propeller indexing

[avio]

Does anyone has a theoretical or other explanation of what is the best position of a propeller on the crankshaft? Are there certain positions which are to avoid? If you give rule of thumbs, please with the reason WHY if possible.
Rgds,
Avio

 

[GregLocock]

"what is the best position of a propeller on the crankshaft?"

I recommend that you put the propeller on the end of the crankshaft.

More seriously, remembering the title of the thread, crankshaft and pistons create a second order force (ie twice crank speed) due to the finite length of the conrod.

In some engines, I6s, H4s, and the like, these are cancelled internally, so if you measure the torsional vibration of a crankshaft on an I6 you won't see much second order.

A two bladed prop is a (potentially) strong, /installation dependent/, source of second order. So you could, in theory, increase the stress in the crankshaft if the second order from the prop is in phase with the second order in the crank.

In practice if you think this is an issue then I'm afraid the optimal index will probably have to be found by trial and error, I don't think it is possible to reliably calculate it without doing some tests first. Note that the inertia of the prop will strongly affect the dynamics of the crankshaft.



Cheers

Greg Locock

 

[WKTaylor]

Avio... This is an important issue for pistion engines.

Acft maintenence manuals will have this info for each model; otherwise contact the engine or prop manufacturer for this info.

Reason: Piston engines have ignition timing criteria related to prop location on crank flange.

NOTE: some prop flanges have locating pins for this purpose.

Gas Turbine engines may have otther prop/flange position critera...

Regards, Wil Taylor

 

[avio]

WKtaylor,
thank you for your reply;
indeed, prop or crankshaft flanges have/can have locating pins to assure a certain mounting position; but I was wondering following what rules this position is determined;
You refer to ignition timing criteria, can you be more precise.
Rgds,
avio