How would the selection of wing anti ice affect the stall speed?

[Luke SkyToddler]

Something that came up in a mate of mine's recent airline interview ... none of us can find any references to it anywhere

(This is assumed to be in relation to a large jet transport).

Guessing that the hot air coming out the tiny leading edge holes, in some way re-energizes the boundary layer, alters the laminar flow, or does it disturb the stagnation point position or what?

[MrBernoulli]

"Guessing that the hot air coming out the tiny leading edge holes ....."

I am not familiar with a system that does that but most of the aircraft types I have flown (including large 4-engine jet) have been relatively 'elderly'. I thought that most wing leading edge hot air systems vented somewhere through/near the trailing edge or wing tip. Could that be where the air modification, if any, might occur? Interesting question but it seems it might be type-specific.

Or is it intended to be e red herring to allow someone to dig themselves a hole at interview? Obviously if you were REMOVING ice with hot air selection then you would be lowering your stall speed. Yes, I know, we usually apply anti-ice BEFORE entering icing conditions. Just thinking aloud

[Flight Detent]

Hi Mr/Luke,

All the airplanes I've operated have the leading edge hot air exiting the leading edge on the underside of the leading edge - and its' still leading edge technology!!

The 737NG has about 16 holes in the underside of each LE, except, of course, for the outboard ones.

And further, for what it's worth, I'm not aware of any aerodynamic advantage, other than ice removal, in having the LE heat on.

If there is, please somebody mention it here!

Cheers,
FD

[BOAC]

Could be the automatic adjustment to stick-shake speeds/speed tape manoeuvre speeds in the 737NG after wing anti-ice has been used?

[Old Smokey]

This is a bit from the far side, but..........

It will have negligible effect upon the power off stall speed, but an effect upon the power on stall speed due to reduced thrust with Anti-Ice ON. This will have implications when full power stall is considered, for example, rotation to PLI in a severe wind-shear encounter.

Regards,

Old Smokey

[FlightDetent]

Supposing aircraft with underslung engines the thrust is offset to CoG and
and somewhat compensates the aerodynamic moment (nose down pitch) of the wing. Thus less downward force needed on the horizontal stab./rudder. And less lift required on the wing to compensate.

Also upward directed thrust line reduces the amount of lift required on the wing.

On tail moutned engines I do not know but just by the looks of it (MD80) the thrust line does not go through CoG either. So the effect will be there as well even if it were in opposite direction.

Now bleed demand for WAI is rather significant and reduces the thrust provided by the engines unless the total power output is increased accordingly. Definitely valid under max / idle thrust conditions and with hard-linked moving throttles.

So, does on does not WAI affect the thrust output in your case ?

Does thrust provided have an effect on the wing lift requirement?

Does the wing lift requirement, as opposed to total lift, influence the stalling speed ?


I am too lazy to think any harder now but if the answer is triple yes, all you need to do is to elaborate on the plus/minus signs of the effects and there goes your answer.

Cheers
FD. (the un-real )


EDIT: why does it always take me 30 minutes to type a posting

EDIT2: for nick clarity

[CaptainSandL]

Luke,

I would agree with your guess about re-energising the boundary layer. Several STOL aircraft use exhaust gas blown over the wings to give extra lift and hence reduce stalling speed.

If the interview was 737 type specific then I am not so sure. The wing anti-ice air is exhausted to the underside of the leading edge



Since the exhausted air speed would probably be slow (unlike engine exhaust gas), this may just give some increase in air pressure beneath the wing.

Incidentally, the 737 NG automatic stall speed adjustments after the use of wing anti-ice, is to allow for the increase of weight & reduction in aerodynamic performance due to airframe ice accretion.

S & L

[lomapaseo]

How would the selection of wing anti ice affect the stall speed?

Something that came up in a mate of mine's recent airline interview ... none of us can find any references to it anywhere

Based on the number of responses above, this question should not be answered definitively in an interview.

Clearly there is no industry wide knowlege of the effect.

Perhaps he heard the question wrong.

[Old Aero Guy]

BOAC comment is correct. Use of wing anti-ice does not change stall speed but many aircraft re-set stall warning and manuever speeds as a pre-caution.

