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#1
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How efficient are our tailplanes?
Hi folks,
In order to get the best performance from the gliders we fly we need the C of G near the aft limit. How much difference does this actually make? Or is it just perception ? For example, If I add 60kgs of ballast to my Discus, I should put 2kgs in the fin tank. How much drag penalty will I incur by not doing so? Do I lose more by adding tailballast & sacrificing stability than I actually gain in reduced drag? (Hope this thread doesn't go on as long as the ballasted pull-up one :-)) Kevin |
#2
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Hi kevin,
No numbers, just some observations. After his first flight in a Duo Discus, a friend of mine said, "This thing won't climb or cruise". To which, I asked, "How much water do you have in the tail ?" He replied, "Nothing" To which, I said, "Fill it up" After his second flight in the Duo Discus (with water in the tail) My friend said, " This is a damned GOOD machine. End of my observstions concerning tail ballast in a ship with 2 pilots of 200+ lbs. each :) JJ Sinclair |
#3
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Hi kevin,
No numbers, just some observations. After his first flight in a Duo Discus, a One of our motorglider pilots pulled many pounds out of the glider nose and liked the results. When I flew my 172 for 400+ hours in 2 years, I noticed that certain C.G.s were better. All the way aft legal was too much, I think because the forward trim was so much that the angle of elevator to trim created a lot of drag. But 3/4 aft worked VERY well. It was amazing how much faster it cruised and how much easier it was to to trim and to flare with 3/4 aft CG. I flew a Twin Commander with a forward CG problem. I didn't notice until I got to fly a different Commander, and the controls were light. It seems this one still had the old radios left in as ballast in the aft. It was a totally different feel. Beware of old weight and balance sheets. The actual W&B may be quite different but the owner may be reluctant to reweigh it, since many aircraft lose quite a lot of useful load when officially reweighed. |
#4
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On 23 Oct 2003 11:48:25 GMT, Kevin Neave
wrote: Hi folks, In order to get the best performance from the gliders we fly we need the C of G near the aft limit. Well. maybe. How much difference does this actually make? As Robert Ehrlich pointed out in a earlier post at some point in the normal operating range of speeds you probably want to organise zero lift on the tail. Moving the C of G will move this point. Or is it just perception ? For example, If I add 60kgs of ballast to my Discus, I should put 2kgs in the fin tank. How much drag penalty will I incur by not doing so? Do I lose more by adding tailballast & sacrificing stability than I actually gain in reduced drag? It is possible to calculate the induced drag produced by the tail giving you 2 Kg of lift in either direction. I'll let someone else do that this time. You don't want to sacrifice so much stability that maintaining accrate attitude becomes difficult or takes too much concentration. I wasn't a believer in tail tanks until I flew an ASW20B with one. Our BD4 goes noticeably faster with someone in the backseat or heaps of baggage there and doesn't become unstable. Mike Borgelt |
#5
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Derrick Steed wrote:
Recommendation for keeping the discus sion short: 1. don't put ballast in wings I never do, since the 2 times I tried it, I had to dump it during the 5 first minutes of flight in order to stay aloft and avoid landing back. 2. put as much ballast as possible in fin tank I would never do that, since I am at the lower weight limit (62.5 kg (myself) + 7 kg (parachute) + .5 kg (various things) = 70 kg) 3. fly As often as I can |
#6
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At 09:30 24 October 2003, Robert Ehrlich wrote:
Derrick Steed wrote: Recommendation for keeping the discus sion short: 1. don't put ballast in wings I never do, since the 2 times I tried it, I had to dump it during the 5 first minutes of flight in order to stay aloft and avoid landing back. 2. put as much ballast as possible in fin tank I would never do that, since I am at the lower weight limit (62.5 kg (myself) + 7 kg (parachute) + .5 kg (various things) = 70 kg) 3. fly As often as I can I think (hope?) Derrick was joking ;-) With a full fin & no ballast I'd be 60kg or so below min cockpit weight!! |
#7
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this has been referenced before from the DG web site,
