HFT CVT for the job? https://secure.wikimedia.org/wikiped...y_Transmission
It's hydraulic, so no belt failures here! It should also work great through its entire range of motion, while not requiring anything special at the extreme ends.
Edit: some cutaway shots of it in this video: http://www.youtube.com/watch?v=5b4_MIe5byI
Last edited by jmccreight; 10-20-2011 at 06:54 AM.
Anybody have any idea how much one of these units weighs or what their operating efficiency is?
The CVT belt/chain type units appear to be rather heavy. Anyone know what they weigh?
They claim in the video that the hydraulic method is more efficient, though I don't know what the weight or exact gains are from such a system. Unfortunately, I also don't have a DN-01 to disassemble, and even if I did, I can't imagine I'd want to.
Vince, you're looking at the wrong curve. Don't look at the efficiency curve, look at the power absorption curve. It goes as the cube of the RPM with minor variations for airspeed and propeller efficiency. (Reference: Fraas' "Aircraft Power Plants", McGraw-Hill 1943, page 88.) So, at full power (not full throttle like on the Lycoming, but at full power) the prop rpm will be very close to its design ("max cruise") rpm even at zero airspeed. (The prop rpm will be a little less than cruise rpm because the prop efficiency is so bad at zero/low airspeeds.)
Then the CVT will allow the engine to spin up to its rated rpm in order to deliver its full power to that prop from brake release all the way up to full-power design airspeed for the prop (and airframe!). However, be careful in your choice of CVT so that you get a range of ratios that will limit the propeller's rpm. You'll want a CVT that is at minimum reduction ratio when you get to cruise airspeed so that your CVT will not allow the prop to overspeed when you glide. Also be sure to use both an engine tachometer and a propeller tachometer, since propellers have operating limits, too.
I think you'll have success with this if you choose components that are matched in terms of horsepower and rpm and put them on an airframe that is matched to the design airspeed of the prop and the horsepower. (I shouldn't have to say that but so many good projects have gone astray on those "details"....) Might I suggest a Cub or Champ as a good place to start when you get tired of the "sled." And try to get an engine/CVT combo that will allow you to use the Cub's same prop for good comparison testing. However, I think that, given the same horsepower as the engine you're replacing, the only advantages you'll see are shorter takeoffs and modestly improved climb. (OH! And a cheaper engine package, too?)
I can't wait to hear a progress report!
Thanks for the reply.
In the article I show the propeller power absorbed curve and compared that to the engine power curve. I was referring to the efficiency vs advance ratio curves to try to convey my thoughts about operating a fixed pitch prop at varying airspeeds.
The main message of the article was that with a fixed ratio PSRU the propeller power required and engine output would only be matched at one RPM and one airspeed. Assuming that you are comparing a fixed ratio PSRU/cruise prop then I agree that the cruise speed would be the same with a CVT. However, it is my contention that, as you say, the takeoff and climb will be improved with the CVT.
I need to get out to the shop where the test stand needs finishing.
Vince Homer
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