Naked Science Forum
Non Life Sciences => Technology => Topic started by: amalia on 08/11/2019 11:16:18
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Daniel got in contact with us with a good question:
If a domestic jet that is travelling from point a to point b at near top speed suddenly enters an area of air moving very fast in the direction of the plane, what would stop the plane exceeding its top designed speed?
Do you know the answer?
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If a domestic jet that is travelling from point a to point b at near top speed suddenly enters an area of air moving very fast in the direction of the plane, what would stop the plane exceeding its top designed speed?
First of all, the speed of an aircraft is relative to the air, so meeting air that moves in the direction of the aircraft would cause its speed to drop lower than 'top speed'. If the air current change is severe enough, it might cause the aircraft to stall.
So if it suddenly encountered a headwind, it would find itself moving at greater than top speed, and would likely slow itself down to bring the speed back down to this top cruising speed.
In a severe case, an aircraft might find itself in conditions where the aerodynamics just don't work at the speed it is moving. I was actually witness to such a situation (from the ground) where mistaken choices by the pilot caused a commercial aircraft (a 727 if I recall correctly) to break the sound barrier, and become unresponsive to attempts to regain control. The plane rolled upside down twice before they implemented the great idea of dropping the landing gear which created enough drag to bring the speed back down to a level where control could be regained. They did an emergency landing at a different airport than intended, but most of the 'injuries' were spilled drinks.
I didn't watch it happen, but I heard the sonic boom.
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There are several maximum speeds for any airplane! As a minimum (!) you will have gear limiting, flap limiting, rough air, normal max cruise, max manouvering, "never exceed" (all marked on the airspeed indicator) and Vdem, the maximum speed at which the aircraft has actually (carefully, accurately) been flown in a test.
In general the rough air speeds will have been designed and demonstrated to retain full control authority and structural integrity for sudden gusts of 20 kt or more (40 kt in some cases) in all directions. "Max man" is the speed at which authority and integrity are maintained up to the specified operational g limits (probably +5 -2g for a small plane, +2 -1 for a liner, lots more for a fighter). I think the other values are selfexplanatory.
And then just to make it interesting, the simplest display is "indicated air speed" - just the actual pressure on a pitot tube. This is then converted to "calibrated", "rectified" and "true" airspeed by the application of geometric, pressure and temperature corrections, the final TAS figure being the speed at which the actual molecules are flying past the window. The difference between IAS and TAS is significant at altitudes above 5000 ft.
Where it all goes wrong is flying into a thunderstorm downburst on final approach. As the cold air and rain reaches the ground, by Sod's Law always about half a mile before the runway, it spreads out, so having reduced speed below "max gear", you encounter a 20kt headwind in addition to the runway wind that you had planned for. It's scary but less of a problem in a light propeller aircraft where you can quickly alter your flight attitude and speed without exceeding your g limits, but big jets have a lot of inertia and the engines don't take kindly to being messed about. Anyway if the auto-throttle has managed to compensate for the gusting headwind by slowing down a bit, the next thing you encounter is rain, snow and hail with a downward wind vector, then just 10 seconds later, a 20 kt tailwind. This seriously erodes the margin between gear limit and stall speed, the latter having increased by the accretion of a ton or more of ice on the wings, and just as your pulse rate recovers, your sleek flying machine suddenly becomes a brick.
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Travelling back from South Korea in a 747 we were fortunate to encounter a tail wind that gave a land speed of 750 mph and enabled us to avoid a refuelling stop at Frankfurt.
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If a plane is going to be exeeding its top air speed, its usually down with the engines running fast , in such a senario the structure of the plane is stressed beyond its design limits. Basically the wings fall off. First instances of supersonic flight occoured on descents, and in some cases the boom broke the plane apart as the planes where not designed for such speed.