Would a 50 mph aircraft holding a ground pattern in a 20 mph cross wind risk stalling by abruptly rolling away from a crosswind?

Question

This is another play on the "down wind turn myth".

We all know a plane flying in a "uniformly moving airmass" is not affected by wind direction and speed.

But a plane following a ground track must compensate for wind direction and speed. In this scenario:

Pilot Bob could not wait to fly his ultralight and went out to practice take-offs and landings on runway 270 with a 20 knot westerly wind from 270. In the pattern, flying heading 180 on the crosswind leg, a heavy crab into the wind had to be applied to hold the ground track.

Instead of adding power, Bob increased AOA to hold altitude (BAD MOVE). Flying near stall, the pilot abruptly rolled and yawed the aircraft away from the crosswind and began to turn into the 090 "downwind" leg.

Could this maneuver significantly change the AOA of the wing, resulting in a stall?

Answer 1

Your question is flawed because it is creating a link between two different things. Let's deal with them one at a time:

In the title question you ask whether a plane holding a crosswind correction in a landing pattern risks stalling by rolling abruptly away from a crosswind. Ignoring for a minute that doing anything "abruptly" imposes some additional level of risk, (depending of course on the severity) the answer is no. Turning into, or away from the wind in a "uniformly moving airmass" does not alter the airspeed of the aircraft. You said as much yourself. When ready for downwind you simply turn to a new heading for that leg as you would normally. (in this case 090 because you will now have a tailwind.)

The very concept of "wind" only matters when you consider the surface of the earth. The airplane doesn't recognize or care what the ground is doing, only the airmass matters.

Now to the second part; would flying near stall speed and then abruptly rolling and yawing the aircraft risk a stall? Yes, absolutely yes! As well as a spin... Those are the main two ingredients of a spin, and every pilot is taught to avoid them, especially down low. Statistically this is probably one of the main GA pilot killers.

So, let's further unlink the two concepts. You seem to be espousing an unbalanced slip approach to maintaining heading and ground track in the crosswind turn, furthermore, doing it right around stall speed! (Sorry, that wasn't you, it was pilot Bob...) This is improper and unsafe technique. You should not attempt to maintain aircraft heading of 180 on the crosswind, you should fly 190, or 195, or whatever heading will allow you to maintain a consistent ground track. You should do it wings level, in balanced flight, at a safe and appropriate airspeed for your aircraft in the pattern. The same airspeed you fly when the airmass is not moving relative to the ground. This wind correction technique is called "crabbing".

Bottom line, it isn't the wind creating any additional risk, it is pilot Bob's improper technique.

Answer 2

Yes. Any abrupt maneuvering near stall speed in any conditions increases the risk of stall. Add yaw (flying an uncoordinated turn), and you further increase the risk of entering a spin.

As for the effects of the wind in this and similar situations: If the wind is steady, it has no effect. John K made an excellent remark in comments about turning close to the ground (in a glider), where strong wind gradient may cause control issues. However, standard landing pattern height is 1000 ft (300 m), and at this height wind gradient for 20 kts (~10 m/s) is only about 1% over a 33 ft (10 m) of height (the table only goes to 150 meters (500 ft) but gradient diminishes as altitude increases further). For a plane with 10 meter wing span in a 30 degree bank turn, the difference in wind velocity over wingtips would be less than 0,2 knots. That order of magnitude is unnoticeable. In the particular case in question, the whole paragraph above is irrelevant, as Bob has already stalled his plane and is heading for the ground. Poor Bob.

Below is a link to an article which very profoundly explains the physics of the downwind turn, the reasons it is not dangerous for, as well as the reasons it is dangerous for (attention, a bee is involved in the examples):

Australian flying: Dragons of the Downwind Turn

Fact 1 - A downwind turn is not dangerous - aerodynamically

Once the aircraft leaves the ground and is flying in a steady wind, its aerodynamics relate only to its movement through the air in which it is flying. Its geographical position is dependent on the wind’s speed and direction – but that is a navigational matter – not an aerodynamic one.

(should a request appear, I'll quote the article more here)

And since this subject has proven to be complicated enough that I have failed to communicate the physics of the downwind turn to several fellow aviators on two previous attempts, a little humour on the subject might lighten up the atmosphere and make the subject easier to grasp:

Flying Magazine: The Last Word on Downwind Turns, Really.

Answer 3

You seem to think that pilot Bob has to have a sideslip at some point in the pattern, because he has to make 90 degree turns in the air, and somehow also has to follow the groundtrack with 90 degree turns.

However, that's not the case (at least not if Bob is anything resembling a good pilot).

If he wants to follow a groundtrack with 90 degree angles, he has to adjust his heading, and the path he flies in the air looks something like this: (the wind is coming from the right)

     <-<-<-<-<-<-<-<-<-<-<-<-<-<-<-<-<-
    /                                 /
   /                                 /
  /                                 /
 /                                 /
->->->->->->->->->->->->->->->->->-

At no point in this pattern is he in a sideslip, or doing something weird. You could do this pattern as well when there's no wind at all, the only difference is that in that case it follows an usual groundtrack. There's just one reason to follow this weird pattern today: because with the wind acting on it, this pattern will be bent into a nice rectangular groundtrack, which is what Bob wants.

And there's no real danger here, except when Bob does stupid things (like flying close to a stall and then entering a turn). The only danger you could argue there is that two of his turns are somewhat longer than the usual 90 degrees.

Related, you seem to think that crabbing is somehow difficult or dangerous. But whether you are crabbing or not makes no difference at all as long as you're not close to the ground. Only landing with a crab is difficult, because you have turn the nose (de-crab) or your landing gear will push you the wrong way.

In a sense, a plane flying due north while over the equator is crabbing heavily, because its nose points due north while its "groundtrack" as seen from space is east-northeast. Because it's moving along the Earth's rotation of ~1,000 mph. But does this matter? No, not as long as you don't want to interact with any satellites or other things outside the Earth's atmosphere.

Answer 4

Crosswind, headwind, tailwind - in whichever direction, it only answers to Einstein in that all airspeed is relative, and relative airspeed is the only relevant contribution to lift generation. All forces and responses are relative to aircraft axes, as also described in this answer. Wind relative to the aeroplane, it can only see headwind when flying!

That is, wind at constant velocity and direction. Start making sudden changes, and now there are effects to lift and drag. A sudden horizontal wind gust exerts an additional force and causes an acceleration, direction of which depends on the direction of the gust. The magnitude of the acceleration depends on the magnitude and duration of the gust, and on the inertia of the plane.

The questions about sudden downwind turns are based on a partial realisation of the above gustiness effect. The thing is, a gust in longitudinal air flow creates an almost instantaneous effect due to the inertia of the aircraft. A manoeuvre on the other hand can never be instantaneous due to again the inertia. In aircraft of several tons and designed and optimised for their mission, manoeuvres are changes that can be considered as being quasi-static. Sudden downwind turns can simply not be sudden enough.

For a bumble bee: perhaps it can spin instantaneously so that its backside points into its flight velocity, but I don't think so.