Wind and stalls (split)

[EtherealN]

How exactly does wind speed affect stall? ;)

 

But yes, pretty much. There obviously is a lot more specific detail that can be ventured into to describe the differences, but that pretty much covers it.

 

But don't make the mistake of thinking that just because of this difference there are not amazing things that could be done even with the SFM. If I remember right, the BMS flight model is roughly equivalent in basic technology to the SFM - but people appear quite happy with it. ;)

 

Moderation note: I split this from http://forums.eagle.ru/showthread.php?t=89885

Edited June 12, 2012 by EtherealN

[WildFire]

wind speed affects every aspect of speed. Could create drag or provide lift, or add speed in the event of a tailwind.

 

I hope you were joking that was kind of a no-brainer..

Edited June 12, 2012 by WildFire

[EtherealN]

wind speed affects every aspect of speed. Could create drag or provide lift, or add speed in the event of a tailwind.

 

I hope you were joking that was kind of a no-brainer..

 

Errr... No.

 

L = Cl * A * .5 * r * V^2

 

L is your lift.

Cl is your lift Coeficcient

A is your wing area

.5 is a constant

r is air density

V is your airspeed

 

Your airspeed is not decreased by having a tail-wind. If you have a tail-wind, your ground speed will increase. If you have wind from front quarter, your ground speed will decrease. But ground speed is irrelevant for whether you'll stall or not.

 

If your stall speed in level flight is 90km/h, and you are flying at 100km/h airspeed, having a 20km tailwind will not make you stall. It will make your ground speed be 120km/h. Your airplane flies through the air, not the ground. ;)

 

Try this in A-10C or P-51D (or FC for that matter): check your stall speeds at varying wind speeds. They will not change.

Edited June 12, 2012 by EtherealN

[WildFire]

Thats a pretty generalized equation, it does not take into account windshear. Also if using a crab angle it affects the efficiency of your airfoils. Your just being nit-picky though and there are lots of scenarios that general equations wont work for, and Im guessing that ED's hardcore engineers do think about all of it. Thats why we have engineers in the first place, to find problems that no one has seen and find solutions to those problems.

 

And I never said airspeed is DECREASED by a tailwind, thats a bit ridiculous. It clearly says "add speed"...

Edited June 12, 2012 by WildFire

[EtherealN]

Thats a pretty generalized equation, it does not take into account windshear. Also if using a crab angle it affects the efficiency of your airfoils. Your just being nit-picky though and there are lots of scenarios that general equations wont work for, and Im guessing that ED's hardcore engineers do think about all of it. Thats why we have engineers in the first place, to find problems that no one has seen and find solutions to those problems.

 

Windshear is not stall. It's when the volume of air has a high velocity differential within a small area. This can conceivably cause a stall, but it would cause the stall through decreasing the V factor in the equation. It can also cause bad things to happen through simply introducing a downwards wind vector over parts of, or the whole of, the aircraft. (My home field has "awesome" windshear conditions with westerly wind, as a side note. It's pretty interesting when your plane suddenly goes 35 degrees right bank attitude when you are 20 meters above ground on final...)

 

What a stall actually is, is that it is a loss of lift at excessive alpha. This can either be caused through maneuvering too strongly (exceeding critical AoA for the aerofoil), or throught airspeed decreasing to the point where the required AoA would exceed critical.

 

I've done plenty of stall maneuvers, IRL, and it does not matter in the slightest whether I'm facing the wind, or having tailwind. The wind is irrelevant because I'm flying through the air.

 

EDIT: Regarding crab angle: this will, depending on exact aircraft type, potentially cause a stall through affecting your Cl and (for aircraft with out straight wings) A. Crab angle can of course also cause your drag to become greater than your L, in which case you are also in trouble. However, it is not "wind speed" that is going on here. It's what you did to the aircraft.

Edited June 12, 2012 by EtherealN

[GGTharos]

Nope. That's why your flight gauge reads IAS, not TAS or GS. You are moving with the airmass and thus your IAS is relative to it. Windspeed does not affect your stall characteristics. It may affect your approach to landing, or your take-off because you now need to mind GS as well, and side-loading moments from cross-winds.

 

But when you're flying around? Winds? Who cares - unless you're minding navigation.

 

wind speed affects every aspect of speed. Could create drag or provide lift, or add speed in the event of a tailwind.

 

I hope you were joking that was kind of a no-brainer..

[WildFire]

Approach and landing is a scenario that has to be programmed with the flight model right?

