Thank you for your input Harry. That is one trigonometric tip that I didn't see. Great idea...

That looks a little low to me, unless the plane is really small with very big control horns.
Could you post the sheet here ?

It's the Trim Sabre, about 1/7th scale.

Oh I see: extremely small flight control angular deflection ( 7 degrees ) with extremely big control horn ( 3,3 cm or nearly 1/2 of the control chord )
Then it makes sense.

One remark here: this kind of setup tends to amplify the influence of the servo slop compared to the control travel.

Are you sure? Any slop in the servo gears produces the same angle of rotation of the servo arm regardless of the linkage, so a control rod attached at a small radius will have a small linear movement compared to one attached at a large radius. Then at the control surface, the small movement of the pushrod attached at a large radius will have make small angular changes in the control surface. Surely a linkage that is attached at a large radius on the servo and a small radius on the control surface, such as for 3D travels, amplifies servo slop?
Harry

You're right Harry. This sentence is extremely badly written/confusing. What I was meaning is that on such a setup, the servo slop becomes significant to the control travel. That is not because of the control link geometry but because of the aero dynamical configuration/ ATV.
7 degrees of deflection is very small plus the servo is only used at +-35 ° of deflection ( 60% ATV ? ). So the slop will be more significant compared to the total servo angular deflection used.

However, maximizing the mechanical advantage WILL minimize the influence of the servo slop, which is what you have done in this case ( de-amplify the servo slop at the control tip ).

I am still surprised as well to see such a small torque value required. I guess that on this servo power segment, the critical factor to choose it would be the servo reliability...

Did you manage to find a servo arm that gives you 0,7 cm of leverage ?

I have been measuring my Aviation Design F-100 (similar size to the BVM F-100) and putting the values into the spreadsheet. Once again I am very surprised at how little power the servos need. The ailerons, flaps and rudder could have used very ordinary servos, not the expensive and power hungry JR8511s that I fitted. The all moving tailplanes will have to wait for Oli's next article, I am looking forward to that. The screenshots below show aileron, flap, rudder in that order, with aileron and rudder calculated at 150mph, flap at max 100mph. As you can see the ailerons only need a servo of 1.4kgcm torque (approx 20ozin I think). Even at 200mph this only rises to 2.4kgcm ( 34ozin I think). The flaps have very little travel in this model hence the low torque required, but even if speed is set to 150mh to allow for forgetting to raise the flaps, the torque required still only peaks at 3.9kgcm ( 54ozin).
I am building the Mick Reeves Lightning next and had assumed I would be using servos like the JR8511 all round, but I will run the numbers through this spreadsheet before making any purchase.

Harry

Hi Harry,
Glad to see that you find the tool useful.

I have been running your numbers in the original Imperial spreadsheet to make sure once again that the metric one that I made up is giving correct figures. No problem.

I find those very low values hard to believe. So I started by doing the same sums with a formula published in the Multiplex catalogue which calculates the torque exerted by the control surface, the figure for Tc in the spreadsheet. It comes out with different numbers but close enough, and and slightly lower than the spreadsheet so the spreadsheet doesn't seem to be undercalculating Tc. Multiplex says Tc = length in cm * chord in cm^2 * Vm/s ^2 * degrees deflection/2000000, and this is the same calculation done in column AE of the spreadsheet other than it using a different approximation factor hence the slight difference in results. Not enough difference to bother about.

But if we carry on with the spreadsheet it generates some odd numbers that I can't explain. Take the example of the aileron in my post above. Tc is max 5.9kgcm, which it operates on the pushrod at a radius of 3cm, so the force applied to the pushrod by the aileron is 5.9/3 = 1.9kg, yet the spreadsheet says Fp max is 0.8kg. It then says that the pushrod is connected to the servo at a radius of 0.8cm, so the servo torque required will be 0.8*0.8 = 0.64kgcm, yet the spreadsheet reports Ts max as 1.4kgcm. Using my calculation, if Tc max is 5.9kgcm which I will accept as being good enough, then on a pushrod at radius 3cm from C and 0.8cm from S, pushrod force is 1.9kg and servo torque is 1.5kg. This is very close to the spreadsheet's Ts max of 1.4kg, but arrived at via a very different mid point, the pushrod force.

