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#1
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poor lateral control on a slow tow?
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 ... Is poor handling at low speed on tow a common experience? 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 |
#2
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poor lateral control on a slow tow?
On Dec 31, 9:40*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 ... Is poor handling at low speed on tow a common experience? *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 Sure, even more with flaps equipped ships! But yaw control seems to be more affected than roll. Can we explain that? Just a little faster please is what doctor ordered here...usually 5 mph more is enough to a more pleasant tow. |
#3
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poor lateral control on a slow tow?
On Dec 31, 4:40*am, "Doug" wrote:
Is poor handling at low speed on tow a common experience? Well not for me, but only because I don't often get towed slow. For my ballasted ASW-28, 65kts indicated in the glider is as slow as I'd like to be. On the few tows that the speed has dropped to 60kts it's barely possible to stay in control in any sort of turbulence. Both pitch and roll control are greatly reduced, I never noticed a problem with yaw control. Slow tows only seem to be a problem for me when I go to an site that uses tow pilots that have no high performance glider experience but they still know better than the glider pilot what speeds are needed. Fortunately there are not too many of those. The worse possible case is the tow pilot that refuses to follow the simple instruction to stay in ground effect until a safe tow speed is reached. Some glider pilots don't realize how hopelessly inaccurate the tug airspeed indicator may be on tow and they ask for the speed they'd like to see in the glider. Bad idea. Far better to ask for a speed x kts or mph higher than they use for the gliders they tow everyday. Andy |
#4
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poor lateral control on a slow tow?
"wladkummer76" wrote in message ... On Dec 31, 9:40 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 ... Is poor handling at low speed on tow a common experience? 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 Sure, even more with flaps equipped ships! But yaw control seems to be more affected than roll. Can we explain that? Just a little faster please is what doctor ordered here...usually 5 mph more is enough to a more pleasant tow. Good question ... the only thing I can think of at the moment is that the tug tip vortices would also generate an inwards 'sidewash' flow above the wake, which would be stronger the heavier and slower the tug was. If you are then for some reason off to one side of the tug the sidewash at the fin from the nearer vortex would tend to yaw you 'nose-out', in the wrong direction. Doug |
#5
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poor lateral control on a slow tow?
Is poor handling at low speed on tow a common experience?
Well not for me, but only because I don't often get towed slow. *For my ballasted ASW-28, 65kts indicated in the glider is as slow as I'd like to be. *On the few tows that the speed has dropped to 60kts it's barely possible to stay in control in any sort of turbulence. *Both pitch and roll control are greatly reduced, I never noticed a problem with yaw control. This is precisely the aerodynamic puzzle. The same standard class glider stalls in the low 40s and is perfectly happy thermaling at 45-50. So why does 60 feel so awful on tow? This does seem a lot worse in standard class than 15 meter, another hint for the puzzle. If it were downwash, one would think that just flying higher would solve it. But that's the best theory I've heard so far. I have seen quite a few contests with tow pilots who had little experience with fully ballasted gliders. It's really important to get the word out to them 1) yes, we really want to tow that fast. 2) get up to speed in ground effect, then start climbing. I distinctly remember that helpless feeling sitting in a tanked up discus, crossing the end of the runway, as the towplane departed what looked like straight up at about 50 knots. On another occasion, every single tow by one pilot was accompanied by a chorus of demands for more speed on the radio. Eventually he piped up "what do y'all want to fly so fast for anyway?'' I guess the 2-33 never wanted to go 70 on tow. John Cochrane |
#6
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poor lateral control on a slow tow?
The worst towing experiences I've had has been during aero retrieves.
Once the tugs (I've seen this behind an L-19 and a Pawnee) reached cruising altitude, it became extremely difficult to stay behind them. I asked them to slow down (went from 85 to 65 kt), got into low tow, slipped and even pulled on some spoiler, but my LS8 (unballasted) was a handful in roll and pitch. Once on a turbulent blue day I deliberately released 25 miles before I had final glide, preferring the excitement of finding a means of staying up rather than continuing on tow. -John |
#7
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poor lateral control on a slow tow?
