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A little more technical this time (back to school!)

Posted by bdk on Mon Nov 11, 2002 09:06:28 PM

In reply top Re: Using your analysis method... posted by Christer on Mon Nov 11, 2002 07:15:36 PM

It has been quite a while since I was in school studying this stuff, but here it goes...

NACA did a number of studies during the war years, using a full size Mustang (as well as other aircraft) to correlate drag reduction theory to actual numbers. They even towed a P-51 aloft with no propeller and all the gaps sealed up and released it, measuring its sink rate with precise instruments to determine its actual drag compared to what was predicted in the wind tunnel (with a much smoother scale model).

The Germans also did full scale wind tunnel (Gottingen?) studies with a 109 where they sealed the gaps and measured how much difference each change made.

Check your local library for copies of the old NACA reports and the book, "Theory of Wing Sections" by Abbott and Von Doenhoff.

The P-51 has a number of advantages compared to other aircraft of the era.

The P-51 has a radiator system that produces a significant amount of thrust in cruise. It was designed to optimize this thrust. Note the lip on the top of the scoop that prevents the radiator from ingesting air disturbed by the aircraft belly skin. Notice how similar it is to modern design (F-16, etc.). Both the 109 and the later Spitfire have dual radiators mounted under the wings. Two is not better than one in this case, as two small radiator systems produce significantly more drag than one large radiator system. Not only was the design of the Spit/109 system poor to begin with (by modern standards), they were mounted to the wings, which disrupted the flow there which further increased wing drag. In a racing P-51, a revised inlet scoop is typically used featuring a reduced inlet area. Higher speeds provide plenty of airflow since the original scoop was designed for adequate cooling on a hot day with high power at low (climb) speeds, loaded with drop tanks, fuel, and ammo. Added to that on the racer is a water spraybar to further increase the cooling efficiency.

P-51 had fully enclosed gear and a retractable tailwheel. The others did not (maybe Spit 24 did?).

A "midget racer" style canopy can be installed in place of the bubble on a P-51D, reducing not only the frontal area but the skin friction due compared to the surface area of a "razorback".

While the P-51 in combat probably had little laminar flow occuring due to the surface finish, in civilian hands, significant laminar flow can be attained (using auto body plastic filler to fair in the panel joints). Rare Bear, the Super Corsair (R.I.P.), and Dreadnought do not have laminar flow wings (by definition), but they actually see significant amounts of laminar flow when properly finished (like Dago Red). Even the T-6 class sees some major benefits. The radial engined aircraft make up for their extra cooling drag and less efficient airfoil section (and larger size in general) through the use of significantly higher continuous power (larger displacement). Most of these are former navy airplanes and needed the extra lift for carrier operations, so they didn't use laminar flow airfoils.

Cees mentioned something about the "drag bucket". Most airfoils have significantly reduced drag as they approach zero angle of attack. I don't know if this is responsible for a stock Spitfire's ability to out-turn and out climb a Mustang or not. Wing loading and the maximum coefficient of lift on the airfoil section also play a part in that equation. Also, if you are in a "Lufbery Circle", the plane with the higher drag may win because it can slow down and reduce the radius of turn for the same G-loading to get a better lead on the pursued aircraft. That is the idea behind maneuvering flaps. What I wonder though is if Dago Red EVER gets out of the drag bucket when turning a 497 MPH lap at Reno. Lift always comes at the price of drag, but the drag bucket is related to angle of attack (the orientation of the relative wind to the airfoil section) only. At nearly 500 MPH around the pylons it doesn't take much of an angle of attack to pull 6 G's compared to the angle of attack to achieve 6 G's at 300 MPH in combat. If the drag bucket theory is true though, just imagine how fast Dago is going in the straights! Maybe 540? 550?

The elliptical wings of the Spitfire only reduce the wing drag by one or two percent compared to tapered wings. This may sound significant, but the wing has very low drag to start with, so this contributes very little to the overall picture when copmpared to the fuselage and tail which only contribute drag (or negative lift) to the system. Clip the elliptical wings and you end up with somewhat of an abbreviated elliptical lift distribution so the advantage is even less.

My conclusion is that you need to install a P-51 scoop on a Spitfire, enclose the wheels/retract the tailwheel, install a small canopy, buy a racing engine, and hope you can land in a 50 kt crosswind at Reno with that narrow gear. Then you are ready!

I would really like to see a competitively raced Spitfire at Reno some year (along with the 3-engined P-38 and the P-82 with the Griffons). Maybe it will happen some day. The variety would be nice. :-)

: Well, Cees had the power output in the equation as well th
: us discussing power to weight ratio and/or power to drag r
: atio, didn?t he?

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