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  #31  
Old 02-08-2011, 11:34 PM
elippse elippse is offline
 
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BTW, if you'd like to get an estimate of what your speeds would be relative to Jim's if you equipped your plane with similar tips, but with different HP, here're some multipliers to get you a ball-park estimate:

160 - 1.0217
180 - 1.0627
200 - 1.1006

These factors are based on the cube-root of the power ratio. For instance, you could expect at 8000' dalt, based on Jim's 193.5 mph, about 203.5 mph with 180 HP or about 213 mph with 200 HP.
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  #32  
Old 02-09-2011, 08:15 AM
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  #33  
Old 02-12-2011, 07:04 PM
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hevansrv7a hevansrv7a is offline
 
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Question Looking at Serendipity

I am not afraid to risk looking ignorant or of making a mistake if it will help me to learn and possibly to help others to learn. This could be one of those times, but here goes.

Paul's friend's airplane gained some speed with new wingtips. Let's take that as a fact. Paul says the parasite drag increased slightly. If I understand his numbers, it is a 1.77% increase in parasite drag. So the speed increase had to come from reduced induced drag and that appears to be what Paul is saying he did. He also says he got a surprise when it worked better than he expected. He attributes the improvement to better Oswald efficiency of the tips. BTW, the CAFE 6A test said the Oswald factor was .851 but Paul said he used .81 for the original tips. At least I think that is what he said.

Unless I'm mistaken on the basics (always a possibility) the parasite drag goes up with the square of the speed and the induced drag goes down with the square of the speed. At the point of best L/D they are equal. For an airplane like the RV, there is not much induced drag at 190 mph TAS at 8000'. It is around 25 induced to 175 parasite for the CAFE test. I don't know any reason to expect that set of rules to change with different tips. (Laminar flow would violate it.)

Paul estimated a 10 or 11 HP improvement. If that is BHP, I agree. At 190 mph at 8000' that means around 8.1 THP which is about 16 pounds of drag net of the expected increase in parasite. Hmm. 175 pounds times 1.77% is about 3 pounds. So we need to reduce the induced drag by about 19 (16 + 3) from a starting point of around 25. Do you see where this is going? If so, you can skip to the last paragraph.
--------------------------------

I was curious, so I created a special spreadsheet set of columns for the drag curve of the CAFE RV6A. Then I created a new set of columns for the same airplane at 1440 pounds instead of 1650, the test weight. Then I modified the parasite drag enough to account for the 2 mph gain going from A model to t.d. Now I have a model of a 6t (not 6A) at 1440 pounds. Then I created another set of columns for the 6e (as in Ellipse). Each set of columns has one for speed, one for parasite, one for induced and one for total drag. The columns for the 6e are set up to allow easy iterative alteration to get numbers to where we want them.

I used the 6e's 6.4 mph gain over Van's numbers at 8000'. I ignored the CAFE performance numbers which are for 180 HP. The 6e has a nominal 150.

Since the CAFE drag numbers are for CAS, I had to figure the CAS numbers for the TAS speeds at 8000'. I then figured THP from the drag times the TAS. The idea is to alter the induced drag, see the reduced total drag such that the same THP gets the correct mph increase. For the 6e, I increased the parasite drag by 1.77%. Then I tried to find the numbers that will give the same THP as the Van's numbers, but at the higher speed (6t vs 6e). I knew that the 6e had a lower stall speed so I expected a lower best L/D, too. This is where I ran into trouble. In order to get the drag and THP numbers to come out right I had to reduce the L/D speed to 53 and the drag at that speed to 33 pounds and the resulting glide ratio was over 40. I don't consider these numbers realistic.

There are obvious problems with this analytical technique. I'm combining the CAFE drag numbers with Van's performance numbers. I'm using conventional techniques for changing from 1650 to 1440 pounds but I could have done that wrong. I could have made a mistake converting from 6A to 6t. None of these look wrong to me, though. Just to put possible error in perspective, the speed for the 6e before the new tips at 8407' DA was 187 mph. If you plug that into my model for the 6t you get less than 1 THP difference from Van's numbers. When I get home next week I can put the whole spreadsheet on the web so that anyone can check it. There are very likely small errors in all of this, but the result for the 6e with increased parasite drag is grossly unreasonable.
----------------------------------------

The 6e performed very closely to the CAFE 9A (can't be precise - too much missing data). That's very good for a shorter wing with longer chord and allegedly less efficient airfoil. I think that something in the design of the 6e improved parasite drag (too). If you use my triangle spreadsheet and use a L/D speed of about 95 mph (a guesstimate of five less than the 100 for the 6t at 1440#) you get a pretty reasonable result. The problem is that when you do that you get lower parasite drag along with lower induced drag and that is not what Paul says he expected. This is merely a guess on my part. I don't know what the experiment did to either kind of drag. When you encounter serendipity it is good to pursue it. It would be good to better understand how those tips actually do what they do.
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  #34  
Old 02-12-2011, 07:59 PM
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Bill Wightman Bill Wightman is offline
 
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The wing tips here increase span and AR. For a tighter analysis of the resulting planform, a vortex lattice model should be done to study the spanwise loading. The Oswald factor is a pretty dumb correction to estimate induced drag on a non elliptic planform, and isn't a good analytical tool.

