Gliders: Weight/ Speed/ Range

[Fred Elliot]

Settle a crew-room puzzlement, ( in simple, ie pilot's, language)please:

We know that heavier, water-ballasted, gliders go further. But, is that because/ why they go faster. Which is the chicken, and which the egg?

Ken Stewart are you out there with your laptop?

Thanks.



There's a nice packet of lamb's sweetbreads for the first answer I can understand.

[Tinstaafl]

The water ballast increases the speed that corresponds to best AoA. This allows the glider to fly faster for the same range (still air). Flying faster minimises the time spent in descending air after spending all that effort to find & use an ascending column of air.

Net result is a reduced height loss after flying through a descending air mass. The downside is the requirement for a stronger updraft to gain altitude.

[erikv]

Fred,

best glide ratio is attained at the speed that gives the best Cl/Cd ratio.
This is a constant for a given wing/fuselage combination. Therefore, the speed you need to fly to get optimum glide depends only on weight (as W = L = Cl x 1/2 p v^2 x S, assuming constant air density).

This means that at higher weight, the best glide speed is also higher, but that the range remains the same. The goal in competition is to fly a given distance at the highest speed, so that it makes sense to take on water ballast.

Erik.


Edit: apparently I need to improve the rate at which I type

[Fred Elliot]

Tins. / Erik

Many thanks for your swift replies. I evn understood Erik's bit with all the numbers in. Not bad for a humble butcher/ pilot.

I win the two pints of guinness at home time and you each get a delicious helping of bullock's offal.

Thanks again.