Futaba radio question

[Bob Richards]

The whole idea of "linear" control throw depends on the entire system. Yes, the pushrod moves straight-line while the servo is circular, thereby the pushrod will move further per degree of servo travel around neutral. However, the same thing happens in reverse at the control surface, the control arm moves in a circular arc. So, if the distance from the hinge line to the clevis pin is the same distance as the clevis-to-servo wheel centerline, then the control surface should move the same degrees as the servo. Linear.
 
I had toyed with the idea of using a pulley mounted on the servo, to get rid of the non-linear response on the pull-pull cables. I even thought about making the pulley elliptical in shape, to build-in expo in the linkage. That would make the center resolution better. Never got around to trying it, but seemed like a good idea to me.
 
Troy is right-on when he talks about maximizing the servo travel to get the most resolution from the system. This is something I wrote about in the K-factor article I wrote way back when.
 
I'm not entirely sure the lift is proportional to the angle of attack. I suspect there is a lift curve, obviously there has to be since at some point the wing will stall, but how linear it is around neutral, I don't know. And this also depends on how linear the lift of the stab/elevator is with the change in elevator position. I remember back in the days before expo, some pattern planes (Banshee) had a diamond airfoil section on the stab, supposedly this made it less sensitive around neutral.
 
BTW, I still have a Banshee kit, NIB, circa 1970?. Wicked looking plane.
 
Bob Richards.


"J.Oddino" <joddino at socal.rr.com> wrote:
My theory on "linear":
Consider the pitch plane. We want to do a loop. We need to change the lift.
Lift is proportional to the wing's angle of attack.
1) We want to change the wing's ANGLE of attack in order to change the lift.
2) We do this by changing the deflection ANGLE of the elevator.
3) The angle of the elevator is proportional to the ANGLE of the servo
output arm.
Therefore the change in lift is proportional to the change in angle of the
servo arm not the change in linear distance of the pushrod.

Regards, Jim

----- Original Message ----- 
From: "Troy A. Newman" 
To: 
Sent: Friday, November 19, 2004 6:46 AM
Subject: Re: Futaba radio question


> Negative expo on Futaba is softer neutral
>
> .................
> and Positive expo on JR is softer neutral
>
> It takes about 20-30% expo just to get the surface to throw linear. The
> reason is the servo is in a rotational output vs. the pushrod which is a
> linear output. If you use large servo arms and have the connection further
> out on the servo wheel this gets minimized some...but you sacrifice
> resolution of your servos because now you have to dial back you rates too
> far to get the actual travel you want. I don't fly my D/R or my flying
rates
> back below about 75%. The reason is you sacrifice the precision of the
> servo. I try to maximize my ATV values and keep the max out of the servo
and
> TX resolution as possible.
>
> To this end you will need about 30% expo just to get a linear output to
the
> surface. I tend to run 40%-50% expo on Ailerons, about 40% expo on the
> elevator and about 60-70% expo on the rudder in my normal flying rates.
>
> I would caution against high expo values like 60-100% you will get a hump
in
> there and it will drive you nuts. Main thing you see is the roll rates
don't
> stay constant...They start slow speed up and then finish slow. The reason
is
> you are moving the stick so far before the surface responds.
>
> Another note that may be significant to some. I fly one rate or Flight
mode
> all the time. I have a upline snap rate (aileron +10%, elevator +10%,
> rudder -10% on the rates), this is only used for upline snaps where energy
> management is critical. And I have a spin mode or condition where the
> elevator rate is higher by 20-30% travel. This is until the break into the
> spin...then the spin is done on the flying rate. The spin mode has more
expo
> on the elevator to give the same feel for flying as the normal flying
rate.
> All other rates stay the same in the spin mode.
>
> All snaps (save the uplines), rolling loops, circles and maneuvers are
done
> on the same rates. By the way these rate are all on the same switch and
its
> easy to prepare for the upline snap. or spin entry...
>
> I think that if I could get used to the higher aileron rate I could fly
the
> upline snaps without a switch for these.
>
> Hope this helps.
>
> Troy Newman
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