SeditiousCanary

OK, I wish there was a more comprehensive servo guide somewhere which covered lock-to-lock throw (in degrees), length of control arms (center-center for inner and outer holes and overall size) and other bit of information only I seem to be interested in...

So, in case it wasn't obvious, my question is about control arm length and rotation rage on servos. After speaking to my local hobby shop, they seem to be steering me toward either Airtronics or JR servos. Previously, I had been looking at Hi-Tech largely because their website is easy to find (A single Google search on "Hi-Tech RC") and the great deal of information they offer about their product. I left with the impression that this is the only place which Hi-Tech excels though.

So my first question is when dealing with micro and submicro servos like the Airtronics 94555Z Microlite / Aluminum Gear Servo, what is the center to center for the control arm?

Second, is my understanding that most servo's total throw range is 160 degrees?
Third, which will work better to actuate ailerons, flaps and air brakes? Push rods, Flex Rods, or something else I don't know about (Pull-Pull is not an option)

I'm asking because I am trying to work out the details for actuating Fowler flaps and air brakes, I mean spoilers, so the total throw would be immensely helpful in the calculations.

Thanks for your input in advance.

Chris

KidVermin

My Feedback: (3)

Each servo comes with 4 or 5 different arms so you can choose whatever throw you need. With the new pooter radios, you can dial in what end point you may need up to about 125%.

To find center, remove servo arm and turn on radio and have that channels trim and stick centered, then replace arm as close to 90 deg. to servo.
I guess 160 deg. is close -- never paid much attention.
For aileron/flaps -- on 40/60 size planes use 2-56 rod. for 120 to 33% use 4-40 rod.

3DFanatic

My Feedback: (5)

Higher end radios go to 140% or more on the EPAs and then another 140% o nthe D/Rs

FHHuber

Generally... it is BEST to mechanicly adjust the linkages so that correct throw is achieved with 100% throw on the computer radio's ATV. There are potential problems with some older servos and going beyond 120%. You don't want to get into those problems. (can destroy the servo position sensing pot)

The servos are DESIGNED for a given throw angle. If you need more throw... you reall need to fix the linkages.

If you set yournormal throw rate below 80% you should change the linkage as you are not getting full servo power. (the mechanical linkage being adjusted will increase torque at the control horn and make the response crisper. also a good defense against alloing fluttter because of the increased servo authority over the surface.)

Computer radios with the ATV ranges they have now are teaching some bad habits. "Good engineering practice" is to make the setting as close to 100% as possible.

Geistware

My Feedback: (16)

Some servos will allow you to remove the end stop limiter and give you 360 degrees of throw.

aimmaintenance

Elaborate please.

DBCherry

My Feedback: (4)

The majority of servos have 120 degrees of throw, retract servos have 180 degrees. The amount of linear throw is based on the length of the servo arm, and there are aftermarket horns available.

The "best" method of connecting the servo norn to the control surface is a short, straight, solid rod. Less chance of "slop" in the control.

Hitec makes reasonably good micro servos (their HS-50's or 55's), and their mini servos are quite good too (HS-81's). The "major" manufacturers, Futaba, JR and Airtronics micro servos are extremely overpriced in my opinion, and no better than the Hitecs.
Dennis-

ballgunner

My Feedback: (1)

Sailboat servos have a different amount of rotation in degrees. The Futaba 3801 and 3802 have 140 deg. There are also sailboat servos that are drum tiype that will rotate through about 2 1/2 revolutions. Most aircraft servos rarely rotate more than about 60 deg. If you note the specs on each one it will give you the speed on two voltages, 4.8V & 6.0V, that speed is for 60 deg. With the newer computer radios the end point travel is adjustable up to the travel limits built into the servo. If you disassemble one you will find a small protrusion on the final shaft that limits the total travel. No matter what the TX you cannot exceed that limit. To get more throw the servo arm used is the answer. Longer arm, more throw. Adjustment is usually a compromise between the servo arm and the control surface arm. Several types of connections including the clevis at the control surface and adjustable attachments on the servo arm will determne the overall amount of throw.

