stall Flashcards
what is a stall?
the breakdown of the smooth airflow over the wing which leads to not enough lift being produced to keep the aircraft airborne.
what is the relationship between the disruption of streamline flow over the upper surface of the aerofoil and stall angle?
to produce lift, air must be able to conform to the shape of the wing and produce a fairly laminar flow. as AoA increases the air will conform less to the shape.
what is the critical angle of attack?
the point where when the angle of attack is increased any further, the wing will stall. this point is usually at 16 degrees AoA.
what is the relationship between the stalling AoA reduction of lift and increase in drag?
as the airflow breaks away from the upper surface due to it passing the stall AoA, the result is a large reduction in lift produced and a significant increase in drag.
what are the main symptoms of a stall? (6)
noise - generally be lower due to the throttle being closed
airspeed - low and decreasing as the aircraft is decelerating
nose attitude - progressively getting higher
control effectiveness - not as effective due to lack of airflow and slipstream
stall warning - will activate
buffet - buffet/vibration felt in the cockpit due to turbulent flow striking the empennage
how does aircraft weight affect stall speed?
with an increase in weight, there’ll be a need for an increase in lift (as lift opposes weight and the critical AoA remains the same) for the equation (L = AoA x A/s) to remain true, then airspeed would also have to increase.
increase in weight = increase in stall speed
how does Load factor affect stall speed?
the increase in apparent weight is the g force used to describe load factor and has the same effect as an increase in actual weight.
therefore increase in LF = increased stall speed.
how does altitude affect stall speed?
as altitude increases air density reduces. so compared to a lower altitude, for the same indicated airspeed at a higher altitude the true airspeed will be greater.
therefore as altitude is increased true airspeed increases, resulting in an increased stall TAS.
how does flap extension affect stall speed?
using the lift formula L = Cl1/2pV^2S
- if weight doesn’t change, lift has to remain constant
- for a given config, wing platform area remains constant
- extension of flap increases camber (Cl) and therefore lift
so therefore IAS (1/2pv^2) must decrease for the equation to hold true meaning extension of flaps reduces stall speed.
how does contamination or damage to the wings affect stall speed?
both things cause airflow flowing over the wings to become turbulent, and to make lift, laminar airflow is needed.
therefore the undamaged sections of the wing need to develop more lift and they can only do that by increasing airspeed.
ice build up also adds weight.
therefore an aerofoil that is contaminated or damaged lead to higher stall speeds.
why is the use of aileron bad near stall?
imagine a scenario, where aircraft is rolled to the right as right-wing, has dropped due to loss of lift
- by using ailerons to try roll aircraft to the left, the aileron will be deflected upwards and the right downwards.
deflecting aileron downwards increases the camber of the wing (right-wing in this case).
increasing camber results in an increase in AoA for that wing and since its already exceeded the critical angle, using aileron will further stall the down going win and further un-stall the upgoing wing.
what is autorotation?
the tendency of an aircraft to roll left or right suddenly due to the aircraft operating near the critical angle of attack.
what are the conditions leading to autorotation?
operating near critical AoA, if one wing stalls before the other, this will cause the downgoing wing to have less lift being produced in comparison to the upgoing wing causing a roll towards the downgoing wing.
what is a spin? and what is typically found while in one?
an undesired aircraft state in which the pre-requisite is for the aircraft to be stalled.
you’ll find that in a spin the aircraft is rolling, yawing and pitching about a central axis simultaneously and that it has a low and fluctuating airspeed and high rate of descent.
what occurs during spin?
when aircraft first enters spin it’ll be in an incipient stage for up to the first two turns which lasts until the spin has fully developed, whereupon it’ll enter what is known as a fully developed spin.
at this point the aircraft is yawing, pitching and rolling around a vertical axis.
