GalenB

My Feedback: (18)

Howdy,

Like many of you I have a 1/4 scale H9 CAP 232 ARF. I have already chosen and engine, a ST 3000K, but now I need servos...

I am wondering if standard 42 oz. in. torque servos are big enough to drive each elevator half if tail mounted using short pushrods? In theory, you get the sum of the servo's torque and have 84 oz. in. of torque on the elevator and that seems like enough. I have some HS-85 servos that have this torque rating and at 5/8 oz they won't add too much weight to the tail.. Can I use them? Should I use them? And if the answer is no -- why not?

I haven't made up my mind just yet what I am going to do, but I want to explore the option of some smaller servos for the elevator. My mental shopping list has HS-605's on the ailerons and elevators, and an HS-646 on the rudder...

TIA for your help and suggestions.

Cheers!

Galen

JohnW

My Feedback: (6)

IMO it really depends upon how you intend to fly this plane and how sensitive you are to blow back. If you keep your throws low and don't fly the plane agressivly 40oz servos are probably fine. If you set up 3D throws and push the plane hard, the 40oz servos will probably allow the elevators to blow back.

I have a 25% CAP set up for 3D and push it pretty hard. I'm using 120 oz servos on each elevator half. Probably a bit overkill but I don't get any blow back .

majortom-RCU

My Feedback: (40)

Check out http://www.multiplexrc.com/calcservo.htm for an interactive servo torque calculator.

GalenB

My Feedback: (18)

Thanks for the link.


I just tried a sample calculation for a elevator half. Since I don't have the model in front of me I guessed at some of the measurments, but I don't think they are too far off.

Here are the numbers that I used:

Control Surface Chord in: 6
Control Surface Length in: 12
Deviation deg: 45
Max Speed mph: 70
Weight oz: 208
Average Wing Cord in: 18
Wingspan in: 73


Here is the result:

Required Servo Torque 172.41 oz-in

Ca 0.07
Speed 31.3 m/s
Wing Load 22.795 oz/sqft
Wing Surface 1314.0 sqin
Ctrl Surf/wing ratio 0.33
Ctrl Surf Area 72.00 sqin

K1 0.377
K2 0.062

172 oz. in for an elevator half seems like a lot. Hopefully my quesses for chord and length are off since my CAP is in the box in the rafters of my garage... Can anyone with a this CAP provide more accurate measurements of the elevators and the average wing chord?

Cheers!

Galen

Wings-RCU

My Feedback: (2)

Galan,

You can use those servos if and only if they are metal gear servos. The nylon gear mini servos will not stand up the abuse the cap will put them through. This is how I know....

I used 225's which are a little bigger and a little stronger then the 85's in the tail of my CG Shokoi (about the same size and weight as your Cap) and had the gears strip in flight from a full throttle lomcevack. (OK.... probably not smart but fun) luckily only one went out and could be flown back to the runway.

Anyway if your going to do that, I would use 2 - 225mg's in the tail. That would still give you mini servos with over 100 oz combined toque along with a durable gear train.

I'm willing to bet that the 85's would self destruct within a few flights on this type of plane. Torque is only one thing to consider when selecting servos.

rctimm99

My Feedback: (34)

Hey Galen:
You have seen me crash.... trouble just finds me :-) I know this topic well.

The math is linear.... so two standards are going to give you about 80 .... Make sure you have the speed you want. Faster servos will make your cap seem lighter.

Speed and MG would really rock... like the HiTech 605 MG..

My 2 cents
Tim

beedriver

Galen,
Your calculations are correct. However, remember that you will not be using full deflection at max speed. For 3D you will be going to full deflection at a MUCH lower speed, say 25? Now recalculate and see what you get.

Walt

GalenB

My Feedback: (18)

Okay, at 35 mph and 45 degrees of deflection torque needed drops to 47 oz. in. That means that HS-225MGs would be okay for each elevator half of this model. While no cheaper than a HS-605MG they are much ligher and faster.

Thanks for all for your help.

Cheers!

