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Spit 72" aileron servos

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Applewood11/08/2018 00:26:03
35 forum posts

I've acquired a tony nijhuis spit 72 electric. Lovely model however I'm unconcerned that the New Power XL-17hmb Servos in the ailerons look a bit on the light side. They are 3.5kg torque mini size. Would appreciate any advice on if they are up to the job before I take it up. I know some of the glider guys have to go down the slim fit servos route because of the wing build dimensions. But this spit is quite heavy and plenty of room. It's a shame to go to standard size servos because I'll have to cut a larger servo hole and build and paint a new servos cover to match. Let me know if you have any advice, my gut feeling is to bite the bullet and go bigger.

Percy Verance11/08/2018 06:26:00
7569 forum posts
145 photos

Hi Applewood

A certain amount of care needs to be taken when selecting servos. I personally wouldn't have chosen servos of the type presently installed in the wing of the Spitfire, given the size and weight of it and the speed at which it's likely to fly. I don't think it's so much the torque output you need to consider, but also the robustness of the servo geartrain. Ultimately, these small servos are unlikely to be up to the job because of the wafer thin gears, and I rather fear that eventually the model might be lost as a consequence.

It seems as though the original builder of the model was on a bit of a budget and didn't quite appreciate just what was required here. It really ought to have had standard size servos in there. It's a large weighty model, and I personally would have fitted somehing like Futaba 3001's at the very least. Sorry it's not what you really wanted to hear because it now means extra work. But it's the reality I'm afraid.......

It's just a shame the original builder didn't at least retain the standard sized servo hatches shown on the plan.......





Edited By Percy Verance on 11/08/2018 06:36:40

Frank Skilbeck11/08/2018 08:12:22
4311 forum posts
101 photos

Here you go, you can use this to calculate the servo torque, it's surprising how little torque is required.

Years ago we would all fit Futaba 148s or 3003s to this type of model, with slightly less torque than these new modern mini servos.

BTW I have used these metal geared servos in my 1/4 and 1/3rd scale gliders and found them to be fine,

Richard Wills 211/08/2018 12:22:25
160 forum posts
7 photos

I would have gone with a standard size servo too if I was building this model. However, I would have no fears about using the minis if they were already in there. They will be fine for the flight loads on a plane such as this. You will however need to check them regularly, especially if you bump the surface on anything, they are not very tolerant of knocks as the gears are so small.

Would it be less work to move up to a 26g park servo? These are tougher than the mini, but still much smaller than a standard?

Applewood11/08/2018 13:10:56
35 forum posts
Thanks all, Which 26 gram servo would you recommend? I've also been looking at the slim fit servos, used in gliders. Cheers
Andrew Ray11/08/2018 13:17:21
665 forum posts
19 photos

Too much hype about servos and servo power, put it into context, Brian Taylor with his similar size championship winning warbirds would use a single standard servo for the ailerons and it would have to drive the linkage.

Also think about lifting say, 3.5kg and then think about applying that to a control surface, it is probably not too dissimilar to the pressure required to operate the controls on a full size Piper Super Cub or Pawnee at it's normal cruise speed.

I wouldn't worry about the servo power from those servos.

Former Member11/08/2018 15:33:25

[This posting has been removed]

Applewood11/08/2018 16:03:44
35 forum posts

Ok cheers guys, thanks for your help.

Nigel R11/08/2018 16:25:06
2483 forum posts
406 photos
Andrew your comparison with a full size cub controls is very wrong.

It might be 3.5kg but the distance component of the torque us massive by comparison to a rc servo
Andrew Ray11/08/2018 19:34:44
665 forum posts
19 photos
Posted by Nigel R on 11/08/2018 16:25:06:
Andrew your comparison with a full size cub controls is very wrong.

It might be 3.5kg but the distance component of the torque us massive by comparison to a rc servo

I don't think the comparison is wrong, it illustrates the point I was making. If you want to be pedantic then you need to consider the 72" Spitfire against the full size Cub flying at 80kts with much much bigger ailerons than the model and the gearing in the controls of the full size not just the relative moments etc. I was merely trying to illustrate a point rather than provide a technical or scientific answer. Anyway this is going off topic and I will leave it at that.

eflightray11/08/2018 20:13:43
560 forum posts
127 photos

I perhaps used slightly over-kill servos on my version of the 72" TN Spitfire, I used the Hextronic HX5010 servos, mainly because I had some, ( 39g,


Nigel R12/08/2018 22:23:07
2483 forum posts
406 photos
Andrew apologies if that came over a bit brusque.

But. What point are you making?

Your comparison seems like apples to oranges here.

A full size cub (or otherwise) would have far more torque required when operating the controls vs a 6' model.

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