I have 2 425BBs, and 2 605s. Which should I use for ailerons? The 605 are faster and more powerful, so from a power perspective, which surface has mor torque demands on it, elevators or ailerons? The rudder already has a 605.

Thanks,
Jet

and what type of flying do you plan to do with it?
DKjens

It's going on my GP .60 sized Extra, w/ .91FX. The plane is coming back from the dead, after I dead sticked it into a tree earlier this year.

I'm doing some Intermediate acrobatics. Lots of big loops, spins etc. I'm trying to learn harriers, elevators, walls, waterfalls etc.. Now that I put it that way, I suspect that you'll advise me to put the faster/higher torque 605s in the tail for the elevator.

JetSD,
Yep, for 3D, put the STRONGER, not necessarily faster, servos on the elevators. The reason here is, that you will need max control surface movement (hopefully your hinge lines are double bevelled), so you will need large servo arms.
DKjens

Hey, cool covering idea on that Extra! I've got to try something like that... oh wait.... Doh!

I'm just a sport pilot, but there's nothing I like better than a crisp, quick, snap roll. A few years ago I bought some FMS servos which I really didn't like, but they sure were quick! I popped them into a 0.40 size Extra and that baby did the quickest snap roll you'd ever want to see. I had people come over to me and say they didn't think it was possible for a plane to rotate that fast.

Anyway, I came to think it was the speed of the servos that helped to push the limit on that plane, and the planes I've used those servos in over the years have always had pretty good snaps.

What's the consensus here? Do fast servos make for crisper maneuvers or is this just a lot of hooey?

I have a set of (the old) FMA coreless servos in my .40-size Modeltech ME-109. .12 transit time, 100+ oz/in of torque, etc. Ridiculous for such a small plane.

But!!! The controls respond *right now* and are accurate. Precision aerobatics are , well, presice. Snaps, spins, tumbles, etc. stop and start exactly on demand, no over-rotating.

Because of thse servos I've learned that speed and precision *do* make a difference in th right airframe. No more mediocre servos in performance planes for me - everything I have now is either coreless or digital. Or both.

Hmm. I haven't had the 109 out for a while. I think I'll add it to this weekend's flight roster

RWH: Thanks! It came out pretty good. I did it pretty much like the pic of the plane on the box. Great Planes .60 size Extra 300S.

For that size plane, I would recommend to run 6 volts and put the 605's on the ailerons and the 425's on the elevator. You have more surface area to control on the ailerons than you do on the elevator.

How many of you folks have seen and used the servo calculation spreadsheet put together by Craig Tenney? It has been uploaded in post 10 in this thread:

http://www.rcuniverse.com/showthread...388&forumid=31

The spreadsheet with some explanation is also available here:

http://www.pongo-air.com/servotorque.html

The reason I ask is that I see a lot of servo recommendations from various people. The recommendations are all across the board with little justification other than "I recommend it" or "It works for me" or "That's not enough torque". Curious, I started searching for a more scientific way to try to estimate servo torque requirements and found this spreadsheet.

While I can't vouch for it nor claim any expertise or ability to judge it's findings, it is very educational - Not only from the aspect of some surprises in it's numbers (like aileron servo requirements for the Lanier Edge 540T) it has some VERY nice features which help you work out optimum servo arm length and offset angles for odd geometries in your setups.

I've seen surprisingly little discussion on this spreadsheet here and elsewhere. I'd be interested to hear some feedback on this spreadsheet from others.

Craig did an outstanding job putting this together. It's quite impressive. Check it out and report your findings here!

