Flight controls power requirements and servo sizing
 

Hello guys.

Following this post I thought that some guys might be interested in a detailed method on how to compute the flight controls power requirements and how to choose your servos accordingly.

Note that the method that I will describe below is part of my latest article in RCJI on the pre-maiden tests and procedures.


First things first: this method uses the invaluable freeware from Craig Tenney: RC calc.
This is a very clever spreadsheet and I will details its use below.

http://www.geohei.lu/olin/data/model...calculator.xls


Servo considerations: why bother choosing exactly the appropriate servo and not use systematically the biggest available ?
For several reasons:
1) The bigger servos idle currents are higher
2) The bigger servos current under reasonable load ( linear response ) are also slightly higher
3) Choosing bigger servos thus means choosing bigger batteries, bigger regulators/power busses ( if required ), bigger cables, higher spec connectors. This means spending more money.
4) Choosing these bigger equipments means a heavier remote control system

Let's illustrate this in practice.
I have done a simple test with my Hangar 9 amp meter and the following servos:
JR 3421
JR 8611A
JR 8711
JR 8711 HV

All have been tested at exactly 5 V under the following conditions: idle with no load, under load at 10 kg.cm.

Here is the picture of the setup:

http://www.geohei.lu/olin/data/model...74_web_raw.jpg

The results for the idle test are:

3421=0,02A
http://www.geohei.lu/olin/data/model...68_web_raw.jpg

8611/9711=0,04A
http://www.geohei.lu/olin/data/model...67_web_raw.jpg

8711HV=0,05A
http://www.geohei.lu/olin/data/model...66_web_raw.jpg

RE: Flight controls power requirements and servo sizing
 

The test under load @ 1 kg.cm:

3421=0,15A
http://www.geohei.lu/olin/data/model...73_web_raw.jpg

8611=0,17A
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8711=0,18A
http://www.geohei.lu/olin/data/model...70_web_raw.jpg

8711HV=0,20A
http://www.geohei.lu/olin/data/model...71_web_raw.jpg

Of course, when coming to a higher weight, the 3421 will enter the non linear curve sooner than the other servos and start showing higher consumption results than the 8611/8711/8711HV before stalling.
So choosing the correct servo is important: it shall not be too big, neither too small.

Here is the method to find out how to choose them.

1) measure all your flight controls and control horn arms

You'll need the average chord, span and horn distance to the control axis in cm.

RE: Flight controls power requirements and servo sizing
 

2) enter the data in the RC calc

Go to the "Super servo" tab.

An example of an aileron computation:

http://www.geohei.lu/olin/data/model...%20optimum.jpg

The average chord goes into the Cavg field on the left of the table
The span of the control goes into the Lavg field on the left of the table
The control horn length goes into Yc

Then you'll need to adjust the control throw in degrees ( Rc+-, usually +-20° ) and the servo throws ( Rs+-, usually +-50° ).
Then enter the speed you intend to fly the plane. You can enter the maximum speed ( for maximum servo power requirements ) and a "realistic speed" for average servo consumtion.

The table above shows a maximum speed/servo power computation for the ailerons ( 200 mph in the Vmax field ).

The output data:
Ts max will give you the maximum servo torque required ( most of the time given at max deflection )
Tc max will give you the maximum hinge moment on the control horn ( this is very useful to check that your hinges are correctly dimensioned )
Fp max is the maximum force exerted on the pushrod and the servo ( this is very useful to check that your pushrods/links/threads/servo tray system and servo fixing are correctly dimensioned )

So when you know Ts max, you just need to add 30% ( friction and control weight and inertia ) and you'll know what servo you need to choose.
In the previous example any servo giving more than 130 oz.in of torque will be fine ( check here for JR servos: http://www.jrradios.com/Content/PDF/ServoChart.pdf )

RE: Flight controls power requirements and servo sizing
 

Now lets talk about current consumption and electrical test.

What you want to do here is use a realistic speed value ( the speed that you will fly most of the time ). This really depends of your flying style.
Lets enter 120 mph into the Vmax field in the previous Rc calc simulation:

http://www.geohei.lu/olin/data/model...%20average.jpg

The interesting output data is Fp max. This gives you the weight that the system is seeing at the control horn distance given in Yc.
On the example above we have 4,2 lbs for Yc= 2cm.
This means that at 120 mph, a full deflection of the control surface ( the aileron here ) will generate an effort of 4,2 lbs at 3/4 inch from the surface rotation axis ( 1,9 kg at 2 cm ).

