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

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?

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

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.

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.

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.

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

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.

On 5/10/2014 5:28 AM, wrote:

Cogent numerical analyses/discussions snipped...

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.

Snip...

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 Knauff...] 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

How cool (it is to me!) that experienced (instructors, number-crunching
engineers, thoughtful and concerned sailplane pilots) are contributing their
insights on this topic in the wake of what may or may not have been (yet
another) avoidable tragedy involving a low-altitude premature release from
aerotow. I'm sure I'm not alone in thanking them for their heartfelt offerings
toward improving soaring's future accident record.

Writing as a non-instructor, erstwhile practicing engineer and longtime pilot
with a lifelong interest in preservation (of self, soaring, general aviation,
nifty hand-crafted machines, etc.) here's my summation-to-this-point of
takeaway points for similarly interested future readers of this thread:
1) Mental preparation matters - be prepared with an executable plan!
2) Begin executing it as rapidly as informedly and safely practicable.
3) Retain control always!
4) Hit whatever you're going to hit, while still under control and flying.

Everything else is of lesser particular (though not necessarily ignorable :-))
criticality: mental flexibility (really, an extension of 1) above); bank angle
(see preceding-post excerpts above...with which I agree); even arguably speed.
Get the fundamentals right, and the details are much more likely to then be
correct...

Based on both numerous/uncounted field/B.S. conversations about soaring safety
aspects - as well as others' previous contributions to this thread - I'm
convinced "many of us (soaring practitioners)" can benefit from seriously
seeking to comprehend, absorb and internalize "fundamental takeaways" from
these sorts of RAS threads, skeptics notwithstanding...

Thanks everyone!!! Keep contributing!

Bob W.

P.S. Please take my above little attempt at summation as nothing more than one
individual's expressing some things he simply deems important...

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