What if torque is insufficient ?

[jpage]

I am building a Dream Aircraft Tundra (high wing, 4 person bush plane). I am trying to decide how much torque is required for the aileron servo.
According to a magazine report, the control stick moves laterally 12" side to side, requiring approximately 8 pounds of force at the limit. This moves the aileron pivot 3", so there is a torque multiplication of 4 times, so the servo would need to produce approximately 32 pounds.
The SV42L servo, with the arm in position B claims 31 pounds of torque, barely under the estimate, for a 3" deflection.
Presumably the greatest force is required at the extreme aileron deflections. Since the arm angle is 60 degrees at the limit, I think there is a slight advantage, increasing the torque available ?
So is this enough torque ?
What happens to the servo if it is overloaded ? Overheat ? Chatter, Quit ? Break the safety pin ?
I don't think I have ever used full aileron deflection in cruise flight. Would I even want my autopilot to do so ?
Comments and advice appreciated.
Jeff Page
Dream Aircraft Tundra #10

 

[Ken_Kopp]

There is a built in safety advantage in not having a servo capable of applying full control authority as that would under MOST conditions be an extreme regime and not one suitable for any auto pilot - IMHO.

Some may disagree and believe the AP should be able to bail the pilot out of any situation, but THIS AP was not designed for that. Its an AID not a replacement to the pilot for workload REDUCTION only.

When the servo's reach their available torque limit they begin to slip with a slight but noticable ratcheting sound/vibration in the stick. This will not break the connection. You'll also see a either the ALT or HDG/TRK/NAV indicator turn yellow (indicating a slipping servo).

Based on the numbers you report the SV42 sounds perfect for normal in flight control forces. I recommend you set the servo torque as low as you can to obtain slip free control inputs under normal flight conditions to build in extra safety margin and prevent the AP from ever OVER-Controlling your plane.

My opinion only -

Ken

 

[dynonsupport]

The servos can't be overloaded, and they won't break or do any damage if they are moved externally. They move freely once too much force is put on them, and you can do this all day with no issues. The whole advantage of this design is that at any point the pilot can override the AP without any clutches or anything else needing to disengage.

The shear pin should only ever break if the servo actually binds up internally.

There's really no difference in high air loads or a pilot moving the stick. The servo will go somewhere it's not commanded, and the AP will try and move it back. If it succeeds in getting the airplane to do what it wants then great, if not, then it will just keep trying.

Because of this, weak servos generally make an airplane wander, since the AP only gets some of the control surface deflection it asked for, and overall the servos command of the control surface is kind of iffy. But as you say, you never use full deflection, and the AP never really does either. Most of the servos in our AP installs can't go full deflection, but that's one of the safety features, since it means it's easily overridden.

It sounds like you've done a good analysis of the servo you need. I'd go with the 42, and if that doesn't work, we'll happily trade servos.

As a side note, why do you think you need the SV42L vs. the standard SV42? Your general concern is torque, and longer arms reduce usable torque. Most people that can use a SV42L can actually use an SV32 (which is lighter and has less control drag).

 

[jpage]

I need the L, because I have a nice pivoting bracket that moves a rod that moves the aileron. Limit to limit movement is exactly 3.0 inches, which matches perfectly with position B of the long arm model for 120 degrees rotation. So I can cleanly connect a simple rod.

I am now starting to consider the SV32L. The deflection force is lowest near the neutral point and increases up to maximum deflection.
I assume the torque specified in the Servo Mounting Instructions is measured perpendicular to the arm. When the arm is at the farthest point from neutral, it has a mechanical advantage, that should provide almost double the torque. So SV32L at B, with 20 x 2 = 40 lbs, that should be more than needed to achieve full deflection, that I don't necessarily desire.
This would be wrong if the specifications for "max" force are really the forces measured at the end of a rod connected to the servo arms at 60 degrees rotation from neutral ??? Could you check how these measurements were made please ?

 

[dynonsupport]

The measurements are based on maximum, actual shaft torque, divided by the arm length. They assume perpendicular force.

The thing to consider is if the rest of the system backs out any mechanical advantage. If you hook a rod between two rotating shafts, the rod experiences different forces as the shafts move, but the shafts have identical torque.

Given that you have a mid-sized plane, and have done the other work, I do agree an SV42L sounds best. Remember that you can always turn the torque down, but you can't turn it above 100%.