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#111
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motorgliders as towplanes
K13s and Bocians (both types I fly regularly) also aerotow in a noticeably
nose down attitude. The Bocian is such a slow glider that it unlikely that any normal tug could fly slowly enough to get close to its stalling speed, without stalling itself, but the K13 definitely starts 'wallowing' below about 52knots, whereas its normal free flight stalling speed is about 36knots. Derek Copeland At 12:15 16 March 2009, Bob Cook wrote: I also noted, in the case of a 2-33 and a Blanik L-13 (the crap that I usually have to fly) that BOTH of these gliders fly NOTICIBLY nose down on tow! (which menas the rope is imparting a nose up force vector, even in normal, tow position. ) Anyone who has flow a 2-33 knows that it takes CONSIDERABLE forward stick force when on tow. I have experienced the "too slow tow" phenomonon in the '33. (never got a too-slow in the Blanik) Again, my point being that I am still not convinced that anyone has completly explained the phenomonon, that we all agree exists! Cookie forward and downward pull (relative) to the glider! =A0 I'd expect high tow to give a downwards pull and low tow to give an upwards one. Seems a lot to me as well. What are these people towing with - chain? Ian |
#112
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motorgliders as towplanes
On 16 Mar, 10:15, Alan Garside wrote:
Can I add another question, why does the adverse yaw at 60 on tow appear to be more than 60 in free flight, many students learing to aerotow have difficulty with the rudder. Do you think it's an aerodynamic problem? I'd always thought, based on my own experience, that it came from having a much clearer indication of yaw (the tow rop) than normal and overcorrecting some PIOs into the system. But I hadn't really considered the aerodynamics. Hmmm. Ian |
#113
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motorgliders as towplanes
On Mar 14, 8:43*am, The Real Doctor wrote:
On 14 Mar, 13:15, Bob Cook wrote: To beter understand how the lift gets less as the climb angle gets greater, let's look at teh "extreme". Consider a glider attached by a nose hook to a huge construction crane. *The crane operator *applies POWER to the lifting cable and the glider is slowly lifted, vertically into the air. Bad example, since tow planes pull - give or take a wee bit - horizontally, regardless of climb angle. Ian Never been towed behind an Ag-cat, have you? Nothing horizontal about that evolution! Kirk 66 |
#114
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motorgliders as towplanes
Sorry Nyal - excessive vocab use. Dimer == related pair of.
In this case two vortices - one off each wingtip that interact to create a roughly symmetrical "geared disk" shape behind the wing. With the downward part of the vortex from each wingtip merging with the downward flow from the other. If you drive behind a (modern / streamlined not SUV) car in the rain or snow you can see the dimer it creates. Formula one and Nascar rear wings also create impressive examples... Nyal Williams wrote: Help; what is "dimer" ? At 14:09 15 March 2009, Bruce wrote: Paul There is a large scale vortex dimer operating behind any aircraft, and particularly behind high wing loading, heavy short winged things like Pawnees. The wake we fly above in high tow is the turbulent propeller wake, but we would have to be impossibly high and/or far back to avoid the downward moving centre section of the dimer. I saw a picture using smoke trails that demonstrates the scale and power of this some years back - http://www.nasa.gov/audience/forstud...ry/Vortex.html There is a more impressive video at http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1 So - given that you are flying in a field of air that has a significant downward component, maybe you do have a higher angle of attack on the wings. Bottom line is that even in the smooth air above the propwash you are still in air affected by the tug. Bruce sisu1a wrote: Agreed. My money is on the towplane wake. I put my monies on the elevator authority/AoA ratio. We fly above the wing wake (USA...) in most cases, in relatively clean air, but sometimes in the clean air below it. Box the wake, it will tell you where it is and where it isn't... But typically glider's noses, on tow, are unnaturally high (and thus AoA is higher...) for a given airspeed, in addition to being more forcefully held there, both effects of course due to the rope's pull. The elevator is the same size whether on tow or free flight though, so the authority it can exert against the countering forces is proportionately lower than in free flight... The fix is the same regardless of why though- more speed... please! (wings rocking in vain...) -Paul |