but it's not long so I'll post it here.. The Optimum C/G of Sailplanes A Caution from Wilhelm Dirks In the 'classic' aerodynamic theory of airplanes the wings generate lift and the tail plane generates stability. Because curved wing profiles are used, the aerodynamic moment generated by the wings, which tries to push the nose down and has to be countered by the tail plane. For this the tail plane has to produce downward force dependent on the airspeed and C/G. The higher the airspeed and the more forward the C/G, a higher downward force is produced. In a rearward C/G the tail plane can even produce a lifting force. Most contest pilots trim their sailplanes to the most rearward permissible position. In theory this improves performance, especially in circling flight one does not have to 'pull' on the stick as much. At the 'German Soaring Symposium' in Stuttgart a paper was presented and discussed which showed these well known facts in graphic detail for several sailplane types. In the first phase it was researched just how much the performance was affected by forward or rearward C/G positions. The permissible C/G values are determined by the designer. A forward C/G determines the size of the tail plane and elevator, i.e. that sufficient force is available in the elevator to make circling flight possible. The aft C/G is determined so that the airplane has satisfactory pitch stability and is able recover from a spin. Generally the calculations for all types showed a performance difference of 1.5 to 2 points between the foremost and rear most C/G positions. That is a significant difference! Is it therefore correct to fly with the rearmost C/G? Let's think about it. Performance Factors The L/D of a sailplane is calculated by the ratio of total lift to total drag. If the tail plane produces downward force the wings have to generate more lift, and that causes increased induced and profile drag, reducing the L/D. In spite of this the rearmost C/G does not necessarily produce better results. The tail plane is not designed to produce much lift. It normally has an almost symmetrical profile. The wing profile is designed to produce lift and is much better at this task. In addition the tail plane produces a disproportionate amount of induced drag because of it's low aspect ratio. The optimum condition would be one in which the tail plane in slow flight produces just enough lift to compensate for the loss of lift of the wings in the fuselage area. That would minimize the induced drag of the sailplane. This condition is obtained, more or less, depending on the design, by the distance of the tail plane from the most rearward C/G position. Flight Characteristics and Safety At aft C/G stability is minimal, especially in the roll axis, and the sailplane must be 'flown' at all times to avoid air speed variations usually encountered when thermaling. Depending on experience and skill, the pilot may tire faster and his concentration can diminish, so that the theoretical advantage is greatly reduced. At the rearmost C/G the sailplane will enter a spin much easier at less than the minimum airspeed then at forward C/G, where spinning in many instances is not even possible. This can have deadly results, especially close to the ground. Different sailplanes react differently, but the tendency is clear. Even an experienced contest pilot should be very wary of choosing the extreme rear most C/G position. At 10:06 24 October 2003, Kevin Neave wrote: At 09:30 24 October 2003, Robert Ehrlich wrote: Derrick Steed wrote: Recommendation for keeping the discus sion short: 1. don't put ballast in wings I never do, since the 2 times I tried it, I had to dump it during the 5 first minutes of flight in order to stay aloft and avoid landing back. 2. put as much ballast as possible in fin tank I would never do that, since I am at the lower weight limit (62.5 kg (myself) + 7 kg (parachute) + .5 kg (various things) = 70 kg) 3. fly As often as I can I think (hope?) Derrick was joking ;-) With a full fin & no ballast I'd be 60kg or so below min cockpit weight!! |
#8
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Just to tag on the ending of the DG article:
'The C/G definitely influences the flight performance and efficiency. A position in the forward half of the C/G envelope produces negative results and should be avoided. A good compromise is a C/G position about 30 to 40 % forward of the aftmost position. Flying with the C/G at the rearmost position is endangering your life. Check your C/G and do a weight/balance of your sailplane, and weigh yourself.' John Galloway At 13:18 24 October 2003, Mark Stevens wrote: this has been referenced before from the DG web site, but it's not long so I'll post it here.. The Optimum C/G of Sailplanes A Caution from Wilhelm Dirks In the 'classic' aerodynamic theory of airplanes the wings generate lift and the tail plane generates stability. Because curved wing profiles are used, the aerodynamic moment generated by the wings, which tries to push the nose down and has to be countered by the tail plane. For this the tail plane has to produce downward force dependent on the airspeed and C/G. The higher the airspeed and the more forward the C/G, a higher downward force is produced. In a rearward C/G the tail plane can even produce a lifting force. Most contest pilots trim their sailplanes to the most rearward permissible position. In theory this improves performance, especially in circling flight one does not have to 'pull' on the stick as much. At the 'German Soaring Symposium' in Stuttgart a paper was presented and discussed which showed these well known facts in graphic detail for several sailplane types. In the first