Subject: Re: Prop planes and speed of sound, was:Fastest Piston Fighter 
From: David Lednicer <dave@amiwest.com>
Date: Jun 10 1996
Newsgroups: rec.aviation.military

The article on the M=.96 Spitfire appeared in "The Aeroplane" 
about four years ago.  Accompanying the article was an "artist's 
conception" of the dive, complete with shock waves.  We had quite a 
chuckle about it here, as I had just completed some CFD calculations on 
the Spitfire and I could see that the artist had omited the strongest 
shock wave on the Spitfire - the one on the canopy!  Yes, the canopy on 
the Spitfire shocks before the wing!

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David Lednicer             | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc.   |   email:   dave@amiwest.com
2133 152nd Ave NE          |   tel:     (206) 643-9090
Redmond, WA  98052  USA    |   fax:     (206) 746-1299

Subject: Re: Mach 0.96 Spitfire dive (was: Fastest fighter and other
From: David Lednicer <dave@amiwest.com>
Date: Jul 15 1996
Newsgroups: rec.aviation.military

Philip Morten wrote:

> Look at the sequence of design drawings for the Type 300, the
> elliptical wing planform is intoduced when it was decided to increase
> the armament from 4 x .303 to 8 x .303.  In order to accommodate the
> outer guns and maintain the low t/c it was necessary to increase the
> chord outboard, hence the elliptical planform.  The mininum *induced
> drag* characteristic of an elliptical lift distribution was a bonus.
> The down side to this was that the wing was difficult to make and
> production of wings ran well behind fuselages in the early days.

	Yes, the elliptical planform did result from the need for 
outboard guns.  More specifically, wing depth for the ammunition trays 
for the outboard guns was needed.  Also, remember that the landing gear 
retracted outboard, so depth was need for the tire too.  The Spitfire 
wing had approximately 2 degrees of twist, so the lift distribution was 
not elliptical.
 
> The high speed characteristics of the Spitfire are due to the low t/c
> rather than the elliptical planform but it was the elliptical planform
> that enabled the low t/c to be maintained over the outer gun bays.
> Remember that induced drag is most significant at high AOA and high
> speed diving flight involves a low AOA.

	Actually, the Spitfire was very slow for its power, so I wouldn't 
say it had good high speed performance (in level flight).  The wing has a 
NACA 2213 at the root and a 2209.4 at the tip.  The Mustang has a 13.8% 
thick root airfoil and a 11.4% tip airfoil.  So yes, the Spitfire has a 
thinner wing, which would help at high Mach.  However, I would estimate 
that the Mustang's airfoils have lower drag at high Mach (something I 
have got to try and calculate someday soon...).  I think that the 
Spitfire's high drag divergence speed, relative to the Mustang is due to 
a combination of features, not just the wing or wing airfoil design.

	For more info, so my two papers:

	Lednicer, D., "A CFD Evaluation of Three Prominent World War II 
Fighter Aircraft," Aeronautical Journal of the Royal Aeronautical 
Society, June/July 1995.

	Lednicer, D. and Gilchrist, I., "A Retrospective: Computational 
Aerodynamics Analysis Methods Applied to the P-51 Mustang," AIAA paper 
91-3288, September 1991.

-------------------------------------------------------------------
David Lednicer             | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc.   |   email:   dave@amiwest.com
2133 152nd Ave NE          |   tel:     (206) 643-9090
Redmond, WA  98052  USA    |   fax:     (206) 746-1299