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Aerodynamics of aero towing
This is a quick observation report. Its really a follow up to a sub-topic
on the "Poor lateral control on a slow tow" thread. Last Friday I was sitting outside the CGC clubhouse, which was at the upwind end of the active runway, when our Pawnee went past at about 200 ft with a Ventus on the line. As it went away I had a clear rear quarter view which made the attitudes if the two aircraft really easy to see. The Ventus looked to be at the same height as the Pawnee, but was clearly flying much more nose-up. I'd estimate that it was at around 4 degrees nose-up relative to the Pawnee. -- martin@ | Martin Gregorie gregorie. | Essex, UK org | |
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Aerodynamics of aero towing
On Mar 7, 2:15*am, Martin Gregorie
wrote: This is a quick observation report. Its really a follow up to a sub-topic on the "Poor lateral control on a slow tow" thread. AIRSPEED! AIRSPEED! It is way more disconcerting when you see the tops of the wings of a Pawnee from the grounf just before the DG300 behind him stalls, releases and and spins in from 100 ft. Last Friday I was sitting outside the CGC clubhouse, which was at the upwind end of the active runway, when our Pawnee went past at about 200 ft with a Ventus on the line. As it went away I had a clear rear quarter view which made the attitudes if the two aircraft really easy to see. The Ventus looked to be at the same height as the Pawnee, but was clearly flying much more nose-up. I'd estimate that it was at around 4 degrees nose-up relative to the Pawnee. -- martin@ * | Martin Gregorie gregorie. | Essex, UK org * * * | |
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Aerodynamics of aero towing
On Mar 7, 7:43*am, Karen wrote:
On Mar 7, 2:15*am, Martin Gregorie wrote: This is a quick observation report. Its really a follow up to a sub-topic on the "Poor lateral control on a slow tow" thread. AIRSPEED! AIRSPEED! It is way more disconcerting when you see the tops of the wings of a Pawnee from the grounf just before the DG300 behind him stalls, releases and and spins in from 100 ft. Last Friday I was sitting outside the CGC clubhouse, which was at the upwind end of the active runway, when our Pawnee went past at about 200 ft with a Ventus on the line. As it went away I had a clear rear quarter view which made the attitudes if the two aircraft really easy to see. The Ventus looked to be at the same height as the Pawnee, but was clearly flying much more nose-up. I'd estimate that it was at around 4 degrees nose-up relative to the Pawnee. -- martin@ * | Martin Gregorie gregorie. | Essex, UK org * * * | We have discussed this many times before. The nose-up attitude of the glider results in extra forces that have to be overcome by the wings and tail surfaces. In particular, modern standard-class ships with a low angle of incidence of the wing have a pronounced nose-up attitude that results in loss of control at speeds well above free-flight stall speed. I find my Discus 2 won't high-tow very well below 60 knots, even though its stall speed is nearer 40 or 45. One strategy I use if the tow plane won't or can't speed up is to descend into low tow, which buys an extra 5 knots or so. Extra speed is great, but not under control of the glider pilot! Mike |
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Aerodynamics of aero towing
On 3/7/2011 7:24 AM, Mike the Strike wrote:
We have discussed this many times before. The nose-up attitude of the glider results in extra forces that have to be overcome by the wings and tail surfaces. In particular, modern standard-class ships with a low angle of incidence of the wing have a pronounced nose-up attitude that results in loss of control at speeds well above free-flight stall speed. I'm having trouble picturing the influence of a low angle of incidence of the wing (I assume that's compared to the fuselage) on loss of control. Why would the wing be affected by the fuselage attitude being a few degrees higher, relative to the air, than on an older glider? -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) |
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Aerodynamics of aero towing
On Mon, 07 Mar 2011 13:14:04 -0800, Eric Greenwell wrote:
