Over-Thick Control Surfaces

[Wacobipe]

I have the books written by Dave Scott of 1st US R/C Flight School, and I have also read an article by him recently in Model Airplane News (I think) where he references this idea and I am curious what everyone else thinks about this:



In these resources Dave proposes the following 2 things to improve aerobatic aircraft performance: A thicker control surface than the flying surface it is attached to (Aileron thicker than wing) and rounding the leading edges of control surfaces. I have attached scans of 2 pages of Sport Aerobatics written by Dave for reference but Let me explain in more detail.



First, Dave says that essentially all modern aerobatic full scale aircraft have thicker control surfaces than the flying surface they attach to (the LE of the aileron is thicker that the TE of the wing for example). The purpose of this is to push the control surface out beyond the turbulent air layer and into the smooth air beyond it. This provides much more precise control and also control at lower airs speeds.



Second, he suggests that a control surface leading edge should be round (not beveled as is the norm in the hobby). This round leading edge of a control surface should be attached to a trailing edge of the flying surface that is squared off, and not beveled or rounded (see images). To clarify let's consider an aileron. The aileron would attach to a squared off rear edge of the wing, and the aileron would have a rounded leading edge. By doing so, Dave says that turbulence over the control surface is reduced providing for more precice control.



Combined, these 2 design considerations improve the correlation between radio inputs and airplane reaction, allow for lower minimum controllable airspeeds, aircraft response is improved in aerobatics (especially on windy days and close to the ground), and the potential for flutter is greatly reduced (he claims sealing the gaps becomes unnecessary).



I am curious what everyone here thinks about these concepts so please throw your 2 cents into the conversation!




20068=12905-Scan 1.jpg

20068=12904-Scan 2.jpg

 

[SleepyC]

Interesting.

 

[tl3]

Interesting indeed. Guys in the pattern world have been advocates of thick trailing edges for years, particularly on the rudder. Some of that has migrated into scale aerobatics with the use of flared trailing edges on rudders. I am about to start re-furbishing a Carden PRO, some of which will include replacing all the hinge stock and rehinging...maybe time to try something different...

 

[Bartman]

what you guys are talking about is what Curtis Pitts did with the S1-11B Super Stinker which is why you'll hear guys referring to the thick ailerons as SS ailerons. The Pitts S1-SS used the SS literally as it is an S1-S with SS ailerons.



Curtis found his inspiration by looking at what the Soviets had been doing and he worked up a formula for his own designs that Kevin Kimball at Jim Kimball Enterprises now holds lock and key to.



The theory behind it, like WacoBipe said, is that it forces the bulging leading edge of the aileron up into the airflow and the more the aileron is deflected the more it extends up above the trailing edge of the wing. It's not so much about reaching up into smooth air as the air at the trailing edge is pretty turbulent, especially at higher angles of attack, but what it does do is it forces the air to re-attach to the aileron and then follow the surface of the aileron before it eventually unattaches again.



The effect isn't extremely pronounced, it's a very turbulent area to begin with so there's only so much you can expect from the trick but it's enough to improve control especially at higher throw angles.



The bulging leading edge also has the effect of forcing the airflow to follow the contour of the aileron when the controls are centered and this has the effect of improving the feel of the centered aileron. a pilot can feel a much more centered position of the control stick letting him/her snap the controls out of center and then right back to center for cleaner maneuvers with that center better defined by the aerodynamics of the airflow over the aileron. it's a feel thing, pilots with lots of time in acro planes can describe it better.



so is there a place for this in RC? not really, imho. we use control throws that are so extreme that it would be difficult to fair the aileron LE into the wing TE while also getting 50 degrees of throw. Most full scale planes don't have the luxury of 50 degree control throws so they use the fat control profiles to improve what they can get from the throw that they do have. Also,the servos center perfectly and our radios have springs to snap the sticks back to center so a pilot wouldn't know the difference from a sloppy aileron and a crisp one.



scale guys might see improved control authority with Aerodynamically Boosted Ailerons (as they're formally called) but for the 3D guys I'd have to guess there's no point to it.



Let's see what other guys with knowledge on the topic have to say. Kevin Kimball is a pretty active RC flyer but I don't think he's a member here yet. If he were here he'd chime in and lay it all out for us, he's pretty good that way.



Bart

 

[Bartman]

the squared off trailing edges helps the airflow to make a clean break with airflow on the top separating off the TE as close as possible to how the bottom airflow is breaking, again to help the aileron feel stable and well centered as the pilot is centering the stick. it can reduce vibrations so maybe there's a benefit for the servos and maybe an RC plane will benefit from controls that center and stop maneuvers as instantaneously as possible?



that's how i understand it at least.

 

[Pistolera]

BTW - the Pitts S1-T had the fatter rounded aileron leading edges, and if I'm not mistaken was the first of Curtis' designs to use this feature. The S1-11 came about several years later.

 

[Terryscustom]

Very interesting. My guess is like the images shown in the first post is that the aileron would have to be rounded LE (center hinged) to have the correct effect so modifying most ARF's would be out to give it a try.

 

[Wacobipe]

And we thought you were just the webmaster and admin Bart! You keep up being a source of knowledge and you'll become as indispensable as SleepyC!



Seriously though, my question was coming from an IMAC viewpoint. I agree completely that this doesn't come to bear with 3D setups due to the extreme throws but my planes tend to have 20-25 degrees of surface throws max as I fly scale/IMAC style. In this case I think you are saying that there is benefit, but perhaps not as much as in full scale (no control feeling in our case for instance), but the approach is quite likely aerodynamically beneficial and certainly not counterproductive.



Interested to hear more points of view as this thread grows. Thanks!

 

[Pistolera]

This is a not-to-scale cross-section drawing of the way my Pitts S1-T ailerons were designed. It was a 1981 factory-built Pitts....the fourth T-model off the line.

20098=12908-Slide1.JPG]

 

[Bartman]

the S1-T was a -C made by Aerotek so it had flat bottom ailerons to go with the flat airfoils that the -C's have.



Wikipedia has a great entry on the line of Pitts biplanes. I've got a thing for Pitts biplanes, I'd like to have an S1 of some variety one of these days but they ain't cheap so I wait, and wait....lol



http://en.wikipedia.org/wiki/Pitts_Special