The P-51 and its laminar-flow wing

[Brooke]

Chalenge, I am ready to wager with you on it.

We can put our money with a mutually trusted third party, draw the pay for three experts from the pot, each expert will render a decision on who is more correct, and the remainder of the pot goes to whoever gets the majority nod of the experts.

The bet will probably need to be about $2k each to cover cost of the experts and still have a profit left for the winner.

Will you take this bet?

[Stoney]

Angle of incendence does indeed matter when you say "This is why a P-51 can raise its nose as much as eight degrees (or a little more) and the drag remains the same".  A P-51 with its nose raised eight degrees has its wing root at 9 degrees angle of attack.  That was the point I was making.

I thought this originally started with a statement about "8 degrees angle of attack"? 

I guess this isn't so much a discussion of the issue as it is an argument between you two.  With that, I'll bid you both adieu.

[Stoney]

In looking at Pyros chart I would have to say the chart defines parasitic drag as only the part of drag associated with surface friction/resistance with the air flow and does not mention profile drag (which as I understand things should double the drag coefficient since profile drag is usually the equal of or slightly greater than parasite drag). In other words the total drag would be 2 x .008 + vortex drag but in reality it would be more. The published zero-lift drag coefficient for the P-51 is .0163 and so please tell me what you think the total drag coefficient for 8 degrees angle of attack would be and explain how it is "substantially more drag."

Then you can explain why you think an airfoil that is nothing like the P-51s has any bearing on the subject.

Actually, the 65215 is pretty close to the D model airfoil.  Probably a 64215 would be closer, but the behavior of all the laminar airfoils plots out the same, so I think its a decent relative comparison.

The profile drag is shown on Pyro's chart--that's the type of drag being represented by the drag polar.  There is no other drag represented except for the "standard roughness" graph, which introduces the skin friction component.

[EDIT]...after thinking further, even the standard roughness is still profile drag...

[Masherbrum]

Chalenge, I am ready to wager with you on it.

We can put our money with a mutually trusted third party, draw the pay for three experts from the pot, each expert will render a decision on who is more correct, and the remainder of the pot goes to whoever gets the majority nod of the experts.

The bet will probably need to be about $2k each to cover cost of the experts and still have a profit left for the winner.

Will you take this bet?

You'd make a bet with Paul Hinds?   Good luck collecting. 

[Ack-Ack]

You'd make a bet with Paul Hinds?   Good luck collecting. 

You can be almost certain that one of Chalenge's experts will be an entomologist.


ack-ack

[Brooke]

I thought this originally started with a statement about "8 degrees angle of attack"? 

I guess this isn't so much a discussion of the issue as it is an argument between you two.  With that, I'll bid you both adieu.

It started with this statement:  "This is why a P-51 can raise its nose as much as eight degrees (or a little more) and the drag remains the same," and whether or not that is correct is the topic of the discussion.

It's not that big a deal if one wants to substitute "raise the AoA of its wing as much as eight degrees" for "raise its nose as much as eight degrees," but the two are technically different by about one degree.  That's all I was pointing out as one small part of my overall discussion.  My feeling that the statement is not correct does not hinge on whether one is taking that to mean 8 deg. AoA or 9 deg. AoA.

[Stoney]

It started with this statement:  "This is why a P-51 can raise its nose as much as eight degrees (or a little more) and the drag remains the same," and whether or not that is correct is the topic of the discussion.

It's not that big a deal if one wants to substitute "raise the AoA of its wing as much as eight degrees" for "raise its nose as much as eight degrees," but the two are technically different by about one degree.  That's all I was pointing out as one small part of my overall discussion.  My feeling that the statement is not correct does not hinge on whether one is taking that to mean 8 deg. AoA or 9 deg. AoA.

Sorry, I misunderstood then...

[Brooke]

Sorry, I misunderstood then...

No problem at all, and you are correct in pointing out that, for angle of attack of the wing, angle of incidence doesn't enter into it.

[Chalenge]

You'd make a bet with Paul Hinds?   Good luck collecting. 

Karaya what is it with you and Mr Hinds? Did he send you to places you havent seen since? Grow up grow a pair and move on your gay bf is long gone.

[Chalenge]

Actually, the 65215 is pretty close to the D model airfoil.  Probably a 64215 would be closer, but the behavior of all the laminar airfoils plots out the same, so I think its a decent relative comparison.

The profile drag is shown on Pyro's chart--that's the type of drag being represented by the drag polar.  There is no other drag represented except for the "standard roughness" graph, which introduces the skin friction component.

[EDIT]...after thinking further, even the standard roughness is still profile drag...

Okay Stoney if its 'pretty close' then I suppose 'substantially more drag' cant be verified.

The point is Brooke is trying to substantiate his argument with data that is not relevant to the P-51D. That leads me to believe that he either doesnt have verifiable evidence (which admittedly would be difficult to find) and/or he is incapable of working it out logically. Thats why he has turned to the betting approach.

