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Question 1: For a coordinated 30º banked turn, for any glider, if flown at the minimum SINK SPEED for the angle of bank, how much does the glider SINK RATE increase over the level flight minimum SINK RATE?
A. 8% more than the level flight minimum sink rate
B. 16% more than the level flight minimum sink rate
C. 24% more than the level flight minimum sink rate
D. 32% more than the level flight minimum sink rate
E. 40% more than the level flight minimum sink rate
F. I do not have a clue
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Greg Palmer
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Question 2: For a coordinated 30ºbanked turn, for any glider, by what percentage does the minimum SINK SPEED increase over the level flight minimum SINK SPEED?
A. 0% the minimum sink speed is identical to the level flight minimum sink speed
B. 3% more than the level flight minimum sink speed
C. 7.5% more than the level flight minimum sink speed
D. 10% more than the level flight minimum sink speed
E. 15% more than the level flight minimum sink speed
F. I do not have a clue
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This is fun!
For #1, my vote is B. 16%.
My reasoning: In a 30% bank, load factor is 1.154Gs, or just about 16% more than the weight of the glider in level flight. As bank angle increases, the lift vector is shifted into the turn and away from direct opposition to gravity. Assuming a constant angle of attack through the turn, effective weight exceeds total lift by 16%, which would subsequently increase the sink *rate* by about 16%,
For #2, my vote is C. 7.5%.
My Reasoning: We know that stall speed increases proportionally to the square root of the load factor. As far as I can tell, this effect should be universal for ALL performance airspeeds (excluding structural limitations like Va, Vra, Vt, Vw, and Vne) Since we’re in a 30 degree bank, we know our load factor should be 1.16, and the square root of 1.16 is 1.077, or just about 7.5% increase over the reference airspeed in the POH.
So, in practice a glider with a minimum skink speed of 40kts at max gross should expect to use 43kts for a 30 degree banked turn.
How’d I do? 🙂
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Some good answers so far.
Question 3: For a coordinated 45ºbanked turn, for any glider, if flown at the minimum SINK SPEED for the angle of bank, how much does the glider SINK RATE increase over the level flight minimum SINK RATE?
A. 9% more than the level flight minimum sink rate
B. 19% more than the level flight minimum sink rate
C. 29% more than the level flight minimum sink rate
D. 48% more than the level flight minimum sink rate
E. 68% more than the level flight minimum sink rate
F. I do not have a clue
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On Mon, Sep 27, 2021, 4:44 PM Sky Soaring <webmaster@skysoaring.com> wrote:
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Question 4: For a coordinated 45ºbanked turn, for any glider, by what percentage does the minimum SINK SPEED increase over the level flight minimum SINK SPEED?
A. 0% the minimum speed is identical to the level flight minimum sink speed
B. 4% more than the level flight minimum sink speed
C. 9% more than the level fl ight minimum sink speed
D. 19% more than the level fl ight minimum sink speed
E. 25% more than the level flight minimum sink speed
F. I do not have a clue
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Last but not least.
Question 5: You need to reverse direction and you want to conserve energy … or, you are in a rope break scenario shortly after takeoff (above 200 ft AGL) and you have decided it is safe to make an approximately 210ºturn and return to the runway, what angle of bank and airspeed will lose the least amount of altitude for ALL gliders?
A. 20º of bank and level flight minimum sink speed
B. 30º of bank and level flight minimum sink speed plus 5%
C. 35º of bank and level flight minimum sink speed plus 7.5%
D. 45º of bank and level flight minimum sink speed plus 18.9%
E. 60º of bank and level flight minimum sink speed plus 20%
F. I do not have a clue
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Hmmm. I seem to recall a relatively steep bank angle during rope break practice. I’ll go with “D”
It seems worth considering that rate and radius of turn factor into the equation. In a shallow turn, your sink *rate* is lower, but distance travelled is longer as is time to complete the turn. In a steep turn, the opposite is true: Sink rate is up, distance travelled is smaller and duration of turn is shorter.
There’s a peak to this bell curve somewhere… 🙂
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“D” of course because a 45º bank is always the most efficient one. Btw: I was at the field this morning to do some work on my trailer and upon arriving at the entrance the gate was wide open! This was at 8:32. I left at 11:45 with no one else showing up.
KarlOn Wednesday, October 6, 2021, 12:13:32 PM CDT, Sky Soaring <webmaster@skysoaring.com> wrote:
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You do read your Soaring Magazine, I hope?
September issue. Answers are: Question 1: C; Question 2: C; Question 3: E; Question 4: D.
Answer Question 5: D. Th e optimum bank angle and airspeed to lose the least amount of energy (altitude) in a completed turn for ALL gliders is 45º of bank and an airspeed equal to the level flight minimum sink speed (for the current operating weight) plus 18.9% ( approximately 20%).
If you don’t still have your copy you can read it here. https://magazine.ssa.org/viewer?y=2021&m=9&p=38&o=0&q=
Log in required.
Stay tuned for more brain teasers to come.
Steve
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