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

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...

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.

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 Sunday, May 11, 2014 3:01:30 AM UTC+12, wrote:
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!

Ahh . yes .. you are correct. That's right for the descent angle, but then you need to adjust that by the ratio of the airspeeds to get the descent rate.


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

With the correction of multiplying the sink rate by "speed needed"/"min sink speed" my spreadsheet gets the above numbers (and same turn times and height loss) for a glider with a min sink of 122 fpm at 42 knots.

On Saturday, May 10, 2014 8:25:35 PM UTC-7, Bruce Hoult wrote:

With the correction of multiplying the sink rate by "speed needed"/"min sink speed" my spreadsheet gets the above numbers (and same turn times and height loss) for a glider with a min sink of 122 fpm at 42 knots.

New summary chart with correction: https://drive.google.com/file/d/0Bw1...it?usp=sharing

The new minimum altitude loss bank angle is...drum roll...45 degrees.

If for some reason you are worried about dragging the wingtip the bank angle that keeps the wingtip the highest is 36 degrees, though obviously this only matters as you sink lower and/or encounter obstacles.

The discussion about offset from the runway is unchanged going from 45 to 60 degrees of bank will reduce the runway offset by 80 feet or so - I don't think it's worth the risk to crank it over that far in most situations. The simple observation is that between 30 and 55 degrees the amount of altitude lost doesn't vary much, so do whatever allows you to control the airplane and get to a laudable spot in go shape to land.

9B

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

On Saturday, May 10, 2014 10:22:28 AM UTC-7, Dan Marotta wrote:
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.

Bruce raised the lateral displacement issue as well. Here is a link to a graphical depiction of the main parameters we've been discussing versus bank angle:

https://drive.google.com/file/d/0Bw1...it?usp=sharing

As to the lateral displacement versus bank angle tradeoff, I have some thoughts:

For the range of bank angles being discussed you are going to end up 400-600 feet (two turn radii) laterally displaced from the centerline (less wind drift - assuming a turn into any crosswind component). Where that puts you will depend A LOT on the airport configuration. If your plan is to land back on a 50-100 foot wide paved runway you will have to turn more than 180 degrees initially then do a reversal to get lined up and the displacement is less of a factor. If you are taking off from the centerline of a 800 foot wide turf runway bordered by 150 foot trees the difference between 400 feet of displacement and 600 feet of displacement matters quite a lot. Have situational awareness and control the airplane - words to live by.

Assuming in most cases the pilot will be trying to get back to a more narrow runway we have the challenge of turning 210-220 degrees on the initial turn and then making a reverse turn to get lined up. The reversal will be when the glider is the lowest so it's probably safe to presume that this turn will be at a lower bank angle (say, 20-30 degrees, perhaps in ground effect). This allows you to avoid performing a cartwheel in front of all your friends. I did this at a high bank angle when I was 11 years old (a story in itself) and can attest that it is not fun.

I'd say the best advice (summarizing the summary) is the following:

- Know ahead of time what your specific plan is for the specific takeoff, not just a general idea - including accounting for crosswinds, obstacles and where you are going to go straight ahead if needed. I have walked fields off the ends of the runways of several problematic airports and it helps a lot - if a field is not an airport it is likely full of surprises.
- Call out the decision height for straight ahead versus 180 while towing out.
- When committing to a 180 make a decisive, controlled and coordinated turn - initially up to 45-55 degrees of bank OR whatever maximum bank angle over 30 degrees you still have rock-solid speed control and coordination. Not being honest with yourself on this point can prove fatal.
- Gradually reduce the bank angle as you approach 180 degrees and continue turning until you can make a smooth turn reversal to the runway centerline.
- Fly the airplane - watch airspeed, coordination, wingtip clearance and obstacles (mine was a fence).
- Try to be aware when a controlled crash is better than the chance you are taking - it can happen quickly.

9B