Explain this and win the prize!

[gripen]

Well, one can draw lines to the picture but the spanwise efficiency is function of the whole wing despite it is called some times  wing tip efficiency. Besides, we talk here about efficiency of the whole airframe as in Perkins&Hage.

Regarding efficiency calculation based on relative lift distribution, it certainly gives relative difference of lift distribution but it takes account only the differences in induced drag, not the viscous. Still, for one reason or another it seem to give suprisingly similar results as polar analysis.

gripen

[Crumpp]

Still, for one reason or another it seem to give suprisingly similar results as polar analysis.

Yeah I am sure we could throw some numbers out there that  could be remarkably similar to polar analysis.  Does not mean they are correct by any means.

What is that saying, an infinite number of Monkeys banging away on an infinite number of typewriters will eventually reproduce the works of Shakespeare...

Crumpp

[gripen]

You can easily test it yourself, besides FW data gives the drag polar:

CwF = Cw0 + K * Ca^2

No need to quess anything.

gripen

[Crumpp]

You should check the polars.

Crumpp

[gripen]

You are most wellcome to post polars here if those are somehow different than given in FW data.

gripen

[Crumpp]

You are most wellcome to post polars here if those are somehow different than given in FW data.

No, Thanks.  I sent them in to Pyro.  Positive HTC will do a fair and impartial set of calcs off them.

Crumpp

[Charge]

"Why do you think Mitchell picked this exact design?"

Mitchell picked that design because he wanted as large wing area as possible and for certain reasons it is the most sensible choice there is if the wing area is considered the most important factor.

You can get nearly the same Cl efficiency from a rectangular wing as seen in many modern day small a/c but the elliptic wing is better for a fighter aircraft because you can build the tip of the wing more rigid than it would be possible in rectangular shape plus the elliptic shape produces less drag.

The negative effect of the elliptic shape is the bad stability of the design in other than level flight which was probably why the Spit had such tall and slim fuselage to act as a huge rudder to counter that imbalance. Further maneuvering stability was produced by the washout which made the wing act more like a rectangular wing as seen in P51 or FW without sacrificing much of the lift qualities of elliptic design as seen in figure presented earlier in this thread.

It should also be noted that that figure shows the Cl for level flight AoA so as AoA increases the Cl for the wingtip rises to , more or less, meet the Cl values of the optimum elliptic profile while the Cl for the root may have already lost its lift due to stall.

-C+

[Angus]

Emm...Charge....wooot?

"You can get nearly the same Cl efficiency from a rectangular wing as seen in many modern day small a/c"

Well, with 30% more induced drag for a full rectangular.

"The negative effect of the elliptic shape is the bad stability of the design in other than level flight"

Never heard that one.
The Spitty had C of G problems, but they had nothing to do with the ellipse.


And:
"Further maneuvering stability was produced by the washout which made the wing act more like a rectangular wing as seen in P51 or FW without sacrificing much of the lift qualities of elliptic design as seen in figure presented earlier in this thread. "

The 190 and P51 have tapered wings, not rectangular. Tapered wing is the cheap way of obtaining something close to the elliptical effect.

Finally, I think I've heard before that the Spitfire wing root stalled before the tips. Anybody?

[gripen]

Seems that Mitchell ended to the elliptic form for several reasons and need to place the wheels, guns and ammunition inside wing are probably among these. Otherwise, it's quite probably that they did not think mass production at all; in the mid thirties very few types reached large production numbers.

Regarding stall, the Spitfire had generally good stall characters like the Bf 109 ie no tendency to tip stall.

gripen

[Crumpp]

Regarding stall, the Spitfire had generally good stall characters like the Bf 109 ie no tendency to tip stall.

Yes it did.  AS Lednicer points out however it would have had a very dangerous stall without the twist.

Although it sacrificed some wingtip efficiency for better stall characteristics, I think the tradeoff was worth it.

It allowed poorly trained pilots to survive and gain experience quickly.  As the NACA trials point out, the Spitfire exhibited an amazing amount of control and stability even during the stall.

Crumpp

[Angus]

Ummm?
"--------------------------------------------------------------------------------
Regarding stall, the Spitfire had generally good stall characters like the Bf 109 ie no tendency to tip stall.
--------------------------------------------------------------------------------

Yes it did. AS Lednicer points out however it would have had a very dangerous stall without the twist. "

Crumpp, your text needs a wee refining.

It was twisted a bit and not completely elliptical.
A wee more induced drag pro square, however maybe less square?, - anyway, the Spit stalled gently when compared to other WW2 planes of similar power.
The Hurricane would dip a wing BTW.

[Crumpp]

Your correct Angus.  I was reading about the Spitfires development.  Seems it was a windtunnel test wooden model in 1935 I was thinking of...

As far as the wingtip efficiency goes:




Key word is probably.

Looking at the shape of the high pressure portion (red) of the wet lifting surface:



The sharp ending is hardly elliptical.  In fact the the FW-190's:



Looks alot closer (red).  Taking into the lower pressure portion of the lifting surface (yellow), it sure looks to me like in the end they will come out very close.

Crumpp

[gripen]

Crumpp,
You have a very strong tendency to do yourself laughable, pressure distribution is not the same thing as lift distribution.

And there is no such thing as "wet lifting surface" except in your creative mind.

gripen

[Charge]

Emm...Charge....wooot?

->Sorry Angus, you obviously knew these things already but there are other people reading this thread, too?

"You can get nearly the same Cl efficiency from a rectangular wing as seen in many modern day small a/c"

Well, with 30% more induced drag for a full rectangular.

->Well, I said that elliptic produces less drag. 30%? Interesting.

"The negative effect of the elliptic shape is the bad stability of the design in other than level flight"

Never heard that one.
The Spitty had C of G problems, but they had nothing to do with the ellipse.

-> I think some of it may have. U gotta read more about elliptic wings maybe?

I believe it was a known profile even before Mitchell decided to use it but others chose not to, and there is a reason why. One thing is its complexity when considering mass manufacture but that is not the only reason.

 Anyway Mitchell did a good job as history shows. The wing design was changed from Spitfire model 21 onwards (IIRC) for some reason. The later design probably rolled better because the aileron design was better in that one.

I forgot to mention that too much lift in the tip portion of the wing is probably not good if the torsional rigidity of the wing is not good enough as a flexing wing can have a negative effect on roll rate and stall characteristics in high speed or G loads. The wing needs to be torsionally stiff.

-C+

[Charge]

"pressure distribution is not the same thing as lift distribution"

If a wing creates lift due altering pressure around it so how does not pressure distribution have nothing to do with lift distribution?
Of course it depends on the vector of the pressure whether or not it produces lift...

If Spitfire had no washout I'd imagine that the red pressure area would resemble that of FW 190 and in higher AoA the red area will extend more to the tip of the wing. I guess the washout was taken into account in the model?

-C+

Edit: http://www.grc.nasa.gov/WWW/K-12/airplane/cp.html ??