I Might Stall If I Turn Downwind. Myth Busted.

[farri]

Hi All,

 

With the amount of missundestanding I read on this issue,I am amazed we don`t have more accidents.

 

The forum is here to exchange views and ideas and that`s fair enough, however this section, which is about, Student pilots and Instruction,should be kept,to,Questions and factual answers,anyone submiting a reply should use the correct facts unless they are asking a question or we are only confusing the issues which makes the situation more dangerous.

 

The Fact,

 

An aircraft will only stall,"WHEN THE CRITICAL ANGLE OF ATTACK HAS BEEN EXCEEDED".

 

Definition.

 

The angle of attack,is,"THE ANGLE OF THE CHORDLINE TO THE RELATIVE AIRFLOW".

 

The chordline,is,"THE LINE THAT RUNS FROM THE LEADING EDGE TO THE TRAILING EDGE OF THE AEROFOIL".

 

The relative airflow,is,"THE AIRFLOW DIRECTLY OPPOSITE THE FLIGHT PATH OF THE AIRCRAFT"

 

That`s it,the stall has nothing to do with the direction of the wind or any reference to the ground or the speed of the aircraft over the ground,the only reference required is the Airspeed Indicator,which if correct will indicate the angle of attact.

 

Anyone who does not see, what I call ,the big picture and understand this yet,should,either,ask more questions or go back to BAK,or a book called,"THE MECHANICS OF FLIGHT" writer by, A.C. Kermode.

 

Please keep the general discussion for the right section.

 

farri. :hittinghead:

 

 

[robinsm]

As an add on to the Ground speed vs Air speed, I was silly enough to go for a flight today, the wind at take off being marginal. After a very rough, not pleasant flight, I turned onto final at IAS of 50 knots. On the GPS my ground speed was showing as 25 knots. The stall speed for the Xair is of 29 knots. Needless to say my airspeed was fine but when the IAS and the ground speed intersected at touchdown, the roll out was very short. Just my 0.2c worth. Remember, ground speed for ground people, airspeed for real people.:big_grin:

 

 

[Yenn]

So with 45 deg of flap on a C172 the angle of attack is vastly different from wingtip to the flapped area. What is the angle of attack and when will it stall?

 

 

[farri]

Yen,

 

The difference in angle of attack at the wing tip on any aircraft is called "WASH OUT" or "WASH IN".

 

"Wash out" is used on most aircraft and this gives a lower angle of attack at the wing tip which allows the wing tip to stall slightly later than the root of the wing making the aircraft more stable in the stall and not wanting to imediatly drop a wing,giving the pilot more time to react.

 

With your C172,refer to the aircraft manual for differences in angle, however, the area where the flaps are will stall first due to it`s greater angle of attack but only when the "CRITICAL ANGLE OF ATTACK HAS BEEN EXCEEDED".

 

farri.

 

 

[Yenn]

So what is the Critical Angle of Attack, and please don't say the angle where the wing stalls.

 

If the C172 had an angle of attack meter fitted it would stall at different angles with and without flaps, wouldn't it?

 

What I am trying to get at is how do we know the critical or any other angle of attack, except by when the wing stalls. We do know that it changes with weight.

 

don't recall ever seeng a critical angle of atack given as an angle tha s masurable by the pilot in any aircraft manual.

 

 

[hihosland]

Washout means that when the inboard portion of the wing stalls the outboard portion has not yet and consequently the ailerons still have some effect.

 

Davidh

 

 

[Yenn]

Washout is built into the wing by the manufacturer. It does not apply to the difference between flapped and unflapped flight.

 

Many people say angle of attack is what we should fly by, and produce the old theories, but I want to know how can the pilot know his angle of attack.

 

l would also like to know what is the chord line. Farri says The chordline,is,"THE LINE THAT RUNS FROM THE LEADING EDGE TO THE TRAILING EDGE OF THE AEROFOIL".

