Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: refutation of the scoop theory, revisited



Read our article. We discuss ground effect.
http://www.aa.washington.edu/courses/aa101/lift.htm>

David Anderson
dfa@fnal.gov




On Sun, 22 Aug 1999, brian whatcott wrote:

At 13:46 8/21/99 -0500, David Anderson wrote:
... we assume that the entire weight of the
plane is supported by the reaction force of diverting the air down and
that this can be calculated with Newton's second law:

F = dp/dt or F = dm/dt *V,

where V is the vertical velocity of the air. For the sake of our
order-of-magnitude calculation we assumed:

mass of plane: 1045kg (2300-lb weight)
Speed: 61 m/s (120 knots)
Angle of attack 4.3 degrees
This yields a value of V AT THE WING of 4.6 m/s
We take half that value as the average V. (V=2.3 m/s)

We have:
dm/dt = (1045 kg * 9.8 m/s/s) / (2.3 m/s)
dm/dt = 4450 kg/s
....
David Anderson
Dfa@fnal.gov



Continuing with this 'teaching' model, let us say that air at STP weighs
1.293 kg/m^3 and so the requisite mass rate implies a volume
affect of 3442 m^2 per second, hence at a forward speed of 61 m/s the
modelled cross section of
downturned air could be 56 m^2.

If we imagine the modelled plane had a span of 7 meters tip to tip
then a possible value for the vertical extent of the downthrust air
'window' (limited to the span) would be 8 meters.

Now it happens that it is found experimentally that certain values of
lift and drag of ordinary airplanes start changing when they approach
within a wingspan of the ground, so this represents a (very) weak
observation that is not in dissonance with Anderson's cheap and
cheerful model.

I expect it is this sort of calculation that gives rise to strong
disagreement.
brian whatcott <inet@intellisys.net>
Altus OK