Reply to Brian Abraham re swept-wing aircraft
G’day Brian. You asked why a swept-wing aircraft stalls at a higher angle of attack than one with a straight wing. I think the best answer is that an aircraft with a lower aspect ratio appears to stall with a higher angle of attack than an aircraft with a higher aspect ratio.
Every fixed-wing aircraft flies in the downwash induced by its own trailing vortices. If the downwash in the vicinity of an aircraft is 10 knots and the aircraft is flying level at 100 knots true airspeed that shows the atmosphere appears to be descending towards the aircraft at a gradient of one in one ten. Ten percent or six degrees. If the wing of this aircraft needs an angle of attack of twelve degrees the angle between the wing and the horizon will be eighteen degrees, not twelve. The effective angle of attack is twelve degrees, but the induced angle of attack is six degrees, so the geometric angle of attack is eighteen degrees.
The downwash and induced angle of attack are what tilts the lift vector backwards so that part of the lift is actually drag – that part of the drag called induced drag. The higher the induced drag the higher the induced angle of attack. Induced drag is highest at slow speeds and on aircraft with a low aspect ratio.
Perhaps the lowest aspect ratio of any manned aircraft is seen on the Anglo-French Concorde with an AR of only 1.5. At low speeds the induced angle of attack on the Concorde causes it to fly so nose-high that the pilots have an inadequate view of the airspace ahead. This is especially true when approaching to land and the pilots need to have the runway in view. The designers of the Concorde gave it a drooping nose so that the pilots had an adequate view in spite of the extreme nose-high attitude. It was the same in the Fairey Delta 2 research aircraft which had a drooping nose.
Delta-wing combat aircraft have an exaggerated nose-high attitude during takeoff and landing because of their low aspect ratio wing.
The spanwise lift distribution of a swept-wing aircraft is far from elliptical, especially at low speeds so its effective aspect ratio is significantly less than its geometric aspect ratio. (Oswald Efficiency Number is much less than one.) As a result, swept wings also appear to have an unusually high angle of attack when flying slowly. In fact, their angle of attack is not unusual but the strong downwash means the angle between the wing and the horizon is significantly greater than the angle between the wing and the approaching air.