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Similar to 15 aerodynamic hazards high speed flight (1)
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15 aerodynamic hazards high speed flight (1)
- 5. One of the really fun things about flying a fast jet is to throw around some fancy terms like “Mach Number”. The FAA wants you to know that the Mach Number is the ratio of: True Airspeed Speed of Sound Mack .82 = 0.82 x Speed of Sound
- 6. As air flows over a wing as you know it speeds up – that’s what causes lift as you know. It’s very possible that even if the airplane is flying slower than the sos, the air across the top of the wing could actually be faster than the sos. At any point when the speed of the air over the wing is called the “Critical Mach” of that airplane.
- 7. Transonic flight is the transitioning from sub sonic to supersonic and some of the local airflow is going supersonic. At any point when the speed of the air over the wing is called the “Critical Mach” of that airplane. TRANSONIC FLIGHT REGIME .75 to 1.2 Mach
- 8. When air goes supersonic it can’t get out of it’s own way and instead it compresses. It’s this compressibility that causes shock waves resulting in airflow separation, very much like stalling, in fact you can’t tell the difference between them when it’s happening. TRANSONIC FLIGHT REGIME .75 to 1.2 Mach
- 9. This airflow separation increases drag, locally destroys lift and changes the distribution of airflow over the wing and therefore changes the distribution of lift.TRANSONIC FLIGHT REGIME .75 to 1.2 Mach
- 10. As speed increases there is a sizeable increase in drag. That drag when you hit the Critical Mach Number is called Drag Divergence. That’s why people don’t fly faster than the CMN because you get a big drag divergence. IF you do fly beyond CMN it may result in the onset of these compressibility effects which could cause you control difficulties. 0.5 M (Mach number) CD (DragCoefficient) 1.0
- 11. • Let’s assume the air goes supersonic at the wing root and you get that separation and consequently the loss of lift. • Well, the wing tips which are behind the wing root continue to fly and that can cause the airplane to pitch down resulting in what they call a Severe Diving Moment known as “MachTuck” …and that is a bad thing.
- 12. Something you see on a relatively faster aircraft… WING-MOUNTED VORTEX GENERATORS Used to delay the onset of flow separation due to shock induced separation, drag divergence and the loss of smooth flow of air. If the smooth flow of air gets disrupted over the aileron – that would a bad thing.
- 13. DUTCH ROLL Sometimes on aircraft with weak dihedral and very much swept back wings can get into this which is known as “Dutch Roll.” The a/c rolls in one direction and yaws in the other, corrects one way and gets worse the other way. It may get started by the a/c hitting gust to get the motion started. A lot of jets have to have one and sometimes twoYaw Dampers to prevent this.