The rudder value is most odd. It shows Fp as 1.3kgcm, which at a servo radius of 1.1cm requires torque of 1.3*1.1 = 1.43 yet the spreadsheet says Ts is 2.9. Since it reports Tc as 7.6kgcm, at a horn radius of 2.5cm I would expect Fp should be 3kg, not the mere 1.3kg it reports. 3kg force on the servo radius of 1.1cm requires torque of 3.3kgcm, the spreadsheet result is 2.9kgcm.

I can see that if the pushrod is not at 90 degrees to the control surface horn then the force on the pushrod will be less than torque/radius but the difference in the above examples will be small and nowhere near enough to account for the difference the spreadsheet shows. Can anyone explain why the Tc values don't compute directly to the Fp values, and the Fp values don't compute to the Ts values?

Harry

I've been thinking about the above, and might have the answer.
The pushrod is not at 90 degrees to the control horn when it matters. Take the example of the aileron above, the pushod may be at 90 degrees to the horn when the control surface is at neutral, but at that time the control is not generating any torque. The control generates max torque when at max deflection, at which time due to rotation of the horn the pushrod is no longer at 90 degrees to the horn. Because it is no longer aligned with the force vector generated by the torque, the force applied to the pushrod will be in the order of cosine of the angle off from the tangent to the horn, so the force on the pushrod will drop as the angle changes away from 90 degrees. Similarly the force applied at the servo arm to convert into torque applied at the servo will be reduced by its angle off the tangent to the radius of the arm, I can't be bothered to work out if sine or cosine but either way it will be well reduced since as you can see the servo arm rotates in the region of 40 degrees. But that works in reverse too, so the servo will need much more torque to be able to generate the same force on the pushrod when it is rotated away from 90 degrees to the pushrod, compared to when it is at 90 degrees

If you are struggling to imagine all that, imagine taking things to the limit. Imagine the control surface deflects 90 degrees, then so does its control horn. At that point no matter how much torque the control exerts on the horn, it has no fore or aft movement so it can't exert a gnat's wotsit of fore and aft force on the pushrod. When the control is only partially rotated, the force the horn can apply to the pushrod is somewhere between full force and that zero force. The same applies when it reaches the servo arm, if the servo had rotated as far as 90 degrees then you could apply all the force in the world but it won't make the servo rotate and the servo will need zero power to resist it - you would need a strong mounting and servo case, but you couldn't make it turn! With servo typically rotating 40 degrees or more, the translation of pushrod force into torque to try to turn the servo is hugely reduced, and that reverses in that the servo needs extra torque to apply a force since its max force is at 90 degrees to the arm and has reduced force at other angles.

That would explain why the spreadsheet gets lower values for Fp than my simple calculation that ignored the angles, and why the low Fp converted into a much higher servo torque requirement.

H

Hi Oliver,

I was reading your thread on this nifty software you've created. Wondering how I go about getting one to help me with my flight control settings?

Couple weeks ago I burnt out two brand new 8611 servos for my flap system. Setting up for landing I reduced airspeed and drop first notch of flaps. Everything fine! Just before turning base leg from downwind I dropped the rest (about 85 degrees). I noted that the airplane abruptly started to turn to the left. I compensated with an automatic trim correction to maintain straight and forward flight. So I increased turbine thrust and decided to do a go around as the sudden banking caught me by surprise. Flipping the flap switch back to first notch I noted there was no response. So the jet goes by me and as we looked it was verified that one flap was lower than the other. Managed to get the airplane back down and upon inspecting the problem both servos where locked. When I turned off the transmitter tried to move the servo arms and still locked. I suspected that the servo motors just burnt up.

I am clueless as to why this occurred. I have put on 5 flight already on this Dragon and never was there a sign of any problems. Every time before every flight I do a thorough inspection of flight controls, batteries, landing gear, turbine, hardware, and so on. I've usually seen these problems before they occur.
I've gone ahead and concluded that maybe my geometry set up was incorrect.
It’s a typical install for flap system.

