Zapata

Play with this NASA airfoil sim

It's neato!

WalkOfShame

As an "incompressible" fluid (water, oil, air in subsonic flight, etc.) is forced to try and compress, it speeds up so that the volume (if you will) of the fluid will remain constant. This increase of speed, lowers to pressure of the fluid. Its the decrease in the "flow-able area" (if that makes sense) that causes the increased speed.

As far as your second question, I don't quite understand what your asking. The boundary layer happens because of Bernoulli's principle. A fluid cannot have a boundary layer if it is not moving.

joepilot

Hi HSLD.

I've just got to jump in on this.

Most of what you have quoted is true, but there are a couple of problems. First, in some cases, Va is not based on the elevator as the primary control problem. In many transport category aircraft the critical control is the rudder, as in the AA accident over NY. There may be some aircraft where the critical control is the aileron, although I don't know of any. Second, in the case of many transport category aircraft, full and abrupt elevator deflection from a trimmed condition will not cause either a stall or an overstress condition even at Vmo. This is because the trimmable horizontal stabilizer has much more control authority than the elevator.

Joe

HSLD

That and paperwork

It's all so confusing, back in the helo days, we just beat the air into submission. Now-a-days, I occupy my time wondering if I could airborne on an infinitely long conveyor belt that matched my wheel speed.

TonyC

The difference between your airplanes is the angle of attack, that is, the angle of the wing to the relative wind. In both cases, the relative wind is horizontal, or parrallel to the ground. The airplane on the ground, with its nose wheel on the ground, has a lower angle of attack. While there is certainly a pressure differential, the fact that it is still on the gorund belies the fact it is not producing sufficient lift to bring the airplane off the ground.

The airplane flying low obviously is generating the lift required, and if the nose were lowered, the ensuing encounter with the ground would be an instant clue as to why. Rotating the nose (pulling back on the yoke) with sufficient airspeed is what causes the wing to acheive an angle of attack to produce lift sufficient to cause the airplane to rise off the ground -- and that's where the fun begins.




In between. The airfoil causes a differential pressure when air passes over it. However, the differential pressure is insignificant in terms of wake turbulence disturbances until that differential is sufficient to lift the airplane off the ground.






.

ryan1234

It is confusing, there is so many exceptions to this and that, and many, many variables - it's hard to say ok, "x" causes lift - without talking about rotational flow, viscosity, etc. Maybe there is a fine line between keeping it simple and making it too simple it comes out all wrong (i.e. equal transit theory).

joepilot

Many aircraft, (the Cessna 182 and the B-52, both with full flaps extended, come to mind), are perfectly capable of generating enough lift with the nose wheel on the ground to lift off in a flat attitude. In these aircraft, if you try to land at higher than the reccommended speed, you will probably touch down nose wheel first.

In these aircraft the angle of incidence (the angle between the longitudinal axis of the aircraft and a line from the leading edge of the wing to the trailing edge of the extended flap) is quite substantial. Therefore rolling down the runway you already have a significant angle of attack, and can lift off without raising the nose. Doing so, of course, is likely to lead to an accident, because if you try to hold the aircraft on the runway with forward elevator, the main gear may lift off first, and this would cause serious directional control problems.

Joe

KC10 FATboy

Here's the simple answer. Yes, the aircraft on the ground is producing some lift. However, the angle of attack isn't great enough to create sufficient lift to overcome the aircraft's weight. As soon as the elevator causes the aircraft's angle of attack to increase, the lift being generated exceeds the aircraft's weight and the aircraft becomes airborne.

If you have ever sat in a window seat near the wing of a 747-400, you will notice something odd. During taxi, the winglet is below you as the wing bends downward due to its weight. As you increase speed for takeoff, the wing rises and before the aircraft is even airborne, the winglet is well above you.

TPROP4ever

Rooster and the egg...ooops did I say that

Yes I know roosters dont lay eggs