SeditiousCanary

Thanks to all for responding. Let me clarify a few things.

I'll start with what I do know.
I know how far I have to throw the flaps and air brakes.
I also know the total throw can be adjusted for by which hole you mount the rod to, in addition which horn is used.
I know there are specialty servos and servos which can be modified to offer more then 180 degrees of throw.

Then there is what I don't know:
One issue is I don't know how far the servo throws.
Another other issue is I am trying to design the linkage to match the throw of the servo (again, because 100% throw is better for the servo and easier to set up as someone pointed out).

I have a few minor details I'm still working out about where various pivot points and tracks will be placed in the wing, but my main issue is designing the linkages without knowing dimensional data more about servos. I was guessing that most servos throw is 120 degrees since (as someone pointed out) speeds are given for 60 degrees, it would be logical to assume this is a center point, with 60 degrees on either side. From the limited amount of R/C experience I have, it seemed highly unlikely there would only be a total of 60 degrees of throw. Horns are another issue. It kills me that not one servo website I have been to talks about where the holes on the various horns they make are located. This seems to be The Most Fundamental Of Information when it comes to selecting a horn. Overall size is nice to plan internal clearance, but not that helpful to me in this case.

FHHuber

The throw result is a mathematical formula... each linkage setup gives a different result. A typical "standard" servo using the various "arms" supplied can give from appx 3/4 inch to 2 inches of pushrod travel. Then that travel is converted back to an angular deflection of the control surface by pushing on the control horn, rotating the control surface about its hinges.

We can't specificly say... this servo gives 1 inch of travel... because you can change how much it gives just by changing which hole you use.

You adjust which holes are used on the servo and on the control horn to achieve the desired throw angle for the control surface being moved. In some cases you use a hole very close to the center of rotation of the servo output shaft and one far out on the thing being controlled (use this for the nosewheel on a trcycle landding gear airccraft) Sometines we use one far out on the servo and close in on the control surface (ailerons on a Fun-Fly airplane so you can do CRAZY maneuvers) Sometimes we try to match the distance from center at the servo to the distance from hinge at the control surface (most surfaces on a typical sport aerobatic model, like a Four-Star-40... at least till you get used to it...)

The issue of control horn and servo arm selection is left to the modeler. (sometimes not the greatest idea...) but you get a plastc bag with about 4 to 6 servo arms with a typical servo. These arms are sized appropriate to typical uses of the servo they come with. The control horns shipped with a typical model will be apporpriate, assuming you use the correct size servo (and servo arm) and you use the kit maker's recommended engine size. (a whole bunch of IF's... and they are often not followed.)

If you build from some good quaility kits and follow the recommendations in the kits, you will get a feel for what is correct. Unfortunately i don't know a good way to explain what is appropriate when in the space that would fit in this discussion forum. (the amount of servos, optional arms, servo horns, and different aircraft is HUGE!)

SeditiousCanary

The question is for a 30 year old model that didn't have any servo recommendations then and would be highly outdated now (it is only designed for one channel and I am converting into a four channel). I'm also asking for a plane I am designing. I guess I'm just kid of hosed till I go buy servos.

I am well aquatinted with the basics of how servo throw works, I'm just trying to get an idea of how much throw I will have so I can align that with the horns I have to custom make for the flaps.

FHHuber

What is the model?... Wingspan? Engine size RECOMMENDED? (probably going to want different but we can get the basic idea of what is needed from there)

we CAN give recommendations based on a specific case.

Conversion of old FF (or early 1 ch) models to 4 channel can usually be done without serious hassles.

SeditiousCanary

It's a Sterling Diamant. It has a 74" wingspan and has no engine.

In the case of the one I am designing, it has to move a 2.5" chord flap back .5" and down 20 degrees. The Diamant will use a similar design, but scalled down. The chord for the Diamant is 4.2", so I was planning on about a 1" flap with about .25" or less backward travel.

