CVT CVT for use on gyrocopter prerotator. Is it possible?

Discussion in 'Transmission / Drivetrain' started by shroomer1966, Jul 22, 2012.

  1. shroomer1966

    shroomer1966 New Member

    Hello all.

    Am in the process of building an autogyro, like this one

    The main rotor is not driven, unlike a helicopter. it freespins. The propulsion is from the rear prop.
    To takeoff, there is a system called a pre-rotator which is engaged and which then spins the rotor head up to about 250+ rpm.
    It is then disengaged, and the machine runs down the runway like a plane until the rotor rpm is high enough to take off.

    I want to build a mechanical pre-rotator system. essentially there is a cog just behind the rear (driven) prop onto which a belt is tensioned. It spins a shaft, and the shaft
    then transfers this power to the main rotor via some universal joints and another set of cogs (which also engage and disengage).

    My reasoning for putting a CVT in the line. Initially it will give a soft start, allow for a smooth transfer of power to the rotor head, and will hopefully allow for a maximum transfer of power at the higher end.

    My query is this.

    The engine is a rotax 582. Engine specs are

    The gearbox reduction is 2.58:1

    I would like to engage the pre rotator from idle up to 5000 rpm, so the shaft from the engine (to apply to CVT) would be running at approx 390 rpm to 1950 rpm. This could be geared up or down dependant on cog size I choose, so I can choose what rpm to send to the CVT primary input drive. The output can also be geared, depending on the cog ratio I apply to the rotor head.

    My query is this.

    Would a CVT be suitable for this task. I only need to apply perhaps 5 hp to the rotor head to get it to spin to speed, I am aiming for 300 rpm.
    Given that the engine output is approximately 45 hp at 5000 rpm, if this was applied (gently, to spin the rotor up slowly) to a small CVT capable of transferring 5+ hp, would the CVT be capable of handling the power without damage.

    If suitable, what CVT would be best. I am looking for the lightest most compact unit that would do the job.

  2. bigoilbob

    bigoilbob Member

    The CVT might actually be less "gentle", because of torque multiplication starting up. Remember, that unless the shaft was designed for rotation/prerotation, it's torque carrying capability is just an extra benefit. Tortional stress also reduces the pull strength of a shaft, and I wouldn't want you dodging a very expensive, lethal, whirlygig during your take off roll. But as is obvious, I actually know little about this. I know that gyrocopter prerotating systems have been around for a long time, but are CVT's used in them? If not, I would find out why not. Not much new under the sun, and why reinvent gyrocopter prerotation technology needlessly? Especially because you might be messing with your health and wealth.
  3. shroomer1966

    shroomer1966 New Member

    Thanks for that. I realize that a CVT has never been used on a pre-rotator before, but then again small light CVT's have not been around for long either,
    so that may be part of the reason. People are putting electric pre-rotation systems on gyro's now as well, only because light, high power lipo batteries have become available.
    They were not there years ago either.
    New tools become available, and people try to adapt them. It may be possible, and it may not, that is what I am trying to find out.

    The pre-rotator system will both engage and disengage from the engine before flight, and indeed even before the take off roll. The CVT in line
    has no bearing on that fact, so it should have no safety implications during flight at all. The CVT may introduce additional forces on the drive shaft spinning the main rotor,
    but I would have thought not. There will only be tortional, not pull stresses involved.

    Any pre-rotation system will be thoroughly tested before it is used.

    Is it possible to adapt a small CVT to use inline on a mechanical drive system of this type? It is only turning a rotor blade instead of a wheel. To my mind, theoretically it should work, and that is what I am trying to find out
    from someone who knows CVT's better than I do.
  4. bigoilbob

    bigoilbob Member

    I'm out. You obviously know much more than me on the subject, But I hope you do lots of due diligence and keep safe. I'm a paramotor pilot, but just totally safety crazy........
  5. shroomer1966

    shroomer1966 New Member

    I fly ultralight trikes as well, am the same way about safety. I did a couple of para-glider courses, but did not have the nerve for it!
  6. Skyliner70cc

    Skyliner70cc Active Member

    Not a fan of CVTs, they don't engage gently. Why overcomplicate the pre-rotator. Just engage it with a manual clutch with engine at idle and slowly bring the rotor rpm up to desired RRPM.

    A small snowmobile or ATV CVT should easily handle any horsepower you throw at it.

    I'm curious, what does a gyro such as that cost to build? I intend to get one eventually as being in a constant state of auto-rotation is kinda neat, ya think?
    Last edited: Jul 23, 2012
  7. Skyliner70cc

    Skyliner70cc Active Member

    Not a fan of CVTs, they don't engage gently. Why overcomplicate the pre-rotator. Just engage it with a manual clutch with engine at idle and slowly bring the rotor rpm up to desired RRPM.
  8. shroomer1966

    shroomer1966 New Member

    The engine idles at around 1000 rpm, so any initial slippage for the rotor head to catch up to the engine rpm would have to be done by a belt.
    With a reduction ratio of 2.58, the speed of the pulley from the prop (to engage the drive shaft) would be around 400 rpm. I was hoping that a CVT might be able to absorb these initial
    rpm without engaging the output drive, thereby saving slippage on the belt, and make for a smoother process.
    Those were my thoughts. The original mechanical system as you described just used belt slippage as a clutch of sorts.