So we can now easily simulate the effort generated at this speed: just put 4,2 lbs of lead at 3/4 inch from the control axis, or 2,1 lbs at 1 1/2 inch .
The servo consumption at this load will tell you what your plane will sustain in an average flight condition.

RE: Flight controls power requirements and servo sizing
 

3) electrical tests

We can now compute this for every flight control.

I then do two types of electrical tests:

The torture test:
The torture test is made with the flight controls loaded according to the maximum possible efforts found in flight ( 100% stick deflection at maximum speed )
In the first Rc calc example we can see that we need 11,7 lbs at 3/4 inch or 5,85 lbs at 1 1/2 inch.

I then put the plane into the sun to simulate the maximum temperature at the field and activate the servo testing feature on the transmitter. I let the routine go for the equivalent period of two flights.
During the routine I check and make note of the receiver and /or regulators temperature as well as the maximum current consumption.
This routine helps me to check how the servos can cope with the maximum flight loads and the system peak consumption in a realistic environment. It also makes me verify that there will be no regulator thermal runaway or receiver thermal lock for the Fubata FASST system.



The endurance test:
The torture test is made with the flight controls loaded according to the average flight speed entered in the last example ( 25% servo deflection at 75% of max speed)

http://www.geohei.lu/olin/data/model...%20average.jpg

In the table above you can see that I've used the slide to adjust the servo deflection at 25% and to read the associated pushrod deflection ( red arrows )

In this example we can see that we need 0,9 lbs at 3/4 inch or 0,45 lbs at 1 1/2 inch.

I then put the plane into the sun to simulate the maximum temperature at the field and activate the servo testing feature on the transmitter. I let the routine go until the batteries are empty 8 or at minimum voltage for Lipos )
I'll repeat this routine 3 to 5 times.
This will run the system in ( getting rid of the early component failures and run the batteries in ).
This routine will give you a very precise idea of how many flights you can do with your set of batteries...

RE: Flight controls power requirements and servo sizing
 

Please note that the examples above have been taken from the large 1/7th scale F18F. The force and torque values exerted on the flight controls might look very high.
These figures have been actually verified and proven to be correct.

Large scale jets are experiencing quite large forces and effort in flight as you can see !!!

Also these techniques should not preclude you from doing a good setup ( use the servo arm distance given in "Ls" field of the Rc calc ) and checking that no servo binding is present.

Once again, for a more detailed and wider scope explanation, please check my article in the December/Janaury RCJI issue...

P.S: to verify the amount of efforts generated in flight you'll need an Eagle three system or similar plugged on one servo channel. Check the average and maximum servo consumption. If you are operating in the linear range ( which should be if you have followed the points above and verified that no binding is present ) then the current consumption can be converted into a torque using the reverse method described on thread 1 .

RE: Flight controls power requirements and servo sizing
 

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RE: Flight controls power requirements and servo sizing
 

Guys I know that it might look a bit complicated but it is in fact quite easy to do once you have understood the principle of it. It takes me about 20 minutes to work out a complete plane.
I believe that it is really worth the effort ( although the manufacturers should do it for us and provide the file/table ready to use ).

I will show a more practical example on Tassos UL when he sends me his data...

RE: Flight controls power requirements and servo sizing
 

Oliver,
thanks for the lesson on using your spreadsheet! Now i know for sure i'm making use of it correctly.

RE: Flight controls power requirements and servo sizing
 

You're welcome Thomas

I have not really covered the geometry of the control link in this thread, but it is fairly self explaining when looking at the spreadsheet.

Note the on the right of it you have a table giving you the offset required for a pull-pull configuration.
Also this does not cover the specific case of pendular stabilizers. I'll make another thread later on to cover this specific topic.

RE: Flight controls power requirements and servo sizing
 

Nice Oliver.

I have also found this one very helpful: http://atlas.csd.net/~cgadd//eflight/calcs_servo.htm

RE: Flight controls power requirements and servo sizing
 

Oliver,

Thanks a lot for explaining the method. This is a great thread, it should be made sticky.

By the way, I love your arcticles in RCJI.

Best regards

Hermann

RE: Flight controls power requirements and servo sizing
 

Thank you everyone.

