Servo question....

[ericrw]

I have in my stock box 4 servos, which I want to fit in my Blackhorse Commander 57" wing span. Two have different specs to the other two. Two have 2.2kg (4,8v) and weigh 14g. The other two have 1.8kg (4.8v) and weigh 9g. My query is which two should I use for the ailerons and which two should I use for the elevator /rudder ? Eric.

[simon barr]

On a model that size and weight, I would go for standard 3 kg torque servos all around...

The Hitec HS 225BB are slightly smaller than standard but have a torque of around 3.5 kg, so they may be an option.

[Concorde Speedbird]

HS 225BBs are superb servos.

CS

[ericrw]

I can`t save money, no matter how I try !!! Eric.

[Gaz Elliott]

Build a shock flier. Greatfor servos like that . Or a pipe lagger by Tom wright cheap fun fly plane. I got one with led lights for night flying .

[Simon Chaddock]

Eric

I could say do you really need a servo on each control sirface that is capable of lifting the entire weight of the plane?

The answer should be no. but it will depend on how much of the servos effort you are happy to loose in the hinges & linkages.

I hate snakes!

[John Muir]

Hi Eric,

The model is specced for 9g/16g servos so they should be fine. I'd put the 9g ones in the wings and the 14g ones on rudder and elevator, unless the instructions state otherwise.

[ericrw]

The apperture for the servo in the wing is only 24x11.6x23.6mm. Unlike the Hitec hs225bb, which requires 32.26x16.76x31.00mm. I`ll wait until I open the Box on Xmas day before I decide which servo to go for !!!

[John Privett]

Posted by Simon Chaddock on 22/12/2012 14:21:31:

Eric

I could say do you really need a servo on each control sirface that is capable of lifting the entire weight of the plane?

Any servo is (theoretically) capable of lifting the weight of the whole plane - but not necessarily very far! The confusion is due to the all-too common misuse of the unit for torque.

The "2.2kg" and "1.8kg" servos that Eric mentions should really be described as servos capable of a torque of 2.2 kg.cm. and 1.8 kg.cm respectively. This means that a pushrod 1cm out from the centre of the servo arm can exert a force of 2.2kg and 1.8kg respectively. If the pushrod is 2cm out from the centre then the force is only 1.1kg and 0.9kg. Conversely if you could manage to get the pushrod only 1mm (0.1cm) out from the centre you'd have a force of 22kg or 18kg - but the pushrod would have very little travel!

The size of aperture that Eric mentions sounds very similar to the size of the Savox SH0257 servos I used on my Sebart Katana 30e. These are 14g servos rated at 2.2kg.cm torque. Eric's model is a bit bigger and heavier but I think I'd tend to agree with John Muir's suggestion - or four of the 14g servos, though I realise Eric only has two of those!

[ericrw]

Thanks chaps ! I appreciate your comments. Most helpful !!

[Simon Chaddock]

I wonder how far the pilot of an 8000lb Spitfire could lift the whole plane with just a single movement of the joystick? I doubt more than 1/4".

I doubt I will convince anybody but the point I am trying to make is that in many applications the servos in RC planes are over specified (by 10?) to overcome the actual aerodynamic forces of the control surface.

[Stuart C]

There is no need to guess. Based on the anticipated top speed of the model and the control surface area, the worst case scenario for servo torque can be calculated. High speed models come out with some terrifying numbers, in which event one can always reduce the travel (often desirable at speed) or reduce the control surface area.