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.
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ELIPPSE-Felix prop on RV-6
- Thread starter elippse
- Start date
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
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
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
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
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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
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 [email protected].
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Seems reasonable
The only way to really know of course is to refly the test at 6,000 ft. density altitude and validate the 1.6% assumption. The 4kt subtraction from my speed for the cooling drag mod is probably OK but the drag is not constant at different velocities - it probably is a higher figure at the 177.8 KTAS value but close enough. A confidential chart I was given with predicted cruise efficiency comparison between my prop and the Hartzell blended airfoil prop with 7496 blades across a speed range of 140-200 KTAS shows a rapid drop off of my prop above 165 KTAS. At my max cruise speed (minus the 4kt cooling drag improvement) 173.8 KTAS the efficiency is 85.4%. At this same airspeed on the chart the blended Hartzell is at 87.8 and climbing with cruise airspeed. I have been reeding Jack Norris' book "Propellers, the First and Final Explanation" and I have seen photos of your propellers on the winning biplanes at Reno for the past few years so I know that the blade area distribution appears to conform to his rule and the speed at Reno validates their effectiveness for obtaining high cruise. I can believe that 90% efficiency at this speed is achievable with your prop.
Bob Axsom
The only way to really know of course is to refly the test at 6,000 ft. density altitude and validate the 1.6% assumption. The 4kt subtraction from my speed for the cooling drag mod is probably OK but the drag is not constant at different velocities - it probably is a higher figure at the 177.8 KTAS value but close enough. A confidential chart I was given with predicted cruise efficiency comparison between my prop and the Hartzell blended airfoil prop with 7496 blades across a speed range of 140-200 KTAS shows a rapid drop off of my prop above 165 KTAS. At my max cruise speed (minus the 4kt cooling drag improvement) 173.8 KTAS the efficiency is 85.4%. At this same airspeed on the chart the blended Hartzell is at 87.8 and climbing with cruise airspeed. I have been reeding Jack Norris' book "Propellers, the First and Final Explanation" and I have seen photos of your propellers on the winning biplanes at Reno for the past few years so I know that the blade area distribution appears to conform to his rule and the speed at Reno validates their effectiveness for obtaining high cruise. I can believe that 90% efficiency at this speed is achievable with your prop.
Bob Axsom
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David-aviator
Well Known Member
Paul,
In reading about the ELIPPSE design, I've wondered if it is related to the Felix bi-camber design and it would appear it is. I've had 2 Felix props, one a 2 blade pusher on the Cozy MKIV and the other a 3 blade fixed pitch experiment with the Subaru engine, both of a bi-camber design.
I really liked the idea of a simple fixed pitch prop with this engine but it did not work out. For lack of a starting point of pitch and diameter, I asked Fred Felix to design it as if it were an O360 180HP engine. Ha, the thing turned up at 1900 rpm and made for quite a long take off roll with the Subaru. In flight engine cooling was terrible so after just 2 flights I sold the prop to a friend with an 0360 RV-4 and he is most happy with it.
Question, is it possible to design a fixed pitch prop to work with a 2:1 reduction and the Subaru engine? Fred Felix said he could have repitched the prop to perhaps turn up 2000 or 2100, but I thought I needed at least 2300 to get it to work for take off, and even at that was inclined to believe it would over speed quickly in flight.
By the way, I sat in on Klaus Savier's talk on props and other things at OSH last summer. That was a very interesting seminar and he touched briefly on why the MT seems to run out of steam on the high end of the performance envelop. It is all in blade design and I'm curious as to why Mt hasn't done something about it.