If you guys have any question about this method, I'd be happy to answer it.
I can also work out the simulation for your plane and show it here as a didactic exercise.

RE: Flight controls power requirements and servo sizing
 

Ok Oliver,

I got the data from my UL and here they are:

Aileron
average cord = 100mm
span = 480mm
control linkage length = 182mm

Flap
average cord = 116mm
span = 342mm
control linkage length = 77mm

average cord = 75mm
span = 390mm
control linkage length = 133mm

Top speed 350km/h
Average speed 220km/h

Thanks alot Oliver,

Tassos

RE: Flight controls power requirements and servo sizing
 

Hi Tassos,

The most important data is the control horn length, not the control linkage length.
I need this data to input the parameters.

RE: Flight controls power requirements and servo sizing
 

Oli,

The control horn length on the servo side or on the flying surface side? And I guess if this is for the flying surface side you consider the length from the hinge? Because the CARF models don't have a hinge in the middle but the hinge is their skin.

Thanks alot,

Tassos

RE: Flight controls power requirements and servo sizing
 

Control horn is on the flight control side
Servo arm is on the servo side
I need the total length from the control surface rotation axis to the ball link hole...

RE: Flight controls power requirements and servo sizing
 

To get another perpective of the deflection needed and the force & time applied one must not forget that whatever value is calculated, the same force will act in the opposite direction on the control surface and directed into the aircraft. My reasoning below is that plane configuration and trim has a great effect on this subject. So here we go........

Any rudder is only a means of creating a change to a steady state of balance.
For arguments sake, an aileron control surface load of 10N will induce a roll by roughly the same force (minus losses) on the wing. If not counter acted by an opposite force ( built in stability in the form of dihedral / by high wing configuration ) the load should get lower after roll initiation.

The same goes for pitch control. The shorter arm of momentum by position of CG in relation to lift center, the less force is needed to bring the plane into an attitude change.
It all about balance as I´m sure we all have experienced when fine tuning a plane from beeing a sluggish feeling beast to a nimble piece of precision by moving the CG rearwards and the result is less net elevator travel and I suggest (not having done any measurements ) less servo load in the process.

Granted, going in 200mph with a 20kg jet and pulling full elevator is going to put som serious loads on everything but there should be a big spread depending on the type of plane and as mentioned CG trim.
An airbrake should produce a constant load consistent with the calculation as would a rear spoiler of a car

It would be very interresting to see a data log of servo power consumtion if any one can share

These are my two of whatever currency you prefer,
Thanks for a great thread Oliver

RE: Flight controls power requirements and servo sizing
 

Your right Surfer.
Unfortunately the tools required to compute a complete plane stability and and total forces involved are not within a modeler's reach.
The Rc calc spreadsheet/method involved is only an evaluative tool but it is meant to be conservative ( ie in real life the force required shall be significantly less than the simulation ).
Furthermore it is a freeware. So don't hesitate to use it. You'll be surprised how useful it is...

RE: Flight controls power requirements and servo sizing
 

Wow, is the servo calculator accurate? I tried it and according to the table, it would requires about 1043 oz/in of torque.
I don't know if JR makes such a servo. :)

RE: Flight controls power requirements and servo sizing
 

Ok here we go once more Oliver with the missing data,

Aileron
average cord = 100mm
span = 480mm
control linkage length = 182mm
control horn length from axis of rotation = 59mm

Flap
average cord = 116mm
span = 342mm
control linkage length = 77mm
control horn length from axis of rotation = 28mm

Elevator
average cord = 75mm
span = 390mm
control linkage length = 133mm
control horn length from axis of rotation = 38mm

Top speed 350km/h
Average speed 220km/h

Tassos

RE: Flight controls power requirements and servo sizing
 

If your trying to figure out the data on a full flying stab.. the spreadsheet doesn't work for those.. maybe oliver can enlighten us on what needs to be done for that :)

RE: Flight controls power requirements and servo sizing
 

Yes, I will use Thomas F-14 as an example. Will do this in a couple of days from now...

RE: Flight controls power requirements and servo sizing
 

Ok Oliver,

I tried this myself and here is what I did for the aileron only.

Am' I doing something wrong? Which parameter is the control horn length which you said is an input? Yc?

Thanks, Tassos

RE: Flight controls power requirements and servo sizing
 

......And this is what I've done for the flaps....

Is this ok?

Thanks alot,

Tassos