#115
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motorgliders as towplanes
I know this is not alt.usage.english, but what is the etymology of this
word? Is it slang? Jargon? Engineer language? Is it Di-mer or Dim-er or dimer... something or other? I'm just a poor musicologist trying to figure stuff out. At 14:27 16 March 2009, Bruce wrote: Sorry Nyal - excessive vocab use. Dimer == related pair of. In this case two vortices - one off each wingtip that interact to create a roughly symmetrical "geared disk" shape behind the wing. With the downward part of the vortex from each wingtip merging with the downward flow from the other. If you drive behind a (modern / streamlined not SUV) car in the rain or snow you can see the dimer it creates. Formula one and Nascar rear wings also create impressive examples... Nyal Williams wrote: Help; what is "dimer" ? At 14:09 15 March 2009, Bruce wrote: Paul There is a large scale vortex dimer operating behind any aircraft, and particularly behind high wing loading, heavy short winged things like Pawnees. The wake we fly above in high tow is the turbulent propeller wake, but we would have to be impossibly high and/or far back to avoid the downward moving centre section of the dimer. I saw a picture using smoke trails that demonstrates the scale and power of this some years back - http://www.nasa.gov/audience/forstud...ry/Vortex.html There is a more impressive video at http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1 So - given that you are flying in a field of air that has a significant downward component, maybe you do have a higher angle of attack on the wings. Bottom line is that even in the smooth air above the propwash you are still in air affected by the tug. Bruce sisu1a wrote: Agreed. My money is on the towplane wake. I put my monies on the elevator authority/AoA ratio. We fly above the wing wake (USA...) in most cases, in relatively clean air, but sometimes in the clean air below it. Box the wake, it will tell you where it is and where it isn't... But typically glider's noses, on tow, are unnaturally high (and thus AoA is higher...) for a given airspeed, in addition to being more forcefully held there, both effects of course due to the rope's pull. The elevator is the same size whether on tow or free flight though, so the authority it can exert against the countering forces is proportionately lower than in free flight... The fix is the same regardless of why though- more speed... please! (wings rocking in vain...) -Paul |
#116
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motorgliders as towplanes
monomer
Dimer Trimer .... Nyal Williams wrote: I know this is not alt.usage.english, but what is the etymology of this word? Is it slang? Jargon? Engineer language? Is it Di-mer or Dim-er or dimer... something or other? I'm just a poor musicologist trying to figure stuff out. At 14:27 16 March 2009, Bruce wrote: Sorry Nyal - excessive vocab use. Dimer == related pair of. In this case two vortices - one off each wingtip that interact to create a roughly symmetrical "geared disk" shape behind the wing. With the downward part of the vortex from each wingtip merging with the downward flow from the other. If you drive behind a (modern / streamlined not SUV) car in the rain or snow you can see the dimer it creates. Formula one and Nascar rear wings also create impressive examples... Nyal Williams wrote: Help; what is "dimer" ? At 14:09 15 March 2009, Bruce wrote: Paul There is a large scale vortex dimer operating behind any aircraft, and particularly behind high wing loading, heavy short winged things like Pawnees. The wake we fly above in high tow is the turbulent propeller wake, but we would have to be impossibly high and/or far back to avoid the downward moving centre section of the dimer. I saw a picture using smoke trails that demonstrates the scale and power of this some years back - http://www.nasa.gov/audience/forstud...ry/Vortex.html There is a more impressive video at http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1 So - given that you are flying in a field of air that has a significant downward component, maybe you do have a higher angle of attack on the wings. Bottom line is that even in the smooth air above the propwash you are still in air affected by the tug. Bruce sisu1a wrote: Agreed. My money is on the towplane wake. I put my monies on the elevator authority/AoA ratio. We fly above the wing wake (USA...) in most cases, in relatively clean air, but sometimes in the clean air below it. Box the wake, it will tell you where it is and where it isn't... But typically glider's noses, on tow, are unnaturally high (and thus AoA is higher...) for a given airspeed, in addition to being more forcefully held there, both effects of course due to the rope's pull. The elevator is the same size whether on tow or free flight though, so the authority it can exert against the countering forces is proportionately lower than in free flight... The fix is the same regardless of why though- more speed... please! (wings rocking in vain...) -Paul |
