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#81
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motorgliders as towplanes
On Mar 14, 9:36*pm, "Mike Schumann" mike-nos...@traditions-
nospam.com wrote: You are confusing a lack of elevator authority with stalling. *The two are completely different phenomenon. Mike Schumann "Mike the Strike" wrote in ... I took a good look at the geometry of my tow today. *The tow line appeared to be pulling down on the nose at about an angle of twenty degrees. At 65 knots, the tow line formed a catenary to the towplane with significant sag in the line. It should be possible to model the angles of the line and angles of attack of the towplane and glider, but I suspect some of the simplistic arguments have not explained the phenomenon because they haven't taken full account of the complex geometry of the tow. At around 50 to 55 knots, I am unable to maintain high tow and sink into low tow with no elevator authority and reduced aileron control. My free-flight stall speed is below 40 knots. *Gurus please explain. Mike Maybe so, but why is poor aileron control in a slow tow similar to that experienced in an incipient stall? What are the symptoms of running out of elevator authority and why does this happen on tow but not in free flight? No-one has satisfactorily explained this. Mike |
#82
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motorgliders as towplanes
Mike,
If the tow line formed a 20 degree angle to the glider, (from tow plane upward to glider) the glider would have to be 72 feet ABOVE the tow plane. Well, really allowing for the angle of climb, it would be 72 feet above the "direction of flight" of the tow plane. If the tow plane could climb at a 20 degree angle you would be level with the tow plane, but this is not possible, If the tow plane climbed at a more reasonable angle of 5 degrees you would be 54 feet above the tow plane. This, in my opinion, would be dangerous! I like to tow with the tow plane's wheels sitting on the horizon. This means that the glider is about 5 feet BELOW the towplane, and the angle of the tow rope is about 1.5 degrees UPWARD from glider to tow plane. I despise the term "high tow" as it implies positioning the glider "high" and above the tow plane. I use the term, ""low tow" for flying below the wake and the term "normal tow" for flying level with, or slightly below, the tow plane, but above the wake. In my opinion flying above the tow plane is forbidden!! Cookie At 04:07 15 March 2009, Mike the Strike wrote: I took a good look at the geometry of my tow today. The tow line appeared to be pulling down on the nose at about an angle of twenty degrees. At 65 knots, the tow line formed a catenary to the towplane with significant sag in the line. It should be possible to model the angles of the line and angles of attack of the towplane and glider, but I suspect some of the simplistic arguments have not explained the phenomenon because they haven't taken full account of the complex geometry of the tow. At around 50 to 55 knots, I am unable to maintain high tow and sink into low tow with no elevator authority and reduced aileron control. My free-flight stall speed is below 40 knots. Gurus please explain. Mike |
#83
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motorgliders as towplanes
Stall is a factor angle of attack, not airspeed.
You could say that your MINIMUM possible stall speed is under 40 knots, but you could easily stall at higher speeds IF you get the angle of attack high enough. Just pull back on the stick harder! Cookie At 04:07 15 March 2009, Mike the Strike wrote: My free-flight stall speed is below 40 knots. Gurus please explain. |
#84
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motorgliders as towplanes
Has anybody actually stalled during an aero tow?
