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Which Servo Sould I Use?

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David Davis13/07/2019 08:24:36
3782 forum posts
724 photos


I am repairing my Baron after the rudder pushrod unscrewed itself in flight. This was the first time that I had fitted carbon fibre pushrods and did not realise that you had to secure them in place with locking nuts! blush

bertie 2.jpg

The Baron is a simple 1.55 metre wingspan three channel trainer originally designed for a 15-19 sized engine. I intend to compete with this model at La Coupe Des Barons in September. In this event there are four flying rounds flown in concert with up to nine other models. Two of these rounds are flown close to the ground while the other two, a pylon race and a "fox hunt" require an element of speed. I had planned on fitting an OS 52 FS, this being the largest sized four-stroke permitted in the competition rules but it was damaged in the crash so I've replaced it with an OS 48 FS Surpass.

Two standard Hitec HS 311s are mounted at the rear of the fuselage. When checking the cg earlier this morning I found that the model was tail heavy though it did not seem to fly too badly on the two flights prior to the crash. Not wishing to take a risk, I have decided to replace the servos with smaller ones in order to move the CG forwards.

The Nature of the Problem.

I do not recall having fitted servos smaller than standard size to any of the models which I have built but a trawl through my souvenirs revealed the following:

  1. Two Hitec servos: a Hitec HS 81 and a Hitec HS 5085MG.
  2. Five smaller Corona CS-939MG Micro servos bought for some long forgotten project which was never even started.

The Hitec HS81 is rated at torque at 6 volts with a speed of 0.09.

The Hitec HS 5085 is rated at and 0.13 speed.

The Coronas are rated at and 0.14 speed.


  1. Are the Coronas powerful enough for the purpose I have in mind?
  2. If I fit the Hitecs, which one should I use for the rudder and which for the elevator or should I buy another matching servo?

Picture of the starters in last year's event for your delictation!

group photograph.jpg

Don Fry13/07/2019 09:21:51
4557 forum posts
54 photos

I would say they are all up to the job, (Assuming free linkages)

Former Member13/07/2019 10:45:40

[This posting has been removed]

David Davis13/07/2019 11:46:25
3782 forum posts
724 photos

Thank you gentlemen.

Nigel R13/07/2019 19:35:51
3981 forum posts
714 photos

Must admit my fix would be two snakes and servos in the wing saddle.

I'd go with hitec 225 rather than the 81 or 85 though.

extra slim13/07/2019 22:46:02
495 forum posts
49 photos

Agree, 81’s are confined to throttle for me, strip too easy. I’ve had a few 82’s with iffy centering, 85’s are much better and 225’s are fine also

David Davis14/07/2019 06:42:44
3782 forum posts
724 photos

I'm going to risk the Coronas to get in some flying practice. I will order another HS 5085 just to be on the safe side.

Piers Bowlan14/07/2019 06:57:25
2167 forum posts
53 photos

The 81s are not ball raced and not really up to it in my view. I agree with Percy, the 85s are great servos particularly if you get the MG version, although more expensive. The corona 939MG work well and I have little doubt they will be adequate. I used one as a replacement elevator servo in a 2.4m e-glider. Size is not everything!

Sparks14/07/2019 09:18:47
269 forum posts
55 photos

Hi David,

A dab of threadlock, such as Loctite, on the nuts is a useful precaution.

Engine Doctor14/07/2019 10:03:14
2534 forum posts
39 photos

Once model is trimmed use threadlock as already suggested or a drip of thin cyano on clevis thread locks it for good. Easily undone with heat( soldering iron etc) for maintenance. You could use nyloc nuts if they are available in 2 mm? But ordinary nuts alway seem to come loose.

Martin Harris14/07/2019 12:03:11
9409 forum posts
255 photos

I've used Corona servos which seem to perform well but has anyone had problems with mounting lugs breaking? I ended up making some aluminium frames for some of mine which I epoxied to the case tops.


ED - yes, 2mm Nylocs are available although well tightened plain nuts seem to work well for me - they come with larger AF dimensions than standard nuts.

Edited By Martin Harris on 14/07/2019 12:14:38

David Davis14/07/2019 14:05:18
3782 forum posts
724 photos
Posted by Sparks on 14/07/2019 09:18:47:

Hi David,

A dab of threadlock, such as Loctite, on the nuts is a useful precaution.

Thank you Richard. How are things at Forton?

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