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

Re: Ground Effect & Gliding Q

Waylon Rowley wrote:
> I was thinking along the lines of an area
> of pressurized air between the wings and water
> creating the additional lift

That effect can exist to some slight extent.  I've observed that when
landing a J-3 (Piper Cub) on top of the Mississippi River levee, I stall
about 2 mph faster than when I land in ground effect (the side slopes of
the levee fall away from the wings and eliminate much of the ground
effect), raising my stall speed from about 38 to perhaps 40 mph, not
enough to matter significantly.

> - something that could be
> bounced off of. Clearly, I was wrong.

Well, you certainly wouldn't bounce off it. You can detect its presence
because you have a slight tendency to fly more nose down while in it and
under power, at least that's the thing that I notice most.

> Why not just
> establish a glide at a low enough HAG to help
> withinduced drag and then, as speed gradually bleeds
> off and aoa increases,gradually lower the HAG very
> slightly and gradually to keep lowering theinduced
> drag as required to remain above stall speed. > That
> makes perfect sense if you are in it for the long
> haul, but what if the evolutionary objective was to
> maintain the highest possible airspeed for a maximum
> given time interval?

Then you would also follow the scenario that I just described.  Works
for pelicans and Quetzes (except the latter weren't using the uplift
from advancing waves).

As an aside, with regard to the dropping out of trees idea -- and
without bothering to do specific numbers -- though it can work for small
animals, if one assumed Qn dropped out of a tree, accelerating
vertically to achieve flight speed, then one would also need to assume
that enough excess speed would be needed to enable the animal to manage
the high g pullout at the bottom.  Say Qn needed to accelerate to
roughly 60 mph to accomplish that.  Then the initial vertical
acceleration would require about 2.75 seconds, during which time the
animal would fall approximately 120 feet.  At that point, he could begin
a high g curved pullout, while descending perhaps another 100 feet or so
before reaching level flight (so as not to wind up stuck in the ground
like a 20 foot long lawn dart).  How many of the trees in the late
Cretaceous reached an average height of say 220-300 feet? Or perhaps
more, if Quetz didn't start from the very top like a big Christmas tree
ornament.  Actually, if the animal did launch from a tree, he would
probably use a very different scenario that isn't so drastically
oriented.  But the tree would still need to be immense.  Were they?

>  If I understand you correctly,
> wouldn't intermittent use of the wings in the low-drag
> ground-effect zone result in a higher airspeed for a
> greater distance relative to intermittent wing use at
> an increased HAG with the same initial velocity?

It would indeed, and would also have the same effect for continuous
flapping.  It is a very useful technique, and Quetz has several features
that caused me to think ground effect was important to him when I first
got interested.  As an aside, Quetzes would appear to have been
intermittent flappers.  Reduction in total drag due to ground effect is
useful, but not always all that great.  At practical heights that leave
room for flapping, total drag will be reduced on the very loose order of
5% to roughly about 25%.