 

Do we assume that it is a straight line, in which case we can work out it's angle in relation to the airflow. Or is it cambered, as shown by airfoil designers, in which case the angle of attack must vary with the camber change. Is the flap part of the airfoil, or not?

 

I have not been flying all that long, nor do I have a degree in aeronautics, so maybe someone can enlighten me.

 

Maybe a good place to start would be http://www.av8n.com/how/

 

 

[Guest pelorus32]

Chord Line: The line joining the leading edge to the trailing edge of the aerofoil. This is an artificial construct that helps us in understanding and analysing aerofoils. It does not necessarily lie within the structure of the wing and yes it is generally a straight line;

 

Camber: This is the curve of the aerofoil.

 

When we deploy flaps or slats or any other aerofoil modifying device we are creating a NEW aerofoil. Very often we are altering - generally increasing - the camber of the aerofoil by deploying flaps.

 

When we deploy flaps the trailing edge of the flap becomes the trailing edge of the wing. So the chord line shifts - often markedly. Have a look at the fully deployed flaps on a B744 to see this effect at a fairly extreme point.

 

So any aerofoil has a critical angle of attack (AOA or alpha) at which it stalls. When we deploy flaps or other devices we are creating a new aerofoil with a new critical alpha. In some cases the change is immaterial. In others, and here the FA/18 is an example, it can be huge. The FA/18 has a "living wing" which can see the a/c continuing to fly at or beyond 44 alpha when we would normally expect a wing to stall at around 14 to 16 degrees alpha.

 

So now to Yenn's very good statement that he - and most of the rest of us - don't know what alpha their aircraft stalls at.

 

This is true and so we use a series of approximations and assumptions:

 

• Firstly we assume that the aircraft is in a 1g situation;
   
   
  
• Then we assume some given weight for the a/c - say 600kg for an LSA at MTOW;
   
   
  
• Then we say that if we maintain 1g and decelerate slowly we will reaching our "stalling airspeed" at say 40 knots.
   
  

 

What we are actually doing is reaching our critical alpha and because we have chosen one particular weight and one particular g loading that alpha is arrived at a particular airspeed - around 40 knots for our example.

 

Next we know that "stall speed" increases with the square root of the load. So if we put that same aircraft into a 2g manoeuvre then we will stall at a speed that is 1.41 times 40 knots or about 56 knots.

 

What's actually happening is that you can get lift from a combination of alpha and velocity. It's just that this wing, at this weight, loaded to 2g reaches a point where at 56 knots airspeed the wing needs to be at the critical alpha in order to generate the required lift.

 

So short answer is that no we don't know what the critical alpha is in practice - the POH may tell us that its 16 degrees or whatever - but in practice we don't know.

 

BUT WE DO: The stick will always be at the same position of rearward travel when we stall. This is the stalled stick position. If you don't know where it is on your aircraft then you should. Go and stall and find it. So the stick is in fact the best possible indicator of alpha and it is remarkably progressive and linear.

 

Please just remember that the aerofoil is a new and different aerofoil with potentially a new critical alpha when you deploy flaps etc.

 

There is also a nice conversation about trim - we are actually trimming our a/c for a particular alpha and you can demonstrate that in your a/c. Later.

 

Regards

 

Mike

 

 

[Guest pelorus32]

Just reading the other posts in this thread rather than just Yenn's 5:47pm.

 

I think that there is some confusion introduced by the notion of washout. Washout is not really an appropriate concept to compare a flapped with an unflapped section of wing.

 

What normal camber flaps do is to increase the coefficient of lift for any given alpha. That means that for any given alpha you can generate more lift (than the same aerofoil if not flapped), thus delaying your arrival at the critical alpha.

 

Regards

 

Mike

 

 

[vk3auu]

"BUT WE DO: The stick will always be at the same position of rearward travel when we stall. This is the stalled stick position. If you don't know where it is on your aircraft then you should. Go and stall and find it. So the stick is in fact the best possible indicator of alpha and it is remarkably progressive and linear."

