Choosing Servos
 

How do I determine the torque required for a control?

I have an old sailplane I need to put servos in and would like to choose the lightest, most economical servos to install. How do I determine the minimum torque required?

Also I have another .60 size plane I am putting together that just requires a single standard servo for the ailerons. I would like to install 2 micro/mini servos to experiment with flaperons. Would torque ratings of 1/2 the standard servo suffice?

TIA, Glenn.

RE: Choosing Servos
 

On a 60 size plane, I would not drop down on the servo recomendations. Your standard servo's are only 12 to 15 bucks each and you can use 2 of them for your ailerons. The smaller servos are usually used in the smaller foam electirc planes and are not designed for the bigger planes. Good Luck, Dave

RE: Choosing Servos
 

I would not use the smaller mini/micro servos on any control surface of a plane that size. The standard servos, as recommended, would be just fine for any control surface on a 60 size plane. Tower has standard servos at about $10.00 each. As for the sail plane, I would think that the same would apply, but it depends on how large the sail plane is. Perhaps someone with sail plane experience can fill you in on that.

Go to Towerhobbies.com and search on standard servos. You will get a list of what they have with the prices. My favorite is the Airtronics 94102Z at $12.95 each. I have quite a few of them, as well as several Futaba, Hitec, and JR/Spektrums. They all work fine, but the economical standard servo at around $10.00 plus or minus two or three dollars will do just fine.

CGr

RE: Choosing Servos
 

In both cases look at the hitec HS225. For the sailplane the standard nylon gears will work fine for a sailplane 2M or larger. For the power plane use the metal gear version.

RE: Choosing Servos
 

I've never tried this, but I have kept this link in my favorites for a long time just in case I ever needed.

http://www.mnbigbirds.com/Servo%20To...0Caculator.htm

Good luck!

RE: Choosing Servos
 

On sail planes I have used the Hitec HS 81s and the HS 55, they don't require a lot. You can use any standard size servo on 40 size planes and most 60 size planes. As the others have stated, a standard analog servo is about $10.00 or $15.00. On 60 size stunt planes I do like to go a bit better then the standard servos so I often buy the low cost digitals but they cost about $35.00 or so. Servo City often has some good sales so when I'm building a new plane I shop around a lot for the best prices. I'm not brand loyal but I find Hitec on sale most often. I haven't ever had a problem with Hitec or Futaba, I also have a few JRs but they don't come up on sale often.

RE: Choosing Servos
 

I like this calc , and have just played a few thing with it. I'm just starting to have to decide my own servos in a few planes. I'd rather be taught HOW to decide rather than be told "which" servo in a particular application. I hope the OP doesn;t mind me piggybacking on this thread. I'm thinking that that may be what the OP wanted as well. Anyways, I have suspected that many of my RTF or RXR planes have had servos that were grossly oversized and I'm not sure why. Maybe I'm wrong about it.

Putting servo or control surface speed aside I did 2 planes and 5 control surfaces on that calc and yielded some surprising results. On the 2 planes the elevator servos seem grossly oversized according to the calc. On the 2 planes the rudder servos seem to be undersized. In fact on the e-stik the rudder servo comes out to be 8 times what it is actually.

I'm really wondering what I did wrong. I used 70mm cord and 160mm length (but it is a rounded rudder) I got the high rate (50) degrees from the manual. I hope someone uses a calc and can help me out.

EDIT I see what I did wrong and N/CM is about 10 times of what KG/CM is. This explains the large rudder servo, but means that the elevator servos are GROSSLY GROSSLY over powered. Still need a lesson in using the calc. [X(]

RE: Choosing Servos
 

A little searching will probably get you a manual for your sailplane with torque recommendations for servos. If you don't know what it is, post a couple of pics in the Q&A forum and you'll probably find out in a few hours. FWIW, I know you want to save money but going ahead and springing for the ball bearing supported servos is money well spent. It's only $15-20 extra for the entire airplane and you'll get servos that can take a lot more abuse before they get sloppy and start wearing out their gears. If you're the money saving type, you probably also are the use servos until they fail type, so going ahead and buying good ones will save you some trouble and some money in the long run. I wouldn't worry about the tougher servos for the sailplane, but definitely for the .60 size sport plane.

