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Author Topic: Why do planes stall when their wings reach a certain angle?  (Read 5387 times)

Marco Ocariza

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Marco Ocariza  asked the Naked Scientists:
   
Hey Chris I'm Marco! I just wanted to ask you how do aircraft fly? And why do they stall when their wings reach a certain angle?

This interests me a lot and it would be great if you could answer it!

Thanks.

What do you think?
« Last Edit: 18/05/2010 21:30:02 by _system »


 

Offline norcalclimber

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Why do planes stall when their wings reach a certain angle?
« Reply #1 on: 19/05/2010 02:21:46 »
Airplanes, in general, achieve flight due to the shape of their wings.  With the right shape, especially the back edge of the wing, air actually pushes the wings up which gives the airplane lift.  If the wings are too vertical, the air can't push the wing "up" anymore and the engine can't fight gravity anymore, so it stalls.

The back of the wings which move are called flaps, and can be adjusted to change the amount of lift generated. 

Interestingly enough, birds have flight feathers and with a slight trim of just a couple feathers they can no longer fly.

(I'm not an "expert", so I hope this explanation suffices)

« Last Edit: 19/05/2010 02:23:39 by norcalclimber »
 

Offline LeeE

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Why do planes stall when their wings reach a certain angle?
« Reply #2 on: 19/05/2010 14:08:08 »
I'm afraid that's not quite right norcalclimber.

Have a read of: http://www.thenakedscientists.com/forum/index.php?topic=31012.msg306466#msg306466
 

Offline norcalclimber

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Why do planes stall when their wings reach a certain angle?
« Reply #3 on: 19/05/2010 17:09:45 »
Thanks LeeE.  That was a much more in depth and accurate answer.  But I have the basics down at least, right?  It is the shape of the wing which allows it to generate lift, and too vertical of an angle it no longer provides lift, then we get a stall?

I am certain I got the bird part right though, because I have hand raised quite a few birds and seen the effects of trimming the flight feathers myself.
 

Offline imatfaal

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Why do planes stall when their wings reach a certain angle?
« Reply #4 on: 19/05/2010 17:55:55 »
Marco and Norcal

How a wing generates lift is highly complex and not fully understood - it has flavours from newton's third law, the bernoulli principle and much besides.  but it is clear lift is not due to solely differing paths lengths/flow velocity.  my physics teacher and others have told me that lift is explained through bernoulli's principle and the asymmetric cross-section of the aerofoil (it's further over the top than round the bottom) - this is such a simplification as to be almost mis-leading. 

wikipedia has the lovely quote from the aeronautical curator of the smithsonian:

It is amazing that today, almost 100 years after the first flight of the Wright Flyer, groups of engineers, scientists, pilots, and others can gather together and have a spirited debate on how an airplane wing generates lift. Various explanations are put forth, and the debate centers on which explanation is the most fundamental. – John D. Anderson

this is a brief summary of the problem - but with no answers cos they get too complex
http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

You are right about the angle of attack - too high an angle (as well as too little speed) will lead to a dramatic lack of lift, a stall.  How clipping flight feathers works - I don't know

Matthew


 

Offline norcalclimber

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Why do planes stall when their wings reach a certain angle?
« Reply #5 on: 19/05/2010 18:03:51 »
  How clipping flight feathers works - I don't know


I'm not sure how the physics of flight feathers works; but basically, the flight feathers are at the tips of the wings, and if you shorten them to where they are about even with the rest of the wing the bird can no longer fly.

Perhaps someone else knows the physics of why?  Maybe I need to post this as a separate question(or search, because maybe this has already been asked).
 

Offline Remo

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Why do planes stall when their wings reach a certain angle?
« Reply #6 on: 19/05/2010 21:22:52 »
Why the wings make a plane fly seems pretty well answered as best as can be understood at the moment, as for the stall, i would say its all thrust, it has nothing to do with the wings, at a thrust in excess of speeds 7miles/s there is no stall, Now before you say wait a plane isn't a rocket!! Think about this

The weaker the plane is, the less steep its stall angle is.
The more powerful the plane is, the steeper its stall angle is, and the longer it can hold its stall angle before falling, so the stall is cause by lack of power/thrust, with enough power you could take a plane straight up then level out 100 000ft no stall.

Now the fact that this theoretical plane would have to break mack 34(like any rocket escaping the earth)and that we are not there yet technologically doesn't mean we will never do it.

The reason stall exists in the first place is we're using the wings to cheat the power equation, were using wings to use an underpowered engine(s) to move the plane through the air, but the wings didnt cause the stall.

 

Offline LeeE

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Why do planes stall when their wings reach a certain angle?
« Reply #7 on: 20/05/2010 16:22:24 »
Marco and Norcal

How a wing generates lift is highly complex and not fully understood - it has flavours from newton's third law, the bernoulli principle and much besides.  but it is clear lift is not due to solely differing paths lengths/flow velocity.  my physics teacher and others have told me that lift is explained through bernoulli's principle and the asymmetric cross-section of the aerofoil (it's further over the top than round the bottom) - this is such a simplification as to be almost mis-leading.

Actually, it is very well understood.  Aerofoils have long been designed using numerical methods, which wouldn't be possible if the principles were not well understood.

