My bird has a rotax 377 nonprovision dual carb engine with a belt redrive of unknown make. It has a three blade ultraprop on it. Its pitched at 10 degrees and getting 6300 rpm on ground whereas ultra prop says it should be pitched at 13 or 14 degrees. Thats a big difference. But they are using a 2.58 ratio gearbox in their examples. So perhaps my gearing is way different?

My question then is, how do deternine the ratio of the redrive? Im guessing you measure the radi of the two pulleys and divide them?

6 inch diameter prop pulley and 3 inch engine pulley would give a ratio of 6/3=2.

Radius 3/1.5 will also give 2. So radius or diameter both should work.Or you can count the turns while turning prop.

I used to fly a chinook wt11 long long time ago and it was powered by a 377 engine and I really mean powered.

It was a points engine/single carb A gearbox. Chinook looks small but it has a huge 35 ' wing.315 lbs empty  505 lbs AUW

60x30 prop .On windy days climbing with full power at 25- 30mph my shoes used to be higher than eye level.

I wouldnt hesitate to use it in a minimax. A good engine.


KK

Yes, that is right. Big wheel diameter divided by little wheel diameter. 6300 on the ground sounds like it might exceed the max RPM in the air. 6100-6200 are the usual best static figures I have seen so an extra degree of pitch might not be so bad.  Using an optical tachometer to get the actual prop speed might be worthwhile.
6000 RPM at 2.58 reduction will give 2325 at the prop. Those are the numbers for the manufacturer's quoted pitch.  A closer ratio (2.4 for example) will give a higher prop RPM so you need to reduce pitch. (in this case, the prop RPM would be 2500 for 6000 engine RPM).
(The tip speed of the prop will go supersonic if it spins too fast. The noise from the prop will increase. A lot. The efficiency will decrease likewise so it is a good idea to be in the right ballpark.)
Keep a close eye on the RPM and EGT in flight as well. Those will tell you if you are hitting the right numbers.

Ok thxs guys. I assume you measure the lands (low points) inside the grooved pulleys? Cause i get a different ratio if I measure low or high points of grooves.

Quoted from Stroid

Ok thxs guys. I assume you measure the lands (low points) inside the grooved pulleys? Cause i get a different ratio if I measure low or high points of grooves.

I think you should measure from the top of groove since belt being constant thickness has to take equal bite out of the

smaller pulley  which is excessive  so the ratio measured from the bottom seems different.



KK

Depth of groove =x Total thickness of belt =y diameter is outside diameter of grooves +(y-x) if you need to be really accurate. The circumference is really at half the thickness of the belt above the top of the grooves but in real life, just measure the outer diameter of each pulley.

Does your redrive use a toothed "Gilmer belt"? Some do. If so count the teeth on each pully instead of the diameter. You could also use an optical tach to read prop speed and engine tach for crankshaft speed.