I'd like to bounce this off some of you...
If the cord of my aileron is 3.5 inches from hinge point to trailing edge.
And if my control horn is 1" tall from the hinge to take off point.
And if I want 15 degree deflection in each direction (I believe this to be 3/4").
Would the pushrod stroke required from the servo be a little less than 1/4" in each direction?
Thanks!
JLK

I use crc throw meters. They are cheap, and available everywhere. And they work great.
Try Central hobbies.
Chris

Chris,
Thanks for the reply but..
I know what my throw will be.
Don't need to measure it at this point.
What I am trying to figure is which inner most hole on the servo output arm am I going to be able to use.
This will determine the placement spanwise of the aileron horn on the aileron.
Thanks again!
JLK

15 degrees deflection at 3.5" chord would be about 0.91" and 0.26" pushrod travel. 3/4" deflection would require 12.4 degrees, 0.21" pushrod travel.

Wazmo,
Thanks!
JLK

Are you serious about this?
That setup would likely have a very low servo power and no solid power to hold around center .plus any slop in linkage wold show up readily
Twere it mine -- I would look at the servo travel at max practical ATV . then select innermost practical servo arm hole -- then increase horn length accordingly .

Dick,
Sorry if I confused.
I am looking for the inner most servo arm hole.
The 1" arm length I was referrering to was the control horn arm length at the aileron.
JLK

Ratio of servo moment arm to control horn moment arm is determined by the inverse ratio of the sines of the rotation angles. You want 15Ëš on the aileron, and set your aileron servos (as well as other control surface servos) to maximum ATV or end-point (whatever your radio system calls it), which is 60Ëš at full stick.

So ratio of the sines (sin60/sin15) is .866/.259, = 3.34, inverted = .298, call it 0.3. So your servo arm linkage point should be 0.3 or 30% of the length of the control horn arm. Call that 25.4 mm for easier measuring (for me anyway), 0.3 x 25.4 = 7.6 mm, which sounds like the closest hole you can find on the servo arm. Anything longer than 7.6 mm will give you more throw than 15Ëš; so reduce the throw by adjusting ATV/endpoint down a bit, or find a longer control horn.

It's not uncommon for me to use the servo wheel rather than an arm, just to get a connection point closer to the hub. As Dick says, going to maximum ATV will reduce slop and increase mechanical advantage at the servo, give better holding power against blowback. It will slow the effective speed of rotation of control surface, but that has never proved to be a problem for me.

Yes - but -one addition - increasing the rotation never really slows the surface movement - the "WHY" is because under load, the load controls the speed of movement -
Buying superfast servos-vs super torque is usually a mistake -
on most modern servos, there is little effective speed difference and a lot of actual torque difference .
testing for speed with no load is folly.
In order to get fast servo movement -under load - lots of current is required.
There is no free lunch as power consumed is directly related to work performed.
The point many miss is that by using a modern digital servo of only moderate power -then using max rotation per surface angle, the power spikes (baaad) can be greatly reduced and make for a better system.

Keith Black posted a REALLY cool Excel spreadsheet that does all the calcs, with any two parameters known, very friendly format. Maybe he'll read this and post it.... it's in here someplace, maybe search on "Control Throw Calculator" ? From Dec, '05, I think.

MajorTom and Dick,
I should set the ATV on my Futaba to 140%?
Won't this increase the "anti-exponential" effect of the rotation of the servo arm?
Do I enhance servo resolution by doing this.
I am really at a loss on this one.
Thanks!
JLK

Too much calculatin-- not enough doin--
The relative arm positions -if both start at center of a given arc- both change at the same rate
The arc of each will be different but still the rate of change will be the same -so forget the math studies here .Unless you would rather fiddle than fly -

what you are after is maximum use of servo power per angle of surface deflection

In actual practice- matching the throws of both elevator halves is far more important and will be much more easily done if you maximize the throws of the servos
setup at 100 degrees or 120 degrees - I will bet money you find the higher throw (servo) more accurate .
All of this is assuming you have two reasonably well matched servos on a radio which really matches outputs.
some very expensive elaborate TXs rare not the greatest at this. also those shi tty servo reversers and cheap extensions can kill an otherwise good setup

Bob, here's a link to the post that contains the Excel throw calculator:

[link=http://www.rcuniverse.com/forum/fb.asp?m=3712684]www.rcuniverse.com/forum/fb.asp?m=3712684[/link]

Keith B

well FWIW -I have yet to see any value in the math approach.
What is going to do a newcomer the most good, is to learn to make matched angles and lengths and always maximize servo travel to surface deflection .
-always-
The idea that faster servos or faster throws (using short servo movements to make large surface movements )is just bad understanding of force/distance relationship.

