Or maybe this for flaps is better.....

I set Rs+ to 0 since there is only one side the flaps deflect to

Tassos

And now that I have done this how can I simulate on ground the air forces on the surfaces? I mean how do I calculate the weight required to stick on the surfaces to simulate as close to reality as possible the forces exerted on the surfaces (like you did on the Phoenix in the last RCJI)

Tassos

Hi Tassos,

The aileron computation is OK.
You might want to input the correct distance of the servo arm axis to control axis ( input Xs ). This will get you closer to your real configuration.

For the flap computation I doubt that your servo only has 50° of rotation for the whole flap deflection. You have correctly entered the control geometry ( Rc+=0, Rc-=45 ) however the servo might move from 40 to 40 degrees for example ( in that case you would have Rs+=40, RS-=40 )
Look at what you get on your plane and copy the geometry into the spreadsheet.

This is explained in post 4 and 5 as well as in the article.

The important output data for this computation is Fp max. This gives you the weight that the system is seeing at the control horn distance given in Yc.
On your example, for the aileron we have 7,6 lbs for Yc= 5,9 cm.
This means that at 200 mph, a full deflection of the control surface ( the aileron here ) will generate an effort of 3,44 kg at 5,9 cm from the surface rotation axis.

So for the torture test you will have to put 3,44 kg of weight at 6 cm from the control axis of the ailerons.
For the endurance test, you'll have to put 1 kg at 6 cm from the control axis ( corresponds to 20% aileron deflection at full speed )

For the flap, I think that you need to rework the simulation a little bit. It does not look close enough from the UL configuration...

Thanks alot for your help.

Got it now!

I think I'm moving to the right direction. I'll also try to do the torture and endurance tests.

Your help is invaluable.

Thanks alot once more.

Tassos

You're welcome Tassos.
Please post the flap simulation with the correct geometry. I'll have a look at it.

Guys.. I usually use 8611 in my jets and in some places 8711.. doing that i never use power box.. but JR stop to sell the 8611 force customer to change for the strongest 8711.
But a big jet usually use 7 servos for the surfaces... (2 - flap, 2 aileron, 2 rear stab, 1 fin), now we have a problem with power

I usually use 2 regulator with 2 battery and plug both in the 14 channel futaba receiver.. but with this number of 8711 i am worry....

any help??

Hi,

I want to check servos linkages for my Tamjets A-4 skyhawk!
Tamjets already suggest which servos to use in his model, but I am curious to see the result from using this program on the A-4.

For now, just to have a go with the inputs on the program I tried the calculation for the aileron.

Aileron size:
average chord = 7.7 cm (considering the chord from the T.E. till the hypothetical hinge line)
length = 26.5 cm

The aileron has a kind of aerodynamic balancing but I didn't consider it in the computation just to make thinks worst for the servo!

As you can see from the pictures the control horn has its outside hole at 1.5 cm from the surface, that will allow for a control horn height of 2.2 cm (Yc) above the hinge line.
The distance between the hinge line and the servo axis is 7.0 cm (Xs = servo distance forward of hinge line) and looks like the servo axis will be placed quite precisely on the chord line.

Tamjets suggest a 7/16" travel up/dn for the aileron, that equals to 1.11 cm up/dn.
I estimate the maximum speed to be around 180 mph, this will be a EDF powered model not a turbine so I guess this is already anough!
I put the servo rotation @ 45 degrees each side otherwise if I use 50 or 60 degrees the servo arm become very short!


Using this computation here's what the program suggest me.

@ 1.1 cm of travel up/dn as requested by the manufacturer give me a surface movement of only 8 degrees each direction.
The servo arm should be 0.4 cm ??? For sure is correct but is silly
@ 100% travel and speed the servo torque required is 9 oz-in ??? How can it be so low?

In the manual they state to use the further hole in the control horn and the closest on the servo arm, but looks like that will bring too much excursions on the surface!!

Am I doing something wrong?
The only thing I change in comparison to the manufacturer is the style of actuation, they call for an exposed control rod while I am using a partially conceived one by placing the servo arm upside down. Please if somebody have suggestions or see something wrong, tell me!


Thank you
Gianluca

Hi Gianluca,

either measure your servo's angle of rotation so that you are sure, or try the calculation again with 40 degrees of servo rotation. 40 degrees is fairly normal, 45 degrees is unusual unless you turn up the travel % in the transmitter. That should allow you to move the pushrod a bit further out on the servo arm.
It is not unusual to find that ailerons on the scale jets even at very high speeds only need very low power servos. Without doing proper calculations, people just assume you need very powerful servos. I still prefer to fit a lot more power than the calculation says, since the calculation can not deal with flutter and while a small, low power servo can move the ailerons, I don't want it to be overcome by flutter that a bigger servo could resist.
Harry

Hi Harry,

thank you for your reply.
The servos I will use are already much more powerful then what suggested by the calculation (JR 3711HV) and in no way I will use a servo with a torque close to the one suggested.
Is too low and weak in the train and as you say the flutter can increase the torque needed to maintain the surface in position exponentially I guess!
I know that is a good thing to increase the ATV on the transmitter to gain resolution and precision, but is not practical in this application because the servo arm is already too small to deal with.
I think the program is quite precise and correct in the calculation but not being made to take into consideration high G-loads maneuver and flutter we have to introduce kind of safety margin or "k" factor like a multiplier? Maybe 1,5 or 2 could give a good safe margin!

Gianluca

In post #3 of this thread Oli suggests 30% safety factor. I use 50%.

