Re: upwash contributes lift

[John Denker <jsd@MONMOUTH.COM>]

At 03:20 AM 8/22/99 -0700, William Beaty wrote:
> I don't understand your reasoning.  In my opinion the "lift/upwash"
> process probably violates Newton's laws.  Here's why I think so:
>
> If an airplane flies into still air,

Let's be careful here, reminding ourselves that the "still air" is in the
far field, not right next to the wing.

> then the parcels of air are not
> moving initially.  The wing approaches, and upwash begins.

OK so far;  in the near field there is upwash.

> When upwash
> first begins, a parcel of air must be accelerated upwards, and this can
> only occur if there is a force-pair between that parcel of air and some
> other object or parcel of air.
>
> Why does this parcel of air initially start moving upwards to become
> upwash?  That's what I need to know.

OK.  Right.  Understanding upwash is crucial.

> If the wing somehow causes the
> initial upwards acceleration of the air-parcel, then Newton's laws require
> that a force-pair exist between that parcel and the wing.

Now this statement appears to gloss over the distinction between direct and
indirect effects (i.e. direct and indirect force-pairs).  The distinction
appears to be very important to those who like to play nit-picky word
games.  This statement is so open to misinterpretation that I will not
comment on it.

I will say this:  Surrounding the wing there is a velocity field and a
pressure field.  These fields are caused in part by the presence of the
wing and in part by the presence of the surrounding air which is a fluid
with density and pressure.  These parts combine in nonlinear ways to create
pressure and velocity fields that are consistent with all of Newton's laws.

> If such a
> force-pair exists, then when the parcel of air is initially accelerated
> upwards to become upwash, the wing must experience an equal downwards
> force and accelerate downwards.

That is false, as the trampoline analogy clearly shows.  If one has
something else to push or pull against, one can get momentum on the rebound.

Air parcels need not travel all the way to the surface of the earth before
rebounding.  They can rebound off the air below (or indeed above) the wing.
Remember the air is a fluid with density and pressure everywhere, not a
bunch of loose particles.  The distinction is important, as discussed in
  http://www.monmouth.com/~jsd/how/htm/airfoils.html#sec_fluid

> If the wing does NOT cause the initial upwards acceleration of the
> oncoming parcel,

Again, does the word "cause" refer to direct or indirect causation?



> So, has the upwash now contributed to the lifting force?  No, because we
> don't know what caused the parcel of air to initially accelerate upwards

This is really poor logic.  It says if "we" don't know the cause of the
effect then "no" the effect doesn't exist.

The absence of knowledge about X is the the same as knowledge of the
absence of X.

And please speak for yourself, rather than saying "we" don't know.  Some of
"us" do know where upwash comes from.