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#51
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poor lateral control on a slow tow?
On Jan 2, 2:49*am, Doug Greenwell wrote:
At 03:11 02 January 2011, wrote: On Jan 1, 10:34=A0am, Doug Greenwell *wrote: At 15:09 01 January 2011, Derek C wrote: On Jan 1, 11:15=3DA0am, Doug Greenwell =A0wrote: At 20:23 31 December 2010, bildan wrote: On Dec 31, 1:06=3D3DA0pm, Todd =3DA0wrote: I too agree with the real or perceived tow handling characteristics. Looking at things =3D3DA0from and aerodynamics standpoint (and I am abou=3D t as far from and aerodynamicist as you can get) it should seem that part of the empirical data would suggest an experiment where you fly a glider equipped with and Angel of Attack meter at your typical tow speeds and record the AoA at various speeds. =3D3DA0Then fly that glider on tow at those same speeds and record the results. Done that - and as nearly as I can see, there's no difference in AoA. I've flown some pretty heavy high performance gliders behind some pretty bad tow pilots - one of them stalled the tug with me on tow. If I'm careful not to over-control the ailerons, there's no problem at all. Heavily ballasted gliders respond sluggishly in roll just due to the extra roll inertia. =3DA0A pilot trying to hold a precise position behind a tug needs and expects crisp aileron response. =3DA0When he doesn't get it, he increases the amount and frequency of aileron with a corresponding increase in adverse yaw. =3DA0If he's less than equally crisp with rudder to oppose the adverse yaw, it gets wobbly. Where did you mount the AoA meter? It's not the angle of attack that's the problem, but the change in local incidence along the wing. =3DA0The overall lift may not change by very much when near to the tug wake, but its distribution along the wing does, with increased lift at the tips and reduced lift at the root - putting the aileron region close to the stall and hence reducing control effectiveness. I agree that increased roll inertia due to ballast is a factor, but since the same factor applies to maintaining bank angle in a thermalling turn I don't see how it can account for a significant difference in handling between tow and thermalling?- Hide quoted text - - Show quoted text - What started the debate at Lasham was using a Rotax engined Falke as a glider tug. This towed best at about 50 to 55 knots (c.f. 60+ knots with a normal tug), but K13s with a stalling speed of 36 knots felt very unhappy behind it, especially two up. In a conventional powered aircraft you pull the nose up (to increase the angle of attack and produce more lift) and increase power to climb, the extra power being used to prevent the aircraft from slowing down. I don't see why gliders should behave any differently, except that the power is coming from an external source. As you try not to tow in the wake and downwash from the tug, I can't see that this is particularly significant, Derek C In a steady climb in any light aircraft the climb angles are so low ( 10deg) that the lift remains pretty well equal to weight. =A0For example = a 10deg climb angle at 60 kts corresponds to an impressive climb rate of 10.5kts - but that would only give Lift =3D Weight/cos(10deg) =3D 1.02 x Weight. =A0You don't need to increase lift to climb - you increase thrust to overcome the aft component of the weight, and the stick comes back to maintain speed ... at constant speed the increased power input comes out as increasing potential energy =3D increasing height. I think a lot of people confuse the actions needed to initiate a climb with what is actually happening in a steady climb. =A0 On your second point, if you are on tow anywhere sensible behind a tug yo= u are in its wake and are being affected by the wing downwash. =A0Wake is n= ot really a good word, since it seems to get confused with the much more localised (and turbulent) propwash. A (very) crude way of visualising the affected wake area is to imagine a cylinder with a diameter equal to the tug wing span extending back from the tug - that's the downwash region, and then in addition there's an upwash region extending perhaps another half-span out either side.- Hide = quoted text - - Show quoted text - "aft component of weight??" Not that this adds anything to the discussion, but.....weight acts in a "downward" direction toward the center of the earth. In a climb, on tow, the "aft" forces are drag (mostly) and a small bit of lift. Anyway, interesting topic.......has been beat to death at our local field...EVERY pilot seems to have had it happen, in all different kinds of gliders......many explainations....not one all-encompassing explaination yet. Cookie it depends on your reference frame - lift and drag are perpendicular to the direction of motion (relative to the air), which is inclined upwards - so if you take 'aft' as relative to the glider flight path rather than the earth, then there is an aft component of weight.