Is there any method (short method not those long engineering calculations) or rule of thumb that can be used to determine how much maximum load will be acting on a control surface ... so that I buy servos accordingly ?

Yes, check the sticky FAQ threads at the top of the Scratch Building and CAD/3D forum for links to online servo load calculators.

Good luck-depending on how you elect to fly a model - I would bet the margin of error is about----500%

I don't know if it's THAT bad but you gotta input some reasonable worst case numbers. Size and throw angles are no big deal. Easy to measure and all. But the speed is the kicker. You need to figure out a reasonable terminal velocity for a longer dive and input that as a worst case. What that speed is depends a lot on the model, power source and prop. And in the end if this is a 3D type model with huge surfaces then you need to figure on YOU limiting the dive speed by not using full throttle or you'll require far larger servos.

I have servos ranging from 300 in ounces torque to 15 in ounces
digital /non digital , on and on.
for just flying around with say a sport 40 powered Cub - and an occasional loop etc., a decent non digital servo -one each for ailerons , elevators , rudder (total 3) at 30 inch ounces of torque -is plenty That is from experience - no calculations - I never calculate -except against sales people -- I SWAG (scientific wild arsed guess) and it works very well -for me.
The recent explosion of huge power output servos has occurred along with huge 3D models and high output batts .
The old "40 in ounce servos were not bad but had lousy power at center -- this is where the latest chips and motors and good gear trains have produced a quite improvement -that fact has not gotten a lot of press
I recently got aholt (!) of some very inexpensive JR Sport mini servos MN48---they are rated at 48 in ozs on 4.8 v - In testing them - resolution is noticable on the model - with each click of trim on my JR 6102 Tx ! That is all that is needed.
My point is that high output servos ane NOT required now to get good power at reduced throws -where 99.99% of flying is done.
going to smaller servos -,I fly 15 in ounce servos on foamies with 100 wat motor setups and these will do fast and furious direction reversals all day long - BUT they do heat up doing those tricks and they have teeny weenie gears. So for just fun easy sport flying -- on 40 sized glow models - stick with mini servos -or larger from the better servo mfgrs .they are cheap ($23.00 street price on the 48's) Trying to calculate servo load may be fun but unless you are a real kluge at geometry and are terrible at linkages - this approach works fine.

What sort of servos are good are a 2 m like the fantasy with large surfaces.
I have JR NES 9011 ... are they any good ?

Depends on how you fly it - actually the inexpensive JR811 digitals will do the job! they are rated at 55 in ozs at 4.8 v -I have used them on 3D 2mm ZDZ40 powered setup -no problem - On the Fantasy -I assume(?) you are not into wild throws
the JR9411SA is reaqlly excellent and smaller -costs more tho 90+ in ozs torque. good digitals are really best for really precise stuff as they have far greater ability to hold a position The effective deadband is far smaller .
The 9011 are good too -but I like the digitals -- I used the 9011 on my Bucker Youngman (four ailerons) 1780 squares.

You got a link to what a Fantasy is?

No -but it is a FAI type pattern airplane
I only made a few thousand of those ----

www.camodel.com.ar --- you can find the fantasy here.


I fly it in pattern and some wild 3D. My new project is a 2 m (designed by me) and has about control surfaces 3 inches wide and full length along the ailerons.
You said 9011 is good but you like digitals. I've already spent a lot on this machine and Was thinking that my 9011 could come in handy. I am using a pull pull setup on my rudder and both elevators have separate servos with pull pull configuration.
And ofcourse the ailerons have simple pushrods but I think there'll be a lot of force on these ailerons when inflight.

For ailerons that large and at the speeds a pattern model flies at what about using two servos on each aileron with one inboard and the other about 2/3 of the way outboard? Lots of power and more support to the surface to help resist flutter? And that way you can get away with a lighter and cheaper servo?

Hello Uby,

Take a look at this link

http://www.jettech.ch/Servo%20Stellkraft-Berechnung%20(97).xls

From a swiss site ,in german, but i think they have the answer to your problem....

It was helpful, but the calculations are for a very small surface (rudder).
The force on the surface can never vary lineraly.

The size of my control surface (aileron) is 3 inch X 36 inch
I engines I am using are OS 160 FX and OS 140 RX (Hitori).
Even if My servo drives in well , My fear is only at one point : does it have enough holding power , I am afriad the gears may break while in flight.

Bmatthews ... I am afraid 2 servos (like the 9011) cost as much as 1 digital servo or high torqe servo. Why not keep things simpler.

If you want max aileron force -for min aileron servo cost - do this:
use one servo per aileron-- small -like the 9011 then----- link the two servos together across the fuselage
This will take some work to get the link (a carbon fiber rod) to both sustain pull /push.
The advantage is that the work of holding the aileron POSITION is now transferred to the interconnection as one aileron can no longer operate independently
Also when operating the ailerons - the load is absorbed by both servos- note that load is not equal on both servos starting a roll.
The amount of power required is actually quite low - because aileron forces -to deflect ailerons- is quite low (notwithstanding 1940's movies ) Once the model starts the roll -the forces actually reduce even further. This ain't a Piper Cub
99% of the work for the aileron servo(s) is holding servo from bumping up and down and if --the ailerons are tightly , mechanically coupled - this load vanishes.
Fotr the armchair set - the ailerons on a 330 EXTRA -a full blown aerobatic craft powered by an A0540 - can be operated easily with two fingers on the stick. This is because they are coupled (as are all small craft and they have aerodynamic assists (spades) and are properly balanced.

uby wrote: What is not linear?

