Stall speed in climbing turn

[Mad (Flt) Scientist]

@italia458

Sorry but you are thoroughly "wrapped around the axle".

Increasing angle of attack DOES NOT increase the stall speed. Increasing load factor does - everyone agrees with that - but you are confusing a coincidental factor with a CAUSAL factor.

This can be seen by considering that I can increase angle of attack independent of load factor - a simple slow-down does that - and the stall speed doesn't change, even though the AoA changes throughout the manoeuvre - the time history of AoA in no way affects the final stall speed for a suitably slow manoeuvre.

Arguing that because load factor affects Vs, and AoA can be associated to load factor, then AoA affects stall is misguided logic. I could as well try claiming that bank angle affects stall speed, because bank angle can be related to load factor under certain circumstances. That would be just as wrong.

AoA does not have a direct effect, or a causal effect, on stall speed. Never has, never will.

[Tourist]

italia,

You are very wrong about the outside wing having a higher angle of attack.

The extreme example of this is a helicopter blade.

The tip is always at a dramatically lower AoA than the root.

[italia458]

Arguing that because load factor affects Vs, and AoA can be associated to load factor, then AoA affects stall is misguided logic. I could as well try claiming that bank angle affects stall speed, because bank angle can be related to load factor under certain circumstances. That would be just as wrong.

I defined a condition and for that condition I specified that angle of attack was changing the stall speed. There is nothing wrong with that statement.

It's exactly the same as saying bank angle increases stall speed - under the condition of level flight, that is 100% correct.

I understand this perfectly well - I would suggest you re-read what I said.

You are very wrong about the outside wing having a higher angle of attack.

The extreme example of this is a helicopter blade.

The tip is always at a dramatically lower AoA than the root.

Really? Well you should write to John Denker and tell him that he's wrong.

I would just suggest you look into the helicopter blade thing again. I would guess that you're actually talking about 'washout' and not a climbing turn in an airplane. The propeller blades on an airplane also have washout.

[Genghis the Engineer]

There are moments when the explanation of a problem is getting much more complicated than it deserves.

Deneker, who so far as I know tends to get his maths right, shows that in a climbing turn [ reference ] the outside wing is at a slightly higher angle of attack. Therefore if the aeroplane is stalled in a climbing turn, the outside wing will stall slightly ahead of the inside wing (which may not have done).

This makes intuitive sense, if only because in my experience most aeroplanes stalled in a turn tend to roll either towards, or through, wings level.


Now, the speed the aeroplane registers as the stall speed is from a single point on the airframe.

So, at a given reference speed, in a single system (the aeroplane) the outside wing stalls before the inside wing. In a certain light then, the outside and higher AoA wing has a lower stalling speed (as it has stalled) than the inside and thus lower AoA wing, which may not have done.

In reality, when you come down to aerofoil level, stall speed is a pointless concept - it only has any validity when you apply it to a whole aeroplane, not to a bit of aerofoil. At the level of an aerofoil: stalling alpha is a useful concept, as is Reynolds number, but stall speed is not.

Between the two wings in a turning aeroplane, the tiny difference between speeds will make a miniscule difference in Re and have absolutely no relevance, since Re only really has appreciable effects with orders of magnitude, not the small percentage change that applies here.

G

[italia458]

Thanks for the explanation Genghis!

[Andy1973]

Yes, No and Maybe

Italia,

For a lot of flight dynamics debates, the tricky bit is asking the right question rather than having the right answers! And I'm not sure that "Stall speed in a climbing turn" is really helping us out here, because it doesn't let us separate out the variables properly.

Once we talk about a climbing-, rather than a level-, turn we open up a world of possibilities. Any amount of g, any amount of lift, and any helical climb angle (just to screw up Mr Deneker's maths).

So, to have a sensible answer, we really need to only change one variable at a time. Are you asking about changing angle of bank at 1 g, or holding a given angle of bank and varying the normal acceleration?

Andy

PS For what it's worth, the Deneker explanation doesn't help much if you're considering a constant angle of bank turn. If the outer wing is traveling faster and at a higher angle of attack, then it would be producing more lift than the inner wing. This would give a rolling moment if you were not balancing this out with some out-turn aileron. This aileron is effectively reducing the angle of attack of the outside wing (excuse the horribly imprecise terminology).

PPS In essence, if your outer wing is travelling faster but you're not rolling, its at a lower angle of attack!

[italia458]

Andy... I agree the question being asked wasn't 'ideal' for all aspects of aerodynamics. But I do believe it was a valid question with a valid answer. In the first paragraph of my first post in this thread, I stated clearly that the stall speed was a function of the load factor and provided the equation for this. Further explanation was to expand on this and give a more detailed look at the condition of a climbing turn.

Once we talk about a climbing-, rather than a level-, turn we open up a world of possibilities. Any amount of g, any amount of lift, and any helical climb angle (just to screw up Mr Deneker's maths).

Agreed. There's also the fact that, in any climb, thrust usually has a downward component which reduces the requirement of lift from the wings. For example, if you were to go from a level flight condition at 150 kts into a climb while remaining at the same speed, the angle of attack would decrease. But I don't believe that these factors that affect climbs and/or turns would affect the comparison of angle of attack between the two sides of the wing.

