Excess Aileron sensitivity around neutral

[Amir Neshati]

Yikes, that's plenty of travel alright.....about 10 degrees with 45% expo on the ailerons feels pretty
good to me....I'm with Vicente, I'd cut down the throws first to about 12 & 10 degrees for high & low
rates respectively and you should feel he difference (that's if you own a JR ;-) ....Not sure what class you
fly, but 10 degrees was just fine for Masters and FAI for me.

How sharply rounded is your TE?......Can you send us a picture?....However, now that I see how much
throw you have on your ailerons, reducing them will be the first, easiest step.

Below, please read my aerodynamics guru friend's response to our thick versus thin TE curiosity. He
is a very knowledgeable fellow in full size & R/C and he and I did a few RPV projects as well. Those of
you who remember the man powered airplane, this fellow was one of the three  engineers on the project...

He and I have talked about this in the past also, a thin/sharp TE is less sensitive around center and a 
thicker/squared off TE is more sensitive around center.....On a 2 meter pattern ship I flew with a sharp
TE, the aileron sensitivity wasn't an issue and trim held perfectly on the ailerons. 

Take a minute and read please, Amir...

The lift of an airfoil is strongly determined by where the flow detaches
at the TE.  This is REALLY strong function.  A few degrees more or less
around a rounded TE can make a huge difference in lift.  there have been
some planes flown where they did a round TE and blew some high pressure
air out of a slot just ahead of the TE, and could control the plane by
varying the amount of air blown.

Here is a somewhat Techie article on the subject:
<http://www.fluent.com/solutions/articles/ja153.pdf>

I worked on a project at Lockheed 25 years ago that was trying to do
that for a helicopter rotor.. and you could stop the rotor in an X
configuration and then use it as a wing.  Since half the airfoils went
backwards in heli mode compared to wing mode, the idea had some merit.

Anyway, for models, if you do that on a control surface, you are likely
to not have as linear response as you would like.

now, sometimes, a sharp TE has problems on models.  The low Reynolds
Numbers mean thick boundary layers, and quite often the control surface
spends a lot of motion wiggling around in the boundary layer with
minimal lift change.  Probably not a problem with 1/3 scale stuff or F3a
stuff, but it can be an issue.

So, a moderately thick, squared off TE is the compromise.  Still sharp
edges, so you know where the flow separates, but the thickness gets it
out of the boundary layer faster.  The CL stunt guys had it right.. flat
flap on the back of a symmetric wing, with a concave cusp at the hinge
line.  Its going to be VERY sensitive around neutral, so good servos,
good linkages are mandatory, and then soften the response as needed in the TX.


  ----- Original Message ----- 
  From: paul 
  To: discussion at nsrca.org 
  Sent: Monday, December 05, 2005 4:11 PM
  Subject: Re: Excess Aileron sensitivity around neutral


  Eric, Regarding mechanical advantage, The radius on the servo arm is 15 mm, the aileron control horn is 25 mm.  The battery shows 5.2 volts under load.

  Jim Ivey,  Aileron deflections are 15 degrees low rate 18 degrees high rate.

  Del and Amir, The trailing edge of the ailerons is sharply rounded.  The TE is 1/8 " width at the end and has a 1/16" radius.  
      
  Thanks,
  Paul
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