my gearbox first test run for V-twin engine

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BBerson

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Why did the dynavibe not work?
It is made to detect a constant rotational imbalance, like a tail rotor that runs smooth. It can't separate the odd firing power pulses from the simple prop rotational imbalance, is my guess. Got a different result each try. It doesn't separate the prop from the engine shaking.
The v-twin runs very smooth with no load. But put a prop on it and it lugs it at full throttle and it shakes like a car engine on a steep grade.
 
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TFF

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You needed one with frequency analysis. Sometimes the place the vibe transducer is placed can matter. It can happen on tailrotors with basic boxes too. Piston tail rotors are not as smooth acting as turbine as they get pulses from the engine.
 

TiPi

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this is not a ”vibration” as such, it is the reaction force of the engine to the combustion pressure. The force that pushes the piston down is also pushing the engine in the opposite direction. It is more pronounced at lower rpms when the time between the pulses is longer. Opposed twins have the same problem (a BWM motorbike at low revs shakes sideways like a vibrator).
 

TFF

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No, I doubt it. You can usually move the sensor around to other locations to null out the unwanted vibe pickup.
 

BBerson

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I think it needs a more rigid mount. The 30 pound fuselage wasn't much to hold it still.
Might try try again someday.
 

FishHawk

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thanks for your info about V-twin. :)

3 reasons, 1 the engine have been put on homebuilt aready and tested (850cc, 65hp) by other homebuilters. 2 it 's convenient for me to get this engine and it is affordable 3, it seems (I am not sure , but just seems to me ) easy to be converted to airplane use.

BTW, working progress was painful slow :(

also , still studying the auto engines. after testing this gearbox, I will have try
Actually, to correct myself, primary vibrations aren't a problem in a 90 degree v twin. The problems are the secondary vibrations from the uneven firing order.
Wouldn't that fatigue the metal? Idk a V Twin never gives me peace of mind.
 

Vigilant1

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Actually, to correct myself, primary vibrations aren't a problem in a 90 degree v twin. The problems are the secondary vibrations from the uneven firing order.
Wouldn't that fatigue the metal? Idk a V Twin never gives me peace of mind.
They are giving good service in many small aircraft with the prop hung right on the crankshaft. Some of these planes have many hundreds of hours of flight time. The conversions have to be done right, like any engine installation. Limit the propeller weight/MOI, don't put significant mass on opposite ends of the crankshaft, give it adequate cooling air and don't exceed reasonable cylinder head temperatures, etc.
The price of some popular air cooled V-twin models, brand new, works out to about $45US per HP. Add in all the needed modifications and even if the price doubles, it is still a pretty good deal for a complete package (engine, induction system, electrical system, starter, etc). And the weight isn't bad, either, at around 2.6 lb per HP for a reliable 4 stroke that burns fuel fairly efficiently. They aren't perfect, but for planes that are engineered with these engines in mind, they do pretty well.
There are quite a few discussions on these engines here, also at sites devoted to the Spacek SD-1 and the Colomban Luciolle (MC-30). For a thread with a lot of good info in one place, see Tipi's B&S 810cc conversion project thread.
 
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