My point is that even the evidence that I have seen from history (wind tunnel tests of the later 40s) are not quantifiable because of the tunnels having inherent turbulence. Some of the tunnels of the day had a turbulence factor of as much as 2.64 which requires not only a quantitative but statistical adjustment rendering any result useless.

Now if you want to look at data that is 'pretty close' then I suggest you do some research on the work of Eppler and Wortmann (pre-Raspet) where Eppler and Wortmann independently tested the NACA 65 415 with the Eppler 266 profile. The double bucket result is very interesting but again... 'pretty close' doesnt cut it.

[Brooke]

Thats why he has turned to the betting approach.

No, it's because of two reasons.  First, is that I said I'd bet your statement was incorrect, initially along the lines of a figure of speech.  But when you wrote "but your large sum probably equates to about $15," I thought "OK, let's go for it then."  Second, no amount of discussion by me, no matter what data I find or what explanations I apply, is going to convince you.  The only way to further progress is to get the opinion of a panel of undisputed experts and have them opine on it.  But that likely will cost some money.  Might as well have a way for whoever is wrong to foot the bill for that.  My suggested wager covers both issues nicely.

You haven't accepted the wager, so I assume that your answer is "no," but that your way of delivering that answer is just not to say anything.

[Chalenge]

Its not likely you have the ability to pay for the only kind of test that would settle it. As I already said... you posted one airfoil that is not the P-51. The P-51 airfoil is different at every station (rib) of the wing and so a single section isnt going to work either way. One of the reasons the 1946 data is 'calculated' is because the individual cl(s) need to be individually tested (probably with a very well designed 'drag rake') and then properly summated in a proper series. Modern tunnels have a turbulence factor of less than 1.06 but its very expensive to do this type of research and for the P-51 alone you will be using the tunnel for a week or more.

Even though I have the kind of money it takes Im not even going to pretend to be bold enough to put that on the line. If you are willing to do that your crazier than you look. 

[Masherbrum]

Karaya what is it with you and Mr Hinds? Did he send you to places you havent seen since? Grow up grow a pair and move on your gay bf is long gone.

Even your former squaddies have come forward.   I'm happily married.   I realize you're under some stress so if you need to "tap" the left foot, leave me out of it.  

[Stoney]

Okay Stoney if its 'pretty close' then I suppose 'substantially more drag' cant be verified.

The point is Brooke is trying to substantiate his argument with data that is not relevant to the P-51D. That leads me to believe that he either doesnt have verifiable evidence (which admittedly would be difficult to find) and/or he is incapable of working it out logically. Thats why he has turned to the betting approach.

My point is that even the evidence that I have seen from history (wind tunnel tests of the later 40s) are not quantifiable because of the tunnels having inherent turbulence. Some of the tunnels of the day had a turbulence factor of as much as 2.64 which requires not only a quantitative but statistical adjustment rendering any result useless.

Now if you want to look at data that is 'pretty close' then I suggest you do some research on the work of Eppler and Wortmann (pre-Raspet) where Eppler and Wortmann independently tested the NACA 65 415 with the Eppler 266 profile. The double bucket result is very interesting but again... 'pretty close' doesnt cut it.

You are exasperating...  I've never read any of Eppler and Wortmann's work, admittedly.  I know Eppler was part of the NLF series airfoil designs.  But I have read Theory of Wing Sections from stem to stern.  You look at enough drag polars and you can see trends.  I've plotted and compared the 45-100 root airfoil plot with the 64218 plot and they're nearly identical.  At the tip, the 64212 is almost identical.  At the P-51D MAC, the 64215 plot is almost identical.  This is assuming that the plot that you got from the UIUC airfoils database for the 45-100 is dead-on accurate.  They are close enough for any reasonable person to use for comparisons.  The differences?  The 45-100 point of maximum thickness is at about 37%, same as the 64000 series.  There is a bit of a variation in the mean lines used for the two, as NACA used the 1.0 mean line versus a shallower mean line used by North American.  I don't know which because its impossible to tell visually.  What does a shallower mean line mean?  That the P-51D pitching moment would be a bit more benign, and add just a hair more profile drag.  I'm talking variations of fractions of percent here.  If I had time (probably take 6-8 hours of work), I could plot a 64418 a=.5, 64415 a=.5, 64412 a=.5 and see how much closer they would be, but I'm not.

I love these discussions when they're educational and enjoy the back and forth.  This one could be, but isn't anymore.

[IrishOne]

Its not likely you have the ability to pay


Even though I have the kind of money it takes

why do you constantly try to make everyone think you are better than them?    get over yourself, cupcake.   did u not see Pyro's post above?!   my moneys on Brooke.  but of course I don't have nearly the $$$ you do, as you so eloquently stated