Jim Smith has just sent me some TAS numbers which he obtained after making and installing a streamlined cowl outlet as shown by Vetterman. Jim has a 150 HP Lycoming RV-6 running on auto gas. He originally changed his two-blade Aymar DeMuth prop for a three-blade Elippse prop, then added triangular wingtips, then reduced the cooling inlet area, added a single LSE ignition, and finally added this fairing on the outlet. He also recently installed an altitude-hold which gives him very consistent TAS numbers within 1 mph. His latest TAS numbers: 203 mph, 2745 rpm, 6000' dalt; 202 mph TAS, 2734 rpm, 8000' dalt; and 199 mph TAS, 2680 rpm, 10,000' dalt. How's that for only 150 HP!!!!He finally broke the 200 mph barrier!
Paul, going back to the report of last spring when it was 193.4 mph at 8000', I re-worked my analysis because the old one was C**P (my bad) and came up with the stuff below.
Please tell me/us if this is close. If it is, then it can be used to evaluate the additional changes.
http://www.vansairforce.com/community/showpost.php?p=511894&postcount=28
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193.4 mph at 8000' on a nominal 150 HP engine, 1440 pounds.
New max L/D speed: 92.8 vs. CAFE 6A at 106.
New glide ratio with "transparent prop": 14.0 vs CAFE 6A at 12.245.
Minimum Drag: 102.6 pounds at 1440 pounds aircraft vs. CAFE's 128.5 corrected for weight and nosewheel.
THP at 193.4 mph: 98.1 vs CAFE 6A would be 103.85.
Total reduction in drag at 8000' and 194 mph is about 7 pounds.
Reduction in induced drag approx. 37%.
This set of data is for 193.4 mph vs. 189.5 for Van's as corrected for weight and nosewheel, using factors derived from CAFE 6A. A difference of about 4 mph. I think your original posting said the gain from tips was greater than that, so a lower starting point?
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If the above is assumed correct, then:
Estimated 81% BHP available at 8000' (no, not 75%!) is 121.11.
Kevin Horton's spreadsheet suggests that 81% is a better estimate.
If BHP is correct, then net prop efficiency would be 80.75% at that speed.
If BHP is assumed 75% at 8000', then prop is 87.2%.
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To move that airplane from 193.4 mph to 202 mph would take an additional 15.2 BHP. Therefor the changes since the wingtips would have that value, total of increased HP and reduced drag.
That's about 12.5%. Not too shabby! And that's above the previous 4 mph for the tips.