Thanks Harry,

that's great, 1.3 will be good

Gianluca

Gianluca, I have put your numbers through the metric version of spreasheet and get similar result to you. I use servo rotation 40 degrees which gives a servo arm of 0.5cm and a torque requirement of 0.7kgcm, which is tiny. That is very similar to the results I get for my own models. You either have to use a longer horn on the aileron so that you can move the pushrod further out the servo arm, or reduce the travels in your transmitter for example at 30 degrees of servo rotation the servo arm still only needs 0.6cm and torque required goes up to 1.1kgcm. If you can get the aileron horn up from 2.2 to 3.5 it helps a lot.

Harry

Hi Harry,

Thank you for coming back on the subject with your numbers, that is a nice confirmations!


Gianluca

Oliver,

When you use the "servo test" function outside in the sun - how do you avoid having the servos over travel ? usually this function moves the servo to its end points regardless of Tx programming..

Oli,

This is re-posted from the Weatronics 'Gigacontrol' thread as you correctly suggested.

I really should go back into this thread and do my own research, and will do when I have time in the next few days. However, until then here's my issue: I have a Eurosport with 2 x servos (8611s) per single elevon, previously matched using a Powerbox SC-12 and 35/72 Mhz radio. Dave Wilshere recommended this setup as it gave a more 'locked-in' feel of control when he did a review in RCJI for the Eurosport about 4 or 5 years ago (!) Now I am converting to a Weatronics 2.4 receiver which could mean I can remove the Powerbox providing the servo power loads don't exceed the receiver's 10 amp limit. Despite the Weatronics' programming capability to match the servos without the PB, I am now considering reverting to one servo per elevon (as indeed most ES owners have) but would like to have a better understanding of the likely servo mechanical and electrical loadings with that setup. Assuming the loadings are within the servo and receiver capabilities, it would be good to revert to a simpler and lighter setup. Two extra 8611s put the balance point further aft so I have cmpensated with nose-weight and it would be good to shed this if possible.

So, any pointers or thoughts will be appreciated.

Cheers, Andy

Yeah I just go over kill on servo torque so its nice and simple. Always more than I actually need

Please have a quick look at post 1 to 5 so that you know what we are talking about.
Then send me the following information for each flight control:
Average chord, span and horn distance to the control axis in cm, distance from the servo axis to the control axis in cm.
I will compute the data and post it here.

What about your power requirements? How do you know how much current each of your "overkill" servo is going to drain in flight?
How do you choose your batteries, power bus size, wire gages, servo plugs ?

well servo wire is all the same size, the plugs are all the same. it doesn't matter what you plug into the the servo its still only going to take so much. I check my batteries after every flight and the simple answer to your question is to isolate servos from your receiver. I run one pack for the receiver and the servos have there own battery. 7.4 volts is plenty for any application, Ions, A123s, Lipos they can all push out more that anything I have ever flown. 40% and below that is, after 25 years I have never lost anything due to power loss from overloading my setup.
Not bashing all this tech stuff by any means i think it is cool you guys can make sense of all that.

No,
Servos have different cables quality.
We've got many wires from 16 AWG to 24 AWG suitable for our use. That is twice the current capacity.


No. The manufacturer plugs are rated at 1 amp, multiplex plugs are rated at 60 amp, Emcotec plugs 100 amps, Ashlock crimp plugs 5 amps, Henson crimp plugs 1 amp, etc...

Wrong.
The current chain upstream the servo will influence the current drop into the servo.


Wrong. This is the kind of thinking that bring the guys to crash a plane and claim that their 2.4G system locked out.
What really matters is the impedence of your pack. A Lipo with high impedence will see itsdvoltage drop dramatically under load. Especially with overkill servos and undersized wire and plugs. To the point that the servo will not move anymore. Isolating the receiver will not help at all.

Well, now that you are into jets I guess it's time to think about it.
The interesting thing about this hobby is that our airframes push the servos and electrical system available on the market to their limit. Understanding how all of this work and being able to characterize your requirements will save you a lot of hassle and make your plane much safer.
It is not just a "cool" stuff. The purpose is to increase safety and predictability.

OK, Oli, here are the measurements for each Eurosport Elevon:

Average chord: 11cm
Span: 60cm
Horn distance (control axis to ball link): 4.2cm
Distance from servo axis to control axis: 9.5cm

If you can run these numbers and let me know I will also use the spreadsheet and see how you got there. Remember, I'm trying to evaluate use of a single 8611A servo per elevon, rather than the two I have now. I would think 120 mph average and 140mph max is realistic for my flying style.

Many Thanks, Andy

Oli or Harry,

I computed my numbers for the Comp-arf Eurosport's elevon(s) on the spreadsheet: Xs=9.5cm, Yc=-4.5cm, Rs=+/-40, Rc=+/-20, Vmax=150, Cavg=11cm, Lavg=60cm.

Now, the 8611 servo has 220oz/in of torque, and my figures produce a Tsmax of 211oz/in, so there isn't much safety factor at max speed and throw, but then these conditions seem unlikely for smooth, conservative flight.

At a stick deflection of 30%, the torque needed is only 80 oz/in, so do you think this setup is realistic? I haven't yet measured static servo currents at these torques.

Cheers, Andy

Andy, using the metric version I get servo torque required 15kgcm, plus safety factor say 30% takes you up to needing 20kgcm. That is pretty hefty. Can you double check and confirm the average chord of the elevon is 11cm?

H