Explain this and win the prize!

[gripen]

Originally posted by Crumpp
What is ridiculus in saying that without the polars you are just guessing?

The problem is that you can't under stand that the formula:

CwF = Cw0 + K * Ca^2

is directly drag polar.

Originally posted by Crumpp
1. Insist on using the taper ratio of a theoritical ellipse for the Spitfire when every aeronautical engineering student knows the wing twist destroyed the elliptical wet lifting surface of the wing.

As noted several times in these discussions, you are the only one who has claimed that the Spitfire has e factor value 1. In addition you have quoted couple contradictory claims for near elliptical lift distribution taper ratio values (0,4 and 0,52 and actually 0,67 for elliptical wing )

Originally posted by Crumpp
2.  Do use the drag polars.

Well, it seems that I'm the only one in this thread who has actually calculated e factors from the drag polars.

gripen

[Crumpp]

Well, it seems that I'm the only one in this thread who has actually calculated e factors from the drag polars.

Yep.  For the Spitfire, Me-109, and P51.  Unfortunately you don't have the FW-190 drag polars and your not getting them from me.

In addition you have quoted couple contradictory claims for near elliptical lift distribution taper ratio values (0,4 and 0,52 and actually 0,67 for elliptical wing )

Exactly.  Why is that?  Because the taper ratio for the Spitfire is impossible to nail down since the wet lifting area in no way matches the elliptical shape of the wing due to the wing twist destroying the benefits of the elliptical tips.  That leaves a wide margin that delivers correct values that are meaningless in the real world.  Very open to data manipulation.

On the other hand, being a much later design and it's designers being well aware of the benefits of elliptical distribution purposely twisted the FW-190's wing to achieve it.

In the real world they were probably pretty close as the 1965 formulas compute them.

In other words without tested drag polars under the same conditions it's impossible to correctly compare performance.

A general formula that does not include taper ratio in this case, as Badboy pointed out, probably yields results that are much closer to real world performance.

Crumpp

[gripen]

Originally posted by Crumpp
Unfortunately you don't have the FW-190 drag polars and your not getting them from me.

Well, the Fw document gives the value of the K and that's all I need. But maybe I'll dug that "Chalais-Meudon" report from somewhere to have even more fun.

Originally posted by Crumpp
Because the taper ratio for the Spitfire is impossible to nail down since the wet lifting area in no way matches the elliptical shape of the wing due to the wing twist destorying it.  That leaves a wide margin that delivers correct values that are meaningless in the real world.  Very open to data manipulation.

Plain nonsense as usual, so now you introduce "the wet lifting area"

Originally posted by Crumpp
In other words without tested drag polars under the same conditions it's impossible to correctly compare performance.

So here you admit again that you don't understand at all the linear stage of the Cd/Cl^2 curve.

Originally posted by Crumpp
A general formula that does not include taper ratio in this case, as Badboy pointed out, probably yields results that are much closer to real world performance.

Well, the generic formulas seem to assume tapered wing so these should work in those cases but as pointed out above, those formulas seem to give too high values even in those cases. If this kind of formula gives about correct result for other kind of wing, it's more or less luck.

gripen

[Angus]

Crumpp, are you referring to the twist as washout?

[JoOwEn]

Originally posted by Angus
I'd say NO.
Aspect ratio is only a function of chord and span.
You'd be missing shape, wingtip shape, thickness and dihedral.

I agree.

[Crumpp]

Well, the generic formulas seem to assume tapered wing so these should work in those cases but as pointed out above, those formulas seem to give too high values even in those cases. If this kind of formula gives about correct result for other kind of wing, it's more or less luck.

As opposed to just guessing at the correct taper ratio?  Your stand is that is more accurate?

Please.

In addition you have quoted couple contradictory claims for near elliptical lift distribution taper ratio values (0,4 and 0,52 and actually 0,67 for elliptical wing )

Which still leaves this unanswered.  What is the guess for the correct taper ratio of the Spitfire?

Crumpp

[Angus]

What Spitfire?
I presume the most produced wing, i.e. from Mk I through MkV all the way to many MK IX's right?

Anyway, you should be able to calculate the taper ratio from chord and span and for a perfect ellipse, which however the Spitfire did not have, but very very close.

[gripen]

Originally posted by Crumpp
As opposed to just guessing at the correct taper ratio?

Nonsense, there is no need to use  generic formulas when the value of the K is known.

Originally posted by Crumpp
Which still leaves this unanswered. What is the guess for the correct taper ratio of the Spitfire?

Nonsense, there is no need to "quess"  when the shape of the drag polar is known (ie the value of the K).

gripen

[Crumpp]

Nonsense, there is no need to use generic formulas when the value of the K is known.

Not talking about the FW-190's.  I am refering to the Spitfires.  Nice bait and switch though.

Nonsense, there is no need to "quess" when the shape of the drag polar is known (ie the value of the K).

Again I was refering to the taper ratio of the Spitfire.

