HIGH LIFT DEVICES Flashcards
(25 cards)
For both take-off and landing, one of the requirements is for a safe margin above the stalling speed ___ VS1 for take-off and _____ VSO for landing). The stalling speed is determined by the CLMAX of the wing, and so to obtain the lowest possible distances, the CLMAX , must be as high as possible.
For both take-off and landing, one of the requirements is for a safe margin above the stalling speed (1.2 VS1 for take-off and 1.3 VSO for landing). The stalling speed is determined by the CLMAX of the wing, and so to obtain the lowest possible distances, the CLMAX , must be as high as possible.
What is a Flap ?
A flap is a hinged portion of the trailing or leading edge which can be deflected downwards and
so produce an increase of camber. For low speed aerofoils the flaps will be on the trailing edge
only, but on high speed aerofoils where the leading edge may be symmetrical or have a negative
camber, there will usually be flaps on both the leading edge and the trailing edge.
What is the difference in the lift produced by a split flap and Plain Flap ?
–The split flap gives about the same increase in lift as the plain flap at low angles of attack, but gives slightly more at higher angles as the upper surface camber is not increased and so separation is delayed. The drag however is higher than for the plain flap due to the increased depth of the wake.
– Also split flap has smaller changes in pitching moments during deflection
What is a Slot ?
The purpose of the slot is to direct higher pressure air from the lower surface over the flap and
re-energise the boundary layer. This delays the separation of the airflow on the upper surface of
the flap. The slotted flap gives a bigger increase in CLMAX than the plain or split flap and much
less drag, but has a more complex construction.
The Fowler flap, moves _____ and then _______, initially giving an increase in _________ and then an _______________. The Fowler flap may be __________-.
The Fowler flap, moves rearwards and then down, initially giving an increase in wing area and then an increase in camber. The Fowler flap may be slotted.
Because of the combined effects of increased area and camber, the Fowler flap gives the greatest
increase in lift of the flaps considered, and also gives the least drag because of the slot and the
reduction of thickness : chord ratio. However the change of pitching moment is greater because
of the rearward extension of the chord.
What is the Effect of lowering Flaps on the CP movement and TP ?
–When flap is lowered, the Centre of Pressure will move rearwards giving a nose down pitching moment. In the case of a Fowler flap, rearward movement of the flap will also cause the CP to move aft, resulting in an even greater increase in the nose-down pitching moment.
–Tailplane effective angle of attack is determined by the downwash from the wing. If the flaps are lowered the downwash will increase and the tailplane angle of attack will decrease, causing a nose-up pitching moment.
–The resultant aircraft pitching moment will depend upon which of the two effects is dominant. The pitching moment will be influenced by the type of flap, the position of the wing and relative position of the tailplane, and may be nose-up, nose-down, or almost zero. For example, on flap extension, a tailplane mounted on top of the fin will be less influenced by the change of downwash, resulting in an increased aircraft nose down pitching moment.
To promote root stall on a swept wing, ________ are used on the inboard section because they are ________________________________________.
To promote root stall on a swept wing, Krueger flaps are used on the inboard section because they are less efficient than the variable camber shown opposite.
How does the Slat increase Lift ?
The effect of the slat is to prolong the lift curve by delaying boundary layer separation until a
higher angle of attack.
–When operating at high angles of attack the slat itself is generating a high lift coefficient because of its marked camber.
–The action of the slat is to flatten the marked peak of the low-pressure envelope at high angles of attack and to change it to one with a more gradual pressure gradient.
–The flattening of the lift distribution envelope means that the boundary layer does not undergo the sudden thickening that occurred through having to negotiate the very steep adverse pressure gradient that existed immediately behind the former suction peak, and so it retains much of its Kinetic Energy, thus enabling it to penetrate almost the full chord of the wing before separating.
– Alleviating effect of the slat on the low pressure peak and that, although flatter, the area of the low pressure region, which is proportional to its strength, is unchanged or even increased. The ‘suction’ peak does not move forward, so the effect of the slot on pitching moment is insignificant.