D = Cd_0 + CL^2/Pi/A/e where e is the Oswald Factor. Pretty unsophisticated.

I must admit I'm surprised these helped at all, but it just goes to show how badly the short wing RV's need more AR to reduce induced drag.
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  #35  
Old 02-13-2011, 11:00 AM
elippse elippse is offline
 
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For induced drag I prefer the simpler CDi=W^2/(Q^2*B^2*pi*e), which shows immediately the contribution to span in overcoming induced drag, and you don't have to do additional calculations involving span and area to arrive at AR. If you set this equal to CDo and solve for Q, you have the estimated speed at which L/D is maximum.
And I must apologize, Harry, but after wading through what you wrote, I'm not really sure what you were driving at. Could you just put into a simple declarative statement what it was I got right or wrong?
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  #36  
Old 02-13-2011, 05:39 PM
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hevansrv7a hevansrv7a is offline
 
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Default What I said..

Quote:
Originally Posted by elippse View Post
For induced drag I prefer the simpler CDi=W^2/(Q^2*B^2*pi*e), which shows immediately the contribution to span in overcoming induced drag, and you don't have to do additional calculations involving span and area to arrive at AR. If you set this equal to CDo and solve for Q, you have the estimated speed at which L/D is maximum.
And I must apologize, Harry, but after wading through what you wrote, I'm not really sure what you were driving at. Could you just put into a simple declarative statement what it was I got right or wrong?
I answer to Harry, too. Just not late for dinner.

The short version: the performance gains observed cannot reasonably be accounted for by reducing induced drag alone.
The model is assumed inviolable and so the data must work within the model. The most reasonable guess is that, somehow, parasite drag was also reduced.
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  #37  
Old 02-13-2011, 07:52 PM
elippse elippse is offline
 
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Quote:
Originally Posted by hevansrv7a View Post
I answer to Harry, too. Just not late for dinner.

The short version: the performance gains observed cannot reasonably be accounted for by reducing induced drag alone.
The model is assumed inviolable and so the data must work within the model. The most reasonable guess is that, somehow, parasite drag was also reduced.
Sorry that I must disagree with your guess, but I think you're making an assumption that you can't support. First off, I must tell you that as far as C.A.F.E.'s numbers on OEF are concerned, I don't consider them to be the last and greatest word on anything to do with airplanes. They did some interesting testing years ago, but I don't credit them for anything beyond that.

You must consider that when testing any individual airplane, the performance numbers that you get apply only to that particular airplane and not apply to the fleet as a whole, as we're talking about homebuilts, not factory-built planes. They are engineering estimates!

Now I asked a well known acquaintance who actually designs real honest-to-God, high performance airplanes and he said that these short, wide wings are known for not being very efficient. In the book Design of the Aeroplane, by Darrell Stinton, 1983, he shows on P. 139 a maximum of 0.82 for a wing planform somewhat similar to an RV-6 with its raked tips if it has sharp rear corners. I'm afraid again that because Darrell actually designs real aeroplanes, that I would take his research over a single C.A.F.E. test.

Darrell gives the rake angle vs AR as 0° for infinite AR, 20° for AR = 6, and 25° for AR= 1 to 5. 'Don't know if that's a linear progression, but doubt it. The references for these rake angles are: Shaw, H. (1919), A text Book of Aeronautics London: Charles Griffin and Company Limited, and Barnwell, F.S. and Sayers, W.H. (1916) Aeroplane Design and a Simple Explanation of Inherent Stability London: McBride, Nast and Company. (Note, Barnwell was the designer of the Bristol F2B Fighter (1916-1917). The RV-6 tips with 1' span and 5' chord are about 11.3° for their 4.8:1 AR. Go figure if that's correct or not!
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  #38  
Old 02-14-2011, 12:35 AM
scsmith scsmith is offline
 
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Default An error here:

The correct form for the dimensional induced drag in units of force is
Di = W^2/(pi * Q * B^2 *e).

So, if you divide by Q again (as shown in the quote) you have an equivalent "flat-plate" drag area. Not a drag coefficient yet. To get CDi, you must also divide by the reference area.

It is often very handy to work in terms of the dimensional forces for a given airplane.


