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#71
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Fatal crash Arizona
On Wednesday, May 7, 2014 6:24:23 PM UTC-7, Bruce Hoult wrote:
No matter what your stall speed or L/D, it turns out the optimum to minimise loss of height in a turn is to bank at 54.7 degrees. Feel free to play. Thanks Bruce - I did play with this a bit. Always helpful to do the math. Several observations pop out from the numbers: 1) The 25-30' height loss for a 180 is small compared to the 200' I always use as the minimum safe altitude to make this maneuver. Not that I'm recommending a smaller margin - there are considerations of sink and wind and clearance for the bottom wingtip in the bank. 2) Speaking of the bottom wingtip in the bank, if you subtract that height difference for each different bank angle you get a height loss for a 180 measured at the bottom wingtip that is actually minimal at a lower bank angle than 54.7 degrees. Obviously this would be most likely to apply at the end of the maneuver, not the beginning, unless there is unusual terrain. 3) Whether you include the wingtip clearance in the calculation or not, the total height loss doesn't vary all that much between 30 and 60 degrees of bank - about 6 feet of difference for the center of the aircraft and only a foot or two of difference at the lower wingtip. The conclusion this draws me to is that the most important consideration in PTT is to make a smooth, coordinated, deliberate turn that you can manage easily - and to make sure not to dig the bottom wing into the ground. Within a pretty broad range, there isn't much percentage in optimizing the bank angle. 9B |
#72
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Fatal crash Arizona
At 07:20 10 May 2014, Chris Rollings wrote:
All completely correct but there is one even bigger problem, most pilots when making a low level turn off a launch failure or to modify a circuit/pattern that has got too low, tend to be looking for/at the place they intend to land with little or no attention to spare for the ASI, attitude or slip/skid indicator, that's why these events are so productive of stall/spin accidents. Training needs to emphasise, GLANCE AT THE ASI EVERY 2 - 3 SECONDS IN THESE CIRCUMSTANCES. Attitude is un unreliable indicator very near the ground, even the smallest undulations in the terrain can give a false impression and just being low can make the attitude look more nose down than it is. Surely best practice is simply to keep the speed on until you have got it all sorted. Certainly in modern slippery gliders. Too much speed is much safer than too little and costs very little in height through a turn. Isn't this what you taught us Chris? |
#73
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Fatal crash Arizona
On Saturday, May 10, 2014 6:49:57 AM UTC-4, wrote:
On Wednesday, May 7, 2014 6:24:23 PM UTC-7, Bruce Hoult wrote: No matter what your stall speed or L/D, it turns out the optimum to minimise loss of height in a turn is to bank at 54.7 degrees. Feel free to play. Thanks Bruce - I did play with this a bit. Always helpful to do the math. Several observations pop out from the numbers: 1) The 25-30' height loss for a 180 is small compared to the 200' I always use as the minimum safe altitude to make this maneuver. Not that I'm recommending a smaller margin - there are considerations of sink and wind and clearance for the bottom wingtip in the bank. 2) Speaking of the bottom wingtip in the bank, if you subtract that height difference for each different bank angle you get a height loss for a 180 measured at the bottom wingtip that is actually minimal at a lower bank angle than 54.7 degrees. Obviously this would be most likely to apply at the end of the maneuver, not the beginning, unless there is unusual terrain. 3) Whether you include the wingtip clearance in the calculation or not, the total height loss doesn't vary all that much between 30 and 60 degrees of bank - about 6 feet of difference for the center of the aircraft and only a foot or two of difference at the lower wingtip. The conclusion this draws me to is that the most important consideration in PTT is to make a smooth, coordinated, deliberate turn that you can manage easily - and to make sure not to dig the bottom wing into the ground. Within a pretty broad range, there isn't much percentage in optimizing the bank angle. 9B I agree with B. Further,advocating very steep turns near the ground, even if technically optimum, is likely to result in a less safe result for a number of reasons. First- few pilots can execute such a turn accurately. Speed control goes to crap as bank gets steeper. Second- The effect of wind shear is much greater at very steep banks. Third- Timing of the turn is much harder at high turn rate usually leading to overshoot. A moderate bank of 30-45 degrees, Tom commonly points out correctly that most pilots, when asked for 45 degrees will come out about 30, is close enough to optimum and much more likely to be executed correctly. I submit that having a plan that includes turning promptly in the correct direction for the conditions is an order of magnitude more important than the bank angle used. UH |