RE: Choosing Servos
 

Thank you everyone for your responses.<div>
</div><div>Lone Star Charles, as guver stated I prefer to learn how to choose so the calc you posted is a very good thing.</div><div>
</div><div>On the 60, I am concerned about the space to mount two servos.  My thinking is that if one 40 oz/in standard servo is sufficient for two ailerons then one 20 oz/in servo should be sufficient for one aileron.  If I end up needing two High Torque mini/micros then that is what I'll get but if they are not needed then I prefer to spend that extra $$ elsewhere.  Once the plane arrives (an ARF supposed to get here Tuesday) I'll be able to measure it up and see.  It's a trainer from Tower.  I don't plan to use the flaperons right away but am trying to plan ahead as I know it's easier to build something like that in initially rather than modify later.  I've read several threads on flaperons and found nothing definitive so I thought I'd leave the option open to myself to experiment with later.</div><div>
</div><div>Jester I do need to post some photos of that sailplane.  I thought it was a Gentle Lady but it is not like the kits shown now and it has spoilers.  I built it while in my teens, during the early '80s.</div><div>
</div><div>Guver I don't mind at all as you were correct in your thinking.  Hopefully someone will help us out here.</div><div>
</div><div>Glenn</div>

RE: Choosing Servos
 

Thanks , I'm gonna play with calc on a number of other planes that already have servos in them to see what results I get. As is common it will probably turn out to be "me" the problem. I can definately see that it is theoretical and any drag , binding, ect. can affect the theoretical number. I also find out how some rate servos isn't the available torque , but rather the torque required to drive the motor while no power is applied. This seems very strange and nearly meaningless to me.

I can sure see that doubling the value from the calc is in order on many apps , maybe even 5-10 times the value? I'm just not sure. I've also looked for some other clacs and they are either gone or not real friendly like this one. Good luck, I'll keep posted if I learn anything. So much to learn about doing planes. [X(] :D

RE: Choosing Servos
 

I have used the Hitec micro servos in the ailerons on race planes but the ailerons aren't used very much so I have gotten away with it. On these planes the ailerons are pretty small strip type and don't require much movement. Micro servos can have the same torque as a standard size but you have to keep in mind the motors are only half the size too so the bigger controls will put more load on the motor.
On your plane it is more common to use flaps instead of flaperons. I use the flaperons on one of my fun fly planes and have them slaved to the elevator. As the plane slows down and I start adding up elevator the ailerons start to come down. The problem with the flaperons is the plane will want to tip stall as it looses the lift when slowing down. If you haven't used them before take the plane up high and play with them so you have plenty of sky so you can turn them off and get the plane back under control. They are fun to play with on windy days. On calm days the plane wants to stall more. Trainer type of planes aren't the best type to use flaperons with. I have more fun with a trainer using flaps. I just cut the ailerons about on third and make the flaps.
Picking servos is just something most of us have learned over time. Weight of the plane, size of the controls, type of plane. I have never used a chart but they may be of great help?
I have built a couple of the Gentle Lady kits and just used micro servos, the HS 81s. Even with an electric motor they don't require a lot of power for control operation. I think they only cost $9.00. The motor and pot don't like the vibration from an engine but I have gotten away with using them for the throttle without problems. Luck??

RE: Choosing Servos
 

Glen what you projected is a perfectly acceptable modification and will work well. When converting an older single servo aileron set up and one that was intended or an ordinary 'standard' Then converting to a modern two servo system with modern 'mini' servos' not only will result in a net thrust improvement per surface you will experiance few of the mechanical losses of a torque rod setup and less sloppiness.

I have done many such conversion on similar airplanes and everyone was an improvement in a number of ways. The servo I would specifially recommend for your specified conversion is the Hitec HS 225. This is a mini and not a micro the thrust is at or near what older standard servos were especially when using 6 volts and at far less weight.

By far the major improvement in these types of conversion is stiffer and short pushrods instead of the soft/springy torque rods or multiple sloppy bellcranks.


John

RE: Choosing Servos
 

I fail to see the point in going with a mini servo personally. The standard size ones can take abuse better and there's no worries about no having enough torque. It's not like weight is an issue on a trainer, so why not get the affordable standard servos that will also be transferable to the next plane when the time comes?

RE: Choosing Servos
 

After reading this thread, most likely the standard size is what I'll go with. You do make good points.

I haven't received the plane yet and I was just looking thought the plans, thinking of what I'd like to do and wasn't sure if there would be room enough to mount two standard size servos. I was thinking that I would get the mini/micro servos coming this way (I have no LHS so all is ordered via 'net) so I wouldn't have to wait another week. But it is supposed to be here tomorrow according to the UPS info so I'm just going to wait and check if the standard servos will fit.

On the sailplane I would like to save weight if possible, that is why I asked about it also in the original post.
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RE: Choosing Servos
 

You have not indicated what kind of airplane you are talking about And that is always important when asking questions. However the fact that you are worried about room tells me the plans are for an older sport design flyer in which torque rods and a single servo was the most common arraignment. These days its difficult indeed to find any modern design beyond a basic genaric trainers that is set up like that.

Now since you seem worried that you won,t have any room for two servos it seems you think that putting two servos side by side is the way to go.

No that arraignment will only detract of many of the advantages and that is less weight elinminating the heavy torque rods as well as eliminating the all important far stiffer linkage. Make no mistake flimsy floopy linkage is indeed a disadvantage.

