Re: pteros have lift-off

[David Peters <davidpeters@att.net>]

Sounds like you've done your homework!  Just a question or two buried  
in the text:
On Jan 14, 2009, at 11:11 PM, jrc wrote:

> Been away from the computer -- just got back.  Re concern about  
> wingtip on downstroke after launch hitting the ground, for talking  
> purposes let's take a Qsp glenoid as the reference rather than cg  
> so that we won't have to worry about the vertical shifts in cg  
> relative to the glenoid during the launch and flapping strokes  
> (animal's cg lowers relative to glenoid during the ground based  
> part of the launch).  Note that I'm not implying that the  
> velocities and coordinates calculated below are actually known to 3  
> decimal places...... :-)
>
> A 20 Kg Qsp carrying a 15% tail upload in level, steady cruise  
> flight at a height above ground (HAG) of 5 feet will flap with a  
> beat period of about 0.6 seconds (frequency 1.667 Hz) doing about  
> 31.58 miles/hour (46.3144 fps). The wingtip flapping amplitude will  
> be about 4.02 feet, with 1.926 feet of that being up above the  
> glenoid, and 2.095 feet being down below the glenoid.


If Qsp has a wingspan of 19-20 feet (is that close?) your figures  
indicate an angle of 11º more or less above and below the horizon  
with a total of 22º. (let me know when I'm substantially off). That's  
not a lot considering that it has much more range to work with. After  
all, at rest the wings are nearly parasagittal, at 80-90º, so they  
can get there if they want to.

> Now, at a launch to that cruise speed, if the glenoid path at the  
> time the manus lifts off is inclined 30 degrees above the  
> horizontal, then the glenoid is rising at a vertical speed of  
> 23.157 fps,


Rising at a vertical speed of 23 fps, on that 30 degree angle means  
rising at a ground speed of 43.3 fps and an airspeed (the hypotenuse)  
of 48 fps or 33 mph. (am I close?) That's a pretty fast initial  
launch speed, much faster, or so it would appear, than what I'm  
seeing in the Albatross launch sequence. And much faster than the  
first second of flight in a pelican or swan, or so it seems. I  
realize that being shot out of a cannon, so to speak, in the case of  
pterosaurs, is a different deal than a running start, as in birds. I  
see in Desmodus " the animal accelerates to a mean take-off velocity  
of 2.38±0.24ms-1." or translated: 5.32 mph or 7.81 fps. and at that  
rate experiences "Peak vertical force can reach 9.5 times body weight  
in approximately 30ms." So if Qsp. is accelerating at roughly 3x  
Desmodus, does it reach a G factor of nearly 29x? Or am I screwing up  
the math?

> and the time it takes the glenoid to reach its maximum height of  
> 8.33 feet above glenoid liftoff height on a ballistic path is 0.720  
> seconds -- this gives the animal enough time to perform a complete  
> upstroke and finish the first downstroke as the shoulder reaches  
> its ballistic peak.

Mike mentioned that wing unfolding at the metacarpophalalangeal joint  
would be delayed somewhat as the manus lifted off the ground to a  
height great enough to enable this. Even so, some lift was generated  
by the innner wing. So, shouldn't this delay and less than optimal  
lift be factored in at the initiation of launch?

> As I mentioned above, at that point in time the glenoid has risen  
> to a point 8.33 feet higher than it was when the manus lifted off,  
> and the WINGTIP at the BOTTOM of the first downstroke is 8.33-2.10  
> = 6.23 feet HIGHER than the GLENOID was at the time of manus liftoff.

And therein lies the problem, because the wing at lift-off starts 80º  
BELOW the bottom of your downstroke. Maximum lift does not occur  
until the wings are fully deployed, so this initial launch has  
trajectory that begins to fade (like all ballistics) immediately  
after launch and must employ the first downstroke before reaching  
that critical wingtip strike height. The question is... will it?

The major questions I would have at this point is 1) why such a small  
amplitude wingstroke at take-off? 2) is the G-force at launch similar  
to what you calculated? and finally 3) does this work with a  
pterosaur with roughly a 3x longer wing. It would work, I suppose,  
with a 22º wingstroke, but is that close to reality? Or just a fudge  
factor to make sure the wingtips don't strike?

Thanks 4 the input. We'll nail this down shortly I think.

David Peters

> Which makes it obvious that the wingtip is never in danger of  
> whacking the ground during a normal launch.  Most other pterosaurs  
> will show similar trends..
> JimC
>
> ----- Original Message ----- From: "David Peters" <davidpeters@att.net
> > All I want now is minimum flight speed and apogee as a multiple  
> > of  the unit specified by CG height at the beginning of the leap.  
> > Been  gone all day. No reply, so far, to those queries.

David Peters
davidpeters@att.net