soontobeover

There are 2 gliders in the same thermal. One is circling normally and trying to gain altitude. The 2nd one is also circling and without falling out of the thermal it is pumping the elevator abruptly at one minute intervals. Which glider will gain the most height after a given time?

Gremlin Castle

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The thermal is putting out a fixed amount of energy at any given point in time so the amount of vertical climb is going to the sailplane that utilizes the rising air most efficiently.
Pumping the elevator may give the perception of rapid climb but the loss of airspeed drops the wing below it's best minimum sink speed so the rate of overall climb decreases until the airspeed comes back up to the optimum l/D or minimum sink rate one of which will allow for the best rate of climb when in a thermal.
In full scale sailplane flying there is a technique used in cross country flying on strong lift days. Flight is done in a straight line from thermal to thermal pulling back on the stick when a thermal is intercepted and then sticking the nose down to re- accelerate when you come out the other side of the thermal.
Don't know if this helps but I spent many an hour flying full scale in the same thermal with other sailplanes. The most coordinated pilots always gained altitude a bit faster than the ones that changed pitch and roll in a less than smooth fashion.

Campgems

To many variables are ignored in your question. Same airframe, weight, balance, radio setup, IE the same plane. Then, thesame pilot skills?

So, based on the variables in your question,the answer is that the glider that has thehighest altitude at the endof the given time would be the winner.

Don

soontobeover

Ihave been pulling back on the stick for years just before crossing the shear of thermals. Iam just trying to validate that pulling back once inside the thermal does not work at all like crossing the shear.

Ihave seen the climb rate increase when normal circling did not work by falling outside the thermal while circling which Cal Tech says works also when I spoke to taras k on the phone.

You give good answers in this section Gremlin thanks.

soontobeover

Thanks to you above also..

abelard

The optimal strategy for climbing in a thermal is to fly at a constant angle of attack. Only one problem: we don't know how to do that. Jets have very good AoA sensors, and use them to good effect (especially in carrier landings!) but making one that will function at glider airspeeds and stand up to the ground-handling environment that goes with them has been elusive so far.

And that's just in full-size gliders; in models, you'd have the added problem of providing a telemetry capability to display AoA on the transmitter.

BMatthews

Let's make some assumptions here so we're all on the same page. First off the gliders are identical in every way. And the portions of the thermal they pass through are the same.

So we come down to the question. Namely is the lift a constant or near constant value throughout the circle or are the gliders passing through shear zones?

If the lift is relatively constant throughout the circling turn then the glider that is flying at the stable angle of attack which is generating the minimum sink airspeed will go up the fastest. Simply put if the glider is descending through the rising air column at the minimum sink angle of attack then it will get the most rapid climb value. The rate of climb being simply the vertical velocity of the rising air minus the sink speed of the glider.

The glider that is allowed to fly a little faster and "pumps" the elevator will not gain as fast due to a few factors. By flying faster it has a higher sink rate so it's not maximizing the potential climb rate of the vertical airflow. And the higher flying speed produces more parasitic drag even though the induced drag is lower. And then you generate even MORE drag when you pulse back on the stick which both produces added control surface drag as well as forcing the wing to an overly high angle of attack and into a much higher lift coefficient and associated higher drag coefficient. Oh sure, the blip of up will cause it to rise up. But at the same time it trades the speed for the altitude so it comes off the top of the blip at a slower speed than it was trimmed for. And by accelerating back up to the trimmed speed it loses much of what you gain with the blip.

Now if the gliders are passing back and forth through some sort of shear zone around the edge of the thermal AND if you time the blips on the elevator to match these transitions then you could potentially gain some energy from the momentary dynamics. But for this to work the flyer has to be keen of eye and judge exactly when to push down and when to pull back to maximise the energy gained by passing through the shear zones. Miss the timing by even a quarter second and you might not gain anything at all.