Stalling in downwind turns
Sorry I am so late coming to this party
I think this is a very simple issue that has got a bit out of hand over the years (and more recently in this thread)
So let’s use the general knowledge and common sense that all pilots have!
If a turn is maintained at a constant angle of bank and constant height and constant power in air that is not moving over the ground (an academic rarity if ever there was one!) the aircraft is going round and round in a state of constant energy. That energy being made up from the sum of its kinetic energy (due to speed over the ground) and potential energy (due to height above the ground).
NB I have deliberately ignored the effect of the loss of weight due to burning fuel in the turn because it is so small - unless you are grunting round turn after turn in full reheat at the local airshow, which is not what we are talking about here.
On the other hand, if the air is not still, but moving at a steady speed over the ground, the aircraft will suffer a fluctuating ground speed after the “steady ” turn is established at “constant” airspeed. This ground speed (and its related kinetic energy) will be a minimum when flying into wind and a maximum when downwind.
This energy change and the problems it produces (thanks to inertia) is what is at the heart of the so-called downwind turn stall issue. I say so-called because this effect (while always having some transient effect on airspeed, related to the transient change in ground speed) will only cause a STALL in quite EXTREME CIRCUMSTANCES. Normally there will be no more than a minor reduction of height or a slight loss of airspeed (both usually so small that I recommend you don’t even think of wasting your time airborne by trying to measure them in real air, which is anything but steady)
OK so what are these extreme circumstances?
An example was what applied to the Bristol Fighter at the Farnborough airshow in 1962 and it will do it every time! Perhaps some of you even saw it hit the ground then?
Imagine you are standing on the tower at Farnborough. It is a lovely day, the wind is nicely down 25 and you are watching the vintage Bristol F2B (D8096) doing its thing. Every day the same, it flies majestically downwind along runway 07 about 50 ft up, then just before the control tower it pulls up into a stall turn yawing round at say 350 ft. Nicely done it goes vertically up (how else) before the rudder goes in, round she goes, check going vertically back down, then pull out of the dive. Fly back down 25 and into the next pass. (Ah Hendon! Those were the days sort of thing).
Let’s think about that. Any stall turn takes the aircraft from its entry heading to its exit heading – a 180 degree change – in the time it takes the nose to yaw from straight up to straight down. Not long. (2-4 secs say) If the aircraft is like the Bristol Fighter above and going downwind when it pulls up it will necessarily be recovering from its dive facing into wind. Thus the airspeed that builds up in the dive, before pull out, gets a nice little addition courtesy of the surface wind when you fairly abruptly level out into that wind.
So now you can guess what happened! On the last day at Farnborough 1962 the wind turned and blew down 07. The Bristol Fighter however was flown to exactly the same visual (ground) cues and so when it tried to pull out from the nice vertical dive it not only didn’t have the benefit of a headwind, quite the reverse, it had some 12 kts up its backside. Fortunately the height margin the pilot had in hand was nearly enough to compensate and so it just clobbered the ground hard at the bottom, modified the undercarriage a fair bit, then bounced back into the air for a wobbly airborne rethink of the eventual landing strategy.
So, is there anything behind this (sudden) downwind turn and stall thing? - yes. Does it normally affect today’s aviator? - NO.