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#141
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poor lateral control on a slow tow?
At 23:51 05 January 2011, ProfChrisReed wrote:
I'm still not convinced by those who propose that the wings of the glider generate no extra lift (or even generate less lift) when climbing on tow. We know that on a winch launch the glider climbs because the wings generate more lift than in level/descending flight. This must be true because there is nothing pulling it up. However, we are told that on aerotow the wings generate the same (or less) lift as in level/descending flight and the tug just pulls the glider up the slope. Does this mean that the tug climbs in the same way, i.e. wings generate only enough lift to carry the weight of the tug, and the prop drags the tug up the slope? This doesn't match what I've read about how aircraft work. L=W only in level flight. I think the tug's wings generate more lift than its weight, and thus it climbs. If this is true, the same must be true for the glider behind it. Bring on an aerodynamicist to show me I'm wrong. Chris Yes, it does mean the tug climbs in the same way - the prop is dragging it uphill. In steady climb or descent the lift is actually slightly less than weight, with the remaning very small component of the weight balanced by the drag (in a glide) or the thrust (in a climb). This can be demonstrated very easily with a vector diagram of the forces acting on an aircraft, and you'll find this picture in every textbook on aircraft performance or flight dynamics. If you'd like to talk to an aerodynamicist, there are several contributing to this thread - but more locally Dr Graham Dorrington in the engineering department at Queen Mary has worked on an electric-powered sailplane http://www.flightglobal.com/articles...ane-flies.html and should be able to convince you. Doug (aerodynamicst & flight dynamicist at City University) |
#142
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poor lateral control on a slow tow?
At 05:22 06 January 2011, Eric Greenwell wrote:
On 1/5/2011 10:52 AM, Andreas Maurer wrote: On Wed, 5 Jan 2011 09:23:29 -0800 (PST), Derek C wrote: Gliders appear to get near to the stall during slow aerotows at much greater than their normal free flight stalling airspeeds. I would suggest that aerotowing must increase the wing loading in some way. I have to admit that I didn't bother to read all the 120+ postings about this topic, so please forgive me if the things that I'm going to post have already been mentioned in this thread. The main factor for the seemingly odd flying characteristics behind the tow plane is the downwash of the latter. Let me explain: The downwash has a significant angle (the air is deflected downwards behind the tow plane's wing to up to four degrees!), but due to the larger span of the glider it only affects the inner part of the glider's wing. (big snip) Andreas' posting was the clearest description for me of the wake effect. I'd love to see "3-D" perspective view of the wake behind a towplane, as I doubt I'm visualizing it well. -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) someone else posted this link before - there's a good picture half way down of the tip vortex behind a typical towplane http://www.sciencebuddies.org/scienc...Zoo_p057.shtml |
#143
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poor lateral control on a slow tow?
At 03:57 06 January 2011, Derek C wrote:
On Jan 6, 2:20=A0am, Andreas Maurer wrote: On Wed, 5 Jan 2011 15:09:39 -0800 (PST), Derek C wrote: Hi Derek, The two most scary aerotows I have ever had we 1) 2 up in a K13 behind a Rotax engined Falke at about 50 knots indicated airspeed Well... 50 kts is pretty slow... 2) 2 up in a K13 behind a 150hp Piper Cub when we visited another site. This tug wasn't very powerful anyway and its pilot seemed to be trying to demonstrate how slowly he could fly. Indicated airspeed slightly under 50 knots. Clear case: Low aspect ratio, wing loading twice of the ASK-13. How much above the stall speed of the Cub? 10 kts at maximum? Scary... I guess you had a word with the tow pilot afterwards. In both cases the glider wallowed about and it seemed very difficult to keep above the wake turbulence/prop wash. Yes, the typical situation for a very slow aerotow. I have not been towed by a Dimona or Katana, but they seem to be a bit faster than the above, so may not give the same problems. I think the problem is more lack of airspeed than the type of the tug aircraft. Well, I guess we both agree that this problem only manifests itself at the low-speed area of the envelope, don't we? The general consensus here in Germany (as well as my own experience) is that an aerotow behind a motorglider is *much* easier to control despite the fact that it takes place at 110-115 kph (60-63 kts) instead of the 130-140 kph (70-75 kts) =A0that are typical for Morane MS 893 and Robin Remorqeur. Of course similar wing loadings result in similar reactions to gusts, which helps to follow the tow plane. Regards Andreas Yes, but 50 knots (93kph) should be OK for a K13 when its stalling speed in free flight is only about 36 knots (67kph). But it clearly isn't! Therefore either the wing loading or the angle of attack (or both) must be greater during an aerotow than in free flight. Cheers, Derek C ... or the distribution of lift over the wing has changed ... |
#144
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poor lateral control on a slow tow?