So if you contemplate using two servos (in the wing with short pushrods) You will indeed realize a net improvement in weight and Stiffer linkage plus you will indeet net typically an 'increase' in thrust per control surface and not a reduction. Two other improvements may well be argualby faster control response and of course far easier linkage adjustments.

So the decision is yours and that is the point the advantages are certainly there but I would not put two servos side by side for ailerons. that is a waste of time.

John

RE: Choosing Servos
 

It is the 60 size Tower ARF trainer that is on sale for about $100 right now. It is the basic genEric high wing tricycle gear rubber-band wing mount Cessna looking aileron trainer. It has a single servo in the wing for the ailerons.  I'm sure the wing will still mount to the fuselage with two servos side by side but I don't know what the effect of the larger hole would be on the strength of the wing.<div>
</div><div>The reason I was wanting to put in two servos was for the future possibility of Flaperons. From looking at the plans two servos side by side seems to me the best, most straightforward method.  I had considered putting one ahead of the other but I think that would weaken the central wing joint considerably more than the side by side method. It seems there is much more to consider than I thought. I think I'll just wait until I receive the plane and see what I have to work with.</div>

RE: Choosing Servos
 

Oh no, don't do that. Flapperons on a trainer won't do much to get you ready for a scale plane that uses flaps, so that's not much of a reason to do this mod. The good reason is so you can mount your servos out at the middle of the control surface and eliminate all the flex that the torque rod arrangement gives you. You're probably right that cutting an extra servo hole in the middle of your wing probably will weaken it. My advice would be to just fly the thing like it was designed and let that be in. If you want to play with flaps, get an Ugly Stick or similar plane that has them and learn that way.

RE: Choosing Servos
 

I quite agree with Jester on the flaperons. They will buy you nothing but will make landings a lot more dangerous but will do nothing in slowing your landing. What happens with Flaperons on a trainer they will inevitably make you use more power on approachs and actually increase you landing speeds or at least hippity hops down the runway. This greatly increases the difficulty in landing for the pilot.

Dedicated flaps that are separate surfaces are better but for primary training they to will only force a steeper learning curve and they are for later if one want to play with them.

Just for giggles here are several of my airplanes that I have converted to surface mount two servo aileron set ups Two of the ships are in fact twin engine airplanes with .25's and those have had long lives with Hs 85's for ailerons and are totally up to the job. The Hs 85 is in fact considered a mini. The third airplane is a Senior Kaydet airplane that I use for most of my students and I prefer to use my own equipment initially anyway now for that trainer I use HS 225's for all five and they have served a very hard life and continue to do so. I do indeed have many other completely successful examples.


John

RE: Choosing Servos
 

My plan was to only program ailerons initially and not programming, let alone using, the flaperons until I was ready and approved to do so by my instructor.   I was simply trying to plan ahead.  The advice I am receiving is appreciated and this idea as is going to be aborted as advised.  I like the idea of waiting until a future plane that has flaps designed in.<div>
</div><div>Thank you John for posting the photos, those are some nice looking planes.</div><div>
</div><div>Glenn</div>

RE: Choosing Servos
 

Since my suggestion was that Mini metal geared servos are good on a .60 size airplane I thought I would post an example. The airplane pictured is a 6 lb Machhi 202 warbird racer. Power is a modded Rossi .61 FIRE long stroke spinning an APC 11X11 at 13K on the ground. Airplane just won season championship in local series. Aileron and rudder servos are HS 225MG and elevator is HS 5645.

RE: Choosing Servos
 

Thats a beautiful and rather unique warbird pylon ship Speed. I too have used similar servo combinations on the warbird racers.

John[8D]

RE: Choosing Servos
 

This is a wing from a plane I will be painting this week with two aileron servos installed. Because of the width of the wing I wanted to use micro servos, the forward part of the wing is only 7/8 thick and the aft section is only 1/2 inch so I mounted them sideways on a hatch. The ailerons are very small and it's just a flat bottom wing so the plane isn't a real fire breather. I used the HS 81s but only because I had several on hand. There are much better micro servos on the market but this is just a cruiser float plane so I should be OK with them. If I really like the plane and money finds it's way into my pockets I will change them for something better in the future. These will be good enough to get me started.

RE: Choosing Servos
 

One of the things the calc helps underscore is how important your "mechanical advantage" setup is for the torque requirements of your servos.

If you use the 45 degree horn movement ( a very low average for my setups ), this represents nominal mechanical advantage.

If instead you set up your linkages so that you use 60 degrees of servo horn movement to get the indicated surface movement, you're maximizing mechanical advantage.
This lowers the torque requirements greatly.

If you have already selected a servo based upon 45 degrees, and opt for 60 degrees of movement, the effective holding power goes WAY up. This is beneficial to prevent blow-back, flutter, etc. while providing more precise control.

Often the suggested servos are WAY over what the plane actually needs for safety reasons, but there is more than one way to achieve that end.