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#21
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poor lateral control on a slow tow?
On Jan 1, 8:07*am, John Cochrane
wrote: The D2 also has a very low angle of incidence and thus a high nose attitude on tow. Could this be part of the issue -- it's just darn uncomfortable to fly with the nose pointed above the towplane? That would also account for why 15 meter seems easier. We fly with flaps; they are interconnected to the ailerons so we're not getting great roll rates on tow. But it does give a nose down attitude so we can see the towplane. I had some really horrible feeling tows in a PW5. The thing felt mushy, nose high, couldn't see the towplane, needed a lot of back stick, afraid it was going to stall etc. Then I realized that when you're going up at over 1000 fpm in still air, keeping ANY part of the tug on the horizon (even wheels) is far too high a position. I dropped down until I could start to feel the wash and then came up a little. It felt much better but the tug seemed WAY UP THERE. Work it out ... at 65 knots and going up at 10 knots, the other end of a 50m rope will be 7.7m above you if you're following the same path. Even guessing 3m to get out of the wake, the tug should still be nearly 5m above you. And maybe it's 11 or 12 knots climb (I can't tell because the vario is pegged), in which case that's another 1 or 1.5 m. Since then I tow with the Pawnee horizontal stabilizer in the same position against the forward parts of the tug no matter what glider I'm in and just ignore the horizon. Even in the DG1000 two-up and climbing at 700 fpm this still results in the tug's wheels being a fraction above the horizon (and I've been criticized for this on biannuals) but I'm still comfortably above the turbulence of the tug's wake. |
#22
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poor lateral control on a slow tow?
Bruce Hoult wrote:
....Since then I tow with the Pawnee horizontal stabilizer in the same position against the forward parts of the tug no matter what glider I'm in and just ignore the horizon. Yes, use the tug as a reference. Using the horizon doesn't work on real hazy days and it doesn't work in the mountains. Tony V. |
#23
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poor lateral control on a slow tow?
On Jan 1, 12:38*am, Tony V wrote:
Bruce Hoult wrote: ....Since then I tow with the Pawnee horizontal stabilizer in the same position against the forward parts of the tug no matter what glider I'm in and just ignore the horizon. Yes, use the tug as a reference. Using the horizon doesn't work on real hazy days and it doesn't work in the mountains. Tony V. John Cochrane has the answer right, at least for standard class ships like the Discus 2. I can verify that you run out of elevator control at tow speeds significantly lower than the free-flight stall speed. The reason is that the tow rope applies a downward thrust at the nose - I have wing tip-camera video that confirms the tow rope has a significant downward pull on the nose. I always try to stay away from tow plane wash, so don't think that's a major component. I've never experienced as marked a behavior in flapped ships, so I put it down to AOA. Mike |
#24
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poor lateral control on a slow tow?
At 06:24 01 January 2011, Anne wrote:
On Jan 1, 12:38=A0am, Tony V wrote: Bruce Hoult wrote: ....Since then I tow with the Pawnee horizontal stabilizer in the same position against the forward parts of the tug no matter what glider I'm in and just ignore the horizon. Yes, use the tug as a reference. Using the horizon doesn't work on real hazy days and it doesn't work in the mountains. Tony V. John Cochrane has the answer right, at least for standard class ships like the Discus 2. I can verify that you run out of elevator control at tow speeds significantly lower than the free-flight stall speed. The reason is that the tow rope applies a downward thrust at the nose - I have wing tip-camera video that confirms the tow rope has a significant downward pull on the nose. I always try to stay away from tow plane wash, so don't think that's a major component. I've never experienced as marked a behavior in flapped ships, so I put it down to AOA. Mike Surely LOW TOW has many handling advantages and I have been very comfortable with this method gliding in Aus. At least the tow rope has an upward componenet. John |
#25
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poor lateral control on a slow tow?
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. |
#26
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poor lateral control on a slow tow?
At 21:47 31 December 2010, Martin Gregorie wrote:
On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote: On Dec 31, 6:19Â*pm, bildan wrote: On Dec 31, 4:40Â*am, "Doug" wrote: As an aerodynamicist/flight dynamicist recently re-soloed after 25 years off, people keep asking me hard questions. Â*One that has come 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 an increased angle of attack out at the tips reducing aileron effectiveness. Â*There'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? Â*I'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. Â*Overuse of ailerons is very common and it makes aero tow 'wobbly'. Â*If 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. Â*Using 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. Â*I haven't noticed any tendency for them to yaw a glider towards 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 to 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. 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@ | Martin Gregorie gregorie. | Essex, UK org | The downwash angle doesn't change much past the tail, and a half to a third of the tug AoA is a good first guess. My modeling suggest that there does seem to be an overall reduction in the glider wing lift (downwash over the centre wing having more of an effect than upwash over the tips), so the glider requires another degree or two in AoA - so feeling even more nose-up to the pilot! |
#27
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poor lateral control on a slow tow?