At 23:51 05 January 2011, ProfChrisReed wrote:
I'm still not convinced by those who propose that the wings of the glider generate no extra lift (or even generate less lift) when climbing on tow. We know that on a winch launch the glider climbs because the wings generate more lift than in level/descending flight. This must be true because there is nothing pulling it up. However, we are told that on aerotow the wings generate the same (or less) lift as in level/descending flight and the tug just pulls the glider up the slope. Does this mean that the tug climbs in the same way, i.e. wings generate only enough lift to carry the weight of the tug, and the prop drags the tug up the slope? This doesn't match what I've read about how aircraft work. L=W only in level flight. I think the tug's wings generate more lift than its weight, and thus it climbs. If this is true, the same must be true for the glider behind it. Bring on an aerodynamicist to show me I'm wrong. A winch launch is very different because (a) the angle between the cable and the direction of motion of the glider is large, and therefore unlike a tow the downwards component of the cable tension is no longer negligible, and (b) the motion is not steady. In this case the lift is greater than the weight because it is partially counteracting the cable tension and weight. The precise balance depends on pilot and winch driver technique. Even so, it is still the forward component of the cable tension force that is doing the work required to raise the glider to its release height. The one (illegal in the UK I believe?) situation in which the glider wing lift is doing the work would be a kiting launch - and here you are in fact extracting the energy required from the wind ... something that a number of researchers are trying to do http://www.ampyxpower.com/PowerPlane.html |
#145
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poor lateral control on a slow tow?
At 18:52 05 January 2011, Andreas Maurer wrote:
On Wed, 5 Jan 2011 09:23:29 -0800 (PST), Derek C wrote: Gliders appear to get near to the stall during slow aerotows at much greater than their normal free flight stalling airspeeds. I would suggest that aerotowing must increase the wing loading in some way. I have to admit that I didn't bother to read all the 120+ postings about this topic, so please forgive me if the things that I'm going to post have already been mentioned in this thread. The main factor for the seemingly odd flying characteristics behind the tow plane is the downwash of the latter. Let me explain: The downwash has a significant angle (the air is deflected downwards behind the tow plane's wing to up to four degrees!), but due to the larger span of the glider it only affects the inner part of the glider's wing. Therefore, if the glider if lying laterally displaced, only one wing is affected by the downwash of the tow plane - four degrees of AoA difference between left and right wing need a lot of aileron to correct. Likeise, if the glider is flying straight behind the tow plane, the downwash *decreases* the AoA of the affected inner part of the wing. Getting the nose up by pulling back will restore the lift of the inner part of the glider's wing, but now the outer parts of the wing have a much higher AoA than they have in free flight. Voila, meet the the conditions for poor alieron efficiency (high AoA!) and tip stall. The downwash is reduced by - wingloading of the tow plane - wing span of the tow plane In other words: The more a tow plane looks like a motorglider (say, a Dimona, or Katana Extreme), the less the flight characteristics of the glider are affected. Anyone who has ever been towed behind a motorglider or a microlight will testify that problems like poor lateral control or running out of elevator don't exist there, despite a far slower tow (55 kts compared to a typical 70-75 kts behind a typical tow plane like Reorqeur or Pawnee). One interesting fact: When Akaflieg Braunschweig flight-tested their SB-13 flying wing (with a back-swept wing), they encountered a nose-down momentum after lift-off that could not be recovered and usually lead to a crash immediately after lift-off. Explanation: The downwash of the tow plane (Robin Remorqeur) hit the inner part of the wing, decreasing its AoA (and lift) and therefore shifting the center of lift backwards due to the sweepback. Increasing the length of the tow rope helped. Greetings from a snowy Germany Andreas Interesting experience with the SB-13. There's a chapter in Eric Brown's book 'Wings of the Weird & Wonderful' in which he describes flight tests of the GAL 56 flying wing glider in 1946. This was a 28deg swept wing with an aspect ratio of 5.8 towed by a Spitfire IX* (!!!) to 20000ft (!!). He describes the opposite effect, with a very strong (often uncontrollable) nose-up pitch on take-off - this was thought to be due to ground effect. In this case the tug span was similar (37ft) to the glider span (45ft), so the wake/wing interaction would be different. Interestingly he also reports that the GAL56 could be flown hands-free on the tow - unless the tug slipstream was entered, in which case all lateral and longitudinal control was lost. Robert Kronfield was later killed spinning this aircraft. |
#146
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poor lateral control on a slow tow?