- Hide quoted text - - Show quoted text - Yes, this is true......but to me it is better to keep the vectors simple. If you apply a component along the line of the fuselage (aft vector) then you have to add in the other component too. What direction? Remember aft is parallel to the glider, not the flight path of the glider. We could in fact break any vector up into any number of components......but eventually you have to combine them again. To me, using the Earth (as horizontal and vertical reference) is best. Then we can easily see the climb angle of the glider, the direction of flight if you will, and the speed. We can also easily see the angle of attack. With this reference we need to apply only 4 vectors (forces) lift, drag, weight, thrust. IF we use the glider itself, longitudinal axis as reference, we right away have 8 vectors to contend with. On tow, if we know any three forces, we can calculate the forth. In gliding flight its only three forces (thrust = 0) so its even easier. Ultimately, if in "steady flight" there is in fact no force acting on the glider......because the sum of all of the components = 0. I like your explaination of climbing aircraft above. Another way to look at it: (speed kept constant) IF thrust is greater than drag, the aircraft will climb, IF thrust = drag, the aircraft will fly level with the Earth. IF thrust is less than drag, the aircraft will descend. If thrust = 0 the aircraft will descent at its L/D angle. (assuming thrust is applied along the direction of flight) Oh yeah...yet another factor from the earlier version of this discussion. The force of the tow rope(thrust) does not necessarily act through the glider's center of gravity. Neither does the drag vector. This can cause a pitching moment, which will require elevator input to counteract. Another factor that can give a different "feel" on tow. Cookie |
#52
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poor lateral control on a slow tow?
On Jan 2, 6:01*am, Derek C wrote:
On Jan 1, 5:27*pm, Doug Greenwell wrote: At 16:43 01 January 2011, Derek C wrote: On Jan 1, 3:34=A0pm, Doug Greenwell *wrote: At 15:09 01 January 2011, Derek C wrote: On Jan 1, 11:15=3DA0am, Doug Greenwell =A0wrote: At 20:23 31 December 2010, bildan wrote: On Dec 31, 1:06=3D3DA0pm, Todd =3DA0wrote: I too agree with the real or perceived tow handling characteristics. Looking at things =3D3DA0from and aerodynamics standpoint (and I am abou=3D t as far from and aerodynamicist as you can get) it should seem that part of the empirical data would suggest an experiment where you fly a glider equipped with and Angel of Attack meter at your typical tow speeds and record the AoA at various speeds. =3D3DA0Then fly that glider on tow at those same speeds and record the results. Done that - and as nearly as I can see, there's no difference in AoA. I've flown some pretty heavy high performance gliders behind some pretty bad tow pilots - one of them stalled the tug with me on tow. If I'm careful not to over-control the ailerons, there's no problem at all. Heavily ballasted gliders respond sluggishly in roll just due to the extra roll inertia. =3DA0A pilot trying to hold a precise position behind a tug needs and expects crisp aileron response. =3DA0When he doesn't get it, he increases the amount and frequency of aileron with a corresponding increase in adverse yaw. =3DA0If he's less than equally crisp with rudder to oppose the adverse yaw, it gets wobbly. Where did you mount the AoA meter? It's not the angle of attack that's the problem, but the change in local incidence along the wing. =3DA0The overall lift may not change by very much when near to the tug wake, but its distribution along the wing does, with increased lift at the tips and reduced lift at the root - putting the aileron region close to the stall and hence reducing control effectiveness. I agree that increased roll inertia due to ballast is a factor, but since the same factor applies to maintaining bank angle in a thermalling turn I don't see how it can account for a significant difference in handling between tow and thermalling?