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#1
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Constant speed or constant attitude?
I would like to hear what other glider pilots have found to be helpful
in trying to center thermals. It is common in gliding books to read that a constant diameter circle, as a product of flying a constant airspeed and a constant bank, is important in centering thermals. If the airspeed and / or bank is allowed to vary significantly the thermal circle becomes an inconstant oval, which can make locating and centering a thermal more difficult. I understand this, and I think it helps in staying in contact with a thermal once I have identified the stronger area in a thermal and am more or less centered in it. But while I am exploring a thermal, while I am trying to get an idea of how the lift is varying around the circle I'm flying, while I'm working my way towards the core of the thermal, I find it more informative to try to fly a constant attitude and bank and allow the airspeed to rise and fall as the lift comes and goes, and not lower and raise the nose in response to the airspeed changes resulting from the lift changes. I find that flying a constant attitude rather than a constant airspeed in this task greatly simplifies the task of locating the stronger lift and moving to it. If I do try to keep the airspeed constant by raising and lowering the nose as I fly through the changing lift around the thermal I usually end up behind the changes in lift and confused about the thermal's structure. By keeping a constant attitude while investigating a thermal I seem to be better able to use the lift-created airspeed changes as markers of the thermal's structure. Am I goofing-up my thermalling this way? |
#2
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"Jim" wrote in message ... I would like to hear what other glider pilots have found to be helpful in trying to center thermals. It is common in gliding books to read that a constant diameter circle, as a product of flying a constant airspeed and a constant bank, is important in centering thermals. If the airspeed and / or bank is allowed to vary significantly the thermal circle becomes an inconstant oval, which can make locating and centering a thermal more difficult. I understand this, and I think it helps in staying in contact with a thermal once I have identified the stronger area in a thermal and am more or less centered in it. But while I am exploring a thermal, while I am trying to get an idea of how the lift is varying around the circle I'm flying, while I'm working my way towards the core of the thermal, I find it more informative to try to fly a constant attitude and bank and allow the airspeed to rise and fall as the lift comes and goes, and not lower and raise the nose in response to the airspeed changes resulting from the lift changes. I find that flying a constant attitude rather than a constant airspeed in this task greatly simplifies the task of locating the stronger lift and moving to it. If I do try to keep the airspeed constant by raising and lowering the nose as I fly through the changing lift around the thermal I usually end up behind the changes in lift and confused about the thermal's structure. By keeping a constant attitude while investigating a thermal I seem to be better able to use the lift-created airspeed changes as markers of the thermal's structure. Am I goofing-up my thermalling this way? That's pretty much what I do. Keeping a stable attitude does seem to help visualize the location of the thermal core. However, small speed changes don't affect the turn diameter nearly as much as small bank changes. See: http://home.twcny.rr.com/ghernandez/turn_rad.htm I try to carefully adjust the bank angle to move the center of my turn towards the stronger lift. Using the typical bank angles and airspeeds, a 15 degree change in bank will either double or halve the turn radius. I will reduce the bank 15 degrees at the weakest point in the turn and after an interval of about 8 second have elapsed, steepen the bank 15 degrees. Plotting this to scale shows that it moves the circle about one diameter toward the lift. This is not my idea, I got it from someone else. Bill Daniels |
#3
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fly the airspeed.