 

Look I was right, in certain conditions it was correct, I already knew that, I was taking all of those into account for the sake of example. Sorry my example didnt sound as intelligent as anyone elses. The point originally about all the conditions that go into an advanced flight model. The stall was just an example. There really was no need to pick it apart.

[EtherealN]

No, you were not correct. Wind speed will never cause you to stall.

 

At all points of flight your aircraft is moving through a volume of air. The velocity of the volume of air relative to the ground - that is, wind - is completely irrelevant to what your aircraft will do.

 

As GG said, wind is relevant in takeoff and landing, but not through causing you to stall. There is a fundamental difference between wind making you stall, and (for example) wind conditions making it unsafe to attempt landing due to requiring excessive ground speed on touchdown. (This is a case where you would either divert, bale out, or make an emergency landing attempt at that excessive ground speed - if you "stall" it is still because your airspeed got too low, not because of the wind.)

 

What you are saying is like saying that running out of fuel will cause you to stall. No it won't. Having run out of fuel and still attempting to maintain level flight will cause you to lose airspeed and you'll eventually stall because of the loss of airspeed, but not because the fuel ran out.

Edited June 12, 2012 by EtherealN

[GGTharos]

Approach and landing is a scenario that has to be programmed with the flight model right?

 

That depends on what you mean. If you mean 'it has to be programmed in as a special case' then no, you're wrong. The aerodynamic modeling takes care of the flying part, and collision deals witht he ground part. If you have some sort of hurricane force tailwind that causes you to touch down at some insane ground speed, you'll have to deal with the consequences of that. But it won't stall you.

If you get suddenly hit by a hurricane up the tailpipe while at approach speeds, well - I don't think you should have been flying that day, but that's not a programatically exceptional case either.

 

Look I was right, in certain conditions it was correct, I already knew that, I was taking all of those into account for the sake of example. Sorry my example didnt sound as intelligent as anyone elses. The point originally about all the conditions that go into an advanced flight model. The stall was just an example. There really was no need to pick it apart.

 

No, you're wrong, and you're wrong in such a way that shows that you don't understand what is going on. Wind speed won't cause a stall since you move with the airmass - your performance is indicated by your IAS, and your IAS is what it is regardless of what the windspeed is. It was nit-picked because you present it from a lack of understanding, at least as I see it.

[effte]

Aaaah, the much dreaded and oft discussed non-significant downwind turn is rearing its ugly head again!

 

But if I am taking off from a conveyor belt, with an aircraft full of chicken, and as I turn downwind all the chicken take flight inside the cabin... ? Will it then be Bernoulli of Newton causing the lift?

 

:sly:

[GGTharos]

What? Wait?! Chicken?

 

We need to account for this new aerodynamic factor!

[GripenNG]

Mind if I come in with a simple question, just so I understand it right also.

 

If the windspeed is about 20 knots head on: that has no affect on stall speed because it moves along the wing as if our ACFT would have moved 20 knots faster generating lift instead - and therefore has nothing to do with stall?

 

Somewhat like flying with a kite, the more wind moving across the wing surface (in the right direction) the less of a chance to stall.

[Jona33]

Mind if I come in with a simple question, just so I understand it right also.

 

If the windspeed is about 20 knots head on: that has no affect on stall speed because it moves along the wing as if our ACFT would have moved 20 knots faster generating lift instead - and therefore has nothing to do with stall?

 

Somewhat like flying with a kite, the more wind moving across the wing surface (in the right direction) the less of a chance to stall.

 

I I understand what you're saying. Yep, that's why you use IAS which is basically measuring how fast the air is moving over your wing. This means you might be able to fly with 0 ground speed and 120 knots headwind, or 120 ground speed and 0 headwind. Air is moving over the wing at the same rate.

[GGTharos]

No, that's wrong again :)

 

Stall is a function of (among other things) your INDICATED AIRSPEED. The wind does not affect this; it affects your TAS and GS.

 

Edit: Sniped :D

[effte]

@GripenNG:

I'd describe it as the aircraft (and the wing, hopefully) travelling at the same airspeed as it would had there been no wind, just 20 knots slower across the ground.

 

But you've got the gist of it - wind has no effect on stall speed, nor will manoeuvring your aircraft in wind have any effect on your flying, as far as aerodynamics go. Only changing wind speeds/directions (e g gusts) will affect you aerodynamically.

 

Gillar nicket. ;)

[EtherealN]

Well GripenNG, having wind in your face will make you have a faster takeoff, since 20 knots wind will in this case cause you to have an airspeed of 20 knots while standing stationary on the ground.