PPS In essence, if your outer wing is travelling faster but you're not rolling, its at a lower angle of attack!

I don't believe that's quite true. The only part I believe that's in contention is when the ailerons are deflected to prevent the airplane from increasing the angle of bank. When you deflect the ailerons, you're essentially changing the deflection angle (downwash) of the airflow. Generally, positive camber is used on wings so that the deflection angle of the airflow is downward (with reference to the chord line) which, in turn, lifts the wing. When you deflect the aileron on the outside wing upwards, that deflects a short band of airflow (the air flowing over the ailerons) over the wing at a slightly less downward angle. You could consider it upwards to make it easier to visualize but it's not necessarily upwards relative to the chord line. This band of airflow that's now being directed at an upwards angle, relative to the rest of the wing, is what balances the lift on the wings and ensures that the airplane does not increase its bank in the turn. The opposite happens on the inside wing where the aileron is deflected down. Since angle of attack is defined as the angle between the chord line and the relative airflow, the angle of attack is not changing. You're just changing the upwash and downwash. This video shows this: Airfoil with movable flap in smoke tunnel - YouTube

You've essentially just changed the shape of the wing which changes the coefficient of lift of the wing (similar to extending flaps) - increasing lift for the inside wing and decreasing lift for the outside wing in a turn, while remaining at the same angle of attack. It's the change in deflection of airflow at the rear of the airfoil that is ensuring you're not increasing bank in a turn.

Like Genghis said - the outside wing is at a higher angle of attack than the inside and that's why when you stall you generally will roll towards wings level or through depending on the airplane.

EDIT: I redid this post and changed a few things. Should be more clear/correct.

For what it's worth, this helps explain the change in airflow pattern at the leading edge of the airfoil in the YouTube video I posted, when the flap is deflected down. https://www.box.com/s/18a7421d629f1346c56b

[italia458]

Mad Scientist....

Arguing that because load factor affects Vs, and AoA can be associated to load factor, then AoA affects stall is misguided logic. I could as well try claiming that bank angle affects stall speed, because bank angle can be related to load factor under certain circumstances. That would be just as wrong.

I understand what you're meaning but I don't agree with exactly what you said. This picture is a performance chart for a BE55. It shows the relationship between bank angle and stall speed. Performance charts usually list the conditions that the chart will be accurate but it doesn't even mention that the airplane needs to be in a level turn for this relationship to be accurate. Disregarding what I think of that, I believe they must have thought that it was unnecessary to include that condition considering that the relationship between stall speed and angle of bank is so well know and I'm pretty sure that everyone to pass a Private pilot flight test knows that 60 degrees of bank = 2G which has a corresponding increase in stall speed. It's a condition that is normally assumed when talking about angle of bank and stall speed.

http://i.imgur.com/wZY2c.jpg

I see nothing wrong with describing the relationship between AoA and stall speed as long as I have specified the conditions in which that relationship is true - which I did.

[Dick Whittingham]

Italia458

The chart in your link is almost certainly referring to load factors and the consequent increase in stall speed IN A SUSTAINED LEVEL TURN. It doesn't say so on the placard but I suppose it assumes that anyone sitting in the pilots' seat knows this. If you are saying that the chart implies anything more than that you are wrong. Likewise, Mad Flt Scientist is, as usual, right

Dick

[italia458]

Dick... I'm not saying the chart implies anything more than that!

I think you should read my post again and the quote I used from Mad Scientist. Mad Scientist said you couldn't say bank angle affects stall speed and I say you can, if you specify the conditions, and I included the picture to help prove my point. You should read the thread again to understand why the 'bank angle affects stall speed' statement came up.

I agree, Mad Scientist is quite knowledgeable and is usually correct. But I am allowed to disagree with someone and I've stated why I disagree in this case.

[Dick Whittingham]

Italia,

It's a comprehension thing. You said "It shows the relationship between bank angle and stall speed". But it doesn't, and that statement is wrong. There is no general relationship between bank angle and stall speed.

Dick

[Tourist]

Italia, my apologies.

You are quite correct about the helicopter blade. (I just went and drew the diagrams!)


However, I think Andy is quite correct about the angle of attack having to reduce to offset the higher airspeed.

You can call it what you like, ie downwash etc, but at the end of the day lift is all about AoA and airspeed, and if the airspeed has increased then the AoA has to reduce to stop a roll.

Yes that is by movement of aileron in a conventional aircraft thus you can talk about deflection angle etc.

You could, however design an aircraft that could rotate the wings independently about their wing spar rather than change their shape.
Such a wing would still require you to have to rotate them to a position giving a lower AoA for the angle of bank to be stable.


No matter how you go about it, in a stable climbing turn to the left, there is a steady rate of roll to the right, which suggests a higher lift on the inside of the turn, which points to a higher AoA not lower. In a stable descending turn to the left, there is a steady roll to the left.

Lift is simply AoA and airspeed. Downwash etc are just confusions.