Cookie At 04:36 15 March 2009, Mike Schumann wrote: You are confusing a lack of elevator authority with stalling. The two are completely different phenomenon. Mike Schumann "Mike the Strike" wrote in message ... I took a good look at the geometry of my tow today. The tow line appeared to be pulling down on the nose at about an angle of twenty degrees. At 65 knots, the tow line formed a catenary to the towplane with significant sag in the line. It should be possible to model the angles of the line and angles of attack of the towplane and glider, but I suspect some of the simplistic arguments have not explained the phenomenon because they haven't taken full account of the complex geometry of the tow. At around 50 to 55 knots, I am unable to maintain high tow and sink into low tow with no elevator authority and reduced aileron control. My free-flight stall speed is below 40 knots. Gurus please explain. Mike |
#85
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motorgliders as towplanes
For purposes of analysis, the angle at which the tow rope meets the glider
is the angle to consider that THRUST is acting on the glider. A rope can only be in tension. It cannot impart any rotational "moments" to the glider. If the tow rope has a big sag or belly in it, consider the angle at which the rope meets the glider, not the angel of the glider relative to the tow plane. The sag in the rope is caused by the weight of the rope itself. Cookie At 04:07 15 March 2009, Mike the Strike wrote: I took a good look at the geometry of my tow today. The tow line appeared to be pulling down on the nose at about an angle of twenty degrees. At 65 knots, the tow line formed a catenary to the towplane with significant sag in the line. It should be possible to model the angles of the line and angles of attack of the towplane and glider, but I suspect some of the simplistic arguments have not explained the phenomenon because they haven't taken full account of the complex geometry of the tow. At around 50 to 55 knots, I am unable to maintain high tow and sink into low tow with no elevator authority and reduced aileron control. My free-flight stall speed is below 40 knots. Gurus please explain. Mike |
#86
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motorgliders as towplanes
On Thu, 12 Mar 2009 22:09:24 +0200, Bruce wrote:
Bruce wrote: In case anyone thought that the MG/UL has too little grunt. Follow the link below. The story behind it is - experienced pilot in getting current again on Janus forgot the thing has a drogue chute. On a check flight the Janus is towed up from an airfield (Orient) at 5100" MSL at around 20 centigrade. As you can see the Samba accelerates slowly - then the drogue chute deploys. (one bump too many ) Tuggie happens to be enormously experienced CFI - uses his head and abuses the Rotax a little. Combination manages to make a circuit and the Janus gets dropped over the threshold. Samba engine did not over heat or suffer any apparent damage, although it was kept in the time limited maximum power range for the entire circuit. For information , low down there are very few options straight out on 36 that would not include reducing the Janus to kit form if he had been dropped before getting back to the runway. http://www.youtube.com/watch?v=4_w3n...eature=related I actually witnessed that incident from a vantage point midway down the runway. I did not notice at the time anyone with a video camera but you can't be to careful these days! I was not expecting to see two sets of wings climbing out from the dip and the end of the runway. I realy thought we would be picking up broken fibre glass that morning. One thing you missed out in the commentry, the Janus' radio was also discovered to be not working. So the pilot did not hear radio calls advising him of the situation. The tug pilot did hear the radio excitement but became aware of the situation when he saw the shadow of the drag chute. The glider pilot first became aware of the drag chute when he climbed out of the glider after the landing. The Samba, with its low mass, low drag airframe coupled with a VP prop certainly has enough excess thrust to provide the aircraft at the end of the rope a decent climb rate. The limitations a - Low power to mass of glider + tug combination, leads to slow acceleration. - Low mass of tug relative to glider leaves little control authority for handling out of possition gliders on tow. - VP prop is a high maintenance item. - The Samba airframe is designed to an AUM limit of 450kg (including two pilots, engine and fuel). It is very light, has lots of carbon and is strong in all the right places, but it has limited resilience for handling club abuse which can test things in ways that the designer did not anticipate. Our club decided not to buy one. We did get feedback from other operators including those at Gariep. Perhaps if you have an owner/tuggie operation and you tow mainly experienced pilots in single seaters, it is a good option. For a club operation with a whole group of tuggies sharing the duties of towing 2 seater trainers with a whole bunch of different instructors and every level of student, I think the Samba may have a hard time. Does anybody know what the certification situation is for towing with ultralight catorgory aircraft (450kg AUW) is in Europe these days? How many tow planes are certified for tugging and what mass of glider are they permitted to tow? Ian |
