In my previous post about my new Xplorer Mark 3 I detailed how I setup the wing for pushrod exits. Now that I have each of the exits cut, the linkages measured, and the servo frame locations set it is time to glue those frames onto the inner skin of the wing. Let’s talk about a Xplorer Mark 3 servo install!
Each step in this build, especially when it comes to the wing has been one where once undertaken cannot be undone. It really isn’t more evident than gluing in those servo frames. Once cured those frames are impossible to get out without some major surgery. Before gluing I made sure to check and recheck all the linkage geometries. I also make sure that I took measurements on one side and applied the same distances on the other side of the wing. This way the flight surfaces should have approximately the exact same movements mirrored one either side of the model. Hopefully that will help lessen any time working out inconsistencies in the radio program later.
Prepping The Servo Bays
Gordan Buckland in his lastest article for the RC Soaring column in the May 2016 issue of Model Aviation went into the importance of zero slop flight surfaces. Within his article Gordon specifically called out Jody Miller’s Xplorer builds as being one of the most precision setups he has ever encountered. I decided early on that following Jody’s advice for installation in every way I could for the Xplorer Mark 3 servo install.
One of the things Jody told me was that as the Xplorer has gotten lighter over each of its iterations is that the skins have gotten thinner. As the servo is attached directly to the skin it makes it possible for flex to be introduced under high loads on the flight surface. To combat this possibility Jody adds a small patch of 5.6 ounce, or similar weight, carbon fiber cloth in the area where the servo frame will be attached. In the above picture I measured out 5.5mm by 6mm areas to be placed into the servo bays. The dimensions came from measuring the area where the servo frame would be plus enough to go out to some sub-ribs.
These sub-ribs are a Jody specialty. They previously appeared in my last article but I really didn’t go indepth about their purpose.
The sub-ribs are cut pieces of Rohacell foam shaped in the same fashion as the airfoil. They’re designed to go to either side of the servo bay to tie the upper and lower skins together for the least amount of flex. The carbon cloth will go across the bay and up the inside of each of these ribs. This adds additional stiffness.
My cuts were off a little bit, but in the above picture you can see the cloth go up the side of the sub rib. In the previous pictures of the cloth there is blue painters tape applied. I tried to make a square with the tape so that when I cut there would be an amount of blue tape going around the outside of the cutout area. This would make it so that the weave wouldn’t separate. Much to my chagrin pulling the tape off also took a generous amount of the outside carbon fiber with it. On the ailerons it meant that I couldn’t go all the way up the sub-ribs. On the flaps I applied what I learned on the ailerons and made my cuts a little bigger. When I pulled tape off that and accounting for the part of the weave that was left stuck to the tape the cloth went up the entire side of the rib.
I should have taken a picture of the servorahmen servo frames for the MKS HV6110. The picture on their website differs in what I actually received. My servo frames had a lip on them extending out maybe 10mm. With the frame sitting in the bay it was a few millimeters away from the bottom skin. I thought that if I could tie that lip to the skin that the whole structure would be very strong and much less prone to flexing. What I ended up doing, and this is partially visible in the picture above is cut out a notch in the sub rib so that the lip of the frame would go into the foam. This tied the rib,the frame, and both skins together into one structure. Post curing this whole area is solid like a rock!
Paul Naton in his F3X build video uses JB Weld to glue his frames directly to the upper skin. I’ve seen my dad use JB Weld to fix a crack in a motor block. That stuff is seriously tuff! Since I had to laminate in the carbon cloth I didn’t want to mix the laminating epoxy with something else. On Jody’s suggestion I used EZ-LAM by ACP Composites to wet out the cloth.
I then used the most horrible epoxy filler I’ve ever had the pleasure of working with, cab-o-sil (fumed silica), to thicken up some of the same epoxy to bond the servo frame to the cloth/skin. In the Florida humidity the cab-o-sil clumped up. I still was in long sleeves with a dust mask and googles so that this stuff wouldn’t get on me. Just opening the container sent the stuff all over the place! When, and oh boy when, I win the lottery I’m going to have an air-conditioned room where I can do this type of work!
It took a good amount of that cab-o-sil to make the what was otherwise as thick as water epoxy to get to a peanut butter consistency. When looking at the mixture my first thought was that it wasn’t going to be nearly as strong as it should be to hold in those critical components. Boy was I wrong! That stuff is very, very tough when cured! I pray I never have to get those frames out of that wing! It’ll probably be easier to order a new tip or center section!
After sanding the bottom of the frames and applying the epoxy mixture to them they went into the servo bay. On the ailerons I applied epoxy to the upper and lower side of that lip on the frame. I also tried to get the epoxy up and over the frame in locations where I could reach around the servo.
The same procedure was used for the flaps. After everything sat for 24 hours it was ready to test out. At the time of this writing I’m still waiting on the flaps to finish curing, but the ailerons are finished. I have never had another RC model with a flight surface as tight as these! Jody knows his stuff!
I still have the tiniest amount of play on one surface. I can directly attribute this to the clevis and pushrod setup. The other tip had absolutely none. I did a basic test of the flight surfaces with my calibration radio program to make sure all the geometries and clearance were correct. I knew that putting together the linkage now for basic testing would get me close but would need to be re-done when the whole wing was on the plane. After the final tweaks in the completed configuration I’ll apply CA to all the different points along the linkage route from the brass horn, the threads on the pushrod, to the servo arm. When fully cured that should eliminate that tiny slop.
Here’s a quick video of the finished product!
After the tidying up of the servo bays in the center panel to remove excess epoxy I’ll have completed the most stressful part of a RC model build that I have ever encountered. In my next post I’ll cover the installation of the servos which drive the tail control surfaces. Once that is completed I should be ready to install the battery, receiver, and prep for the first hand launches. If all goes well I’ll have a world class “J” plane flying by the end of the week!