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When both packets arrive at the trailing edge, they will have picked up some downward speed.īehind the wing, both packets will continue along their downward path for a while due to inertia and push other air below them down and sideways. It also has to change its flow path, because the cambered and/or inclined wing will push the air below it downwards, creating more pressure and more bouncing from above for our packet below the wing. Flow over the lower side of the wingĪ packet of air which ends up below the wing will experience less uplift and acceleration, and in the convex part of highly cambered airfoils it will experience a compression. Since camber allows for a gradual change of the contour, it is more efficient than angle of attack. This could either be camber or angle of attack - both will have the same effect. Therefore, supersonic lift is no longer caused by camber and curvature, but by the aircraft's inclination toward its direction of movement which causes a pressure increase on the lower side of the wing.īack to subsonic flight: Here, lift can only happen if the upper contour of the wing will slope downwards and away from the initial path of the air flowing around the wing's leading edge. Now the air in the streamtube decelerates and contracts from the increase in density, again allowing the aircraft to squeeze through. At supersonic speed this expansion becomes dominant, and luckily is accompanied by an increase in density when the flow slows down. At the speed of sound, the thinning from acceleration is exactly balanced by the expansion of the stream tube from the drop in density, and the aircraft cannot squeeze through as easily as before - this is the sound barrier. The streamtube will contract less, so more air needs to move away to make space for the approaching aircraft. This fast-flowing, low-pressure air will in turn suck in new air ahead and below of it, will go on to decelerate and regain its old pressure over the rear half of the wing, and will flow off with its new flow direction.Īpproaching the speed of sound, this stretching is accompanied by a thinning of the air - density decreases as speed increases. This requires even lower pressure, to make the molecules change their direction. Reluctantly, the packet will change course and follow the wing's contour. Once there, it will "see" that the wing below it curves away from its path of travel, and if that path would remain unchanged, a vacuum between the wing and our packet of air would form. Spreading happens in flow direction - the packet is distorted and stretched lengthwise, but contracts in the direction orthogonally to the flow. Due to the acceleration, the packet will be stretched lengthwise and its pressure drops in sync with it picking up speed - at least at subsonic speed. The packet of air will rise and accelerate towards the wing and be sucked into that low pressure area.

See it this way: Above and downstream of a packet of air we have less bouncing of molecules (= less pressure), and now the undiminished bouncing of the air below and upstream of that packet will push its air molecules upwards and towards that wing. Now to the airflow: When a wing approaches at subsonic speed, the low pressure area over its upper surface will suck in air ahead of it.

The more bouncing, the more force they exert on their surroundings. Pressure means that air particles oscillate all the time and bounce into other air particles.Inertial means that the mass of the particle wants to travel on as before and needs force to be convinced otherwise.To get to the bottom of it, it might help to look at airflow at a molecular level:Įvery air molecule is in a dynamic equilibrium between inertial, pressure and viscous effects: At higher angles of attack, the top of the wing becomes a "leeward" side, where air has to be pulled from above in order to fill in the shadow (of sorts) behind the wing. They generate almost all of their lift via varying their angle of attack. Acrobatic planes, on the other hand, tend to have roughly symmetrical wings because they want to be able to fly upside down. In general, we find the long haul aircraft (such as 747s) gain as much lift as they can from the curvature of the wing because getting lift through angle of attack means you pay a cost in drag. This effect can also be caused by modifying the angle of attack. The top of the wing is more often more convexly curved than the bottom - that's why the pressure isn't symmetric. The low pressure is typically caused by the curvature of the wing. The answer is really that it's a bit complicated because there's many ways to cause the effect, for varying aircraft. (for a complete story, here's a good link)