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Disturbances during High Speed Flight

Updated on July 29, 2013

Transonic Flight

In order for an aircraft to fly safely and efficiently at transonic speeds, certain features must be incorporated into the design. These features will reduce the effects of compressibility. A large problem that is associated with high speed flight is the additional drag that is created by the compressed airflow as it moves over the aerofoil. If this drag is not reduced then the maximum speed of the aircraft is drastically lowered. In subsonic flight there are many different types of drag (profile, parasite and induced) but with higher speeds you encounter shock drag.

Consider an ordinary tail plane and elevator. At subsonic speeds an elevator depends on the complete change of flow for it's effectiveness. The change of flow occurs over the both tail plane and elevator when the elevator is raised or dropped. The actual forces on the tail plane are much greater than on the elevator itself. Once a shock wave is formed a movement of the elevator will not affect anything in front of the shock wave, this means you have to rely on the elevator itself to control the movements of the aircraft. However the elevator is in the turbulent airflow region which suggests it may be ineffective in controlling movements. At higher speeds the forces behind a shock wave cause the elevator to become almost immovable.

Shock Drag

As you know that a shock wave forms when the critical mach number is reached. Once this occurs the an area of compressibility also occurs behind it, this will be accompanied by a turbulent airflow that will cause the boundary layer to break away in a similar fashion to the subsonic condition that will result in a stall. In order to distinguish between subsonic and transonic stalls we refer to this condition as shock drag.

Once a shock stall occurs, the turbulence that is produced by the shock wave creates a longitudinal trim disturbance. This is usually a nose down movement. The area of turbulence is large and so the corrective action needed is difficult to achieve. IT's difficult because most of the turbulence is near the ailerons and buffeting / vibrations cause big problems.

Sweepback

The formation of a shock wave depends on the speed of the airflow across the section of the aircraft that is perpendicular to the leading edge. By sweeping back the leading edge of the wing, the velocity of airflow is reduced and the formation of a shock wave is delayed. This will increase the mach number at which the critical mach is reached.

The problems with sweepback is that it allows wing tip stalling and excessive stability. Skin friction effects are greater on sweptback wings as the airflow moves along and across the wing. This creates a tendency for the boundary layer to thicken and separate towards the wing tips. This increases and decreases of lift.

Tip stalling: When a wing is swept back it causes a change in the distribution of lift, a larger proportion is carried by the outer parts of the wing. As the boundary layer tends to change direction and flow towards the tips, it will interact with the airflow at the tips causing an early stall.

Deep Stall

Swept wings will tend to pitch up at the stall. At some sections you get a high drag rise and a forward movement of the centre of pressure after this stall occurs. The lift becomes decreased and the aircraft begins to sink. This increases the angle of attack pushing the nose upwards even more. As the angle of attack increases further and further, the elevator or other types of pitch control becomes less effective. The aircraft will reach a point were it is in a stable equilibrium were no pitch control can stop the movement and the aircraft is locked in stall. After this point pitch control is completely in effective and unless the aircraft has another way to produce a large nose down movement then the aircraft will continue until impact with the ground.

Recovery from this is only possible before the aircraft becomes locked in stall, a strong nose down force needs to be produced in order to prevent the ineffectiveness of the elevator. To pull a rapid pull up is needed to recover but if performed to quickly it will re enter the stall state.

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