If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#121
|
|||
|
|||
poor lateral control on a slow tow?
At 09:25 05 January 2011, Derek C wrote:
On Jan 5, 12:00=A0am, " 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 th= e 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.. =A0Cosine 0 =3D 1 =A0 =A0so lift =3D100% glid= er weight 5 degree climb (reasonable tow climb angle) =A0 Cosine 5 =3D .996 =A0 =A0= so lift =3D 99.6% of glider's weight 45 degree climb (unlikely but just for demonstration) =A0 cosine 45 =3D . 707 =A0so lift would be only 71% of glider's weight 90 degree climb =A0 Cosine 90 =3D o =A0 so lift would be zero. If we keep the airspeed constant, the drag shoud be constant....so the only variables are lift and thrust. =A0 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 There are two components to the energy required in this case - (1) the energy required to overcome friction (which will indeed be slightly less, because of the reduced reaction force perpendicular to the slope), (2) the energy required to lift the load up a given height (NB this assumes that you are pulling the load at a constant speed - otherwise we would have to take kinetic energy into account as well) (1) can be reduced to (near) zero by reducing friction - using rollers for example, or in your alternative example of a bicycle - the equivalent effect in a glider on tow is reducing drag by careful streamlining or increased aspect ratio. (2) is fixed, and independent of speed or slope angle - raising any object a given height requires a fixed amount of energy (= mass*acceleration due to gravity*height change). Both components of the energy input are provided by you pulling the load up the slope. A glider on tow is exactly the same. The wing lift corresponds to the reaction force between the surface and the load. The drag corresponds to the friction force between the surface and the load. The tug corresponds to you pulling the load - and is doing all the work against friction and gravity. The lift/reaction force does no work - all it does is stop the load sinking into the ground or the glider falling further and further below the tug. Imagine a perfect glider with no drag* on tow (= pulling a load up the slope with no friction, or a perfect bicycle) ... what happens if you release the rope (or stop pedalling)? If the wing lift were responsible for the climb rate then you would carry on climbing until you ran out of atmosphere (or hill) * fortunately not currently available in the shops, since it would ruin the sport! |
#122
|
|||
|
|||
poor lateral control on a slow tow?
On Wed, 05 Jan 2011 09:04:12 +0000, Doug Greenwell wrote:
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? On an ASW-20 flaps and ailerons move together so the trailing edge remains straight with the stick central in the flying flap settings: +8 (thermal) through -9 (max negative flap). When stick is moved laterally the flap deflects half as far as the aileron. In landing flap settings the ailerons mover to -8 degrees - what the RC glider guys call 'crow mode'. This reduces tip stalling tendencies and the handbook says this also increases drag. -- martin@ | Martin Gregorie gregorie. | Essex, UK org | |
#123
|
|||
|
|||
poor lateral control on a slow tow?
At 13:09 05 January 2011, Martin Gregorie wrote:
On Wed, 05 Jan 2011 09:04:12 +0000, Doug Greenwell wrote: 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? On an ASW-20 flaps and ailerons move together so the trailing edge remains straight with the stick central in the flying flap settings: +8 (thermal) through -9 (max negative flap). When stick is moved laterally the flap deflects half as far as the aileron. In landing flap settings the ailerons mover to -8 degrees - what the RC glider guys call 'crow mode'. This reduces tip stalling tendencies and the handbook says this also increases drag. -- martin@ | Martin Gregorie gregorie. | Essex, UK org | Ok - so that would help in reducing stall speed slightly, but would not help with the spanwise lift distribution. Is the aileron/flap interconnect a standard arrangement, or are there flapped gliders without it? |
#124
|
|||
|
|||
poor lateral control on a slow tow?
Eric has the right idea.
The forces acting on a glider towed upwards in steady climbing flight are necessarily at equilibrium. The vectors lift + weight + drag + thrust add up to zero. Otherwise, as some of the dafter ideas proposed here imply, the glider will accelerate continuously toward outer space. Think of pulling a wheeled trolley with a rope up an inclined plane. The steeper the incline, the greater is the tension in the rope until, in the limit, with a vertical incline, the rope supports the entire weight and the wheels none. At practical climb angles (less than 1:5) the required wing lift should be slightly reduced from level flight by the cosine of climb angle. On that basis a glider on tow should stall at, or less than, the same airspeed as straight and level flight off tow. I think pilots who feel alarmed about stalling on tow may be misled by cues from the nose high attitude combined with the need to maintain position in turbulence behind the tug. Doesn't mean it feels good though :-) Ian Grant (PEng and D2 driver) On Jan 5, 2:15*am, Eric Greenwell wrote: 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) |
#125
|
|||
|
|||
poor lateral control on a slow tow?
