AirTalk.org Aviation discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Aerodynamics of carrying water Goto page 1, 2, 3, 4  Next          AirTalk.org Forum Index -> Sailplanes View previous topic :: View next topic   Author Message Gene Whitt Guest Posted: Tue Oct 11, 2005 2:55 am    Post subject: Aerodynamics of carrying water Y'all, Been many years on rec.aviation.student but even more years since gliding. Unable to explain the 'why' of water ballast to increase performance in gliders to argumentative airplane student. I need a simple explanation in 25 words or less. [email]gwhitt (AT) ix (DOT) netcom.com[/email] Gene Whitt Back to top Tony Verhulst Guest Posted: Tue Oct 11, 2005 3:22 am    Post subject: Re: Aerodynamics of carrying water Gene Whitt wrote: Quote: Y'all, Been many years on rec.aviation.student but even more years since gliding. Unable to explain the 'why' of water ballast to increase performance in gliders to argumentative airplane student. I need a simple explanation in 25 words or less. [email]gwhitt (AT) ix (DOT) netcom.com[/email] Gene Whitt For the long answer, see http://home.comcast.net/%7Everhulst/GBSC/student/ballast.html The real short answer is - see the polar at the end of the above article. One curve is with ballast, the other without. The slightly longer answer is that a glider's best glide, for instance, occurs at one speed. Increase the weight and that same best glide (more or less) now occurs at a higher speed. You can go faster and maintain a better L/D than you would without ballast. Tony V. Back to top Pete Brown Guest Posted: Tue Oct 11, 2005 3:26 am    Post subject: Re: Aerodynamics of carrying water Gene: 1: A glider's best glide ratio is unaffected by its weight. 2: However, a heavier glider flies and sinks faster at the same glide angle than a lighter one. 3: When lift conditions are strong, the pilot accepts the higher sink rates to achieve higher speeds over the ground. Pete Gene Whitt wrote: Quote: Y'all, Been many years on rec.aviation.student but even more years since gliding. Unable to explain the 'why' of water ballast to increase performance in gliders to argumentative airplane student. I need a simple explanation in 25 words or less. [email]gwhitt (AT) ix (DOT) netcom.com[/email] Gene Whitt -- Peter D. Brown http://home.gci.net/~pdb/ http://groups.yahoo.com/group/akmtnsoaring/ Back to top Nyal Williams Guest Posted: Tue Oct 11, 2005 6:01 am    Post subject: Re: Aerodynamics of carrying water The best illustration used to be: Remember when you rode your coaster wagon downhill alone and also with a buddy in it? It always went faster with two people. Who knows anything about coaster wagons anymore? At 03:30 11 October 2005, Pete Brown wrote: Quote: Gene: 1: A glider's best glide ratio is unaffected by its weight. 2: However, a heavier glider flies and sinks faster at the same glide angle than a lighter one. 3: When lift conditions are strong, the pilot accepts the higher sink rates to achieve higher speeds over the ground. Pete Gene Whitt wrote: Y'all, Been many years on rec.aviation.student but even more years since gliding. Unable to explain the 'why' of water ballast to increase performance in gliders to argumentative airplane student. I need a simple explanation in 25 words or less. [email]gwhitt (AT) ix (DOT) netcom.com[/email] Gene Whitt -- Peter D. Brown http://home.gci.net/~pdb/ http://groups.yahoo.com/group/akmtnsoaring/ Back to top Andy Guest Posted: Tue Oct 11, 2005 10:52 am    Post subject: Re: Aerodynamics of carrying water Gene Whitt wrote: Quote: Y'all, Been many years on rec.aviation.student but even more years since gliding. Unable to explain the 'why' of water ballast to increase performance in gliders to argumentative airplane student. I need a simple explanation in 25 words or less. [email]gwhitt (AT) ix (DOT) netcom.com[/email] Gene Whitt A ballasted sailplane has more energy at a given altitude and airspeed than an unballasted sailplane. (potential + kinetic). At cruise speeds the energy is dissipated predominantly to overcome parasite drag which is independent of weight. Start with more energy, expend the same to overcome drag, so give up less altitude. Edit to 25 words. Andy Back to top boblepp@aci.on.ca Guest Posted: Tue Oct 11, 2005 12:38 pm    Post subject: Re: Aerodynamics of carrying water Vigorously throw pingpong ball. Throw golf ball as vigorously. Walk to pingpong ball. Stop. Can you see golf ball yet? Think. Ponder. Consider. Massier = Energier (25 words plus punctuation, not all words are real English) Back to top Tony Verhulst Guest Posted: Tue Oct 11, 2005 1:00 pm    Post subject: Re: Aerodynamics of carrying water Gary Emerson wrote: Quote: We have a WINNER! I think not (though I do like the answer ). While "Massier = Energier", the difference in golf ball / ping pong ball performance is explained by ballistics (see "sectional density" and "ballistic coefficient") and not aerodynamics. I am *not* an expert, i could be wrong. Tony V. Quote: Vigorously throw pingpong ball. Throw golf ball as vigorously. Walk to pingpong ball. Stop. Can you see golf ball yet? Think. Ponder. Consider. Massier = Energier (25 words plus punctuation, not all words are real English) Back to top Gary Emerson Guest Posted: Tue Oct 11, 2005 1:16 pm    Post subject: Re: Aerodynamics of carrying water We have a WINNER! [email]boblepp (AT) aci (DOT) on.ca[/email] wrote: Quote: Vigorously throw pingpong ball. Throw golf ball as vigorously. Walk to pingpong ball. Stop. Can you see golf ball yet? Think. Ponder. Consider. Massier = Energier (25 words plus punctuation, not all words are real English) Back to top Derrick Steed Guest Posted: Tue Oct 11, 2005 3:16 pm    Post subject: Re: Aerodynamics of carrying water Gene Whitt wrote: Quote: Y'all, Been many years on rec.aviation.student but even more years since gliding. Unable to explain the 'why' of water ballast to increase=20 performance in gliders to argumentative airplane student. =20 I need a simple explanation in 25 words or less. [email]gwhitt (AT) ix (DOT) netcom.com[/email] Gene Whitt=20 For the long answer, see=20 http://home.comcast.net/%7Everhulst/GBSC/student/ballast.html The real short answer is - see the polar at the end of the above=20 article. One curve is with ballast, the other without. The slightly longer answer is that a glider's best glide, for instance,=20 occurs at one speed. Increase the weight and that same best glide (more=20 or less) now occurs at a higher speed. You can go faster and maintain a=20 better L/D than you would without ballast. Tony V. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Did you mean to say in the same sentence that the L/D is the same, but better? The polar is magnified in the speed and sink directions by the square root of the ratio of the weights, so that (simplistically) the L/D is the same but occurs at a higher speed and sink rate. In other words, for the same start height both the heavy and the light glider will hit the ground at the same place, but the heavy glider will get there first (so the golf ball/ping pong ball argument patently doesn't work). And actually, the best L/D is slightly better on the heavy glider - the reason is due to the fact that the heavy glider is operating at a higher Reynolds number and has a slightly higher coefficient of lift as consequence. Look at the LAK12 spec. and you'll see that it's advertised L/D at max weight is 50 whereas at min weight it's 48, that's true of other gliders too even if the manufacturer doesn't make a big deal of it. Rgds, Derrick Steed Back to top Eric Greenwell Guest Posted: Tue Oct 11, 2005 3:29 pm    Post subject: Re: Aerodynamics of carrying water Gene Whitt wrote: Quote: Y'all, Been many years on rec.aviation.student but even more years since gliding. Unable to explain the 'why' of water ballast to increase performance in gliders to argumentative airplane student. I need a simple explanation in 25 words or less. [email]gwhitt (AT) ix (DOT) netcom.com[/email] Gene Whitt Try this: The important points are the cruise speed goes up, the sink rate goes up, but the increased sink rate is insignificant on a strong day. For why the glide angle remains the same with different aircraft weights: * the best L/D occurs at one particular angle of attack (AOA is the important parameter here) * for low weights, this AOA will produce a low airspeed; at higher weights, higher airspeeds For why this is beneficial, given the glider won't climb as quickly in a thermal at higher weights; for example, with a 20% weight increase with ballast: * you get a 10% increase in cruise speed * you get a 10% increase in sink rate, but that's only 2.5% decrease in the climb rate (unballasted sink rate of 150 fpm while thermalling) on a 600 fpm day If you get these points across, you can generalize the "best L/D" to other AOAs, and talk about how we don't fly at the best L/D on a good day. -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA Back to top Bill Daniels Guest Posted: Tue Oct 11, 2005 4:27 pm    Post subject: Re: Aerodynamics of carrying water "Gene Whitt" <gwhitt (AT) ix (DOT) netcom.com> wrote Quote: Y'all, Been many years on rec.aviation.student but even more years since gliding. Unable to explain the 'why' of water ballast to increase performance in gliders to argumentative airplane student. I need a simple explanation in 25 words or less. [email]gwhitt (AT) ix (DOT) netcom.com[/email] Gene Whitt All good answers. I would add that an examination of two polars for a particular glider, one ballasted and one not, will show a crossover airspeed above which the sink rate with ballast is less than without - very counterintuitive. Most modern gliders flying in conditions with greater than 2Kt thermals will spend 80% of their X/C time flying at airspeeds above the crossover point where the sink rate is reduced by using ballast. Flying fast with reduced sink is dramatic. Bill Daniels Back to top Pat Russell Guest Posted: Tue Oct 11, 2005 4:38 pm    Post subject: Re: Aerodynamics of carrying water On 11 Oct 2005 03:52:06 -0700, "Andy" <a.durbin (AT) netzero (DOT) net> wrote: Quote: Edit to 25 words. "A glider is a gravity-powered machine. The heavier it is, the more power it has." 16 words, beat that! -Pat Back to top Andy Guest Posted: Tue Oct 11, 2005 4:43 pm    Post subject: Re: Aerodynamics of carrying water The polar documents the phenomenon, it does not explain it. Any attempt to explain the use of ballast by reference to the polar should leave any intelligent person asking - but why does the polar do that. I believe the effect can only be explained by considering the system energy exchange. Once that is understood the effect on the polar family is intuitive. Back to top Andy Guest Posted: Tue Oct 11, 2005 6:10 pm    Post subject: Re: Aerodynamics of carrying water But I need the same amount of power to go the same speed don't I? Why does more power reduce my sink rate? I think you'll need more than 16 words. Andy Back to top Derrick Steed Guest Posted: Tue Oct 11, 2005 6:31 pm    Post subject: Re: Aerodynamics of carrying water The polar documents the phenomenon, it does not explain it. Any attempt to explain the use of ballast by reference to the polar should leave any intelligent person asking - but why does the polar do that. I believe the effect can only be explained by considering the system energy exchange. Once that is understood the effect on the polar family is intuitive. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ but loses all the effects due to Reynolds number which is an aerodynamic phenomena and the original question was about the aerodynamics, not the dynamics. 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