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#41
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poor lateral control on a slow tow?
On Jan 1, 3:00*am, Doug Greenwell wrote:
At 23:25 31 December 2010, Andy wrote: On Dec 31, 1:47=A0pm, Martin Gregorie wrote: On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote: On Dec 31, 6:19=A0pm, bildan *wrote: On Dec 31, 4:40=A0am, "Doug" *wrote: As an aerodynamicist/flight dynamicist recently re-soloed after 25 years off, people keep asking me hard questions. =A0One that has com= e up recently is why a heavy glider on tow feels horrible, but thermalling in the same glider at lower speeds is fine? (see also Mike Fox's article on aerotowing in the October issue of S&G). I did some calculations, and I reckon it's probably due to the tug wing wake (tip vortices generating a downwash inboard, upwash outboard) changing the lift distribution on the glider wing - with a= n increased angle of attack out at the tips reducing aileron effectiveness. =A0There's possibly an interesting academic research project here, but it's always best to get a reality check first .. Is poor handling at low speed on tow a common experience? =A0I'd appreciate any thoughts/comments/war stories ... particularly bad tug/glider/speed combinations, incidents of wing drop during a tow etc etc? Doug Greenwell I suspect, but can't know unless I flew with you, that you are unconsciously trying to "steer" the glider with ailerons. =A0Overuse o= f ailerons is very common and it makes aero tow 'wobbly'. =A0If you consciously use rudder to aim the nose at the tug's tail and just keep the same bank angle as the tug with ailerons, it might work better. Wake effects are generally favorable if you stay at the right height relative to the tug. =A0Using a slightly higher tow position can sometimes help a lot. The tip vortices rotate inward above the propwash which, if allowed to do so, will drift the glider to the center position and help keep it there. =A0I haven't noticed any tendency for them to yaw a glider towa= rds a tugs wing tip.- Hide quoted text - - Show quoted text - There was a debate on our club forum about why gliders feel uncomfortable on slow tows that are still well above their normal stalling speed. We think the answer is that the glider is being asked t= o climb with the tug providing the thrust via the rope. The glider is still effectively in free flight and therefore has to fly at a greater angle of attack for a given airspeed to produce the extra lift for climbing. Hence its stalling speed is somewhat increased. If the tug's downwash field extends back far enough to include the glider, its AOA will be relative to the downwash streamlines. Add the downwash angle to the climb angle of the tug-glider combination will make the glider look quite nose-high to its pilot. =A0 I know that the downwash angle is roughly 1/3 of the wing AOA at 4-5 chords behind the wing, i.e. about where the tailplane is, but not what its angle might be at the end of a tow rope. -- martin@ =A0 | Martin Gregorie gregorie. | Essex, UK org =A0 =A0 =A0 | I'd be surprised if the flow field from the towplane wake is significant for gliders in normal high tow position. I do wonder if the "sluggish controls" effect is to some extent psychological because flying formation requires much more precision than normal slow flight off tow. I'm most uncomfortable when I find myself slow and below the towplane and need to climb up. Unless the glider is accelerating vertically, I'm pretty sure that steady climb requires the same amount of lift as steady glide. Steady climb is not the same as accelerating climb. (F=3DMxA so if the lifting force exceeds the glider's weight by definition it accelerates vertically). The towplane provides thrust to overcome the frictional and lift- related drag losses, but unless you are well below the towplane the force on the rope is, for all practical purposes, horizontal. If you have a cg hook you will get a modest nose-up pitching moment from the rope, but this is a trim issue more than an AOA issue I believe. The tension on the rope could also provide some counter-force to rudder and elevator inputs, but I don't think you'd feel much for small angular displacements. 9B It is surprising, but part of the problem is the word 'wake' ... in order to generate lift a wing has to move a fair amount of air around (although let's not start the bernoulli argument now!), so its influence on the surrounding atmosphere extends a surprising distance away from it. Tip vortices are also a very stable flow structure, so don't begin to break up or decay for a very very long way downstream. The climb angles are too small to make a significant difference to the lift required from the glider wing (assuming the tow rope is straight), since the effect on lift goes with the cosine of the angle On the other hand, if the tow rope is not straight then there could be a significant lift component from the tension force (going with the sine of the tow rope angle) ... but you would have to be quite a long way above the tug to make a big difference. Span differential, the nature of wake roll-ups, and effects in the larger free stream. An airfoil moving through a viscous media makes quite a disturbance. Among other things, it results in relative upwash upstream in the flow field, downwash aft in the flow field, and effects which are vertically displaced in the flow field as well. However, lateral influence in the flow field- outside the wake rollup at the tips- is of special interest here. Wake rollups with vorticity do not spread the energy spent in achieving pressure equilibrium very efficiently. That is why displacing the event over a larger area such as laterally (as in more span) or vertically (as in the case of winglets) makes the wing more efficient. Since the wing doesn’t do a very good job of inducing lift beyond the tip on the other side of the wake rollup, the downwash