Thursday, April 26, 2007

TFXX /F111 Page 3

How the body evolved

It is not my intent to show how the entire configuration developed, but the airplane was , in my opinion, determined by the criticality of the high speed dash mission. This set limits on just how large the maximum cross section area could be.

Everything else being equal, for supersonic flight the most critical element is the maximum frontal area and the change in shape along the length of the airplane.. This is not so for subsonic flight where total wetted area is just as important. For a body of revolution there is a theoretical shape that provides the minimum wave This is referred to as a Sears-Haack body. Of course the real airplanes are much more complex shapes than a simple body of revolution, like a missile, but the concept of Sears-Haack requires the cross section areas to change smoothly along the length of the body.

This applies not only to the body of the airplane, but to the wings and empenage. The figure below first shows a typical cross sectional plot of an airplane and wing combined. Next to it the body has been area ruled, that is reduced in cross section where the wing joins the body. The result is the curve has been smoothed out and the maximum cross section reduced in value. A number of airplanes in the early 1950’s ,like the F11 and the F106, used this concept to reduce drag particularly in trans sonic flight.

Of course there is a limited amount of “area-ruling” that can be done depending on the unique requirements of the design. For the TFX, the important considerations were to minimize the maximum cross sectional area and to do what ever could be done to keep the plot of the area as smooth as possible. .


It was believed ( prior to wind tunnel testing) the maximum cross section area would have to be below some value that I have long since forgotten, but lets say about 48 square feet for illustration purposes. Now this is not just the body, but also includes the wings, and further the area distribution had to be smooth.

This was a continuous design process. There must have been hundreds of these plots made as all the airplane parts were added.

What determined the minimum area needed.

Once the crew station, nose radar, the engine and inlets ,and the bomb bay were configured, all that was left was the landing gear. All those things above pretty much determined just how much space was available for the landing gear. Fitting the main landing gear into the little area that was left for it was the most difficult problem in the whole design

Layout after layout was made trying all kinds of schemes to get a usable landing gear in the available space. As I recall both GD and Grumman engineers gave it a try, until one General Dynamics designer came up with what he called the “fomoco “ gear. Fomoco derived from Ford Motor Company. He said it was the Ford front wheel suspension system that gave him the idea!

Now I believe there are (or were some44 years ago) no better landing gear designers in the world than at Grumman, considering carrier landings. But there never would have been a F111 without his ‘fomoco’ design.

Interestingly, for all production airplanes the landing gear was supplied by Grumman.

During the proposal there was a continual update on just how well the General Dynamics TFX was meeting the Mach 1.2 dash mission of 200 miles.

Rumors had that Boeing was having troubles and was only making about 150 miles. I believe we thought the GD design would guarantee 190 miles or so/

Just a few days ago I found a wonderful story on the internet providing a complete and accurate history of the TFX program. It is the work of George Spangenberg who was the director of new aircraft evaluation for US Navy. Although I never actually met him, we designers knew it was he we had to convince more than any one else in Defense Department iif we were going to get the contract award. Now it is also true that he clearly thought that Boeing should have been selected for the TFX, but was overruled by a whole bunch of politicians including, of course Lynden Johnson.

If you read through this reference you will see the complete story of the program with just about all the pertinent statistics of the TFX story .