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#111
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poor lateral control on a slow tow?
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#112
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poor lateral control on a slow tow?
At 13:32 04 January 2011, BruceGreeff wrote:
Thanks Martin I did use the "vector" word talking about the winch case - because the cable has mass (our steel cable is ~150Kg so not insignificant) and there is a pull at a downward angle. At top of launch cable angle approaches 90 degrees to fuselage - If you want proof look at one of the videos on you tube. http://www.youtube.com/watch?v=v2Qh95I_YM0 http://www.youtube.com/watch?v=np8OGPZ2pvE As Doug Greenwell points out - there is constant acceleration on winch launch because the flight path is curved, describing a horisontal S. I don't know how what magnitude the acceleration has, but subjectively it is only significant in the brief rotation to steep climb, and possibly on the level out if you are less than smooth... Generally it is a relatively small change from a little over 1g to a little under 1g at release. Anyone have the maths capability to calculate for a known situation? Cheers Bruce On 2011/01/04 1:43 PM, Martin Gregorie wrote: On Tue, 04 Jan 2011 10:57:01 +0200, BruceGreeff wrote: Of course - the angle that the flight path can make relative to the ground is proportional to the excess power available - hence the low rate of climb behind the cub, versus the extreme angle on a winch. Aerodynamics guys - Am I confused? Sounds fair to me except that you omitted two fairly significant forces: - the weight of the cable - the tension in the cable. Both will add to the load carried by the wing. The tension should add a fairly constant load to the wing once the glider has rotated into full climb since the throttle setting remains fairly constant[*] from rotation until the glider is near the top, but the effective cable weight will increase as more of it is lifted off the ground and then as the whole cable gets closer to vertical. [*] this is true on a calm day but is obviously incorrect in the presense of turbulence or a significant wind gradient. -- Bruce Greeff T59D #1771 & Std Cirrus #57 Hopefully this post will go this time ... I did try modelling a winch launch some time ago. It's very dependent on the assumptions you make on piloting technique and winch control (constant power, tension, cable speed?), but in general once you've transitioned into full climb the accelerations as felt by the pilot seem to be very small ( 0.1g). Hence the danger of overstressing, since you've no physical indication of the high wing loads due to the cable tension & weight. |
#113
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poor lateral control on a slow tow?
On 1/3/2011 11:51 PM, Darryl Ramm wrote:
On Jan 3, 8:54 pm, Eric wrote: Imagine an extreme tow, a 50 knot airspeed, but climbing at 35 knots (45 degree angle). The tow rope is providing 70% of the force holding the glider in the air, so the wing needs to supply only 30% of the force. Or imagine a really extreme, vertical tow: all the force required to keep the glider moving steadily through the air is provided by the towrope/towplane, and none by the wing. I think you are trying to push this argument up an incline with a rope. :-) But I'll take your points into consideration next time I'm vertically towing behind a helicopter. I'm serious! But, let me add this constraint to make the idea easier to absorb: the glider pilot flies the tow so the rope is always parallel to the fuselage. In level flight, the rope pull equals the drag; the lift equals the glider weight. Rope force vector and weight vector are at right angles. In a 50 knot airspeed, 35 knot climb (45 degree angle of climb), the rope vector and the glider weight vector are now at an obtuse angle, so some of the rope force is supporting the glider. Stating it another way: we know the rope is pulling a lot harder, but the glider is not accelerating, so what force is opposing the rope pull? It can't be additional drag (glider is still going only 50 knots airspeed); it can't be the lift (regardless of it's value), because that's acting almost entirely perpendicularly to the rope. So, what force is opposing all that extra rope pull? I say - it's the weight of the glider (about 70% of the weight). Another way to imagine the situation, using the helicopter to provide a 50 knot airspeed tow, rope always parallel to the glider fuselage: * level flight, wing lift = weight of glider * vertical flight, wing lift = 0 (or the glider won't have right rope angle) So, in between level flight and vertical flight, there must be a region where the wing lift is less than in level flight, right? I'm saying there is a continuous reduction in the lift the wing must provide as the climb angle increases. Only two months till March flying starts...gotta solve this problem while we still have time! -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) |
#114
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poor lateral control on a slow tow?