 

So assuming you need 100 knots to get enough lift to take off, at that point you only need to add 80 knots. Whereas if you had 20 knots of tailwind you'd have to add 120 knots.

 

When airborne, what gives you lift is not how the airmass is moving relative to the ground (that is, wind), it is how the aircraft is moving relative to the airmass - that is, airspeed. Incidentally also why you use indicated airspeed in the air: this automatically adjusts for a decreasing R in the lift equation as you gain altitude (or simply different pressure conditions). If your stall speed is 90 knots, you can know that that's the indicated speed you need to be careful with no matter if you're in low pressure, high pressure, low altitude, high altitude etcetera.

 

It is perfectly possible to fly calmly and nicely with the nose pointed any (horizontal) direction you want in a 10000km/h wind. I'm not sure how you'd take off or land in any kind of aircraft we know how to build, but assuming you never have to do either of those - flying is just fine, and you'll still mind the same indicated air speed as ever, even if you will in some circumstances fly backwards relative to the ground. (There are actually fixed-wing aircraft that can sort of fake a "hover" simply through their rediculously low stall speed - give them a good frisky wind, turn into it, and you can sit happily over a helipad until your fuel runs out or the wind calms down... :P )

 

EDIT: lol @ pileup... :D

[EtherealN]

@GripenNG:

I'd describe it as the aircraft (and the wing, hopefully) travelling at the same airspeed as it would had there been no wind, just 20 knots slower across the ground.

 

To add to the above: this is of course also one of the interesting aspects when turning an aircraft - the wings will fly at different speeds through the air. This makes it theoretically possible to stall one wing, but not the other. If this happens during a landing cirquit, you're in deep trouble (though not for long).

 

It is also why you will often find yourself needing some opposite aileron during a turn to maintain your bank angle.

[GripenNG]

Ah yes now I undestand (I think)! So basicly:

 

airmass speed over wing = Indicated Air Speed

higher IAS = more lift

more lift = flying

 

Airmass speed could be retained from either wind or by engine power. No matter what gives you this lift (wind or engine) you could still be airborne according to the IAS...while GS reads zero.

 

And stall has nothing to do with this! Since you can fly without moving as long as the wings are provided with a moving airmass. So saying that windspeed has something to do with stalls (which relates to another problem) is just simply wrong! Am I right now? :)

 

@effte: Bästa nicket som finns haha! :gun_smilie:

[effte]

Only that unless your aircraft is tethered to the ground like a kite, your engine will still have to provide the power to oppose the drag, even when your ground speed is zero. As previously stated, no aerodynamic effect from the wind. :)

[GGTharos]

Yeah you're getting it now. For flying, the only important thing is your IAS (in terms of speed). You stall if your IAS gets too low, not your GS.

[Bucic]

My first thought was - 'Why EtherealN would ask an insignifficant question?' :)

[WildFire]

I was talking about angle of attack, in such there are some cases where wind can be a factor in disrupting airflow over the wings, typically at very low speed and with a bad angle of attack and other associated angles (it is very circumstantial).

 

As far as standard conditions flying is concerned, yes wind doesnt matter. Again the original point was that there are a lot of things to consider in an advanced flight model. The fact that this was attacked as being absolutely wrong when there are conditions in RL that this is true, even if extremely rare due to the amount of wind needed and the particular geometry/attitude of the aircraft, really is sad. Of course in jets its not really an issue by design but light aircraft are obviously more susceptible. Bad case of thread hijacking, which is why its now its own thread, but it really wasnt needed. Debates over the most simple of things get attacked so quickly here.

 

And Tharos your right it doesnt need to be modeled, I was only throwing a bunch of stuff out that could have been used in flight model calculations. Of course it wasnt meant to be an officiall thesis or anything.

[aaron886]

No, that's wrong again :)

 

Stall is a function of (among other things) your INDICATED AIRSPEED. The wind does not affect this; it affects your TAS and GS.

 

I have to just point out that this is wrong. (Again, GGTharos, AGAIN!) Indicated airspeed does change with wind. With a perfect airspeed indicator, parked on the ground facing into a 20 knot wind, your airspeed indicator would read 20 knots indicated airspeed.

[GGTharos]

Yeah, and obviously right there you will stall, too.

 

I have to just point out that this is wrong. (Again, GGTharos, AGAIN!) Indicated airspeed does change with wind. With a perfect airspeed indicator, parked on the ground facing into a 20 knot wind, your airspeed indicator would read 20 knots indicated airspeed.

[Panzertard]

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Otherwise the thread will be closed.

 

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