Stalls in a crosswind

[733driver]

Dear PPRuNers,

Apologies for the strange thread title.

I came across an interesting discussion in another forum, which, to my surprise, actually made me think.

I am probably missing the obvious here but I thought I'd ask.

Someone claimed that during stall exercices in a particular light aircraft, one wing would drop if there was a crosswind. Then another poster comes in and says: There is no such thing as a crosswind inflight, as you are moving within an airmass which limits the relevance of crosswinds to take off and landing.

Now, I think that may not be an accurate statement.

Imagine a ferry that wants to cross a river. There is a current downstream so if the ferry skipper points the nose directly at the other side of the river, he will drift downstream. So he points the nose a little upstream to correct for drift. Now, doesn't that mean that the ship experiences a force on the sidewall on the upstream side? Only if it is headed directly parallel to the current does it not experience any forces on either side, right?

So that must have an affect on how an aircraft handles in a stall, or not?

I know this is a very basic question and I may well be missing the obvious but I am interested to hear your opinions.

[Dont Hang Up]

Sorry but you are missing the obvious
'doesn't that mean that the ship experiences a force on the sidewall on the upstream side? '
Nope. Think about it.

[Ex FSO GRIFFO]

G'Day Mr '733'....

Would that have been a 'Recreational' type site by any chance??

Many Many misconceptions on that or those sites....

If you would really like to get a 'handle' on this, then may I recommend a book by A. C. Kermode - The Mechanics Of Flight', or go and spend some quality time with your favourite Flight Instructor.

When an aircraft is airborne, the actual direction of the wind relative to the ground, has no affect on the aerodynamics of the flight of the aircraft THROUGH the air - though it may appear vastly different to an observer who is FIXED on the ground.

The aircraft is simply moving in a body of air - the fact that this body of air is in itself moving over the ground - in whatever direction - has no affect on the aircraft - whose RELATIVE airflow is still coming from the front, in normal flight.

In the stalling sequence, the RELATIVE airflow is still from the front - as it always will be. Even in a 'sideslip' manoeuvre - the airflow is still from the 'new' front - opposite to that in which the aircraft is travelling.

Try and imagine it.

I have demonstrated a Cessna's ability to 'hover' and actually move backwards - to observers on the ground! The mere fact that the Westerly wind was in excess of 45 knots and I flew the aircraft directly into this wind at about 45 knots or so, was 'lost' on the observers at the time.

Looked impressive though, I am sure. Can I fly a Cessna backwards in normal flight? NAH! (Not unless its vertically DOWN....BUT....NOT FOR LONG!!)

In the 'canoe across the river' scenario, the path of the boat relative to the flow of the water is exactly the same as in the aircraft situation.
But - relative to an observer on the river bank - the ILLUSION is that the canoe MAY APPEAR to be moving 'sideways' is because the observer on the bank is STILL - however the wake of the canoe in the water is the same as in still water.....

Unless the canoe is 'tethered' in some way, there is no difference to the water flow around the canoe in forward motion - the bow wave will be the same, as will be the wake.
Think about it.

Best regards,:ok::ok:

[733driver]

Thanks to both of you. I was of course aware that an aircrafts movement relative to the ground as wittnessed by an observer on the ground has no relevance to the aerodynamics involved. I also find it easy to imagine an aircraft standing still or travelling backwards if the groundspeed as a result of TAS-wind speed is 0 or negative. I somehow found it more difficult to resist the tempation to include "vectors" into the equation that somehow split the total velocity into two smaller vectors, one oposing the "current flow" and one pointing in the direction of travel. But of course, that too is a ground based observation.

I guess if the ship, or aircraft had an additional propeller acting on a different axis (like sideways) that would change the aerodynamics and result in a sideload on the aircraft much like tying the boat, a scenario where again the ground becomes relevant.

I guess relative wind is the term that explains it best and we have all had it explained to us at some stage. I guess sometimes trying to picture something can cause more confusion if one is not using the correct model.

