I'm thinking of putting twin aileron servos in my Goldberg Cub so I can try flaperon mix on landing.

Using two standard-size servos seems like overkill and a lot of weight when the load is split over the two servos. But I've never heard of anyone trusting S3101/Hitec HS-81 mini/micro type servos in gas-powered aileron service, even on a sedate model like a .60-size Cub. Most of the references to mini's are regarding electrics and park flyers.

What's people's experience with servo sizes? Are gas fliers just stuck in a standard-size rut, or are mini servos to weak to drive an gassie's aileron?

If it were me I'd stay with standard size servo's. Price difference is about the same and even though the servo load will be light, the extra assurance that the servo provides is an increase in safety. Long term thought is that the servo's could always be transfered to another airplane down the road

Cheers,

I have a Sig Kavalier that had one Futaba 3004 servo in the wing center connecting to torque rods. That servo has 42 oz of torque, I think. I put in 2 of the Hitechs side by side, one for each torque rod. So, that gave me more power per aileron than the single Futaba standard servo. It is ridiculously easy to do and I actually saved about .1 oz in weight. Plus, I have stronger servo power to the ailerons and can set aileron differential if I need to....or flaperons. I only had to shave off less than 1/8 inch of the bay for both servos to fit.

Thanks. Which Hitec servos are you using?

I got 2 Hitech 81 micros. They are around $17 at Servo City or at my LHS. I have flown the Kavalier a few times since doing that and it flies much better now.

I used dual HS81MG as aileron servos on my 60 size top flight mustang.... =) Worked fine. But perhaps the were a bit weak. But On a cub they would work fine i think.

I have to agree. I think the HS 81 puts out only 36 oz of torque. I am building my first 60 sized plane and I'm settling for around 60 something oz for each servo.

Then I can recomend hitecs HS 255MG. 3.9kg (54oz) @ 4.8volt. OR 4.8kg (67oz) @ 6.0volt. I have four of these in my 60size yak.. (dual aileron, dual elevator.)

Edit: forgot to add speed data: 0.14s @ 4.8volt, 0.11s @ 6.0volt

Lighter, smaller AND stronger then a standard servo.

I bought the servos some time back for the new kit. I got Hitech 5475s. They are digital with 61 oz at 4.8v. I didn't really know about some of these smaller servos when I bought these. I like digital servos.

Altough the Hitec Hs-225 servo is more powerful than a standard, the gear train is weaker. Its a great servo and probably much more than needed in a 60 size Cub. I used standard servos in my 1/4 Cub and it did great until I got cocky. Beautiful flying bird!

Carl

Are you kidding? The 225MG;s are not weaker then a standard servo. Metal gear you know...

Try this handy Servo Torque Calculator to answer your questions.
http://www.morgantech.com/Servo%20To...0Caculator.htm

Don't forget that on a cub with scale ailerons, if you use them as flaps you will *increase* the risk of tip stall. If its semi-scale and has full span stip ailerons then I wouldnt worry.

The HS-225 servos should do ok. I've stipped the plastic gear version on 40 size 3D ships and wont use them in that application anymore, but on up to 40 size sport planes with smaller surfaces and throws they do great.

John, this servo torque calculator is an answer to a design issue I've been pursing for some time, so thank you so much. Is this your work?

Manufacturers catalogs detail servo power, and offer some choices in quality of materials (bearings, gear material, etc.) but very little application data. I think we're often selecting them based on guesswork or conservative experience, or just following what everyone else does like sheep. (Until they strip a gear and crash, of course!)

I wanted some rational engineering estimates of the power required to move control surfaces, and now I have it. Now I can select the servos based on the need.

(And then go do a figure "9" due to dumb thumbs. :-))))

What a great hobby!

- Balsacutter

That is a great calculator, John. Thank you. It's interesting how it shows that as servo arm throw is increases, so does the mechanical advantage. A linkage set up so that 50° of servo travel gives an intended surface throw of 15° requires less torque than a linkage where the servo only travels 40° to provide the same surface deflection. As others have explained, keeping your EPA as close to 100% is to your advantage.