#74
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Fatal crash Arizona
On Saturday, May 10, 2014 10:49:57 PM UTC+12, wrote:
On Wednesday, May 7, 2014 6:24:23 PM UTC-7, Bruce Hoult wrote: No matter what your stall speed or L/D, it turns out the optimum to minimise loss of height in a turn is to bank at 54.7 degrees. 2) Speaking of the bottom wingtip in the bank, if you subtract that height difference for each different bank angle you get a height loss for a 180 measured at the bottom wingtip that is actually minimal at a lower bank angle than 54.7 degrees. Obviously this would be most likely to apply at the end of the maneuver, not the beginning, unless there is unusual terrain. You also can't change bank angle instantaneously. I think it makes sense to peak at around 60 degrees of bank as you're about halfway through the turn, and decrease it by the time you're only 45º or so from having reversed direction. 3) Whether you include the wingtip clearance in the calculation or not, the total height loss doesn't vary all that much between 30 and 60 degrees of bank - about 6 feet of difference for the center of the aircraft and only a foot or two of difference at the lower wingtip. The biggest difference between 30º and 60º isn't the couple of meters of difference in height lost, but the 100m difference in lateral displacement at the end of the turn. That means you have to turn further and take more time and height to get back in line with the runway (assuming you don't just have a very wide airfield), and increases the chances of finding yourself on the wrong side of some obstacle. I think it's completely reasonable to expect students to be able to do a crisp 180º reversal turn using between 45º and 60º of bank more or less instinctively before they get to solo. You use such turns all the time when ridge soaring, either when you discover you've gone too far and got into sink, or just to end up no too far out in front of the ridge. |
#75
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Fatal crash Arizona
On Saturday, May 10, 2014 10:49:57 PM UTC+12, wrote:
3) Whether you include the wingtip clearance in the calculation or not, the total height loss doesn't vary all that much between 30 and 60 degrees of bank - about 6 feet of difference for the center of the aircraft and only a foot or two of difference at the lower wingtip. I've added the wingtip calculation. The bank angle that keeps the wingtip the highest varies with the wingspan .... it's about 39 degrees with 18m wingspan, and 41 degrees with 15m. The difference in wingtip clearance between this 39º or 41º bank angle and 55º is 1.8 ft for 15m and 2.7 ft for 18m. If you're low enough for this to make a difference then you probably shouldn't be turning back :-) I still think the horizontal displacement from the centerline is the biggest factor. |
#76
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Fatal crash Arizona
On 5/10/2014 7:24 AM, Jim White wrote:
Certainly in modern slippery gliders. Too much speed is much safer than too little and costs very little in height through a turn. From the standpoint of preventing stalls, I can only agree. But if your goal is to actually make it back to the runway, that advice should be tempered with a bit more information! Remember that turn radius increases with the SQUARE of airspeed. That means that a small increase in airspeed will result in a significant increase in turn radius. As the pilot, your goal in that situation should be to shoot for the proper airspeed for the situation. Not too much, but certainly always keeping a margin above stall speed. By all means don't risk a stall! But remember that too much airspeed could add to your troubles. |
#77
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Fatal crash Arizona
Don't do 60 degree banked turns close to the ground! I don't want to tell someone that's he's wrong, but I think that some people may think that the conclusions are right just because there are some numbers behind them, and start practicing rope breaks with 60 deg banks.
