I took Dan Schaefer's advice and went out over the weekend to test the
glide ratio for my IV=P.  I'm satisfied now that the best glide happens
at about the same numbers reported in the IV's operations manual.
I found in my data quite a bit of data scatter but the trend was there
and L/D max occurred at approx. 115 knots.  Correcting for the weight I
was flying to gross weight would put the best glide a little less than
120 kts. The best L/D computed to 8:1.
I did most of the testing with the throttle pulled all the way back to
idle with the assumption that the best glide speed would be the same
speed regardless of whether the engine was idling or off.  For my last
glide I pulled the mixture to lean cutoff with the expectation of
increased descent speed.  Interestingly, I couldn't discern any
noticeable change in the descent rate.  Quite frankly, I couldn't
discern any change in engine noise either.  The only way I could tell
that the engine wasn't still running was the fuel flow went to zero.
The oil pressure remained in the low 40's and was sufficient to enable
the prop to be pulled to full coarse pitch.  The change in descent rate
was amazing - to the tune of about 40% reduction in descent.  The L/D
ratio computed to 11:1 with the prop in this configuration.
As Dan suggested, this test is something that everyone should do to
"characterize" the performance of their particular airplane.  BTW, when
I pulled the mixture to idle-cutoff, I had a 10,000 foot runway 7000
feet beneath me.

Hal Woodruff
IV-P