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This styrofoam airplane has two positions for stabilizer. When put in the upper one, the plane flies in a circle.

Is the stabilizer now stalled because the decalage angle is very big?What happens with the airflow over wing and stabilizer in this configuration?

enter image description here

user707264
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2 Answers2

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Declage controls the AOA to which the static stability forces will be focused with no control input; in other words, the hand's free AOA/speed. A higher declage angle is the same as increasing elevator input and holding it there.

It changes the trim force balance (nose down pitching moment from the main wing, C of G relative to wing center of pressure, and other appendages that generate pitching moments, opposed by the nose up pitching moment from the tail) to occur at a higher AOA. Airplanes with moveable horizontal stabilizers, like most jets and a lot of light planes like the Cessna 180/185 family, have what is in effect, adjustable declage as their method of changing trim. What you are doing by sticking the tail surface in the different slot, they achieve by cranking a screw jack to move the front of the same surface up and down for the same result.

So the tail won't stall at the moderate angle variation there, it'll just drive the main wing to a higher AOA to achieve trim equilibrium, and your glider will trim to a slower speed. If designed properly, if you keep increasing the declage, the main wing will be driven to its stalling AOA, and the aft Center of Pressure shift that occurs at stall will cause the nose down pitching moment to overpower the tail's downward lifting force and the glider pitches over before the tail's airfoil itself stalls.

The result is, when you throw it with the tail stuck in the upper slot, it will pitch up harder for the same throwing velocity as it seeks its higher trim AOA/lower flying speed (if you throw it at exactly its trim speed, it's already in trim when you release it and will just glide straight without pitching).

So bottom line; the lower slot is a "fast slot" and the upper slot is a "slow slot". The glider will trim to a lower AOA and a higher speed on the lower slot, and vice versa on the slow slot. That's the practical result of the two options.

John K
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  • With "slow slot" it is almost impossible to fly in straight line ,without adding weight to the nose.It throw it fast it will loop,if you throw it too slow it will fall to the nose and if you throw it with trim speed for "slow slot it will fly only 2 m. Slow slot use for looping,glider behave like boomerang,always come to you,make circle of about 2m radius – user707264 May 16 '21 at 17:31
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    It's looping because it's going way faster than trim speed and it's seeking to slow down. – John K May 16 '21 at 18:59
  • Can you post all links where is explained why pitch up happens when exceed trim speed? – user707264 May 16 '21 at 19:09
  • https://aviation.stackexchange.com/q/72253/42636 – Jpe61 May 16 '21 at 20:46
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The decalage angle of the upper horizontal stablizer slot was measured as arcsin × (.4/2.0 cm) = 11.5 degrees. This symmetrical airfoil will have a high coefficient of (downward) lift when thrown, which would immediately increase the angle of attack of the wing (raise the nose).

11.5 degrees Angle of Attack may be close to stalling, but even if it were stalled it would continue to generate lift.

The only way it could remain stalled would be if the weight were way too far forward. The glider would just crash into the ground.

If your Center of Gravity was set slightly forward of the wing center of lift (read instructions for your plane as to the where CG should be), the tail will pitch your wing to its correct angle of attack, and both AOAs will be well below stall AOA.

So, if you want to throw that glider off a hill, use the upper slot, set weight forward, and it will fly at a constant speed until it runs out of altitude. Do not try to throw it too hard when you launch it.

If you want to throw your glider hard for distance (without a hill), move the weight back and use the lower slot. This glider will fly at at variable speed until drag slows it down.

CG (weight) setting is critical in both cases. Try moving it back and forth until the plane flys the way you like it. (Some free flight models allow you to move the wing. This is a similar approach).

Robert DiGiovanni
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  • This glider dont have option to set/move c.g. but you can add some extra weight at nose if you want play with c.g....Yes if you want that glider fly in straight line with upper stabilizer position,you must add lots of weight at nose.As always this works only for one speed,when you exceed trim speed it start pitch up. – user707264 May 16 '21 at 17:22
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    In flight the 11.5 degree angle is effectively shared between the wing and the stabiliser; the wing’s AoA will increase and so the stabiliser’s AoA will be smaller (smaller in magnitude, as it’s negative). – Frog May 16 '21 at 20:16
  • @Lost in Space paper clips work well as CG adjusters. How about posting a picture of your entire plane (and who makes it)! – Robert DiGiovanni May 16 '21 at 21:50
  • I don't think CG shifting is nerded – Abdullah is not an Amalekite May 18 '21 at 08:44
  • @Abdullah just as in a hang glider, placement of CG is critical to trim settings. One may make that stabilizer variable, but with FF its easier to move the weight a bit. – Robert DiGiovanni May 18 '21 at 11:54