ELIPPSE propeller for RVs

[hevansrv7a]

Originally Posted by SHIPCHIEF

Yes;
This aircraft may not have been equal to the CAFE aircraft, so that point is moot.
The something you are missing is that this is a comparison of propellers on a single aircraft without other mods. This is literally the "apples - to- apples comparison" that everyone demands.

Paul went out of his way in earlier posts to say that the aircraft in question has the same drag (2.32 pounds FP equiv.?) as the CAFE airplane. As a generality, I agree that no two planes are the same.

I agree so much that it's why I developed my own methods for determining the drag curve in flight instead of the tape measure and calculator method which did not work so well on the other airplane that Paul referred to above. My methods are documented in the links below. Anyone can test them and evaluate their validity.

Two props on the same plane - good. Assuming the prop that's being compared is good - not so much. Not all apples are good. I believe someone mentioned a Catto? Van's official specs on the 6A says 199 mph on 180 HP at GW. It's pretty well accepted that many of our planes equal or beat Van's numbers and it's one of the reasons that Van is highly regarded for his conservative, honest numbers.

[Ron Lee]

My vague recollection is that I gained 3-4 MPH when I went to a Catto three blade from my Aymar Demuth two blade. I can't claim that it was a perfect comparison but if close, that suggests maybe a 1 MPH improvement of the ellipse prop over my Catto prop. That is within the margin of error of my measurements.

[Kyle Boatright]

[quote=elippse;473136]

Originally Posted by Kyle Boatright

Paul,
I'd love to see a trial where all of these factors are recorded and posted comparing your design against a known good prop by Catto, Aymar Demuth, etc.QUOTE]

Jim Smith ran several tests of his three-blade vs his previous Aymar-Demuth. Here are the results he obtained using two-way GPS-measured TAS:

<snip>

Paul, do you have manifold pressure data to add to the other data?

Thanks,

[hevansrv7a]

What if the CAFE 6A went exactly as fast at 8407 DA as the airplane in Paul's example (187 mph)?

Then, assuming a 150 BHP engine, the prop is 81.5% efficient or less for that speed and altitude.

You can use the 6A spreadsheet on my info page (link below) to work this or other combos. Here is the part that is not on the spreadsheet:

For a nominal 150 BHP engine and using 2700 rpm and ambient density at 8407', the BHP available is 116.45. If you add 1" of MAP for the ram effect that some of us get, then the BHP available is 121.46. I used the lower BHP figure. If you use the higher figure, the prop is thus less efficient. This method of guesstimating HP is, I think, the same one Paul uses, based on a previous discussion in this forum.

[Kyle Boatright]

Originally Posted by hevansrv7a

What if the CAFE 6A went exactly as fast at 8407 DA as the airplane in Paul's example (187 mph)?

Then, assuming a 150 BHP engine, the prop is 81.5% efficient or less for that speed and altitude.

You're working too many variables. We don't know that airplane's drag matched the one in the CAFE test, AND we don't know how many HP the engine was generating - RPM was off the chart and no MP was published.

We can say that Paul's prop was more efficient at the top end than the other prop, but that may be the only conclusive result of the test.

For another data point (again, not well enough supported by specifics to make it a fair comparison), my 160 HP RV-6 with an Aymar/Demuth prop beats all of those numbers under those conditions and with the RPM at 2700 or less. I do think my airplane is a little slicker than most.

[hevansrv7a]

Originally Posted by Kyle Boatright

You're working too many variables. We don't know that airplane's drag matched the one in the CAFE test, AND we don't know how many HP the engine was generating - RPM was off the chart and no MP was published.

We can say that Paul's prop was more efficient at the top end than the other prop, but that may be the only conclusive result of the test.

For another data point (again, not well enough supported by specifics to make it a fair comparison), my 160 HP RV-6 with an Aymar/Demuth prop beats all of those numbers under those conditions and with the RPM at 2700 or less. I do think my airplane is a little slicker than most.

Oops, my mistake. Paul actually said that the RV-6 he was working with was even lower drag than the 6A:

"This was borne out with the data reduction of Jim Smith's RV-6 with my original two-blade. I estimated that his engine, which I assumed was 160 HP, produced only 150 HP at sea-level, which turned out to be the actual rating. I also estimated that his parasite drag area was 2.15-2.20, which turned out to be close to the CAFE RV-6A estimate of 2.32 which I found out later when told about the test that was written-up in Sport Aviation."

He gave the RPM as 2700 which is 100% of rated. Quote follows (my emphasis):
"Elippse three-blade 64-74, average of two flights, 1440 lb-
4000', 4440' dalt, 2740 rpm, 193 mph
6000', 6440' dalt, 2738 rpm, 192.3 mph
8000', 8407' dalt, 2700 rpm, 187 mph
10,000', 10,400' dalt, 2655 rpm, 184.4 mph
ROC, 2000'-10,000', 916 fpm, static rpm 2125, 95 mph IAS, 1440 lb."

Also, you don't need the MAP if you know it was WOT. That's why I used ambient pressure at that density altitude. If I understood the earlier thread, this is also how Paul figures BHP or is at least compatible with it. With a factory airplane, there is some MAP loss, but with RV's it is common to have MAP equal or higher than ambient.

This correction would lower the prop efficiency from the estimate that I calculated, assuming that Paul is right with his estimate. That's because if the airplane has lower drag then the THP is lower thus the efficiency is lower. And to top it off, the CAFE test was at 1650 pounds, not 1440, further lowering the efficiency calculated result.

[elippse]

BTW, you might be interested in this little tid-bit from Jack Norris' propeller book. On PI-28-BkII, he writes "Now, if we try loading a constant angle of attack, ao and CL, prop blade, elliptically vs. radius, like a wing, with far higher dynamic pressure, q, outboard, we get very narrow chords outboard, a wild shape curving to excessive chords inboard..." "But if we superimpose this elliptical lift distribution on the half teardrop radial lift distribution of an ideal B-G-T prop, we can see that moving the blade loading more inward, we would logically lose less off the tip, better than the B-G-T ideal!!!" "With a wild looking prop, we can beat the Classic Betz Ideal!"