[quote=ucodegen][quote CDMA ENG]The reason, they explained, that you dont go to full power and point the aircraft down has everything to do with the wing design. The aircraft’s wing, if it is going to fast, start to build a shock wave over the leading edge and interrupts the airflow to the extent that the wing will lose lift.[/quote]Compressibility effect occurs trans-sonic/supersonic not low speed. This is why there were several accidents when trying to break the sound barrier. The shock wave prevented sufficient airflow from getting to the control surfaces. Next Miramar airshow, take a look at the leading edges on F18, F16s. You’ll find they are very narrow/thin. Compare that to a commercial sub-sonic plane, which has a fairly thick leading edge and to private planes which also have a fairly thick leading edge compared to their size and weight. The thicker leading edge gives an advantage due to a larger range of Angle of Attacks(AOA) that the wing will operate over (angle of attack being the angular difference between a line through the width of the wing and the vector representing the direction that the air is impacting the leading edge).
Compressibility is also the reason why supersonic aircraft try to have a smoothly transitioning cross section – ie have a coke bottle shape where the fuselage actually gets a little thinner in cross section when near the wings.
What happens at high AOAs is that as you approach stall, the laminar flow on the back(top) of the wing actually detaches from the wing surface. When that happens, you loose lift from the wing. There are all sorts of tricks done to prevent this. Next time you are on a flight, take the window seat next to the top of the wing. You’ll see small little blades sticking up from the top, near the chord of the wing(highest point, thickest in cross section) going lengthwise along the wing. You’ll notice that they are at a slight angle to the airflow. This is to create a vortex near the top wing surface that ‘glues’ the laminar flow down to the surface of the wing. If on the flight, you are on takeoff or landing and there is a moderate amount of moisture in the air – you can actually see the vortexes. Another thing to note, is that when flaps on commercial aircraft are deployed near their fullest, the leading edge of the wing actually pulls away from the wing and drops down. This is to bleed high pressure air from under the wing and ‘jet’ it close to the surface to ensure laminar flow at high Angle of Attacks.
There is another piece of the wing that you may see move. It will be near the fuselage of the wing is a large panel that tilts up from the top of the wing. Most of the panels will be in close to the fuselage. This is the ‘spoiler’ and it does what its name says. It ‘spoils’ the airflow, killing lift and increasing drag (also acting as a brake) when it comes up. It is useful at low speeds because the ailerons at the tips don’t have as much a bite on the airflow at low speeds. It has more of a ‘brake’ effect at higher speeds. They are mostly on the part of the wing nearest to the fuselage because they have a very large effect on the aircraft. If they were further out to the tip, they could easily flip or roll the aircraft.[/quote]
What a great post. Thanks ucodegen.
