Posts: 3612
Joined: Mon May 31, 1999 4:44 am

Another lift question

Sat Mar 06, 1999 4:17 am

For those of you who like military aviation, you should know the "knife pass" displayed on every air shows.
During that display the plane isn't either on the normal position or the upside down position. It's in a middle position and I wonder how can the plane fly without the wings surface.
I think lift here doesn't work because the wings axis is not parallel with the ground, it's perpendicular.
I know for example in a dog fight, the plane can do any thing the pilot wants, as long as the structural limits are not pushed too much, but in this case the G forces do the work. In the knife pass the pilot is under 1G, flying in a straight line, so, how can the plane be sustained.

Imagine this F-16 flying staight ahead for 2Km. What surface keeps it sustained?

Mirage, Faro, Portugal
Posts: 6633
Joined: Tue May 18, 1999 11:10 am

RE: Another lift question

Sat Mar 06, 1999 4:50 am

You probably have noticed that aircraft doing knift edge flight are never horizontal, they are always pointed to the sky somewhat. The lift is created by the downward component of thrust ( and Newtow's 3rd Law (air striking the fuse and tail surfaces). In the picture, you can see that the wings are not totally verticle, thus still creating some downwards lift (even though not much), which is why the aircraft's longitudinal axis can stay fairly horizontal.

RE: Another lift question

Sat Mar 06, 1999 4:55 am

Lift isn't relevant to the ground. It's the relitive to the airflow. Relitive airflow is the air that hits the wings. Angle of attack is the angle of which the wings hit the relitive airflow. Lift is dirived from the angle of attack. The higher the angle of attack the more lift. In the case with the F-16 in the knife pass, the plane is (asuming) straight and level flight. The control surfaces are completely changed around. The rudder now acts as the elevator, elevators as the rudder but ailerons still act as the ailerons.. Useally the plane is parallel to the ground, but many high performece aircraft can fly in manuvers which many other's can't. After writing this I got a little confused, so if I can be corrected please do so.
Posts: 3612
Joined: Mon May 31, 1999 4:44 am

RE: Another lift question

Sat Mar 06, 1999 5:02 am

That picture isn't a good example because that F-16 can be turning and not on a straight flight.

But I conclude that the pilot apply rudder to keep the plane nose pointing to the sky (just a little).
Posts: 29881
Joined: Wed Jul 07, 1999 11:27 am

Vortex generated lift.

Sat Mar 06, 1999 5:24 am

I am glad that you posted a picture of an F-16 because it is a perfect example of desining the shape of an aircraft to support lift and a great example of Vortex generated lift. The shape of the wing fillets and strakes on modern aircraft (F-16, F-18, MIG-29, SU-27, Griffin, Concorde) are made to generate a vortex that starts at the base of the wing and then flows over the top of the wing when the aircraft is moving. The way that the vortex rotates is such that it creates a low pressure area over the body and wing of the aircraft. Of course just like on a normal airfoil the wing is drawn into this area of low pressure. The wing of these modern fighters are really thin because they are optimised for high speed flight. If you where to design a airfoil that produces lift in the normal manner it would have to be quite thick to produce the lift that the Vortex system provides and would slow the airplane down considerably. Also the vortex enables the aircraft to fly at much higher angles of attack (60-70 degrees nose up) then would be possible with a standard airfoil.

There is some really neat footage taken of one of the concorde aerodynamic models during a wind tunnel test. You can clearly see the vortexes clearly rotating over the wing.

On prop airplanes it is just a matter of what we said in the earlier aerodynamic posts. You just need to increase the angle of attack of the lifting surface. Except in sideways flight the fuselage is providing the lift not the wing. One advatage the fuselage has over the wing is that all the air around it is moving at much higher velocity then over the wing due to the fact the fuselage is completly in the propwash. Mose fuselages actually have pretty good airfoils shape. They are blunt at the engine and taper down to a point at the tail. Delmar Benjamin's Gee Bee replica is reported to have very good knife edge flight charicteristics. If I ever get my pictures of it in flight posted it is pretty easy to see why. It is just one big symetrical airfoil from the bottom. In his case he just has to hold enough angle of attack so the body produces enough lift. In jets with normal wings it works the same except that the jet doesn't have the airflow from the prop assisting the creating of lift. So they have to maintain both higher airspeed and a higher angle of attack to maintain altitude.

I Said in the early posts that the book Aerodynamics For Naval Aviators is about the best reference book for aerodynamics out there. I recommend it.

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