A little information on the jet that maybe of help:

Xtreme Jets Dragon
Total weight: Have not weighed it but guessing about 35 lbs wet
Servos: JR 8611's all around (Brand new)
Turbine: JetCat P160SX
Flap settings: First notch (takeoff) about 15 to 20 degrees, Second setting (landing) 85 degrees
Number of flights: 5

Can you give me some of your expert advice on this issue? After that flight losing flaps and not symmetrical it shook me up pretty bad. I've never had such an incident like this before with a model jet. Had one while piloting a full scale aircraft and it was not nearly as tough to fly as a model jet is, Guess I’m even now, LOL!

Hopefully this device you've created could be useful.

Thank you for your time and looking forward to doing some flying with you next time you visit Houston Texas.

Regards,

Orlando

Hi orlando,

Could you please send me the following data:

Flap length
Flap average chord
Control horn length from the rotation axis of the control to the link axis
Mid deflection in degrees ( I know that full is 85 degrees )
Distance from the servo rotation axis to the flap rotation axis
Distance from the servo rotation axis to wing chord

Thanks.

Hi Orlando,

Here is a rough guess of what you might have on your Dragon.

Flap length: 30 cm
Flap average chord: 10 cm
Control horn length from the rotation axis of the control to the link axis: 2 cm
Mid deflection in degrees ( I know that full is 85 degrees ): 25 degrees
Distance from the servo rotation axis to the flap rotation axis: 9 cm
Distance from the servo rotation axis to wing chord: 1 cm


Please confirm that the geometry is correct ( the diagram in the middle ) and send me the data requested above. You'll then have something close to the real case...



The JR 8611A is a great servo that has never deceived me. I have used it for about 7 years in most of my jets applications. I have never seen any of these servos burning. So something must be seriously wrong here.

Hi Oliver,

Thanks for your response! I'm a bit unprepared as I did not take a real look at the numbers you need.
Right now one of my wings has the servo installed and the other doesnt. I've not installed the hi tec servos I plan on using.
Also, my JR servo still attached to the wing is stuck in the down position which it never reached full deflection.

Maybe I can give you the data to the failed servo and then install the Hi Tec servo and give you that data as well. I'd have to complete that project tomorrow as its late here.
I will post the data here tomorrow after work.

Thanks,

Oliver,

Very interesting figures on the rough estimate you provided. It seems like i've been exceeding the servos capability of withstanding. (if i'm reading it correctly). I realize this is a rough estimate but if the data I will provide later on is near what you've determined then for sure I will require another approach.

Thanks

That would be great Orlando. Also a picture of the servo stuck in position with the hatch removed and link connected would help in understanding the problem.

Just to confirm it, all the tools posted here are completely freeware. They are at the disposition of the community and everyone is welcome to ask help from here.
This post is intended to help people in understanding flight controls requirements and is a complement of my two articles posted in the previous and actual RCJI publication.

The tools are available from my ftp here ( imperial units ):
http://www.geohei.lu/olin/data/model...20imperial.xls

and here ( metric units ):
http://www.geohei.lu/olin/data/model...0metric%20.xls

The stabilator pivot point calculation sheet is available here:
http://www.geohei.lu/olin/data/model...stabilizer.xls

Oliver,

Thanks for the freeware. Originally I could not download it from the one you posted at the beginning of the thread.
I will have the flaps going when I get back from work.

Regards,

Orlando

Flaps are an interesting problem for a model. In the full-size the plane has flap limiting speeds and the pilot has an ASI. A model should always be slowed down before applying flap, but the airspeed will be inconsistent without an ASI and you can't be sure at what slower speed each individual pilot will choose to lower the flap. Even then, we don't actually know what airspeed the model is doing with each individual pilot. Therefore do you pick a speed for the calculation that has a massive safety margin, such as the expected top speed in level flight, say 150mph? The result of that - a combination of a flap's large chord, huge deflection that is typical of flaps, and high airspeed is likely to produce a very high torque demand. The effect of airspeed is dramatic, so if you can be sure that you will slow down to some arbitrarily chosen speed say below 100mph, the torque demand will fall substantially. Calculations will become a good deal more accurate once we have the GPS module giving ground speed, it will allow us to come close to the actual airspeed instead of the guessing that mostly happens at the moment.
H

Hi Oli,
Thanks for the software links. In the stabilator pivot point calculator is the measurement of the root chord measured from the centre of the fuselage (like measuring the wing CG) or from the point were the stabilator meets the fuselage?

