wing servo installation - RC Groups
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Feb 15, 2003, 03:05 PM
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wing servo installation

Let's think about this!!
Why do we put the wing servo bays on the bottom of our aircraft wings!! I really wanna know! It makes no sense to me, so somebody give me their reasoning.
This is my thinking: first and foremost, putting the horns and linkages on top gets them away from being snagged on landing, which sounds good to me. Second, pulling to get aileron up, typically the most important movement for an aileron, makes more sense than pushing. Third, putting the horn and linkage on top of the wing, where the flow is already more turbulent, makes more sense than creating turbulence down below. Lastly, by putting the servo in from the top, if it comes loose it will at least sit in place instead of hanging from the wing in flight...I guess if you don't pre-flight that could be a bad thing!!!
I can see why flap linkages should go on the bottom, but it seems like our aileron linkages would do better on top, but maybe it just doesn't look as pretty??
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Feb 15, 2003, 04:08 PM
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Andy W's Avatar
Disturbance of the airflow on the bottom of the wing is less critical than that on top?
Feb 15, 2003, 05:53 PM
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pda4you's Avatar
I agree with all you say - Looks is the only reason I can think of......

Feb 16, 2003, 11:34 AM
Launch the drones ...
The top of the wing is critical to producing lift (for planes that spend most of their flight time right side up), not the bottom. Ever notice how the military loads up the bottoms of their aircraft's wings, but never - not ever - the tops?
Feb 16, 2003, 01:33 PM
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I dont mean to be obvious:
but if you are bombing things below you, why would you put the bombs on top of the wing!!!
In reality, flow across both surfaces should be of similar importance, because without flow over both surfaces, you can't fly. Did you ever notice how much money the military spends getting those ugly wheels away from the bottom of the wing after take-off. And all hardpoints and munitions are given aerodynamic treatments. Seriously, according to "old rules" the bottom should be as impediment free as possible, the top isn't as important...its turbulent anyways. We could even put a flow fence at the aileron servo bay on top, to enhance flow over the top of the wing, come on, I'm just wondering out loud.
Feb 16, 2003, 01:58 PM
Launch the drones ...
Bombs go down. Air to air missiles go straight ahead at launch. Which side of the wing do the air to air missiles always get attached to?
Feb 16, 2003, 01:58 PM
soholingo's Avatar

Re: well,duh

Originally posted by singnplayguy
I dont mean to be obvious:
but if you are bombing things below you, why would you put the bombs on top of the wing!!!

We could even put a flow fence at the aileron servo bay on top, to enhance flow over the top of the wing, come on, I'm just wondering out loud.
hehehe... that's funny. I often wondered about this my self. My first thought is to put the linkage on top, for plane durability.

Maybe it's one of those things that people do and just don't question. You know sorta like tradition.

You know I think it has to do with appearance...
Feb 16, 2003, 02:03 PM
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I've had to load rockets and tow missiles on helicopters, and have seen armament guys load fighters....I sure as heck ain't gonna climb up on the wing to load em on!!! And...the f-16 originally had its main the wing tips??
Feb 16, 2003, 02:06 PM
Launch the drones ...
I don't think the military is worried about appearance - only flight characteristics.

Also - where are the engines mounted? The tops or the bottoms of the wings? Even with the problem of keeping the engines off of the ground - they hang them from the bottoms of the wings. Why do they do this?
Feb 16, 2003, 02:11 PM
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Andy W's Avatar
Missiles don't fly off, they are dropped or physically forced away (explosive bolts) from the airframe before they throttle up. Hence the need to mount underneath, or at least, on the tips.
That said, airflow over the top is more critical to lift..
Feb 16, 2003, 02:12 PM
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but really.....

there is a more important reason for putting air-to-airs below the wing. Consider launch and potential misfire situations. Most missiles actually are freed from the aircraft just prior to actual firing...that could create a problem if it was sitting on a hardpoint above the wing. also, what if the missile didn't you want gravity pulling it down to potentially strike your wing,tail or other aircraft surface, or would you rather it just fell down to the ground in your area of operations, useless, but harmless to you!!
Feb 16, 2003, 02:28 PM
Launch the drones ...
Yeah - I guess I wouldn't want my launchables on the top of the wing either. But I stand by my other statement - airflow over the top is more important than the bottom - and engine placement (typically, if not always, below the wing) proves this point.
Feb 16, 2003, 02:34 PM
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but the real matter is....

if I were going to put engines on wings, I would want the inlet in the high pressure area, not the low pressure area, also smoother flow, not disturbed flow...for performance reasons...but the real point is..doesn't putting the aileron servos in from the top make more sense....forget the bull...hooey about the flow being more important on just ain't so!! Which part of a cardboard box is more important, inside or outside?? take one away, what do you have left?? Even if you subscribe to the top flow theory, a flow fence right outboard of the pushrod/horn would probably improve performance...has anybody tried this?? Is there any real data?? Our sailplanes don't have hardpoints,jet engines,etc,...but they do have servos!!!!
Feb 16, 2003, 02:49 PM
B for Bruce
BMatthews's Avatar
Of course all this is just guessing unless someone with some windtunnel research time show up here but....

If you go back to the early jet age there were lots of in and top of wing attempts. Also I seem to recall seeing overwing ferry tanks on, I think, F89's or the early Republic swept wing jets.

Aside from ruining the upper surface flow I suspect that singnplayguy has the answer. It's just a practicality thing and makes more sense to hang stuff rather than perch it.

But the British, always being up to an engineering oddity, DID use overwing launch rails for the missles on the Electric Lightning.

But our models aren't that hard to turn over to service....

One guy locally that designed his own sailplanes used upper surface horns for the ailerons. I was standing there one day when someone asked about it. His reply was that it wouldn't get caught in the grass and potentially be damaged before he noticed it. Now THAT was a good solid practical reason to me. His flap horns, as you suggest, were lower surface mounted so as to pull. The horn also being placed backwards so as to offer a better arc of movement for the extreme throw.

I like the covers on the lower surface so there's no "turbulator" edges to cause an early area separation. Mind you, a Monokote cover over a local area isn't going to cause any issues. But I can't see why the pushrod couldn't angle up through the upper surface via a nice little streamlined rod and horn fairing. I sort of lean towards that flying buddy's reliability issue for tip mounted horns.

But if that same lightweight fairing was made a trifle heavier and used on the lower surface it could do double duty as a fairing and protective skid.

Lots of options depending on how you like it. Either way I would like to hope that the craftsmanship and "safety" issue of keeping the loosened servo from dangling under the wing wouldn't ever crop up...
Feb 16, 2003, 03:26 PM
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rcav8r2's Avatar
Personally for our use, I don't think it much matters... I routinly mount the servos on the tops of the wings on my glow fun flys. Why? Keep the servos from getting slime on them as I use tuned mufflers and the exaust goes right were the servos are.

And say you mount them on the bottom... Do you fly upside down? Sure you do ( well maybe) and do you notice? I know I don't.

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