ISON Tandem AirBike Builder's Log

March, 2001
Are we tired of the words "flaperon" and "Junkers" yet?


I spent a lot of time in AutoCAD this month reviewing the inherent compromises in the Tandem's flaperon control system, and deciding how to best tweak the system for the Junkers flaperon addition. (Please note: "Compromise" is not a negative word here. Airplanes are by nature full of compromises, and it is the designer's job to optimize them all in a synergistic manner.)

The drawing below shows my analysis of the stock Tandem aileron/flap control system (click on image for an enlargement).

All dimensions were taken from the plans and from my pre-welded kit parts. Impressively, the "approximately 30° of flaps" specified in the plans works out to 30.04°! The aileron differential is nearly 2:1 with no flaps, and less prominent as the flaps are deployed as shown in the drawing.

Those who will be using the stock flaperon configuation might want to be aware of the 154° dimension in the upper right corner of the drawing. This is the worst-case angle between the upper pushrod and the flaperon moment arm at 30° flaps and full down aileron.This angle is getting uncomfortably close to a straight line, and therefore will provide very poor leverage. This geometry may add to the stiffness of the ailerons, but with full flaps the airspeed will be relatively low, so maybe it won't be as bad as it looks. I just wanted to point out this observation so builders are aware of it.


Here is the new control system geometry I've developed for the Junkers flaperons (click on image for an enlargement). This design has its own compromises, but all of the arms are perpendicular to their pushrods in the middle of their travel (the mixer arms are now angled instead of straight in line with each other). This should give optimal leverages at 16° flaps, with equal and opposing compromises on either side of that. In the upper right corner you can see the worst-case flaperon pushrod angle is now 121° instead of 154°.

The new aileron "yoke" is now smaller and includes only a tiny amount of differential action. This is for a compromise between Chris Heintz's spec of +13/-13° of aileron throw, and the flaperon mixer's tendency to decrease up-aileron and increase down-aileron as the flaps are lowered. With this yoke geometry, the ailerons will have a small amount of positive differential at 0° flaps and no differential at 32° flaps. (The reason for the double-vision on the Junkers surfaces is because the outer half of each surface is rigged with less incidence than the inner half to cause the wing's inner areas to stall first.)

Other compromises in this design involve increasing the flaperon surface control horn radius slightly and reducing the motor attach-point radius on the mixer slightly. This was necessary to reduce the aileron travels while increasing the flap deflections. Constructive comments on these control system tweaks are welcome, as always. Please post a message to the Tandem Talk board if you have any comments.


Here I'm drilling mounting holes for the new flaperon brackets, using a custom jig which accurately locates the bracket's hinge point and angle in relation to the rib and trailing edge. All five hinge ribs were filled with 1/4" ply behind the aileron spar, with ply gussets applied where the brackets attach. Two AN-3 bolts attach each bracket. The only other wing modification is a small diagonal brace in front of the aileron spar, where the aileron leading edge rib would have gone.

This bracket-attach design is lighter than what I would have done if left to my own devices, but when I apply heavy simulated loads to the brackets by hand everything feels phenomenally strong. When the Tandem's gross weight is divided by the new total wing area (160 sq. ft.) I get a wing loading of 5.75 lbs./sq. ft. Applying this to the total Junkers area of 18.8 sq. ft., it appears that each hinge point will only be seeing a nominal load of around 11 lbs. So the rigidity this structure is exhibiting is very confidence-inspiring at this point, and it appears to be easily up to the task.

I have to thank my parents for the right-angle "drill" shown in the photo above. It's actually a "Skewdriver" right-angle screwdriver that they gave me for Christmas several years ago, and this is the second time I've used it. It has a 1/4" hex drive output, into which I inserted a length of 1/4 hex key, onto which I inserted a 1/4" socket, into which was super-glued a long 3/16" drill bit. It worked great, and I don't know how else I would have gotten around the diagonal members to drill these holes. Oddball tools really come in handy every now and then. Thanks Mom & Dad!


Here's a view from the wing root showing all five brackets in place. The nearest bracket counters the pushrod's bending moment on the end of the flaperon, and brackets 3 & 4 support the seam between the flaperon halves. I had these brackets laser-cut at a local CNC fabrication shop from AutoCAD drawings. The laser leaves a slightly rough edge, so the holes were cut undersize by the laser and then accurately sized with reamers on my drill press.


I'm really committed to the Junkers retrofit now -- off goes the extra rib bay Wayne helped me add initially (photo credit to Robert). If I had fully appreciated how long the stock wing is already, I would have looked sooner for alternatives to making the span even longer (for low-speed towing operations). I'm happy to be back to the original wingspan now.



Always try to keep the number of landings you make
equal to the number of take-offs you've made.