Are big servos necessary?

[Scotto]

Im assuming there is a vibration reason why the almost 50 gram size servos are used. The actual loads dont look like anything a 9g servo couldnt do. Would the vibration kill or confuse metal gear ~9 gram servos?

 

[Flying Monkey fab]

Im assuming there is a vibration reason why the almost 50 gram size servos are used. The actual loads dont look like anything a 9g servo couldnt do. Would the vibration kill or confuse metal gear ~9 gram servos?

A lot of gas/balsa designs are older and that was standard back in the day.
Lots goes into consideration of size, more than I fully understand but while a 50 might not be necessary I'd sure never use a 9 on anything with an engine other than 1/2a.
I'm sure vibration factors in but with speed comes back pressure on control surfaces.

 

[Merv]

Big planes need big servos.
Smaller planes need smaller servos.
Tiny planes need tiny servos.

 

[quorneng]

Servos were initially big and heavy, it was all there was so there is a tendency to stick with the original specified size.
The way I look at it is to ask:-
Do you really need a servo with a torque rating that could lift the whole weight of the plane?
Is it wise to have the servo power to be able to move the control surface to maximum deflection regardless of the airspeed?
How much of the actual servo torque is 'consumed' by the linkage & hinges?

A human pilot in even a modest size light aircraft is pretty puny but nevertheless seems to manage well enough!

 

[Piotrsko]

My big servos from the 1970s were made from discrete transistors instead of chips like today. My servos from the '60s had just a motor, switches, gears and a couple wires, were 4 times bigger and heavier. Planes were sized accordingly which isn't needed today. My 9 gram servos have more speed and power than a "steering" servo from that era and will pull the hinges out if the throws aren't set correct.

@quorneng is right, heed his advise, BUT YMMV

 

[Bricks]

I don`t think I would want to put in a 9 gram servo featuring 16.6 ounces of torque into a .40 size glow plane. When you consider weight and size of the control surface and the very small drive train, nope nada ain`t going to happen..

When the rudder on my bashed 17 NGH gasser 3D Ugly Stick in knife edge would stall my HT-475HB servo in a high speed knife edge at 76 oz of torque bigger to a point is better. I know it was not linkage as it would stall in both directions if I slowed down it was fine.

 

[Scotto]

Thanks guys. I should be more specific. Im more curious about the vibration than the torque. The throttle for example needs almost no torque at all but on the used planes I bought it has the same servos. I get that its not worth taking a risk with a tiny servo on a throttle, but Im just curious if small ones from electric planes would fail immediatly from the vibration of a glow engine.

 

[Scotto]

the rudder on my bashed 17 NGH gasser 3D Ugly Stick in knife edge would stall my HT-475HB servo in a high speed knife edge at 76 oz of torque bigger to a point is better. I know it was not linkage as it would stall in both directions if I slowed down it was fine

Sounds like it needs an aerodynamic balance.

 

[quorneng]

I suspect it is the vibration effect of the mass of the linkage that would likely damage the gear train of a small servo. The servo itself would no more suffer from vibration than a bigger one, indeed it could be argued that the lighter components in such a servo would be less effected at any given vibration level.

 

[Piotrsko]

I don`t think I would want to put in a 9 gram servo featuring 16.6 ounces of torque into a .40 size glow plane. When you consider weight and size of the control surface and the very small drive train, nope nada ain`t going to happen..

When the rudder on my bashed 17 NGH gasser 3D Ugly Stick in knife edge would stall my HT-475HB servo in a high speed knife edge at 76 oz of torque bigger to a point is better. I know it was not linkage as it would stall in both directions if I slowed down it was fine.

Series of comments: does it actually make that much torque in your installation? My stuff makes advertised max torque at 6 volts not the regulated 5. Torque is also measured in distance so 76 oz torque is kinda meaningless by definition unless you are measuring it just against a spring scale in which case it's just force.

And yes my 9 gram makes better force than my os max series 2 full size "digital" servo from 1971, my futaba full-size one from 1980, but 25 oz/in isn't a high standard. moving barn doors in a hurricane takes a poop pot load of force and you're correct a 9 gram probably won't cut it. For rudder elevator or small surface ailerons on a 1/2A gasser, they work just fine up to flutter speeds.

 

[Bricks]

Series of comments: does it actually make that much torque in your installation? My stuff makes advertised max torque at 6 volts not the regulated 5. Torque is also measured in distance so 76 oz torque is kinda meaningless by definition unless you are measuring it just against a spring scale in which case it's just force.

And yes my 9 gram makes better force than my os max series 2 full size "digital" servo from 1971, my futaba full-size one from 1980, but 25 oz/in isn't a high standard. moving barn doors in a hurricane takes a poop pot load of force and you're correct a 9 gram probably won't cut it. For rudder elevator or small surface ailerons on a 1/2A gasser, they work just fine up to flutter speeds.

When looking at the question of the OP comparing a 50 gram servo to a 9 gram and thinking they could be switched, like I said more then likely not going to happen.

 

[Draftman1]

That’s just how big they were, 9 gram didn’t exist back in the day, it wasn’t till the lightweight electrics started coming out for the smaller servoes.

If you had a large 1/4 scale plane, you would use a servo that was a little larger yet, called 1/4 scale servos

 

[Thomas B]

One thing to remember is that all servos in glow and gas powered models need vibration isolation, usually done with rubber grommets and things in the rubber grommet to prevent over-tightening the grommet, which negates the vibration protection.

Having said that, vibration from linkage could certainly affect a servo, smaller ones more than larger servos.

Always best to balance props and spinners to help reduce vibration.