Engine Balancing
One thing that everybody with a 60 size helicopter notices is how the engine vibrates and shakes at idle. The shake does not go away at high rpms, it is simply masked. Remember with vibrations the amplitude of response (how much it shakes) decreases with increasing frequency (rpm). The reason smaller engines don't have the buzz is there is less mass moving in the engine.
My readers who have followed me over the years know that I have had some success with modifying engines, but one thing that I have never had good information on is balancing. When one removes weight from the piston or crankshaft what changes with the balance? I could never say one way or the other.
That changed a month ago. Model Aviation has a new U-Control Speed columnist who is very smart, Scott Newkirk. See, you really should read everything out there, not just helicopter stuff. In the September 2001 issue Scott discusses crankshaft balancing and gives an analytical formula. I talked to Scott to get more details. As with any formula dealing with single cylinder engines it would be difficult to prove, but Scott says it has worked very well for him over the years. Oh, by the way Scott, what rpm do you guys run your engines at? The answer: 42-44,000 on the ground (no that is not a misprint).
At last a crankshaft formula that can be measured. The Thunder Tiger 70 engine in the Raptor was a great engine to try out Scott's formula on. While the 70 runs well, it does have a vibration that is very noticeable at idle. This is not a slur on the Thunder Tiger engine, the YS and most 60s have a bad vibration at idle too.
The formula is the crankshaft counterbalance should balance out the weight of the piston, wristpin, piston ring if used, circlips if used, the small (piston) end of the conrod, and half the weight of the large (crankshaft) end of the conrod.
Weighing the large end of a conrod. The wristpin is used to support the small end on the balsa support. The balsa is sized to hold the conrod level when the balance pan is in the balanced position. With the weight of the large end known, the weight of the small end is then the total weight minus the weight of the large end.
This is an easy formula for helicopter fliers to use. We already have High Point balancers and triple beam balances which are the tools used. Photo #2 shows how I weighed the large end of the conrod. The weight of the small end of the conrod is then the total weight minus the weight of the large end. On the 70 the small end weighs more than the large end because it is wider. A bob is then made that equals the required weight and is hung from the crankpin with the crankshaft on the High Point. Photo #3 shows a crankshaft on the High Point.
A crankshaft on the High Point. This is an O.S. .46 crank and conrod. Note the crank balances the weight of the conrod and wristpin but no piston. The counterbalance will not balance the weight of the components when the piston is added, similar to the Thunder Tiger 70. Again the counterbalance is as big as it can be which means that weight must be removed from the crankpin side to attempt to make the counterbalance meet Scott Newkirk's formula.
How did the stock Thunder Tiger components measure up to this formula? The crankshaft counterbalance did not come close to balancing out the components listed. The design of the counterbalance is such that it is already as big as it can be so it seems that it is a manufacturing decision to not balance the crank better. Since the counterbalance cannot be made bigger the alternative is to remove material from the crankpin side, which may be difficult to do in a production process and still keep costs down.
My Thunder Tiger 70 crankshaft after balancing. The outlines show where the metal was when I started. It required a huge amount of grinding to meet Scott's formula. The engine runs much better with the balanced crank than with the stock crank.
Photos #4, #5, #6 show my crankshaft after I removed enough material to satisfy the formula. I do not have a before picture but the drawn lines on the picture show where the original outline was.
With this crankshaft the engine runs great and has very little vibration at idle, lending credence to the accuracy of Scott's formula. Scott tells me he got this formula from a Russian technical publication on U-control speed engines.
My Thunder Tiger 70 crankshaft after balancing. The outlines show where the metal was when I started. It required a huge amount of grinding to meet Scott's formula. The engine runs much better with the balanced crank than with the stock crank
.