[Lemurian]

Someone claimed that during stall exercices in a particular light aircraft, one wing would drop if there was a crosswind. Then another poster comes in and says: There is no such thing as a crosswind inflight, as you are moving within an airmass which limits the relevance of crosswinds to take off and landing.
The only reason you can have an assymetric airflow over the wings is called *SIDE-SLIP*, not crosswind.

[airfoilmod]

Except... an A/C crabbing, wings level (skidding). Here is a flight path not 90 degrees to Long/axis. Induced Yaw. Asymmetric airflow? yes. And not a Slip (which requires Roll input other than 0 degrees). The skid causes what that wag in the other quote forgot, there can be a crosswind in flight. If there can be a skid (crosswind), a wing can drop at Stall, and depending how robust the design of the A/C, a resultant Spin.

[john_tullamarine]

And not a Slip (which requires Roll other than 0 degrees)

I think not .. level flight ... rudder ... cross aileron to maintain wings level ref a fixed observer .. you can have a significant slip but no roll ref the fixed observer

For any who think that such conditions are suitable for stalling .. please do run a search on "John Farley" and read some of John's words relating to stall/spin ....

[barit1]

Speaking of which -

In my college days I got a bit of dual in a WWII PT-22 trainer that had a bit of sweepback in the wing. We did slow flight, stalls, the usual training exercises.

Then my instructor (Prof. Irwin Treager, author of the powerplants text) said "try a stall approach with a little cross-control". I did, approaching power-off stall very carefully, when suddenly we did a snap roll! :eek: So fast, we had rolled 360 before I had time to think, let alone recover.

The lesson was obvious; keep up your airspeed in a slip!

[airfoilmod]

I reread my description, what do you see of aileron? I describe a skid. You describe Slip, and relative to a fixed (neutered) observer. Yet your implication is some thing is all wet. Will you explain? Also, why is it you think I suggested cross controls, relevance?

Airfoil

barit1: in your "manouver" describe your loss of altitude?

[LH2]

PFJI, but regarding the original question: if you go through windshear (vertical or horizontal), doesn't the inertia of your aircraft mean that initially you may indeed experience a cross wind, just as you experience changes in airspeed, etc.?

I really don't have a clue when it comes to aerodynamics, so I apologise if the above is too daft a question :O

[airfoilmod]

Your intuition is absolutely correct. In a shear it is possible to have an increase in Airspeed when encountering outflow on the nose, then a neutral zone, followed by a tailwind that can in fact drop your Airspeed low enough to cause a Stall. Same is true for the opponents of crosswind. Proximity to the ground does not create or eliminate the known behaviour of aerodynamics, subject to some well understood concepts like Ground effect. Wind Shear was unknown when I started flying, as were microbursts.

Airfoil

[john_tullamarine]

I describe a skid

Not trying to be confrontational at all ...

Excluding short lived dynamic events, generally associated with perturbations in the airmass or aircraft oscillations not under the control of the pilot, an aircraft will see the air coming head on (considering lateral angles) .. unless the pilot does something to cause that direction to alter.

About all the pilot can do (let's exclude post departure flight) is to yaw the aircraft by using varying combinations of rudder/aileron .. the end result is that the airflow vector moves to one side or the other. Aerodynamically, this is slip, regardless of how it's done, from which side of the nose the airflow is coming, and regardless of bank as seen by a fixed observer.

Where a lot of confusion comes in is in respect of pilot speak .. we talk of skid (wind coming from the outside of a turn), slip (wind coming from the inside of a turn), side-slipping with crossed controls, forward slipping (I think that is the US term for the final approach type of side-slip) etc... etc ..

End of the day it's all much the same .. if the wind is off the nose as seen by the pilot, the aircraft is slipping .... stall in that circumstance and the probability of excitement increases.

I recall (very clearly) my first exposure to this .. as an ab initio pilot (Victa Airtourer) we were having a look at spirals and incipient spins as I recall .. end result was that Cec got me to wind up into a reasonably tight turn for an accelerated stall .. and then put in a bootful of rudder as we got into buffet ... an aerodynamics lesson which remains clearly imprinted in my now not so youthful brain ....

