Gusts and stalling

[Tony Jones]

Another contributor has stated that gust can not cause a wing section to stall. I beg to disagree. Consider the following situation:

You are on final approach with your model on a hot calm sunny day with nice white puffy clouds. Your model’s stalling speed is 10mph and you are flying with a very reasonable 50% margin at 15mph. (Doesn’t matter what the actual numbers are – it’s just a normal approach).

When you are at about 20ft, the hot air that has been lying on your field’s surface breaks away upward to form a new thermal. Air therefore moves in from around the field’s perimeter to replace it. So your model suddenly finds itself in an air mass that is moving at, say, 10mph in the same direction. Because it has inertia in relation to the mass of the earty, it can not accelerate instantaneously. So its airspeed drops to 5mph. What will happen? If you agree with me that it will stall, then IMHO a gust has caused a stall.

So when the wind is light and flukey (Wot – move the pits again?) or thermal activity is present, the prudent pilot will fly his final approach just that little bit faster – even if it means a longer rollout.

In full size commercial aviation, windshear gives the same practical result as a gusting wind and an airliner can easily lose 20kt of airspeed on late finals in the twinkling of an eye. Autopilots and autothrottles often can’t respond to the changing conditions rapidly enough – which is why many firms recommend pilots to fly manual approaches, keeping an eagle eye on the ASI and RCDI, whenever  windshear is forecast. Nice to know the human can still beat the computer on the odd occasion!  

That’s my 2p. Hope someone finds it useful and/or informative.  

Best wishes

Tony

 

 

 

[Gemma Jane]

Go post that on PPRuNe Tony, so I can really shoot you down ... how daft can you get.

 

Sections stall at an angle of attack not an airspeed. I already explained this to you by PM. What on earth are you hoping to prove.

 

A gust can cause changes in lift they can not stall a section.

[Former Member]

[This posting has been removed]

[Gemma Jane]

Tony is making the mistake of claiming to be an 8000 hour pilot Eric when he doesn't understand what the stall is. I simply don't believe that he can come on this forum and firstly state on a thread that it is a breeding ground for amateur nonsense then post this thread.

 

 

An aeroplane sat on the ground with wind blowing from behind does not have stalled wings. It simply doesn't have any lift.

 

 

The stall is a specific aerodynamic event which occurs when the critical angle of attack of the section is exceeded usually aoa ~ 16 degrees positive.

 

 

It is typified by the break away of the boundary layer from the main section and accompanied by a massive reduction in lift.

 

 

Gusts do not and cannot cause this event. The reason for flying a faster approach in gusty conditions is to reduce the danger of rapid sink (reduction in lift) and not to prevent a 'gust induced stall' - which simply does not happen.

Edited By Phil Wood - Moderator on 14/07/2009 20:38:45

[PatMc]

I've demonstrated that it's the AoA not airspeed that's responsible for a stall by using a glider at a steep slope site. By taking the glider high & behind a ridge then diving - pulling out to fast flat flight & crossing the ridge dead into wind at about head height. Despite the fact that the glider is travelling much faster than it's normal trimmed speed & roughly flying S&L it will often stall as it meets the rising air from the slope.

Ergo it's a high speed stall caused by a high AoA resulting from the angle of the local airstream.

Edited By PatMc on 14/07/2009 18:55:23

[Tony Jones]

Gemma

I wish you were able to express your disagreement without being offensive but I guess you can't. At no time have I said you were an amateur. However, you have previously called my opinions 'nonsence' (sic) and you are now suggesting that I am lying in my profile. Sad. And all this started because I said you had been charitable in your assessment of the original article. In other words, you had been a bit too nice. I'm really sorry you interpret that as severe criticism.

 

Eric

I take your point, but gusts are sudden changes in the windspeed and velocity and when a thermal breaks off the result is not necessarily smooth and consistent. But as long as the gust reduces  your airspeed faster than the prop can restore it, you get a large loss of lift and rapid descent. The automatic reaction, as you say is to pull up elevator to arrest the descent, causing a stall.  IMHO (again) most people would regard a  wing which does not have airlfow flowing over it from LE to TE to produce lift as stalled.

 

That's absolutely it from me. Gemma clearly considers this subject to be her personal domain and I don't need the rudeness. Bye all.

 

[Tony Jones]

Oh, and one last thing for the record Gemma. You did not reply to my original PM - sent in an attempt to take our discussion private.

I have just checked my email server again, there if nothing on there from you.  

Edited By Phil Wood - Moderator on 14/07/2009 20:35:57

[Gemma Jane]

No, it all started when you came on here ranting and picked the wrong person to have a go at Tony.

 

You are no pilot as nobody who has flown solo would have your misconceptions regarding the stall.

 

IMHO (again) most people would regard a  wing which does not have airlfow flowing over it from LE to TE to produce lift as stalled.