So, in between level flight and vertical flight, there must be a region where the wing lift is less than in level flight, right? I'm saying there is a continuous reduction in the lift the wing must provide as the climb angle increases. Only two months till March flying starts...gotta solve this problem while we still have time! -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) Yeah....you got it......the lift is the cosine of the climb angle times the weight......... level.....0 degrees climb.. Cosine 0 = 1 so lift =100% glider weight 5 degree climb (reasonable tow climb angle) Cosine 5 = .996 so lift = 99.6% of glider's weight 45 degree climb (unlikely but just for demonstration) cosine 45 = . 707 so lift would be only 71% of glider's weight 90 degree climb Cosine 90 = o so lift would be zero. If we keep the airspeed constant, the drag shoud be constant....so the only variables are lift and thrust. as the thrust vector gets bigger, the direction of flgith gets steeper climb, and the lift vector gets smaller. Cookie |
#115
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poor lateral control on a slow tow?
In very slow flight without flaps my 1-35 drops into a stall long before it gets as bad as a tow does at 60 statute mph. You would think that I would have stalled out of the tow too. Perhaps the wallowing around on tow is just the turbulent air on the ailerons and not an imminent stall at all. (Think rotor in wave or turbulence behind a hill on a smaller scale) Are there any reports of incidents where a glider drops into a stall on a slow tow or are there just complaints of glider pilot annoyance? (I agree it's not fun) For example, if the air turbulence was going "down" on the right side just when you try to bank "left" that would make the controls feel sluggish. At some angle of bank, assuming that everything else was symmetrical, the two ailerons would be in different parts of the turbulence, confusing the situation. Has anyone tried some flaps in an integrated flap machine (which reduces stall speed) to see if the wallowing goes away? Unfortunately, my trailer is in a snowbank. |
#116
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poor lateral control on a slow tow?
On Jan 4, 7:13*pm, AGL wrote:
Has anyone tried some flaps in an integrated flap machine (which reduces stall speed) to see if the wallowing goes away? With every flapped glider I've flown, negative flap improves aileron response fairly dramatically. Positive flap does lower the stall speed a little. I've flown a 20 meter Nimbus 2C ballasted to 11 lbs/sq ft wing loading behind a tug pilot accustomed to towing 2-33's. The speed was low enough to need +1 flap but it didn't wallow. The tug pilot turned off his radio when he got tired of me yelling for more speed than what he "knew" was right. |
#117
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poor lateral control on a slow tow?
At 22:15 04 January 2011, Eric Greenwell wrote:
On 1/3/2011 11:51 PM, Darryl Ramm wrote: On Jan 3, 8:54 pm, Eric Greenwell wrote: Imagine an extreme tow, a 50 knot airspeed, but climbing at 35 knots (45 degree angle). The tow rope is providing 70% of the force holding the glider in the air, so the wing needs to supply only 30% of the force. Or imagine a really extreme, vertical tow: all the force required to keep the glider moving steadily through the air is provided by the towrope/towplane, and none by the wing. I think you are trying to push this argument up an incline with a rope. :-) But I'll take your points into consideration next time I'm vertically towing behind a helicopter. I'm serious! But, let me add this constraint to make the idea easier to absorb: the glider pilot flies the tow so the rope is always parallel to the fuselage. In level flight, the rope pull equals the drag; the lift equals the glider weight. Rope force vector and weight vector are at right angles. In a 50 knot airspeed, 35 knot climb (45 degree angle of climb), the rope vector and the glider weight vector are now at an obtuse angle, so some of the rope force is supporting the glider. Stating it another way: we know the rope is pulling a lot harder, but the glider is not accelerating, so what force is opposing the rope pull? It can't be additional drag (glider is still going only 50 knots airspeed); it can't be the lift (regardless of it's value), because that's acting almost entirely perpendicularly to the rope. So, what force is opposing all that extra rope pull? I say - it's the weight of the glider (about 70% of the weight). Another way to imagine the situation, using the helicopter to provide a 50 knot airspeed tow, rope always parallel to the glider fuselage: * level flight, wing lift = weight of glider * vertical flight, wing lift = 0 (or the glider won't have right rope angle) So, in between level flight and vertical flight, there must be a region where the wing lift is less than in level flight, right? I'm saying there is a continuous reduction in the lift the wing must provide as the climb angle increases. Only two months till March flying starts...gotta solve this problem while we still have time! -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) Right ... it's the weight opposing the rope pull, because the tug is doing all the work of raising the glider to a higher altitude - difficult to demonstrate without a diagram! |
#118
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poor lateral control on a slow tow?
