Flight controls efforts simulation for the FEJ F-18F
 

Hello guys,

I have made a simulation on flight controls setup with Craig's woksheet.

Some specific points:

I have entered a max speed of 200 mph ( 100 mph for the flaps ).
All control surfaces dimensions have been meticulously measured.
The elevator surface have been computed as followed. The surface forward of the control axis has been substracted from the surface aft of the axis.

The results:

Optimized aileron setup:

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

Note that the optimum servo arm length is 0.9 cm. The maximum torque required at 20 degress of aileron deflection is 97 oz/in

I have simulated a non optimized setup that reqires 220 oz/in at 20 degrees of deflection ( JR8611 ).

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

A servo arm length of 1,4 cm is the maximum allowable for the JR 8611

Optimized flaps setup:

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

Servo arm length: 1 cm. Maximum required torque at 35° deflection and 100 mph: 195 oz/in.

Non optimized setup:

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

Note that the maximum allowable servo arm for the JR 8611 with a 35° flap deflection at 100 mph is 1,1 cm

RudderOptimized setup:

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

The optimum servo arm length is 1,4 cm. Max torque required at 200 mph and 25° deflection: 142 oz/in

Rudder non optimized ( pull-pull connection with 2,5 cm servo arm as recommended by FEJ)

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

The JR 8411 stall limit si reached at 10° of deflection at 200 mph !!!

Elevator optimized setup ( non mechanical pull-pull setup as explained in the F-18F thread )

http://www.geohei.lu/olin/data/model...%20optimum.jpg
The JR 8711 limit is reached at full deflection at 200 mph. At 50% stick deflection ( more realistic ) at this speed, the torque required is 312 oz/in. Within JR DS8711 specifications.


Non optimized setup ( 3,6 cm pull-pull setup as recommended by FEJ ):


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

The 8711 maximum capacity is reached at 29% stick deflection at 170 mph. This is not satisfactory.

I strongly recommend to all 1/7th scale F-18F the following:

Fully balance your elevators.
Use the JR DS8711 as a minimum.
DO NOTUSE THE FEJ RECOMMENDED PULL-PULL SETUP.
Use instead the setup that I have recommended in the F-18F thread ( servo arm length 1,5 cm or smaller in the pull-pull configuration with attachement points offset ).

For the other controls, set your servo arm attachement point as close as possible to the servo axis ( check the maximum values above )

Guys, please check the original file and play with it. All suggestions are welcome...

http://www.geohei.lu/olin/data/model...20controls.xls

RE: Flight controls efforts simulation for the FEJ F-18F
 

LOL where have I heard that before :-)

Seriously, great post, this spreadsheet looks as good or better than the one I have been using (which is not free).

Does this software take slop into account? I like these tiny servo arm lengths for a lot of reasons but it seems to magnify the effects of slop.

Thanks again for the great post.

RE: Flight controls efforts simulation for the FEJ F-18F
 

LOL sorry for the suggestive sentence http://www.rcuniverse.com/forum/js/f...ey/msn/lol.gif ...

You're perfectly right Matt. The short arms do "amplify" the servo slop. It is very easy to understand. A 0,05" slop over a 1" inch travel is 1/25 in ratio. A 0,05" slop over a 1/2" travel is 1/10 ratio and become not so negligible. In that case, the slop on the control induced by the servo will be doubled compared to the previous setup.
This is why I mostly use JR DS 8611A for most of my big planes. Only a couple of $ more expensive than the DS8411, almost the same size, and much more powerful with no slop.
This freeware does not take the slop into consideration in this page. However ther is a "buzz" page that computes the flutter resonnance. Have a look at the complete spreadsheet here:
http://www.geohei.lu/olin/data/model...calculator.xls

Courtesy of Craig Tenney.
The spreadsheet was originally posted on the Michigan State College of Engeneering and I assume that he was a student there. Unfortunately have never been able to reach him...

RE: Flight controls efforts simulation for the FEJ F-18F
 

The difficult part of this simulation is to find the correct elevator surface. The software only take into account a traditional control surface behind the hinge. So I had to compute an equivalent surface.
I first ended up with a value of 20x30 cm but then discovered that the center of lift of the control was close enough from the rotation axis to use 15x30 cm ( check the values in the non optimized elevator setup ).
Guys if you find something better, let me know.

The main purpose of this exercise was to show that the conventional setup for the elevator ( symmetrical pull-pull setup ) is reaching the DS8711 limits extremely quickly.

As a matter of fact, with these sizes of jets and no matter what brand, we are reaching the limits of the best servos available on the market for integral elevator control.

My conclusion is that the elevator system setup is critical. So is the battery system that will drive these big consumers at full load.

I have had a report of a person crashing a big F-16 due to an elevator failure. This is the reason why I did this computation.

Once again, here is the setup that I use on the F-18F:

http://www.geohei.lu/olin/photos/Mod...%20neutral.jpg

The binding is controlled by offsetting the servo side application points of the ball links forward of the rotation axis by 15° ( not shown on this picture ). This needs a specific aluminium servo wheel available here:
http://www.airwildpilotshop.com/More...uct&id=288
This product is called Uni-hub and is perfect for the purpose. Drilling/tapping required.