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Stabizer Pressure
John Leidle:
Trying to understand the amount of pressure force on the stab during turns. Wonder if I use a conventional stab setup with it 1/2" above the wing CL or built the stab inline with the wing CL would pressure be greater either way ?
John L.
Brent Williams:
--- Quote from: John Leidle on April 19, 2020, 11:47:47 AM --- Trying to understand the amount of pressure force on the stab during turns. Wonder if I use a conventional stab setup with it 1/2" above the wing CL or built the stab inline with the wing CL would pressure be greater either way ?
John L.
--- End quote ---
"Pressure," meaning what exactly?
This can lead to other important questions that I hope others can comment on.
Effort to use, ie, hinge moment?
Effectiveness?
Ability to stabilize after the turn?
Size of tail for CG toleration?
Location of tail in "clean or dirty" air? Down wash?
Aspect ratio and pressure/effort differences?
John Leidle:
During the turn a certain amount of pressure is put on the stab surfaces to stabilize the aft section similar to what the elevators are doing.
Tim Wescott:
I can tell you how to calculated, it, but it's complerkated (and very likely wrong, because I'm not an aerodynemicist).
It's easiest for a flapless plane -- just look at some center of pressure plots, and you quickly realize that the CP of a symmetrical wing stays pretty darned close to 25%. So in a turn, it's as if you're concentrating the entire weight (not mass) of the airplane at the MAC of the wing.
For a stunter going 25 meters/second (55 mph) on a circle of radius 20 meters (that works out to about 64 feet handle to canopy), the acceleration in a loop is 8g, give or take a bit, plus the acceleration due to gravity. So at the bottom of the loop it's 9g.
So a 4 pound stunter weighs 36 pounds at the bottom of the loop. If you assume a 12-inch chord and a CG at 20% of the wing chord, then you're putting a moment of 22 inch-pounds onto the plane that has to be overcome by the stab. If MAC of the stab is 22 inches back from the MAC of the wing, then the stab + elevator needs to carry one pound of force (wheelward, in an inside loop) to maintain the circular motion (note that I don't know how much of this is carried by just the stab -- I'm already above my pay grade here, and that's more so).
The further forward the CG is, the more load the stab has to carry -- but if I did my ciphering right, it doesn't look like the numbers are too bad.
Brett Buck:
--- Quote from: Tim Wescott on April 19, 2020, 03:54:26 PM --- If MAC of the stab is 22 inches back from the MAC of the wing, then the stab + elevator needs to carry one pound of force (wheelward, in an inside loop) to maintain the circular motion (note that I don't know how much of this is carried by just the stab -- I'm already above my pay grade here, and that's more so).
The further forward the CG is, the more load the stab has to carry -- but if I did my ciphering right, it doesn't look like the numbers are too bad.
--- End quote ---
The static case is relatively small. Starting/stopping the corner is where the real load comes. I have some notes somewhere.
Brett
p.s. I get something between 6 and 11 lbs, depending on how hard you think you hit the corner. Not trivial, which is why you occasionally see stabs fold.
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