immediately aft of the wing is significantly greater than the downwash aft of the wing and a meter or two outboard of the tips. This lateral downwash differential is preserved in the aft flow field, albeit to lesser degrees with increasing distance until the free stream reaches unity. However, when being towed slow and heavy it doesn't take much to create a noticeable effect. In the case of a tow plane with 10-11 meter span towing a glider of 15 meter span, the downwash aft of the towplane and inboard on the glider span is greater than the free stream field meeting the tips and the ailerons. The effect is that a glider under tow can behave more like a design with wings geometrically twisted in the wrong direction- with ailerons operating near the stall. The effect increases with increasing downwash required of the towplane. One way to check this effect would be tow behind a motorglider of greater span than your glider. This should provide for a better match of downwash angles across your span.Get all the climb you can for a given airspeed. Time your roll rates. Then tow behind a conventional towplane at the same speed and same climb rate as the first case. Time and compare roll rates. You can also check numerically by calculating the rolling moments and taking into account the assymetrical lift distributions using the methods of Multhopp and Redeker. However, arriving at the effective angles of attack across the span, in a modified and vertically displaced flow field 200 feet aft of the tow plane might be rather difficult. Several angle of attack probes positioned in front of your wing and distributed along the span would likely be the better approach. If a towplane could push rather than pull a glider, the effect would be reversed and the aileron authority would increase. Best Regards, Gary Osoba |
#42
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poor lateral control on a slow tow?
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 |
#43
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poor lateral control on a slow tow?
At 19:29 01 January 2011, Gary Osoba wrote:
Span differential, the nature of wake roll-ups, and effects in the larger free stream. An airfoil moving through a viscous media makes quite a disturbance. Among other things, it results in relative upwash upstream in the flow field, downwash aft in the flow field, and effects which are vertically displaced in the flow field as well. However, lateral influence in the flow field- outside the wake rollup at the tips- is of special interest here. Wake rollups with vorticity do not spread the energy spent in achieving pressure equilibrium very efficiently. That is why displacing the event over a larger area such as laterally (as in more span) or vertically (as in the case of winglets) makes the wing more efficient. Since the wing doesn=92t do a very good job of inducing lift beyond the tip on the other side of the wake rollup, the downwash immediately aft of the wing is significantly greater than the downwash aft of the wing and a meter or two outboard of the tips. This lateral downwash differential is preserved in the aft flow field, albeit to lesser degrees with increasing distance until the free stream reaches unity. However, when being towed slow and heavy it doesn't take much to create a noticeable effect. In the case of a tow plane with 10-11 meter span towing a glider of 15 meter span, the downwash aft of the towplane and inboard on the glider span is greater than the free stream field meeting the tips and the ailerons. The effect is that a glider under tow can behave more like a design with wings geometrically twisted in the wrong direction- with ailerons operating near the stall. The effect increases with increasing downwash required of the towplane. One way to check this effect would be tow behind a motorglider of greater span than your glider. This should provide for a better match of downwash angles across your span.Get all the climb you can for a given airspeed. Time your roll rates. Then tow behind a conventional towplane at the same speed and same climb rate as the first case. Time and compare roll rates. You can also check numerically by calculating the rolling moments and taking into account the assymetrical lift distributions using the methods of Multhopp and Redeker. However, arriving at the effective angles of attack across the span, in a modified and vertically displaced flow field 200 feet aft of the tow plane might be rather difficult. Several angle of attack probes positioned in front of your wing and distributed along the span would likely be the better approach. If a towplane could push rather than pull a glider, the effect would be reversed and the aileron authority would increase. Best Regards, Gary Osoba Absolutely - I used a simple vortex lattice method (AVL) to come to the same conclusion. For the relatively short spacing between glider and towplane the lack of a wake roll-up model in this code probbaly doesn't affect the adverse change in spanwise lift distribution on the glider wing - however, modelling any more complex interactions between tug and glider vortices would need a proper CFD study. The other interesting element is the effect of bank - with 15m+ wing spans it doesn't take much of a roll angle to put one tip right into the tug wake, leading to yet more asymmetric effects. Most experimental wake vortex interaction studies (eg recent Airbus A380 studies) have used models in big ship tow tanks to get long vortices, but because glider and tug are so close, we could probably use a large wind tunnel. Another possibility would be to fly a motor glider behind a tug aircraft, in order to take out any effect of rope angle. It would probably be difficult to get someone to fund it though, since the solution is simple - tow faster :-) basic conclusions towthis doesm |
#44
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poor lateral control on a slow tow?