 

Not so. That the position of the stick will depend on a couple of different things.

 

First of all, it will depend on where the centre of gravity is and that may well vary according to what moment the luggage or fuel are producing. It will also depend on how much flap you are using. For instance, my flaps produce a fairly strong nose down attitude, which combined with a CoG in the forward half, results in a complete inability to get the AoA high enough to stall.

 

Also a reply to another comment.

 

Inertia has absolutely no dependence on wind, only ground speed. If you are flying your trusty Thruster at 45 knots into a 45 knot wind, your ground speed is nil and so is your inertia.

 

David

 

 

[farri]

Yen,

 

There has been several attempts to try to give you some answers by several people,Peloris32 being the most lengthy.

 

There is too much throry involved to be able to cover the subject fully on this forum,

 

As you asked about flaps,I was assuming that you understood what the chordline was and that the chordline changed as the flaps were deployed.

 

As for the stalling angle, the designer works all this out,that`s what I meant by the aircraft manual, the stalling angle is of no importantance to the pilot flying the aircraft because, as you point out, we don`t have an angle meter,the closest we have is an airspeed indicator,therefore you need to know the airspeeds the aircraft stalls at.

 

I pointed out washout because I didn`t know if you were aware of it,maby I missunderstood your question,it may be someone with a degree in aeronautics that you need.

 

I hope you find the answer to your question,

 

farri.

 

 

[farri]

Vk2auu,

 

Can you please tell me what you mean by your statement on inertia of the thruster,

 

I don`t understand what you are trying to say.

 

farr.

 

 

[Flyer]

we don`t have an angle meter,the closest we have is an airspeed indicator,therefore you need to know the airspeeds the aircraft stalls at.

Please understand that this indicated airspeed that an aircraft stalls at is relative to a 1g loading.

 

An aircraft can stall at ANY speed if you exceed the critical angle of attack which is about 16 degrees as already pointed out.

 

this is why you have to be careful in a dive recovery or a stall recovery that a secondary stall isn't introduced.

 

Whilst I haven't done it in a full size aircraft, I have induced high speed stalls in model aircraft ( wingspan 2.1 meters approx) and it produces some interesting flight characteristics....snap rolling is just one and it takes a hell of a lot of will power to get off the elevator....:raise_eyebrow:

 

Regards

 

Phil

 

 

[Guest pelorus32]

It will also depend on how much flap you are using. For instance, my flaps produce a fairly strong nose down attitude, which combined with a CoG in the forward half, results in a complete inability to get the AoA high enough to stall.David

G'day David,

 

that's why I said please be aware that with flaps deployed you are flying a different wing. However many of the aircraft that we fly, in the clean config do not have a great C of G movement and the stick represents the best indicator of alpha that we do have. Same is true in a flaps down config but the position may be different.

 

Regards

 

Mike

 

 

[vk3auu]

If the bloody thing isn't moving, it has no inertia. What can be simpler than that?

 

David

 

 

[vk3auu]

Please ignore the previous statements about inertia. I must have been having a Senior's moment. What I meant to say is that it has no kinetic energy.

 

David

 

 

[farri]

Flyer,

 

keep in mind I was originaly trying to address the post,"I Will Stall If I Turn Downwind" and I can`t cover every detail of the principals of flight each time I reply.

 

The G forces make no difference to the stalling angle,only to the stalling speed,when an aircraft is coming out of a dive it wants to continue in the direction it was traveling, if the pilot pulls it up too steeply the G force indeed goes up however because the aircraft wants to continue on it`s original path,this path now becomes the new angle of attack, so if that angle of attack is exceeded the aircraft will stall, if not it won`t.

 

I preffer not to talk about the stalling angle in angles of degrees because the angle at which an aerofoil stalls can not be determined by the pilot, from the aircraft.

 

I have already said a couple of times the airspeed indicator is simply a reference point,it will not stop anyone from stalling,however it`s the best we have at present.