Sorry the attached image is so small. RCU made me shrink it a lot. It's a tease to get you to download the spreadsheet! ;-))

Im with geistware on this one. But the question is on this size plane why add all that extra tail weight. Use a Y pushrod or a bellcrank and put a Hitec 545 (or 605) in the middle. Or use 1 servo in the rear and join the elevator halves. 2 605's in the tail would put 3.5 oz of weigt on the tail. That may require as much as 10 oz of lead to balance on the nose depending on setups I had a MW lil cap and used a hitec 545 with a Y pushrod so that each half was adjustable. As long as you keep the elevator servo arm centered to the fuse you wont get any off set throw. Personally I dont think 605's center well enough to put on an elevator but thats just my opinion

I'm a bit leery about running 6 volts. I've heard the unless you use a regulator you'll get glitches until the battery is depleted a little. Plus, doesn't the battery drain faster?

I put the 605's in the tail last night, and the plane balances 3/4" behind the recommended CG. That's w/o one of the wheel pants and wheels, an no nose weight. Does anyone think this is too far off? I almost think that it could go farther back. If that's a bad idea, someone stop me, or I'll hold you responsible .

When I built it I moved the Elevator servos from the recommended location because I did a very scientific study on how pushrod length affects rigidity, and how this affects the precision of the manuvers I like to . . . OK, I did it because I couldn't help it! They just look so cool back there!! Plus it was a kit!! I can't seem to build a kit w/o putting in my 2cents.

Jet

I didn't get in on the subject of whether these servos are suitable or not for this plane, because that wasn't the question, but since it has been raised, I'll give my $.02. I would not use the 425 on a 60 size that's going to be wrung out. For aerobatic flying, I would agree with Geistware, 605 on aileron and 425 on elevator. For 3D, I'd use the 605 on aileron and get one 545 for the elevators, or mount two 525 in the rear for elevators.
My reason initially to use the 605 for elevator was, that when you put a large arm on the elevator servo, to crank the elevators to +/- 45-70 degrees, which is my idea of 3D elevator set up, the servo is going to work hard to drive the elevators to these extremes, and the 425 would not like that at all.
I run 6V NickelMetal on all my planes/servos w/o regulator, and I don't get any jittering. I run it to Futaba, JR, Hitec and even Cirrus Coreless, all w/o problems.
I fly my S3G Extase with Cg 2"-2.5" behind recommended location, and I am still moving it back, (recommended is 5", I am at 7"-7.5" behind LE he he) but this is a long tail moment 3D plane, and probably much more forgiving than regular aerobats.
DKjens

JetSD,
After my first 1/4 scale, I have been flying with 6 volt unregulated packs for 2 years. The 2001 TOC pilots all used unregulated 6 volts packs except for 3 of them. I have heard on some of the high end servos, a topped off charge may cause glitches. I have had this only on 1 HS-605 and it would stop long before I could get to the runway and take off. The extra power is worth the cost of the extra cell to me.

Yes and yes.

The glitching isn't much of an issue if you're using analog servos.

Digitals *really* don't like being run off the 7+ volts that a freshly-peak-charged 5-cell pack puts out... a regulator is cheap insurance against a servo burning itself up.

When compared with a 4-cell pack of identical capacity, your max battery life will decrease by approx. 20%.

I just may switch to 6V. If it's good enough for the big boys blah blah blah.

I flew before with 425's all the way around, with no safety problems. So for now I think I'll try it w/ 605's on the elev., 425's on the ailerons, 605's on the rudder (of course) and the high torque digital on the throttle. (Model Airplane humor, yuk yuk yuk.)

I'll switch them around after a few flights and see which I like better. Actually, I'll probably end up breaking down and puting 605's all the way around. Or maybe the 525 elevator idea.

Jet

For 3d work I don't think you can have too much servo on the rudder. I have a 1/4 scale Sukhoi that isn't that much larger than the G.P. Extra. I am getting blowback in knife-edge with a JR 8411. Wow. I used to have a 4721 on it and I could see the blowback with the naked eye. With the 8411 you can still see it with Binoculars or video playback. At the very least spring for a cheap Hitec digital at Servocity for $54.

Jason,

Would you mind giving me the dimensions of your rudder and an estimate of the plane's speed when it's blowing back? I'd like to plug that in to estimate the accuracy of the calculations in the servo estimation spreadsheet I have.