#117
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motorgliders as towplanes
On 16 Mar, 13:28, " wrote:
On Mar 14, 8:43*am, The Real Doctor wrote: Bad example, since tow planes pull - give or take a wee bit - horizontally, regardless of climb angle. Never been towed behind an Ag-cat, have you? 265 horse Pawnee count? Nothing horizontal about that evolution! Tug wheels on the horizon, glider just above the prop wash, just like everything else. Ian |
#118
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motorgliders as towplanes
On 16 Mar, 13:00, Derek Copeland wrote:
... but the K13 definitely starts 'wallowing' below about 52knots, whereas its normal free flight stalling speed is about 36knots. OK, here's my latest theory. Gliders have bigger wingspans than tugs. Therefore the outer bit of each glider wing is in the upwards moving bit of the tug's tip vortices, and the centre bit is in the downwards going bit. Effective result: much higher angle of attack at the tips, particularly since the nose has to come up to maintain AoA at the centre. Hence wash-in, tips near stall, downgoing aileron actually stalling, reduced control effectiveness, wallowing. Questions: does it happen as much out to one side hen boxing the wake? Does it happen when the tug - a motorglider - has the same span as the tug? Ian |
#119
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motorgliders as towplanes
On Mar 8, 11:03*am, Brad wrote:
I know this has come up before in RAS. But thought I would bring up the subject again. For a club looking at long term projections, which at some point will include either sticking a "new" engine on a Pawnee or getting rid of it, does it make sense to start evaluating getting a 2 place motorglider to serve as a tug and also as a touring/training tool? Can a MG tug pull a loaded 2 place Blanik on a standard day at SL, from a 1800' grass strip? Or perhaps such a tug could be used to tow the members single place ships, and the heavier/ 2-place ships stow behind the clubs remaining pawnee? We are blessed with 3 towplanes in our club, there are upcoming factors that will/are causing us to look at several different scenarios and am wondering if tossing a MG into the mix might be one such solution. Brad Ten - twelve years ago we had a little 4-man club that towed a fully- loaded Blanik L-13 off a *paved* runway with a Piper Pacer. I think it had a 135hp engine. I think the ground roll on asphalt for the Pacer was about 2000'. We had grass available, and really didn't want to find out how long the ground roll would be, partly because the obstacle-clearance distance was very poor due to the anemic climb rate. On a hot day, it might be 1 kt, and really needed thermals to get up. There was no climb in sink. Our field has nearby landable farm terrain, and believe me, I always made sure I knew exactly which field I'd be landing in if the Pacer had an engine problem, and had a hand on the yellow handle all the way to 500+ ft agl. It's the "fully loaded Blanik" phrase that give me pause more than the "motoglider as tug" phrase. As other responders have implied, the proof of the pudding is very much in the eating. Testing is always a good idea. |
#120
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Aerodynamics of Towing
OK, here's my latest theory. Gliders have bigger wingspans than
tugs. Therefore the outer bit of each glider wing is in the upwards moving bit of the tug's tip vortices, and the centre bit is in the downwards going bit. Effective result: much higher angle of attack at the tips, particularly since the nose has to come up to maintain AoA at the centre. Hence wash-in, tips near stall, downgoing aileron actually stalling, reduced control effectiveness, wallowing. Questions: does it happen as much out to one side hen boxing the wake? Does it happen when the tug - a motorglider - has the same span as the tug? Ian This thread is interesting and I thought it deserved a new title, even though I have nothing to really add to it at this point (except that rudder effectiveness is also reduced with a tethered nose...) -Paul PS. the rope's tugging of the nose is quite severe at the outside positions while boxing the wake, and greatly affects control authority (especially in draggy fat ships with ineffective controls like a 2-33...), so the experiment you propose might not yield as much insight as we'd like. |
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