Skymaster F-18 build thread
 

As I've been building my Skymaster Bug, I came across a few things that were not very clear and thought I'd help out new builders with a few tips, pictures, and ideas. I have the primer grey version and the pics show a lot of rough spots from my own sanding/filling, so please ignore - the quality of the glass work is great as is.

I am finishing my Bug in Marine low-viz grey (from a TBD local example, thanks Buck for the support!) and hope to have it flying by the Waco jet event in September.

To start, here's the aileron servo mounting I used. The wing is very thin at the tip and is just thick enough to fit a JR 3421, but not thick enough to use a traditional servo mount. So I decided to use the "hatch servo mount" method. The aileron hatch is not strong enough as is, so I vacuum-laminated 3 layers of 1 oz. unidirection carbon using West System resin, with each layer at a 90 degree bias to each other. After curing, I scuffed the carbon and Aeropoxied some maple blocks to the hatch.

I also cut some vertical grain 1/2" square hard balsa blocks between the top and bottom wing skins using Aeropoxy - making sure the skins were scuffed with 100 grit paper and cleaning with acetone. These blocks are also used to mount the servo/hatch; after drilling 1/16" pilot holes, I threaded the holes using 7/16" long allen-head servo mounting screws (from Dreamworks R/C), then removed them and "hardened" the threads with thin CA. I then repeated this step.

Afterwards I routed out the servo arm slots (I'm using the JR heavy duty nylon arms, #JRPA215) in the hatch/mount using a drill press and a small Unibit. I'll document this process later, it makes a painful task (for me, at least) a snap. I also used a 3/8" Dremel sanding drum to open up the edges of the access hole to clear the mounting blocks, etc. where needed.

RE: Skymaster F-18 build thread
 

Please keep it coming!!!!!!:). I am working on my too!
David
OKC

RE: Skymaster F-18 build thread
 

Next are the flap servo mounts...here I used analog JR5131 servos, as I don't see the need for a big digital servo for this application. I decided not to use the standard Skymaster rod-drive method and opted to mount the servos in the wing. I used a large Dremel cutoff wheel to cut the aluminum drive rod flush with the root edge of the aileron, and glassed over the drive slot opening in the fuse with some 2 oz. glass from the inside.

The flap servo access hatch is about 2" wide and 2.5" long, and is located about 4.25" up from the hinge line and 2" in from the wing root. It is angled such that the servo is perpendicular to the hinge line. I cut out the hatch as usual using an X-Acto knife and hardened the edges of the opening and hatch with thin CA and sanded slightly using a fine grit sanding block for a good fit.

I decided to mount the servos to the inner (top surface) wing skin using RJRC (Rob Janiger) servo mounts. A 4131/4721/DS8411 servo will fit flush with the bottom surface of the hatch if you use a 1/4" thick balsa or ply spacer, basically cut out to the size of the access hole, with a slight fore/aft taper in thickness sanded into it (about 7/32" thick at the rear edge, 1/4" at front). The servo mounts are Aeropoxied to the balsa block, which is then Aeropoxied to the wing skin (after scuffing and cleaning, of course!). The hatch does not need reinforcement as it is not load-carrying, so I just glued in some 1/8" ply triangular mounting plates to the corners of the hatch opening...these are not shown in the photos.

(Note - I did not have the RJRC servo mount bases on hand so I just used his brackets and hardware, and Aeropoxed some maple mounting blocks to the balsa spacer. If you do use his mounts, I think a 3/16" spacer plate would be required, as the plastic mount base is about 1/32" thick).

RE: Skymaster F-18 build thread
 

Next on to the rudder servo mounts. The fins are extremely thin, the only JR servo that fit was a DS168. This has a somewhat unique mounting method that is a bit different than usual, using three tabs that are mounted midway through the width of the servo case.

I first epoxied in some 1/8" triangle plates in the corners to mount the hatch, then used a Dremel sanding drum to open up the access hole as needed for clearance. To mount the servo, I cut some 3/4" long by 5/16" wide mounting mplates from 1/8" ply. I then used some vaseline to coat the servo bottom and sides to make sure the servo did not get glued in (been there before [:@]) and mounted the blocks to the servo using the short screws supplied with the servos. Note that the mounting holes need to be a bit off-centered for the blocks on the sides of the servo - this will make sense when you have the servo in hand.

I then Aeropoxied the servo mounts in place (again, scuff and clean the mounting surface first!). I found that the hatches were too thin to fit flush with the outer surface of the fin, so I cut some 1/2" wide triangles from 1/32" ply and Zapped these to the inside corners of the hatches. I then cut the servo arm holes in the hatch, drilled the servo hatch mounting screw holes, and mounted the hatch.

