hevansrv7a
Well Known Member
Paul designed new tips for Jim Smith's RV-6 and was surprised that the performance gains were even better than expected. Good work!
But, I was still curious. So I reviewed the available information.
First, I have seen this picture of the tips and they appear to be tapered on the trailing edge as well as the leading edge:
But this is what they looked like at AirVenture2011:
I don't know for sure that there are two versions. I also don't know, if there are two, which one produced the 193.4 mph at 8000'.
Paul thinks that the CAFE RV6A's Oswald factor is .81 rather than the .85 they computed. That would be, roughly, within the 4% margin of error they cited. Taking those numbers and carefully recomputing them, I get .8493. It's in the "triangle tool" on my website: LINK if anyone wants to review it. CAFE's numbers balance to their speed and fuel flow, so I think they are probably pretty close. However, I, too, think that number seems high for a low AR, constant chord wing.
If I use +4% induced drag for the CAFE RV6A and leave parasite drag the same, I get an Oswald of .817 which is very close to Paul's result and within the stated CAFE report margin of error. I used that in the steps described below after first doing it with the original CAFE numbers.
As I described in an earlier post, I used factors from Van's specifications to change those numbers, step by step, to match Jim's airplane.
First I compensated for the lighter weight (1440 vs 1650). Then I compensated for the lack of a nose wheel. That gave me a model that can be compared to Jim's plane (lower the drags for faster TAS, match the HP). Then I built the model for Jim's plane going 193.4 mph at 8000' as cited by Paul in two earlier postings. I got an L/D of 13.5 (14.0) vs CAFE's 12.25 and a speed for L/D max of 94.4596 (92.78) instead of CAFE's 106 mph. It's a very substantial reduction in induced drag. I used Paul's estimate of slightly increased parasite drag.
From there, I computed the Oswald factor for Jim's plane to compare to the CAFE plane, both ways. The numbers for my math model of Jim's plane differ depending on which version of CAFE numbers I start with. CAFE's Oswald with the 4% correction above is .817 and Jim's is .8048. Oops! This is one of the cases for which the triangle tool was designed; it comes up with an unreasonable result and compels the re-examination of the input data. This suggests that the orginal CAFE data is better.
The original CAFE Oswald is .8496 versus Jim's .865. In other words, the increase in wingspan from 23' to 26' explains a lot of the reduction in induced drag, but there is also another 1.8% improvement in Oswald due to the shape of the tips. That's a more reasonable result since Paul's design is based on well documented research.
Paul said the Oswald is .91. Although my version is lower, it still represents a significant improvement. Paul is using more sophisticated ways of computing this. I am not knowledgeable enough to argue that one method is better, but the basic method appears to be well accepted: Oswald = Weight_squared / ( q x span_squared x Pi ) / Drag_induced.
I am using that basic formula, often cited on this forum and elsewhere. It's embedded in the triangle tool if anyone wants to review it. To put Jim's plane into the tool, change the span to 26, the two weights to 1440, the VLD to 92.78 and the vertical to horizontal ratio to 14.0.
You can also change the altitude to 8000 and the speed to 193.4. That speed is before the recent drag improvements that took Jim over 200 mph. The prop efficiency, SFC and fuel flow don't matter in this particular analysis but you can change them iteratively to see "what if". If anyone wants to review how I derived the VLD and Drag_min for Jim's plane, PM or email me and I will share the spreadsheet with explanation. It would be very beneficial if we could get those two numbers on Jim's plane in real-life, but for now, this is what we have.
Congrats to both Paul and Jim. I just play with numbers; they actually experiment. Thank you!
PS - I showed Jim's tips (the photo) to a very well known professional aerodynamicist whose recent successes would be recognized if I named them. He immediately said that he liked those tips. FWIW.
But, I was still curious. So I reviewed the available information.
First, I have seen this picture of the tips and they appear to be tapered on the trailing edge as well as the leading edge:
But this is what they looked like at AirVenture2011:
I don't know for sure that there are two versions. I also don't know, if there are two, which one produced the 193.4 mph at 8000'.
Paul thinks that the CAFE RV6A's Oswald factor is .81 rather than the .85 they computed. That would be, roughly, within the 4% margin of error they cited. Taking those numbers and carefully recomputing them, I get .8493. It's in the "triangle tool" on my website: LINK if anyone wants to review it. CAFE's numbers balance to their speed and fuel flow, so I think they are probably pretty close. However, I, too, think that number seems high for a low AR, constant chord wing.
If I use +4% induced drag for the CAFE RV6A and leave parasite drag the same, I get an Oswald of .817 which is very close to Paul's result and within the stated CAFE report margin of error. I used that in the steps described below after first doing it with the original CAFE numbers.
As I described in an earlier post, I used factors from Van's specifications to change those numbers, step by step, to match Jim's airplane.
First I compensated for the lighter weight (1440 vs 1650). Then I compensated for the lack of a nose wheel. That gave me a model that can be compared to Jim's plane (lower the drags for faster TAS, match the HP). Then I built the model for Jim's plane going 193.4 mph at 8000' as cited by Paul in two earlier postings. I got an L/D of 13.5 (14.0) vs CAFE's 12.25 and a speed for L/D max of 94.4596 (92.78) instead of CAFE's 106 mph. It's a very substantial reduction in induced drag. I used Paul's estimate of slightly increased parasite drag.
From there, I computed the Oswald factor for Jim's plane to compare to the CAFE plane, both ways. The numbers for my math model of Jim's plane differ depending on which version of CAFE numbers I start with. CAFE's Oswald with the 4% correction above is .817 and Jim's is .8048. Oops! This is one of the cases for which the triangle tool was designed; it comes up with an unreasonable result and compels the re-examination of the input data. This suggests that the orginal CAFE data is better.
The original CAFE Oswald is .8496 versus Jim's .865. In other words, the increase in wingspan from 23' to 26' explains a lot of the reduction in induced drag, but there is also another 1.8% improvement in Oswald due to the shape of the tips. That's a more reasonable result since Paul's design is based on well documented research.
Paul said the Oswald is .91. Although my version is lower, it still represents a significant improvement. Paul is using more sophisticated ways of computing this. I am not knowledgeable enough to argue that one method is better, but the basic method appears to be well accepted: Oswald = Weight_squared / ( q x span_squared x Pi ) / Drag_induced.
I am using that basic formula, often cited on this forum and elsewhere. It's embedded in the triangle tool if anyone wants to review it. To put Jim's plane into the tool, change the span to 26, the two weights to 1440, the VLD to 92.78 and the vertical to horizontal ratio to 14.0.
You can also change the altitude to 8000 and the speed to 193.4. That speed is before the recent drag improvements that took Jim over 200 mph. The prop efficiency, SFC and fuel flow don't matter in this particular analysis but you can change them iteratively to see "what if". If anyone wants to review how I derived the VLD and Drag_min for Jim's plane, PM or email me and I will share the spreadsheet with explanation. It would be very beneficial if we could get those two numbers on Jim's plane in real-life, but for now, this is what we have.
Congrats to both Paul and Jim. I just play with numbers; they actually experiment. Thank you!
PS - I showed Jim's tips (the photo) to a very well known professional aerodynamicist whose recent successes would be recognized if I named them. He immediately said that he liked those tips. FWIW.
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