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The Impossibility of Flying Heavy Aircraft Without Training



 
 
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  #331  
Old March 8th 06, 11:12 PM posted to rec.aviation.piloting,rec.aviation.homebuilt,rec.aviation.student
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Default lift, wings, and Bernuolli

A jet of water merges from a hose pipe of a cross sectional area 5x 10^-3 m^2 and strikes a wall at right angles. Calculate the force on the wall assuming the water is brought to rest and does not rebound. (Density of water = 1.0 x 10^3 Kg m^-3)

After explaining the simple calculation which gives a force of 45N it goes on to say; " in practice the horizontal momentum of the water is seldom completely destroyed and so the answer is only approximate."


Is this a US book? This is why the US lags in science.

The original question is ok (after all, in physics we use cylindrical
cows, frictionless surfaces, and point masses). But the comment at the
end is very misleading. The momentum is never "destroyed". It is
actually transferred to the wall, and thus to the earth. What they are
probably trying to say is that there is usually some rebound of the
water, and it sprays all over the place rather than becoming embedded
like machine gun bullets in sand... which would have been a better example.

Any changes to the entire earth as a result are insignificant.


Depends whether you are trying to understand the fundamental physics or
just trying to calculate an answer.

Jose
--
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  #332  
Old March 10th 06, 01:07 AM posted to rec.aviation.piloting,rec.aviation.homebuilt,rec.aviation.student
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Default lift, wings, and Bernuolli

On Wed, 8 Mar 2006 at 23:12:19 in message
, Jose
wrote:

The original question is ok (after all, in physics we use cylindrical
cows, frictionless surfaces, and point masses). But the comment at the
end is very misleading. The momentum is never "destroyed". It is
actually transferred to the wall, and thus to the earth. What they are
probably trying to say is that there is usually some rebound of the
water, and it sprays all over the place rather than becoming embedded
like machine gun bullets in sand... which would have been a better example.

I have got to be quick here - busy the next couple of days!

Their statement may be a bit sloppy but the fact is that because the
wall is very rigid and firmly fixed the ground an accurate calculation
of the force can be made by assuming the momentum is destroyed. The
effect on the earth is so small that it is minuscule compared to the
practical result.

Any changes to the entire earth as a result are insignificant.


Depends whether you are trying to understand the fundamental physics or
just trying to calculate an answer.


I thought the basic principles have been stated and stated! It is
whether or not we have a clear understanding of lift or not that seems
to be going around in circles. What happens to the earth a long way away
from the aircraft is, for all practical purposes insignificant and not
worth worrying about. It certainly does not prove that down wash has
nothing to do with lift for example as has been stated.
--
David CL Francis
 




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