Light pole vs. plane wing

[phunk]

lenbrazil said:

phunk said:

I think you're wrong about levers there, it's linear not exponential. But the point is still the same, takes much less force to break it free if you apply the force to the top of the pole.

Are you sure, I thought it was based on the length of the arc which increases exponentially??

Circumfrence is directly proportional to radius, not exponential. c=2*pi*r

[jaydeehess]

lenbrazil said:

papageno said:

For lenbrazil, the force in an impact is estimated from the change in momentum, non kinetic energy. (See, for example, classical kinetic theory of gases.)

[/i]
[/quote]
Do you have enough data to do some calculations? Presumably the poles would have a negligible impact on the impact of the plane. Would the relative force of the impact be the same?[/quote]

Momentum is mass times velocity
Force is mass times acelleration OR change in momentum over the time involved.

You state that it was established that the force on the wing was 32000N. Lets assume that the wing was in contact with the pole for 15 feet of travel(afterwhich the pole is either bent or the wing is pretty much past the pole anyway)
At 530 MPH it would have been in contact with the plane for 0.02 seconds.

Now the change in momentum is 32000N X 0.02sec =640Kg-m/sec
530MPH= 235 meters/sec
aircraft mass is about 140,000 Kg
The pre-lightpole impact is 235 X 140000 = 32900000 Kg-m/sec

So the change in the aircraft's momentum would have been 0.002%

Someone please ckeck my math, but such a small change in momentum is hardly going to be felt. Since one can assume the mass of the plane remains constant through the impact with the pole a 0.02% change in momentum means a 0.02% change in velocity. At 530MPH that is a reduction in velocity of about 0.1 MPH

[Eddie Hitler]

I think this all comes down to inertia. The wing chord - ie the distance from the leading edge of the wing to the trailing edge, for a Boeing 757 is probably in the order of 4 - 5 metres at the wing root (somone might be able to firm that up a bit...). This is not a hollow structure. It has frames, the main wing spar (a very large, very strong flexible aluminium girder), the skin, loads of hydraulic and service ducts and plumbing, electrical wiring. It is actually a hugely strong structure which is designed to be flexible under the various loads imposed whilst strong enough to lift the aircraft, weighing about 50 -60 tonnes. A Lamp post (what we call them back in the old country!!!) is a steel tube with a wall thinkness of about 1/16 - 1/8 inch steel, and a maximum diameter of no more than about 4-5 inches on average. Even the average family car can knock one over. They tend to be made of tin foil thick steel. A Boeing 757's wing, even if you knocked a lamp post with one in some sort of freak accident, wold probably only sustain the most minor of damage. This one was doing 400 knots when it hit the Lamp post. I'm quite glad in a way that Lamp posts are designed to be accident friendly. Imagine hitting one which didn't bend in your car?

[PhantomWolf]

A Lamp post (what we call them back in the old country!!!) is a steel tube with a wall thinkness of about 1/16 - 1/8 inch steel, and a maximum diameter of no more than about 4-5 inches on average.

Um, most are aluminium or fibreglass these days.

[echnaton]

Um, most are aluminium or fibreglass these days.

Not where I live. Here steel rules, even in the new decorative poles.

[jaydeehess]

Ok, inertia.




I_{(moment of inertia)} of a long thin column in this case.
A lamp post in question is about 12 meters tall and weighs in at about 250 Kg so
I=(250 X 12²)/3
I = 36000 Kg-m²

The torque on the lamp post is
T=r X F where r is again the length of the post and the force is that applied by the 32000N impact with the wing.
T=12 X 32000
T= 384000 Kg-m²/s²

T=I X a Where a is angular acceleration

384000= 36000 a
a= 10.7 radians /s²
Once again , the wing is in contact with the post for only a fraction of a second 0.2 s

so the rotational velocity is 2 radians/sec an instantaneous velocity at the top of the post of about 24 m/s (86 Km/h)

That is if I am recalling my first year physics from 30 years ago correctly

[Eddie Hitler]

Lampposts are made of steel in England. It's either because it's cheaper or because they are being used by aliens as a secret base to plan the invasion of the moon.

Not sure which...

[Eddie Hitler]

Actually, there's a concrete one outside my Mum's house. That would be REALLY emotional if you hit that with your car...