On Jan 5, 10:33*am, Doug Greenwell wrote:
At 09:25 05 January 2011, Derek C wrote: On Jan 5, 12:00=A0am, " 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 th= e 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.. =A0Cosine 0 =3D 1 =A0 =A0so lift =3D100% glid= er weight 5 degree climb (reasonable tow climb angle) =A0 Cosine 5 =3D .996 =A0 =A0= so lift =3D 99.6% of glider's weight 45 degree climb (unlikely but just for demonstration) =A0 cosine 45 =3D . 707 =A0so lift would be only 71% of glider's weight 90 degree climb =A0 Cosine 90 =3D o =A0 so lift would be zero. If we keep the airspeed constant, the drag shoud be constant....so the only variables are lift and thrust. =A0 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 There are two components to the energy required in this case - (1) the energy required to overcome friction (which will indeed be slightly less, because of the reduced reaction force perpendicular to the slope), (2) the energy required to lift the load up a given height (NB this assumes that you are pulling the load at a constant speed - otherwise we would have to take kinetic energy into account as well) (1) can be reduced to (near) zero by reducing friction - using rollers for example, or in your alternative example of a bicycle - the equivalent effect in a glider on tow is reducing drag by careful streamlining or increased aspect ratio. (2) is fixed, and independent of speed or slope angle - raising any object a given height requires a fixed amount of energy (= mass*acceleration due to gravity*height change). * Both components of the energy input are provided by you pulling the load up the slope. A glider on tow is exactly the same. *The wing lift corresponds to the reaction force between the surface and the load. *The drag corresponds to the friction force between the surface and the load. *The tug corresponds to you pulling the load - and is doing all the work against friction and gravity. *The lift/reaction force does no work - all it does is stop the load sinking into the ground or the glider falling further and further below the tug. Imagine a perfect glider with no drag* on tow (= pulling a load up the slope with no friction, or a perfect bicycle) ... what happens if you release the rope (or stop pedalling)? *If the wing lift were responsible for the climb rate then you would carry on climbing until you ran out of atmosphere (or hill) * fortunately not currently available in the shops, since it would ruin the sport! *- Hide quoted text - - Show quoted text - To take your points above in order: 1) Gliders, at least decent ones, are pretty low drag anyway. 2) Kinetic energy from the tug is being used to raise the mass of the glider up against gravity, so that it gains potential energy. Once that source of kinetic energy is removed (i.e. you pull off tow), the mass will stop going up and will start to descend due to the force of gravity acting downwards. To maintain forward momentum gliders have to continually descend through the air in which they are flying. Gliders appear to get near to the stall during slow aerotows at much greater than their normal free flight stalling airspeeds. I would suggest that aerotowing must increase the wing loading in some way. Derek C |
#126
|
|||
|
|||
poor lateral control on a slow tow?
On Fri, 31 Dec 2010 11:40:53 -0000, "Doug"
wrote: 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? Yes, it is common. I use to fly mainly at competitions, and among the 5-10 tow pilots, there's always at least one who, despite being briefed by the towmaster, flies too slowly. In my personal experience, it happened to me 3 times in a double seater (Janus B and DuoDiscus). I don't remember any occurrence in my single seater. I can describe it as being unable to raise the nose. As the towplane was flying below 100 km/h, I just couldn't match the climbing rate with the glider, so I was more and more into the propwash. A gentle pull up wouldn't work; pulling more hits the stop and the glider feels like it's sinking. I also cannot find an easy and believable explanation for this phenomenon. I didn't recognize a lack of _lateral_ control, anyway. aldo cernezzi |
#127
|
|||
|
|||
poor lateral control on a slow tow?
At 17:23 05 January 2011, Derek C wrote:
On Jan 5, 10:33=A0am, Doug Greenwell wrote: At 09:25 05 January 2011, Derek C wrote: On Jan 5, 12:00=3DA0am, " 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 th=3D e 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.. =3DA0Cosine 0 =3D3D 1 =3DA0 =3DA0so lift = =3D3D100% glid=3D er weight 5 degree climb (reasonable tow climb angle) =3DA0 Cosine 5 =3D3D 996 = =3DA0 =3DA0=3D so lift =3D3D 99.6% of glider's weight 45 degree climb (unlikely but just for demonstration) =3DA0 cosine 45 =3D3D . 707 =3DA0so lift would be only 71% of glider's weight 90 degree climb =3DA0 Cosine 90 =3D3D o =3DA0 so lift would be zero. If we keep the airspeed constant, the drag shoud be constant....so the only variables are lift and thrust. =3DA0 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 There are two components to the energy required in this case - (1) the energy required to overcome friction (which will indeed be slightly less, because of the reduced reaction force perpendicular to the slope), (2) th= e energy required to lift the load up a given height (NB this assumes that you are pulling the load at a constant speed - otherwise we would have to take kinetic energy into account as well) (1) can be reduced to (near) zero by reducing friction - using rollers fo= r example, or in your alternative example of a bicycle - the equivalent effect in a glider on tow is reducing drag by careful streamlining