Solved 9g servos torque at voltage at servo arm

[ProfessorFate]

I just decided to buy a bunch of these 9g analog servos and wondered what torque we have depending on connection.

Rule of thumb I've heard is you need at least the number of inch oz per control surface area ( elevator, aileron, rudder ).

FT has a video and thread on servos

https://www.flitetest.com/articles/servos-101

explaining that the 25oz force is AT 1 inch on the servo arm, but the arms supplied only reach out maximum 15mm.
This is good. I like to start with
13mm which is about 1/2 inches out from arm center and holes are exactly 2mm spaced. Simple conclusion is at 1/2 inch you get double the 25 oz torque specification.

Since the 4.8v torque isn't listed I approximated using hi-tech spec ratio 67 oz and 54 oz at 4.8v.

See my chart. We may have some controllers without programmers and assume lower 4.8v supply. If you have the esc programmer, you can adjust to 6v.

 

[ProfessorFate]

How much does your FT Spitfire weigh?
Here's my estimate...not quite done yet

Better looking spec sheet on those 9g servos

I made a numbering mistake leaving out the 1st hole at 5mm, but, it's too short anyway.

So my torque figures start at the 2nd hole at 7mm

FT Eddie told me you really can run 2 DC brushless motors on ONE motor controller

It's possible to get more thrust for less power running 2 motors VS one for same load

 

[Inq]

I'm sure you know all this, but I wanted to clarify with pictures for people new to these Engineering principles. From what I can see you are not describing torque output as a function of horn position. You are describing force output. You only get the "torque multiplication" depending on the ratio of horn positions - on the servo versus the control surface horn. In the following illustration:

1. First pair of horns have the wire at the outer most hole on both horns. The maximum torque being applied to the rudder/elevator/aileron is equal to the servo's maximum torque.
2. Second pair of horns have the wire at the inner most hole on the both horns. The maximum torque being applied to the rudder/elevator/aileron is equal to the servo's maximum torque, but the force in the wire is 15/7 times higher. The first pair would be a better solution as you would have less chance of buckling the wire when pushing.
3. Third pair of horns have the wire at the inner most on the servo, but the outer most on the control surface. This does apply more torque to the control surface at a ratio of 15/7 to the servo's maximum torque.

The third choice makes sense... uses less servo torque and thus less energy and thus longer flight times. However, it does reduce your control surface extremes, so you have to be aware of what your plane is needing as far as angle limits.

For instance, the servos can do +/- 60 degrees. On my trainer, the specifications are only to deflect the elevator +/- 15 degrees. Thus, I can use a hole on the servo side 1/4 the length of the elevator side. I can get a maximum of 4x torque to apply to my elevator. I also get the added advantage that I use the full resolution of the servo and thus have more discrete angles between maximum deflections.

Now for high-performance planes like your Spitfire or even worse the huge deflections used on the FT F-22 Raptor, you may need +/- 30 degrees on the surface and thus you can only use a 2x torque multiplier on your horns.

Hope this helps someone and that I didn't just muddy the waters worse.

 

[Bricks]

No where in the description does it say where max torque is taken from, my guess is right at the gear ( manufacturers like doing this it looks so good ) so as you extend out the amount of torque it can deliver will be less..... lever effect.

 

[Inq]

Torque is always defined at the center of the shaft. Note that the units are always a force multiplied by a distance (ft-lbs, in-lbs, N-m, g-mm). When you use a torque wrench... you apply a force at some distance away from the center of the socket. The socket is experiencing a torque of that force * that distance. Vice versa when the servo is applying that torque to the push-rod, the push-rod is experiencing a force (not a torque). That force is the torque divided by the moment arm which is the length from the center of the servo arm to the offset hole you have selected.

At the other end (the control surface horn) you select the moment arm and the force is multiplied with that moment arm to create a torque about your hinge-line. As shown above, you only get a torque multiplication if you use different moment arms on the servo horn versus the control surface horn. You also get a proportional reduction in travel angle of the control surface.