The one you keep asking me to decide on here:

In addition you have quoted couple contradictory claims for near elliptical lift distribution taper ratio values (0,4 and 0,52 and actually 0,67 for elliptical wing )

So which "guess" should I take?

Anyway, you should be able to calculate the taper ratio from chord and span and for a perfect ellipse, which however the Spitfire did not have, but very very close.

Actually according to Lednicer it was not very close.  It was slightly better than the other two but not a fantastic lead.  Certainly nowhere near a perfect ellipse.

Crumpp

[gripen]

Originally posted by Crumpp
Not talking about the FW-190's.  I am refering to the Spitfires.  Nice bait and switch though.

Nonsense, wind tunnel data of the Spitfire gives the shape of the drag polar. And the Fw data gives the value of the K. There is no need to quess.
 

Originally posted by Crumpp
The one you keep asking me to decide on here:

Nonsense, you have quoted two different values for taper ratio which gives near elliptical lift distribution. It's your problem.
 

Originally posted by Crumpp
So which "guess" should I take?

You made those contradictory statements, it's your own problem.

Originally posted by Crumpp
Actually according to Lednicer it was not very close.  It was slightly better than the other two but not a fantastic lead.

Nonsense, Lednicer analyzed span loading of the wing but he did not give any values.

gripen

[Crumpp]

Nonsense, you have quoted two different values for taper ratio which gives near elliptical lift distribution. It's your problem.

Nonsense,

Your Spitfire calculations are Nonsense because you use the taper ratio for a theoretical ellipse when the Spitfire does not have a perfect elliptical wingtip.  The twist ruined it.

Nonsense, you have quoted two different values for taper ratio which gives near elliptical lift distribution. It's your problem.

Nonsense, I quoted a range for values.

You made those contradictory statements, it's your own problem.

Nonsense, Your English is not good enough to understand what I wrote.  Get a better dictionary.

Nonsense, Lednicer analyzed span loading of the wing but he did not give any values.

Learn to read again. Lednicer commented on the efficiency factor.  The Spitfire is only "slightly ahead" and ruined the benefits of elliptical construction by twisting the wing to fix the harsh stall.


Crumpp

[gripen]

Originally posted by Crumpp

Your Spitfire calculations are Nonsense because you use the taper ratio for a theoretical ellipse when the Spitfire does not have a perfect elliptical wingtip.  The twist ruined it.

If I know the shape of the drag polar, the value of the K can be easily determined as pointed out in the PerkinsHage. There is no need to quess or even know the taper ratio.

Shortly you have no idea what you are talking about.

Originally posted by Crumpp
I quoted a range for values.

No, you quoted two contradictory claims for taper ratios which should give near elliptical lift distribution:

Your post 09-11-2004 02:50 PM:
"However, the formulae presented by Professor Wood is for a taper ratio of 0.57 which yields an almost elliptical lift distribution, "

Your post 09-18-2004 04:01 AM:
"A taper ratio of 0.4 most closely approximates the elliptical."

Originally posted by Crumpp
Learn to read again. Lednicer commented on the efficiency factor.  

Here is the direct quote again:
"loading distribution is not elliptical, though it is probably the most optimum of the three from the induced drag standpoint".

Besides, you are the only one in these discussions to claim e factor 1 for the Spitfire as can be verified from here.

gripen

[Angus]

Of this:
"Actually according to Lednicer it was not very close. It was slightly better than the other two but not a fantastic lead. Certainly nowhere near a perfect ellipse. "

Building a perfect ellipse is easily doable, but to get there, the aspect ratio will suffer instead of the induced drag gain by getting all the way to the ellipse.

Why do you think Mitchell picked this exact design?

[Crumpp]

Building a perfect ellipse is easily doable, but to get there, the aspect ratio will suffer instead of the induced drag gain by getting all the way to the ellipse.

Read Lednicer's quote on the fact designers were well aware of the benefits of elliptical distribution and it was easily achieved by twisting the wings.  This occurred in the later designs of the FW-190 and the P51.

The Spitfire on the other hand had an elliptical distribution already.  That distribution caused a nasty and dangerous stall.  To change the stall characteristics so they were workable, the Spitfire designers twisted the wing to cut down on the elliptical distribution.

Thereby bringing all three aircraft in a similar catagory for wingtip efficiency.  Sure the Spitfire has the lead but it's not a commanding lead for this characteristic.  It's lower wingloading is the main contributor to it's fighting characteristics not it's efficiency factor.

Crumpp

[Angus]

I don't have that report, but I'd like too.  
Anyway, from some source (think it was a pilot) I heard that actually the Spitfire wing would NOT stall all at once, which would then be a normal sideffect, since a complete ellipse should stall all at the same time.  Am I right there?
The genius part with the Spitfire is perhaps exactly there, it's a clever compromize to acquire low wing loading with the lowest possible drag. The low wingloading pays for itself instead with good climb and good turnrate and good ceiling.
The second genius part would than be that it's almost uncompressable, not sure why exactly though.
The penalty: Less absolute top speed.