    The CVT's used on a lot of small scooters (125cc 4 stroke) generally absorb lower rpms, and then gently engage as you increase the engine speed. I was hoping to do a similar thing with
    the CVT on the gyro.

    Cost of gyro's? Depends very much on how you approach it. I am building it (or getting it built) off plans. Main costs are rotor head, rotor and engine. First 2 will be new I assume, then the rest is up to you how you go about it. Probably figure somewhere in the 10 -15K USD range if you do a lot of the work yourself. Some of the earlier Bensons or similar could probably be built a lot cheaper.
  9. buggy064

    buggy064 New Member

    I got those plans for a gyrobee it does not recomend preroter it makes it hevy more than a altralight. I did see like a starter motor push a button engages a sproket turns the rotor build up speed I thank I seen something like that on there web site it seems like it doesent takle to long if you push it with your hands.
  10. HeadSmess

    HeadSmess Well-Known Member

    what a peculiar thread....

    i like autogyros :) two rc, the hub from a towed glider plus half the frame, and a kite. yep, a kite. something about the way they turn on the rotor hub like helis rather than following a curve like fixed wings.

    limited full size experience. other than cutting up some of the reinforcing ribs from the main blades from an old one the other day, i havent touched one.

    so, a prerotator. all ive seen is people doing it the old fashioned way. couple of good pushes and let off the handbrake with full throttle... it obviously doesnt need much power if you can do it one handed from the seat...

    so, i would think a cvt is more than viable, if only adding weight. an autogyro is already ineffecient enough! why add what, in my mind at least, is just extra luggage? anyways, i digress.

    250 rpm is FAST when its a 7 metre rotor! and once moving, the rotor naturally accelerates to optimum speed. so i dont think, unless you want perfect STOL, you have to achieve that speed.

    as you only need a very small amount of power, you dont really need to worry about torque caabilities of the rotor shaft. simple gyroscopic forces in flight would be more than a small rotator could produce. 5hp is on the verge of overkill! more like 300 watt cordless drill in low gear with a redux belt drive for some "slip". they make a neat cannister if removed from the case ;) and dont weigh that much. even have torque slippers on some so they wont overdo it. it will sit there and click like its driving a screw home until the rotor matches speed or you turn it off...
  11. buggy064

    buggy064 New Member

    I'm still planning my gyro copter I was thanking of making two frame's maybe three to help pay for the more expensive parts I found some 6061 extruded aluminum at a decent price and a small machine shop that's not to expensive I also found a site that sells used parts, I thank gyrobee has a site where the sell a prerotator.
  12. HeadSmess

    HeadSmess Well-Known Member

    talking to a mate that used to do this a lot...

    talk of an alloy sprocket/ gear on hub, and some type of heavy duty cable drive with a gear/sprocket one end, and a rubber wheel that gets pressed up against engine flywheel. the shaft from a bent shaft brushcutter will do it with some wangling to fit.

    other people get away with electric motors.

    remember a hi-end cordless drill can break your wrist in low gear! chuck in a 3:1 redux via chain/pulley and some type of slipper clutch should do it to.

    yeah ok, a pre rotator is handy so you dont have to do a lap or two of the runway first...

    i think a lot of it will do on the hub setup. do the blades flap on a pivot while the shaft stays upright, or does the whole shaft/rotorhead flap? the first can be frame mounted with no issues, the other type would need to be mounted so it can pivot with the shaft.

    lol. all them people that hate alloy frames with engines...a planes just pop riveted together! beat that :)
  13. beatlegeo

    beatlegeo Guest

    Just watch out for the "Magic" 254lb limit, or you'll be jumping through the "$$$" Hoop! bg
  14. Frankenstein

    Frankenstein Well-Known Member

    Uh... So helicopter like things and motorized bicycles have something in common? Uhm OK sure. Well the only small cvt I have experience with is the nuvinci hub, as far as gently getting the force from the motor to engage the props, well I'd go old school and go with a clutch, slowly letting the clutch engage with the hub, or better yet, letting the hub engage with the prop. I think if a very small frictional rubber surface was put onto the hub, then slowly allowed to begin rubbing the prop shaft then the cvt could ease it's way into engagement. The hub also has a freewheel mechanism, so if the rotor spins too quickly the cvt is "disengaged" from the engine preventing drag. Note073016_1.jpg
  15. Frankenstein

    Frankenstein Well-Known Member

    Maybe the new part if this drawing helps, the hub also accepts some odd belt system too.
  16. darkfighter

    darkfighter New Member

    There is this cheap 50cc scooter by this Chinese company called Tao Tao, which uses a simple CVT technology to get good milage, whilst being street legal in all 50 states. However, there are known to be electrical issues and requires a substantial amount of tuning to be fully effective. This would be even harder without mass production testing techniques that you do not have access to, and even harder considering the performance curve of rotax and ultralight aircraft engines.
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