Water ballast

This comic simply came out of a conversation about how gliders use water ballast to increase the speed of best glide. I had never heard of such a thing before even though I have a glider rating. In my defense, the gliders I flew were not exactly competition material and when you build time, there was no need to arrive at the destination any sooner. Also, it wasn’t on any test I took.

So, we figured that, if we hadn’t heard about this before, Chuck wouldn’t have either.
The concept didn’t make sense to me right away, but after some digging (google is your friend) I found pretty good explanations for it. It appears Chuck found a less gooder explanation… or, and this is more likely, never looked it up in the first place and immediately envisioned what he can do to make the plane faster.
What I found interesting as well was, that the water ballast can also be used to move the CG aft which might give the glider more distance. Aft CG uses less downforce on the stabilizer and elevator which makes the aircraft more efficient. (If the CG is at the front limit, you will need to pull the control stick back in order to fly level, therefore you have more drag) This is a concept on faster planes and jets as well which I was familiar with. More efficiency on a glider means further distance. Makes sense.
The things you learn when you draw comics about chickens flying airplanes…
And now you know as well, LOL.


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6 comments on “Water ballast
  1. Karel A.J. ADAMS says:

    G is a dirty old villain who peeks into your cleavage AND makes photos AND offers the photos for sale. While trying to appear friendly all the while, of course.

    There are better and more correct search engines available.

  2. Steffen says:

    Funny you didn‘t get that during glider exam. So I guess you didn‘t have your license in germany.
    But in fact, it increases cruise speed for certain powered planes, too.
    A Dimona TMG flown with one person has a cruise of 180km/h. With two taller guys, a little overload (ahem) and same power setting, it is 205. Just because you are heavier, have a lower lift coefficient and don‘t fight the laminar bucket.
    But a Cessna, hrmm, no 🙂

  3. Steffen says:

    sorry, higher lift coefficient of course.

  4. Mo Davies says:

    My glider flying was all low-performance in Grunau Baby, Schleicher Ka2 and Ka4, Govier and such aircraft whilst a British soldier in Germany in the 1960’s. But I crewed for pilots in the UK National Gliding Competition after leaving tha military. Apparently the “done thing”, was to release the ballast just before the landing. The sometimes very long “Final Glide” performance often relied heavilly upon water ballast and many competition pilots used to compete in who could drop their water nearest to the judges. Needless to say, the judges were not too keen on this aspect of the competition.

  5. ThisGuy says:

    Can confirm as a glider pilot, carrying water is done quite often in the more high performance gliders for cross-country flying and competition. It’s pretty pointless in gliders with an L/D (polar) curve that drops fast at higher speeds. The basic idea is that you shift the entire polar curve towards slightly higher speeds, meaning your optimum cruising speed between thermals is faster than without the extra weight. The downside is that you climb slightly slower, so the name of the game is finding the flying speed so that you gain more time by flying faster than you lose by having to spend more time climbing.

    The thing about the C/G is true, but not entirely correct. Gliders that can carry water ballast have a separate tail tank in the vertical stabilizer. This is filled before the flight by calculating how much water to put in to put the CG in the most optimal position such that with the (loaded with water) wing load and expected cruising speed between thermals the horizontal stabilizer is providing as little drag as possible (either from having to push or pull the stick).

    Get it wrong though and the aircraft might be unrecoverable from a stall/spin without dumping the water load. The dump valves are set up such that it’s only possible to dump the wing tanks once the tail tank has been opened first.

    Personally I mostly prefer flying older planes with character over the new rocket ships.

  6. Jon Steensen says:

    Another reason to add water in the tail, is that a plane that is too nose heavy is more stable than nessesary, and that can make it annoying to fly.
    The reason for this is, that when you enter an updraft, it will add an upward component to the airflow over the wing, that will change the angle of attack of the wing and increase the lift.
    A stable plane will react to this, by trying to reduce the angle of attack back to its original value, and hence it will pitch down and speed up when hitting a thermal (and likewise it will slow down when exiting a thermal).
    We do not want this to happen, as we in general would like to slow down in thermals in order to fly more effeciently, and to reduce the circling radius in order to stay as close to the center of the thermal as possible -since that is where the updraft is typically the strongest.
    A plane with neutral stability will not act like that, but instead the increase of lift will just cause and upward acceleration until the angle of attack is reduced to a level where the lift again outbalances the weight of the plane.

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