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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?
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.
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) :
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:
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).
I'm tuning in more than 3 years late because I'm not fully satisfied with the answers here. Yes, Lnafziger, when you want to stay up as long as possible, the plane should be as light as possible. But sometimes you need to get down fast: This is when water ballast is added.
Force is right: Water ballast speeds everything up. But there is more to it.
Also StallSpin has a good point: Higher wing loading equals less disturbance by gusts.
But there are two points which should be considered as well:
Higher speed means higher Reynolds number. Since this number shows the ratio of inertial to viscous forces, it means that friction drag is relatively lower. The consequence is that the glider with the higher wing loading really flies a little further than the light glider when both fly at their best L/D speed. The difference is not huge but gives the heavier ship another speed advantage when it can leave the last thermal one turn earlier than the lighter glider.
But the higher Reynolds number makes an even bigger difference at low speed: Roll control is much improved with water ballast. At the Reynolds number range typical for the outer wing of a glider at low speed (much less than one million) the speed increase improves stall resistance and control power markedly.
Friction drag coefficient of a flat plate over Reynolds number (picture source). The curve for a glider is between the fully laminar and the fully turbulent ones. Note the double logarithmic axes.
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.
Force's answer is very good. But the statement "additional mass only increases cruise speed, but not the travel distance", true for any one glide, doesn't take account of the fact that conditions suitable for soaring typically exist for a limited time each day - so increasing cruise speed definitely does increase distance.
StallSpin's point about reduced effect of turbulence on a ballasted glider is significant. This is best seen when flying a ridge, which in strong wind can be very rough. The ballasted glider, suffering less acceleration imposed by the rough air, can fly faster and lower, where the horizontal wind component is less, requiring a smaller crab angle.
Another factor the existing answers don't mention: if you are flying a two-seater glider alone, you might want to add ballast to correct your center of gravity.
Gliders are light, so a missing person can have a significant effect on the center of gravity. Two-seaters are optimized for flying with two people aboard. I've even seen lead ballast being used in the nose of a glider when a very thin and small trainee was flying with a heavy-set instructor in the back seat.
In addition to all the good content in all the other good answers, one more point should be made: when the airmass is moving horizontally and/ or vertically, the glide ratio over the ground is different than the glide ratio through the airmass, and therefore the glide ratio over the ground is different than the L/D ratio.
When gliding into a headwind, the maximum obtainable glide ratio relative to the ground is higher when the glider is heavy than when it is light. You easily verify this for yourself: starting with the second diagram in this related answer, extend the horizontal axis left far enough to include the origin of the graph. Now put your pencil on the point (x=50 kph, y=0). Starting from this point (x= 50 kph, y=0), the slope of a line drawn tangent to the airspeed-versus-sink-rate curve is the highest obtainable glide ratio in a 50 kph headwind in air that is neither rising nor sinking. You can see that the line drawn tangent to the ballasted curve is flatter (i.e. has less slope) than the line drawn tangent to the unballasted curve.
When we consider that when a glider flies a task that returns to the starting point on a windy day, it invariably spends more time flying with a headwind component than with a tailwind component, this is not a trivial point.
Naturally, this effect is even more pronounced if we draw our tangent line from (x=100 kph, y=0), representing the best achievable glide ratio when flying against a 100 kph headwind.
When slope-soaring a radio-controlled miniature glider in strong wind, it is not uncommon to encounter conditions where a lightly-loaded glider has difficulty making any forward progress at all and just sinks almost vertically down to the ground, while a heavily-loaded version of the same aircraft can be flown much closer to the max L/D angle-of-attack and thus can race forward at high speed while maintaining altitude or climbing.
Similarly, if we take the graph discussed above and extend the y axis upward so that extends into positive values for y, and start drawing our tangent line from the point (x=0, y=.2 m/s), we can find the highest obtainable glide ratio relative to the ground in the presence of a .2 m/s downdraft and zero headwind/ tailwind. Again the line drawn tangent to the ballasted curve is flatter (i.e. has less slope) than the line drawn tangent to the unballasted curve. In a downdraft, the maximum obtainable glide ratio relative to the ground is higher when the glider is heavy than when it is light. Since the air between thermals is often sinking to some degree, this is not a trivial point either. One instance where a glider pilot is most likely to be interested in maximizing his or her glide ratio over the ground is when he or she is flying in sinking air, and in this situation ballast helps.
The same method can used to find the maximum obtainable glide ratio relative to the ground in air that is sinking and includes a headwind component. In this case ballast really helps a lot-- the maximum obtainable glide ratio relative to the ground will be much higher in the ballasted glider than in the unballasted glider.
One point not explicitly mentioned in other answers is that on a strong soaring day, no one flies at best L/D. Suppose the lift is strong and climbing is no problem. Ballast up to max gross. Cruise between thermals at 100 knots. The sink rate at the same airspeed will be much higher if no ballast is carried.
You can easily see this by checking the polar diagrams provided in this related answer. Look at the second diagram-- the graph of sink rate versus airspeed for three different wing loadings. 100 knots is about 180 kph. At the heaviest loading, the sink rate at this airspeed is 1.8 m/sec, and at the lightest loading the sink rate at this airspeed is 3 m/sec. That is a 66% higher sink rate.
When flying at some given airspeed that is well above best L/D speed, ballast does in fact increase the distance made good for the same loss of altitude.