At 21:47 31 December 2010, Martin Gregorie wrote:
On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote: On Dec 31, 6:19Â*pm, bildan wrote: On Dec 31, 4:40Â*am, "Doug" wrote: As an aerodynamicist/flight dynamicist recently re-soloed after 25 years off, people keep asking me hard questions. Â*One that has come 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 an increased angle of attack out at the tips reducing aileron effectiveness. Â*There'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? Â*I'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. Â*Overuse of ailerons is very common and it makes aero tow 'wobbly'. Â*If 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. Â*Using 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. Â*I haven't noticed any tendency for them to yaw a glider towards 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 to 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. 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@ | Martin Gregorie gregorie. | Essex, UK org | The downwash angle doesn't change much past the tail, and a half to a third of the tug AoA is a good first guess. My modeling suggest that there does seem to be an overall reduction in the glider wing lift (downwash over the centre wing having more of an effect than upwash over the tips), so the glider requires another degree or two in AoA - so feeling even more nose-up to the pilot! |
#28
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poor lateral control on a slow tow?
At 20:23 31 December 2010, bildan wrote:
On Dec 31, 1:06=A0pm, Todd wrote: I too agree with the real or perceived tow handling characteristics. Looking at things =A0from and aerodynamics standpoint (and I am about 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. =A0Then 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. A pilot trying to hold a precise position behind a tug needs and expects crisp aileron response. When he doesn't get it, he increases the amount and frequency of aileron with a corresponding increase in adverse yaw. If 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. The 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? |
#29
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poor lateral control on a slow tow?
At 06:24 01 January 2011, Anne wrote:
On Jan 1, 12:38=A0am, Tony V wrote: Bruce Hoult wrote: ....Since then I tow with the Pawnee horizontal stabilizer in the same position against the forward parts of the tug no matter what glider I'm in and just ignore the horizon. Yes, use the tug as a reference. Using the horizon doesn't work on real hazy days and it doesn't work in the mountains. Tony V. John Cochrane has the answer right, at least for standard class ships like the Discus 2. I can verify that you run out of elevator control at tow speeds significantly lower than the free-flight stall speed. The reason is that the tow rope applies a downward thrust at the nose - I have wing tip-camera video that confirms the tow rope has a significant downward pull on the nose. I always try to stay away from tow plane wash, so don't think that's a major component. I've never experienced as marked a behavior in flapped ships, so I put it down to AOA. Mike Possibly two (or more) different handling problems on tow then ... 1) Running out of nose-up elevator authority when in a 'high' high tow position due to a combination of increased AoA required due to tug downwash and downward force component from the rope + a nose down pitching from the rope 2) degradation of lateral control due to changes in spanwise lift distribution I've certainly sparked some interest here - considering it's New Year :-) |
#30
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poor lateral control on a slow tow?
At 23:35 31 December 2010, Eric Greenwell wrote:
On 12/31/2010 3:40 AM, Doug wrote: As an aerodynamicist/flight dynamicist recently re-soloed after 25 years off, people keep asking me hard questions. One that has come 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 an increased angle of attack out at the tips reducing aileron effectiveness. There's possibly an interesting academic research project here, but it's always best to get a reality check first ... The wake behind a climbing towplane should be well below a glider in the high tow position. How can it lift it's weight and the glider, if the wake isn't descending? Recall one of the exercises a student does while learning to tow is to start in the high tow position, then move straight down until the wake turbulence is felt. With the usual 150' to 200' rope, it's way below the high tow position. So, I don't think it's wake turbulence, IF the glider is in the high tow position. I think a big part of the answer is the pilot's perception of "attitude": the glider has an additional attitude (relative to the horizon) over it's normal angle of attack because the glider and tug are ascending at about a 6 degree angle. T His is perceived by the pilot as a very nose high attitude, and makes him feel uncomfortable; that, along with reduced control response, makes him feel it's worse than it really is. Usually, this happens close to the ground, making the perception even worse. The unusually nose-high attitude can keep the glider pilot from using enough up elevator, with the consequence that he does sink down into the wake. That will make the situation actually worse, not just perceptually worse. But, it's because he is far from the high tow position, not just because the speed is slower than normal. This analysis obviously assumes a high tow as the normal situation, and may not apply to the situation where low tow is the norm. -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) The wake does descend, although this is not necessary for a wing to generate lift (otherwise wind tunnels would not work!) ... actually, the downwash is a consequence of a reduction in lift and increase in (induced) drag for a three-dimensional wing. However, the turbulent prop wash also descends with it, so setting a tow position on the basis of a reasonable distance above the prop wash would automatically position you close to the tip vortices. PS I've only ever come one other Greenwell outside the North East of England ... any relation? |
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