# Re: The actual running Archie paper...

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John V Jackson wrote:

> "...thrust is the only force that exerts work on _Ax_ along its entire
> take-off run..." ...well, all right, but...

They're using the mathematical description of work.  No vertical movement, no
vertical work.  That doesn't mean no energy has been expended.

> The text to figure 1 included: "During the downstroke the airfoil aligns
> itself to the upcoming airflow by rotating aeroelastically, trailing edge
> up,..."  ...but get this... "...owing to the pressure build up between the
> wing and the ground."

This puzzled me too.  The animal is operating in a speed regime where air is
essentially incompressible.  Also, Japanese aerodynamicists demonstrated back in
the 30's that ground effect often reduces lift rather than increasing it, and
quantify the effect.  I can post the reference if any of you want to see it.

> However they describe exactly the same thing happening in the horizontal
> direction, indicating wing thrust as progressively growing as the bird
> gathers speed.  I don't think so!

In my planes (and all planes with a fixed pitch prop), thrust decreases with
increasing airspeed.  Maximum thrust occurs when you are sitting still prior to
starting your takeoff roll.  In my strip model for calculating pterosaur wing
forces, thrust also decreases with increasing airspeed unless the kinematics are
adjusted to increase thrust relative to lift.  But fairly early on, for both
pterosaurs and birds, diminishing returns set in and thrust again starts
declining with increasing airspeed.

> If you've got to push against something that's actually going backwards
> relative to you, you can't push so hard as you accelerate.  That's why
> rockets go faster than jets.

John, this last went over my head.  What do rockets and jets push against thats
going backwards relative to them, and what does it have to do with their
relative speed?  It sounds like you are saying that rockets and jets move by
pushing against the atmosphere or their own exhaust gases, and of course, we all
know that's not the case.  Would you rephrase it for me so I can be sure what
you mean?Jim

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