ELIPPSE-Felix prop on RV-6
 

Jim Smith of Wichita, KS has been doing TAS testing of his ELIPPSE-designed, Fred-Felix-made, three-blade propeller on his stock 150 HP, auto-fuel RV-6. His best speed was at a density altitude of 7000' where the four-run, GPS-derived, TAS average was 191.6 mph at 2741 rpm, with high and low speeds of 193 mph and 189.5 mph. His average ROC from takeoff at 1350' to 10,000' was 800 ft/min at 95 mph IAS and 1440 lb., and a recent ROC test yielded an average of 1032 ft/min from 2000' to 10,000' at rpm ranging from 2300 at 3000' to 2175 at 10,000', 1440 lb and 95 mph IAS.

Good numbers
 

Good top speed in spite of available hp and limited over-revving.

Does he have any numbers for his previous propellor?

Has he tried ...
 

different climb speeds?

95 MPH is a little slow compared to most RV-6's, especially with a fixed pitch prop. More speed gives you more RPM, which gives you more speed, which gives you more climb, which gives you more...you get the idea.

RV-6's typically have a best ROC speed more like 105 to 110 MPH

Scott: Is that with a CS or FP prop? My computer program, which solves for both induced and parasite drag on my various aircraft models, shows RV-6 best L/D at about 95 mph. If you are using a fixed-pitch prop, the efficiency is low at low airspeeds and so as you allow the plane to climb at a higher speed, the prop efficiency goes up and the rpm goes up, which in turn will give more power. Additionally, at the higher speed, the induced loss of this very low aspect-ratio wing (4.8:1) will be lower. Lower induced loss, higher power, more ROC. But, with a CS prop with a reasonably constant efficiency, the plane should climb better at best L/D! Tests of my three-blade prop on my Lancair show an efficiency of at least 82% at 110mph, 2410 rpm. That's also best L/D speed on my Lancair. So even though the efficiency of the prop goes up from that speed, L/D is going down, so I get sort of a flat spot on my ROC vs IAS.

Status Questions
 

Has there been any improvement in top end speed? Do you feel the 191.6 TAS average is the limit you would expect from an RV-6 with 150 hp at 2741 RPM at 7,000 ft density altitude with auto fuel?

Bob Axsom

I don't have anything else to compare this performance with; I thought that those of you who fly RV-6s, who have done TAS tests using GPS, would be able to say what their results were. Jim's testing showed much better ROC from surface to 10,000' than his two-blade, 1032fpm avg vs 700fpm avg. Estimated TAS performance for a prop of this efficiency using 180HP and 200HP, without accounting for the decreased induced drag at the higher speed nor the increased MAP would be, for 8000' dalt and 2700 rpm: 201.4 mph at 180HP and 208.6 mph at 200HP.

I can give you some numbers
 

Paul you will have to make all of the necessary adjustments. My RV-6A Has been modified to minimize cooling drag and the wing tips were replaced with some of my own design reducing the span to 1.5 ft less than stock (3 ft less than my normal cross country configuration). With a Hartzell C/S (C2YK-1BF/F7666A4, 72 in dia) using 7666-4 blades, on O-360-A1A, wide open throttle, leaned approx. 100 deg. rich of peak on the hotest cylinder (#4 approx. 1300 F), 2730 RPM EI digital instrument with 10 RPM resolution. The temperature at 6,000 ft pressure altitude was 16C so I flew at 4,700 ft pressure altitude for a 6,000 ft. density altitude, trimmed for straight and level flight, all vents closed etc. per the US Air Race Handicap procedure (http://www.us-airrace.org) tracking 000, 120 and 240 degrees magnetic. Five 20 sec, interval GPS ground speed readings in knots were:

000 - 182, 182, 182, 182, 183 = 182.8 kt. avg
120 - 170, 169, 168, 169, 168 = 168.8
240 - 183, 182, 181, 181, 180 = 181.4

These averages, based on readings that do not satisfy the USAR requirement of 1 kt max variation but good enough for my work, were plugged into the National Test Pilot School spread sheet to mathematically eliminate the wind effects and the speed of 177.8 kts TAS was calculated.

The differences that have to be considered that I can see beside the lower horsepower on your test plane are the tricycle landing gear on my plane, his vertical stabilizer/rudder may be larger (mine is the original small design), and the landing gear fairings may be different (my nose is flat sided and the MLG are the newer "eggs" pressure recovery style), the weight (mine is 1141 lbs with the new tips), the 1.5 ft shorter wingspan with the new tips, and the cooling drag modifications in the lower cowl of my plane. The cooling drag modifications increased the speed by 4 kts. The reduced wing span on my plane increased the speed by 3 kts but my initial wingspan was 1.5 ft longer than stock because of the 9" tip tanks I have on each wing between the end of the wing and the stock wingtips. Manifold Pressure approximately 24.5" (analog gauge with some paralax - recurring number not actually recorded on this particular flight). My HP (rated at 180 at sea level and 2700 RPM) was probably around 155 during this test.

This may be of some help but it certainly is not a straight apples to apples comparison.

Bob Axsom

For comparison, the best TAS I have seen in my 160 hp carbed RV-6A with an E-mag, using a Sensenich 80" prop at 10,000' is around 193 mph. The airplane has only the 'regular' drag reduction/speed enhancements. This was a one way calculation using IAS and indicated OAT, no other data available. 191mph on 150hp sounds very good to me, especially if it removes the 2600 rpm rev limit. I normally climb at 120mph for better engine cooling.

Pete

Bob: I've found that with my Lancair, my speed and rpm change 0.6%/1000'. With Jim's plane, I found that the rate was higher, averaging about 1.6%/1000'. I think the difference is due to the difference in induced drag from the 4.8:1 AR of the -6 to the 8.05:1 of my Lancair. I calc'ed the CDI of his plane at the different test altitudes, and it showed quite a change as the speed and density went down. I asked Jim if he noticed a much higher nose-up angle as he went higher, and he told me that it definitely did. If I use that 1.6% to obtain a speed correction from 7000' to your 6000', I get 194.7 mph TAS, and 2783 rpm. Taking the HP and rpm difference into account, I would estimate his speed at 205.6 mph at 6000', 2730 rpm, or 178.6k to compare with your 177.8k. Decreasing your speed by 4k for the cooling mod would give 173.8k vs 178.6k. That would indicate a prop efficiency increase of 8.5% from the ELIPPSE to the Hartzell! That would be the same as going from an efficiency of 83% to an efficiency of 90%! Does this seem reasonable? 'Course, there's your nosewheel and the wing shortening to be considered also. I think I sent you the analysis that said that at 200 mph TAS with a wind of 25 mph, even if it was at almost right angle to your course, it would only give an error of 1 mph. As I exlained in this analysis, if you get forecast winds at the altitude you are going to do your testing and use this to set up an into-the-wind GPS ground track and a with-the-wind GPS ground track, averaging these two runs will give you as good a TAS value as you can obtain with the piloting uncertainties. For my runs I use course and altitude-hold autopilots. If anyone would like to obtain my TAS testing analysis for putting on a thread, please contact me at elippse@sbcglobal.net.

I'll have to study that
 

Taking care of "honey do's" right now (washing car) but I will look at it later. I also use the auto pilot track and altitude hold for the tests (it is a necessity for the cross country air races).

Bob Axsom