Stefan Jeglinski says:
<begin quote>
I don't think that anyone would argue that my trolley guy is not using
a reaction engine as that word has been used in this thread. From this
standpoint, the wing does the same thing (reaction engine) as per item
number one above. OTOH, I think John's argument is that ultimately, the
wing stays up because the air eventually hits the ground. And indeed
that seems to be a consideration. But in the same way that my guy need
not hit the wall with the ball to go, I see this not as a requirement
but only as a consideration (in agreement with Michael Edmiston I
think).
<end quote>
I agree. If John is claiming that the wing is not a reaction engine
because it is the air hitting the earth that is responsible for holding
up the plane, then I have a problem. In Stefan's trolley example it is
not the hit on the wall that makes the trolley go. Just as Stefan's
ball would not have to hit the wall, John's downward-directed air would
not have to hit the earth. Yes it happens, but that's not what holds
the plane up.
Do we have a problem with our definition of a reaction engine? A car
accelerating from a stop sign might be a reaction engine if we view it
as grabbing roadway ahead of the tires and throwing it behind the
tires. We would probably be reluctant to do this because our usual
thinking about a reaction engine includes the idea that the matter
"thrown out" is not connected to anything, and the roadway is certainly
connected to the earth. [Unless we are spinning our tires and throwing
stones like my son occasionally does. Perhaps that makes him think he
is piloting a rocket rather than an ordinary car.]
Furthermore, many examples of reaction engines "throw out" matter that
used to be part of (or carried with) the vehicle. If this is important
then I suppose we might want to define a reaction engine as a device
that obtains its thrust by propelling, in the opposite direction, mass
that used to be part of itself. In this case, rocket exhaust and
bullets and baseballs probably all qualify, but the air through which
an airplane flies does not qualify.
Where do we draw the line? Personally I am inclined to think that the
"exhaust" emanating from a reaction engine ought to emanate as "free
matter," but I am not inclined to require that it used to be part of
the vehicle. In that case the air thrust downward by the wing
qualifies, but the roadway thrust backward by the car does not qualify.
But I can certainly imagine other people might prefer a
reaction-engine definition requiring that the exhausted mass was at one
time on-board the vehicle.
However, please note this... jet engines do not qualify as reaction
engines if we require that the exhausted matter was once carried by the
jet. Yes, the jet-fuel is carried on board, but the oxidizer is not.
As the jet fuel combusts to make CO2 and H2O, it is easy to see that
the vast bulk of the exhausted "burned material" is oxygen. But even
more than that, the exhausted gas is mostly nitrogen. So, the matter
propelled out the back of a jet engine is mostly air... air that came
from outside the craft... air that was sucked in, compressed, heated,
and thrown out the back... in many respects just like the air that was
sucked and propelled downward by a wing. In fact, part of the
operation of the jet engine involves hundreds of "wings" that make up
the turbines that inhale and compress the air prior to ejection.
So I am inclined to think: if a wing is not acting as a reaction
engine, then neither is a jet engine. But I much prefer to say they
are both reaction engines.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817