[airfoilmod]

airflow is from the outside, as you say. A crab is a cruise skid and has penalties in fuel and time, as you can imagine. In such a condition, there is no need (nor desire) for aileron. A slip is a descent device (forward) and very safe, though controls are crossed. I have slipped a small A/C with foot on the stop and stick full side. nose down, full flaps VSI 1200. A full slip at high speed and back stick is a lomcevak.

A slip is also useful in landing crosswind, though descent is to be monitored closely and Vs as well. One wheel landings are a thing of beauty, as is a well executed and abandoned crab at T/D.

rgds Airfoil

[barit1]

barit1: in your "manouver" describe your loss of altitude?

Since we were at 3000' or so, the altitude loss was minor - no more than a normal power-off stall. It was over very quickly as soon as I neutralized controls.

However, if we were at 100' on short final, it wouldn't have been "minor"! The PT-22 had a reputation... :rolleyes:

[Lemurian]

Have some funny notions in that place of yours.
There is no airflow assymetry in a crab, you're just setting the drift for the track you fly. Your airflows are symetrical on your wings. Otherwise, control would be needed.
Ball and needle would be centered in a crab. but for a ground observer, you'd be flying sideways.
Like John, for me, any form of assymetry is a slip although if you asked me, I would qualify a skid or a slip by the ball position.

[airfoilmod]

Ball out to the skid, in towards the slip. I disagree about airflow. Any airflow that doesn't split the nose is interrrupted by the fuselage and can even cause a slight deflection in the ball, (though not continuous). If this is parsed too much, forgive me. The difficulty is in John's statement that slight perturbations present an upset to the rule, air is fluid, and aerodynamics begs a certain understood starting point, i.e. "balanced", unaccelerated, etc. Crab infers biased flight in a range from 0-90 degrees. It is this bias and necessary drift "correction" that causes the shaded symmetry over the airfoil.

rgds Airfoil

[gr8shandini]

I'm not sure what "shaded symmetry" is, but I can assure you that there is no sideslip in a crab. A crab is basically similar to the canoe situation stated earlier. If you wanted to track straight across a river, you'd have to point somewhat upstream to counteract the current, but you don't have to keep exerting force to keep the bow pointed that way.

I'm not sure why the subject got so confused over the years with forward slips, side slips, skids, etc., etc. It's all the same thing. If you simply think of the relative wind as a vector, it's direction relative to the aircraft axes can be described with two angles. Alpha (or angle of attack) and beta (angle of sideslip). Roll doesn't factor into it, so all the minor distinctions between all those terms above are essentially meaningless when it comes to how the aircraft is going to respond aerodynamically.

[Lemurian]

crab is just a drift (angle) close to the ground where the pilot visualises it.
Enough is enough. Same thing to me, the observer.
Biased, indeed !

[cwatters]

The original question is really about stalling with yaw on.

I once witnessed a glider incident that demonstrated what can happen rather neatly.. The glider was returning to land but had got slightly low. There was a modest crosswind and he was crabing to track the runway centerline. He also appeared to be flying slowly trying to "stretch the glide". He made it over the grass strip ok but when he kicked the rudder to remove the crab the downwind wing stalled (or lost lift anyway) and dumped him in from about 10 foot up. Fortunately he was ok.

[Lemurian]

I once witnessed a glider incident that demonstrated what can happen rather neatly..

Yes. Good example, and lucky escape.
Ask now a glider pilot where his/her thread of wool-slip indicator indicates a symetrical flight or not.
Maybe i'm biased, though.
Cheers

[john_tullamarine]

slight perturbations

.. only qualified my comment to keep in mind that we need to distinguish between steady and unsteady flow .. dynamic response of an aircraft can confuse the simpler explanations ...

thread of wool-slip indicator

although outside my normal ambit .. I believe common in the fast jet fraternity as well ?