On Jan 1, 6:00*am, Doug Greenwell wrote:
At 23:25 31 December 2010, Andy wrote: On Dec 31, 1:47=A0pm, Martin Gregorie wrote: On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote: On Dec 31, 6:19=A0pm, bildan *wrote: On Dec 31, 4:40=A0am, "Doug" *wrote: As an aerodynamicist/flight dynamicist recently re-soloed after 25 years off, people keep asking me hard questions. =A0One that has com= e up recently is why a heavy glider on tow feels horrible, but thermalling in the same glider at lower speeds is fine? (see also Mike Fox's article on aerotowing in the October issue of S&G). I did some calculations, and I reckon it's probably due to the tug wing wake (tip vortices generating a downwash inboard, upwash outboard) changing the lift distribution on the glider wing - with a= n increased angle of attack out at the tips reducing aileron effectiveness. =A0There's possibly an interesting academic research project here, but it's always best to get a reality check first .. Is poor handling at low speed on tow a common experience? =A0I'd appreciate any thoughts/comments/war stories ... particularly bad tug/glider/speed combinations, incidents of wing drop during a tow etc etc? Doug Greenwell I suspect, but can't know unless I flew with you, that you are unconsciously trying to "steer" the glider with ailerons. =A0Overuse o= f ailerons is very common and it makes aero tow 'wobbly'. =A0If you consciously use rudder to aim the nose at the tug's tail and just keep the same bank angle as the tug with ailerons, it might work better. Wake effects are generally favorable if you stay at the right height relative to the tug. =A0Using a slightly higher tow position can sometimes help a lot. The tip vortices rotate inward above the propwash which, if allowed to do so, will drift the glider to the center position and help keep it there. =A0I haven't noticed any tendency for them to yaw a glider towa= rds a tugs wing tip.- Hide quoted text - - Show quoted text - There was a debate on our club forum about why gliders feel uncomfortable on slow tows that are still well above their normal stalling speed. We think the answer is that the glider is being asked t= o climb with the tug providing the thrust via the rope. The glider is still effectively in free flight and therefore has to fly at a greater angle of attack for a given airspeed to produce the extra lift for climbing. Hence its stalling speed is somewhat increased. If the tug's downwash field extends back far enough to include the glider, its AOA will be relative to the downwash streamlines. Add the downwash angle to the climb angle of the tug-glider combination will make the glider look quite nose-high to its pilot. =A0 I know that the downwash angle is roughly 1/3 of the wing AOA at 4-5 chords behind the wing, i.e. about where the tailplane is, but not what its angle might be at the end of a tow rope. -- martin@ =A0 | Martin Gregorie gregorie. | Essex, UK org =A0 =A0 =A0 | I'd be surprised if the flow field from the towplane wake is significant for gliders in normal high tow position. I do wonder if the "sluggish controls" effect is to some extent psychological because flying formation requires much more precision than normal slow flight off tow. I'm most uncomfortable when I find myself slow and below the towplane and need to climb up. Unless the glider is accelerating vertically, I'm pretty sure that steady climb requires the same amount of lift as steady glide. Steady climb is not the same as accelerating climb. (F=3DMxA so if the lifting force exceeds the glider's weight by definition it accelerates vertically). The towplane provides thrust to overcome the frictional and lift- related drag losses, but unless you are well below the towplane the force on the rope is, for all practical purposes, horizontal. If you have a cg hook you will get a modest nose-up pitching moment from the rope, but this is a trim issue more than an AOA issue I believe. The tension on the rope could also provide some counter-force to rudder and elevator inputs, but I don't think you'd feel much for small angular displacements. 9B It is surprising, but part of the problem is the word 'wake' ... in order to generate lift a wing has to move a fair amount of air around (although let's not start the bernoulli argument now!), so its influence on the surrounding atmosphere extends a surprising distance away from it. Tip vortices are also a very stable flow structure, so don't begin to break up or decay for a very very long way downstream. The climb angles are too small to make a significant difference to the lift required from the glider wing (assuming the tow rope is straight), since the effect on lift goes with the cosine of the angle On the other hand, if the tow rope is not straight then there could be a significant lift component from the tension force (going with the sine of the tow rope angle) ... but you would have to be quite a long way above the tug to make a big difference. Winter has certainly arrived in the northern hemisphere.Shall we next discuss how many angels can dance on the head of a pin? |
#45
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poor lateral control on a slow tow?