 

Well wake up honey your HO couldn't be more wrong.

Edited By Phil Wood - Moderator on 14/07/2009 20:35:13

[Former Member]

[This posting has been removed]

[Martin Harris - Moderator]

At the risk of being accused of trolling, I must say that I'm finding this debate "interesting" (see the thread on Martin Bedding's article) and I'm surprisingly being drawn towards supporting Tony's viewpoint - although I accept my opinion may be partially or badly informed!

 

I think I understand the basics of stalling, having studied very basic aerodynamics in the past and I do appreciate that the angle of attack determines the airspeed.  However, what I think Tony's point is is that when encountering a sharp edged gust, the relative airflow changes resulting in an almost instantaneous change in angle of attack, thereby in some circumstances exceeding the critical angle and stalling the wing - hence the normal practice of increasing airspeed (reducing angle of attack) when in the circuit and anticipating gusty conditions.

 

 

 

[Gemma Jane]

Martin don't you start.

 

I've been in an aircaft landed in 35 kts gusts how much was added to the airspeed?

 

35 kts? No nearer 5 Kts

 

If any of this was true increasing the airspeed wouldn't help would it, because the 'gust' would still have the same aoa and 'stall' the wing whatever airspeed you had.

 

It isn't the reason for increasing the airspeed at all.

 

The reason for increasing the airspeed is to prevent a sudden sink leaving a smoking hole in the ground.

[Martin Harris - Moderator]

Gemma,

Firstly, I should say that I thoroughly respect your opinions based on both your qualifications and the quality of your responses on this forum and I'm trying to understand this subject and not to impose my opinion.

 

I wonder if the difference in opinion is that you are considering gusts as purely horizontal components in the direction of motion of the aircraft and that I am visualising their vertical and sideways components?

 

I feel that close to the ground in turbulent air and wind gradients, local effects throw a few spanners into normal theory with which I think we are in agreement.  I certainly wouldn't argue that a reserve of energy and increased control effectiveness due to increased speed on the approach is a must have in turbulent conditions.

Edited By Martin Harris on 15/07/2009 01:20:17

[BB]

Stall

 

Nasa studies

 

BB  (Very ineresting thread) 

 

[Bert]

Posted by Gemma Fairchild on 14/07/2009 23:24:11:

 

 

The reason for increasing the airspeed is to prevent a sudden sink leaving a smoking hole in the ground.

 

[Myron Beaumont]

BB

Fascinating links .   Now I know  why I have never done a "proper" flat spin !

I didn't slall the wing(s)  to start with . Just showing my ignorancs I suppose  

My only comment about stalkling is actually not about stalling . Has anyone ever flown a hang glider off a sharp edged clff face /to top ground interface & landed just behind it on the top . You don't half drop quickly ! but of course that is 'cos of turbulence . You should never do it  but some have tried '

Myron

[BB]

 

I go to the cliffs edge forthwith Myron.  I like this ol'vid aswell  

 

BB

 

 

[Gemma Jane]

Posted by Martin Harris on 15/07/2009 01:19:58:

Gemma,

Firstly, I should say that I thoroughly respect your opinions based on both your qualifications and the quality of your responses on this forum and I'm trying to understand this subject and not to impose my opinion.

 

I wonder if the difference in opinion is that you are considering gusts as purely horizontal components in the direction of motion of the aircraft and that I am visualising their vertical and sideways components?

 

I feel that close to the ground in turbulent air and wind gradients, local effects throw a few spanners into normal theory with which I think we are in agreement.  I certainly wouldn't argue that a reserve of energy and increased control effectiveness due to increased speed on the approach is a must have in turbulent conditions.

Edited By Martin Harris on 15/07/2009 01:20:17

 

 

I am not only considering the horizontal component.

 

 

 

What is the wind?

A large body of air moving over the ground

What is a 'gust'

It's a body of air moving over the ground within the larger body of air known as the wind but at a higher relative velocity

What happens when an aircraft hits a gust head on?

Nothing much other than due to the inertia of the aircraft the airspeed indicators shows a higher airspeed and the aircraft tends to produce more lift.

Why are gusts dangerous during a landing?

Because the airspeed indicator was showing a higher reading when you hit the gust. The danger isn't the gust, it is flying out of it and suddenly being at a lower power setting than required to maintain the approach and experiencing sink. This is compensated for by pilots by increasing the airspeed on approach by approximately one half of the difference between the ambient wind speed and the reported gust velocity.

What happens when a gust hits an aircraft from the side to the airspeed?

Nothing the aircraft simply moves with the body of air of in relation to the ground its forward velocity relative to the air mass is unaltered and the aoa in relationship to the free stream velocity is unchanged.

What happens when a gust hits an aircraft from behind?

Nothing other than the aircraft is accelerated over the ground. The aircraft is flying in the body of air already, it simply moves with it as the air accelerates

What happens if an updraught comes directly up from the ground?

