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martinboehme

AoA, stall, and weight...oh my!

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I'm having a hard time putting together a couple of facts I've read separately. First, I know that every wing has a critical angle of attack above which laminate airflow ceases and it stalls. This AoA is unaffaceted by weight, air density, temp, wing loading, and anything else one can think of. On the other hand, stall speeds vary with real weight or wing loading (such as conferred by extreme manuevers). For a given aircraft, the lighter it is the slower the airspeed at which it will stall and the heavier it is(real or through g-force) the higher speed it will stall. Why is this? For example, assume a pilot is flying straight and level at a fixed AoA relative to the relative wind at gross weight and her aircraft stalls at (say) 65kts. After burning much fuel, her aircraft weighs less and she could slow to 60kts before stalling in this attutude. Doesn't 60kts imply that she could raise the nose and AoA a little higher before disrupting the laminer flow---violating the principle that the critical AoA is constant in all conditions? What am I missing

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Here is a very basic explanation.The point you are missing here is that an airplane has a given amount of lift for every AoA and speed combination. For an example lets say an airplane currently weighs 1500 lbs. with a gross weight of 2000 lbs. and is flying at 60 knots with a 5 degree AoA producing a straight and level flight path. This would mean you are creating roughly 1500 lbs of vertical lift component (forget induced drag for now) to keep you at the same altitude. Now when you load the airplane to the 2000lbs gross and fly at the same specs 60 knots 5 degree AoA you still only produce 1500lbs of lift, 500lbs below what you need, this means you will descend. In the lift equation ( L = Cl * A * .5 * r * V^2 ) there are three variables you can change as the pilot; wing area (flaps and slats), Velocity, and Coefficient of lift (AoA has a big part in determining CL). Keeping the same wing config. you can increase your speed to produce more lift, or which is more common is to increase your AoA and this will get you closer to the stall critical AoA or past it for the weight.

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I believe that you will find that the attitude has change as the weight changed and the attitude assumed to maintain 65 knots, loaded, is the same as the attitude required to maintain 60 knots, light. R-

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Thanks guys. Cessna, I think I understand your point but let me tackle it a bit further. I understand that AoA and speed produce lift and that one could trade speed for AoA (and vice versa) to acheive the same lift. Further, a heavier aircraft needs more lift, thus one or both must increase to maintain straight and level flight if weight has increased. Failure to supply sufficient lift produces a descent not a stall however. A stall can only be produced by exceeding the critical AoA, which doesn't vary with weight, temp, speed, or anything else. I also understand that, at heavier weight, a pilot might be more likely to increase the AoA to the point of stalling. But that would simply be pilot mismanagement, not an inherent property of the aircraft or a principle of flight, wouldn't it? Maybe I'm making a simple thing too complcated, but I thinks what is puzzling me is the apparent contradiction that says that the AoA at which an aircraft will stall has nothing to do with speed and weight (i.e., depends only of the angle with which the wings meets the relative airflow) yet the speed at which an aircraft stalls (i.e., exceeds the critical AoA) will vary with weight. Rob seems to be saying that weight will affect attitude, but that seems a tangential issue if I understand correctly. Sorry I'm so dense.

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>Thanks guys. Cessna, I think I understand your point but let>me tackle it a bit further. I understand that AoA and speed>produce lift and that one could trade speed for AoA (and vice>versa) to acheive the same lift. Further, a heavier aircraft>needs more lift, thus one or both must increase to maintain>straight and level flight if weight has increased. Failure to>supply sufficient lift produces a descent not a stall however.>A stall can only be produced by exceeding the critical AoA,>which doesn't vary with weight, temp, speed, or anything else.> I also understand that, at heavier weight, a pilot might be>more likely to increase the AoA to the point of stalling. But>that would simply be pilot mismanagement, not an inherent>property of the aircraft or a principle of flight, wouldn't>it? Correct>Maybe I'm making a simple thing too complcated, but I thinks>what is puzzling me is the apparent contradiction that says>that the AoA at which an aircraft will stall has nothing to do>with speed and weight (i.e., depends only of the angle with>which the wings meets the relative airflow) yet the speed at>which an aircraft stalls (i.e., exceeds the critical AoA) will>vary with weight. Rob seems to be saying that weight will>affect attitude, but that seems a tangential issue if I>understand correctly. Sorry I'm so dense.This is the hard part and is beter explaned futher with vector visuals to help demonstrate what is going on. You seem to have a good grasp on what is happening because you were able to connect all the information in the first paragraph.Here is a good place to start:http://wright.nasa.gov/airplane/lifteq.htmlI didn't read it completly so I don't know if it goes into the stall issue. Another great book is Flight Theory for Pilots by Charles E. Dole.