Galen

Cometlover

I have a CG Sukhoi with a Saito 1.50 for power.
I have two 605 MG's on the elevator, one 605 per aileron and a 615 on the rudder. They all seem to be doing fine.
But I don't really fly 3D style, so speed isn't all that important to me. I do operate them with a 6 V pack, so the speed is quite acceptable to me.

Olafsson

I use HS425 servos in my Excitation and they works great, I have
tried the miniservos HS225 and Jr E381 and I can tell that the gearbox won´t last. So go for standat size servos , exept for the ruder.
Regards
Tomas

Diablo-RCU

My Feedback: (1)

I have a Sig Cap, ya different plane but close enough for servo requirements. It has 42 oz-in Futaba coreless nylon geared servos on ailerons and elevators (4 total) + a 110 oz-in Airtronics coreless servo on the rudder. The plane flies fine, no blowback, no flutter, no issues. Plenty of control authority. Also, it has a ST 2300 motor and is fast.

If you take a poll, you'll find that people stick a wide range of servos in the same plane. Most people overspec the torque requirements...which is fine, but adds cost. You certainly don't need metal gear servos. Unless you plan on kicking your ailerons every so often.

Wings-RCU

My Feedback: (2)

Diablo,
I would certainly disagree with you concerning metal gears. We are not talking about the full size servos here but mini servos.

Several of the nylon gears in a HS 225, HS 85 & HS 81 are less than 1/16 in thick and have gear teeth smaller than the size of a pinhead.

I think you can get by with weaker full-size nylon gear servos - as you have done-, but when you put a more powerful mini with nylon gears in to a 120 size aerobat, you are asking for trouble.

So....go for the standard size servo and suffer the weight consequences, or get the metal geared minis.

Radman

You have a hanger 9 cap with a ST-3000 on it. Your plane will probably weigh anywhere from 11-15 lbs ready to fly. You also have probably close to $800. dollars into it. Why would you scrimp on servos? The smallest i would go would be the 605 hitecks!!! Why take a chance on a dinky servo. The 605's have 80 in oz of torque. and with 4-40 rods you would have a bullit proof system that you won't have to wonder about.

The 605's cost what, Maybe 10 bucks more than those other ones. IMO it just isn't worth losing a 800$ + airplane because ya scrimped a few bucks!!!
Always go with the best you can afford!!! You actually save a lot of time and grief in the long run.

EJB

I have a GB Sukhoi (11lb.) with a ST 2300 using standard 43oz.BB servos(Hitec 425-$15) & they work just fine.Rudder has a 605 MG.The 425's actually are more precise & center better than the 605's that I use on my 80" gassers.I have seen many other 120 size planes using standard servos without any problems.

GalenB

My Feedback: (18)

Radman,

I think you missed the point of the question. I am not trying to scrimp as cost is not the issue. I am building this model for a very low cost, but $10.00 per servo is not a problem. I want to explore how much servo is really needed to do the job. I know I can always over build -- been there, done that, but I want to understand where the boundaries are. As I progress to larger and larger models I think this knowledge will be valuable.

I would like to tail mount the elevator servos so weight is a small concern, but not a large concern.

If the Multiplex calculator is to be believed then most are putting way too much servo on each control surface of this model. The calculaor says that 42 oz. in for the elevatos and 22 oz. in for the ailerons are sufficient for 3D flying...

Thanks to all who have responded so far.

Cheers!

Galen

majortom-RCU

My Feedback: (40)

At the risk of stirring up controversy, I will offer a little non-expert comment here. I suggested the multiplex formula because I expect these people to know what they're talking about, but that is something I must take on faith, not being a rocket scientist myself.

There is another servo torque formula, sort of a simplification of what I expect would satisfy a professional engineer, that I picked up on some website that I no longer recall, but I liked the simplicity of the formula and have used it for my own guidance. The formula is:

servo torque req't in ounce-inches = (control surface length in inches) x (squared control surface width in inches) x (angle of deflection in degrees) x (squared speed in mph) / 430,000

If I apply this formula to an elevator of 2" chord, 10" length, deflected 25 degrees @ 50 mph I get 2x2x10x25x50x50/430000 = 5.813 oz-in. That strikes me as a reasonable number, considering a moderate speed and a moderate deflection. The formula tells me that as the speed increases, the torque requirement increases by the square of the speed, which seems reasonable. Also, as I put on more control surface width, the torque requirement increases with the square of the width, but only by the first power of added length.

