Are big servos necessary?

Scotto

Elite member
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

Elite member
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.
 

quorneng

Master member
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

Master member
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

Master member
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

Elite member
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

Elite member
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

Master member
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

Master member
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

Master member
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

Active member
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

Member
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.
 

JennyC6

Elite member
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.
My go-to throttle servo is the venerable plastic geared Flite Test 9-gram servo. No reason to use anything fancy or heavy to open a nitro carb. The vibration doesn't harm them one iota.
Big planes need big servos.
Smaller planes need smaller servos.
Tiny planes need tiny servos.
Different parts of the airplane require different servo specs. A nitro carb barrel is fine on a 9-gram servo whether you're opening the carb of an Enya 09 IV or a Super Tiger G3250. You'd want different servos on the flight controls between the airplanes carrying those two engines, but as far as opening the carb is concerned, a standard little 8 dollar plastic geared Flite Test 9-gram is more than enough. Hell you could use a FT 5-gram if you wanted to really pinch grams.
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
I've seen that with my VK Cherokee 60. That thing was designed so far back in the past that RC itself was in its infancy and is set up for reed radios. I'm putting a handful of modern servos in it instead...three Futaba S3004s for flight controls and an FT 9-gram for throttling the Super Tiger G60-16 up front...and it's astonishing how much empty space there is in the cabin of that thing! And of course the servos I'm putting in it outperform what was available in the early 1960s by orders of magnitude...
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.
I do it all the time, but I don't do it on flight surfaces. I use the right size servo for the right job; on my 40-size ships I use something like a Futaba 3004 or U-400 for flight surfaces but if the task is simply opening the carb of a Magnum 52 4-stroke or an OS 46ax or something? Yeah a FT 9-gram is perfectly fine for that and is what I use. Don't need to throw 50 grams of servo in to do the job 8 grams of servo will do.
Hell, I ever get an airframe for my Super Tiger G3250, I'll have a humble Flite Test 9-gram plastic-gear servo on the throttle of a 32cc giant-scale bird!

That may be a huge engine, but it's still a nitro engine, and the perry carb on it requires little more than a gnat's fart to open and close. So again, no reason to use 50 grams of servo for an 8 gram job.

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.
It works fine. It's what I do on ALL of my planes. I fly exclusively nitro and I've never had any issues with using plastic gear FT 9-grams as throttle servos. I even had one survive a crash that destroyed everything forward of the rear headrest except the engine itself! Bent the 2-56 pushrod into a prezel, ripped the servo off the fuse wall, but that little trooper still works without flaw.

I don't take any steps to vibe damp them, either. I usually just remove the stickers, sand the side of the servo rough, then CA it right to the balsa somewhere close to the carb.

Those little servos are absolute tanks and are ideal for opening and closing nitro carbs. No need to spend money and weight on a 50 gram standard servo to do a job an 8 gram servo can do just fine.

Of course, I also only use known to be reliable servos. FT/EMax, Futaba, etc. You get those 2.99 Wish.com specials, buy one of those unknown Amazon brands, whatever? You're on your own. I wouldn't trust those things to run the needles on a gauge in an FPV model's scale cockpit, nevermind anything important. When a genuine Emax/Flite Test 9-gram servo is only 8 bucks, when a genuine Futaba S3004 is only 14 bucks, I really don't see any reason what-so-ever to cheap out on servos. I still fly S3003s/4s that are older than I am!