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Explain Water Ballast Effect (was aerodynamics of gliding)
Darryl Ramm wrote:
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. I thought the best L/D stayed about the same. It just occurs at higher speed with ballast. 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. OK. Thanks for that. Is a proper explanation that complicated? I was hoping to avoid a $140 answer. :-) Regards, -Doug 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 |
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