At 15:05 31 December 2010, John Cochrane wrote:
Is poor handling at low speed on tow a common experience? Well not for me, but only because I don't often get towed slow. =A0For my ballasted ASW-28, 65kts indicated in the glider is as slow as I'd like to be. =A0On the few tows that the speed has dropped to 60kts it's barely possible to stay in control in any sort of turbulence. =A0Both pitch and roll control are greatly reduced, I never noticed a problem with yaw control. This is precisely the aerodynamic puzzle. The same standard class glider stalls in the low 40s and is perfectly happy thermaling at 45-50. So why does 60 feel so awful on tow? This does seem a lot worse in standard class than 15 meter, another hint for the puzzle. If it were downwash, one would think that just flying higher would solve it. But that's the best theory I've heard so far. I have seen quite a few contests with tow pilots who had little experience with fully ballasted gliders. It's really important to get the word out to them 1) yes, we really want to tow that fast. 2) get up to speed in ground effect, then start climbing. I distinctly remember that helpless feeling sitting in a tanked up discus, crossing the end of the runway, as the towplane departed what looked like straight up at about 50 knots. On another occasion, every single tow by one pilot was accompanied by a chorus of demands for more speed on the radio. Eventually he piped up "what do y'all want to fly so fast for anyway?'' I guess the 2-33 never wanted to go 70 on tow. John Cochrane In the normal tow position I don't think you can get high enough to get away from the wing wake without tipping the tug on its nose. On the other hand, my simple modelling suggests that a typical low tow position should be a lot better. 15m vs standard class ... Forgive my ignorance (I've never flown a flapped glider), but presumably a 15m glider is towed with the flaps deflected, which would tend to unload the tips and hence maintain aileron authority at high incidence. Would the ailerons typically be drooped at this flap setting? Doug |
#8
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poor lateral control on a slow tow?
On Dec 31, 8:05*am, John Cochrane
wrote: The same standard class glider stalls in the low 40s and is perfectly happy thermaling at 45-50. Not mine, at least not when ballasted. Most of my flying is done where the thermals are small cored and often rowdy. 50kts would only work in a very smooth thermal that was big enough to use a low bank angle. However, the D2 which does seem to climb well at low speeds seems to be really sensitive to being towed slow, at least that I understand from one of the local pilots. Maybe D2 owners could comment on that? Andy |
#9
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poor lateral control on a slow tow?
Sure, even more with flaps equipped ships! But yaw control seems to be more affected than roll. Can we explain that? Just a little faster please is what doctor ordered here...usually 5 mph more is enough to a more pleasant tow. I'm going to assume your flying with a nosehook, in which case the big tether is resisting motion in the yaw axis... whereas for roll the towrope runs directly down the longitudinal axis, leaving airspeed/AoA as the dominant external forces affecting roll. The rope also holds the glider at unnatural attitudes for given airspeeds, which I think contributes a lot to the lack of perceived (less than usual for that particular airspeed) aileron authority, contributing greatly to making the glider feel horrible on tow, despite being well within it's stall speed. Not as an endorsement for or against this arrangement but from an aerodynamic prospective rather, a CG hook leaves the yaw axis pretty free to swivel accordingly and likely reduces the rope induced AoA affecting aileron authority as well *once in a steady climb. (*not to be confused with the up pitching tendency these hooks are famous for during the initial acceleration of a launch...) -Paul |
#10
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poor lateral control on a slow tow?
At 16:35 31 December 2010, sisu1a wrote:
Sure, even more with flaps equipped ships! But yaw control seems to be more affected than roll. Can we explain that? Just a little faster please is what doctor ordered here...usually 5 mph more is enough to a more pleasant tow. I'm going to assume your flying with a nosehook, in which case the big tether is resisting motion in the yaw axis... whereas for roll the towrope runs directly down the longitudinal axis, leaving airspeed/AoA as the dominant external forces affecting roll. good point - a nose hook would tend to improve yaw and pitch stability on tow, but have little effect on roll, so it's difficult to split out the effects of aerodynamics and tow cable dynamics. I did some work years ago on towed bodies (aerial targets and sonar fish) and the coupling effects between cable and body motion get really complicated. The rope also holds the glider at unnatural attitudes for given airspeeds, which I think contributes a lot to the lack of perceived (less than usual for that particular airspeed) aileron authority, contributing greatly to making the glider feel horrible on tow, despite being well within it's stall speed. yes - I'm sure there must be a pyschological element as well Not as an endorsement for or against this arrangement but from an aerodynamic prospective rather, a CG hook leaves the yaw axis pretty free to swivel accordingly and likely reduces the rope induced AoA affecting aileron authority as well *once in a steady climb. (*not to be confused with the up pitching tendency these hooks are famous for during the initial acceleration of a launch...) -Paul I think the rope effects on stability with a nose hook are generally favourable - its the aerodynamic effects of the tug wake that cause the problems |
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