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 |
#91
|
|||
|
|||
poor lateral control on a slow tow?
On Jan 4, 12:58*am, "
wrote: On Jan 3, 6:30*pm, ProfChrisReed wrote: It seems to me that increased AoA must be a very large part of the cause. Imagine you are flying free @55kt. You have a sink rate of, say, 1.5kt. Now you are on tow, again @55kt, but this time the combination is climbing @5kt. Your wings are generating 6.5kt more lift than in free flight, and must therefore be at a substantially higher AoA. Additionally, the faster you are climbing (in still air) the greater the AoA must be for you to keep station with the tug. I fly an Open Cirrus, towing from the C of G hook without ballast, and never experienced this at my previous club which had a Citabria tug. My current club has a Pawnee, and I have from time to time felt the tow was too slow because the controls felt mushy and the glider wallowed about, feeling as if it was close to the stall. The Pawnee climbs much faster than the Citabria. If in addition the tug's slipstream imparts a downward flow to the airmass, even more lift and higher AoA is required. Actaully, comparing climbing steeply say, 10:1 on tow, to gliding at 40:1, *the lift vector is (a tiny bit) SMALLER during the tow! During the 10:1 tow, lift would be 99.5% of the glider's weight, while during a 40:1 glide, lift would be 99.97% of the glider's weight! (the missing 0.5% on tow is made up by the thrust vector...the missing 0.03% in glide is made up by the drag vector. Cookie Cookie- Hide quoted text - - Show quoted text - If you had a really powerful tug that was capable of climbing vertically, then the glider would just be dangling on the end of the rope and would not have to produce any lift. The tension in the rope would be equal to the weight of the glider plus any drag components. While this is not a very likely scenario, I do think that the thrust vector must be greater in a 10% climb than you are claiming. Derek C |
#92
|
|||
|
|||
poor lateral control on a slow tow?
On Jan 3, 11:30*pm, ProfChrisReed wrote:
It seems to me that increased AoA must be a very large part of the cause. Imagine you are flying free @55kt. You have a sink rate of, say, 1.5kt. Now you are on tow, again @55kt, but this time the combination is climbing @5kt. Your wings are generating 6.5kt more lift than in free flight, and must therefore be at a substantially higher AoA. Additionally, the faster you are climbing (in still air) the greater the AoA must be for you to keep station with the tug. I fly an Open Cirrus, towing from the C of G hook without ballast, and never experienced this at my previous club which had a Citabria tug. My current club has a Pawnee, and I have from time to time felt the tow was too slow because the controls felt mushy and the glider wallowed about, feeling as if it was close to the stall. The Pawnee climbs much faster than the Citabria. If in addition the tug's slipstream imparts a downward flow to the airmass, even more lift and higher AoA is required. I had a high tow behind a 180 hp Piper Super Cub (a fairly slow tug) to practice aerobatics in a K21 on Sunday. The nosehook on the K21 is situated under the nose, just in front of the nosewheel, and I was flying it solo. For the last thousand feet of the tow the airspeed dropped to about 56 knots and I got the same symptoms as described above. The normal free flight stalling speed for a K21 is only about 39 knots, and I had no problems in the early part of the tow when the airspeed was 60+ knots. If the glider feels as though it is close to the stall, then it probably IS close to the stall! Derek C |
#93
|
|||
|
|||
poor lateral control on a slow tow?
On Jan 3, 8:54*pm, Eric Greenwell wrote:
On 1/3/2011 8:10 PM, Darryl Ramm wrote: On Jan 3, 5:23 pm, "twocoolglid...@juno. com The rate of climb is strictly a factor of the power available. * More powerful towplane = faster rate of climb......lift on the glider's wing, and the *towlane's wing stays practically constant, therefore the angle of attack is just about constant. It is the climb angle (direction of flight) which changes with power, not the AoA. Cookie Ugh? The glider is flying, the towplane is not dragging the glider up an incline. If the combination is going up faster (=steeper climb rate/ angle) then both aircraft wings are generating more lift and they get this this from some combination of increased AoA and airspeed. The more powerful towplane may allow both aircraft to fly at an increased AoA and overcome the associated drag. The increased climb angle comes from the increased lift. Assuming a constant airspeed means all the increase is coming from an increase in AoA and the more powerful towplane thrust is offsetting the increased drag. I'd be interested to see an explanation of any other way of generating an increase in climb angle without increasing the lift of the glider and/pr towplane. Actually, I do think the towplane is pulling the glider up an incline! The flight path is inclined, and the towplane is the only one that can provide the force. In fact, I think the lift required *decreases* with increased climb rate during tow! How could that be? The tow rope provides some of the force needed to hold the glider in the air. 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. Let the games begin! -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) 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 think Chris Reed well nailed the (somewhat bleeding obvious when you think about it) issue here with AoA and handling on slow tow. Darryl |
#94
|
|||
|
|||
poor lateral control on a slow tow?
