Limitations 14

For a given gust speed and aircraft TAS, the increment in the load factor depends on the increase in CL per change in angle of attack due to the gust (the slope of the lift curve). If the lift curve has a steep slope, the ‘g’ increment will be greater. Factors which affect the lift curve are aspect ratio and wing sweep.

C L	HIGH ASPECT
	RATIO

LOW ASPECT RATIO

(or sweepback)

Figure 14.8

Wings having a low aspect ratio, or sweep, will have a lower lift curve slope, and so will give a smaller increase in ‘g’ when meeting a given gust at a given TAS.

High wing loading reduces the ‘g’ increment in a gust. This is because the lift increment produced is a smaller proportion of the original lift force for the more heavily loaded aircraft.

For a given TAS and gust speed, the increase of lift will be proportional to the wing area. Therefore, the increase in load factor is inversely proportional to the wing loading.

	Wing Loading =	Weight
		Wing Area

For a given aircraft the only variables for load factor increment in a gust are the aircraft TAS and the gust speed.

Effect of the Gust on Stalling

If an aerofoil encounters an upgust, it will experience an increase in angle of attack. For a given gust velocity the increment in angle increases as the aircraft TAS decreases. If the angle of attack is already large (low speed), the increment due to the gust could cause the wing to stall. There is thus a minimum speed at which it is safe to fly if a gust is likely to be met so as not to stall in the gust.

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Limitations 14

Limitations

Operational Rough-air Speed (VRA / MRA)

For flight in turbulence an airspeed must be chosen to give protection against two possibilities: stalling and overstressing the aircraft structure. Turbulence is defined by a gust of a defined value. If this defined gust is encountered, the aircraft speed must be:

•high enough to avoid stalling.

•low enough to avoid damage to the structure.

These requirements are fulfilled by calculating the stall speed in the gust and then building in sufficient strength for this speed.

The key is the chosen value of the gust, as this will dictate­the strength required and therefore the aircraft weight. The gust velocity is associated with the design speed, VB, and the vertical value of the gust is 66 ft per second. Encountering a gust before the pilot is able to slow the aircraft, plus the possibility of hitting a gust if the aircraft is ‘upset’ at high speed, must also be taken into consideration. Because these probabilities are lower however, progressively lower values of gust velocity are chosen at the higher speeds. These values are 50 ft per second at the design cruise speed VC and 25 ft per second at the design dive speed VD.

The design gust values of 66, 50 and 25 ft per second for gusts at the design speeds of VB, VC and VD have existed since the early 1940s. In the UK they were established as a result of the earliest “Flight Data Recorder” results. Modern flight recorder results and sophisticated design analyses continue to support the original boundaries of the design gust envelope.

Generally, design for strength is based on calculating the increase in load on the aircraft as a function of an instantaneous increase in angle of attack on the wing page 469.

On large aircraft, additional allowances have to be made for several reasons:

•The greater dynamic response due to increased structural flexibility.

•The possible implications of the smaller margin between actual cruise speed and design cruise speed.

•The significance, in the more advanced designs, of the effects of build-up of gusts and unsteady flow generally.

•The frequency of storm penetrations.

•The implications of the limited slow-down capabilities.

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Limitations 14

All design speeds, and design gust values, are EAS. But, remember: the increase in angle of attack due to a gust is a function of the TAS of the aircraft and the TAS of the gust.

The choice of rough-air speed to be used operationally must be consistent with the strength of the aircraft. At the same time the aircraft must comply with both minimum stability and control criteria. There is also the important consideration of what maximum speed reduction can be achieved in a slow-down technique. A typical chart of the speeds to which the roughair speed is related, is shown below in Figure 14.9. The illustration is drawn for a single (mid) weight. Line AB is the 1g stall speed.

50	B	I	L	E
40	R			P		S
					N
						H
30
							K
20								14
10								Limitations
0	A	C		O	M	G	J
100	200				300		400
	SPEED - KNOTS EAS

Figure 14.9

Line CE is the stall speed in a 66 ft per sec gust.

(This assumes the 66 ft per sec. gust up to maximum altitude. Note that point E would represent an extremely high true airspeed gust value).

Line GHI is the VMO/MMO line.

Line JKL is the VDF/MDF line.

Line MN is an example of a maximum strength speed line for a 66 ft per sec gust.

Line RS is the 1.3g altitude.

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14 Limitations

Limitations 14

At all speeds above the line CE the aeroplane will sustain a 66 fps gust without stalling and at all speeds below the line MN the aeroplane is strong enough to withstand a 66 fps gust. The rough-air speed therefore should lie somewhere between these two speeds, and the line OP gives equal protection between accidentally stalling and overstressing the aircraft.

The line MN is a curious shape because different parts of the structure become critical at different altitudes. This line is actually the lowest speed boundary of a collection of curves at the higher speed end of the chart.

Because of the obvious attraction of a single speed at all altitudes up to that at which the rough-air speed becomes a rough-air Mach number, the line could be adjusted slightly so as to avoid any variations with altitude. As turbulence is generally completely random, this halfway speed would give equal protection­ against the 50-50 probability of being forced too fast or too slow.

It has been stated that the diagram is drawn for a mid weight. The effect of weight change in terms of the lower and upper limits to rough-air speed is, of course, significant, but selfcancelling. At low weights the stall line for a 66 ft per sec gust falls to lower speeds and the maximum strength speed line increases to higher speeds. There is therefore no point in attempting a sophisticated variation of VRA with weight.

The maximum altitude limit does, however, vary significantly with weight, and also varies for the level of manoeuvre capability chosen. A 0.3g increment to buffet is not too much protection in severe turbulence. A lower altitude will therefore be required for a higher level of protection, and, for a given level of protection, a lower altitude will be required for higher weights.

Landing Gear Speed Limitations

The landing gear will normally be retracted as soon as possible after take-off to reduce drag and increase the climb gradient. There is no normal requirement for the gear to be operated at high IAS so the retract and extend mechanism together with the attachment points to the structure are sized for the required task. To design the gear for operation at high IAS would unnecessarily increase structural weight.

VLO: the landing gear operating speed is the speed at which it is safe both to extend and to retract the landing gear. If the extension speed is not the same as the retraction speed, the two speeds must be designated as VLO (EXT) and VLO (RET).

When the gear is retracted or extended the doors must open first. The doors merely streamline the undercarriage bay and are not designed to take the aerodynamic loads which would be placed on them at high IAS. Consequently VLO is usually lower than VLE.

VLE: the landing gear extended speed. There may be occasions when it is necessary to ferry the aircraft with the gear down, and to do this a higher permissible speed would be convenient. VLE is the speed at which it is safe to fly the aircraft with the landing gear secured in the fully extended position. Because the undercarriage doors are closed, VLE is normally higher than VLO.

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