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BobBonahoom

Jim Allen or Andy Brown Type Question?

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I need to make a flywheel with reduced diameter in order to fit an engine into a boat. I was thinking of making the new one from brass assuming it's higher density would allow a smaller diameter, but present the same moment or inertia to the engine. Not smart enough in physics to figure this one out.

 

My Aluminum wheel is 1.25 " dia at the rear X 1.0" dia at the front X 0.85" long.

 

Has anybody ever done the math on this?

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https://skyciv.com/free-moment-of-inertia-calculator/

 

You'll have to break down the flywheel into segments, i.e. the portion behind the staring belt groove, the starting belt groove and the portion in the front of the starting belt and add the moments of inertia. Then repeat for the denser material of a smaller dia as an iterative process to get a match. I wouldn't worry about the angle sides of the v belt groove, just average out the angles to make three rectangular cross section donuts of material.

 

Brass is ~3 times denser than aluminum. I'll take a swag that if you reduce the dia by 3/4 to 2/3 you'll be in the ballpark.

 

If you can get the clearance you need by shaving 1/8" or less off the aluminum flywheel, I'd got that route first.

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I need to make a flywheel with reduced diameter in order to fit an engine into a boat. I was thinking of making the new one from brass assuming it's higher density would allow a smaller diameter, but present the same moment or inertia to the engine. Not smart enough in physics to figure this one out.

 

My Aluminum wheel is 1.25 " dia at the rear X 1.0" dia at the front X 0.85" long.

 

Has anybody ever done the math on this?

 

Does it have a tube section or is it mostly solid with a Vee groove?

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Hi Bob

I just make up a stub shaft to hold the aluminum one in a lathe and reduce the diameter to fit your needs.

Thanks John

 

John, how much do you think I can take off before I lose ability to launch the boat?

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You might be surprised at how little flywheel is needed, Ive taken some down to the point that that there is just the belt groove for starting. With the new style high rake prop I doubt launching will be an issue.

Thanks John

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Hi Bob

I just make up a stub shaft to hold the aluminum one in a lathe and reduce the diameter to fit your needs.

Thanks John

 

 

John, how much do you think I can take off before I lose ability to launch the boat?

. On a heat race boat? Don’t worry about it at all. In 1987 I was running a picco 45 with a steel flywheel. I purchased the aluminum flywheel when it came out which was the same dimensions. I launched the same props. There is a difference but you have to start looking for it to find it in most cases. If you have a good strong engine you are probably not on the edge with your setup. Shave it and go run. Jeff Lutz

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I will give my thoughts & knowledge on this question when more time allows. For now, it is common knowledge that a larger diameter flywheel , no matter the material, will always have greater energy to help in keeping the engine turning, once it is at WOT. The larger diameter flywheel will also make the launching of large diameter , high pitch propellers easier. The larger diameter will also allow a throttled engine under load to pipe back up. Throttle response & the ability to get back on the pipe will happen slower with a large diameter flywheel compared to a smaller diameter made of the same material.

 

It is also very important that whatever flywheel is mounted to the crank shaft is turning at a radial run out of no more than .0002" total indicator run out! The photos show the 2.000" diameter, precision knurled, flywheels used on the 27 cc gas & .95 cu in nitro engines. A metal cavity is added to the gas rigger for the flywheel to clear the hulls bottom. Notice that the aluminum flywheel used on the nitro rigger has been extended to give sufficient energy to help pipe the engine up.

 

 

Jim Allen

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Edited by Jim Allen

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I will give my thoughts & knowledge on this question when more time allows. For now, it is common knowledge that a larger diameter flywheel , no matter the material, will always have greater energy to help in keeping the engine turning, once it is at WOT. The larger diameter flywheel will also make the launching of large diameter , high pitch propellers easier. The larger diameter will also allow a throttled engine under load to pipe back up. Throttle response & the ability to get back on the pipe will happen slower with a large diameter flywheel compared to a smaller diameter made of the same material.

 

It is also very important that whatever flywheel is mounted to the crank shaft is turning at a radial run out of no more than .0002" total indicator run out! The photos show the 2.000" diameter, precision knurled, flywheels used on the 27 cc gas & .95 cu in nitro engines. A metal cavity is added to the gas rigger for the flywheel to clear the hulls bottom. Notice that the aluminum flywheel used on the nitro rigger has been extended to give sufficient energy to help pipe the engine up.

