Re: Battleship downforce and JumboJet downwash.

[brian whatcott <inet@INTELLISYS.NET>]

[Brian]
> > Though pilots notice distinct floatation effects in low wing planes when
> > landing, it is usual to model an effect on lift for any plane when height
> > above the ground gets less than about one wingspan.
At 15:26 12/11/99 -0800, Bill Beaty wrote:
> I don't understand.  Do you mean that we traditionally explain only the
> lift during ground-effect mode, but not the lifting force created during
> flight at high altitudes?   I'm discussing a bumblebee which hovers at
> many wingspans height.

Flight simulators change the coefficients controlling flight dynamics
within a wingspan of the ground. This effect is inadequate to moderate a
600 fpm rate of descent to a greaser however.
 In light low-wing airplanes, students are alerted to the tendency to
'float' along a runway.

Your example of a bee hovering at 10 cm was not quite comparable -
I suppose this might represent 5 wingspans or so?
 Here (certainly in contrast to an airplane at several thousand feet),
 you observe a direct ground reaction from the Bee's wings, but it is
 probable the bee does not get much relief, as high as this.

> ... The air
> launched by the bee might never reach the ground, but its momentum would
> still be communicated to the ground.

Think of the air commotion as a warming, with a consequential
pressure rise.   This is a question of a statistical approach rather than
visualizing individual pool balls in collision.

> Ah, I know where temperature comes in.  In the long run, the jet of air
> must collide with the ground inelastically. ///

I think you are on shaky ground with this idea of an inelastic collision.
 The statistic mechanics approach ( and I think Einstein was involved?)
models gas pressure with the force associated with the change of momentum
provided by molecules reversing speed elastically at a wall - treated
simply in three axes.

> /// is the physics of a hovering
> helicopter radically different than the physics of a hovering Harrier jet?
> (Some say that the helicopter flys by pressure-difference, and it need not
> fling any air downwards.)

> William J. Beaty

This brings us neatly back to a preceding topic - wherein it was related
that Feynman had some conceptual difficulties with a concept that you at
 least proposed to work in the correct direction - I speak of the water
jet, and its reversal when water is sucked back.

I bring this up, because one of the scholarly papers Bob Sciamanda pointed
 out in the AJP refered to a comparable effect. It goes like this.

Fancy a water hose playing into a barrel of water.
You notice the water is rotating, and the water hose appears to be reacting
little force. You see ( though perhaps Feynman couldn't) that the reason so
little force was acting, was because there was little change of momentum by
the water exiting the hose - its delta v was low - the water was already
 moving. This was mentioned in one of those papers as a confusing factor.

 Now you visualize a helicopter in hover (not in ground effect for
simplicity, just in hover high up)   The downwash air in the rotor wash
soon circulates and rolls back in on top.   Reduced change of momentum.
Increased power required to hover - compared with forward movement.
 You see a similarity with the water barrel, I'm sure.

This is not to say a Harrier in hover cannot rebreathe its efflux -
but I think the effect is less pronounced.

Brian


brian whatcott <inet@intellisys.net>
Altus OK