How big before multiple servos on a control?

[Bartman]

A big question I've always had is how big can you go before you need multiple servos on your control surfaces?



I think it's pretty safe to say that 30 to 40CC planes (about 70 to 80" wingspans) are going to be fine with a single, metal gear servo on each control surface including the rudder. Keeping in mind that 3D flying requires more servo power than Sport/Scale flying (because of the extreme control throws), what is a reasonable rule of thumb for when it's time to put two servos together?



I'm a sport flying kind of guy so I've been able to get away with single servo control surfaces on a plane up to 70cc but I also understand that you hardcore 3D guys probably couldn't count on a plane to survive that long if you did.



Adding a quick explanation to this first post, control surfaces become increasingly harder for the servo to move as things like airspeed, surface size, and the amount of deflection go up. It's easy to move a small surface, on a slow plane, a small amount. A big surface though that's on a fast plane will need a ton of power behind it for an abrupt maneuver that requires a lot of surface deflection. It all has to do with the air pressure that is acting on the control surface as it's being deflected from its neutral position up or down into the airflow around it.



So, for a first time Giant flyer, what's a safe and reliable rule of thumb?



Bart

 

[ericb]

On my 91" Extra I am just fine with single servos on each control surface. Once you get into the 100CC size then you need multiples on the ailerons for sure. I use a single strong servo on the rudder and elevators in the 100CC size. 150CC size then multiples on rudder also. After that is out of my league, so others will have to answer.

 

[SleepyC]

Most planes will dictate what they want. The designers have that figured out.

With the newer HV monster torque servos realistically a lot of planes up to 35% can get away with 1 servo,(same with rudder) but it then depends on where the servo pockets are. If it's an arf, fill the holes. If you are building 35% i'd use 2 and 40% I'd use 2 or 3 depending on what servos.

 

[Ken-H]

Hard to answer that because of all the variables. Servo's are getting very powerful almost to the point of insane. It really comes down to how much force is required for the given surface, how strong or ridged that surface is, how much can the hardware hold, and how much load can the servo pocket hold. The servos now a days can just about move mountains so now with the most powerful, how much can the air frame take with a single mount.

 

[Bartman]

that's a good point Ken. last i checked we were into the 250 to 300 oz-in range for servo power which is way overkill in a 30cc airframe but makes one sero possible in larger planes.



@SleepyC, referring to the plans is always best practice so I entirely agree with you. the thread was more for sake of discussion as new guys (if they're anything like me when i was starting) will want to read a lot and get some of these things sorted out before picking a size and model to try first.



Bart

 

[Chris.Puckett]

Surface stiffness is a big factor. With how light the aircraft are built now, there is flex. This flex leads to flutter. By adding multiple servo points it reduces the amount of flex in the surface. I have run single servos on 35% and currently have single JR MPH83Ts on the elevators of my 60% L4. While not an aerobatic aircraft it does have very large surfaces. Servo arm length and proper geometry are key as well.

 

[Bartman]

that's another great point Chris, single big servo can only be considered if the flight control itself has enough rigidity to be actuated from one point.

 

[SleepyC]

Another thing I am seeing making a comeback is split aerilons. 1 servo for each half. Besides not having any servo fighting issues, you can perform some cool mixes like crow, or flaps or ?!??! With the gyros out now, I bet some interesting things could happen with split aerilons!

 

[Islandflyer]

With servos now easily available with 500oz torque and above, this is the far from the weakest point in the chain of surface control. Hard to believe that when the JR 8411 first came out, the TOC guys flew 40% airplanes with just 150oz servos, and they were thrilled to have that!

In addition to the variables mentioned by Ken, with today's equipment, a lot depends on the mechanical stress requirements generated by the pilot's ability and flying style:

- Kyle "The Alien" can do his awesome 3D on a 40% with those old 150oz servos.

- The more aggressive pilots (like Gaby, Jase, Spencer) need over 400oz on just 100cc planes...

 

[Terryscustom]

SleepyC;660 wrote: Another thing I am seeing making a comeback is split aerilons. 1 servo for each half. Besides not having any servo fighting issues, you can perform some cool mixes like crow, or flaps or ?!??! With the gyros out now, I bet some interesting things could happen with split aerilons!

I'm working on covering a "little" plane I just built with split ailerons!! Finished my first one a few months ago and will never build another foam core wing without split.....it is SOOO much fun!

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