[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Ground Effect & Gliding Q



Waylon wrote:

> My question is
> whether a somewhat accomplished glider could leap from a tree with
> wings folded, open them above the water,

It could, but why not open them when leaping out of the tree?  As an
aside, it would take an awfully big tree to hold either type of Quetz if
that is what Q stands for in the subject line.

> skip off of it (now with
> wings tucked in again), and repeat this process until it had traversed
> the pond or lake it meant to cross.

Why would this technique assist the animal in any way?  Ground effect
follows the Biot-Savart law and works mostly to reduce induced drag with
the amount of reduction increasing as the HAG is reduced. It does not
necessarily increase lift, and at some ratios of chord/HAG, lift is
actually reduced.  Think of ground effect primarily as a means of
improving the overall lift/drag ratio by reducing drag.

> This style of flight (assuming it
> was aerodynamicly efficient and evolutionarily plausible) would look
> like that used by some birds today, IINM. From there, it is a very
> short step to powered flight. Thoughts?

I don't see any way that the technique described could have increased
efficiency enough to help, and it sounds counterproductive at first
blush.  Why not just establish a glide at a low enough HAG to help with
induced drag and then, as speed gradually bleeds off and aoa increases,
gradually lower the HAG very slightly and gradually to keep lowering the
induced drag as required to remain above stall speed.  This is the
technique that pelicans use when riding the lift off the shoreward side
of advancing waves as they approximately parallel the shoreline.  You'll
see them gradually descending as they gradually increase their aoa until
they finally reach the aoa limit and have to flap again.  All of this
takes place with very small change in HAG.  As an aside, in all of my
quetz flight calculations, I make HAG a variable so I can quantify the
ground effect in different conditions.
All the best,
JimC