Also, as mentioned previously, the aerofoil doesn't need to be asymmetric to generate lift as a symmetric aerofoil will also generate lift when used with a positive Angle of Attack.  However, relying upon a positive AoA for lift has two major drawbacks for passenger and cargo aircraft: increasing the AoA also increases the effective cross section of the aircraft, and the resultant drag, which  means that more power is needed for the same cruise performance.  In addition, flying with any significant AoA will be uncomfortable for passengers and increases the risk of any cargo shifting, which would upset the balance of the aircraft.  For these reasons, asymmetric aerofoils tend to be used for these types of aircraft as they allow them to be flown more 'level'.

Symmetric aerofoils tend to be used in aircraft that are likely to be performing aerobatics, such as military fighters and sports aerobatic aircraft, where maintaining steady and level flight isn't such a major concern.

The most common misunderstanding, which is still frequently cited, is that the lift comes from the 'downwash' generated by the aerofoil.  However, downwash occurs behind the wing and a quick consideration of delta winged aircraft, such as Concorde, the Vulcan and the French delta winged Mirages show that downwash cannot be the primary means of lift as the lift would be acting behind the aircraft and not close to the Center of Gravity.  There is also the observation that if lift was derived from downwash then an equivilant force to the weight of the aircraft would be exerted upon the ground when the aircraft was flying low, crushing anything beneath it: once again, this is seen not to happen.

It is also frequently stated that the lift generated by a wing acts perpendicularly to the ground i.e. directly upwards, but this is also untrue.  The Wright brothers discovered, by measuring the force needed to anchor their development gliders, that the lift generated by the wings is perpendicular to the surface of the wing, and not the ground, and that most of the lift comes from the forward part of the aerofoil, with the result that the direction of lift is actually angled slightly forwards, reducing the amount of power they would need to make their powered aircraft work.  If it were not for this fact, the Wright brothers' engine (which they actually designed themselves, as all other engines available at the time didn't have a high enough power to weight ratio) would not have been powerful enough to propel their aircraft.
 

Offline norcalclimber

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Why do planes stall when their wings reach a certain angle?
« Reply #8 on: 20/05/2010 16:55:26 »
Why the wings make a plane fly seems pretty well answered as best as can be understood at the moment, as for the stall, i would say its all thrust, it has nothing to do with the wings, at a thrust in excess of speeds 7miles/s there is no stall, Now before you say wait a plane isn't a rocket!! Think about this

The weaker the plane is, the less steep its stall angle is.
The more powerful the plane is, the steeper its stall angle is, and the longer it can hold its stall angle before falling, so the stall is cause by lack of power/thrust, with enough power you could take a plane straight up then level out 100 000ft no stall.

Now the fact that this theoretical plane would have to break mack 34(like any rocket escaping the earth)and that we are not there yet technologically doesn't mean we will never do it.

The reason stall exists in the first place is we're using the wings to cheat the power equation, were using wings to use an underpowered engine(s) to move the plane through the air, but the wings didnt cause the stall.



Seems to me that while LeeE's explanation of lift is an excellent one(thank you very much), Remo is right about the stall being a result of an underpowered engine(s) using wings to "cheat" gravity.
 

Offline LeeE

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Why do planes stall when their wings reach a certain angle?
« Reply #9 on: 20/05/2010 16:59:55 »
...as for the stall, i would say its all thrust, it has nothing to do with the wings, at a thrust in excess of speeds 7miles/s there is no stall, Now before you say wait a plane isn't a rocket!! Think about this

The weaker the plane is, the less steep its stall angle is.
The more powerful the plane is, the steeper its stall angle is, and the longer it can hold its stall angle before falling, so the stall is cause by lack of power/thrust

I'm afraid this explanation is not correct.  When a wing stalls is down to the design of the aerofoil, and to a large degree, the weight that the wing is supporting.

A stall occurs when the air flow over the wing cannot change direction quickly enough to follow the upper surface of the wing and as a result, detaches from it, at which point it stops generating lift.  With a simple, non-laminar-flow aerofoil, even in 'level' flight, the airflow is not smooth over the full chord of the wing but detaches from it, starting at the rear of the aerofoil.  As the Angle of Attck (AoA) increases, the point at which the airflow detaches moves forwards along the chord of the aerofoil, reducing the amount of wing that is generating lift, and eventually the airflow will detach almost immediately, with the result that hardly any lift is being generated, at which point you have a total stall.

A stall can also occur when the amount of weight the wing has to support exceeds its lifting capacity.  This is not likely to occur in civilian or cargo aircraft under normal circumstances but was a problem for some early jet fighter aircraft when pulling very tight turns, the G-force generated by the turn effectively making the aircraft weigh too much for its wings to support.  This type of stall is known as a G-stall.

Quote
...with enough power you could take a plane straight up then level out 100 000ft no stall.  Now the fact that this theoretical plane would have to break mack 34(like any rocket escaping the earth)and that we are not there yet technologically doesn't mean we will never do it.

Modern fighter aircraft tend to have power to weight ratios that exceed unity: the amount of thrust they produce can exceed the weight of the aircraft.  When this is the case, the aircraft doesn't need to rely upon lift to gain altitude but can simply rely upon its excess thrust.  Neither does the Earth's escape velocity need to be exceeded to reach high altitude by this method; as long as the thrust exceeds the weight, the aircraft will climb and with more thrust the aircraft will climb more quickly.  The current altitude record for jet aircraft (taking off from the ground) is 37,650 meters (123,523 ft), set in 1977 by the E-266M, which was a development version of the Russian MiG 25.  Newer fighters have broken various time-to-altitude records i.e. by climbing to a particular hight more quickly, but none have yet broken the absolute altitude record for air breathing jet aircraft set by the E-266M.
 

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Why do planes stall when their wings reach a certain angle?
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