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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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    $\begingroup$ Apart from all the speed benefits, dumping ballast shortly before landing makes for great photos. Occasionally it has also been used to irritate competitors (^_-) $\endgroup$ – yankeekilo Jan 7 '14 at 21:34
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    $\begingroup$ @yankeekilo: Haha, good points! $\endgroup$ – Lnafziger Jan 7 '14 at 22:08
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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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    $\begingroup$ Another downside/caveat of higher loading is higher stall speed. $\endgroup$ – yankeekilo Jan 7 '14 at 21:31
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    $\begingroup$ 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. $\endgroup$ – Force Jan 10 '14 at 3:24
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    $\begingroup$ 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. $\endgroup$ – yankeekilo Jan 10 '14 at 9:26
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    $\begingroup$ @PeterKämpf Do you have a source for that? $\endgroup$ – Force Apr 19 '14 at 11:09
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    $\begingroup$ @Force: Reynolds number increases with speed, and the friction drag coefficient goes down. Flying at a higher wing loading means flying at a higher speed regime and at a lower friction drag coefficient, thus better L/D than at the same polar point at lower speed. This is basic aerodynamics - what source do you need? $\endgroup$ – Peter Kämpf Mar 2 '17 at 17:59
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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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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:

  1. 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
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.

  1. Tactics: Water ballast is used mostly in competitions, and when several aircraft share one thermal, every pilot waits for the others to fly off to the next thermal. Watching the others tells her/him where the best route for minimum altitude loss is. This even makes the highest pilots in the thermal open their speed brakes, just to avoid leaving the thermal first. With water ballast the climb speed is reduced (higher sink plus bigger turning radius conspire to reduce the climb rate of the glider significantly), so the pilot with water ballast will even have a tactical advantage in the climb phase by flying a heavier ship.
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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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  • $\begingroup$ Probably a lot, given that gliders are generally much much lighter than internally-powered aircraft of the same size. $\endgroup$ – Sean Mar 23 at 3:09
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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.

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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.

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The point missed by all the previous posts is that on a 'good day

It

' 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. Check the ploar diagram for 100 knots at max gross. It shows a sink rate of about 1.8 m/sec. Now check the sink rate at 100 knots with no ballast. It shows about 3 m/sec. That is 66% higher sink rate than at max gross weight. The ballast does in fact increase the distance made good for the same loss of altitude.

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    $\begingroup$ "Check the ploar diagram for 100 knots at max gross", that would be nice if you could include this document, because that's a strange thing that the range can be increased just by adding weight... if it was true, commercial aircraft would carry more passenger on a greater distance for a lower cost. $\endgroup$ – mins Mar 2 '17 at 18:15
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    $\begingroup$ For me to believe this answer you'll need a very good source to back it up. $\endgroup$ – Notts90 Mar 2 '17 at 19:17

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