Van's has changed Hartzell recommendations

[Bob Axsom]

The difference is reported to be small but when you are struggling for fractions of a knot, throwing away a whole knot is unacceptable. My prop is "only" three knots slower than a 7496 and yet I try to rationalize on the order of $10,000 for 7496 blades (haven't succeeded yet).

Bob Axsom

[stanman]

can anyone tell me the difference between the HC-M2YR-1BFP as compared to HC-C2YR-1BFP.....F7497 blades for both....the C2YR seems to be the reccomended prop by vans....RV8...T/W...IO360 Superior...i have an opportunity to buy the M2YR.......rgds to all...stan

[flion]

'C' is the standard hub, 'M' is the extended hub (BTW, Hartzell has an application guide online and the model designators are explained near the front). I'm not sure exactly what that translates to (though the guide seems to indicate that there is an extra 3.5" between the flange and the parting line on the hub) but I expect you can look forward to spinner fit issues and possibly some extra weight up front.

[DanH]

<<Is the conventional wisdom that tuned exhaust has only a negligible impact on prop vibration and stress?>>

In my opinion, yes.

Tuned pipes and high compression pistons both add horsepower, but they do it in very different ways. The key issue regarding propeller vibration is an increase in the amplitude of a forcing frequency, ie variation in force applied to the crankshaft as it rotates through 720 degrees.

Tuned pipes (hopefully) improve cylinder scavenge and filling (negative pressure during overlap) and reduce pumping losses (negative cylinder pressure following blowdown as well as less overall backpressure, ie no muffler).

Improved cylinder filling should increase gas pressure above the piston during combustion and increase force to the crank during that period of rotation. However, I don't think we get any huge improvement in filling; we don't have the necessary cam timing to make pipe tuning really effective.

Reduced pumping loss would tend to reduce crankshaft torque variation in that part of the cycle, a good thing for a prop. And I think backpressure reduction is the primary source of horsepower improvement in our case.

High compression pistons are another animal. They increase crank torque variation during compression (slow the crank). They increase gas pressure during combustion and thus again increase crank torque variation (acccelerate the crank). If they weigh more due to increased crown height (I dunno, do high compression Lyc pistons weigh more?), they would increase recip inertia and again increase crank torque variation, in particular during the ex/in period around TDC.

So, I suspect a lot more variation with HC pistons than with tuned exhaust. But I could be wrong. Trouble is, ain't much data available. 720 degree crankshaft torsional measurements tend to be closely held information, as are propeller vibration plots.

<<I'm just lookin' to learn something.>>

Me too.

[aerhed]

Nice, there Dan-o.