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Optimum thermalling speed display



 
 
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  #41  
Old August 15th 17, 11:33 AM posted to rec.aviation.soaring
Dan Daly[_2_]
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Default Optimum thermalling speed display

On Wednesday, October 3, 2012 at 3:42:53 PM UTC-4, Tony V wrote:
On 10/2/2012 11:56 PM, wrote:
Hello fellow glider pilots,

There has been a study of tradeoffs between bank angle and speed.


The Soaring Flight Manual (sadly no longer in print) had a nice diagram
that showed these tradeoffs:

http://home.comcast.net/~verhulst/GB...TurnRadius.JPG

Tony "6N"


When I try the link I get:

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  #42  
Old August 15th 17, 07:24 PM
Squeaky Squeaky is offline
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OK,

I wonder about speeds/angles to fly, so I play with calculators too, then try it out.

Here's why. As mentioned, if speed stays the same, increased bank results in decreased radius. Decreased radius results in getting to the strongest part of the thermal. Therefore it should be simple--bank it up right? Except, as you increase bank, you increase AoA, you increase stall speed, and you increase sink (and min sink speed).

Simple example, so my Glider stalls @ 36 Knots; min sink is 41kts (Level flight). So I enter a turn at 35 degrees of bank at 41 kts. And I get a 427 foot diameter turn. BUT, my stall speed is now 40, and my best min sink speed is now higher than 41. Does the increase in the thermal lift at this turn diameter/airspeed-offset the higher sink by not flying at the new min sink (which would increase the radius at the same bank?)??? Should I stay slow or speed up? Can I handle/manage the 1 knot stall margin, do I have full tail/turn authority at this speed?

Or part two. Same aircraft. now 45 knots, 40 degrees of bank. My turn diameter is now 430 (about the same as previously--huh)) but now I'm at the real new min sink value and it's a little less sink than previously. Seems better there now--less sink, same circle, more room over stall speed (now 41 knots) so It doesn't seem being behind the speed curve makes any sense.

But let's turn tighter because 45 degrees is always best right?? So I feel a surge in the thermal, at 45 knots so I roll to 45 degrees of bank and my circle shrinks to 360 feet diameter. But my stall speed is now 43 knots, and my sink has increased again... I'm almost out of turn/tail authority, so if I do not increase speed, I'll actually lose the ability to maintain 45 degrees and slow/stop turning (not really stall...). So I speed up for safety margin, and a better min sink rate to keep my 45 degree bank and I speed up to 50 knots. Now my turn is diameter is 445 feet... worst diameter yet. Maybe I only speed up to 48 knots--five over stall and still leaving some tail/turn authority, my diameter becomes 410 feet, not much different than what I had all the way through this discussion, but at a higher sink rate for the aircraft--does 20 foot diameter offset the increased sink rate at 45 degrees of bank?? Not that I've seen on the east coast.

But I'm East coast. And based on a lot of different calculations, including making the same ones that the SSA magazine went over a few months back, My best turn is 45 knots at 40 degrees of bank, and in a surge of lift, I tighten to 45 degrees of bank for a bit until I have to shallow the bank back to 40. Not much margin, but at 45 degrees of bank, if I slow that 1 knot I do not stall and fall out of the sky, the plane stops turning and I have to shallow out, or if I don't understand and unconsciously pull to keep the turn going, I watch the yaw sting point up as the nose slices.... at which point I bury the stick in my lap (I'm heavy) to keep the turn going as the speed rises and I regain flying properly...

I do wonder given a thermal strength which a computer could figure, knowing my polar, can it tell me, as Kirk suggests, what speed/bank I should be at to minimize sink and turn radius for the observed thermal performance?
 




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