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#1
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Did we decide if ballasted pullups are higher????????
OK, here we go, just to exite trolls, believers, non-believers, math
fereaks, the lot: Off course wet pullups go higher. In the end, a pullup is conversion from kinetic energy (speed) to potential energy (altitude). Soaring is all about trading one of these for the other. Picture two identical gliders (please, no Discus/Duo Discus (sorry, couldn't resist)) both at 100 feet, 250 Km/h. Their kinetic energy is derived from their speed (equal in this setup) and mass. So the heavy one has the most kinetic energy. -Which one can then obtain the highest potential energy ? Elementary, my dear Watson. Happy soaring, Lars Peder Replace the obvious by a dot to respond via e-mail "Scott Correa" skrev i en meddelelse ... OK people, what was the verdict. I'm sure some logger equipped pullups were made. Who wins?? Wet or dry. I still think wet pullups go higher, but I can't prove it. Scott. |
#2
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In article ,
"Lars Peder Hansen" wrote: OK, here we go, just to exite trolls, believers, non-believers, math fereaks, the lot: Off course wet pullups go higher. In the end, a pullup is conversion from kinetic energy (speed) to potential energy (altitude). Soaring is all about trading one of these for the other. Picture two identical gliders (please, no Discus/Duo Discus (sorry, couldn't resist)) both at 100 feet, 250 Km/h. Their kinetic energy is derived from their speed (equal in this setup) and mass. So the heavy one has the most kinetic energy. -Which one can then obtain the highest potential energy ? Except that potential energy is proportional to both altitude *and* mass. IOW, double the mass, and you double the kinetic energy at a given speed, but you also double the potential energy of the change in altitude. Elementary, my dear Watson. If you think about it a moment, the correct answer is "Elementary, my dear Galileo" (think dropping balls of different masses, and then reversing the experiment). Happy soaring, Lars Peder Replace the obvious by a dot to respond via e-mail "Scott Correa" skrev i en meddelelse ... OK people, what was the verdict. I'm sure some logger equipped pullups were made. Who wins?? Wet or dry. I still think wet pullups go higher, but I can't prove it. Scott. -- Alan Baker Vancouver, British Columbia "If you raise the ceiling 4 feet, move the fireplace from that wall to that wall, you'll still only get the full stereophonic effect if you sit in the bottom of that cupboard." |
#3
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On Thu, 25 Sep 2003 13:38:01 -0500, "Scott Correa"
wrote: OK people, what was the verdict. I'm sure some logger equipped pullups were made. Who wins?? Wet or dry. I still think wet pullups go higher, but I can't prove it. What Alan, Mike and m said. Strictly from a potential/kinetic energy standpoint, it's a wash. Not APPROXIMATELY a wash --- EXACTLY a wash. However, there at least two more factors. First, the wet ship has to pitch up to a higher angle of attack as it starts the pullup, in order to achieve a given upward acceleration. This will extract a penalty in the form of energy removed by induced drag. Once the change of direction is completed, both pilots can reduce their angle of attack to the zero-lift point, giving an ascending parabola which will maximize the altitude reached. In this condition the only drag is parasite drag, which will be the same for both ships; consequently the wet ship has an edge here because its larger mass will decelerate less for a given drag force. So the answer to the question depends on which of these effects is larger. It shouldn't be hard to settle with an empirical test: just have two ships pull up simultaneously in line-abreast formation. rj |
#4
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Lars Peder Hansen wrote: OK, here we go, just to exite trolls, believers, non-believers, math fereaks, the lot: OK I'll bite Off course wet pullups go higher..... Picture two identical gliders (please, no Discus/Duo Discus (sorry, couldn't resist)) both at 100 feet, 250 Km/h. Their kinetic energy is derived from their speed (equal in this setup) and mass. So the heavy one has the most kinetic energy. -Which one can then obtain the highest potential energy ? Elementary, my dear Watson. Happy soaring, Lars Peder The heavier one does have more energy. Then again, it requires more energy to lift a heavier object. And the additional work required to lift a heavier object to the same height as a lighter one is directly proportional to the difference in mass. -- Peter D. Brown http://home.gci.net/~pdb/ http://groups.yahoo.com/group/akmtnsoaring/ |
#5
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"Mike Borgelt" wrote
Dear God! Didn't anyone pay attention to physics lessons in high school? Actually, I skipped a lot of physics & math lessons in high school, to go soaring with a friend from my class. ( I guess you figured that out by now ;-) We had a math teacher who eventually figured out the connection between blue skies with cumulus, and the two of us being absent from class. One day he called the gliderport, just to let us know what chapters to read for next week. -I was the unlucky one who picked up the phone... Happy soaring, and safe pullups to exactly the same altitude no matter what mass you fly at, Lars Peder |