- Hide quoted text - - Show quoted text - What started the debate at Lasham was using a Rotax engined Falke as a glider tug. This towed best at about 50 to 55 knots (c.f. 60+ knots with a normal tug), but K13s with a stalling speed of 36 knots felt very unhappy behind it, especially two up. In a conventional powered aircraft you pull the nose up (to increase the angle of attack and produce more lift) and increase power to climb, the extra power being used to prevent the aircraft from slowing down. I don't see why gliders should behave any differently, except that the power is coming from an external source. As you try not to tow in the wake and downwash from the tug, I can't see that this is particularly significant, Derek C In a steady climb in any light aircraft the climb angles are so low ( 10deg) that the lift remains pretty well equal to weight. =A0For example = a 10deg climb angle at 60 kts corresponds to an impressive climb rate of 10.5kts - but that would only give Lift =3D Weight/cos(10deg) =3D 1.02 x Weight. =A0You don't need to increase lift to climb - you increase thrust to overcome the aft component of the weight, and the stick comes back to maintain speed ... at constant speed the increased power input comes out as increasing potential energy =3D increasing height. I think a lot of people confuse the actions needed to initiate a climb with what is actually happening in a steady climb. =A0 On your second point, if you are on tow anywhere sensible behind a tug yo= u are in its wake and are being affected by the wing downwash. =A0Wake is n= ot really a good word, since it seems to get confused with the much more localised (and turbulent) propwash. A (very) crude way of visualising the affected wake area is to imagine a cylinder with a diameter equal to the tug wing span extending back from the tug - that's the downwash region, and then in addition there's an upwash region extending perhaps another half-span out either side.- Hide = quoted text - - Show quoted text - So why did a K13 feel on the verge of a stall at 50 knots on tow? All the classic symptoms of a stall were there, including mushy controls, wallowing around and buffeting. If you got even slightly low it seemed quite difficult to get back up to the normal position. Lack of elevator effectiveness is yet another sympton of the stall! Fortunately we have given up aerotowing with the Falke. It just seemed like a good idea at the time because its flying speeds are more closely matched to a glider; in theory anyway. Derek C good question - which suggests that something more complicated was going on? * Lack of elevator effectiveness is not really a symptom of stall as such .. it's a symptom of low airspeed. *So for buffeting and mushy, ineffective elevator to be happening at an indicated airspeed of 50-55 knots I'm wondering whether the tailplane was stalling rather than the wing? In this case you'd a tug with a wing span of a similar size to the glider (14.5m to 16m), which would put the tug and glider tip vortices very close together. *Two adjacent vortices of the same sign tend to wind up round each other and merge quite quickly - if this happened with the two sets of tip vortices it would generate an increased downwash near the tail and push the local (negative) incidence past the stall angle. I'd be the first to admit this is getting rather speculative - but these possible interaction effects would be amenable to some fairly straightforward wind tunnel testing *... a good student project for next year!- Actually the only totally reliable sysmptom of being stalled is that the elevator will no longer raise the nose. The elevator should still be effective at 50 knots, so it's more likely that the wing is close to the stall. The stall is only strictly related to the angle of attack. During a aerotow climb the wing has to support an additional weight component as well as drag, so the effective wing loading may well be increased, requiring a greater angle of attack for a given airspeed. Going 10 knots faster seems to cure the problem. Derek C- Hide quoted text - - Show quoted text - 'Actually the only totally reliable sysmptom of being stalled is that the elevator will no longer raise the nose.' HUH? Many cases possible where we could have full elevator and not be stalled. (I demonstrate this is 2-33 and grob 103 and ask-21. All you need is heavy pilot (forward CG) and gentle stick back to the stop. Glider will mush, but not stall. Elevator will not raise the nose........wing does not have angle to stall. On tow the only additional "weight component" would be a downward component to the tow rope (thrust). Since the tension on the tow rope is fairly low........it should not have a big effect, but there is some effect. But yeah, that extra 10 knots makes all the difference in the world. (I remember occasionally getting a "slow tow" when flying a 2-32 with three aboard..........what a handful!!! Cookie |
#53
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poor lateral control on a slow tow?