On Jan 1, 10:34*am, Doug Greenwell wrote:
At 15:09 01 January 2011, Derek C wrote: On Jan 1, 11:15=A0am, Doug Greenwell *wrote: At 20:23 31 December 2010, bildan wrote: On Dec 31, 1:06=3DA0pm, Todd =A0wrote: I too agree with the real or perceived tow handling characteristics. Looking at things =3DA0from and aerodynamics standpoint (and I am abou= 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. =3DA0Then 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. =A0A pilot trying to hold a precise position behind a tug needs and expects crisp aileron response. =A0When he doesn't get it, he increases the amount and frequency of aileron with a corresponding increase in adverse yaw. =A0If 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. =A0The 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. *For example a 10deg climb angle at 60 kts corresponds to an impressive climb rate of 10.5kts - but that would only give Lift = Weight/cos(10deg) = 1.02 x Weight. *You 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 = 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. * On your second point, if you are on tow anywhere sensible behind a tug you are in its wake and are being affected by the wing downwash. *Wake is not 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 |
#46
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poor lateral control on a slow tow?
On Jan 1, 10:11*pm, "
wrote: On Jan 1, 10:34*am, Doug Greenwell wrote: At 15:09 01 January 2011, Derek C wrote: On Jan 1, 11:15=A0am, Doug Greenwell *wrote: At 20:23 31 December 2010, bildan wrote: On Dec 31, 1:06=3DA0pm, Todd =A0wrote: I too agree with the real or perceived tow handling characteristics. Looking at things =3DA0from and aerodynamics standpoint (and I am abou= 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. =3DA0Then 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. =A0A pilot trying to hold a precise position behind a tug needs and expects crisp aileron response. =A0When he doesn't get it, he increases the amount and frequency of aileron with a corresponding increase in adverse yaw. =A0If 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. =A0The 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. *For example a 10deg climb angle at 60 kts corresponds to an impressive climb rate of 10.5kts - but that would only give Lift = Weight/cos(10deg) = 1.02 x Weight. *You 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 = 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. * On your second point, if you are on tow anywhere sensible behind a tug you are in its wake and are being affected by the wing downwash. *Wake is not 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- Hide quoted text - - Show quoted text - Just looking at the vectors..........lift + drag + weight + thrust(tow rope)... must = zero 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. A lot more than if the thrust(tow rope) was pulling along in the direction of flight. So...the angle of attack has to be higher at a given speed on tow than it would be in free flight at the same speed..... plus all that other stuff already mentioned.......... Cookie |
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poor lateral control on a slow tow?
Suffice to say the glider is being towed at an artificial angle of attack
compared to free glide so requires more speed on tow. Heavy standard class probably the worst needing 70-75kts on tow but thermalling happily at 60kts. Re low tow, we use it in Australia, it feels more stable [to me] and we release in low tow with no problems. |
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poor lateral control on a slow tow?
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poor lateral control on a slow tow?
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poor lateral control on a slow tow?
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 |
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