#87
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motorgliders as towplanes
On Sat, 14 Mar 2009 18:08:23 -0700, The Real Doctor wrote:
The behaviour of the Pirat on aerotow is rather different, by the way. At low speeds it handles very nicely, but if the tuggy is a bit enthusiastic the ailerons get horribly heavy and ineffective. Quite the opposite of wallowing, really. At the same speed off tow they are light and responsive. I'm guessing that the tug downwash affects the centre of the wing more, effectively increasing the washout. Meanwhile, I just cajole tuggies into flying with the CHT just below the red ... How does the downwash angle vary with position along the wing span? Since the lift distribution varies across the span its unlikely that the downwash angle is constant. As an additional factor, even a single seat glider's span is larger than the tugs span, so the glider's tips (and ailerons) will be in a somewhat different airflow to the inboard sections of its wing. If the downwash angle behind the tug reduces as you move outboard along its wing then that's effectively washin at the glider's tips, i.e. an increased AOA, so during slow tow the glider may be nearly tip stalling, which would account for the symptoms people have mentioned: poor aileron response and apparent loss of lift. However, it doesn't explain the behavior of Ian's Pirat, so I must have missed something obvious. -- martin@ | Martin Gregorie gregorie. | Essex, UK org | |
#88
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motorgliders as towplanes
On 15 Mar, 04:07, Mike the Strike wrote:
At around 50 to 55 knots, I am unable to maintain high tow and sink into low tow with no elevator authority and reduced aileron control. My free-flight stall speed is below 40 knots. *Gurus please explain. Type and hook position, please ... Ian |
#89
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motorgliders as towplanes
On 15 Mar, 05:30, Bob Cook wrote:
If the tow line formed a 20 degree *angle to the glider, (from tow plane upward to glider) the glider would have to be 72 feet *ABOVE the tow plane. Only if the tow rope were straight. He has already said that it was in a clearly visible catenary curve. Ian |
#90
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motorgliders as towplanes
Stick well back and won't further raise the nose, controls ineffective and
a feeling of 'wallowing'. All classic symptoms of being close to the stall! If you tow on a belly hook (all my own glider has) the stick has be further forward to prevent the glider trying to 'winch launch' and directional control during the ground run is a bit more difficult, but the other symptoms stay the same. Derek Copeland At 00:45 15 March 2009, Big Wings wrote: Hmmm. In a glider with C of G in the correct place the tail plane will be producing a down thrust, hence the wing will need to generate lift greater than the weight of the glider when in more or less level flight. The more back stick, the more down thrust and the harder the wing will need to work to maintain level flight. Now my Discus turbo stalls in free flight in the low 40s (kts). With the engine out and the thrust line well above the fuselage, thus pushing the nose down, the best climb speed in the POH is given as 49 to 54 kts depending on weight. If I let if fall to about 45 kts it is still perfectly controllable - just less efficient. However aerotowing on the nose hook at that speed would not be a happy experience. I have never done a slow tow on a belly hook so don't know if the symptoms are the same. What I have observed on slow tows, and has been reported by others in this thread, is that the ailerons are ineffective, the glider tends to wallow, the stick is a long way back and the nose high - even though the speed is above the normal 1G stalling speed. These seem to be symptoms of an accelerated stall or incipient spin - but the rope pulling ahead seems to stop the glider rotating into a spin with the associated wing drop. I wish I knew the answer, but if I have the symtoms of an accelerated stall in more or less straight level flight above the 1g stall speed that sugests that the wing is generating more lift than the weight of the glider for some reason; if the stick is near the back-stop at 50 kts there is more elevator downforce so the wing will have to compenstate for that. The pull from the rope may be slightly down if I'm in high tow, especially with a long heavy rope with a slight bow in it, but to raise the stall speed from say 42 to 50 kts (* 1.2) is equivalent to an increase in load on the wing from 1g to about 1.4g. At a take-off weight of say 475 kg this is equivalent to an additional 190kg or another 418 lb. Where could this come from??? The downthrust from the elevator fighting the tow rope? |
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