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Where exactly does the flaperon position change over from positive to negative? What is the reference, or datum line, that defines the two expressions?

Johannes Czernin

If only it were that easy, a simple dimension or angle. Trouble is it isn't, I can tell you roughly what a short wing Avid would be and even make a stab at estimating a big winger.

Where the problem lies is that each wing is constructed by an individual, slightly different washout, slightly different dihedral, quite different fabric tension and to cap it all a vague method of fixing the flaperon final location.

The net result is all the wings fly just slightly differently and so what for one is zero degrees between the two chord lines needs a small change on another to bring about a state of rig.

In both marques the methods of initial rig described will get you to a good start point after that only methodical flight testing will determine the final settings. I have a slightly different method which employs a jig so the rig of each flaperon is set in relationship to its wing rather than the aeroplane as a whole.

The relationship between the two surfaces is complex and has more to do with biplane theory than normal finite wing theory. A biplane or triplane for that matter enjoys a theoretical aspect ratio which is in fact larger than its physical value. Dependent on the stagger distance the aerodynamic center of the wing lies forward of its normal position by a small amount, typically around 23% of chord of the equivalent single wing position rather than the more normal 25%. The fact that the flaperon is adjustable allows the AC to be shifted slightly. In a normal state of rig the aircraft CG is located ahead of the AC to an extent that the moment plus wing pitching moment is balanced by the tail downforce. This provides the necessary positive pitch stability to counter upgusts etc. With both marques we can determine the trim by two means the more normal being to adjust the elevator trim tab to increase or reduce the stick free down force required for stability. However we can also increase or reduce the moment by altering the flaperon setting.

Quite why you may ask, well under ideal conditions the weight and balance is as the designer calculated reality often has it that conditions are different. A plane may fly OK but have a larger tail down force than strictly necessary. The result will be a lot of trim drag which is inefficient. By altering the “zero” setting of the flaperon the extra downforce might be eliminated with quite small angular changes with respect to the main wing chord line. A change in loading may then require another slightly different setting.

This thing can be taken to extremes, by setting a negative angle with respect to the main wing to the point that the tail download actually reverses and becomes a net upload which will unload the main wing. Under clear air conditions and with good piloting skills this is no great problem, no worse than flying a competition glider, but the natural stability will have been traded off. The aircraft will now be flying at good efficiency aerodynamically, lower wing loading equals reduced induced drag, lower tail loading equals reduced induced and trim drag. This is probably the condition most often termed as “getting up on the step” but simply is a state where the overall drag total is lower and so the plan “jumps” up to a new airspeed where thrust is again balanced by drag but for what is effectively the same power setting.

In general setting a less than “zero” setting will shift the AC forward, nose will rise because of excess tail force and vice versa with positive settings. The trim and flaperon work closely together and judging the chances is a fine art and works best in clear air. If the nose rises too much airspeed will bleed off as the plane tries to climb so the normal trick is to push forward slightly as you go negative to maintain altitude whilst waiting for the airspeed to rise again. With practice you get there quite quickly but it is a fine art and doesn't suit turbulent air.

Always a good check is to land off with the cruise elevator trim tab unchanged and see how much is drooping. Five or six degrees is probably about right, anything more means it is having to work real hard.

One of the big disadvantages of the simple wing manufacturing method is that only geometric washout is employed. Without any form of aerodynamic washin, by increasing camber toward the tip, the wings are essentially single speed wings. That is they reach best efficiency at one speed only, having some small adjustment of the flaperon allows this fixed point to be altered to suit conditions or rig.

For the beginning just concentrate on getting the flaperons exactly balanced and rigged per the book. Then fly the plane, checking for assymetry in lift, tendency to drop a wing in the stall, any yaw imbalance etc. then play around with small changes in setting. By rigorously checking cruise settings and true airspeed some improvement will be seen I am sure.

More confused?

Graham Laucht

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