 

farri.

 

 

[Yenn]

farri. My questions and comments were very much tongue in cheek. I don't care what anyone says all this talk of critical angle of attack is pure theory. Foe an ultralight p;lot the way to prevent a stall is by monitoring airspeed, attitude and feel. Airspeed and attitude are not necessarily 100% correct but they may be the best we have.

 

 

[Yenn]

vk3auu.

 

The stick will always be at the same position of rearward travel when we stall.

 

If that is the case how can I stall an aircraft with the stick forward?

 

You may not believe it possible, but it can be done and usually the stall speed will be higher than from normal level flight.

 

If you can answer this you may see where we have all been in the past with this subject. If not it is a good brain teaser.

 

 

[Guest pelorus32]

G'day Yenn,

 

inverted flight will do it.

 

Mike

 

 

[eastmeg2]

Flying a Trike will do it . . .

 

 

[Mazda]

An aicraft can stall at high speed. I've also seen zero on the ASI and counted a bit, but the aircraft is not yet stalled.

 

The aircraft sure will stall with the stick forward, I've done it. If the pilot has only been taught to push the stick forward to recover, they could end up holding the aircraft in an inverted stall.

 

The aircraft will not be stalled (upright or inverted) if the critical angle has not been exceeded - so if you are ever unsure put the stick in a neutral position. It then cannot be in an upright or inverted stall.

 

 

[farri]

farri. My questions and comments were very much tongue in cheek. I don't care what anyone says all this talk of critical angle of attack is pure theory. Foe an ultralight p;lot the way to prevent a stall is by monitoring airspeed, attitude and feel. Airspeed and attitude are not necessarily 100% correct but they may be the best we have.

Yen,

 

I`m very pleased at what you have said,you had me worried for a while, had this gone on any longer that is exactly what I would have said,leaving out the bit about "Pure Theory",however.

 

Because this section is for student pilots,also,it is extremely important that they thoroughly understand the definition of the stall so they can understand why it occurs and hopefully avoid it.

 

Theory only helps us to understand what is happening,it does not fly the aircraft,the pilot does, and a complete awareness and understanding of the situation and the aircraft,by the pilot, is required to fly saftely,this is easier done with a clear understanding of the theory involved.

 

Let`s not say we don`t care what anyone say`s about the critical angle of attack because there have been too many deaths because of this,in the time I have been flying,I would like to try and reduce the numbers.

 

farri.

 

 

[Guest Andys@coffs]

Flying a Trike will do it . . .

Except that if your upside down in a trike the wing stalling is the least of your worries (generally, excepting those darwinian attempts at loops, russian roulette!)

 

In fact least is probably incorrect, perhaps more correct would be the wing stalling is only one of a whole heap of things that are trying to kill you.....

 

Andy

 

 

[farri]

As an add on to the Ground speed vs Air speed, I was silly enough to go for a flight today, the wind at take off being marginal. After a very rough, not pleasant flight, I turned onto final at IAS of 50 knots. On the GPS my ground speed was showing as 25 knots. The stall speed for the Xair is of 29 knots. Needless to say my airspeed was fine but when the IAS and the ground speed intersected at touchdown, the roll out was very short. Just my 0.2c worth. Remember, ground speed for ground people, airspeed for real people.:big_grin:

Robinsm,

 

If the aircraft encounters a sink, it will travel downwards as well as forward therefore the "RELATIVE AIRFLOW" is now coming at the aircraft from below and the "ANGLE OF ATTACK" has increased bringing the aircraft closer to the "CRITICAL ANGLE OF ATTACK" and a stall.

 

On final approach the airspeed should be high enough to allow for any unexped sink and wind shear,wind shear being "Any change in wind speed or direction"

 

A good airspeed to use is,"The stall speed of the aircraft + 50% of stall speed + 1/3 of the wind speed" this should cover most situations,Never use your ground speed,keep the GPS for navigational use.

 

farri.