I went through the spread sheet and I found everything correct except for the calculation of the "enhanced sink rate". The sink rate is not just the unbanked rate multiplied by the load factor! Some manufactures include a circling polar. Using the circling polar of an asw-24 with a total weight of 750 lbs, the speed and sink a at 30 deg: 45 kt, 150 ft/min at 45 deg: 50 kt, 205 ft/min at 60 deg: 59 kt, 345 ft/min Now, the time and height lost to complete a full turn (divide by two for a 180 deg turn): at 30 deg: 26 secs, 64 ft at 45 deg: 16 secs, 56 ft at 60 deg: 11 secs, 65 ft The differences are not much, anyway, but it is easier to bank and unbank at 45 deg. I gotta go and can't expand, but I'll try to get the numbers based on the real formulas at a later time. In the mean time, you can check Fred Thomas book, Fundamentals of Sailplane Design, pgs 64 and 65. The minimum sink rate in a turn will be higher than the minimum sink rate in level flight by a factor of 1/(cosangle)^1.5 |
#78
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Fatal crash Arizona
On Saturday, May 10, 2014 5:28:51 AM UTC-6, wrote:
On Saturday, May 10, 2014 6:49:57 AM UTC-4, wrote: On Wednesday, May 7, 2014 6:24:23 PM UTC-7, Bruce Hoult wrote: No matter what your stall speed or L/D, it turns out the optimum to minimise loss of height in a turn is to bank at 54.7 degrees. Feel free to play. Thanks Bruce - I did play with this a bit. Always helpful to do the math. Several observations pop out from the numbers: 1) The 25-30' height loss for a 180 is small compared to the 200' I always use as the minimum safe altitude to make this maneuver. Not that I'm recommending a smaller margin - there are considerations of sink and wind and clearance for the bottom wingtip in the bank. 2) Speaking of the bottom wingtip in the bank, if you subtract that height difference for each different bank angle you get a height loss for a 180 measured at the bottom wingtip that is actually minimal at a lower bank angle than 54.7 degrees. Obviously this would be most likely to apply at the end of the maneuver, not the beginning, unless there is unusual terrain. 3) Whether you include the wingtip clearance in the calculation or not, the total height loss doesn't vary all that much between 30 and 60 degrees of bank - about 6 feet of difference for the center of the aircraft and only a foot or two of difference at the lower wingtip. The conclusion this draws me to is that the most important consideration in PTT is to make a smooth, coordinated, deliberate turn that you can manage easily - and to make sure not to dig the bottom wing into the ground. Within a pretty broad range, there isn't much percentage in optimizing the bank angle. 9B I agree with B. Further,advocating very steep turns near the ground, even if technically optimum, is likely to result in a less safe result for a number of reasons. First- few pilots can execute such a turn accurately. Speed control goes to crap as bank gets steeper. Second- The effect of wind shear is much greater at very steep banks. Third- Timing of the turn is much harder at high turn rate usually leading to overshoot. UH Ground reference maneuver training teaches pilots how to very accurately fly turns at low altitudes. |
#79
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Fatal crash Arizona
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#80
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Fatal crash Arizona
Let's not forget the time it takes to complete the turn or the distance
covered over the ground. Without getting into the math, I can comfortably say that, using a 30 deg bank will take longer to complete the turn and leave you further from the runway. Likewise, at higher elevation airports your sink speed will be higher (think true airspeed) so height loss will be greater than at sea level. wrote in message ... On Wednesday, May 7, 2014 6:24:23 PM UTC-7, Bruce Hoult wrote: No matter what your stall speed or L/D, it turns out the optimum to minimise loss of height in a turn is to bank at 54.7 degrees. Feel free to play. Thanks Bruce - I did play with this a bit. Always helpful to do the math. Several observations pop out from the numbers: 1) The 25-30' height loss for a 180 is small compared to the 200' I always use as the minimum safe altitude to make this maneuver. Not that I'm recommending a smaller margin - there are considerations of sink and wind and clearance for the bottom wingtip in the bank. 2) Speaking of the bottom wingtip in the bank, if you subtract that height difference for each different bank angle you get a height loss for a 180 measured at the bottom wingtip that is actually minimal at a lower bank angle than 54.7 degrees. Obviously this would be most likely to apply at the end of the maneuver, not the beginning, unless there is unusual terrain. 3) Whether you include the wingtip clearance in the calculation or not, the total height loss doesn't vary all that much between 30 and 60 degrees of bank - about 6 feet of difference for the center of the aircraft and only a foot or two of difference at the lower wingtip. The conclusion this draws me to is that the most important consideration in PTT is to make a smooth, coordinated, deliberate turn that you can manage easily - and to make sure not to dig the bottom wing into the ground. Within a pretty broad range, there isn't much percentage in optimizing the bank angle. 9B |
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