On 3/7/2011 7:24 AM, Mike the Strike wrote: We have discussed this many times before. The nose-up attitude of the glider results in extra forces that have to be overcome by the wings and tail surfaces. In particular, modern standard-class ships with a low angle of incidence of the wing have a pronounced nose-up attitude that results in loss of control at speeds well above free-flight stall speed. I'm having trouble picturing the influence of a low angle of incidence of the wing (I assume that's compared to the fuselage) on loss of control. Why would the wing be affected by the fuselage attitude being a few degrees higher, relative to the air, than on an older glider? I should add that there was plenty of speed - the Pawnee had its tail well up. From where I was sitting its fuselage looked to be pretty much horizontal. The Ventus looked to be well under control - it was flying very steadily. I ascribed most of the glider's nose-high attitude to the fact that it was flying in the downwash field behind the tug. Don't forget that the downwash behind a lift generating wing is roughly 1/3 of the wing's AOA, so this was actually a great visualisation of the downwash. -- martin@ | Martin Gregorie gregorie. | Essex, UK org | |
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Aerodynamics of aero towing
On Mar 7, 2:14*pm, Eric Greenwell wrote:
On 3/7/2011 7:24 AM, Mike the Strike wrote: We have discussed this many times before. *The nose-up attitude of the glider results in extra forces that have to be overcome by the wings and tail surfaces. *In particular, modern standard-class ships with a low angle of incidence of the wing have a pronounced nose-up attitude that results in loss of control at speeds well above free-flight stall speed. I'm having trouble picturing the influence of a low angle of incidence of the wing (I assume that's compared to the fuselage) on loss of control. Why would the wing be affected by the fuselage attitude being a few degrees higher, relative to the air, than on an older glider? -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) The angle of incidence of the wing (relative to the fuselage) is a lot lower on modern standard class ships. The result is that they have a pronounced nose-up attitude when flying slowly - you can see this both on tow and when thermalling. On aero-tow, a nose-mounted tow hook will result in a downward force on the nose that has to be compensated by an upward force from the tailplane and extra lift from the wings. The point at which you lose control under these dynamics is at a lot higher speed than the free-flight stall speed. The effect doesn't seem to be as strong with older gliders or flapped ships, but definitely is a lot less in low tow. Mike |
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Aerodynamics of aero towing
On 3/7/2011 2:47 PM, Mike the Strike wrote:
On Mar 7, 2:14 pm, Eric wrote: On 3/7/2011 7:24 AM, Mike the Strike wrote: We have discussed this many times before. The nose-up attitude of the glider results in extra forces that have to be overcome by the wings and tail surfaces. In particular, modern standard-class ships with a low angle of incidence of the wing have a pronounced nose-up attitude that results in loss of control at speeds well above free-flight stall speed. I'm having trouble picturing the influence of a low angle of incidence of the wing (I assume that's compared to the fuselage) on loss of control. Why would the wing be affected by the fuselage attitude being a few degrees higher, relative to the air, than on an older glider? -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) The angle of incidence of the wing (relative to the fuselage) is a lot lower on modern standard class ships. The result is that they have a pronounced nose-up attitude when flying slowly - you can see this both on tow and when thermalling. On aero-tow, a nose-mounted tow hook will result in a downward force on the nose that has to be compensated by an upward force from the tailplane and extra lift from the wings. The point at which you lose control under these dynamics is at a lot higher speed than the free-flight stall speed. The effect doesn't seem to be as strong with older gliders or flapped ships, but definitely is a lot less in low tow. The incidence difference is less that 5 degrees, isn't it? That would mean the nose hook was only about 6" higher than otherwise. I just can't see how moving the rope up 6" is going to change the forces significantly. There is a bigger difference between a CG hook and nose hook vertical separation, and nobody comments on how they lose control sooner with the nose hook. I sure didn't notice any difference on my previous glider, an ASW 20 B, where I used neutral flap to tow. Maybe the difference between older and newer gliders is wing loading? -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) |
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Aerodynamics of aero towing
On Mar 8, 4:24*am, Mike the Strike wrote:
