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Thread: CVT-PRSU what do you think?

  1. #11

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    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.

  2. #12

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    Hydraulic CVT

    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?

  3. #13

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    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.

  4. #14
    David J. Gall's Avatar
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    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!

  5. #15

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    Power and efficiency curves

    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

  6. #16

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    I did find another link fully explaining the Honda HFT (on their own site, of course!) http://world.honda.com/motorcycle-picturebook/HFT/

    On page two, there's a nice "how it works" section. However, I looked in the parts catalog and the whole assembly is about 4 grand as a "replacement part." For reference, the whole bike new runs about 9 grand now, so that's a good chunk of what's there. The included engine is only 60 horse, so it might or might not be usable in some kind of Frankenstein conversion. Hopefully this isn't too much of a diversion from your project, but I kind of think something belt free would be less failure prone. (Or maybe more so? Hard to say.)

  7. #17

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    I have thought about this topic for a long time. We chatted about it some on Yahoo! group small4-strokeengines, but not as deeply as we have here. My original idea wasn't to functionally replace a constant speed prop. I was attracted to the CVT PSRU because of these factors:
    - I was involved with small aircraft with engines in the 30-50 hp range.
    - the CVT is already manufactured with parts robust enough for the engine hp, torque and rpm, assuming you use the same engine.
    - Depending on how much you have to modify to get it to work (if even possible) it may be lighter than extant PSRUs.
    - if you are working from a donor vehicle for the engine then the CVT probably comes along free, perhaps giving a $$ savings.
    - I was looking for, hoping for SOME benefit from the 'variable' feature. It was never my expectation that this would become the major focus of such an install.
    - you have the benefit of thousands of times more in-service hours for a typical CVT than a typical after-market limited production PSRU. Compared to scooters and quads, any aircraft PSRU is limited production.
    - you benefit from an existing pool of trained mechanics (limited value here), and an expensive parts distribution system in place.
    - because of the years of racing CVTs there exists a mature body of knowledge on tuning these CVTs, assuming the knowledge transfers somewhat intact into aircraft apps.

    Thus I think the diverging discussion to the Honda HFT misses the point. Oh, it's interesting, but it is essentially a different discussion. Yes some cars and heavier utility vehicles use CVTs but I was focused on lighter/cheaper quads and motorscooters with CVTs.

    On the reliability of the CVT PSRU... I have no clue. I *do* know that Honda was sued because a drive belt failure on a Honda Reflex 250cc scooter on a busy freeway resulted in the rider being struck by a car and loosing a leg. The drive belt failed because of excessive wear. It was not a premature failure because of some engineering weakness. Honda won but it was a squeaker. They recommend replacement at 12k miles now though extrapolation of wear indicates that the belt should be functional out to 18-24k miles. Depends on the rider of course because some have already exceeded these distances with their belt. My point is that MANY scooters use these CVTs. Many man-miles are delivered by them. People successfully depend on them with stakes nearly as high as in aircraft apps. For the most part they are trouble-free given normal suggested maintenance. I expect the same level of reliability in aircraft.

  8. #18

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    Also the CVT response to rpm and load can be modified extensively. There are a number of approaches to this some of which can even be combined for even more control over response.

    If weight was a huge factore the 'variable' feature can be sacrificed in order to save weight. It is easy to lock the sheaves in place. Some mods to save weight:

    - remove the fly-weights.
    - typically there are a few other parts associated with the moving fly-weights that could be tossed if the fly-weights were removed, such as any retaining parts and the cover. The cover is a stressed part as the weights exert presure on it.
    - remove the centrifugal clutch
    - remove the large spring on the driven side of the CVT.

    After removing all of these extraneous parts you basically have just a PSRU, depending on how you have to lock the remaining clutch parts in order to still transfer power. There are few parts here that are not on the average PSRU. You would still have the down-side of lower efficiency which might be a deal-breaker. You would have to address the cooling issue no matter which direction you went with the CVT- you could keep the stock housing to use the stock cooling system or maybe a free-air system of some sort.

  9. #19

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    Cvt

    Removing the variable feature kinda defeats the purpose. As far as weight reduction, the flyweights actually weigh only a few onces. It would appear that without the springs and flyweights the unit would require some sort of "shims" to set the ratio. In any case, I'm sure that a cog belt PSRU would be more efficient than using a locked CVT.

    Along this line. I just read an article in the Nov. 1987 "Kitplanes" about Holcomb's Perigee aircraft testing. They went to a variable pitch prop for several reasons, one was to keep the engine from overspeeding with the fixed pitch prop they initially used. A CVT would eliminate this possibility is the prop was choosen correctly. Even if the engine was a little underpropped, the ability of a CVT to "overdrive" would still keep the rpm in range.

    Vince Homer

  10. #20

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    I might be wrong in my thinking, but suspect the CVT in aircraft is a answer looking for a question

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