How to determine which servos to use?

[Grant Smith]

I'm trying to decide which servos to use in my next model and I wondered if anyone knew of a formula to use to calculate the forces that need to be over come by the servo to move control surfaces. I know it's a factor of surface area, deflection and speed but how is this all combined together to work out the pull the servo needs?

thanks

Grant

[vintage1]

I think the pragmatic approach is to ask other people what they use in similar sized models actually :)

You ought to be able to start working it out for yourself using basic wing data, angle of attack (deflection) lift (torque to flap the surface) and relate that to the speed.

I can't remember whether lift goes up as the square or cube of airspeed, but its a power law anyway, so the biggest problems seem to be associated with high speed models.

[stuartaw]

Grant,

There is a tool called Servocalc: http://atlas.csd.net/~cgadd//eflight/calcs_servo.htm

But - I'd be a bit cautious about the numerological approach here. I believe the servo killers are usually control binding problems or flutter. Even if you can work out a theoretical torque based on a steady airflow, I bet it doesn't hold as the airflow separates.

Like vintage1, I generally just follow the herd. I just hope someone at the front knows what they're doing.

Stuart

[ChrisP]

I went overboard with weight savings attempts on my Ultra 930-7 powered Thunder Tiger Champion 30 aerobatic model. AUW about 2.9 kgs. I used 2 micro servos for the ailerons which were not up to the job.
On the first flight I found that I couldn't turn the model and it flew into the power lines which are about 150 yards from the field.
I think I'm wiser now.

[Ray Lowinski]

The old rule of thumb was to use 1 in-oz per 1 sq-in of surface to be moved. Of course this was for simple sport planes. I still use this to get in the ball park.

1"x20" ail. = 20sq"
2 ail. on one servo = 40sq"
One 47in-oz standard servo would suffice.

It all depends on the type of model you plan and the type of flying you will do. Remember to give yourself a margin of error too.

Keep 'em hummin'

Ray

[ChrisP]

Originally posted by Ray Lowinski
The old rule of thumb was to use 1 in-oz per 1 sq-in of surface to be moved
Nice rule of thumb Ray which I will definitely use in the future. The only problem on my side of the pond is that our servos are specified in N cms instead of inch ounces.

To save anyone else cranking the maths this works out to :
0.11 N cms of servo torque per sq. cm. of surface area. Not quite as easy to remember as your formula so I wrote a mini Excel file for my PC.

With a conversion factor of 1 inch ounce = 0.706 N cm your 47 inch ounce standard servo would be rated as a 33 N cm servo over here.

Thanks a lot !

[rorywquin]

deleted some nonsence that I has written - sorry !