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#1
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Question of the day
A given glider is at level flight, IAS= 100 knots.After a pull-up, will it
achieve more height gain with 100 liters (100 kgs) of ballast than with empty ballast???? Réjean |
#2
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The difference in height will be negligible.
The glider's energy, both potential & kinetic is proportional to Mass so the height gain for a given loss of velocity will be the same. However the ballasted glider will have a better sink rate at 100kts than the unballasted one so during the few seconds of the pull-up it will 'lose' less height. On the other side of the equation the un-ballasted glider will be able to pull up to a lower speed, so it's change in velocity will be greater so the resulting height gain may be more. (My money - if I had any - would be on the un-ballasted one) |
#3
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Steve B wrote:
I am curious... what amount of altitude are the different gliders able to get from a high speed pull up? 15 m 18 m 25 m? Neglecting drag and other minor inconvencies: m/2 v1^2 + mgh1 = m/2 v2^2 + mgh2 I think I can leave the rest to you. (Hint: It's something between 100m and 150m, depending on v1 and v2.) Stefan |
#4
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I think you'll find the polar on a 15m glider is quite
a lot flatter than that of a model. So at the entry to the pull up the model is probably losing height faster than the full-size ship. The proportion of ballast you can carry in a model is probably higher.In my Discus, weighing 320kg dry (with me in it!) I can carry about 200kg of ballast i.e about an extra 60%. The models I used to fly 'when I were a lad' weighed 1-2kg, but could carry about 5kg ballast ('cos I was less worried about pulling the wings off). This results in a much greater benefit at high speed than you'll get in the full-size object :-) I also suspect the ballasted one is travelling faster in the first place. How are you measuring the speed of your models at the point you're starting the pull up? At 18:30 08 September 2003, Jim Vincent wrote: (My money - if I had any - would be on the un-ballasted one) From my many years experience flying radio control slope ships, there is a heck of a difference in performance based on the amount of ballast. Without ballast I can get maybe one vertical roll; with ballast I can get at least three or four. Jim Vincent CFIG N483SZ |
#5
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Kevin Neave k wrote in message ...
The difference in height will be negligible. Not true. A full load of water makes a HUGE difference in pullup altitude gained The glider's energy, both potential & kinetic is proportional to Mass so the height gain for a given loss of velocity will be the same. Again, wrong - check your basic physics. You even say that the energy is proportional to mass. Therefore, more mass, more energy, more altitude gained. You appear to be confusing velocity with mass. However the ballasted glider will have a better sink rate at 100kts than the unballasted one so during the few seconds of the pull-up it will 'lose' less height. True, but the crossover point is quickly reached so this effect is probably negligable. On the other side of the equation the un-ballasted glider will be able to pull up to a lower speed, so it's change in velocity will be greater so the resulting height gain may be more. If you pull up below the ballasted sink rate crossover speed, sure a heavy glider will gain less. But at those speeds neither glider will gain much anyway. The real test is what you can gain at redline. (My money - if I had any - would be on the un-ballasted one) Too bad, I love a sure thing! Kirk Stant LS6-b (which loves ballasted pullups!) |
#6
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I just tried this - but had to use a pair of bicycles.
Me (unballasted) and my boss (ballasted) on similar bikes at the same speed coasted up a small hill. He was going significantly faster at the top. Happens every lunch time so must be true. At 13:30 08 September 2003, Szd41a wrote: A given glider is at level flight, IAS= 100 knots.After a pull-up, will it achieve more height gain with 100 liters (100 kgs) of ballast than with empty ballast???? Réjean |
#7
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Jim
Next time, challenge your boss on an endless hill, not a small one!!! It is obvious that at T=0 , if both your boss and you hit me, your boss will hurt me more than you:-)))). Gee! Are we trying to prove that it easier to move heavier load up the hill or what ???. "Jim Britton" a écrit dans le message de ... I just tried this - but had to use a pair of bicycles. Me (unballasted) and my boss (ballasted) on similar bikes at the same speed coasted up a small hill. He was going significantly faster at the top. Happens every lunch time so must be true. At 13:30 08 September 2003, Szd41a wrote: A given glider is at level flight, IAS= 100 knots.After a pull-up, will it achieve more height gain with 100 liters (100 kgs) of ballast than with empty ballast???? Réjean |
#8
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I think Kevin's right. (at least to the first order).
In a pullup you trade kinetic energy for potential energy, so (neglecting friction effects) the physics are (mV^2)/2=mgh -- kinetic equals potential energy. Or (solving for h): h=(V^2)/2g (the mass cancels out). You can try to add the drag parts back in, but the time is so short, I don't think it will not add up to much. I think maybe ther reason people associate ballast with taller zoomies is because the cruise speeds with ballast are higher. For my ship a McCready 10 pullup yields 700 feet with full ballast, 530 feet dry, but the speed is 15 knots higher. At 21:00 08 September 2003, Kirk Stant wrote: Kevin Neave wrote in message news:... The difference in height will be negligible. Not true. A full load of water makes a HUGE difference in pullup altitude gained The glider's energy, both potential & kinetic is proportional to Mass so the height gain for a given loss of velocity will be the same. Again, wrong - check your basic physics. You even say that the energy is proportional to mass. Therefore, more mass, more energy, more altitude gained. You appear to be confusing velocity with mass. However the ballasted glider will have a better sink rate at 100kts than the unballasted one so during the few seconds of the pull-up it will 'lose' less height. True, but the crossover point is quickly reached so this effect is probably negligable. On the other side of the equation the un-ballasted glider will be able to pull up to a lower speed, so it's change in velocity will be greater so the resulting height gain may be more. If you pull up below the ballasted sink rate crossover speed, sure a heavy glider will gain less. But at those speeds neither glider will gain much anyway. The real test is what you can gain at redline. (My money - if I had any - would be on the un-ballasted one) Too bad, I love a sure thing! Kirk Stant LS6-b (which loves ballasted pullups!) |
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#10
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