CVT airplane CVT

[jeb99]

Hi, I've been building an airplane for the last 15 years (whew!) I've got a Subaru EA81 turbo (should develop about 120 hp)
with a 2:1 belt reduction on it. It hasn't flown yet, but runs great (90% done - 90% to go type of thing). Anyway, wondering
if anybody out there has adapted a CVT for airplane or swamp buggy ? I want to let the prop "idle" when the
engine does - but keep the prop rpm constant while in flight. Normally this would be done by changing the pitch
on the prop blades. It seems like keeping the engine rpm at a constant - oh, say 2000rpm or 3000 or 4000 while
the prop turns 2500 would be really cool.
Anybody out there know of people who have done this type of setup? Info? Links to pages?

 

[bigoilbob]

Hi jeb99;

Know nothing about this CVT setup, but if you have not done so already, do one piece of housekeeping. Research how much your prop will flex under a normal duty cycle. I think that would mean from a full throttle ground start to a higher altitude cruise. My understanding is that many props, especially home built wood props, change pitch significantly, and thereby naturally give you some of the effect you want. Why add weight/complexity needlessly?

 

[jeb99]

I've got a 3 blade warp drive propeller - carbon fiber - not sure it flexes at all!
You're the first reply - seems like nobody has attempted this.

 

[bigoilbob]

As a paramotor pilot - nice prop! But I'll bet it does flex. Not a sign of weakness - just the opposite. It's a design goal for the makers. I have not done this myself, but I would e note the manufacturers and ask the question. You might be surprised at how much they know about their product. Even if they have not wind tunnel tested it, against different "flight speeds" and prop speeds, they could sim it with software from any reputable aeronautical engineering school. And I'm guessing they have......

 

[kallsop]

I am no expert, but I would think the added weight of a CVT is a major drawback in an airplane. What you are asking for sounds like a mechanical governor, or ECU, so the engine and prop run at a set speed.

 

[loquin]

It flexes. All materials flex under load.

It would let you adjust your engine between it's max torque and max power RPMs, while keeping the prop at a constant velocity. But, unless you have an adjustable pitch prop, I'm not sure that you'll get much effect.

However, weight would be an issue, as others have said. Industrial V-Belt adjustable ratio solutions exist, but for 120 HP, I'm sure you're looking at an automotive type CVT, which usually use a metal belt...

 

[bigoilbob]

Hi Loquin;

You're right when you say that all materials flex. But I am guessing that the natural flex of the CF prop will have "much" effect. Why? Because most fixed pitch props (even those behind your radiator) are designed from the get go to do just that. For flying, you want max flex at your standing start, for take off. But at altitude, cruising, you want less/no flex. You know about what these loads are going to be, you know the elastic modulus of your composite material, so you make it thick enough, and pitch it to flex as much as you want when you want and as little as you want otherwise. I'm no aero historian, but I think even the Wright Bro's designed for this in their wind tunnel, and WW1 props were definitely designed to exploit this fortunate material property. Jeb99's prop suppliers have it all over the older designers, and their design is probably much more efficient. As for the CVT, the auto engineers have a saying. "The best part in a car is the part you can leave out without significant consequence". I'm betting the CVT qualifies.