CafeenMan

Well Said. [sm=thumbup.gif]

ballgunner

My Feedback: (1)

There used to be some control horns made especially for the movement of Fowler type flaps. To me that sounds similar to what you are trying to do. Investigate the possibility of having the control horns do the work and then adjust your servo arm throw accordingly.

FHHuber

don't need special gadgets to do a semi-Fowler flap.

This is how I hinged the flaps on a Kadet Sr. Note the pushrod pushes and pulls the LE of the flap, not the control horns being used as the hinges. worked VERY nicely.

feihu-RCU

Sed...
Perhaps a picture is worth a thousand words - so, see the attached pic which shows how to translate rotational degrees to linear throw and linear throw back to rotational degrees. I assume by now that you know how to set up your servo for stop to stop degrees of rotation. In the case below, I have set the servo for +/- 45 degrees ( 90 deg total throw) and a standard control arm hole of 1/2"; this translates into 1/2" linear throw. The belcrank in the wing acts as a throw reducer to accommodate a smaller aileron control horn. By drawing in the +/-20 deg aileron (or whatever deflection you want) and locating the linear throw, the control horn length from the hinge point to the pushrod point is determined to be 3/4 inches. A slight offset of the aileron horn causes the differential throw for adverse yaw correction.
You can use this same concept for determining and adjusting your flap control.
If you need further help, then you will have to give me specifics - such as your servo degrees of rotation, servo arm length, and what kind of output throw or rotational degrees you want. I assume you have decided on your mechanism for the fowler flap movement and just want a particular throw.
feihu

SeditiousCanary

I'll post a pic of what I am trying to do once I get home. It will likely make this a little easier to understand.

I have two competing ideas, one uses the track idea where the flap follows two grooves cut into a rib. The other uses a linkage to turn the flap down and a groove move the flap back. One of the issues is only having about .3 inches to work with for total height. It's a 25% of chord flap on a 6 inch chord with a 25% of chord airbrake starting at about 60% of chord. There is a .5" overlap of the airbrake and flap. Like I said, pics to come...

SeditiousCanary

feihu,

I am designing the mech now, which is the problem. You're pic is exactly what I have been struggling with, namely, "How long are arms and 90 deg control horns?" I can handle the math part (AutoCAD 2004 rules!), but I need to know how to apply it for my needs. I figured out a curve that would seem to be correct based on the chord as it would rotate when the flap is extended. Without building a small test setup, I can't tell if it would work. Let's say that I plan on buying some plywood and making some test ribs this weekend...

FHHuber

don't reinvent the wheel doing the fowler flap action... for small planes at slow speed, the control horn makes a fine flap hinge.

You can change the rrelationship of kick-back to angle by moving where the hinge line is fore and aft compared to the retracted flap LE position. furhter back hinge = more kick-back of the flap. further down = more kickback. (for the same angle of deflection) The hinge line being further back would ALSO induce the front of the flap to rise. (useful when making flaps act as airbrakes at 90 deg deflection in a sailplane.)

SeditiousCanary

Ha! Picture!



Here you can see all open, all closed, brake open and flap open. Barring that, you can see the basic path I hope to get out of the flap.
[link=http://homepage.mac.com/mikejames/rcu_flaps_site/highlift_images/thunderai_meridian/flap3view.jpg]This article[/link] is what I was basing my idea off of. I had thought about using push rods to do the rotation of the flap.

Dang wheels... They're all in need of improvement.

feihu-RCU

[quote]ORIGINAL: FHHuber

don't reinvent the wheel doing the fowler flap action... for small planes at slow speed, the control horn makes a fine flap hinge.

How simple can you get?

feihu

SeditiousCanary

Yea, I have seen the control horn trick. I have considered it, but the design is for a duration sailplane with a six foot wingspan with a seven inch chord (so far). I am trying to keep everything inside the wing to reduce drag, look cooler/cleaner and prove that my brain is capable of complex problem solving. I am OK with there being exposed wire or linkages with the flaps down, but I really want to end up with a clean look when everythign is closed.

I bought a sheet of 3/16" ply to make some templates for gang sanding the ribs on my Diamant today and figured I could use the rest to make a few testing models.