On Jan 4, 2:14*pm, "
wrote: Just some real fast and dirty assumptions.........say your climb angle is 5 or 6 degrees.......200' rope. * Rope could easily sag 10' in the middle 10 *feet* ???!!!! When I watch a glider taking off, the tow rope is entirely clear of the ground as soon as the tow plane has taken up slack and started to accelerate, and this continues to be true when the glider is just off the ground (in case you argue there is more tension when the glider is rolling on the ground). The sag might or might not be a bit more than 10 inches, but it's certainly nowhere near ten feet! |
#95
|
|||
|
|||
poor lateral control on a slow tow?
Chris
As I understand things - you are confusing climbing with vertical acceleration. While flying without vertical acceleration within the performance available to gliders (I.e. constant vertical speed, modest climb angles, tiny descent angles) the wing has to support pretty much one glider weight (1g*mass). This is true whether you are gliding at 1:nn, towing behind an anaemic cub at 9000" density altitude, or screaming skywards behind a turbo Cmellak (Zlin 37). All that changes is the angle your flight path makes relative to the ground. While the rate of climb may seem significant it has nothing to do with AoA. So - the flight path angle to any given frame of reference is largely irrelevant to the AoA required. The AoA required is dependant on many things - as I understand it these include - - the Cl of the wing, - relative velocity of the air over the airfoil, - relative density of the air, - the surface area of the wing - the force it must support. The force it must support will vary slightly depending on the vector of force applied by the propulsive device. This could be an engine, a tow plane or gravity. But it is only significant for one propulsive tool I am aware of - winching does involve a short period of significant vertical accelleration. In the transition from the initial climb to the steep climb part of a winch launch the accelleration changes from ~1g to about 2g. So the wing has to generate enough lift to generate the force to change the flight vector - for a short while the AoA is high, close to the ground. If it goes wrong here the prospects for a stall are good. In the steep climb the angle described by the flight path relative to the ground can easily reach 45 to 50 degrees, but the AoA on the wing remains constant. It will be supporting a constant about 1g after the transition. The bending moment on the wing root is higher for reasons related to where the winch vector is applied, and to the direction and magnitude of that force, but this is not the load the wing must support. As the glider has no significant vertical acceleration, the wing is aerodynamically supporting a constant ~1g. (It must be a little higher because of the added component of the winch force vector normal to the wing) 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? Bruce On 2011/01/04 1:30 AM, ProfChrisReed wrote: It seems to me that increased AoA must be a very large part of the cause. Imagine you are flying free @55kt. You have a sink rate of, say, 1.5kt. Now you are on tow, again @55kt, but this time the combination is climbing @5kt. Your wings are generating 6.5kt more lift than in free flight, and must therefore be at a substantially higher AoA. Additionally, the faster you are climbing (in still air) the greater the AoA must be for you to keep station with the tug. I fly an Open Cirrus, towing from the C of G hook without ballast, and never experienced this at my previous club which had a Citabria tug. My current club has a Pawnee, and I have from time to time felt the tow was too slow because the controls felt mushy and the glider wallowed about, feeling as if it was close to the stall. The Pawnee climbs much faster than the Citabria. If in addition the tug's slipstream imparts a downward flow to the airmass, even more lift and higher AoA is required. -- Bruce Greeff T59D #1771 & Std Cirrus #57 |
#96
|
|||
|
|||
poor lateral control on a slow tow?
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. -- martin@ | Martin Gregorie gregorie. | Essex, UK org | |
#97
|
|||
|
|||
poor lateral control on a slow tow?
At 23:30 03 January 2011, ProfChrisReed wrote:
It seems to me that increased AoA must be a very large part of the cause. Imagine you are flying free @55kt. You have a sink rate of, say, 1.5kt. Now you are on tow, again @55kt, but this time the combination is climbing @5kt. Your wings are generating 6.5kt more lift than in free flight, and must therefore be at a substantially higher AoA. Additionally, the faster you are climbing (in still air) the greater the AoA must be for you to keep station with the tug. I fly an Open Cirrus, towing from the C of G hook without ballast, and never experienced this at my previous club which had a Citabria tug. My current club has a Pawnee, and I have from time to time felt the tow was too slow because the controls felt mushy and the glider wallowed about, feeling as if it was close to the stall. The Pawnee climbs much faster than the Citabria. If in addition the tug's slipstream imparts a downward flow to the airmass, even more lift and higher AoA is required. In a steady climb (or descent) lift is very close to being equal to weight, even taking account of tow rope inclination. You don't need extra lift to climb, you need extra thrust to increase potential energy. (A pull-up or zoom climb is different, because in this case you are trading speed for height). The Pawnee is a significantly heavier aircraft than the Citabria, so would generate stronger tip vortices at a given tow speed, and hence have more effect on a glider behind it. |
#98
|
|||
|
|||
poor lateral control on a slow tow?
|
#99
|
|||
|
|||
poor lateral control on a slow tow?