 

 

Jim Allen

 

Jim,

 

I clamp down my nitro engines and keep iterating on tightening the collet until the run out is as low as I can get it. Is there a better way to do that?

 

Nice belly pan. Did you make that?

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Bob,

 

I formed the belly pans in an aluminum die. When I machine a flywheel, the brass or aluminum stock is mounted to a shortened crankshaft that is the full diameter out to the taper. This piece is mounted in a precision four jaw chuck. The cut down crankshaft is carefully zeroed to + or - .0000" runout. It will be used to set the precise angle needed on the lathes compound when cutting the taper in the back end of the brass or aluminum flywheel stock & it will be used to machine the front side of the flywheel piece. Notice the oversize stainless steel piece the is shrunk into the flywheel stock as a first step. It will be finished machined to a size that causes the collet body to be located in two axes precisely each & every time it is mounted. The collet body is held to the flywheel face with the two 6-32 screws shown. "IT SHOULD NEVER BE MOUNTED BY USING A MACHINED SCREW THREAD"! The collet body has a hardened & ground seat for its collet to sit in & a 32 pitch thread for very aggressive tightening. The wrench size on collet body & the clamp nut is 5/8".

 

I machine the entire back side, including the tapered seat, "V" groove, "cut" knurl if wanted, final diameter & the 8 mm through hole. All of these machining operations will be dead concentric to the tapered seat that was cut in the beginning operation. Turn the piece around, use the zeroed crankshaft (.0000" runout) & mount very "tightly" the piece to be machined on the front side with a 8 mm screw. Now, the oversize shrunk in stainless steel piece will be machined to size as well as the front side of the flywheel. The front side surface will be square & concentric to the locating hub. I use .5000" + .0002" or .0003" interference fit for the 1/8" wide locating hub that locates inside collet body.

 

The collet body is machined & the collet seat in the collet body is hardened & ground. Again, I start with the back side of this piece & use the two 6-32 holes in the piece to mount it to a zeroed (.0000") holding fixture. The clamping collet, which is split from end to end, is also hardened & ground in a holding fixture. Splitting the collet from end to end cause it to close over its entire length, giving a very high clamping force! There is a hardened steel thrust washer used between the clamping nut, which is not hardened, & the hardened split collets back end. All of the pieces shown are interchangeable between nitro engine flywheels & gas engine flywheels with the universal mounting system used. I can use the same type collet body with this locating system to mount a ball & pin type universal on any flywheel. Cables that are used in these split collets are solid over the distance they sit in the split collets with their ends silver soldered solid. The collets can also be used with solid shafts. The .0002" runout I referred to is measured at the output shaft.

 

I have used the same collet assemblies without any failures for more than 10 years. Ask yourself how much runout can a 2.000" OD, 8 ounce flywheel withstand when operating at 20,000 to 30,000? Better yet, how much runout can the cable withstand before it begins to adversely affect the performance of the boat?

 

Jim Allen

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Edited by Jim Allen

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Hi Bob,

after i read this tread i know , this crazy ,bad formula about inertia . Who want to know this shit. But yes we need it to get a answer .

It took me fife days of my liftime to do all this complicatet math . Inertia is 1/2 mass multiplicatet with the square of the half dia of the flywheel. The lengh of the flywheel is includet in the mass. We know also the dens of brass and of aluminium . brass is 8,5 kg /dm and aluminium is 2,7 kg/dm. This are european ,but itˋs the same with american . So after some mixing the formular we get somthing 4 root of a number . The result is at the end that you can simply use the relation 1,33 for alu to brass converting.

So for example say you have a flywheel with a dia of 4 cm than difidet it by 1,33 and you will get the dia of the flywheel made of brass that has now only 3 cm dia, same lengh. In American dia say it has 2 inch dia aluminium / 1,33 = 1,5 inch dia brass. So for allever the relation number is 1,33 . You know this flywheel in brass has now the same inertia as befor the flywheel made from aluminium. The waight of the smaler brass flywheel is higher than the aluminium flywheel . But for the smaler dia you can get a deeper c.g. or lower angle of attace for a straight propshaft or , or , or.

 

Happy Amps Christian

Edited by Christian Lucas

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