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Flashcards in High Speed Flight Deck (24):

Speed of sound only varies with the ..., and it can be said that this speed is proportional to ...

air temperature

the square root of the air’s absolute temperature


speed of sound formula =

38.94 x square root of absolute temp


0 degrees C is what in kelvin?


K = C + 273


what does mach number define?




what is Mfs?

free stream Mach no, which is the speed of the undisturbed airflow ahead of the aircraft, before the shape of the wings and fuselage have accelerated or decelerated the local airflows


what happens to Mach number in level flight and constant IAS with varying temp?

Mach no will stay constant when flying at constant flight level and IAS despite varying temp. Eg if temp rises, LSS will rise, but so will TAS at a constant IAS, because higher temperature reduces air density


how is a shockwave formed?

As the aircraft reaches M1.0 it is travelling at the same speed as the pressure waves it is creating, which are now unable to move ahead of the aircraft and the individual pressure waves pile up into a single, powerful pressure wave just ahead of the aircraft known as a shockwave

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In a shockwave the air is violently compressed and the result is:

Greatly increased pressure, resulting in increased density and temperature. The speed of airflow is decreased and the LSS is increased, due to higher temp


what happens to total pressure in supersonic flow?

some of the dynamic pressure energy of the airflow is converted to heat so the result is that total pressure decreases in a supersonic flow


what happens to the speed of airflow behind a normal shockwave?

It causes a speed reduction of the airflow so that behind the shockwave the air slows back to subsonic values. This change is helped by the fact that the increase in air temp increases the LSS


what is wave drag?

The energy accounted for by the rise in pressure is recoverable to the aircraft, but the energy accounted for by the rise in temperature is lost, so there is always extra drag 


what causes the normal shockwave to form ahead of the leading edge rather than on it?

The air that is hitting the nose is being brought to rest at the stagnation point, which causes it to compress and increase in temp

This rise in temp means that the LSS in the area just ahead of the nose is higher than in the free stream, and so Mach no here remains less than 1 and allows the shockwave to move ahead, clear of the nose


what is Mdet?

detachment Mach number - the Mfs at which the shockwave attaches  

If the aircraft continues to accelerate, the speed will eventually exceed the influence of the temp rise due to compression at the stagnation point and the shock wave will attach to the nose

Mdet = M1.3


when does the leading edge shockwave become oblique?

when Mfs is increased beyond Mdet


what is Mach angle formula?

see image

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how to fing oblique wave cone angle?

double the mach angle

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What happens to speed of flow behind an oblique shockwave?

Is it still supersonic?

it does slow down but remains supersonic


what is critical mach number, Mcrit

the free stream Mach no at which, for the first time, the local flow becomes sonic somewhere on the aircraft

whenever an aircraft exceeds Mcrit shockwaves will be present


what is the effect of increasing wing camber on Mcrit?

reduces Mcrit


what is the effect of increased alpha on Mcrit?

decreases it


how is Mcrit proportional to mass?

inversley proportional to mass

Increased mass means more lift is created by flying at a higher angle of attack, so more mass decreases Mcrit

As fuel is burnt through the flight Mcrit will increase as mass reduces


what effect does moving CG forward have on Mcrit?

Moving the CG forward has the same effect as increased mass, so if we move the CG forward Mcrit decreases


The transonic regime is defined as ...

the speed range in which all three speeds of flow can be found around different parts of our aircraft


The supersonic regime does not actually start until above ...


Below this speed the bow wave ahead of the nose slows the airflow to subsonic speed.

Above Mdet the shockwave attaches and turns oblique, after which all flows remain supersonic. All flows must be supersonic before the aircraft speed is described as supersonic

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