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#21
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THANK YOU UDO!!!
I think your sim. has the rights equations planted within it's program!!!! I am the one who started this debate. Quite fun uh!!! For your info. ,since french is mother tongue, I posted the same question on a French forum. Hell is raised over the pound too. But the balance is in favour of a tie ( no advantage to the ballasted). We stated than any difference less than 50 feet is a tie. (That,s what you can read on your alti. with minimum precision). Must of people rely on their intuition on this side " If it's twice as heavy, it has twice the energy, and it will coast to twice the height, since the same drag force is acting to deplete it's kinetic energy reservoir of twice the size. " Do not forget that the heavy glider has a bigger job to do and its is exactly equivalent to the extra energy!!!!! More thoughts for your intuition 1. will you need a bigger force to change the direction of travel of a heavier object than a light one. 2. Intuitively, the wings of the heavy will bend more. Is the force needed to bend the wings translate in lost enrgy for the glider? More for your consideration, better L/D is no advantage when your nose is pointed to the zenith. Lifts acts perpendicular to axis, it is actually pulling you away from optimal trajectory. Lastly, using the original data, if the heavy glider go say 75' (25m) higher, it takes (100kgs*9.81 m/s2*25 m= 25 000 Joules of energy to do that job. So conversely, it is the extra job done by drag (....what else...)on the dry glider. I cannot fit this in any equation!!!!MAGIC!!!!!! BravoQuebec "Udo Rumpf" a icrit dans le message de ... I am not able to comment on the math questions, instead I used the simulator. I used the ASW27 model with and without water. I selected a trim speed in both cases (empty and full) of 180 km/h flaps stayed in original position through out the manoeuvre. The pull ups were with max stick deflection till an optimum trajectory for both gliders was achieved. Once the trajectory was established the glider was allowed to fly/coast to the top. The results over many test runs showed a 12% advantage on average for the unballasted Glider. Regards Udo While Udo doesn't state the numerical value of the difference, I bet it's only 30-40 ft. you are correct. it works out to about 39 feet. Udo |
#22
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"Andy Durbin" wrote in message om... (Chris OCallaghan) wrote in message om... 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? So far I have not seen anyone consider the fact that, at the same (high) speed, the unballasted glider has a significantly higher sink rate at the start of the pull up. The initial conditions are not the same. Andy (GY) The sink rate for the fully loaded glider(190 litres) = 1.2m/s for the none ballaste version 1.65m/s Udo |
#23
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Hi Udo,
When you ran the Sim, how much ballast were you carrying, & what speed did you pull up to in each case? At 02:30 18 September 2003, Udo Rumpf wrote: The sink rate for the fully loaded glider(190 litres) = 1.2m/s for the none ballaste version 1.65m/s Udo |
#24
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Hi Derrick,
The principles are indeed simple. The original post stated two gliders with & without 100kgs ballast starting at 100kts. The heavy glider is indeed losing height more slowly than the light one at this speed, (But the difference is only about 1m/s). This difference is only maintained for the duration of the pullup (About 4-5 seconds) and will be diminishing as the speeds drop off. In addition we're not pulling up to a standstill 'cos that's an untenable position for most gliders, in which case the light glider gains an advantage because it can fly a few knots slower than the heavy one. So in the end I believe it's too close to call!! (Cynically I believe that the original post of 100kts/100kgs was deliberately chosen to make it too close to call) Cheers At 11:30 18 September 2003, Derrick Steed wrote: In response to post number 39: The math is a bit complex, but the physical principles are simple. 1. In the glide the heavier glider goes faster for the same glide angle (read Frank Irving - he's a gliding aerodynamicist) 2. In the pull up the same is true (It's just a higher wing loading 3. In the zoom subsequent to the pull up there are, principally, two forces at work: i) that due to gravitational acceleration - this is proportional to mass and so both gliders decelerate at the same rate (if this were the only factor then they would both zoom to the same height provided that they both started the zoom at the same height and speed) ii) that due to drag, this is primarily proportional to speed and not proportional to mass, the result is that the heavier glider decelerates at a slower rate than the light glider and so goes further (= higher). Rgds, Derrick. |
#25
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Kevin,
If you are talking about 1m/s difference in sink rate, then I would think that is a huge difference. Even unloaded, my gliders' polar shows me sinking at about 2.5m/s at 100kts. Rgds, Derrick. |
#26
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check previous post.
Udo When you ran the Sim, how much ballast were you carrying, & what speed did you pull up to in each case? At 02:30 18 September 2003, Udo Rumpf wrote: The sink rate for the fully loaded glider(190 litres) = 1.2m/s for the none ballaste version 1.65m/s Udo |
#27
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check previous post.
Udo When you ran the Sim, how much ballast were you carrying, & what speed did you pull up to in each case? At 02:30 18 September 2003, Udo Rumpf wrote: The sink rate for the fully loaded glider(190 litres) = 1.2m/s for the none ballaste version 1.65m/s Udo |
#29
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Udo Rumpf wrote:
The sink rate for the fully loaded glider(190 litres) = 1.2m/s for the none ballaste version 1.65m/s And the vertical kinetic energy converted to altitude (the only place where we can take the energy in a glider) would be h=v^2/(2g) so for 1.2 and 1.65 m/s the altitude losses would be 0.073 and 0.139 m respectively. Not a signifigant factor I would say. Plus, we would have to take into account the fact that in the end as well (after levelling out) their sink rates will be different. But still no signifigant effect... Jere jere at iki.fi |
#30
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Hi Todd,
You seem to be of the opinion that in the accelerated part at the start of the pull-up the ballasted glider has a vast advantage over the un-ballasted one. Have you any evidence to support this? I would have thought that the effort required to accelerate the extra ballast would give the advantage to the light glider at this point - but yes that's just my opinion. As far as I'm concerned the overwhelming maths here is good ol' conservation of energy where speed is traded for height & the two come out equal. Yes there's some drag involved but the actual drag forces are pretty small on modern sailplanes & the time in which they have to operate is pretty small. Three questions for you:- 1) How long do you think the pull-up lasts? 2) What sort of difference do you think there is in the respective sink rates - during the pull, during the climb & the push over at the top? 3) What sort of difference do you think there is in the height gained? Finally I've suggested a couple of times that someone with a Duo / ASH25 / Nimbus D go & do the tests At 15:42 18 September 2003, Todd Pattist wrote: Kevin Neave wrote: The heavy glider is indeed losing height more slowly than the light one at this speed, (But the difference is only about 1m/s). Where do you get that number? From the polar measured at 1G? That's the wrong polar. The glider is not operating at 1 G for much of the pullup. This difference is only maintained for the duration of the pullup (About 4-5 seconds) and will be diminishing as the speeds drop off. I regret to say that this analysis is bogus. It just tells us what would happen if the gliders flew side by side for 4-5 seconds. Of course that difference is nominal, but they aren't doing that, they are flying at a varying G-load through the pullup. You can't wave your hands and ignore that difference. So in the end I believe it's too close to call!! You have no basis other than your opinion. You need to do the math or the experiment. You've done neither. Todd Pattist - 'WH' Ventus C (Remove DONTSPAMME from address to email reply.) |
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