RE: Skymaster F-18 build thread
 

The biggest pain so far was mounting the stabs and stab horn/clamp and marking the spot on the stab rods to drill for the 4-40 bolt that prevents the stab horns from rotating out of alignment. This step really kicked my ass because my hands are too big to fit up the tailpipes, and there is not much room to work from the inside through the engine cover hatch. I finally came up with a quick way to keep all the "bits" (nylon washer, aluminum control horn/clamp) in alignment with the pivot blocks while you insert the stabs. Good thing, because the cat and my wife were both about to run away! ;)

I did not take a lot of pictures but here's the process....

First locate a 3" to 4" long piece brass tubing with the same OD as the ID of the brass pivot blocks/bushings, and round the outer corners of one end. Next, use some rubber cement (or similar) to tack the nylon spacer to the side of the horn. Use a large hemostat (or a pair of long needle-nose pliers with a rubber band around the handle) to "grab" the horn; angle the horn at 10-15 degrees off vertical, and with one hand carefully move the horn into the "pocket" in the stab pivot block structure. With your other hand, take a 5" or so long small Phillips screwdriver, Balldriver/ Allen wrench, small brass tube, etc. and insert it from the outside of the fuse through the stab pivot bushing, being careful to make sure you go through the ID of the horn, and then continue through the inner pivot block/bushing. Remove the hemostat. Pushing up on the screwdriver, get all of the "bits" to align against the top of the shaft, and using the other hand, carefully insert the brass tube from the inside and slowly push it through to the outside of the fuse.

Whew! Now you just slide the stab in, pushing the brass tube out...everything will now stay in alignment. Trust me, much easier than it sounds, but this was a toughie to do without this trick. It will probably take a few attempts to get everything right - making sure the horn is oriented and aligned the correct way, etc.

The idea of all this is to clamp the horn in alignment with the stab at 0 degrees so you can drill the stab rod for that pesky 4-40 bolt. This is where my fat hands became a problem...I just could not reach in from behind to get a Allen wrench in far enough to completely tighten the clamp bolts. So I cheated - I drilled a couple of 1/4" holes in the bottom of the fuse on each side, to allow a standard 3/32" balldriver to reach. Basically the holes should be about 1/2" in front of the centerline of the stab pivot and in just enough from the sides so that they cannot be seen from the side. You could use a very long 3/32" ball driver to reach in from the rear but I could not get enough torque this way; reaching in from the bottom allows a 6" long T-handled wrench to be used.

To get the stabs aligned properly with the horn, I used a panel line on the fuse that conveniently runs through the centerline of the stabs and taped them in alignment, aligned the horns (i.e. at 90 degrees to the stab centerline), and then used a long 1/16" drill bit, through the holes drilled in the fuse bottom, to mark the stab rods. Make sure the stabs are fully inserted before you do this! Then just disassemble the whole deal and use a drill press to drill at the marks (be carefull to drill the holes straight and centered)...and go through the whole monkey-motion again to re-install the stabs and horns. :( Use loctite on the screws.

Next, install the stab servos - you'll need a long 1/16" drill bit to drill the pilot holes for the servo mounting screws. BTW, the allen head servo mounting screws were MADE for this application! (Note that my servo is installed BACKWARDS in this photo). I decided to replace the stock hardware with 4-40 rods and Sullivan clevises, soldered on one end. FWIW, the length overall is 4.25" from clevis pin to pin.

RE: Skymaster F-18 build thread
 

Nice job Dbarrym! [sm=thumbup.gif] I know Todd W. is currently working on the manual for the F-18, but it will take some time to complete.

I found some of the same issues with the Gripen. The ONLY servo that would work for the rudder was the DS 168. I actually made a mount for it and Hysol'ed it in. It more or less is the same exact concept as your mounting except I may have slightly more glueing surface. The elevons servos were also an issue but I used traditional mounts anyway. They do stick out slightly, but it's not too bad.

Keep up the good work, maybe I should do one for the Gripen :D.

George

P.S. Make sure you look at the gear plates and surrouding area.

RE: Skymaster F-18 build thread
 

Barry,

Great job! The construction tips you gave really helped. We understand that the Skymaster instruction manual may be a bit too simple since it was something we put together in a rush. As George mentioned, Todd is helping us out to revise the instruction manual. The new manual should much clearer than the current one. Thank you for helping Skymaster make improvements to our manual. Please continue to post more tips for us, because we always appreciate the help.

George,

Besides use the JR DS168 for rudder, Multiplex Super FL Digital servo (5363) is also another option.

RE: Skymaster F-18 build thread
 

Thanks Eric, happy to help! I am still missing the nose wheel (only have one) and the bypass and I am ready to mount them...hate to beg but pleeze could you send them?

It's a great kit. One thing I need to think through is how to retain the fuel tanks, other than just Gooping them in. How do you keep the nose tank in place? For now, I have just used some foam to wedge it in at the top , along with a Velcro strap at the bottom.

Barry

RE: Skymaster F-18 build thread
 

Barry,
It send yesterday by Priority. You should receive it soon. Sorry for the delay.[&:]

For retain the fuel tanks in place , we usually use a Velcro strap with some CA glue.

