ISON Tandem AirBike Builder's Log

December, 2000 -- Flaperon Solutions


Most of the time I spent on the project this month involved zeroing in on a solution to reduce the aileron stick forces while retaining (and maybe even enhancing) aileron authority at very low airspeeds. I had almost decided to cut the flaperons into outboard ailerons and inboard flaps, when some fellow wingnuts came over to see the project after our December EAA chapter meeting. All agreed that we couldn't be sure (without some skilled analysis) how much aileron authority would remain after the aileron surfaces were reduced from full-span to 75" or 90" as envisioned. They encouraged me to take another look at Junkers-style flaperons, so that's just what I did.

My research of Junkers-style flaperons quickly led me to the Zenith Aircraft Company web site, and I especially enjoyed the aerodynamic lessons found in the Chris Heintz Design College pages. The animation* above demonstrates the special wing design that Mr. Heintz has employed with great success on his STOL designs, such as the CH 701 and CH 801. For the purpose of this discussion, ignore the leading edge slat but notice how the Junkers surface always operates in undisturbed air. Also, the slot between the wing and the aileron creates a clean airflow over the top of the aileron which keeps the aileron "flying" and effective even at high angles of attack. Conventional ailerons stall easily at high angles of attack because they are operating in the disturbed partial-vacuum caused by the airflow's detachment from the wing's top, as seen in the top right of the animation above. (* Animation source: STOL CH 801 Design)

As far as I can tell, Mr. Heintz is THE MAN when it comes to designing this kind of control surface for light aircraft, so I decided to contact him and see if he would be interested in helping out with my dilemma. As the site states, "Aeronautical engineer Chris Heintz, the designer of Zenith Aircraft Company's line of kit aircraft, is one of the most qualified and knowledgable light aircraft designers today..."

It took me about a week to figure out that although Zenith Aircraft Company is relatively near to me in Mexico, MO, Mr. Heintz lives way up north of Toronto where he operates Zenair, LTD. After I finally got my letter and airfoil profile in his hands, he was very kind on the phone and willing to help for a reasonable fee. He agreed that retrofitting the Junkers surfaces onto the Tandem wing is a good solution to my problem. And he sees no problem with using stock aluminum flaperon parts from Zenith's CH 701 kit -- something I was really hoping for. He is going to optimize the location of these 8" chord flaperon surfaces below the trailing edge of the Tandem wing, and draw up his recommendations for mounting the flaperon brackets into the wing structure. I should point out here that this means I will NOT be cutting the stock (wood) ailerons off the wing panels as designed -- they will instead remain a rigid part of the wing structure.


Detail of flaperon bracket on the CH 701 wing

After studying the location of the flaperon hinge point in the photo above, you will notice that when the surface rotates upward it also moves forward, which causes a reduction in effective wing area. Conversely, when it rotates downward its rearward motion causes an increase in wing area. I see a couple of benefits from this. One, it complements the ailerons' job of reducing lift on one wing while increasing lift on the other. And two, it increases the overall wing area when both surfaces are deflected downward as flaps, similar to how Fowler flaps work.

But the main reasons for adapting the Junkers flaperons to my Tandem tug are:

  • An enhanced ability to aerodynamically balance the surfaces and bring the aileron stick forces down to the desired level, and
  • The ailerons will always be operating in clean air (not disturbed by the flow over the wing or masked by its boundary layer). This will provide solid roll authority down through low airspeeds and high angles of attack.

Mr. Heintz says we still want the flaperons to be full-span, so this is good news for adaptation to the Tandem's stock control system. Right now it appears that the new flaperons can be activated by the same mixer and pushrods that drive the flaperons in the stock configuration. Interestingly, Mr. Heintz says "you can design differential into the ailerons if you really want, but it isn't necessary." He says that this will be a "rudder airplane" regardless, and aileron differential will have little effect on adverse yaw.