On Jan 6, 9:50*pm, Doug Greenwell wrote:
A winch launch is very different because (a) the angle between the cable and the direction of motion of the glider is large, and therefore unlike a tow the downwards component of the cable tension is no longer negligible, and (b) the motion is not steady. * In this case the lift is greater than the weight because it is partially counteracting the cable tension and weight. *The precise balance depends on pilot and winch driver technique. *Even so, it is still the forward component of the cable tension force that is doing the work required to raise the glider to its release height. * It's pretty easy to show that in the early part of full climb on a winch launch when the cable is horizontal and neglecting cable weight: tension in the cable = glider weight * tan(climb angle) lift required from the wings = glider weight / cos(climb angle) The latter is identical to the lift required in a turn with the same bank angle as the winch launch climb angle. Thus at 45 degrees the tension is the same as the weight and the lift is 1.4 At 60 degrees the tension is 1.73 times the weight and the lift is twice. When using a tension-controlled winch, what tensions are actually used? |
#147
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poor lateral control on a slow tow?
On Jan 5, 8:45*pm, Andreas Maurer wrote:
On Wed, 5 Jan 2011 15:51:05 -0800 (PST), ProfChrisReed wrote: L=W only in level flight. Wrong. The correct term is L=W in STRAIGHT flight. If both forces are not equal, the resulting flightpath is going to be a circular arc. It's simple vector addition. Regards Andreas Well to get anal......straight and level.....(unaccelerated flight.) Also best to consider L+W+T+D=0 (Unaccelerated flight, climb, level or gliding) Once we have acceleration (change is velocity and or change in speed) then L+W+T+D do not =0 We now have "unbalanced" force(s) Cookie |
#148
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poor lateral control on a slow tow?
On Jan 5, 6:51*pm, ProfChrisReed wrote:
I'm still not convinced by those who propose that the wings of the glider generate no extra lift (or even generate less lift) when climbing on tow. We know that on a winch launch the glider climbs because the wings generate more lift than in level/descending flight. This must be true because there is nothing pulling it up. However, we are told that on aerotow the wings generate the same (or less) lift as in level/descending flight and the tug just pulls the glider up the slope. Does this mean that the tug climbs in the same way, i.e. wings generate only enough lift to carry the weight of the tug, and the prop drags the tug up the slope? This doesn't match what I've read about how aircraft work. L=W only in level flight. I think the tug's wings generate more lift than its weight, and thus it climbs. If this is true, the same must be true for the glider behind it. Bring on an aerodynamicist to show me I'm wrong. Chris. Winch towing is different than aerotowing on a couple of points. On winch the glider goes through a couple of accelerations (change is airspeed and or direction)..."arcing flight path". The glider also must carry the weight and drag of the winch cable. But the glider climbs on winch tow because the winch is imparting energy to the glider. "thrust" if you will. A glider climbs on tow only because of the energy imparted into it by the tow plane. A towplane climbs because of the energy inmparted into it by the engine, from the fuel. Its a study of potential energy (fuel) being converted into kenetic energy (thrust) being converted back into potential energy (height) A glider can only climb due to the following........ Tow plane Winch Thermal Wave Ridge lift Its own power source (motorglider) Being carried up a hill by a human (hang glider) Etc these are all external sources of power. To climb we need "excess horsepower".....not excess lift Excess lift will cause a change in flight direction and or change is airspeed. Cookie |
#149
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poor lateral control on a slow tow?