I find my Discus 2 won't high-tow very well below 60 knots, even though its stall speed is nearer 40 or 45. *One strategy I use if the tow plane won't or can't speed up is to descend into low tow, which buys an extra 5 knots or so. I suspect you're flying the tow too high behind a powerful tug that is climbing at a steep angle e.g. flying with the tug on the horizon rather than just above its slipstream. The rope can't possibly make enough of a difference to the lift needed from the wings to make a difference of 5 knots, but it can make you run out of elevator power. If low tow is ok then a high tow just above the slipstream will be too. |
#9
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Aerodynamics of aero towing
On Mar 7, 5:47*pm, Mike the Strike wrote:
On Mar 7, 2:14*pm, Eric Greenwell wrote: On 3/7/2011 7:24 AM, Mike the Strike wrote: We have discussed this many times before. *The nose-up attitude of the glider results in extra forces that have to be overcome by the wings and tail surfaces. *In particular, modern standard-class ships with a low angle of incidence of the wing have a pronounced nose-up attitude that results in loss of control at speeds well above free-flight stall speed. I'm having trouble picturing the influence of a low angle of incidence of the wing (I assume that's compared to the fuselage) on loss of control. Why would the wing be affected by the fuselage attitude being a few degrees higher, relative to the air, than on an older glider? -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) The angle of incidence of the wing (relative to the fuselage) is a lot lower on modern standard class ships. *The result is that they have a pronounced nose-up attitude when flying slowly - you can see this both on tow and when thermalling. *On aero-tow, a nose-mounted tow hook will result in a downward force on the nose that has to be compensated by an upward force from the tailplane and extra lift from the wings. The point at which you lose control under these dynamics is at a lot higher speed than the free-flight stall speed. *The effect doesn't seem to be as strong with older gliders or flapped ships, but definitely is a lot less in low tow. Mike- Hide quoted text - - Show quoted text - I suspect you intended to say the tail was generating more down force to over come the down vector on the nose. Simple math based on a guess at tow rope tension of, say 50 lb or so ( 1000 lb glider/40 to 1 and double for sake of discussion), would lead me to guess the down force on the nose to be not more than a few lb.(maybe 10?). Assume a tail moment arm of 3 times the nose arm and the extra lift the wing needs to generate is maybe 15 lb. at the most. Your observation that the glider flies better in low tow makes sense in that you are in cleaner air. FWIW on a rainy winter night. UH |
#10
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Aerodynamics of aero towing
On Mar 7, 2:47*pm, Mike the Strike wrote:
*On aero-tow, a nose-mounted tow hook will result in a downward force on the nose that has to be compensated by an upward force from the tailplane and extra lift from the wings. The point at which you lose control under these dynamics is at a lot higher speed than the free-flight stall speed. *The effect doesn't seem to be as strong with older gliders or flapped ships, but definitely is a lot less in low tow. Wow. and all these years I was thinking that the center of gravity was ahead of the center of lift; that there was a downward force 'forward' of the wing by the limits on CG location; and that the horizontal tail had to make a downward force to counteract the nose dropping tendency. I think they called this Trim Drag, but I could be confused. I am blond. I better watch out for those nose-hook mounted gliders and their scary tendencies. Maybe that's why the folks at EASU changed the design requirements from aerotowing on CG towhooks to requiring them to be aerotowed on 'forward' hooks. And that forward hook placement was under the knees ......(even lower on the ariframe than those dastardly true nosehooks. How does that fit into the discussion?) But hey, all that trim drag from different hook locations, gee, must change the wing loading and subsequent stall speed by what...... a knot? Geesh. Spend a few bucks on a few extra Spring tows, and get past your winter nerves. Or break open a chapter of Langewische. Or travel to a site that's still flying all winter long and don't let the rust accumulate. Think : First flight with a CFI-G for 2011. Promote annual refresher training.... and less accidents. Chuckling, Cindy B www.caracole-soaring.com |
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