That way you take advantage of the gusts as you fly through stronger or weaker lift. Al "Jim" wrote in message ... I would like to hear what other glider pilots have found to be helpful in trying to center thermals. It is common in gliding books to read that a constant diameter circle, as a product of flying a constant airspeed and a constant bank, is important in centering thermals. If the airspeed and / or bank is allowed to vary significantly the thermal circle becomes an inconstant oval, which can make locating and centering a thermal more difficult. I understand this, and I think it helps in staying in contact with a thermal once I have identified the stronger area in a thermal and am more or less centered in it. But while I am exploring a thermal, while I am trying to get an idea of how the lift is varying around the circle I'm flying, while I'm working my way towards the core of the thermal, I find it more informative to try to fly a constant attitude and bank and allow the airspeed to rise and fall as the lift comes and goes, and not lower and raise the nose in response to the airspeed changes resulting from the lift changes. I find that flying a constant attitude rather than a constant airspeed in this task greatly simplifies the task of locating the stronger lift and moving to it. If I do try to keep the airspeed constant by raising and lowering the nose as I fly through the changing lift around the thermal I usually end up behind the changes in lift and confused about the thermal's structure. By keeping a constant attitude while investigating a thermal I seem to be better able to use the lift-created airspeed changes as markers of the thermal's structure. Am I goofing-up my thermalling this way? |
#4
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As for this part, Dick Johnson gave an interesting talk about it at a
safety talk at the Region 9 Championships this year. He gave aerodynamic and safety reasons to always use a small slip while thermaling. I can't recall all the details, but have been doing it ever since then and I like it. Larry Pardue 2I I used a little slip ( inner wing forward) before I was flying with winglets. Now with winglets it is different. Most winglets only tolerate max + or - 5 deg. change in angle of attack from optimum. I try to fly now with the string as closely as possible in the centre. Udo |
#5
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When I thermal, there generally seems to be wind,
and since the glider doesn't climb quite as fast as the thermal, I seem to do better slipping or changing bank angle to fly a little into the headwind during each turn (kind of like turns around a point in power flying). I started using slips in thermals after I flew with a competition pilot and noticed he did this a little when thermalling in steeper banks. I also sometimes slip a little "into" a thermal. I suspect this happens mostly because I'm getting fatigued and my coordination is getting worse and I'd rather slip into it than skid. But there may be some aerodynamic reason for slipping a little. It seems that at the extreme (in a 90 degree bank), a slip is better than coordinated flight since it exposes more fusealage area to the thermal when C.G. is forward. There is also some coriolis(SP?) effect, so I notice on .igc traces of extended thermalling that the thermal circles a little as it rises. I also fly by the rule that the center third altitudes of a thermal often provide the best lift. Sometimes I fly in the upper third if I expect to cross a sink area, but the middle third has been pretty good for me. I definitely trim for the thermal, and I've never had a consistent thermal greater than 8 knots, and I haven't found constant banks greater than 50 degrees useful. Some of this comes from discussions with Serge Serfaty, a fellow glider pilot. But he wasn't a big fan of any uncoordinated flight :P I've also had to use shallower banks or leave thermals because I was getting dizzy or tired or hot or couldn't track a fellow glider. I think these are other factors that vary based on the pilot. |
#6
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"Larry Pardue" wrote in message ... As for this part, Dick Johnson gave an interesting talk about it at a safety talk at the Region 9 Championships this year. He gave aerodynamic and safety reasons to always use a small slip while thermaling. I can't recall all the details, but have been doing it ever since then and I like it. Larry Pardue 2I Dick's point was that a glider with dihedral has an increased angle of attack on the leading wing in a slip. (Try bending a #10 envelope to simulate a wing with dihedral. Look at it from the front and then yaw it as in a slip - you'll see the AOA difference between the wings.) If you use a slip instead of top aileron, the wing's airfoil works better without the deflection of the ailerons. You are also holding top rudder, which in the event of a stall, would make the glider roll out of the turn - easier to recover. Really good talk, Dick. Bill Daniels |
#7
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fast as the thermal, I seem to do better slipping
or changing bank angle to fly a little into the headwind during each turn (kind of like turns around a point in power flying). Hmm! Don't understand this and haven't ever done it. The best way to explain this is to show an extreme example. Assume a constant wind from the North at 10 knots, and a stationary source thermal. Also assume that the lift in the thermal is exactly the same at every altitude, and the thermal has constant diameter. The thermal is now a column that tilts south as it rises, but the column never moves, rather like a tall leaning tower of Piza (sp?). It remains fixed relative to the ground. Now assume that a perfectly centered glider in the thermal has just enough lift to remain at a given altitude. If the glider keeps exactly the same bank angle and pitch and rudder on every 360, the glider will drift with the wind and exit downwind of the thermal and begin to sink. So the pilot should extend the upwind time and decrease the downwind time, to fly a perfect ground reference circle and remain in the thermal. The ASEL practical test asks pilots to shallow the bank into the wind and steepen it on downwind to accomplish this. I'd guess a lot of competition pilots probably don't consciously think of this, as their actions to center a thermal are so subtle that changes in airspeed and direction with altitude override the primitive assumptions presented here. Mark Boyd |
#8
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Your example is not working.