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Thanks again Cessna. I enjoyed the article. Your are correct however, it doesn't address stall. My question was how to reconcile two seemingly contradictory assertions (found in separate locations in this month's AOPA Pilot Training magazine): 1. Stall is due entirely to exceeding the AoA and has little to do directly with weight or speed (a point also made repeatedly in Stick and Rudder by W. Langeweische), and 2. Stall speeds (on the airspeed indicator and POH) are given for max gross weight because they are slightly lower at lighter weights.

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For statement 1; that is true but I believe more for extreme flight outside the normal flight envelope. For statement 2; is more inline with what many of us are taught when we get our licenses to fly your speeds because in normal conditions the speeds will most likely keep you out of trouble, and when it does all go haywire to remember to lower the angle of attack to the realitive wind to recover from the condition.

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G'day,You can't see the forrest for the trees. Individually all of what you are saying is basically correct except the breakdown of laminar flow is dependant on angle of attack only! Airspeed has nought to do with stalling. In your post what you do not realise is that in both the 60kts and 65kts examples you quote the aircraft wing is at the same angle of attack (ie stalling angle) Take a very simple example. Two identical aircraft formation flying side by side, straight and level at the same airspeed. One aircraft is loaded to max gross weight the other is much lighter.If these aircraft are at the same IAS then how does the heavy aircraft produce the greater lift it requires for support? Answer - the heavy aircraft is flying at a HIGHER angle of attack than the lighter aircraft.OK if both pilots now start reducing throttle and slowing down then as they slow both aircraft will have to start increasing their angle of attack in order to maintain lift (as a constant) for straight and level flight. (due to the decreasing IAS ). But at any given IAS the heavy aircraft will always be at a higher angle of attack than the light aircraft.If both aircraft continue reducing power (increasing angle of attack) then eventually the HEAVY aircraft will reach it's stalling angle of attack and the aircraft will stall. Note: at this moment the lighter aircraft will be flying because its angle of attack has not yet reached the stalling angle.Thus the lighter aircraft is able to slow a little more before it then also reaches the stalling angle and stalls.Thus for a given aircraft weight does NOT affect the stalling angle but will affect the IAS at which the aircraft stalls.Cheers,Roger

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>G'day,>>You can't see the forrest for the trees. Individually all of>what you are saying is basically correct except the breakdown>of laminar flow is dependant on angle of attack only! >Airspeed has nought to do with stalling. In your post what you>do not realise is that in both the 60kts and 65kts examples>you quote the aircraft wing is at the same angle of attack (ie>stalling angle) Yeah they are at the same angle of attack and speeds but different weights which would produce a descent in the heavier aircraft and it would be opperating on the 'backside of the power curve' and in a descent in nose high attitude. I don't know where I was saying either aircraft was stalled. I was describing the situation where the pilot will mistakenly raise the nose and then pass the critical AoA instead of increasing speed to match the lift vector to that of gravity.

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>Thus the lighter aircraft is able to slow a little more before>it then also reaches the stalling angle and stalls.>>Thus for a given aircraft weight does NOT affect the stalling>angle but will affect the IAS at which the aircraft stalls.>Click! I get it Roger! In other words: Any given (including the critical) AoA is reached at a slower speed in a lighter aircraft. Seems obvious to me now. This is how airspeed affects stall but stall depends on a critical AoA alone and this critical AoA does not depend on airspeed (or other things). That also makes some of what Cessna was saying clearer to me. Thanks to you and Cessna for all the help. You guys are great resources.

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G'day Cessna Flyer,my post to cowlflaps original post>>You can't see the forrest for the trees. Individually all of>>what you are saying is basically correct except the>breakdown>>of laminar flow is dependant on angle of attack only! >>Airspeed has nought to do with stalling. In your post what>you>>do not realise is that in both the 60kts and 65kts examples>>you quote the aircraft wing is at the same angle of attack>(ie>>stalling angle) >your reply to my post>Yeah they are at the same angle of attack and speeds but>different weights which would produce a descent in the heavier>aircraft and it would be opperating on the 'backside of the>power curve' and in a descent in nose high attitude. I don't>know where I was saying either aircraft was stalled. I was>describing the situation where the pilot will mistakenly raise>the nose and then pass the critical AoA instead of increasing>speed to match the lift vector to that of gravity.Sorry mate there is confusion. My reply you quoted above was to the original posters post (cowlflaps) and not to your post.Cheers,Roger

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> Failure to supply sufficient lift produces a descent not a stall however.Thought I should point out that if you're continually supplying less than sufficient lift, this will produce not just a normal descent with constant rate but an _ever increasing_ rate of descent. The reason is that the forces on the aircraft aren't balanced -- there's more pulling down on the aircraft than pulling up on it -- so there's a net force acting downwards, which will result in an acceleration downwards. In a normal descent with constant rate, lift and weight are balanced.Martin

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