80 HP Yamaha Gear Box

Gyro Jim

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Dec 8, 2007
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60
Location
Cherry Valley
Hi Guys,
My name is Jim Sharer. I live in upstate NY. I built and fly a Little Wing with a 912 HP up front, a delightful aircraft with a great little engine.

Over the last year, a gyro friend and myself have been working on a Yamaha Phazer 80 HP snowmobile engine conversion. We have it mounted on my buddy George's Air Command gyro. George is a long time gyro pilot with 800+ hours. We have gotten it to the point where it is up and running well except for a persistent vibration. It's a high-frequency vibration, a buzz, that carries down through the air frame. My best guess at this point is that it's a harmonic vibration that is occurring between the 2 gear boxes. The Yamaha 80 has a built in gear reduction. The only solution I can think of trying is to change the gear ratio in the Rotax c-box. This hopefully may upset the harmonics, eliminating the vibration. I have been in contact with Todd (Racer) about it and I think we have exhausted all other possibilities. I have also talked with Dick Dougherty, he is also working on a 80-HP Yamaha but is not quite as far along.

So, my question is: is there anyone out there that has a gear set we could borrow, rent or buy? We are now running a 2.62 to 1 gear set. I would like to try a 3.47 or perhaps a 3.00 would do the trick. Thanks.

Gyro Jim
 
Rotax has a 4 to 1 gear set too. could that be used and get rid of the yam gears all together?

what rpm is this engine going to use in flight, 10,000? 10000/4 is 2500 not a bad max speed for a prop.
 
Hi Jim

I looked at that possibility (harmonic resonance) when I considered this engine and I am afraid that this is a bad "spook" that will not go away. changing the gear ratio will only move the frequency of the resonance. This kind of resonance will eventually crack or break something. It may be in the engine or on the airframe, especially the long control rods or in the tail at the single bearing (the longest unsupported distance of the airframe).

Tony
 
Moving the resonance is the idea

Moving the resonance is the idea

I think moving the resonance is the idea, hopefully you move it outside of the normal operating rpm of the engine. There have been many engines that have a red lined RPM range on the tach. I wish you all the best in getting this to smooth out

Hi Jim

I looked at that possibility (harmonic resonance) when I considered this engine and I am afraid that this is a bad "spook" that will not go away. changing the gear ratio will only move the frequency of the resonance. This kind of resonance will eventually crack or break something. It may be in the engine or on the airframe, especially the long control rods or in the tail at the single bearing (the longest unsupported distance of the airframe).

Tony
 
I don't have my hands on it, so I can guess was well as the rest of you, but you may want to check the advance on your ignition timing. If it is too far advanced before top dead center, then the firing will create a bad torsional vibration that will radiate through the gearbox and your prop, and you feel it in the airframe. Your prop wants to stay a constant RPM, but your engine wants to speed up and slow down every firing from the high compression.

This was a problem I solved with the special engine I had built for our two-place Voyager-500 helicopter 12 years ago, by installing a programmable electronic fuel system.
 
Hi Guys,
My name is Jim Sharer. I live in upstate NY. I built and fly a Little Wing with a 912 HP up front, a delightful aircraft with a great little engine.

Over the last year, a gyro friend and myself have been working on a Yamaha Phazer 80 HP snowmobile engine conversion. We have it mounted on my buddy George's Air Command gyro. George is a long time gyro pilot with 800+ hours. We have gotten it to the point where it is up and running well except for a persistent vibration. It's a high-frequency vibration, a buzz, that carries down through the air frame. My best guess at this point is that it's a harmonic vibration that is occurring between the 2 gear boxes. The Yamaha 80 has a built in gear reduction. The only solution I can think of trying is to change the gear ratio in the Rotax c-box. This hopefully may upset the harmonics, eliminating the vibration. I have been in contact with Todd (Racer) about it and I think we have exhausted all other possibilities. I have also talked with Dick Dougherty, he is also working on a 80-HP Yamaha but is not quite as far along.

So, my question is: is there anyone out there that has a gear set we could borrow, rent or buy? We are now running a 2.62 to 1 gear set. I would like to try a 3.47 or perhaps a 3.00 would do the trick. Thanks.

Gyro Jim


I'd like to see you start another thread with some photos of your Little Wing.
 
I am not so sure there is a issue due to having two sets of reduction ratios... I mean, a reduction ratio is nothing more than reducing shaft speed, same as a manual gearbox in a car or truck or tractor.

There could be a problem with the ratio and prop and engine not getting along, and what your wanting to do, changing the ratio would help. Just find someone locally with a C gearbox and different ratio to swap the back half of there gearbox and the small gear for a test run, shouldn't take more than a hour to switch it all out and run the engine.

To switch just the gears requires special tools to remove and reinstall the big gear on the prop shaft. The little gear is a easy swap, but the big gear is a pain.
 
Thanks, guys.

There is a clutch between the 2 gear boxes. The RK400, same as Todd uses on the 3 cylinder Yamaha. The engine turns around 11,000 RPM and it makes just under 350lbs. of thrust. It would be a nice little power plant if we could get it worked out. It is computer controlled and fuel injected. George has flown it around the pattern a few times but we do consider it un-air-worthy in it's present condition. The gear box in the engine is a 1.6 to 1 ratio and it is incorporated into the case. It would be hard to eliminate. Our hope was changing the C-Box ratio would do it.

I'll have to post some pictures of my Little Wing.

GyroRon, your suggestion is exactly what we had in mind, but there is no one around here with a C-Box we could swap out. Do you have one? We would pay all shipping and give you a deposit as collateral.

Gyro Jim
 
Yeah, I got one, but mine is in use! Why not ask Todd, he may have a extra gearbox laying around, elsewise I am sure someone on the forum here has one they can spare for a week or two while you test it out
 
moving the prop

moving the prop

I know that IVO says if you are having harmonic problems, you can rotate the prop by a bolt hole on the hub and it will be in a different position in relationship to the engine power pulses. Its free and it might be worth a try

Thanks, guys.

There is a clutch between the 2 gear boxes. The RK400, same as Todd uses on the 3 cylinder Yamaha. The engine turns around 11,000 RPM and it makes just under 350lbs. of thrust. It would be a nice little power plant if we could get it worked out. It is computer controlled and fuel injected. George has flown it around the pattern a few times but we do consider it un-air-worthy in it's present condition. The gear box in the engine is a 1.6 to 1 ratio and it is incorporated into the case. It would be hard to eliminate. Our hope was changing the C-Box ratio would do it.

I'll have to post some pictures of my Little Wing.

GyroRon, your suggestion is exactly what we had in mind, but there is no one around here with a C-Box we could swap out. Do you have one? We would pay all shipping and give you a deposit as collateral.

Gyro Jim
 
Jason... sorry but that makes no sence, as he has a clutch in his gearbox. Think about it......
 
Jim, Please varify that your clutch drive is running straight and true to your output shaft and please make sure the center input shaft in your gearbox lines up exactly in line with the output shaft of your motor. if any of these things are off even by just a little bit that could cause your vibration.
Start with the basics then work your way down the chain from there.
Todd
 
What size prop?, At 11,000 rpm that will be about 2625 rpm. if I did the math right. Maybe its over reving the prop. 350lbs sounds low thrust for 80 hp doesn't it?
 
at 2625 prop rpm, you could spin a 80" prop and the tips would still only be .82 mach, .88 to .92 is the most efficient range. As per--
http://www.pponk.com
 
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