On Jan 2, 2:49*am, Doug Greenwell wrote:
At 03:11 02 January 2011, wrote: On Jan 1, 10:34=A0am, Doug Greenwell *wrote: At 15:09 01 January 2011, Derek C wrote: On Jan 1, 11:15=3DA0am, Doug Greenwell =A0wrote: At 20:23 31 December 2010, bildan wrote: On Dec 31, 1:06=3D3DA0pm, Todd =3DA0wrote: I too agree with the real or perceived tow handling characteristics. Looking at things =3D3DA0from and aerodynamics standpoint (and I am abou=3D t as far from and aerodynamicist as you can get) it should seem that part of the empirical data would suggest an experiment where you fly a glider equipped with and Angel of Attack meter at your typical tow speeds and record the AoA at various speeds. =3D3DA0Then fly that glider on tow at those same speeds and record the results. Done that - and as nearly as I can see, there's no difference in AoA. I've flown some pretty heavy high performance gliders behind some pretty bad tow pilots - one of them stalled the tug with me on tow. If I'm careful not to over-control the ailerons, there's no problem at all. Heavily ballasted gliders respond sluggishly in roll just due to the extra roll inertia. =3DA0A pilot trying to hold a precise position behind a tug needs and expects crisp aileron response. =3DA0When he doesn't get it, he increases the amount and frequency of aileron with a corresponding increase in adverse yaw. =3DA0If he's less than equally crisp with rudder to oppose the adverse yaw, it gets wobbly. Where did you mount the AoA meter? It's not the angle of attack that's the problem, but the change in local incidence along the wing. =3DA0The overall lift may not change by very much when near to the tug wake, but its distribution along the wing does, with increased lift at the tips and reduced lift at the root - putting the aileron region close to the stall and hence reducing control effectiveness. I agree that increased roll inertia due to ballast is a factor, but since the same factor applies to maintaining bank angle in a thermalling turn I don't see how it can account for a significant difference in handling between tow and thermalling?- Hide quoted text - - Show quoted text - What started the debate at Lasham was using a Rotax engined Falke as a glider tug. This towed best at about 50 to 55 knots (c.f. 60+ knots with a normal tug), but K13s with a stalling speed of 36 knots felt very unhappy behind it, especially two up. In a conventional powered aircraft you pull the nose up (to increase the angle of attack and produce more lift) and increase power to climb, the extra power being used to prevent the aircraft from slowing down. I don't see why gliders should behave any differently, except that the power is coming from an external source. As you try not to tow in the wake and downwash from the tug, I can't see that this is particularly significant, Derek C In a steady climb in any light aircraft the climb angles are so low ( 10deg) that the lift remains pretty well equal to weight. =A0For example = a 10deg climb angle at 60 kts corresponds to an impressive climb rate of 10.5kts - but that would only give Lift =3D Weight/cos(10deg) =3D 1.02 x Weight. =A0You don't need to increase lift to climb - you increase thrust to overcome the aft component of the weight, and the stick comes back to maintain speed ... at constant speed the increased power input comes out as increasing potential energy =3D increasing height. I think a lot of people confuse the actions needed to initiate a climb with what is actually happening in a steady climb. =A0 On your second point, if you are on tow anywhere sensible behind a tug yo= u are in its wake and are being affected by the wing downwash. =A0Wake is n= ot really a good word, since it seems to get confused with the much more localised (and turbulent) propwash. A (very) crude way of visualising the affected wake area is to imagine a cylinder with a diameter equal to the tug wing span extending back from the tug - that's the downwash region, and then in addition there's an upwash region extending perhaps another half-span out either side.- Hide = quoted text - - Show quoted text - "aft component of weight??" Not that this adds anything to the discussion, but.....weight acts in a "downward" direction toward the center of the earth. In a climb, on tow, the "aft" forces are drag (mostly) and a small bit of lift. Anyway, interesting topic.......has been beat to death at our local field...EVERY pilot seems to have had it happen, in all different kinds of gliders......many explainations....not one all-encompassing explaination yet. Cookie it depends on your reference frame - lift and drag are perpendicular to the direction of motion (relative to the air), which is inclined upwards - so if you take 'aft' as relative to the glider flight path rather than the earth, then there is an aft component of weight.- Hide quoted text - - Show quoted text - Lift is perpendicular.......drag is parallel........ |
#54
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poor lateral control on a slow tow?