The assumption is that if wing anti-ice has been activated, there may be ice on the unprotected portions of the wing. If there is ice on the unprotected wing, stall speed could be increased leading to the need to increase stall warning and manuever speeds and a subsequent amber band change.

[Flight Detent]

OAG/BOAC - Yes we knew that, but that was not the question!

Also, Wing Anti-Ice is activated for other reasons as well, not directly associated with LE ice formation.

Cheers, the real FD

[BOAC]

We are all only trying to help, FD(real) , since no-one can understand what on earth the 'question' could have been!

NB To Luke - next time you post an enigma, please give a little more information; a/c types operated by interviewing organisation would have been useful, plus job being applied for? It is apparent that no-one here can think how it could cause more than a fraction of a knot change, so we have to assume that the question as you posed it has been incorrectly relayed. How about checking back to 'your friend' and maybe someone can answer you?

[Luke SkyToddler]

Thanks for all the replies guys ... at least if it comes up in one of my interviews I'll know to say "I don't know"

The airline in question operates B747-400 and A330-340. My mate says he's positive that was the question that was asked, but he's Scandinavian with English as a second language, so I guess it's possible that the question was misunderstood.

[overeasy]

It will make no difference in itself. If one considers the secondary effects of removal / prevention of ice accumulation; this will have a big impact on stall speeds.

[FlightDetent]

All this reminds me of a story/myth you'll probably have read zillion times before hence the small font:

--------------


Great Moments in Physics

The following concerns a question in a physics degree exam at the University of Copenhagen:

Question: "Describe how to determine the height of a skyscraper using a barometer."

One student replied:

"You tie a long piece of string to the neck of the barometer, then lower the barometer from the roof of the skyscraper to the ground. The length of the string plus the length of the barometer will equal the height of the building."

This highly original answer so incensed the examiner that the student was failed. The student appealed on the grounds that his answer was indisputably correct, and the university appointed an independent arbiter to decide the case. The arbiter judged that the answer was indeed correct, but did not display any noticeable knowledge of physics. To resolve the problem it was decided to call the student in and allow him six minutes in which to provide a verbal answer which showed at least a minimal familiarity with the basic principles of physics.

For five minutes the student sat in silence, forehead creased in thought. The arbiter reminded him that time was running out, to which the student replied that he had several extremely relevant answers, but couldn't make up his mind which to use.

On being advised to hurry up the student replied as follows:

"Firstly, you could take the barometer up to the roof of the skyscraper, drop it over the edge, and measure the time it takes to reach the ground. The height of the building can then be worked out from the formula H = 0.5g x t squared.
But bad luck on the barometer."

"Or if the sun is shining you could measure the height of the barometer, then set it on end and measure the length of its shadow. Then you measure the length of the skyscraper's shadow, and thereafter it is a simple matter of proportional arithmetic to work out the height of the skyscraper."

"But if you wanted to be highly scientific about it, you could tie a short piece of string to the barometer and swing it like a pendulum, first at ground level and then on the roof of the skyscraper. The height is worked out by the difference in the gravitational restoring force T = 2 pi sqroot (l / g)."

"Or if the skyscraper has an outside emergency staircase, it would be easier to walk up it and mark off the height of the skyscraper in barometer lengths, then add them up."

"If you merely wanted to be boring and orthodox about it, of course, you could use the barometer to measure the air pressure on the roof of the skyscraper and on the ground, and convert the difference in millibars into feet to give the height of the building."

"But since we are constantly being exhorted to exercise independence of mind and apply scientific methods, undoubtedly the best way would be to knock on the janitor's door and say to him 'If you would like a nice new barometer, I will give you this one if you tell me the height of this skyscraper'."

The student was Niels Bohr, the only person from Denmark to win the Nobel prize for Physics.

---------------

My point? You need to know what answer is expected in first place, otherwise you could go on for forever having a hard time proving your valid yet not-correct answer. Isn't this the same company that made for a nice thread the other day (same style) about weight implications on gliding range?

Cheers,
FD. (the un-real )

[wondering]

@ FlightDetent

Excellent

[Old Aero Guy]

Sorry that I tried to give too much information and the implied conclusion was hidden.

To be very direct, use of thermal anti-ice will not increase an airplane's stall speed.