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HomeMy Site✈️Aircraft🚁HelicopterAntiques or Out of Business › What motor for GMP Cricket Heli
12-26-2015 07:13 PM  3 years ago
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solo

rrApprentice

San Antonio Tx, USA

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What motor for GMP Cricket Heli
Restoring a GMP cricket Heli. What is a good motor for it. Currently looking at the K & B .28.
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12-26-2015 10:43 PM  3 years ago
R.J.

rrVeteran

SF bay area, CA USA

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The popular engine at the time was an OS .28 FSR-H, but if you can't find one of those, then look for an OS .32 FSR-H or SX-H.
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12-26-2015 11:06 PM  3 years ago
madgadget

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St Louis MO

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32
OS 32 on 20% w/.003 shim will run smooth as butter
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12-28-2015 12:11 AM  3 years ago
gorhambirds

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Hickory Hills Illinois, usa

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Cricket Engines
Crickets will run well on a 32, but in my opinion, is way too much power, and you don't get much run time on a tank. If you don't use a heli engine, you need to run something that you can get a heat sink for. If not, it will surely overheat. There were no 30 size heli engines when the Cricket first came out way back when. On ebay, there is a guy in Hong Kong selling an original OS MAX heat sink that fits the 25FSR engine. That engine was the engine the Cricket was designed around. Perfect amount of power. There are a couple of used examples on ebay too. I run mine on an old K&B 3.5 ( .21 ) airplane engine with an old Associated R&A heat sink head. I use the Tatone / GMP special made muffler, as shown in the GMP literature, and it rips !!!!

Also, if you do use a heli engine, you will get better cooling if you rotate the head so that the fins are on a diagonal instead of in line with the longitudinal axis.

Steve Z
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12-31-2015 05:01 AM  3 years ago
solo

rrApprentice

San Antonio Tx, USA

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Just put an O.S. .28 Heli engine with heat sink head on my Cricket. Not ready for flight just yet. I noticed when I spin the fly bar by hand the "cone start" wobbles up and down; pretty significant. Also from the engine front bearing to the part that the clutch sits on; there's a wide gap; is this normal. I will try and load a pic.
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01-01-2016 03:31 PM  3 years ago
gorhambirds

rrApprentice

Hickory Hills Illinois, usa

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Clutch Wobble
This is my favorite subject !

First of all, when you spin the rotors ( fly bar ) by hand, the engine shaft SHOULD NOT TURN AT ALL. If it does then your drive belt tension is too much, or the clutch bell is dragging on the shaft somehow, or both. Remedy that problem first.

Now, as fine as a machine as the Cricket is ( was ), there were some awful design decisions incorporated in it. One of these was the flywheel set up. Now, considering that, back in its development, there were no specific helicopter engines to base a design on, Gorham had to choose from several popular engines, then available, and develop a kind of " general solution " to the problem. Had he been able to select a particular example or two, he could have easily designed the flywheel to incorporate a precision bore, or a tapered collet, something applicable to today's designs. Anyway, there is a way besides " blind luck " to yield a successful running set up. It may seem a little extreme to some, but this is the way I do it.

You need to be able to rotate the crankshaft freely, with no internal resistance from compression. This will allow the complete and EASY crankshaft rotation required to detect clutch shaft wobble.I do this by removing the head, piston rod assy, sleeve, and backplate. Also, you will need an assortment of materials to use as shims. My collection includes: K&S brass shim material, different thicknesses of aluminum kitchen foil, paper, thin cardboard, gasket material, and tapes ( scotch tape, masking, etc ).

Generally, the problem starts with the flywheel itself. The bore in the flywheel is usually larger than the crankshaft that it mounts on. Now if the flywheel is not PERFECTLY centered on the engine drive washer, when you tighten the crankshaft ( prop ) nut, the flywheel will be "pulled " to on side or another, causing it to not run true. Also, using any spacer washers between the flywheel and the drive washer will worsen the problem if they are not properly aligned.

Start by assuring any spacer washers are aligned with the back of the flywheel itself. When using more than one spacer washer, I align them with each other and epoxy them together to make a single " thick " washer, which removes one spot of misalignment. If the spacer / flywheel assembly fits loose on the crankshaft, wrap that area of the shaft with a thin piece of aluminum foil or tape. You may have to experiment to see what fits the best. Once the slop is removed as best as can be accomplished, tighten the prop nut. Now, if there is wobble detected when you rotate the crankshaft, as the instructions in the service bulletin state, loosen the prop nut, adjust the position of the flywheel ( turn it ), tighten the prop nut, and check again. If you cant get the flywheel running perfectly true, get it as close as can be. Now spin the flywheel again and carefully note which way the flywheel " tilts " as it rotates. Once that is detected, carefully loosen the prop nut and place a shim, selected from the above materials, and place it between the flywheel and the first shim washer directly opposite the direction of the flywheel tilt. Tighten up the assembly again and check for wobble. I its gone, stop. If its better, but still wobbles a little, try a thicker or double shim in the same place. If the tilt is now in the opposite direction, use a thinner shim. Once the flywheel is made to run true using this method, mount the clutch assembly and check the wobble at the end of the shaft. If a wobble is detected, re mount the clutch 180 degrees and check again. If the wobble is less leave it there and add the appropriate thickness shim opposite the wobble, repeating the same procedure used on the flywheel, until the clutch shaft runs true. The clutch is a little more cumbersome to shim as it bolts on in only two places. Placing shims between the clutch and the flywheel outside the mounting bolt locations will adjust the clutch shat in an in line direction along the line drawn between the two mounting bolts. To adjust the shaft in the other directions the shims must be placed in the appropriate areas on the sides of the bolts.

This procedure may seem rather over bearing to some, but it has always worked for me. In all of my years flying Crickets, I have NEVER had the luck that some apparently have had whereas I just " bolted it all up " and it ran true, NEVER. I can even recall that, on my very first Cricket ever built back in '79, I returned the engine I purchased, an OS 25FSR, to the hobby shop complaining of a bent crankshaft. That's how bad the wobble was for me.

As stated above in another post, I run a K&B 3.5 on my machine. The drive washer is such a small diameter, and the face of it that contacts the flywheel is not a flat surface. It has a slight taper as it goes towards its edges. There was no way, even with the above procedure, that I could get the flywheel to run true. So, I opted to replace the drive washer and shim spacer with an Associated K&B 3.5 rc car flywheel from back in the 70's. The flywheel attaches to the crankshaft using a tapered collar. The cricket flywheel rests up against the car flywheel. It was very close, but I still had to shim the clutch. It runs great.

One more word of advice. Once you get the flywheel running true, tighten the crap out of that prop nut. Whether you spend hours or minutes going through the above procedure, one backfire on the engine will loosen the prop nut and all will be lost. Please send me a PM if anyone has questions on this. I'm happy to help.

Steve Z
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