#6
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I'm not sure I follow your reasoning about the heavy
glider having the advantage in the 2G case. In normal flight at high speed we have relatively little induced drag, and the major component of our total drag is profile. With it's higher reserve of energy the heavy glider gains the benefit 'cos even though the induced drag is higher it's a small proportion of the total. If we now start to pull G the induced drag for both gliders goes up, it now becomes a more significant proportion of the total for each glider so surely the advantage of the heavy glider is reduced? (And I notice that you have admitted you're in the 'Heavy Glider Wins' camp) :-) At 21:06 26 September 2003, Todd Pattist wrote: The advantage of the heavy glider (in terms of lost altitude) remains throughout the increased G-load portion of the pullup. Think of the two gliders suddenly doubling their weight (2G pullup). The ballasted glider would have a lower sink rate in the 2G case for the same reason that it has a lower sink rate in the 1G case. |
#7
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In article ,
Kevin Neave k wrote: I'm not sure I follow your reasoning about the heavy glider having the advantage in the 2G case. In normal flight at high speed we have relatively little induced drag, and the major component of our total drag is profile. With it's higher reserve of energy the heavy glider gains the benefit 'cos even though the induced drag is higher it's a small proportion of the total. If we now start to pull G the induced drag for both gliders goes up, it now becomes a more significant proportion of the total for each glider so surely the advantage of the heavy glider is reduced? But how hard are you pulling? Minimum drag (min sink) for the dry glider is probably at around 50 knots, so to get the same AOA at 110 knots you have to pull well over 4G. Min sink for the wet glider might be -- what -- 55 knots? So you're still talking 4G at 110 knots to get that AOA. If you're only pulling 2 or 3 G then you'll be above best L/D speed as well. The heavy glider still clearly has an advantage. -- Bruce |
#8
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Drag is the thing here...
My ASW24, being lighter than, say, a ASK-21, will get 200mts (600ft) from a high speed pull up. The ASK will get 100 to 130mts. A Blanik will get much less. A ballasted ASW-24 gets more from the pull up than I do unballasted. In our kind of flying drag is everything! Why an ASK-13 flyes less than a -21, a -24, a -25 and so on? How can we explain such large differences in performance? Drag is reduced for the newer gliders. You are all right with the math, and in the total energy equations mass is nonrelevant as it is a constant. But you're all disregarding the effect of drag and you cannot do that! That is the reason why all the fancy math you're doing does not match with our real life experience. And if a theorical reasoning does not match with reality, then it's obvious that the theory is somewhere wrong. So please if you want to get into math please do account drag and fluid mechanics into it. If you simplify it so much, it will be inaccurate enought to be false. Ballasted gliders will go higer because of the increased mass, more penetration and more energy for the same aerodinamic drag. "Bruce Hoult" escribió en el mensaje ... In article , Kevin Neave k wrote: Lots of maths snipped... |
#9
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The original post way, way, way back specified 100kts
& 100kgs ballast. Why will none of the 'Heavy Glider Wins' contingent actually give any numbers as to what they think the advantage here is? At 09:24 29 September 2003, Jose M. Alvarez wrote: Drag is the thing here... My ASW24, being lighter than, say, a ASK-21, will get 200mts (600ft) from a high speed pull up. And how much EXACTLY will it get from 100kts? A ballasted ASW-24 gets more from the pull up than I do unballasted. How much ballast EXACTLY are we talking about here? And EXACTLY how much extra height do you think you'd gain in a pull up from 100kts. You are all right with the math, and in the total energy equations mass is nonrelevant as it is a constant. But you're all disregarding the effect of drag and you cannot do that! And you're disregarding TIME, there's simply not enough time in which the slight difference in performance of the ballasted/unballasted glider has to operate. That is the reason why all the fancy math you're doing does not match with our real life experience. And if a theorical reasoning does not match with reality, then it's obvious that the theory is somewhere wrong. I think you're confusing 'perception' with reality here So please if you want to get into math please do account drag and fluid mechanics into it. If you simplify it so much, it will be inaccurate enought to be false. My analysis, including drag, but with some approximations (All in favour of the heavy glider) is on the way Ballasted gliders will go higer because of the increased mass, more penetration and more energy for the same aerodinamic drag. Ballasted Gliders average higher cross country speeds, finish faster, and with the pilot feeling better about life! That's why the perception is that ballasted gliders gain more in the pull-up!!! :-) |