It is also reasonable to my mind that I am not going to apply full stick to a plane travelling at maximum speed and expect it to hold together for any length of time. If you're going fast and want to change direction, you either feed in a little stick and take a wide turn, or you slow down and take a quicker turn.

If you're doing a stationary hover and want to give full 45 degrees of elevator, I expect you'd need to calculate your airspeed over the elevator not at zero miles per hour (because you're hovering) but at let's say 50 mph of prop blast going past the elevator--more or less the same speed you'd get in level flight with the same throttle setting.

So we have standard practice telling us that standard torque of ~ 40 oz-in is adequate for .40-.60 size flying, "high torque" of 60-70 oz-in is adequate for quarter scale, and "giant scale" is recommended for larger than quarter scale. We know there are thousand of planes flying happily under this scheme of things. We know there is a safety margin built into these numbers that is seldom, maybe never exceeded--barring depleted batteries and other snafus.

I am as curious as the next guy as to how far these rules will bend before they break. To me it is the essence of good design & construction to build strength and supply power adequate to the extremes of whatever loads and stresses can be anticipated, and just slightly shameful to overbuild, which any noodlehead can do.

So: where is the expert who will tell us what we want to know? Who is willing to fly planes to destruction, with telemetry gear to tell him what the loads were up to the point of failure?

beedriver

Galen,
As you stated that you are on a learning curve as you progress to larger aircraft - here is something else you may want to consider. I prefer to go a little larger than required with servos, I just like a larger margin of error. Also, as you get larger with your planes you may want to consider using a bellcrank set up for your rudder system, even if you are only using one servo. With a pull -pull system, when you tighten the cables you put a lot of strain on the servo grommets, case lugs and fasteners. Over time this can lead to problems with isolation and/or case distortion and bearing damage. Use a good bearing bellcrank and you can keep the cables tight and you won't be constantly adjusting tension. Your servos will thank you.

Just another $0.02, I think that brings me up to, almost, a nickel.

Walt

JohnW

My Feedback: (6)

Galen, I just wanted to point out some danger spots in assuming the maximum servo load would occur when flying 3D at low speed. While the multiplex calc claims ~40 Oz/in for your 3D calcs, it also predicts that a 15 degree elevator deflection at 70 MPH requires 58 Oz/in for your model. Manouvers such as snaps may suffer from control authority if you use 40 oz servos if you put faith in the multiplex calc.

Multiplex's calculator also probably dosen't take into consideration mechanical disadvantage caused by setting up a plane for 3D rates and then flying low to medium throws at much higher speeds. This is what I was getting at when I mentioned the 40 oz servos probably won't be enough if you set up 3D.

For comparison, I have flown my 25% CAP (Sig w/YS140L) with both 80oz and 120oz servos. With the 80 oz I could not detect blowback on the elevator, but I could on the rudder, especially in knife edge loops. On the 120 oz servos, the rudder didn't blow back anymore but the big suprise for me was that the elevators were much crisper. Both the 80 and 120 servo's were aprox the same speed, servo design, etc and linkages/throws were the same for both setups. The only major diff was torque.

Again, IMO the servo required really depends on how you fly, how sensitive you are to control response, blowback, your throttle management, etc. Reguardless of which way you go, enjoy your new CAP... should be a blast to fly.

limeybob

My Feedback: (6)

Check out a new servo torque calculator at
pages.sbcglobal.net/limeybob

Based on real data

Limeybob

Stick Jammer

Another thing to keep in mind is that if you can set up the control linkage so that the servo arm is shorter than the control horn (measured from the hinge line to the pivot point), then you will gain some mechanical advantage from the servo. The shorter you can keep the servo arm in relation to the the control horn and still get the amount of throw needed without overdriving the servo, the more force the servo will be able to overcome on the control surface.