Sure, rudder area is 65 sq. inches and speed is 8500 rpm on a 16 X 8 prop, APC. Maybe 70% eff.?

Servo is connected to a belcrank which is pull/pull to surface. Servo runs through a full travel at 130 percent on JR radio to get 40 degrees travel in each direction on the belcrank and surface. In other words the servo has a mechanical advantage. I did this to get full resolution and torque. I didn't like what I saw when running the pull pull directly on the servo. It was not using enough throw. The bellcrank setup works much better, but I am still seeing some blowback. No wonder the TOC guys run so many rudder servos!

For all you people talking about voltages above 4.8 and NiMh batteries, I would like to suggest you take a look at Cermarks website. The site doesn't have half of what they carry, but they specialize in RC electronics, have excellent quality, good pricing, AND, they treat their customers well. Unlike at least one of the manufacturers named in a previous post.

Silversurfer

I have a Futaba 9402 on my rudder and I think I am getting blow back. I never thought about recording it and reviewing the footage. I will have to try that one. What I notice in knife edge is that the plane will porpoise from time to time and I have to make minor adjustments with the rudder to minimize the effect.

Geistware,
Another thing that I found that can happen is flexing of the tail surfaces. I had a real bad pitch couple and loss of rudder authority at extreme rudder applications that was greatly helped by flying wires. You might give that a look too.
I landed one plane that seemed to have suddenly lost rudder authority and trim and found that the vertical stab was broken and one 1/4" stick and the Monocote was the only thing holding it together. It was a miracle I didn't loose it. One good vertical downline would have done it in! I have also used carbon fiber laminates on planes that I didn't want flying wires. The only downside to the laminates is that they must follow the understructure for maximum benefit. If you run them over large areas of open sheeting they flex in compression and cause stress risers that can crack the sheeting.

Jason, without dimensions, I estimated the rudder chord average was 8" and the height was 8". That's a big rudder. Does this include the fin?

I plugged in 40deg control surface movement and 140deg servo arm movement.

Prop calc estimated 65 mph.

The simple servo calculation came up with 80 oz/in torque required.

Horizon claims 155 oz/in torque for that servo. Hmmm.... I wonder if the propwash increased the local airspeed over the rudder. This spreadsheet looks like it may need a fudge factor on the elevator/rudder.

Lengthening the cord while maintaining the same surface area increases the servo load. How close was I on the dimensions?

The rudder is 14 inches tall with a chord of 3.75 at the top and 6.75 at the bottom. It also has a aero counterbalance of roughly 2x3 inches or so. Also, the RPM estimate was conservative. I usually tach the engine at 9000 full throttle on the ground. I don't know how much it unloads in the air. I am usually at 3/4 throttle or so, but with these 2 strokes the throttle is not at all linear, so who knows what rpm it is really running. It seems like the blowback occurs a second or so into rudder application. Maybe it is an issue with sustained current heating up a component or a battery going flat. I am running 1800 mah. Nicads with 5 cells, so I doubt the battery is at issue. I guess you can't count it out though!
Furthermore, the Servo rotation is much less than 140 degrees for 40 degrees of surface deflection. It is closer to 70 degrees.

BGI, I am curious how the calc. works out with the revised servo throw? I bet it is close or exceeds the servo's capability huh?
Maybe the spreadsheet is accurate afterall?

Jason, we're OK with the deflection of servo and rudder in degrees. When I said 140 deg I meant end-to-end which is 70 one way.

The longer length and shorter chord reduced the calculated servo torque down to 55 oz in at 70 mph. A 16x8 at 10000 RPM will get you about 76mph. At that speed the spreadsheet says 63 oz in. Hmmm...

You say you get blow-back a second or two after holding? Do you think you're losing power through to the servo? In that case, a stronger servo may not help.

I just wonder if the spreadsheet formula is OK but your setup has some holding problem. I dunno... Now I'm starting to think it's time to find a wind tunnel.