Keith, Dick and Majortom,
Thanks!
Dick,
Even though the two change at the same rate, the servo arm and control horn, won't the "feel" soften as the arms deflect resulting in this "anti-expo" I'm talking about...softer feel the more you move the stick and more sensitive around center?
It would take some amount of expo just to get to a linear feel, no?
Majortom,
I appreciate the math. I am ashamed to say my work area doesn't lend itself to accurate full setup mockup. Knowing that I have the correct arm and horn lengths enables me to position them right the first time...I hate redo!
Keith,
I did a search and couldn't find your spreadsheet.
Thanks for the link!
Once again, thanks to all!
JLK

Yep, that's the one - and I use it a LOT - THANKS!!!

Dick - as for too much math - it's like majortom wrote - using the calc program lets me know what I need mechanically to end up in the "close" ballpark range of travels. For example, setting the TX at 140% or 150%, then knowing what mechanical distances are needed for what control travels at maximum is handy. It helps make things a little faster when setting up a new plane for me.

It also is VERY helpful in letting someone know if they are setting themselves up for disaster with too little (or too much) travels.

However - said spreadsheet won't do a flippin' thing for knowing the leverage forces and understanding the relationships....I've seen many planes (although VERY briefly) that had SHORT arms on surfaces, and LONG arms on servos, and ended up in the trash bin in short order.

It's pretty simple... where possible, use a ration of 1:1 servo arm takeoff to control arm takeoff (hinge line to clevis attach) or GREATER. Longer on the CONTROL surface.... shorter on the servo arm.... ALWAYS okay...

But one must be willing to invest in servos with power needed.

Hallo Dick,

I have always respected your views here on the Pattern Forum. I also agree with the technical issue in that more servo throw is always better.

My disagreement comes for your attitude towards a math approach. Being and mechanical engineer in the aviation field ( 50% of my dailly work is control system design for full size gliders) I feel you are wrong. Maths (if done correctly) will always get you to an answer faster and more accurate and will allow you to get a feel for a problem before you start fiddeling.

Some systems are so complex that no amount of fiddeling will allow you to understand or optimize it. A correct mathematical model will point out stuff that you cannot obtain from practical experience although practical experience is VERY important.

Still what you say about more servo throw is corect.

Attie

JLK- the expo thing is is not as you are visualizing it .
To get the proper control feel , is another thing.
For most seasoned flyers , a competition model needs some softening around center - using modern servos setup correctly
On older stuff we did what you are trying to do - reduce throws to make it manageable.
These older servos had a softness around center -simply because they were unresponsive around center due to electronics available
Modern stuff reponds instantly to a squeeze of the control stick-in fact on my Funtana -I can watch any surface move by simply moving the trims one click . Each succeeding click moves either way desired --all this on $35.00 servos .
So setup you model for some moderate throw - this stuff of "7 degrees UP and 8.5 degrees down" etc., is bull shat. just make moderate throws with at least 100% of servo travel (default travel). Ad in some expo -on the switches - say - 50% on aileron - 30% on elevator and 50% on rudder - now fly it - learn how it feels at different speeds - then adjust to suit.
Trying to follow some magic recipe for throws is futile- Ballpark it and go fly.

Attie,
I have experience with Dick back in the '80s when I flew two of his Dalotels.
I think where he's coming from is not that he's anti-math but that he's been doing this for so long and has been so successful this stuff like I struggle with is intuitive to him!
JLK

Attie -- math is fine-- providing one knows what to do with the results .

As an engineer - you must know other engineers who know how to look up and provide any technical answer - But how to apply it?
That is another thing altogether
Look at the earlier posting on this question.
The math is right -
However ---how would you get the controls to work properly , using such tiny servo throws?
I spent years designing pneumatic controls for high speed machinery -also the operator controls and devices which interfaced them - The system "feel" was crucial to an operator using it.
I am sure you see the same thing with the gliders . operator "feel" is 1-2 and 3 in getting the setup right
The math is a small part of it.

HI Dick, JLK

You are absolutely right. Math tools are great to give you an understanding of the problem. Final setup (where there is a human in the control loop) must be adjusted to personal "feel". If it is possible to define the required "feel" mathematically, then the math models may be able to allow final setup to be calculated beforehand. It is however difficult to define a "feel".


Regards
Attie

Oh good--that's very reassuring--There I was thinking that my need for more and more expo was due to my logarithmically increasing age!