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I'd like a bash at this:
Can we get some assumptions out in the open first... And before I get toasted, I am not being patronising, I just like to get assumptions clear and out in the open. 1. throughout we stick to one glider type which is, e.g. i) why? because it would be like comparing apples and oranges otherwise ii) so, lets assume it is an asw23, or it is a pik20b, or ... ii) consider two cases: ballasted (= greater mass) and unballasted iii) otherwise, the configuration of the two gliders is identical except for the amount of ballast they are carrying 2. potential energy = mass x gravitational acceleration x height, so i) for a given height the ballasted glider has more potential energy than the unballasted glider 3. kinetic energy = 1/2 x mass x speed x speed, so ii) for a given speed the ballasted glider has more kinetic energy than the unballasted glider 4. total energy = potential energy + kinetic energy i) from the above we now know that, for a given height and speed, the ballasted glider has a greater total energy than the unballasted glider 5. from the gliders polar and the basic arithmetic of ballasting we know that, above a certain speed (i.e. the speed at which the sink rates of both the ballasted and unballasted gliders are the same), the ballasted glider will be travelling faster, for a given sink rate (= rate of energy loss), than the unballasted glider, below that speed the unballasted glider will be losing energy at a lower rate than the ballasted glider i) this effect is due to the increase in the wing loading of the glider ii) the same effect would apply (approximately, because the wing bending would be somewhat different) to the unballasted glider in accelerated flight (e.g. during a pull up) iii) assuming the above, for a given speed the ballasted glider will be sinking at a lower rate than the unballasted glider (e.g. the rate of energy loss is lower) - don't believe me? look at the polar 6. provided we stay above that "certain speed" (which is determined by the wing loading and so will be higher if the wing loading is higher) i) for a given speed the ballasted glider will always be losing (potential) energy at a lower rate than the unballasted glider ii) this will be true regardless of whether the glider is in steady (i.e. straight line) flight or in accelerated (e.g. turning or pulling up) flight iii) in fact the difference in the rates of loss will be even greater in accelerated flight So far, so good (I hope). Now lets ignore the glide segment and just consider the pull up and the subsequent zoom. 7. for two real gliders, of the same type, same configuration, one ballasted more than the other, during the pull up i) assuming the two gliders start at the same height and the same speed ii) both gliders increase their wing loading in the same proportion to their mass during the pull up iii) I think that, given ii, they will follow the same pull up curve as a result, but iv) throughout the maneuvre, the ballasted glider will be losing energy at a lower rate than the unballasted glider v) so it should come out of the pull up higher and having lost less energy than the unballasted glider (i.e. it will start the zoom faster than the unballasted glider) 8. during the zoom (at zero g), if both gliders started at the same height and speed i) both will gain potential energy, and ii) both will lose kinetic energy, but at a rate proportional to their masses due to the effect of gravitational acceleration, and so iii) the gliders would rise to the same height if they were in a vacuum throughout, but iv) they are not in a vacuum, they are gliding (probably at a reduced wing loading), so v) provided they are flying above that "certain speed", the ballasted glider will be losing energy at a lower rate than the unballasted glider, and so it will zoom higher I admit this is a somewhat qualitative argument, so would someone like to put figures on it? Rgds, Derrick. |
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