On Mar 18, 8:20*am, Doug Hoffman wrote:
Derek Copeland wrote:
In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
turbo, a glider IS essentially gravity powered. The resultant force of
gravity plus wing lift, angled very slightly forward, opposes drag. Thus a
glider runs down a very slight slope through the air. The less drag there
is, the flatter the glide angle becomes.

Nicely worded answer.

On several occasions I've had (non glider pilot) friends ask me why does
it help when we make our gliders heavier with water ballast. *Seems
counter-intuitive.

I'm thinking that a proper explanation is in terms of the gravitational
force in a similar fashion to what you describe. *Higher mass = higher
gravitational force (F=MA). *Hence the glider is "pulled down the slope"
by a higher force. *The glide angle is no better, but we can glide
faster at essentially the same glide angle which is an advantage (normal
caveats about thermal climb ability trade-off). *A more complete answer
might also discuss the higher drag at higher speed interplay, but that
could probably be left out as a simplification. *Perhaps a further
discussion of the classic experiment where in a vacuum a feather and a
rock will fall to earth at the same rate because the acceleration of
gravity is a constant (I know, but within limits it *is* a constant).
But in the presence of air the "air-drag" on the feather is relatively
high compared to the relatively low gravity "down-pull" due to its low mass.

Comments on this explanation are welcomed/sought. *I thought I would
find a well worded description of this in Reichmann but it isn't there
that I can see. *TIA

Regards,

-Doug

Remembering that the power for the glider is coming from the
gravitational potential energy, so it is correct that a higher mass
glider has more energy and this is where the increased L/D does come
from. However you can't do an analysis quite like that to explain the
results. A good discussion of this for L/D for powered aircraft is is
in "Mechanics of Flight" by Phillips, if you read the "Power Failure
and Gliding Flight" chapter you will get a pretty good picture of what
happens, even if not really discussing glider wind loading.

Google books has extracts on line at

http://books.google.com/books?id=6-_...nics+of+flight

and available from Amazon.

http://www.amazon.com/Mechanics-Flig.../dp/0471334588

It's expensive but very good.

Darryl