The aircraft moves upward with the gust. The aircrafts horizontal airspeed through the updraught is identical to it's airspeed before it flew into it and the aoa in relationship to the free stream again remains unaltered. Therefore nothing much happens other than the pilot feels and sees the increase in altitude.

 What happens if an aircraft flies into a downdraught?

 

 

The aircraft is carried down with the moving column of air. A dangerous situation in a low powered aircraft as the rate of sink can be greater than the aircrafts rate of climb. (i.e. mountain waves and the lee side of hills)

 

 

Localised turbulence Martin is a different issue. Here the sudden changes in air movement can ovecome the control authority of the aircraft, or more likely the skill level of a pilot under high work load close to the ground. Turbulence can plonk the unwary down in an adjacent field upside down. But the wing didn't stall.

 

 

Windshear, well that is an often misused term. True windshear is what happens in CB. Again not an issue regarding the stall but it can easily turn an aircraft on its back. Or in severe cases cause structural failure. When the wings are no longer attached to the fuselage the aoa and the stall becomes rather academic

 

Edited By Gemma Fairchild on 15/07/2009 10:00:08

[PatMc]

Posted by Myron Beaumont on 15/07/2009 08:44:26:

BB

 Has anyone ever flown a hang glider off a sharp edged clff face /to top ground interface & landed just behind it on the top . You don't half drop quickly ! but of course that is 'cos of turbulence . You should never do it  but some have tried '

Myron

 I've got no experience of hang gliding but I think what you describe is the same effect as I mentioned but without the fast forward impetus.

I've often seen slope soarers launched at what appears to be a good level attitude then instantly stall & come back over the pilots head out of control. The advice given is usually to walk down the slope a few yards if that's possible & launch from there. If that's not possible launching downwards at maybe 30 degrees or so usually gets the model away OK.

[weasel]

 

  i am with gemma on the stalling subject. i have a P.P.L for light aircraft and have flown 1000hrs. mostly  on flexwing microlights. i have flown in some very gusty and thermic conditions, and that a gust of wind on finals to land has never,  stalled my wing but produced more lift, not a stall. that would only happen when there is not enough air flowing over the wing,  so a gust of wind is actually given you more lift not a stall..  correct me if i am wrong.. regards weasel......

[Doug Ireland]

I remember an incident that happened to me as a student pilot at Tayside Flying Club back in the 70's. I was solo circuit bashing in a Cessna 150 and was on finals for another go-around; surface wind was down the runway about 5kts. At about 100' I had 20 degrees of flap and around 60 kts IAS when the left wing suddenly dropped to what I guessed was about 60 degrees. Full opposite on the wheel brought the wing back up and I managed to get down in one piece; other club members on the ground didn't think I was going to recover in time.

 

The popular theory was "wind-shear".

[Gemma Jane]

Happens to me all the time in the PA-28 Doug. It's just dropped a wing due to an air pocket. Scared the life out at me the first time, in the end you just pick the thing up and don't think about it. The dropped wing wasn't  stalled. If it had have been 'full opposite on the wheel' would have finished you for good.

 

The one that really got me was a landing, all three wheels down on the ground and I made the mistake of thinking that was the end to it. Then the gust hit me. Aeroplane was lifted back in the air 20 ft with practically no airspeed. The rest was a bit of a blur, but I'm still here and didn't bend the aeroplane. Did however touch back down 45 degrees to the runway centreline. Fortunately I think instinct made me put the nose down and then flare again so I didn't wheelbarrow it.

 

I've a friend who had the same experience as you just before touch down. It took her nearly 100 hrs dual to get over it and fly solo again.

[Myron Beaumont]

BB

Love your basic video on flight !

Shame it's not watched by all the so-called experts

I'm starting to foam at the mouth  

[Gemma Jane]

That's a great video, probably responsible for a million misconceptions in pilot training manuals. Awful lot of words to describe two simple equations though

 

Myron women invented foam - OK that probably isn't a fact but it is what you do with it that counts.

[Martin Harris - Moderator]

I don't dispute any of the theory quoted Gemma, and I've argued long and hard with clubmates over the principles of a goldfish swimming in its bowl while being taken for a walk to the local shops and similar analogies to explain flight in a air mass.

 

What I am visualising is the following scenario:

 

...as the air mass is moving downwards. isn't the net result an increase in angle of attack while the aircraft is overcoming inertia effects and moving entirely into the downdraft?

 

 

Edited By Martin Harris on 15/07/2009 13:48:12

[Tim Mackey]

Both Phil and I have politely asked you guys and gals to keep things civil in this seemingly controversial debate over on the original thread...same here please

I have already had to delete two postings, and deal with some unpleasantness

Edited By Timbo - Moderator on 15/07/2009 15:56:09