At 02:13 05 January 2011, AGL wrote:
In very slow flight without flaps my 1-35 drops into a stall long before it gets as bad as a tow does at 60 statute mph. You would think that I would have stalled out of the tow too. Perhaps the wallowing around on tow is just the turbulent air on the ailerons and not an imminent stall at all. (Think rotor in wave or turbulence behind a hill on a smaller scale) Are there any reports of incidents where a glider drops into a stall on a slow tow or are there just complaints of glider pilot annoyance? (I agree it's not fun) For example, if the air turbulence was going "down" on the right side just when you try to bank "left" that would make the controls feel sluggish. At some angle of bank, assuming that everything else was symmetrical, the two ailerons would be in different parts of the turbulence, confusing the situation. Has anyone tried some flaps in an integrated flap machine (which reduces stall speed) to see if the wallowing goes away? Unfortunately, my trailer is in a snowbank. That's a useful comment: the aerodynamic modelling I did suggests that the lateral control problems on tow should be different (worse) than those in a typical stall, because the wing is stalling at the tips rather than the root. No-one has yet admitted to actually stalling or dropping a wing on tow - so the effect seems to be annoying rather than dangerous. Flaps should (theoretically) improve matters by (a) reducing stall speed and (b) shifting the spanwise lift distribution inboard and unloading the tips. However, if the flaps are integrated with the ailerons then the associated aileron droop would counteract (b). |
#119
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poor lateral control on a slow tow?
At 02:48 05 January 2011, bildan wrote:
On Jan 4, 7:13=A0pm, AGL wrote: Has anyone tried some flaps in an integrated flap machine (which reduces stall speed) to see if the wallowing goes away? With every flapped glider I've flown, negative flap improves aileron response fairly dramatically. Positive flap does lower the stall speed a little. I've flown a 20 meter Nimbus 2C ballasted to 11 lbs/sq ft wing loading behind a tug pilot accustomed to towing 2-33's. The speed was low enough to need +1 flap but it didn't wallow. The tug pilot turned off his radio when he got tired of me yelling for more speed than what he "knew" was right. Sorry if this is an obvious question (never flown a flapped glider), but with an integrated flap system what is the relative movement of the ailerons and flaps? Presumably the ailerons don't move at all for negative settings? Doug |
#120
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poor lateral control on a slow tow?
On Jan 5, 12:00*am, "
wrote: So, in between level flight and vertical flight, there must be a region where the wing lift is less than in level flight, right? I'm saying there is a continuous reduction in the lift the wing must provide as the climb angle increases. Only two months till March flying starts...gotta solve this problem while we still have time! -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) Yeah....you got it......the lift is the cosine of the climb angle times the weight......... level.....0 degrees climb.. *Cosine 0 = 1 * *so lift =100% glider weight 5 degree climb (reasonable tow climb angle) * Cosine 5 = .996 * *so lift = 99.6% of glider's weight 45 degree climb (unlikely but just for demonstration) * cosine 45 = . 707 *so lift would be only 71% of glider's weight 90 degree climb * Cosine 90 = o * so lift would be zero. If we keep the airspeed constant, the drag shoud be constant....so the only variables are lift and thrust. * as the thrust vector gets bigger, the direction of flgith gets steeper climb, and the lift vector gets smaller. Cookie So according to you, pulling a load up a 10 degree slope should require less energy than pulling it on the flat! Anybody who has ever ridden a bicycle can tell you that is not the case! For a glider on tow, the combined vector of Lift and Thrust (provided by the tug) has to equal the combined vector of weight plus drag. As the glider is not rigidly connected to the tug, the extra lift has to come from its wings (at least at moderate climb angles). For a given airspeed this can only be done by increasing the angle of attack. Hence you are closer to the stalling angle. I am not sure that this is the correct explanation, but it seems to fit the observed facts. Derek C |
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