#10
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OK Folks here we go with the maths, including that
old chesnut drag, and before any of you start to pick nits I'm going to use kilograms as a unit of 'Force' to avoid confusing our American friends who seem to use pounds as both a unit of mass and force. But I'm assuming that the analysis I'm doing here is for our own planet earth where G is about 9.8m/s/s and the difference in Gravitational field between the start and end of the pull up is negligible. Let's start with a Glider weighing 300kgs with a stall speed of 38kts (19m/s), and a best L/D of 40:1 at 60kts. If we now add 100kgs of ballast all the numbers are multiplied by sqrt(4/3) so we have a stall speed of 44kts (22m/s) and a best L/D of 40:1 at 69kts Now I believe that at Best L/D the Profile and Induced drag are equal (And I'm sure someone will correct me if I'm wrong). So for our light glider at 60kts we have a total 'drag' of 7.5kgs (300/40) which means that we have Induced = Profile = 3.5kgs I also believe that Induced Drag goes down with the square of the speed (i.e 1/Vsquared) (Again I'm sure I'll be corrected!) and that profile goes up with the square of the speed. Induced drag is also proportional to wing loading so our Heavy Glider will always have 4/3 the induced drag of the light one. So going back to our light glider... At 100kts our induced 'drag' is now (60/100) * (60/100) * 3.75 = 1.35kgs And our profile is (100/60) * (100/60) * 3.75 = 10.41kgs. Total 'drag' = 11.76kgs Assuming that adding the ballast doesn't alter the shape of our glider too much then for the heavy glider Induced = 1.35*4/3 = 1.8kgs Profile = 10.41kgs Total = 12.21kgs. Now we're ready to pull up & I'll make a few assumptions here. 1) That we're not going to pull up to below the stall speed of either glider. This is a reasonable assumption, pulling up to an airspeed of 0kts followed by a spin recovery and return to normal flight almost certainly hands the 'advantage' to the light glider (It'll hit the ground less hard!!) 2) We're going to pull up into a 45deg climb & maintain a straight line up to our recovery speed. This is not a ballistic trajectory 'cos in order to maintain this straight course the wings will have to generate some lift and so will upset my next assumption, which is... 3) We can ignore changes drag!! This is a pretty big assumption but here goes... At the same level of 'G' the induced drag is propotional to the wing loading i.e the heavy glider will have 4/3 times the induced drag. However the 'Energy Fuel Tank' of the heavy glider also has 4/3 as much as the Light one so I think the two effects cancel out (And once again I'm sure someone out there will correct me!). Secondly, as the speed drops off our profile drag will also be reduced with the square of our speed, in some ways this makes up for ignoring the increasing induced drag required to maintain our straight 45deg climb. Thirdly, making these assumptions about drag gives a slight advantage to the heavy glider. And Lastly it makes the maths simpler!! So here we go... Both gliders start to pull 2G. The induced drag for each is doubled (i.e goes to 2.7kgs for the light, 3.6 for the heavy) but since the change is not significant compared to the total I'm ignoring it! And we start to climb (I'm assuming that the time taken to transition from level flight to 45deg climb is small so any transient effects in our 2G pull won't be significant) The retarding force for the light glider is now 300kgs * 1/sqrt(2) due to gravity plus the 11.76kgs due to drag = 212.13 + 11.76 = 223.89kgs So our deceleration will be 223.89/300 * 9.8 = 7.31 m/s/s For the heavy glider we have 400 * 1/sqrt(2) for gravity plus 12.21kgs due to drag = 295.05kgs So our deceleration will be 295.05/400 * 9.8 = 7.23 m/s/s Now finally we're going to continue up our 45deg climb to our respective stall speeds, all of which is done at Newtonian rather than Einsteinian speeds So V*V = U*U - 2*a*s :- where V is final velocity, U is initial, A is acceleration and s is distance travelled. So ((U*U) - (V*V)) / (2*a) = s For the light glider V=19m/s, U=50m/s, a=7.31m/s/s ((50 * 50) - (19 * 19)) / (2 * 7.31) = 146.31 metres. So Our height gained = 1/sqrt(2) * 146.31 = 103.46m For the heavy glider V=22m/s, U=50m/s, a=7.23m/s/s ((50 * 50) - (22 * 22)) / (2 * 7.23) = 139.24 metres !! Height gained = 98.46 metres !! ------------------------------------------------------------------ --------------------------- OK, So there's some assumptions in the above, but I think all of them were made in favour of the heavy glider. But I say once again, for a pull up from 100kts with 100kgs of ballast, 'It's too close to call'... Over to you Todd :-)) |
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