Dear gentlemen,
I know that 161 kts out of an 0-235 sounds a bit unlikely, but it turns out to be true. Let me tell you how it happened.
Originally I had a B&T 62 x 66 prop that gave about 143 kts at 2700 rpm but i noticed that by allowing the engine to rev out through 2850 rpms and higher only about 5 extra knots was available. This is not what you would necessarily expect as the 100 rpm from 2600 to 2700rpm gave just over a 10 kt increase in speed. Something fishy going on here!
Taking the prop off and measuring the pitch at 4 inch intervals radially showed a pitch that varied somewhat erratically from 75 inches at the root through 65 inches at 2/3 rds radius to about 55 inches at the tip. A bit of calculation showed that at 2700 rpm and 143 kts, the tips were flying at -12degrees incidence. This tends to explain why my long Ez wouldnt go any faster, as at higher rpms the tips were starting to oppose the rest of the prop.
At this point a good friend stepped in with a loan of a "Henry"prop from his own soon to be finished Long EZ. Measuring this prop showed a consistent 68-71" pich at all stations, so a copy was made for experimental purposes. Initially at full diameter of 63" my engine bogged down at about 2200 rpm static and wouldnt get much past 2600 rpm at any altitude. Not good, but the airspeed was 153 kts! Worth persevering with.....
Then followed a number of sessions of reducing the diameter by 1" at a time, fining the tip profile and rebalancing, until the static rpms came up to 2370 and the airspeed was at 157 kts at 2650 rpms. At this, it was till a little difficult to get to 2700 rpms at 8500 ft.
The next fix was a home brew ram air system that increased static rpms to 2420. Now I can get to 2800rpm at altitude anytime! Some selective polishing and aerodynamic tweaks have yielded the extra knot or so to give me 161 kts at 8500 feet at 2700 rpms, though from habit, i do cruise at 2650 rpms and 158 kts at 8500 feet, generally.
best regards
Carolyn