or increased aspect ratio. (2) is fixed, and independent of speed or slope angle - raising any objec= t a given height requires a fixed amount of energy (=3D mass*acceleration d= ue to gravity*height change). =A0 Both components of the energy input are provided by you pulling the load up the slope. A glider on tow is exactly the same. =A0The wing lift corresponds to the reaction force between the surface and the load. =A0The drag corresponds = to the friction force between the surface and the load. =A0The tug correspon= ds to you pulling the load - and is doing all the work against friction and gravity. =A0The lift/reaction force does no work - all it does is stop th= e load sinking into the ground or the glider falling further and further below the tug. Imagine a perfect glider with no drag* on tow (=3D pulling a load up the slope with no friction, or a perfect bicycle) ... what happens if you release the rope (or stop pedalling)? =A0If the wing lift were responsibl= e for the climb rate then you would carry on climbing until you ran out of atmosphere (or hill) * fortunately not currently available in the shops, since it would ruin the sport! =A0- Hide quoted text - - Show quoted text - To take your points above in order: 1) Gliders, at least decent ones, are pretty low drag anyway. 2) Kinetic energy from the tug is being used to raise the mass of the glider up against gravity, so that it gains potential energy. Once that source of kinetic energy is removed (i.e. you pull off tow), the mass will stop going up and will start to descend due to the force of gravity acting downwards. To maintain forward momentum gliders have to continually descend through the air in which they are flying. Gliders appear to get near to the stall during slow aerotows at much greater than their normal free flight stalling airspeeds. I would suggest that aerotowing must increase the wing loading in some way. Derek C 2) that's exactly the point! The energy from the tug (not its kinetic energy, but the work done in pulling the tow rope) is being used to increase the potential energy of the glider ... the glider wing lift is not contributing to the increase in potential energy because it is perpendicular to the direction of motion and hence does no work. |
#128
|
|||
|
|||
poor lateral control on a slow tow?
At 17:25 05 January 2011, cernauta wrote:
On Fri, 31 Dec 2010 11:40:53 -0000, "Doug" wrote: 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? Yes, it is common. I use to fly mainly at competitions, and among the 5-10 tow pilots, there's always at least one who, despite being briefed by the towmaster, flies too slowly. In my personal experience, it happened to me 3 times in a double seater (Janus B and DuoDiscus). I don't remember any occurrence in my single seater. I can describe it as being unable to raise the nose. As the towplane was flying below 100 km/h, I just couldn't match the climbing rate with the glider, so I was more and more into the propwash. A gentle pull up wouldn't work; pulling more hits the stop and the glider feels like it's sinking. I also cannot find an easy and believable explanation for this phenomenon. I didn't recognize a lack of _lateral_ control, anyway. aldo cernezzi Interesting - most people are reporting problems with lateral control (which seems to have a reasonably simple explanation), but running out of nose-up pitch control also seems to occur ... and is harder to understand. Did you notice any kind of change in elevator control force before you hit the stops? Did you experience this effect with any specific type of tug? Derek Copeland has decribed a similar loss of elevator authority when towed by a motor glider. Doug |
#129
|
|||
|
|||
poor lateral control on a slow tow?
On 1/5/2011 7:11 AM, Doug Greenwell wrote:
Is the aileron/flap interconnect a standard arrangement... Kinda-sorta, "Yes, but..." or are there flapped gliders without it? ....because the answer to this question is also (if unequivocally so), "Yes." (I've owned 3.) Regards, Bob W. P.S. Very e-e-enteresting discussion with (apparently ) real potential to clarify some folks' understanding of things. I remain in the F = Ma camp! |
#130
|
|||
|
|||
poor lateral control on a slow tow?
At 17:59 05 January 2011, Bob Whelan wrote:
On 1/5/2011 7:11 AM, Doug Greenwell wrote: Is the aileron/flap interconnect a standard arrangement... Kinda-sorta, "Yes, but..." or are there flapped gliders without it? ....because the answer to this question is also (if unequivocally so), "Yes." (I've owned 3.) Regards, Bob W. P.S. Very e-e-enteresting discussion with (apparently ) real potential to clarify some folks' understanding of things. I remain in the F = Ma camp! Thanks - so the question of whether flaps make things better or worse on tow is also going to be 'it depends' ... :-) It's certainly been unexpectedly interesting - having only recently returned to gliding I'm having a load of fun improving my flying, but also a load of fun trying to understand what is happening from an academic/technical point of view. You can't argue with F=ma ... but which 'F' and which 'a' and what direction they are in is another matter! |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
another poor man's car engine conversion | jan olieslagers[_2_] | Home Built | 19 | February 22nd 09 03:49 PM |
Poor readability | Kees Mies | Owning | 2 | August 14th 04 04:22 AM |
Poor Guy | Bob Chilcoat | Owning | 6 | July 17th 04 06:45 PM |
I'm grateful for poor people who are willing to murder & die | Krztalizer | Military Aviation | 0 | April 20th 04 11:11 PM |
Concorde in FS2002: No lateral views | A. Bomanns | Simulators | 3 | July 19th 03 11:33 AM |