If there is no wind, a perfect thermal would rise vertically and you fly constant circles to stay in. If you have a constant wind, the whole airmass - including the thermal - drifts with the wind. If you stay with your constant circles, you drift at the same speed as the thermal so you stay perfectly centered. Just basic vector addition. Corrections are made because there is no ideal thermal, but corrections are made into the core, regardless of the direction of wind. Corrections into the wind are made if the thermal is of orographic, i.e. rotors. -- Bert Willing ASW20 "TW" "Mark James Boyd" a écrit dans le message de ... fast as the thermal, I seem to do better slipping or changing bank angle to fly a little into the headwind during each turn (kind of like turns around a point in power flying). Hmm! Don't understand this and haven't ever done it. The best way to explain this is to show an extreme example. Assume a constant wind from the North at 10 knots, and a stationary source thermal. Also assume that the lift in the thermal is exactly the same at every altitude, and the thermal has constant diameter. The thermal is now a column that tilts south as it rises, but the column never moves, rather like a tall leaning tower of Piza (sp?). It remains fixed relative to the ground. Now assume that a perfectly centered glider in the thermal has just enough lift to remain at a given altitude. If the glider keeps exactly the same bank angle and pitch and rudder on every 360, the glider will drift with the wind and exit downwind of the thermal and begin to sink. So the pilot should extend the upwind time and decrease the downwind time, to fly a perfect ground reference circle and remain in the thermal. The ASEL practical test asks pilots to shallow the bank into the wind and steepen it on downwind to accomplish this. I'd guess a lot of competition pilots probably don't consciously think of this, as their actions to center a thermal are so subtle that changes in airspeed and direction with altitude override the primitive assumptions presented here. Mark Boyd |
#9
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If you have a constant wind, the whole airmass - including the thermal -
drifts with the wind. Depends Ground disconnected bubbles may drift. Ground connected weaker thermals may have a slight tilt. Ground related strong thermals can be rock steady, with the wind blowing around it Chris Melbourne "Bert Willing" wrote in message ... Your example is not working. If there is no wind, a perfect thermal would rise vertically and you fly constant circles to stay in. If you have a constant wind, the whole airmass - including the thermal - drifts with the wind. If you stay with your constant circles, you drift at the same speed as the thermal so you stay perfectly centered. Just basic vector addition. Corrections are made because there is no ideal thermal, but corrections are made into the core, regardless of the direction of wind. Corrections into the wind are made if the thermal is of orographic, i.e. rotors. -- Bert Willing ASW20 "TW" "Mark James Boyd" a écrit dans le message de ... fast as the thermal, I seem to do better slipping or changing bank angle to fly a little into the headwind during each turn (kind of like turns around a point in power flying). Hmm! Don't understand this and haven't ever done it. The best way to explain this is to show an extreme example. Assume a constant wind from the North at 10 knots, and a stationary source thermal. Also assume that the lift in the thermal is exactly the same at every altitude, and the thermal has constant diameter. The thermal is now a column that tilts south as it rises, but the column never moves, rather like a tall leaning tower of Piza (sp?). It remains fixed relative to the ground. Now assume that a perfectly centered glider in the thermal has just enough lift to remain at a given altitude. If the glider keeps exactly the same bank angle and pitch and rudder on every 360, the glider will drift with the wind and exit downwind of the thermal and begin to sink. So the pilot should extend the upwind time and decrease the downwind time, to fly a perfect ground reference circle and remain in the thermal. The ASEL practical test asks pilots to shallow the bank into the wind and steepen it on downwind to accomplish this. I'd guess a lot of competition pilots probably don't consciously think of this, as their actions to center a thermal are so subtle that changes in airspeed and direction with altitude override the primitive assumptions presented here. Mark Boyd |
#10
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Bert Willing wrote:
Your example is not working. If there is no wind, a perfect thermal would rise vertically and you fly constant circles to stay in. If you have a constant wind, the whole airmass - including the thermal - drifts with the wind. If you stay with your constant circles, you drift at the same speed as the thermal so you stay perfectly centered. Just basic vector addition. Corrections are made because there is no ideal thermal, but corrections are made into the core, regardless of the direction of wind. Corrections into the wind are made if the thermal is of orographic, i.e. rotors. Nevertheless Helmut Reichman in his famous book says the same thing as Mark James Boyd. Except he mentions also a case where you have to do the opposite. You are right that the thermal drifts with the wind but the glider sinks in the thermal. You may either figure the thermal as an oblique column of rising or a sequence of bubbles rising while drifting downwind and so connected by an oblique line. In both cases sinking in the thermal will bring you below it, and in order to get back into the column or the next bubble, you have to move upwind. The case above mentionned where you have to do the opposite is the case of a continuous column with a strong maximum, below which the lift is weaker and converging, as well a weaker and diverging above it. In this case, despite your sink in the thermal, it will bring the glider in the upper part of weaker lift, and at this height the stronger lift is downwind. A thing that Mark James Boyd made me discover is that the needed upwind move is higher with lower climb speed, up to completely cancelling the drift when the climb speed is zero. |
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