<<In reviewing the WOT performance curves for O-360-A, I note that they
converge to 0 power at 59,000 ft. This would equate to around 0.24 inHg at 0
C.>>

An interesting observation.  However, one problem that I have observed in
looking at POH performance is that in some cases it looks like the data was
excessively extrapolated.  Like they made a couple of measurements and then
just filled out the rest of the chart.  In the case you bring up the offset
in manifold pressure readings is only 0.24 inches.  Sounds more like someone
was extrapolating to zero rather than giving actual data, as I don't see how
the friction horsepower could be so low as to account for only .24 inches of
manifold pressure.  I previously guessed 4, which is probably on the high
side.  2 or 3 might be more reasonable.

Gary Casey

Gary,

 

There certainly is a fair amount of interpolation/extrapolation in these performance curves and I confess that I added additional extrapolation to get down to the 0 power condition. Further, I agree that the frictional power loss at full power is substantially greater than 0.24 inches. In fact it is probably in the neighborhood of 6 or 7 inches. But, the majority of the frictional loss varies with the engine speed and load. For instance, the bearings and the ring/cylinder interface are hydrodynamic and their power consumption will vary with the cube of engine speed. Their power consumption will also vary directly with the cylinder pressure and hence, this portion should be a constant fraction of the MAP. I was only trying to get an idea of the fraction of power that is independent of engine load.

 

I found it at least a little curious that all of the rpm traces from the chart seemed to converge to the same point on the 0 power line. This is counterintuitive as I would expect at least some of this friction to vary with rpm as stated above. It may be that some other loss is dominating the result or that the Lyco chart is "crap".

 

Probably the best way to determine what power to use for the non varying friction is to drive an exemplar engine with an electric motor and measure the electric power it draws. Anyone have an engine laying around they want to experiment with? Set it up with WOP and whatever rpm you what to know the answer for.