phase it was researched just how much the performance was affected by forward or rearward C/G positions. The permissible C/G values are determined by the designer. A forward C/G determines the size of the tail plane and elevator, i.e. that sufficient force is available in the elevator to make circling flight possible. The aft C/G is determined so that the airplane has satisfactory pitch stability and is able recover from a spin. Generally the calculations for all types showed a performance difference of 1.5 to 2 points between the foremost and rear most C/G positions. That is a significant difference! Is it therefore correct to fly with the rearmost C/G? Let's think about it. Performance Factors The L/D of a sailplane is calculated by the ratio of total lift to total drag. If the tail plane produces downward force the wings have to generate more lift, and that causes increased induced and profile drag, reducing the L/D. In spite of this the rearmost C/G does not necessarily produce better results. The tail plane is not designed to produce much lift. It normally has an almost symmetrical profile. The wing profile is designed to produce lift and is much better at this task. In addition the tail plane produces a disproportionate amount of induced drag because of it's low aspect ratio. The optimum condition would be one in which the tail plane in slow flight produces just enough lift to compensate for the loss of lift of the wings in the fuselage area. That would minimize the induced drag of the sailplane. This condition is obtained, more or less, depending on the design, by the distance of the tail plane from the most rearward C/G position. Flight Characteristics and Safety At aft C/G stability is minimal, especially in the roll axis, and the sailplane must be 'flown' at all times to avoid air speed variations usually encountered when thermaling. Depending on experience and skill, the pilot may tire faster and his concentration can diminish, so that the theoretical advantage is greatly reduced. At the rearmost C/G the sailplane will enter a spin much easier at less than the minimum airspeed then at forward C/G, where spinning in many instances is not even possible. This can have deadly results, especially close to the ground. Different sailplanes react differently, but the tendency is clear. Even an experienced contest pilot should be very wary of choosing the extreme rear most C/G position. |
#9
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Would anyone out there like to suggest a figure for
how much induced drag is generated by the tailplane of a typical glider when producing 10kgs of download? |
#10
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And to reference the next article from the DG website
which rpeorts on actual Akaflieg flight testing: 'Add on after the soaring symposium Stuttgart 2000 and Braunschweig 2001: Members of the Akaflieg Braunschweig have once again focused on achieving an optimal center of gravity. They have therefore completed a number of test flights with two ASH-25 which had been carefully inspected, set up and checked. Both planes were equipped with highly complex measuring equipment due to the expected marginal rate of differences between the two planes. One was declared to be the reference plane, the other was designed to allow altering the center of gravity before every flight. The test flights have been completed with various speeds and flap settings. The results were later evaluated by computer. The specific results are of no real interest for us. A common pilot flies at various speeds with a variety of flap settings during a normal long duration flight. What was of interest to us was the average result of the test flights. And even without the complete set of results one thing can be noted: The aft position of the center of gravity is not desirable in order to achieve a really good result. The best results were achieved about 30 - 35 % in front of the aft limit of of the specifications for the center of gravity. That is caused by the elevator since it is not in a neutral (streamlined) position with a centered C/G. Instead it has to produce lift (an up elevator deflection) which it's not actually designed for resulting in increased drag. Some of our customers who want participate in competitions ask us to carefully weigh the aircraft and set it up at 98% aft center of gravity. Although we generally fulfill all of our customers wishes this is one we should not comply with. The sailplane will not fly any better by achieving this extreme aft center of gravity. It will simply react more nervously to pitch and roll inputs. In fact, should the pilot loose 2 liters (1/2 gal) of water during a long flight (de-hydration!), the sailplane has exceeded the specified limits of C/G and control in all axes becomes very sensitive. Please choose a center of gravity that is 30 to 35% before the rearmost limit of the aircraft's specification. This will grant an optimum in safety and performance.' John Galloway At 13:18 24 October 2003, Mark Stevens wrote: this has been referenced before from the DG web site, but it's not long so I'll post it here.. |
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