Having read a number of posts to the effect that delta wings do not stall, I was left feeling vaguely uncomfortable. I am not a pilot, but one of my specialisations in my degree course was aircraft aerodynamics, so I decided to go back and have a look at my text books.
I have in front of me a graph plotting lift coefficient against angle of attack for two different wings: high and low aspect ratio (essentially the difference, from a lift perspective, between conventional and delta wings).
For the purposes of simplicity, I'm going to assume that a normal airliner wing is pure trapezoidal and that concorde is pure delta. The truth is somewhere in between, but it's close enough to demonstrate my point.
A trapezoidal wing starts to exhibit flow separation at high angles of attack (12-15 degrees) which leads to flow break-down and finally stall, with the lift coefficient markedly dropping off thereafter.
In contrast, the delta wings exhibits flow separation at even low angles of attack, but the vortices thus produced behind the leading edge are stable and actually contribute to the lift, with flow reattachment occuring at some point on the wing before the trailing edge. This stable leading edge vortex formation contunues up until angles of attack near 25 degrees where the lift coefficient starts to drop off again.
This behaviour is simply explained by the fact that, at high angles of attack (greater than 10 degrees), the leading edge vortex turns away as a free vortex in the main flow direction and, although it continues to provide lift, it increasingly creates reverse flow areas and stagnation zones in the wingtip area. This effect can be readily seen in the creation of tip vortices which are often visible when condorde lands at high alpha.
I suppose it depends a lot on your definition of the word "stall" - which is not actually a precise engineering term. If you define it as the point where your lift coefficient against alpha curve turns the corner, then yes, delta wings do stall eventually.
This graph doesn't even tell the whole story though, because it assumes a constant airflow across the wings (constant airspeed).
Another factor in the difference between high and low aspect ratio wings, which is very important in this instance, is that the drag coefficient increases very much quicker in a delta wing with high angles of attack, and even more so with the formation of tip vortices, which are basically just a waste of energy (pretty, but wasteful
).
A delta wing aircraft that is low on power, such as concorde was, really cannot afford to go to high angles of attack because of the dramatic effect on airspeed. Even a delta wing, with its better performace at high alpha, will produce less lift if the airspeed drops off. Without a whole load of power at your disposal, increasing angle of attack is an inefficient trade-off to gain a little short-term lift for a lot of airspeed - much more so than with a conventional wing. Sooner or later gravity will inexorably take over and the aircraft will spin out of the sky.
If that's not stalling, then I don't know what is!!
[edited to add a sentence for increased clarity]
[ 01 September 2001: Message edited by: Covenant ]
[edited to add graph]
[ 01 September 2001: Message edited by: Covenant ]