On Jan 3, 11:10*pm, Darryl Ramm wrote:
On Jan 3, 5:23*pm, " wrote: On Jan 3, 6:30*pm, ProfChrisReed wrote: It seems to me that increased AoA must be a very large part of the cause. Imagine you are flying free @55kt. You have a sink rate of, say, 1.5kt. Now you are on tow, again @55kt, but this time the combination is climbing @5kt. Your wings are generating 6.5kt more lift than in free flight, and must therefore be at a substantially higher AoA. Additionally, the faster you are climbing (in still air) the greater the AoA must be for you to keep station with the tug. I fly an Open Cirrus, towing from the C of G hook without ballast, and never experienced this at my previous club which had a Citabria tug. My current club has a Pawnee, and I have from time to time felt the tow was too slow because the controls felt mushy and the glider wallowed about, feeling as if it was close to the stall. The Pawnee climbs much faster than the Citabria. If in addition the tug's slipstream imparts a downward flow to the airmass, even more lift and higher AoA is required. I also disagree with you statement that the AoA *must be greater if you climb more rapidly......not so.... Assuming a constant airspeed.... The rate of climb is strictly a factor of the power available. * More powerful towplane = faster rate of climb......lift on the glider's wing, and the *towlane's wing stays practically constant, therefore the angle of attack is just about constant. It is the climb angle (direction of flight) which changes with power, not the AoA. Cookie Ugh? The glider is flying, the towplane is not dragging the glider up an incline. If the combination is going up faster (=steeper climb rate/ angle) then both aircraft wings are generating more lift and they get this this from some combination of increased AoA and airspeed. The more powerful towplane may allow both aircraft to fly at an increased AoA and overcome the associated drag. The increased climb angle comes from the increased lift. Assuming a constant airspeed means all the increase is coming from an increase in AoA and the more powerful towplane thrust is offsetting the increased drag. I'd be interested to see an explanation of any other way of generating an increase in climb angle without increasing the lift of the glider and/pr towplane. Darryl- Hide quoted text - - Show quoted text - Do some simple vector diagrams....lift, weight drag and thrust.....do these for various climb, (and glide) angles)... you will soon see that the greatest lift occurs at "level" flight......the steeper the climb, or the steeper the descent, the less the lift. You will see that to achieve steep climb 9with constant airspeed) , we need only to increase the thrust vector.......more power required. In level flight, power is required......lift = weight lift tapers off to zero as we approach straight up flight, and / or straight down flight. In descending flight (gliding) less and less power is required as we steepen the descent, until we reach best L/D, when no power at all is required. What you are thinking about is trying to increase climb angle with a fixed amount of power.......to do this we have to increase the angle of attack, and of course fly slower........Vx....Vy.......come in to play now.......best rate, best angle ....at fixed power, we vary speed and AoA to achieve Vx or Vy or whaterver rate or angle we want..... But if we have a very powerful towplane, we can climb fairly steeply, at a normal towing speed, and the AoA will be fairly low, certainly not near stall.... But remember the premise of this discussion.......towing at a speed at which the glider performs nicely in gliding flight, yet had control issues in towing flight.....but speed constant. A glider which will "glide" nicely at say 50 MPH, may not tow nicely at 50 MPH.............vector diagrams will show that the lift is nearly the same in both cases, therefore the AoA is about the same........so some or all of the other factor discussed in this thread must come into play. But rapid climb rate = high AoA is a fallacy. High climb rate = high power is correct. Cookie |
#100
|
|||
|
|||
poor lateral control on a slow tow?
At 07:51 04 January 2011, Darryl Ramm wrote:
On Jan 3, 8:54=A0pm, Eric Greenwell wrote: On 1/3/2011 8:10 PM, Darryl Ramm wrote: On Jan 3, 5:23 pm, " The rate of climb is strictly a factor of the power available. =A0 Mor= e powerful towplane =3D faster rate of climb......lift on the glider's wing, and the =A0towlane's wing stays practically constant, therefore the angle of attack is just about constant. It is the climb angle (direction of flight) which changes with power, not the AoA. Cookie Ugh? The glider is flying, the towplane is not dragging the glider up an incline. If the combination is going up faster (=3Dsteeper climb rate/ angle) then both aircraft wings are generating more lift and they get this this from some combination of increased AoA and airspeed. The more powerful towplane may allow both aircraft to fly at an increased AoA and overcome the associated drag. The increased climb angle comes from the increased lift. Assuming a constant airspeed means all the increase is coming from an increase in AoA and the more powerful towplane thrust is offsetting the increased drag. I'd be interested to see an explanation of any other way of generating an increase in climb angle without increasing the lift of the glider and/pr towplane. Actually, I do think the towplane is pulling the glider up an incline! The flight path is inclined, and the towplane is the only one that can provide the force. In fact, I think the lift required *decreases* with increased climb rate during tow! How could that be? The tow rope provides some of the force needed to hold the glider in the air. 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. Let the games begin! -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) 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 think Chris Reed well nailed the (somewhat bleeding obvious when you think about it) issue here with AoA and handling on slow tow. Darryl That's the problem with aeroplanes of any sort - the bleeding obvious is not always right. I mean, it's obvious that if I'm a bit low on approach I can stretch the glide by pulling back a bit more ... and a bit more ... and .... |
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 |