I had some old figure lying around showing the results of measurements of the hinge moment coefficient for a plain full span hinged flap where the flap chord is 25% of the total chord. On the vertical axis is the hinge moment coefficient, on the horisontal axis the control surface deflection is shown and the different curves shows the results for different wing angle of attacks. If I remeber correctly the flying surface was rectangular with a NACA009 symmetrical airfoil.
As you can see the hinge moment coefficient varies almost linearly with control surface deflection.

The hinge moment can be calculated from the following formula:

Moment = 1/2 * air density * (air speed)^2 * (control surface chord)^2 * (control surface length) * hinge moment coeffiecient.

In order to simplify your calculations you can use the calculator available at [link=http://www.csd.net/~cgadd/eflight/calcs_servo.htm]http://www.csd.net/~cgadd/eflight/calcs_servo.htm[/link]. The results of this calculator are in good agreement with results obtained from the figure below.

One warning: The calculator available at http://www.csd.net/~cgadd/eflight/calcs_servo.htm seem to underestimate the torque by almost a factor of 2. I haven't checked why.

I agree with Dick Hanson that the error margin for simplified calculations will be quite big.


/Red B.

Well .......... I did ise the calculator , It is iving me results of over 100oz/in of torque at 45 deg deflection. If this is still underestimated by a factor of 2 .... How can I use my 9011 with 60oz/in of torque ???


Has anyone here Used the 9011 with 2m planes or the funtana 90 ?

On a prop plane the prop will act as a very effective speed brake in a dive even at full throttle. If you take the pitch speed (pitch in inches x RPM x 60)/12 x 5280 = MPH and figure on a 40% overdrive in a dive that should give you a pretty decent max speed to work with. That should allow for the engine being driven above it's normal speed and for the negative slip of the prop disc in the dive. So use 1.4 times the pitch speed and see what the load calculators say.

Does that seem reasonable in your estimation Dick?

is the formula (pitch in inches x RPM x 60)/12 x 5280 = MPH

or is it (pitch in inches x RPM x 60)/(12 x 5280) = MPH

Is this an accurate speed calculating formula ? ... (that I've been looking for from some time)
What is the factor 5280 ?

feet ( 12 x 5280 feet)
I got lost on the prop braking formula-
If I understand you - you are considering the engine reaches some peak rpm and the prop speed also becomes fixed there.
prop efficiency is figured at 140%?
why?
The prop does act as a brake but simply going by pitch numbers , I get hopelessly lost
two different brand 22x8 props on my 50 cc engine act completely differently
One does 7750 static and 8400 level flight atmax airspeed the other does 6700 static and 7600 level flight max airspeed
I don't know the difference in max airspeed but the slower revving one feels faster!
IF--- I were to try for a terminal V dive it would likely be terminal for something -but I suspect the faster revving prop -in this case -- would be braking more than the slower one .
To sum up
I can see how a given prop could be calculated to act at a brake IF it held at a given rpm -I just don't see that happening on most setups the two strokers will rev pretty freely-at least mine will
Did I miss the point of the question?

140% is just a judgement call. In a dive the engine is being overdriven by the prop and that is why it slows the airplane down. Basically the engine will be free running other than prop drag so it'll rev to a higher figure than in level flight. From how the pitch changes on my models I sort of figured the revs go about 20% past the level flight RPM's and then factored in an additional 20% of negative prop slippage. All highly non technical but without actually arranging to measure the full throttle dive speed I could not think of a better option.

Uby, your second formula with (12x5280) is the right one. Sorry for the confusion. 5280 is the number of feet in a mile. So the top part of the equation gives you the number of inches per hour traveled (assuming no slippage) and the lower part under the "/" division line changes that from inches to miles.

The formula gives you the pitch to RPM speed but there's slippage with any model design such that the model will never reach the pitch speed in level flight. Typical prop slippage in steady state level flight is dependent on how clean the airframe is but 7 to 10% for speed models is fairly typical and 15 to 25% is probably typical for high drag trainer types in level flight. A cleaner pattern model is likely in the 15% range. But it's all dependent on the drag of the airframe. There's no really accurate speed formula for any model.

That's a fact Jack!
Control line models are the easiest test mules for trying to establish prop efficiencies -
The flight path is fixed and drag/speed is a constant and easily controlled path.
This allows one to get a very direct comparison on thrust developed on different props-or engines -or compare wing section drag -etc.,etc..
Call it a poor man's wind tunnel but I will take results done this way over any scaled down /extrapolated results from a billion dollar wind tunnel.
I am loath to use formulas which have never been tested in actual case scenarios.
On prop braking -- the exhaust system used on a given engine can assist in braking but most two strokers don't generate much braking force --a very flat pitch prop helps as it can rev like mad and still be resisting forward motion
Now -if you use an electric motor and back EMF ,you can really do some effective braking!
The new Hybrid cars do this extremely well- getting better fuel (gasoline) mileage around town than on the highway. The energy use is in a closed loop - still lots of waste heat but far better than the old gas buggies.