Quote:
Originally Posted by elippse View Post
For induced drag I prefer the simpler CDi=W^2/(Q^2*B^2*pi*e), which shows immediately the contribution to span in overcoming induced drag, and you don't have to do additional calculations involving span and area to arrive at AR. If you set this equal to CDo and solve for Q, you have the estimated speed at which L/D is maximum.
And I must apologize, Harry, but after wading through what you wrote, I'm not really sure what you were driving at. Could you just put into a simple declarative statement what it was I got right or wrong?
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  #39  
Old 02-14-2011, 05:34 AM
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hevansrv7a hevansrv7a is offline
 
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Default Off Point

Quote:
Originally Posted by elippse View Post
Sorry that I must disagree with your guess, but I think you're making an assumption that you can't support. First off, I must tell you that as far as C.A.F.E.'s numbers on OEF are concerned, I don't consider them to be the last and greatest word on anything to do with airplanes. They did some interesting testing years ago, but I don't credit them for anything beyond that.

You must consider that when testing any individual airplane, the performance numbers that you get apply only to that particular airplane and not apply to the fleet as a whole, as we're talking about homebuilts, not factory-built planes. They are engineering estimates!

Now I asked a well known acquaintance who actually designs real honest-to-God, high performance airplanes and he said that these short, wide wings are known for not being very efficient. In the book Design of the Aeroplane, by Darrell Stinton, 1983, he shows on P. 139 a maximum of 0.82 for a wing planform somewhat similar to an RV-6 with its raked tips if it has sharp rear corners. I'm afraid again that because Darrell actually designs real aeroplanes, that I would take his research over a single C.A.F.E. test.

Darrell gives the rake angle vs AR as 0° for infinite AR, 20° for AR = 6, and 25° for AR= 1 to 5. 'Don't know if that's a linear progression, but doubt it. The references for these rake angles are: Shaw, H. (1919), A text Book of Aeronautics London: Charles Griffin and Company Limited, and Barnwell, F.S. and Sayers, W.H. (1916) Aeroplane Design and a Simple Explanation of Inherent Stability London: McBride, Nast and Company. (Note, Barnwell was the designer of the Bristol F2B Fighter (1916-1917). The RV-6 tips with 1' span and 5' chord are about 11.3° for their 4.8:1 AR. Go figure if that's correct or not!
I am not arguing about tip efficiency. All I'm saying is that you can't get that much of a performance increase from induced drag alone without changing the max L/D speed and ratio to a degree that is very unlikely in this case. Try your own drag curve estimates for that airplane (which just happens to perform, pre-tip mods, at the same speeds with the same engine as the Van's prototype).

Do you really think CAFE's drag curve can be off from Jim's airplane that much? Or do you think that the rules for how drag changes with speed are not "true"? Is the airplane now at L/D max at 53 mph when it was around 100? Does it now have a 40:1 glide ratio?

The model is a framework within which the observations must fit. That model does not care about tip efficiency as a separate factor. Parasite drag goes up, induced goes down, each as the square of the velocity. They cross at max L/D and drag min. If you don't accept this, say so. If you do, then do the curves and see what you get.

Here I am saying that you found a way to reduce parasite drag with a tip change and you don't like it? Better you should try to figure out how you did it.
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  #40  
Old 02-14-2011, 09:37 AM
elippse elippse is offline
 
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Quote:
Originally Posted by hevansrv7a View Post
They cross at max L/D and drag min. If you don't accept this, say so. If you do, then do the curves and see what you get.

Here I am saying that you found a way to reduce parasite drag with a tip change and you don't like it? Better you should try to figure out how you did it.
If you'd look at the AIAA paper that I reference you'd see that they found that the overall wing efficiency went up; this is more than just reduced induced drag due to increased span. The wing's induced drag decreased even more than would be obtained from the equation for CDi. Since the other terms in the equation are fixed - weight, Q, pi, and span, the only thing that must have changed is the OEF! They show that the wing's L/D increased with the swept-slashed tips.

Howard, Harry, the references show that in order to obtain an OEF of 0.82 for a rectangular wing that the tip would have to be at a 20° sweep angle; the RV-6 is 11°, so I stand with my use of 0.81, which is still very good, and I totally reject C.A.F.E.'s 0.85. You know, I really looked hard but couldn't find any plane that C.A.F.E. designed. Does that mean I dismiss them totally? No, they have done some interesting testing, but between the testing and analyzing the results there is a great gulf. I know a lot of people in aero who sort of look down their noses at some of C.A.F.E.'s conclusions.

Everything that is done in analyzing data relies upon making certain assumptions, as you did in accepting C.A.F.E.'s 0.85 OEF even though it flies in the face of other analysis. Now it is up to you and your assumption of reduced CDo to show exactly how the rest of the airplane's drag went down as a result of installing these wingtips. When you do, send the results to the SARL mailing list so that these guys who love to race can make their planes go faster.

I really am pleased with the results of the tips, but even more so is Jim who went through a lot of anguish in whether the effort was worth the time and cost of $600-$800 to make them. As the title of this posting said, the outstanding results he obtained, much better than I had estimated, really puzzled me and came as a surprise until I was able to find how the wing's lift distribution, tending more toward the ideal elliptical with its OEF of 1.0, could also have an OEF tending toward more toward 1.0 than the assumed 0.81.
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