Stalling

Question 1

The position of the separation point depends on the intensity of the …

Answer 1

adverse pressure gradient

As alpha increases and pressure at the point of max camber falls, the adverse pressure gradient (the return to higher pressures at the trailing edge) becomes more intense, and the separation point moves forward

This separation means that CL will not continue to rise as alpha increases but at some point, will begin to fall – this is the critical alpha

Question 2

Elliptical wings get fairly uniform loss of lift across the span, whereas straight rectangular wings stall at the … first and tapered wings at the … first

Answer 2

root

tip

Question 3

what is low speed aileron reversal?

Answer 3

At high alphas, a downward movement of an aileron to raise the wing will increase camber and can also increase alpha well beyond critical alpha and therefore cause a full stall over the wing

Lift will reduce, causing the wing to drop, giving an opposite roll response to the one intended

Question 4

stalling speed varies with … of the load factor and the increased stall speed in manoeuvres is called …

Answer 4

varies with the square root of the load factor

accelerated stall

Question 5

stall warning should begin no later than the greater of …kt or …% of IAS before the level flight stalling speed

Answer 5

5

5

Must be audible or a clear indication – simple airframe buffet approaching the stall is considered a clear indication

Question 6

how does Vs vary with changes in weight?

Answer 6

stalling speed will vary with the square root of the change in weight

Question 7

how does Vs vary with changes in LF?

Answer 7

varies with the square root of the LF

Question 8

what is the design manoeuvre speed?

Also known as …

Answer 8

The stalling speed at max load factor, also known as VA

Question 9

What is the effect of high altitude on Vs as an IAS?

Answer 9

The size of aerodynamic forces on an aircraft depends on IAS, therefore, the lift we require to support a given weight will be generated at the same IAS, despite any variation in altitude

So, we find that at lower altitudes, the IAS at which we stall remains constant with increasing altitude, although TAS will be steadily increasing

TAS will eventually rise to a point where our Mach number becomes significant, and the CLmax that we can achieve starts to decrease, while critical alpha also decreases

The result is that at higher altitudes our stalling IAS will start to increase, slowly with increasing altitude

Question 10

what spanwise flow do swept wings have on top and on bottom?

Answer 10

root to tip both top and bottom

Question 11

what spanwise flow do stright wings have on top and on bottom?

Answer 11

root to tip on the bottom and tip to root on the top

Question 12

what is the significance of the spanwise flow on wing tip stall on swept wing aircraft?

Answer 12

Swept wings have a component of flow from root to tip on both the upper and lower surfaces

This flow meets the air coming around the tip from below and the result is a pool of stationary or slow moving, low energy air over the tip which, at high alpha will readily separate into disturbed flow. The result is that the tip stalls first before the rest of the wing

Question 13

when a swept wing aircraft stalls at the tip first, what happens to CP and why is this undesireable?

Answer 13

Loss of lift at the tips will cause a large forward movement of the CP, which will cause the aircraft to pitch up, the exact opposite of what we want for stall recovery

Question 14

how do wing fences help to prevent tip stalling first on swept wings?

Answer 14

prevent some of the spanwise flow that leads to a pool of stationary or slow moving, low energy air over the tip which, at high alpha will readily separate into disturbed flow

Question 15

what are sawtooth or notch sections on the leading edge for?

Answer 15

can be used that they will only change the airflow when high alpha is reached, at which point a vortex is created behind them. The span wise flow becomes wrapped up in the vortex and is prevented from reaching the tips

Question 16

what are vortilons?

Answer 16

vortex generators mounted on or around the engine pylon. They too do not affect the airflow until high alpha, when they create a vortex on the upper wing surface near the engine, which diverts the outward spanwise flow at its source, the wing root

Question 17

what is deep stall?

Answer 17

occurs on T-tails when the aircraft stalls and is at very high alpha, the wings blank off the airflow to the tail and control in pitch is lost

Question 18

what is autorotation?

Answer 18

When a roll is started close to critical alpha, the increased effective alpha on the down going wing can exceed critical alpha and it will stall. The reduction of lift on that wing will intensify the roll, which cannot be stopped with aileron, which will actually stall the wing further as the local alpha is increased further by the down going aileron on the down going wing

With the down going wing now fully stalled, this will give a large increase of drag on that side and will start to yaw the aircraft in the direction on the roll – the aircraft is yawing and rolling uncontrollably towards the down going wing which is known as autorotation and is the beginning of a spin