High Speed Flight

Control Buzz

If a shock wave is situated near to a control hinge, a control movement may cause the shock wave to move over the hinge, resulting in rapid changes of hinge moment which can set up an oscillation of the control surface called control buzz.

Buffet

In the same way that separated airflow prior to a low speed stall can cause airframe buffet, shock induced separation (shock stall) at high speed can also cause buffeting.

Aerodynamic buffet is a valuable stall warning, but it can damage the aircraft structure. Because of the higher dynamic pressure when an aircraft is operating in the transonic speed region, any shock induced buffet will have a greater potential for severe airframe damage. High speed buffet must be completely avoided.

The aircraft must therefore be operated in such a manner that a (safety) margin exists before aerodynamic buffet will occur.

If the variables which affect both high speed and low speed stall are considered it will be possible to identify the conditions under which buffeting will occur and a chart can be drawn to show all the factors involved. This is called a ‘Buffet Onset’ chart (illustrated in Figure 13.26) which is used by flight crews to ensure their aircraft is operated at all times with a specified minimum buffet margin.

In Chapter 7 it was shown that stall speed is affected by several factors. In this study of low speed stall combined with high speed buffet, the factors to be considered are:

•Load factor (bank angle).

•Mach number.

•Angle of attack.

•Pressure altitude.

•Weight.

•CG position.

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High Speed Flight

Factors Which Affect the Buffet Boundaries

Stall Speed

As altitude is increased at a constant EAS, TAS will increase and outside air temperature will decrease, causing the local speed of sound to decrease. Mach number is proportional to TAS and inversely proportional to the local speed of sound (a):

TAS M = a

Therefore, if altitude is increased at a constant EAS, Mach number will increase. At low speed CLMAX is fairly constant, but above M 0.4 CLMAX decreases as shown in Figure 13.21. Refer also to Figure 13.12 for the reason why CLMAX starts to decreases at speeds above M 0.4.

Figure 13.21

From the 1g stall speed formula:

VS1g = √ ρ L

½ CLMAX S

It can be seen that as CLMAX decreases with increasing altitude, the 1g stall speed will increase.

EAS

Figure 13.22

Figure 13.22 shows the variation with altitude of stalling speed at constant load factor (n). Such a curve is called the stall boundary for the given load factor, in which altitude is plotted against equivalent airspeed. At this load factor (1g), the aircraft cannot fly at speeds to the left of this boundary. It is clear that over the lower range of altitude, stall speed does not vary with altitude. This is because at these low altitudes, VS is too low for compressibility effects to be present. Eventually, VS has increased with altitude to such an extent that these effects are important, and the rise in stalling speed with altitude is apparent.

As altitude increases, stall speed is initially constant then increases.

An altitude (Alt1 in Figure 13.22) is eventually reached when there is only one speed at which the aircraft can fly, since increasing or decreasing speed or banking the aircraft will result in a stall. In the case of a 1g manoeuvre, this altitude is called the ‘Aerodynamic Ceiling’. If the aircraft were allowed to ‘drift up’ to this altitude, the aircraft will stall. Not a pleasant prospect for a modern high speed jet transport aircraft. This state of difficulty is also called ‘coffin corner’. Refer also to Figure 13.25.

Note: The recovery in CLMAX at supersonic speeds is such that it may still be possible to operate above this ceiling if enough thrust is available to accelerate the aircraft to supersonic speeds at this altitude.

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CONSTANT

MACH NUMBER

EAS

Figure 13.23

Load Factor

Because load factor increases the stall speed, curves like the one sketched in Figure 13.22 can be drawn for all values of load factor up to the maximum permissible ‘g’, and together they constitute the set of stalling boundaries for the given aircraft. Such a set of curves is shown in Figure 13.23. Superimposed on these curves are dashed lines representing lines of constant Mach number, showing how high Mach numbers can be achieved, even at relatively low EAS, at high altitudes.

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Stall boundaries set a lower limit to the operating speed, according to the load factor. In the case of a high-speed aircraft, there is also an upper limit which is due to the approach of shock stall and the associated buffet which occurs if the aircraft enters the transonic speed range. The limits associated with these effects give the buffet boundaries.

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CONSTANT MACH

BUFFET

BOUNDARY

STALL

BOUNDARY

EAS

Figure 13.24

Mach Number

For a given aircraft there is a Mach number which, even at low angle of attack, cannot be exceeded because of the onset of shock stall. Figure 13.23 shows the EAS corresponding to this Mach number falling as altitude increases, so the range of operating speeds is reduced at both ends.

Angle of Attack

However, there is a further effect which makes the buffet boundary a more severe limit than that suggested by a curve of constant Mach number. As the EAS associated with a given Mach number falls with increased altitude, so the required CL, and hence angle of attack, increases. This results in a reduction in the Mach number at which buffeting occurs, which results in a further reduction in the permissible airspeed. This effect is made worse as the high angle of attack stall is approached, and by the time the buffet boundary intersects the stall boundary the limiting Mach number may be well below its value at a lower angle of attack, as Figure 13.24 illustrates.

Also, an increase in load factor (bank angle) requires an increase in lift at a given EAS, hence an increase in angle of attack and a further reduction in limiting Mach number.

Thus the greater the load factor (bank angle or gust), the more severe the limitation due to buffeting.

There is a set of buffet boundaries for various load factors (bank angles), just as there is a set of stall boundaries.

The restrictions on speed and ‘g’ can be summarized in the form of a single diagram in which load factor is plotted against EAS, shown in Figure 13.25.

Figure 13.25

Pressure Altitude

At sea level there is a stall speed below which the aircraft cannot fly. As load factor increases, so does the stall speed (proportional to the square root of the load factor). The curve of ‘g’ against EAS modifies the low speed stall boundary. It will continue to rise until the ‘limit load factor’ is reached (Chapt. 14). The ‘limit load factor’ must never be exceeded. At the high speed end, when g = 1, there is a limiting speed which must not be exceeded because of shock induced buffet. As the load factor increases, so does the CL at given speed, and the limiting Mach number falls, slowly at first and then more rapidly. This defines a buffet boundary, which eventually intersects the boundary of maximum permissible ‘g’ to constitute an overall envelope like the outer curve depicted in Figure 13.25. Thus the aircraft may operate at any combination of speed and load factor within this envelope, but not outside it.

At altitude the situation is similar. However, at altitude the equivalent stalling speed increases

with ‘g’ rather more rapidly than at sea level, because of the Mach number effect on CLMAX. Also, the buffet boundary becomes much more severe.

Above a certain altitude the buffet boundary may intersect the stall boundary at a value of ‘g’ lower than the structural limit, as shown in Figure 13.25. This ‘point’ is another representation of “coffin corner”.

Weight

The weight of the aircraft also affects the envelope. An increase in weight results in an increase in stall speed, and the stall boundary is moved to the right. It also results in an increase in angle of attack at any given speed, so that the Mach number at which buffeting occurs is reduced, and the buffet boundary is moved to the left. Finally, increase in weight implies a reduction in the maximum permissible ‘g’. Thus all the boundaries are made more restrictive by an increase in weight.

CG Position

Forward movement of the CG increases stall speed so the buffet boundaries will be affected in a similar way to that due to weight increase.

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