What is the sequence of deploying the LE and TE flaps and why ?
Lowering a trailing edge flap increases both the downwash and the upwash. For a high speed aerofoil, an increase of upwash at the leading edge when the angle of attack is already fairly high, could cause the wing to stall. The leading edge device must therefore be deployed before the trailing edge flap is lowered.
When the flaps are retracted the trailing edge flap must be retracted before the leading edge
device is raised.
Why are flaps selected for Take-off ?
Takeoff distance depends upon unstick speed and rate of acceleration to that speed.
a) Lowest unstick speed will be possible at the highest CLMAX and this will be achieved at a large flap angle,
b) But large flap angles also give high drag, which will reduce acceleration and increase the distance required to accelerate to unstick speed.
c) A lower flap angle will give a higher unstick speed, but better acceleration, and so give a shorter distance to unstick.
Thus there will be some optimum setting which will give the shortest possible take-off distance. If leading edge devices are fitted they will be used for take-off as they increase the CLMAX for any trailing edge flap setting.
Explain the Flap setting for Landing.
Landing distance will depend on touchdown speed and deceleration. The lowest touchdown speed will be given by the highest CL MAX, obtained at a large flap angle. Large flap angle will also give high drag, Figure 8.19, and so good deceleration. For landing, a large flap angle will be used. Leading edge devices will also be used to obtain the highest possible CL MAX.
With the flaps lowered, the stalling speed will:
a) increase.
b) decrease.
c) increase, but occur at a higher angle of attack.
d) remain the same.
B
When flaps are lowered the stalling angle of attack of the wing:
a) remains the same, but C L max increases.
b) increases and C L max increases.
c) decreases, but C L max increases.
d) decreases, but C L max remains the same.
C
With full flap, the maximum Lift/drag ratio:
a) increases and the stalling angle increases
b) decreases and the stalling speed decreases
c) remains the same and the stalling angle remains the same
d) remains the same and the stalling angle decreases
B
When a leading edge slot is opened, the stalling speed will:
a) increase
b) decrease
c) remain the same but will occur at a higher angle of attack.
d) remain the same but will occur at a lower angle of attack.
B
The purpose of a leading edge droop is:
a) to give a more cambered section for high speed flight.
b) to increase the wing area for take‑off and landing.
c) to increase wing camber, and delay separation of the airflow when trailing edge flaps are lowered.
d) to decrease the lift during the landing run.
C
Lowering flaps sometimes produces a pitch moment change due to:
a) decrease of the angle of incidence.
b) movement of the centre of pressure.
c) movement of the centre of gravity.
d) increased angle of attack of the tailplane.
B
Which type of flap would give the greatest change in pitching moment?
a) Split
b) Plain
c) Fowler
d) Plain slotted
C
A split flap is:
a) a flap divided into sections which open to form slots through the flap.
b) a flap manufactured in several sections to allow for wing flexing.
c) a flap which can move up or down from the neutral position.
d) a flap where the upper surface contour of the wing trailing edge is fixed and only the lower surface contour is altered when the flaps are lowered
D
If the flaps are lowered in flight, with the airspeed kept constant, to maintain level flight the
angle of attack:
a) must be reduced.
b) must be increased.
c) must be kept constant but power must be increased.
d) must be kept constant and power required will be constant.
A
If flaps are lowered during the take‑off run:
a) the lift would not change until the aircraft is airborne.
b) the lift would increase when the flaps are lowered.
c) the lift would decrease.
d) the acceleration would increase.
B
When flaps are lowered the spanwise flow on the upper surface of the wing:
a) does not change.
b) increase towards the tip.
c) increases towards the root.
d) increases in speed but has no change of direction.
C
If a landing is to be made without flaps the landing speed must be:
a) reduced.
b) increased.
c) the same as for a landing with flaps.
d) the same as for a landing with flaps but with a steeper approach.
B