[Tourist]

Genghis

"This makes intuitive sense, if only because in my experience most aeroplanes stalled in a turn tend to roll either towards, or through, wings level."

There are many other possibilities for this.

For example, as soon as the aircraft stalls in a turn, it is no longer balancing the forces of "G" against gravity to leave you in balanced flight. ie you have gone from a balanced turn, to flying in a straight line with one wing down thus there is a sideslip towards the inner wing potentially giving it greater lift.

Or maybe the differential ailerons/rudder required to maintain a stable turn effect things?

[Pull what]

Going on what John Farley has just emailed me(agreeing with Genghs) the clue is in the fact that no aircraft can turn without a turning force.

So I would say then we are still talking about an increase in loading because there must be a centre seeking force which in turn must increase the loading.

In relation to a full power climbing turn stall in a SE prop A/C, surely there must be a RAF difference due to slipstream over the roots but is that equal over both roots in a balanced climbing turn?

[Genghis the Engineer]

Going on what John Farley has just emailed me(agreeing with Genghs)

It distresses me occasionally that I no longer go to any of the same meetings at JF, the arguments we could have were wonderful. If he's agreeing with me now, I must have finally learned something.

G

[Tourist]

Sorry to drag this back to angle of attack, but I have been looking at Denkers document and having a think.

Can somebody please explain to me where I am wrong in thinking that part of his document is flawed.

In his section on climbing and descending turns linked to earlier, he says

"In a level turn both wingtips are moving horizontally. In a climbing turn, both wingtips will be climbing, but they will not make equal angles to the horizon. This is because the climb angle depends on the ratio of the vertical speed to the forward speed. As a result of the different climb angles, we get different angles of attack for the two wingtips. The geometry of the situation is shown infigure 18.6 (in the chapter on spins)"

I have given this a lot of thought, and I don't believe that the angle to the horizon or climb or descent has anything whatsoever to do with angle of attack.

In the referred to figure 18.6, he shows that in a spin or descent the outer wing has reduced angle of attack, and says that in a climb this changes.

I think this is wrong.

I don't think motion relative to gravity/centre of the earth is in anyway pertinent, rather I think that relative airflow is everything.

I don't believe that an AoA sensor on the wing would notice any difference between a climbing or descending turn or indeed level.

If figure 18.6 is true in a descent, if you now point the nose up above the horizon, what has changed? Lift is not relative to the horizon, it is relative to the wing/AoA.

I don't get it.

Somebody please explain slowly.

[Background Noise]

Because in a turn, the outside wing is going faster. More forward speed gives a lower 'relative airflow' which equals higher AOA.



Same applies in reverse in a descent, the outside wing has reduced AOA.

[Machdiamond]

Tourist, I am with you on that one.

Angle of attack only relates to the angle between the trajectory and the body axis. The outside world does not matter at all (downwind turn anyone?).

Where the earth reference system gets involved is in defining the direction of the weight vector, which affects the left and right wing equally.

There are only three things that affect local angle of attack differently on your right or left wing in a turn: yaw rate, roll rate and aileron deflection. Also the propeller slipstream but I'll leave that one aside.

You will exhibit a yaw rate in a turn, which means lower speed for the inside wing thus higher local angle of attack. Roll rate is zero once a turn is established, thus no effect. Aileron deflection in a sustained coordinated turn will be positive or negative depending on the turn rate and the aircraft stability parameters. On most airplanes in shallow turns you will hold a bit of aileron in the direction of the turn so that means your outside wing will have increased local angle of attack. If you slow down or pull g's and stall in that configuration, your outside wing will stall first.

In a steep sustained and coordinated turn, with most airplanes, you will have to hold opposite aileron. This is to compensate the reduced lift of the inside wing which now has a much lower speed. This increases the inside wing angle of attack which is already high to start with, so your inside wing will stall first if you slow down (or pull more g's).

I hope this clears some confusion.

[Tourist]

Background Noise.

Thanks for that, I think I get the whole helical motion thing.

If I have it right,

Both wings climb or descend the same amount, but one travels further to do it, thus if both wings have the same mounting angle on the fuselage, one has a higher angle of attack.


This does, however leave us with a different problem.

If the outer wing in a climb has higher airspeed and AoA, and lift is dependent upon AoA and lift, then how come there is no change of angle of bank, ie balanced lift?

I still believe the statement "the outer wing has a higher AoA in a climbing turn" is incorrect.

What it should say is "the outer wing in a climb would have a higher AoA, but to maintain a constant angle of bank in a climbing turn, the pilot differentially adjusts the AoA of the wings using the ailerons to reduce the outer wing lift to match the inner wing"


This, of course is actually slightly simplistic, because in a climbing turn to the left, the aircraft is actually in a continuous roll to the right, so the lift on the right wing must actually be slightly lower. The converse is true in the descent.

[cwatters]

Outer wing has the down going aileron so that might be one reason.

I think it's more complicated than that though. Some years back I saw a glider accident where the inner wing stalled first. Possible because the combination of large span and tight skidding turn meant the inner wing was slower and partly blanketed