On Jan 5, 1:52*pm, Andreas Maurer wrote:
On Wed, 5 Jan 2011 09:23:29 -0800 (PST), Derek C wrote: Gliders appear to get near to the stall during slow aerotows at much greater than their normal free flight stalling airspeeds. I would suggest that aerotowing must increase the wing loading in some way. I have to admit that I didn't bother to read all the 120+ postings about this topic, so please forgive me if the things that I'm going to post have already been mentioned in this thread. The main factor for the seemingly odd flying characteristics behind the tow plane is the downwash of the latter. Let me explain: The downwash has a significant angle (the air is deflected downwards behind the tow plane's wing to up to four degrees!), but due to the larger span of the glider it only affects the inner part of the glider's wing. Therefore, if the glider if lying laterally displaced, only one wing is affected by the downwash of the tow plane - four degrees of AoA difference between left and right wing need a lot of aileron to correct. Likeise, if the glider is flying straight behind the tow plane, the downwash *decreases* the AoA of the affected inner part of the wing. Getting the nose up by pulling back will restore the lift of the inner part of the glider's wing, but now the outer parts of the wing have a much higher AoA than they have in free flight. Voila, meet the the conditions for poor alieron efficiency (high AoA!) and tip stall. The downwash is reduced by - wingloading of the tow plane - wing span of the tow plane In other words: The more a tow plane looks like a motorglider (say, a Dimona, or Katana Extreme), the less the flight characteristics of the glider are affected. Anyone who has ever been towed behind a motorglider or a microlight will testify that problems like poor lateral control or *running out of elevator don't exist there, despite a far slower tow (55 kts compared to a typical 70-75 kts behind a typical tow plane like Reorqeur or Pawnee). One interesting fact: When Akaflieg Braunschweig flight-tested their SB-13 flying wing (with a back-swept wing), they encountered a nose-down momentum after lift-off that could not be recovered and usually lead to a crash immediately after lift-off. Explanation: The downwash of the tow plane (Robin Remorqeur) hit the inner part of the wing, decreasing its AoA (and lift) and therefore shifting the center of lift backwards due to the sweepback. Increasing the length of the tow rope helped. Greetings from a snowy Germany Andreas Best explaination so far! Cookie |
#150
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poor lateral control on a slow tow?
On Jan 6, 5:24*am, "
wrote: On Jan 5, 6:51*pm, ProfChrisReed wrote: I'm still not convinced by those who propose that the wings of the glider generate no extra lift (or even generate less lift) when climbing on tow. We know that on a winch launch the glider climbs because the wings generate more lift than in level/descending flight. This must be true because there is nothing pulling it up. However, we are told that on aerotow the wings generate the same (or less) lift as in level/descending flight and the tug just pulls the glider up the slope. Does this mean that the tug climbs in the same way, i.e. wings generate only enough lift to carry the weight of the tug, and the prop drags the tug up the slope? This doesn't match what I've read about how aircraft work. L=W only in level flight. I think the tug's wings generate more lift than its weight, and thus it climbs. If this is true, the same must be true for the glider behind it. Bring on an aerodynamicist to show me I'm wrong. Chris. Winch towing is different than aerotowing on a couple of points. *On winch the glider goes through a couple of accelerations (change is airspeed and or direction)..."arcing flight path". * The glider also must carry the weight and drag of the winch cable. But the glider climbs on winch tow because the winch is imparting energy to the glider. "thrust" if you will. A glider climbs on tow only because of the energy imparted into it by the tow plane. A towplane climbs because of the energy inmparted into it by the engine, from the fuel. Its a study of potential energy (fuel) being converted into kenetic energy (thrust) being converted back into potential energy (height) A glider can only climb due to the following........ Tow plane Winch Thermal Wave Ridge lift Its own power source (motorglider) Being carried up a hill by a human (hang glider) Etc these are all external sources of power. To climb we need "excess horsepower".....not excess lift Excess lift will cause a change in flight direction and or change is airspeed. Cookie- Hide quoted text - - Show quoted text - Oops! Forgot the most important part: On winch the thrust is not along the direction of flight as it is in aerotow. The more the thrust is angled away from the direction of flight, the more lift needs to be generated to compenstae. Cookie |
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