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#11
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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. |
#12
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Andy,
see my notes earlier in the thread. There's not much penalty for pulling from high speed so long as you don't go to too quickly to high AOA (bigger penalty in induced drag). 2g to 30 degrees nose up is typical. The you ease off to 1g until you start push gently to attitude at your desired exit speed. Did your calculations include the losses to friction throughout the manuever? Your altitude difference seems a little too low. I would expect about a 20 to 30 foot difference for a pull from 100 knots to 60 knots, ie, a normal "test the strength of the core" pull up. At any rate, if we get any decent weather, I'll be sure to make some runs with a lighter glider and tender the real world results. The original poster was looking for some real world feedback. Right now all I can offer is that when ridge soaring, if I have water and another 27/V2 is empty, I'll outpace him by at least 10 knots on a hundred mile-per-hour day. Roughly 10 to 12 percent more speed at the same sink rate/drag. A big spoonful of pure, sweet hubris. |
#13
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Hey Chris,
I used the factory polar to estimate altitude loss at redline with and without ballast. The difference over 0.25 nm was 26' (105' - 79'). As you bleed off airspeed the difference in sink rate declines, so the actual difference in altitude loss should be less than 25' - maybe more like 15'. This ignores G-related losses, which should be low if the Gs are low (i.e. not too radical a pullup). Of course too gradual a pullup and you don't get maximum altitude gain because the parasite drag will accumulate. 9B At 23:00 15 September 2003, Chris Ocallaghan wrote: Andy, see my notes earlier in the thread. There's not much penalty for pulling from high speed so long as you don't go to too quickly to high AOA (bigger penalty in induced drag). 2g to 30 degrees nose up is typical. The you ease off to 1g until you start push gently to attitude at your desired exit speed. Did your calculations include the losses to friction throughout the manuever? Your altitude difference seems a little too low. I would expect about a 20 to 30 foot difference for a pull from 100 knots to 60 knots, ie, a normal 'test the strength of the core' pull up. At any rate, if we get any decent weather, I'll be sure to make some runs with a lighter glider and tender the real world results. The original poster was looking for some real world feedback. Right now all I can offer is that when ridge soaring, if I have water and another 27/V2 is empty, I'll outpace him by at least 10 knots on a hundred mile-per-hour day. Roughly 10 to 12 percent more speed at the same sink rate/drag. A big spoonful of pure, sweet hubris. |
#14
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I'm sorry Kate but maths beats 'Feminine Logic' every
time. If two gliders start at the same height & speed & accelerate to the same speed then they'll lose pretty much the same height & follow the same trajectory. And before anyone else brings up golf balls & ping-pong balls may I remind people that your average sphere is a much much draggier shape than your average sailplane, and the ratio of masses is way way way greater than the ratio for a glider with / without ballast At 00:18 16 September 2003, Glider Kate wrote: Boys You seem to be forgeting one or two things!!! If two identical sailplanes with identical weight pilots but with sailplane a) carrying water ballast and b) dry. Set off in still air, side by side at the same speed, say 45 knots and accelerate at the same rate, to a new identical speed, say 100knots. By the time they reach the new speed, sailplane a) will accelerate faster and travel further and lose more height than glider b). No need for maths just a bit of feminine logic Bye............... Kate |
#15
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And you miss the point, if I am on a final glide I don't care what height I
started from, what I am interested in is how fast I can make the glide to the goal. The height I can pull up to determines how low I dare go near the goal bearing in mind that I want to pull to a safe height for my approach and landing. The point is that the ballasted glider will not only get there faster, it will also pull up higher = ballasted glider wins. Rgds, Derrick. |
#16
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Well, at least we've got everyone on the same theme now. It's the
drag. Why don't you guys in Phoenix do a little testing and we'll do the same here at M-ASA. I think we all agree that the heavier glider has a significant drag advantage at high speed, and will gain additional altitude. But how much, exactly? |
#17
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It's worth noting that the heavier glider has more energy stored kinetically and potentially. Whatever the difference in height is, it's due to drag, and heavier glider needs to give up less altitude for any given amount of energy required to overcome drag. Since Newton was hit by the apple, it is very unfortunate that everywhere on this universe, any system trying to carry a load away from the center of the planet will have to work harder than one travelling light. This is intuitive enough. I wonder what is catching here. For any ten feet of height, the heavy system will have to work more than the light system. True, our ballasted glider has more money in the bank at the start, but it will have to spend more on the way up. I hope that money comparaison will help. There is no way around this fact. Travelling towards the center of the planet is another ball game. The reason that for a given angle of glide, the heavy will go faster is that the weight being larger, it's component parrallel to the direction of travel is bigger. That simple. This is the motor. Drag is "induced". it is not running the show.!! Hope this help. Bravo Quebec |
#18
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inclined ramp. There are two things slowing them and limiting the height they coast to, gravity and drag. Gravity has the same effect on both. Drag is higher on the heavy glider, but it has more kinetic energy, so the higher drag has less proportional effect on the distance up that ramp that the glider will travel. Todd Please admitt that the heavy has a bigger job to do. You just need more energy to carry a heavy load up. Please stop denying that, or the buildings around us will start to soar ;-)))) Bravo quebec |
#19
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#20
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While Udo doesn't state the numerical value of the difference, I bet it's only 30-40 ft. That makes sense, considering the expected 3-400' delta h. So this would be a tie, and quite satisfying, considering that everybody I know in the soaring world, including H. Reichman ;-)), believes that the ballasted glider would go noticebely higher. I never tested it, but along with anybody that did a little maths, I could not force-fit the extra ballasted height into the equations. The debate is still on, and we deeply wish that someone could run a "scientific" test. This is fun!!! Bravo Quebec |
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