Debunking Glider Spoiler Turns Causing Spin Thinking

On Friday, June 26, 2015 at 12:36:00 PM UTC-7, wrote:
Ditto the above question-- unless the L/D ratio is really poor, the stall speed in a 45-degree banked turn ought to be about 1.189* the stall speed in the same configuration in wings-level flight. S

This is probably an unneeded addition, but for the sake of completeness, I would change "45-degree banked turn" to "45-degree banked LEVEL turn in still air". Lots of us fly 45-degree banked turns in thermals without pressing the stick back much.

On Friday, June 26, 2015 at 7:57:19 PM UTC-7, Jim Lewis wrote:
On Friday, June 26, 2015 at 12:36:00 PM UTC-7, wrote:
Ditto the above question-- unless the L/D ratio is really poor, the stall speed in a 45-degree banked turn ought to be about 1.189* the stall speed in the same configuration in wings-level flight. S

This is probably an unneeded addition, but for the sake of completeness, I would change "45-degree banked turn" to "45-degree banked LEVEL turn in still air". Lots of us fly 45-degree banked turns in thermals without pressing the stick back much.

Is there some suggestion here that the glider behaves differently in an updraft? Surely not.

Re "level"-- in a glider? If the flight path is horizontal with respect to the airmass, you are decelerating, not in a steady-state situation. To maintain horizontal flight, you'll have to keep moving the stick aft at a certain rate, as the airspeed bleeds off. Will the glider reach the stall angle-of-attack at a different airspeed than if you just ease the stick ever-so-slowly aft, so that the rate of deceleration is negligible, and you stay very close to a steady-state glide?

At any given instant of time where the glider is at some given angle-of-attack, the L/W and Na/W values (Na=Net Aerodynamic Force) will be slightly smaller in the steady-state glide than in decelerating level flight, so the airspeed at the stall angle-of-attack will be slightly less in the steady-state glide than in decelerating horizontal flight, but the difference will be trivial for normal sailplane L/D ratios. In the decelerating level flight case, the L/W value will be the same as it is on the top line (infinite L/D) of the table I posted on June 25, and the Na/W ratio will be slightly larger. For any given glider at some given angle-of-attack, the ratio between L, D, and Na will be the same in the decelerating level flight case as in the steady-state glide case. Looking at the table I posted June 25, for glide ratios of 10:1 or better you can see that there is no discernible difference between L and Na. There's also no discernible difference between the value of L, or Na, at a 10:1 L/D, and L, or Na, at an infinite L/D. So I'd say there's no way you'll be able to tell the difference between the stall speed at the slow deceleration rate required to maintain exactly horizontal flight in still air, versus the extremely slow deceleration rate you'd use to reach the stall angle-of-attack while staying very close to a steady-state glide.

S

I'm afraid I don't know enough to really understand your posting.

I can address a couple of your statements though ( I think ):

Does a glider perform differently in an updraft? Seems like it, at least in the degree of back stick needed to maintain a circle. This is no surprise, I'm sure. The "updraft" contributes to the force needed to reduce the downward acceleration of the glider, so less lift force is needed for the job, so less increase in AoA is needed. I'm probably missing your points here and just describing what you already know. I'm sorry about that.

What about a "level" turn in a glider? As far as my very limited experiences have shown me, a "level" turn - a turn during which the glider does not loose altitude - is what I hope for in a thermal. Actually, I hope to gain altitude in a thermal but that doesn't happen for me very often. Does the glider's airspeed decrease in such a "level" thermal? Not if the "updraft" is adequate.

I'm sorry I don't follow your statements about approaching stall speed gradually or slowly. Over my head I'm afraid.

Does a glider perform differently in an updraft? *Seems like it, at least in the degree of back stick needed to maintain a circle. *This is no surprise, I'm sure. *The "updraft" contributes to the force needed to reduce the downward acceleration of the glider, so less lift force is needed for the job, so less increase in AoA is needed.

An updraft is a velocity, not an acceleration. What you are saying, is much like saying a glider handles differently in a headwind, crosswind, or tailwind. Remember the famous "walking slowly down on a rising escalator" analogy-- does it feel any different as you walk downward, on an escalator that is rising than on an escalator that is stopped?

All unaccelerated reference frames are equally legitimate-- it doesn't matter whether the escalator is stopped or moving up or moving down, it feels the same to walk on in each case. It only feels "funny" for a second or two when you step off the escalator into a new reference frame.

Food for thought...

I'm at a loss to explain your perception. Nothing comes to mind that would create that illusion-- unless it's just that when you are rising rapidly, you relax a bit and are no longer so worried about maintaining the absolute minimum sink rate relative to the airmass.

S

PS re recent few posts above-- there might be a correlation between your not getting that stick well aft while thermalling, and your experience of rarely gaining altitude while thermalling! When you are at a safe altitude, gently take it all the way to the stall buffet while thermalling, and then back it off a bit.

Then try the exact same thing at the same bank angle, when no longer in the thermal. If you are expecting some difference in airspeed, pitch attitude, or stick position, then you might be surprised at what you discover.

S

Thank you. I'll give it a try.