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*'Actually the only totally reliable sysmptom of being stalled is that the elevator will no longer raise the nose.' HUH? * Many cases possible where we could have full elevator and not be stalled. *(I demonstrate this is 2-33 and grob 103 and ask-21. All you need is heavy pilot (forward CG) and gentle stick back to the stop. *Glider will mush, but not stall. *Elevator will not raise the nose........wing does not have angle to stall. .. whoa - depends on who's defining "stall". The FAA definition is indeed that when the aircraft does not respond in the direction of the control input that it's done. When you can no longer move the elevator up, you're done. Nose doesn't respond in direction of aft stick deflection, you're stalled. I don't remember exactly the way they word it, but the result is that touch the elevator limit, that's it. Slow entry rates result in higher stall speeds. Forward cg's give higher stall speeds. Trim settings (on some configs) affect stall speeds. Weight, etc., etc. The scene that seems the most insidious is the slow entry rate. They sneak up on you, kind of like a slow tow. |
#55
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poor lateral control on a slow tow?
On Jan 1, 8:29*pm, "
wrote: Then.....if the tow rope provides a forward and Downward pull........ (which was pretty much proven in an earlier discussion, by virtue of the 'sag" in the rope, the angle at which the rope meets the glider) * *then lift has to be GREATER than what you might at first think. * I was not part of that earlier discussion and I certainly don't accept that conclusion. All I have read here is that the D2, because of its very low angle of incidence, may have a downward pull on the nose (and even here downward would mean below the glider longitudinal axis, not necessarily below the horizon). I'm quite sure that my ASW 28 being towed on the CG hook has no downward force on the nose. When I do tow in gliders with a nose hook I'm quite sure there is no significant downward pull from the rope. Maybe it all depends on what you call high tow. I've seen may pilots tow tens of feet higher than I regard as normal high tow. Andy |
#56
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poor lateral control on a slow tow?
On Jan 2, 10:38*am, Andy wrote:
On Jan 1, 8:29*pm, " wrote: Then.....if the tow rope provides a forward and Downward pull........ (which was pretty much proven in an earlier discussion, by virtue of the 'sag" in the rope, the angle at which the rope meets the glider) * *then lift has to be GREATER than what you might at first think. * I was not part of that earlier discussion and I certainly don't accept that conclusion. All I have read here is that the D2, because of its very low angle of incidence, may have a downward pull on the nose (and even here downward would mean below the glider longitudinal axis, not necessarily below the horizon). *I'm quite sure that my ASW 28 being towed on the CG hook has no downward force on the nose. When I do tow in gliders with a nose hook I'm quite sure there is no significant downward pull from the rope. *Maybe it all depends on what you call high tow. *I've seen may pilots tow tens of feet higher than I regard as normal high tow. Andy |
#57
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poor lateral control on a slow tow?
On Sun, 02 Jan 2011 07:38:29 -0800, Andy wrote:
All I have read here is that the D2, because of its very low angle of incidence, may have a downward pull on the nose (and even here downward would mean below the glider longitudinal axis, not necessarily below the horizon). I'm quite sure that my ASW 28 being towed on the CG hook has no downward force on the nose. Hmmm, My Libelle glides at around 55 kts with the trim full forward so should need its nose held down a bit when being towed at 60-65 kts on the nose hook. Its possible that I am holding the nose down - all I can say is that I'm not aware of doing so once I'm off the ground, stabilised behind the tug and waiting for it to unstick, gain speed and start to climb. There is a noticeable catenary in the tow rope and, since that is a thin, flexible rope the pull on the nose hook will be at the same angle as the rope leaves the nose and not on the direct line between my nose-hook and the rope attachment point on the tug. This probably puts the force line above the glider CG and so is contributing a nose down moment. FWIW I estimate that climbing at 600 fpm at 60 kts is a 5.67 degree climb and that the tow rope tension is 37.62 kg for my glider (10 kg is drag due to the glider and the rest is due to the glider hanging from the rope). However, I don't know rope weight or exact length or how to calculate the sag in the rope and hence can't estimate the distance of the force line above or below the CG. -- martin@ | Martin Gregorie gregorie. | Essex, UK org | |
#58
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poor lateral control on a slow tow?
