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#1
September 11th 03, 08:27 PM
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I wish I'd never got into this...
The original question stated a glider doing 100kts
with/without 100kgs of ballast. Ignoring the stalling speed for the moment, Does anyone out there think the pull-up takes more than 4 or 5 seconds? Does anyone out there think that the sink rate of the unballasted glider at 100kts is more than 1m/s greater than the ballasted one? Finally does anyone out there think that the difference in height gain for the ballasted glider is more than 4 or 5 metres? If you think the answer to the third is Yes but the answer to the first & second is No, please let me know why!! And if your answer to 'Why ?' is ''cos I've done pull-ups higher with ballast' I want to know exact entry speed, exact exit speed, climb angle, exact amount of ballast, and exact heights gained with & without ballast (preferably backed up with a logger trace!). (And NO comparing two different gliders, just 'cos they have the same wing section & are doing rolling manoeuvres during the pull-up doesn't count) :-) Kevin 
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#2
September 12th 03, 02:19 AM
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Kevin,
What we appear to be seeing here is how strong preconceptions and/or expectations can override actual events. I could have sworn I would get a better zoom with ballast, but I now realize that it doesn't make sense. And I know all about Gallileo's experiment - I surprized my 15 year old daugher with it just a few weeks ago. But not having put much thought into it, I didn't connect the two. Amazing, you can learn something on RAS, occasionally! It would be fun to poll the general soaring population about this - I asked a really good pilot friend about this and his immediate answer was "Of course you will go higher with water". Now, I still have a gut feeling that there are some other forces acting that make it seem that a ballasted pullup goes higher - because it sure feels like it does! Must have something to do with Flat Earth Theory.... Kirk 66
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#3
September 12th 03, 01:59 PM
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#5
September 12th 03, 08:59 PM
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Kevin,
the problem isn't wholly ballistic. That is, it's not quite the same as firing projectiles of different mass straight up at the same initial speed. Remember, your sailplane has drag that varies with speed. Additional mass reduces total drag at higher speeds. That's why we carry water ballast when racing. Thus, during your pull up, the ballasted glider will have less drag than the unballasted one, and will therefore gain additional altitude. Is it alot? No. But significant. On a side note, Galileo never dropped objects off the tower of Pisa. Though he did some work with inclined planes, he recognized that friction would skew his results (think of dropping a ping-pong ball next to a golf ball...), and Catholic dogma didn't leave much room for fault when it came to heresy. Instead, he created a thought experiment. He postulated that if heavier objects fall faster, a heavy object tied by a string to a lighter object and thrown from a tower should pull the lighter object faster, and the lighter object would impede the acceleration of the heavier one. However, once tied by a string, they were a single object of greater mass and should therefore outpace the individual objects. This demonstrated the fallacy of the argument for "greater attraction" and saved him the embarrassment of having to demonstrate a flawed experiment to anti-empiricists.
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#6
September 15th 03, 04:55 AM
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I just entered this thread so apologize if having not read all the posts
this has been mentioned already. Let me quote Helmut Reichmann from Cross Country Soaring pp 63-64: "Starting in contests with full water ballast is always a good idea. If the run through the gate is made at high speed, heavier sailplanes will gain more height than light ones in the subsequent pullup". Although I find it impossible to argue with the math presented might it be that we are oversimplifying things? With all due respect a modern sailplane is a long way from a rock or pendulum. I'm certainly not an engineer but have flown for long enough and have done enough high speed pullups at the finish and on course to feel fairly certain that the altitude gained is substantially greater. But even more to the point if you don't believe this then was Reichmann wrong? Maybe the translation was poor? Geez my bubble is bursting! Send help! Casey Lenox KC Phoenix
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#7
September 15th 03, 04:05 PM
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If I look at my '27's polar and assume it takes about
0.25 nm to execute a pullup, I get less than 25 feet of difference due to the higher sink rate at redline for the unballasted case (at 154 kts the ballasted L/D is 19 and unballasted L/D is 14). This doesn't count for the losses associated with the Gs of the transition to the pullup, but I would think that you'd generate more induced drag to change the (vertical) direction of the heavier glider. I, too, have always flown with the belief that ballasted gliders would get more altitude on a pullup from the same speed - but I can't come up with any aerodynamic or physics rationale to support it. 9B At 04:00 15 September 2003, Kilo Charlie wrote: I just entered this thread so apologize if having not read all the posts this has been mentioned already. Let me quote Helmut Reichmann from Cross Country Soaring pp 63-64: 'Starting in contests with full water ballast is always a good idea. If the run through the gate is made at high speed, heavier sailplanes will gain more height than light ones in the subsequent pullup'. Although I find it impossible to argue with the math presented might it be that we are oversimplifying things? With all due respect a modern sailplane is a long way from a rock or pendulum. I'm certainly not an engineer but have flown for long enough and have done enough high speed pullups at the finish and on course to feel fairly certain that the altitude gained is substantially greater. But even more to the point if you don't believe this then was Reichmann wrong? Maybe the translation was poor? Geez my bubble is bursting! Send help! Casey Lenox KC Phoenix
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#8
September 15th 03, 11:57 PM
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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.
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#9
September 15th 03, 09:44 PM
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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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#10
September 16th 03, 08:32 AM
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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.
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