With thanks once again.

Reuben

Does G loading have any bearing on torque required? Not sure..........probably having one of my many brain farts

Hi Oliver,

I've taken the measurements you requested. These measurements are from the old servos that are burnt out. I'm not sure if this would work since I cannot bring them to the neutral position unless I take out the servo and reposition the arm. I can do that as well if you wish. Hopefully this will work, if not I can go with changing them around.

Flap Length: 460mm
Flap Average Chord: 100mm
Control Horn Length from rotation axis of control to link: 25mm
Mid Deflection in Degrees: 42.5 degrees
Distance from servo rotation axis to flap rotation axis: 106mm
Distance from servo rotation to wing chord: 11mm

I will have to purchase some control arms of longer length for the Hitec servos. Dont have any arms at the moment!

I'll play with this freeware you've provided but after looking at your figures you provided your pretty close. Hope that the measurement for the "Distance from the servo rotation to wing chord" is correct. I'm assuming this is the measurement from the clevis to the hole on the flap control horn.

Look forward to your reply

Regards,

Orlando

I guess it must Alex, though given the weights involved in our model control surfaces it is probably of minimal effect compared to the aerodynamic forces.
The typical model control surface is front hinged and not mass balanced about the hinge line, so the weight tries to make it fall. The force will be a multiple of the G load. The servo has to counter that force, so it does have hold the surface weight against G load. In some cases this will assist the servo, in others it will add to the power required.
Consider a flap that is down, the airflow is trying to push it back up to neutral and its weight is trying to pull the flap down, the servo has to provide enough force against the balance of the flaps aerodynamic less its weight forces. If G is increased, the weight pulling the flap down will increase, thus helping the servo work against the aerodynamic force that is trying to push the flap back up. But you can easily understand that the aerodynamic force is much stronger than the light weight of the flap, so the aerodynamic force still dominates the equation.
On the other hand, consider up elevator, the servo has to hold up the weight and hold it up against the aerodynamic force trying to push it down, again an increase in G will increase the weight but this time it adds to rather than subtracts from the aerodynamic force, but once again in most cases the weight even under several G is likely to be a fraction of the aerodynamic load.

Something that occurred to me with all moving tails is not so much the aerodynamic load which can be minimised, but the rotational momentum due to the weight. My F-86 tails are not balanced about the pivot, are light, but the servo is always buzzing slightly against the weight it has to hold. On my F-100 the tails are mass balanced about the pivot, that took a fair bit of lead, so when you move the tail it takes a good bit of force to get it moving, and then to stop it at the correct position. I haven't yet done Oli's calculations for the tailplane, but if it is well designed and made it should present a surprising small aerodynamic load but that doesn't take account of the weight which in the case of a mass balanced AMT could present quite a load for the servo trying to start and stop it, and may become the dominating factor or at least significant enough to warrant being calculated in addition to the purely aerodynamic force.

H

OK Orlando,

I've loaded the worksheet with your data.
Everything pretty much depends on what kind of servo travel you are using.
With a +- 30 degrees travel you need a servo arm of 5 cm and the maximum torque required is 190 oz-in:



With a +- 40 degrees travel you need a servo arm of 3,7 cm and the maximum torque required is 133 oz-in:

Hi Oliver,

Thank you very much for your advice. This program really adds true value to really knowing where one is at with there setup. I suspected something was wrong and clearly I see it.
I'm still learning each of the defenitions and playing around with the software. But it looks like I will do some experimenting with linkages and arms.
I will have to locate a copy of the artical you wrote for RCJI. I'm sure all the defenitions and a crash course on how to really use the software. Its been awhile since i've done this and the internet has really turned out very useful for getting back the basics.

I'll let you know soon as I get the Dragon back up in the air with my experiments.

Kind regards,

As you can see on the example above, it is very important to use as much of the servo travel as possible. Increasing the travel by 10 degrees here drops the torque requirements by 60 oz-in.
Also note that the simulation was done at 60 mph. Increasing the extension speed will dramatically increase the load on the control surface.

Finally I have edited the link to the imperial units file ( the .xls extension was missing in the link ).