On Jan 1, 12:44*pm, Free Flight 107 wrote:
On Jan 1, 3:21*am, Doug Greenwell wrote: At 06:24 01 January 2011, Anne wrote: I've certainly sparked some interest here - considering it's New Year :-)- Hide quoted text - And I mignt add this is a very fast moving discussion too! While I was loging in 2 messages were posted.. Concerning the Tow Plane position while on tow, two of my CFIs have said to position yourglider as if you were going to Machine Gun the pilot of the Tow Plane. this is equivelent of aligning the horizontal of the TP with a portion of his foweward fuslage, like the wheels on a Pawnee. Works great in all conditions I've come accross in 15 years flying 8 different types from 2-33 to Duo Discuss. Never been criticized for it either in BFRs. Wayne Without a gunsight, how do you do that? ;^) I don't understand why the high tow position is taught by reference to the towplane or horizon, when what should be taught is how to find the correct tow position (just above or below the wake, which is actually the propwash). Simple - once safely airborne (usually before the towplane), just ease down until you feel the towplanes turbulence, then ease up a bit. THEN look at the towplane and pick whatever convenient references you need to maintain this vertical alignment. Any significant change in towplane speed will require a readjustment of the tow position (normally only a factor if on an aerotow retrieve). Obviously, if you only tow behind the same towplane on every flight, you will quickly learn where to position your glider. But if you have a variety of towplanes, or are towing behind something different (Agcat, Wilga, AN-2, whatever) for the first time, you can use this process to find the correct position quickly. Many US instructors seem to only teach HOW to do something without going into WHY it is done. As a result, there are a lot of "shortcuts" being taught, and a lot of poorly trained pilots, IMHO. A result of not having a standardized curriculum, a la BGA, perhaps? Kirk 66 |
#59
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poor lateral control on a slow tow?
-
Yes, this is true......but to me it is better to keep the vectors simple. If you apply a component along the line of the fuselage (aft vector) then you have to add in the other component too. What direction? Remember aft is parallel to the glider, not the flight path of the glider. We could in fact break any vector up into any number of components......but eventually you have to combine them again. To me, using the Earth (as horizontal and vertical reference) is best. Then we can easily see the climb angle of the glider, the direction of flight if you will, and the speed. We can also easily see the angle of attack. With this reference we need to apply only 4 vectors (forces) lift, drag, weight, thrust. IF we use the glider itself, longitudinal axis as reference, we right away have 8 vectors to contend with. On tow, if we know any three forces, we can calculate the forth. In gliding flight its only three forces (thrust =3D 0) so its even easier. Ultimately, if in "steady flight" there is in fact no force acting on the glider......because the sum of all of the components =3D 0. I like your explaination of climbing aircraft above. Another way to look at it: (speed kept constant) IF thrust is greater than drag, the aircraft will climb, IF thrust =3D drag, the aircraft will fly level with the Earth. IF thrust is less than drag, the aircraft will descend. If thrust =3D 0 the aircraft will descent at its L/D angle. (assuming thrust is applied along the direction of flight) Oh yeah...yet another factor from the earlier version of this discussion. The force of the tow rope(thrust) does not necessarily act through the glider's center of gravity. Neither does the drag vector. This can cause a pitching moment, which will require elevator input to counteract. Another factor that can give a different "feel" on tow. Cookie There are a multiplicity of possible axes systems that are used in flight dynamics - which one you use (and what you call it) depends on where you are (US, UK, rest of world), and which one makes the sums simpler! The equations of motion for climb and descent in free-flight are exactly the same - just some terms disappear, or change sign. On tow is another matter, since the tow rope angle introduces an additional force, and a constraint on the motion. DLR, the German aero reseach instititute, did do some work in 1999 on the longitudinal dynamics of aerotowing, looking at the effect of downwash, rope forces and hook position on pitch stability ... at least I think they did - I have the report, but need to get it translated! Doesn't look to contain anything concrete on lateral stability though. |
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