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Teacher needs help: In Plane Crash, Loss of Momentum Still a Mystery."

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Teacher needs help: In Plane Crash, Loss of Momentum Still a Mystery."

Old 15th Feb 2009, 05:33
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Teacher needs help: In Plane Crash, Loss of Momentum Still a Mystery."

http://www.nytimes.com/2009/02/15/ny...5crash.html?hp reports that flight 3407 crashed on approach, with the headline "In Plane Crash, Loss of Momentum Still a Mystery."

I am an Australian high school physics teacher and my heart goes out to all the families involved. However, my small group of physics students will want to discuss this incident on Monday - their current unit of work is aviation based, and a number of them want to go onto a career in aviation.

Would you please critique my analysis of a possible scenario, based on newspaper reports to date. I want to introduce linear momentum, angular momentum and conservation of momentum.

Information in current news reports (15/2/08):
The aircraft involved in the accident was a Bombardier DH8.
It was on approach in icy weather.
It crashed in a flat attitude, with very low horizontal velocity, facing backwards along its track.

My interpretation based on readings suggested in other PPRuNe posts, the Nasa video on tail icing and other websites on stalls/spins:
The aircraft entered a spin of some description, and as a result its forward, linear momentum, was transferred into angular momentum. The linear momentum didn't magically vanish, but the difference in airflow over each wing caused a turning moment due to friction force acting more on one wing than the other. As a result, the forward motion of the aircraft was translated entirely into rotational motion, and the effective ground speed quickly became zero.

I am not trying to preempt, or second guess the air accident investigation. I'm not going into any detail of what the passengers experienced in the accident, so I'm not looking at the details of how a tail stall can cause a departure from control. I'm just looking at an analogue of the situation where a model aircraft hits a pole with one wing and spins in horizontal plane. In that case, the plane wouldn't orbit the tree, it would retain some linear momentum. Would any real create a force feedback system where the wings work to 'draw in' any linear momentum and turn it ALL into angular momentum?
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Old 15th Feb 2009, 17:52
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It's pretty difficult to use any kind of aircraft dynamics (whether a crash or a simple manoeuvre) to illustrate basic kinetics such as momentum, because the "laws" about conservation of momentum apply to a closed system, and an aircraft is an open system, with momentum being in effect continuously exchanged between the aircraft and the atmosphere.

So, for example, for entry into a spin (of the classical, training, type, not an accident) the aircraft would start by slowing down (by reducing engine power, so that the drag of the air slows the plane down). Then, at a low enough speed and high enough angle of attack you'd pull back on the stick (increasing angle of attack, more drag, slowing down faster) and kick the rudder to start the aircraft yawing. If it goes correctly the yawing motion and pitch up combine to create a rolling motion where one wing stalls more than the other, and you'll get a three axis rotation (pitch roll and yaw). Because the aircraft is now in a "disroganised" state aerodynamically, the drag will be huge and the plane will lose most if not all of its forward velocity. Its also not generating much lift, so will start to drop, and the motion will become a fairly steep descent. The aircraft will keep gyrating as it falls as long at the controls are positioned correctly, and the forces to keep it spinning against the natural drag of the air are extracted by means of lift generated on the different bits of the aircraft.

So it's not really a case of converting one form of momentum to another. Bear in mind too that the air around the aircraft is being disturbed too, and the whole closed system is conserving momentum.

Sorry if thats not very helpful or encouraging, but you've picked a pretty tough (though Ill agree interesting) way to illustrate mechanics.
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Old 15th Feb 2009, 17:57
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Doubly Ugly, you refer to the recent accident in Buffalo.
I would suggest caution in any ‘momentum’ analysis as the facts leading to the accident are far from clear. The aircraft was flying the approach in icing conditions, but beyond that there is only speculation.
A tail stall is a rare and somewhat unique problem in some aircraft, which should not normally be encountered when flying within accepted safety boundaries. A tail stall would not necessarily (IMHO most unlikely) result in a spin; ‘conventional’ or otherwise. However, this possibility should be excluded, but if encountered would probably result in a high vertical descent rate. A flat spin is a rare form of spin with results similar to your description, but involving all 3 axes. I do not know if the aircraft type is susceptible to a flat spin. The reported facts add little support for any of these outcomes.
From a physics aspect, this and similar accidents involving icing might have more relevance with super cooled liquid water droplets transitioning to ice when striking the aircraft.

Re momentum, see Chapter 18 Stalls and Spins.
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Old 15th Feb 2009, 18:05
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I think is is brilliant that you are trying to use practical, real events to make physics more interesting. There was an excellent article in Saturday's Telegraph by James May advocating the use of Scalectrix racing to teach science in schools.

Link to the article is http://www.telegraph.co.uk/motoring/...calextric.html

It might be a bit easier than an aircraft stalling and spinning.
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Old 15th Feb 2009, 21:28
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I have nothing to add as far as your classroom physics go but I would like to congratulate you on the time and effort you put into the kids in your class. If 50% of teachers were as dedicated as you it would be a different country!
Nice work.
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Old 15th Feb 2009, 21:52
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Originally Posted by Basil View Post
As an operational pilot, all our procedures permitted some ice/frost on lower surfaces but, of course none at all on the upper surfaces.
Now, since, on a conventional aircraft, the tailplane or horizontal stabilizer produces a downforce should not our procedures have permitted frost on top but required a clean surface underneath the tailplane?
Logically, yes, the lower surface of a tailplane is likely more critical than the upper, and so you'd be more concerned about that.

However, there are reasons to allow no frost at all on a tailplane. Firstly, how did it get there? There's no cold fuel, so it's can't be due to that. And if it's the airframe that's cold, then any frost on one side may mean fuel on the other. So having seen some frost, how sure are you that there's no more? Finally, there are times you need control power in both directions on a tail, which means frost on either side might be bad news.

The allowances for ice/frost on lower wings are really there because of the difficulty of preventing it when there's cold fuel. As an aerodynamicist, I'd like there to be none, period. But the practical risk of non-environmental frost is low enough to grit your teeth and bear it. (Note that here again, it's only non-environmental fuel frost - if the conditions themselves are frosty, it's usually not allowed, for the same "if its there it could be anywhere" reasoning)
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Old 15th Feb 2009, 21:53
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Old 16th Feb 2009, 04:42
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The aircraft entered a spin of some description, and as a result its forward, linear momentum, was transferred into angular momentum.
Whoa! Stop there. Let's take Occam's razor to this. Allow no other factors than the conversion of one...'display' of an aircraft's energy into another.

IF, this aircraft did suffer an conventional stall, much of it's forward energy would be lost before it hit the ground. I very much doubt that it would be converted into angular momentum, but dissipated into the air. Some of this would be immediately converted into heat, some of the remainder, would be expended by disturbing a mass of air, altering the water/vapor ratios, then more slowly converting into heat.

The rest, I would think a small proportion, would be converted into angular momentum.

Imagine this aircraft in space. A lamentable 134kts relative to a marker buoy. Now by some mysterious force, convert all of its forward energy into angular momentum and see just how fast the aircraft would be flat-spinning.
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Old 16th Feb 2009, 09:37
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Thanks for all your replies folks. We worked on "hunt the momentum" instead. It was a pretty successful lesson, and I was pleased to see that the kids were able to place the air as the other partner in the Newton pairs. We did spend some time talking about the victims and what they went through. Sometimes the laws of physics aren't fun after all.
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