Question to aerodynamic engineers

[PROTECengr]

I was recently watching a show on discovery channel about aircraft last week, and learned some interesting stuff. I am not an aerospace engineer, but a mechanical, but I do have a basic knowlege of aerodynamics. This show was talking about the F16 and eurofighter having aerodynamically unstable airframes, and they have to use fly-by-wire systems to correct instabilities to keep the things in the air. This is all done to increase their agilities. My question is that if you aren't designing a stable airframe, how much of your knowlege of aerodynamics is actually applied in designing an unstable aircraft?

 

[Wittenborn]

Nearly all of it is applicable. Whether the resulting airplane is stable or not, the basic fundamental principles apply. The most fundamental form of longitudinal stability is the location of the center of gravity with respect to the aerodynamic center. I can take an existing airplane (which is stable) and move the horizontal tail around to make it unstable. But the aerodynamics are the same.

In the case of fighters, the more "stable" the airplane, the less maneuverable it is (without couputer feedback, that is).



Regards,
Grant
Aerospace Engineer

 

[jamescornford]

A delta wing has poor lift at subsonic speeds, and thus design difficulty is greatly increased. If you are able to consider a standard wing, it would be easier .

 

[WhiteRabbit]

Fundamentally, there is a fine balance between control (maneuverability) and stability. If you lose stability you gain maneuverability and vice versa.

My Stability and Control teacher used the analogy of having a marble on the inside of a bowl (concave section). You can move the marble but it will always return to it’s origin (the center of the bowl) and that was referred to as a stable system.

If you turned the bowl upside down and put the marble on the convex side you can move the marble much easier but the system would be inherently unstable. Aerodynamically, very stable aircraft have a tendency to correct themselves to straight and level (if given enough altitude and speed and trimmed properly) and very maneuverable aircraft will not naturally self correct themselves without input from the pilot or computer.

Obviously, fighter and high performance aircrafts are required to have maneuverability as opposed to a 747 which requires stability.

You will find that commercial transport and most GA aircraft are designed for stability (dihedral wings, aerodynamic moments and CG moments, cambered airfoils, etc) while high performance aircraft either have the opposite or reduced characteristics for aerodynamic stability.

Some high performance/stunt aircraft have airfoils are not cambered and are symmetric which results in poor cruise aerodynamic properties for a wide range of flight criteria which are desired by the airlines for fuel efficiency.