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So I was looking at the description of a ASW 27 B glider and ran across this statement:

Two water tanks in the wing plus a further 35 liter tank in the fuselage enable the ASW 27 B to carry more water ballast than any other 15 m glider and also give it the widest range of wing loadings

If a glider is trying to stay aloft as long as possible, wouldn't it be better to be light? Why would you add ballast and be able to dump it?

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Apart from all the speed benefits, dumping ballast shortly before landing makes for great photos. Occasionally it has also been used to irritate competitors (^_-) –  yankeekilo Jan 7 at 21:34
@yankeekilo: Haha, good points! –  Lnafziger Jan 7 at 22:08

3 Answers 3

up vote 21 down vote accepted

Mass doesn't affect the maximum distance, only the maximum endurance.

For example, image two identical planes A and B: A weights 50kg less than B. Assuming no wind (horizontal / vertical) and speed of best glide, both gliders will land at the exact same spot.

The lighter airplane A however will arrive later than B, as the speed of best glide is less than for B. In conclusion you can say, that additional mass only increases cruise speed, but not the travel distance.

Glider competitions are most of the time a route you have to fly in the shortest time possible. So that means, if you have a higher speed of best glide, you can fly faster in competitions.

The only downside to having a higher weight is, that your liftrate in thermals will be decreased and due to the higher speed it is harder to center the thermals.

It is to some extend also possible to shift the Centre of Gravity (CG) with the added load. The further it is to the aft limit, the higher your maximum distance is. This is because you will have less down-force from the stabilizer required. (If the CG is at the front limit, you will need to pull the control stick in order to fly level, therefore you have more drag). However I think this is rather a positive side effect and most of the time the water is used for flying faster.

Source: I am a glider pilot and currently doing my ATPL training.

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Okay, thanks. I changed it in the answer. –  Force Jan 6 at 2:21
Another downside/caveat of higher loading is higher stall speed. –  yankeekilo Jan 7 at 21:31
Yes, I kind of implied that with "higher speed it is harder to center the thermals", but you are absolutely right. Most of the time you center thermals just above the stall speed. –  Force Jan 10 at 3:24
There are two factors: higher stall speed and higher sink ratio. Both have adverse effects on circling, esp. in weak/narrow thermals. But I mentioned higher stall speed mainly because of the additional care you have to take e.g. near terrain (ridges) or in case of a sudden landing which may not leave time to dump properly. –  yankeekilo Jan 10 at 9:26
Do you have a source for that? –  Force Apr 19 at 11:09

In addition to the other answers, let´s look at this L/D(=E) diagram of the enticing DG-1000 from DG Flugzeugbau (but fear not, 'tis true for all gliders) :

enter image description here

The best L/D ratio is equal for different wing loadings, but is occuring at different speeds - the higher the load, the higher speed. You can also see that the minimum/stall speed is also higher for higher loads.

The next diagram shows the polar curve: enter image description here

You can see that the minimum sink rate occurs at lightest load. The heavier the load, the longer you will have to circle in the same thermal for a given height gain.

The loading is a tradeoff between higher average speed and less efficient climbing. In case of strong thermals and/or long glide intervals, the optimum moves toward more, in weak conditions towards less or no ballast. The good thing is that you can dump water rather quickly (also partially), so that in a competition you usually tend to fill up (and dump in case) rather than start light (the Quintus e.g can take up to 250 liters!)

Aft ballast in the vertical tailplane is sometimes used to balance a forward CG caused by water in the wings - depending on your ship, partial dumping can be problematic.

Of course there are many philosophies and tactical debates concerning the "water or no water" dispute, but once you´ve overtaken an identical, lighter ship with full wings and no height loss, you get to see how much fun ballast can be (until the next thermal, that is).

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Great answer, thanks for sharing! –  Lnafziger Jan 7 at 23:36

Force's answer is pretty much the answer, but also consider that mass = inertia. If you weigh more, you are less likely to be disturbed by any given outside force (turbulence). A lighter plane is more maneuverable but it will also bounce around a lot.

I cannot comment on how much of an effect the ballasts in question have on this for a glider, though.

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