Home made prerotor

Doug Riley

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A prerotator in the 1-2 hp range will spin the rotor maybe 150 RPM. Therefore, your redrive must take the engine's RPM down to that speed. Hence, typical reduction ratios are more like 20:1 or more.

Those that are driven off the prop hub are downstream from the main redrive. Therefore, they might use a ratio closer to 10:1.
 

Rehan K.Janjua

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A PREROTATOR.

Nice, actually very nice work.

A launching Pad is the best answer to prerotate the rotor.
The engine, reduction drive and part of the drive system stays on the ground.

14lbs is light for a good high Rrpm system.

Have fun and fly safe.
 

WHY

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HI George

NICE ! now your talking serious turkey, in my way of thinking , this is better than jump take off , very short but does not require the very precise timing of a jump.

Tony
 

Alan_Cheatham

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A gearboxed goped prerotator has been done, as long as the second stage reduction at the head is the correct ratio it should work although it's heavier I'm sure. The best solution is the go with a belt drive and eliminate the chain as in this thread of Matt Pearson's Dragonfly prerotator: http://www.rotaryforum.com/forum/showthread.php?t=24979 much lighter and safer if it breaks and is sucked through the prop. If you look carefully you'll notice that Matt's prerotator looks like GGFirestar's shown at the top of this thread, that's because GG used Matt's earlier goped prerotator on Superfly as an inspiration, and with my prodding Matt used GG's simpler mounting design when he built Dragonfly's prerotator.

Another consideration is the goped engine to be used, there are many on the market but most of the generic ones are in the 2 hp range, the engine of choice is the GP460 which is a race engine producing 4.25 hp (more with a tuned pipe), which will get a set of 23 ft Dragonwings to around 220-230 rpm range with an overall gear ratio of 55:1.

.
 

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Earthboundmisft

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Alan, that pre-rotator in your last photo is my gyro. Current performance is about 160 rrpm, with the 33cc mill. Results are smooth, and predictable. I hope to step it up, as I learn to get a handle on this machine. Mike.
 

ggfirestar

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I made a couple more I bought one of those $69.00 engine off ebay and with some stuff around the shop made one that would do 147 rpm would have done better with different sprockets but I was trying to make one for under $100.00 bucks.

Ive never had a chain to go threw the prop I made this one with a chain guard
on it just in case I thing the belt is a good thing the chain and sprocket are just easier to come by.

The one in the picture here is a GP460 there more bang for the buck.
 

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nice work. I tend to disagree with the other about the socket ujoint; as long as you keep it greased, it should be plenty robust for this application. They're much stronger than the flex shafts used on weed eaters, and how often do these little motors break one? There's only a couple of models that I know of that will, and I'm working on making one into a prerotator for me. Just keep an eye on it if it starts getting play in it, I'd change it out just on the wear principle.
 

Danough

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A gearboxed goped prerotator has been done, as long as the second stage reduction at the head is the correct ratio it should work although it's heavier I'm sure.

Probably heavier as you say, but very sleek and clean looking in the photo you posted. And easier to install for the mechanically challenged like me. Here's another interesting idea--you could use a gearbox with a continuously variable transmission like this one http://fancyscooter.com/item00478-scooter-part-ninjia-cvt-transmission.html bolted onto a GP460. Seems like it should spin up a lot faster than direct drive and it will choose the most optimal gear ratio automatically. Just a thought--its probably already been done as well. I'm new to all this.

-dan
 

Alan_Cheatham

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There are limits as to how fast you want to accelerate the rotor as anytime the rotor is under power it puts torque and mechanical pressure into the airframe. Prerotators are typically designed with some form of "soft start" to limit these stresses and the nature of the two stroke engine combined with a single ratio drive works very well as a soft start.

.
 

GyroChuck

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There are limits as to how fast you want to accelerate the rotor as anytime the rotor is under power it puts torque and mechanical pressure into the airframe. Prerotators are typically designed with some form of "soft start" to limit these stresses and the nature of the two stroke engine combined with a single ratio drive works very well as a soft start.

.

Reminds me of a story Tom Milton told me. This fella was unhappy with the rotor brake and wanted to improve it. He converted to a bicycle brake setup. It worked really well. Except it twisted and bent his mast when he tried it.
 

ggfirestar

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YA I guess you could just nail it and screw up ever thing but a little common sense goes a long way
 
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