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Home FPV/UAV flight tips, how to and guidelines Servo stress load on wings.

Servo stress load on wings.
Wednesday, 29 October 2014 20:39

I'm sure you've seen some plane have single servo to operate both side of the aileron per-wingspan and while other operated on two servo where one servo operate the left aileron and another one operate the right aileron per-wingspan.  Let examine the differences  between them.  Along the way do read some article right here (http://homepages.paradise.net.nz/bhabbott/Servo.html)  to get some necessary info before we go on for further understanding on servo operation.

 

SINGLE SERVO operation. (Two aileron left and right operated on single "centralized"servo)

Pros:

  • Cheaper to assemble and lighter. Less current power consumption of aileron operation.
  • Load balance between two side of the servo horn connected to both side of the aileron push rods are well balanced. Zero stress load.

    Perfect example:  You got two girlfriends (left aileron and right aileron) keep pulling your hand (servo horn) both in the opposite direction repelling from each other. You (the Servo) hardly able to budge but staying rooted on the same spot (Zero load) because the force of two girls (ailerons) pulling your hand each on opposite direction both are pulling with the same amount of force.  You don't have to struggle to counter either direction of the pull  so you don't feel the strain. 

Cons:

  • Cannot easily customized to program both aileron as Spoileron or artificial Flaps.
  • Lack of 'Aileron Differential' capability.
  • The amount of aileron movement on each side are not 'symmetrical' due to servo horn angle since the servo located at center wing.
  • If the aileron servo fail there will be no roll or banking capability. The only substitute will be the plane's main Rudder to slightly assist roll and banking to take place on fail aileron control.  If the servo fail to "center" back the servo horn control to normal neutral position (0 degree) the plane will roll on its roll axis and bigger rudder surface control needed and elevator action to compensate the error but it will be a temporal solution.

img ttrlfpv servoload single ailerons

 

DUAL SERVO operation.  (Where each aileron have its own dedicated servo to control the movement)

Pros:

  • Can be easily programmed to create Spoileron feature, artificial Flaps and other innovative surface control capability since both side of the ailerons are independent.
  • Redundancy; should either one of the aileron fail another remaining surviving aileron servo on opposite side will compensate and counter the roll and banking the plane provided the aileron control surface big enough to compensate the error if the failed servo horn fail to return the control back to neutral position (0 degree).
  • The amount of aileron movement on each side are 'symmetrical' because of direct/straight angle of contact with servo horn.
  • Suitable for precision aerobatic flights and 3D maneuver.

Cons:

  • High stress load on the servo because the servo are subjected to one direction load pressure from aileron control surface.  The push and pull operation with no opposite load to balance the other side of servo horn causes stress to the motor especially when surface control constant bashed by heavy winds leading to coreless motor can and MOSFET chip overheat and fail.  Usually occur more often on bigger aileron surface.

    Perfect example:  Imagine you are the "SERVO" and both of your hands are servo horn.  You have a wife (1x Aileron), she pull your hands to the left side begging too bring you out for shopping.  No body pulling your hand on the right side so nobody help to counter back the pull force.  You felt the hard tug (Load = Wind pressure) from your wife and you almost collapse towards her direction but you struggle to counter will all your might   So you counter act by pulling back into opposite direction so that you don't fall off (consume more Current/Amps) and keep rooted on the same spot. But your wife was so strong she manage to pull you hard toward her direction (Wind pressure load on aileron), you become so tired/weak then fell down (Overheated coreless motor/Burned MOSFET) and being drag off towards the door and off you go shopping with your wife. Game over. 

img ttrlfpv servoload dual ailerons1

 

Simple Experiments
If you wanted to know how much load can your servo take on different (single/dual) servo setup on your wing try this:

  1. Turn on your plane as usual. Release your aileron stick to neutral.
  2. Now put your finger or thumbs under both ailerons at the same time both with equal pressure.
  3. Apply some pressure to simulate air/wind loading on aileron control surfaces.  You should hear your servo starts to jitters after few seconds as it receive some pressure load.  Try hold it on for 10~30 seconds.
  4. If you are lucky the servo will just get warm, if you're unlucky its either jammed due to coreless motor/MOSFET overheating heat or burned.
  5. Then release your finger or thumbs from the control surface (ailerons) to end the test.

Now you knew why you shouldn't have a heavy plane or payload especially if you have big aileron surface.

 

Conculsion
Actually both have their advantages that suits best on their individual flight application and requirement, we cannot assume everything is perfect since there are many grade of aftermarket/commercial servo available for consumer.  Single servo are good for long range, high stamina, cheap and low power consumption plane while dual (independent) best for temporal precision flight application such as 3D and aerobatics. A stiffer aileron hinge with auto return to neutral position springs works best to reduce wind turbulence load stress and the inability for certain servo to return to neutral "0" (Zero) degree after motor/MOSFET fail or burned.  But most important thing when considering buying a servo do check the following attributes before you check in those servo under the wings because it may save your neck and pro-long its lifespan.

Check list servo attributes:

  • Servo pull/push torque (pull weight).
  • Current peak load and current consumption (A) ...you don't want to kill the BEC do you?
  • Servo weight
  • Ability for servo to "center" back after failure.
  • Motor type (Coreless, traditional brush or brushless).
  • Servo horn rigidity/plastic thickness especially for single servo operation for both ailerons.

Good luck.

Last Updated on Wednesday, 29 October 2014 20:46
 
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