He also said that these new surfaces will increase the wing area, and since the increase is at the aft part of the airfoil this will move the center of lift slightly aft. This generally means that the tail surface areas will need to be increased proportionally (or moved rearward). Although ISON did make my tail feathers extra-large already, it is possible that adding the Junkers surfaces will create the need for even more tail area. But now I'm starting to wonder if I should chop off the extra 15" I added to each wing panel and just let the new flaperons provide the extra wing area I'm seeking. If I went back to the stock wingspan of 31'-4" and added the Junkers flaperons, the effective wing area would still be 3% higher than the extended wing with the original plain flaperons, as shown by this chart:





31'-4" (stock) Plain (stock) 141 sq ft - 6.4 lbs/sq ft
31'-4" (stock) Junkers 157 sq ft** +11% 5.7 lbs/sq ft
33'-10" Plain (stock) 152 sq ft +8% 5.9 lbs/sq ft
33'-10" Junkers 169 sq ft** +20% 5.3 lbs/sq ft

(** I'm estimating the effect of the Junkers on the wing area. From photos it appears that about 25% of the flaperon surfaces will be ahead of the wing's trailing edge, so I didn't count that part. If the forward 25% counts toward wing area, these two figures could be as high as 162 sq ft and 175 sq ft respectively.)

I've asked Mr. Heintz to suggest the best configuration of all the above parameters for my given mission profile. Hopefully he will offer specific suggestions for wingspan/wing area and tail surface area.

I talked with Wayne Ison about the Tandem's ailerons, and he said that the present system is a compromise in favor of a simple control system. He attributes the aileron stick forces -- which are higher than on his other designs -- to the longer wing span, higher wing loading, and also to the torsional twisting inherent in these long surfaces. Since they are driven at their inboard ends, there is more aileron throw at the root than at the tip, and this is backwards from how it should be. Ideally, of course, we'd prefer more throw at the tip and less at the root.

Wayne thinks that the span of the Tandem's ailerons could be reduced with no adverse effect on roll authority. He's seen miniMAXes that had been made with ailerons that were 25-30% of the wing span, and "they seem to work OK." He said (off the cuff) that ailerons at least 72" long should be OK on the Tandem. His suggestion for their connection is to run torque tubes from the ailerons to the wing roots, and then connect the aileron pushrods to the torque tubes with lever arms. This wouldn't be too difficult of a project if no flaps were desired inboard of the ailerons. If flaps were employed, the aileron torque tubes could be routed inside the flap torque tubes (Wayne thinks the BD-5 works this way), but we're now talking about a fairly sophisticated re-engineering and building project. I wouldn't recommend trusting your life to a system like this without a thorough load test, as wing flexure could easily cause a bind in the torque tubes and lock up your ailerons. Wayne said a person could certainly run a cable system to the surfaces "if you can afford the expensive pulleys and turnbuckles" required for the job.

If I had already cut my ailerons off the wing panels, I would feel pretty good about further investigating the plan of chopping the ailerons into shorter surfaces. I think I'd find an aerodynamicist who can confirm this modification as a good idea, and then get to work redesigning the aileron and flap hinge points. I haven't cut the ailerons off, though, and have my heart set on the Junkers surfaces for my plane. I also believe that it will be easier to add the Junkers than to re-hinge the stock ailerons and flaps and modify the control system.

Wayne had no problem with my working with Mr. Heintz on this modification, so I guess that settles it. I have sent Mr. Heintz all of the information he should need to design the Junkers retrofit, and I expect to have the results from him around the end of January. I may catch up with other aspects of my life during this down-time, so my shop may be quiet for a while. Who wants to deal with a 23° workshop anyway? Perhaps it will be warmer in January. Let's hope...

Aside from building one whole rib (with one left to go), this is about all I have to show for the month of December. But I think it's a significant step which will lead to reasonable aileron stick forces, and a much nicer balance with the elevator stick forces. Who knows -- perhaps it will even transform the Tandem into more of a sportster like the single-place AirBike. Time will tell, and the clock is ticking...

I'm gonna close out this installment with some pictures of Junkers flaperons on various airplanes. The first 8 photos are from the Zenith site -- if you like these shots, go check out their photo archives, because there are LOTS more. If I was to say today what my next airplane project will be, it would have to be a CH 701! Looks like loads of fun. The last two photos are from the Skystar site.

Happy Holidays, everybody.







Seeing how the Junkers flaperons fold over backwards, perhaps they won't interfere with the Tandem's stock wing-fold set-up after all.




Now THAT is a short-field landing!

The surfaces on this Kitfox are hinged on top. You can't see it here, but there are two counterweights projecting forward from each aileron (flaperon?) for mass balance. I wonder what Chris Heintz knows that these guys don't?

Skystar's new ultralight Kitfox. Don't know if these surfaces have the counterweights.