RE: Skymaster F-18 build thread
 

Thanks Eric!

I posted this in another thread, but makes sense to do here as well.... here's some notes on mounting the scale nose gear (i.e. strut) using the Robart retract unit...

I had to trim the front edge of the nose gear mounting plate back about 3/16" to provide clearance for the steering arm, to move the retract back a bit so the nose wheel(s) would clear the front of the nose gear opening.

I also fabricated a simple bulkhead mount from 3/4" high by 1/8" thick lite ply (it is non-load bearing) for the rear nose gear following support strut - here's some pics that make this clear.

Barry

RE: Skymaster F-18 build thread
 

When I started this thread, I did not mean for it to become a Skymaster ad...however, I just used a Handi-Bond CA product (Skymaster's parent company) and wanted to post an example of a great use for the Super CA Gel...a tube of which was included with my kit.

I was gluing some flap hatch mount ply mounting tabs in the wing with epoxy and was frustrated when a couple of them would not stay put. Out of frustration I remembered the CA gel and figured I'd try it..I've used thick CA to do this before but was not happy with the results.

The gel makes it easy - just apply to the tab, hold in place for a couple seconds, use a scrap of thin ply to clean up any excess at the hatch edges, and clamp for a couple minutes. I had both wings done in 5 minutes, waited an hour and did a pull test - they are in good!

Thanks Danny and Eric - I'll be using this stuff again.

Barry

RE: Skymaster F-18 build thread
 

Observation about the Main Gear retracts: I'm just getting ready to mount the retracts and notice in the "up" position (wheels inside airframe) that the main wheels hit the wood plate inside and won't allow the retract to "lock" in the up position. It appears I will have to add a plywood shim to the retract mounting plates to raise the retract to allow the wheel to fully go up into the fuse and to allow the retract to lock.

Anyone else see this happening?

Thanks

Nick

RE: Skymaster F-18 build thread
 

George,
The photo show wrong way to hook up the ball linkage to control horn.
Change it out before it will snap.
Tam

RE: Skymaster F-18 build thread
 

Thanks Tam for your input and experience with those horns breaking. I'll tkae care of it.

George

RE: Skymaster F-18 build thread
 

I love ball links, but I have to agree with Tam, that type of installation put a large torqueing moment on the horn.

I like to install 2 horns and put the ball link in between. Called a double truss..BVM horns. Works nice.

RE: Skymaster F-18 build thread
 

OK, here's a couple pics of how I use a small diameter Unibit (stepped drill bit) to rout out the servo arm slots in the hatch covers. FWIW, I have been using the same set of Unibits for at least 15 years now and would not be without them - many, many uses, especially for cutting clean holes in thin ply and composites. They work by "slicing" from the side as they rotate, allowing them to be used as a router bit when mounted in a drill press or hand-held drill. In this application, a drill press is needed.

First, mark the area to be cut out on the hatch...I mount the servo with arm in the hatch and move the arm through it's range of motion, marking the fore and aft limits and adding about 1/16" of clearance on each end. Draw a line through the centerline of the slot.

Next, I adjust the table of the drill so that I have about 1/2" or so of clearance between the bit and the hatch. Next, set up a fence on the table (I use a piece of 1/4" ply) and clamp it to the table such that the drill is centered on the long axis of the cutout. To make the cut, use your left arm to pull down on the drill press handle, while firmly holding the hatch against the table and fence with your right hand. Drill down through the hatch on one end until you reach the correct slot width. Now the critical part - slowly push the hatch along the fence, carefully keeping the hatch tight against the fence and table, until you cut through to the other end. Note that you should always cut with the piece moving away from you, as the rotation of the bit will help to keep the hatch tight against the fence.

That's it! In this case, I aligned one edge of the slot with the edge of the hatch, note the clean semi-circle at both ends of the slot. Hard to do by hand.

Barry

RE: Skymaster F-18 build thread
 

Looks Good Barry!

RE: Skymaster F-18 build thread
 

Yes, me.

I have the same problem, and don't know what to do about it...

Anyone got a solution to get the mains to lock in the up position?

RE: Skymaster F-18 build thread
 

The way that my friend has fixed it is to cut a whole the plate that the wheel hits. Do bear in mind that the wheel needs t go about an inch further up in order for the lock to engage.

RE: Skymaster F-18 build thread
 

I don't think that is possible on mine as I would have to cut away a lot of the supporting former as well.....[&:]

RE: Skymaster F-18 build thread
 

Thanks Kenny! You are kicking bottom on your giant Bug build.... you'd have a whole squadron of these SM Bugs in the air by now! Going to be interesting at Fresno, lots of -18's there.

I haven't figured out the main gear locking issue, I did not notice this when I mounted mine, but I did space them up on an incline (tapered 1/8" ply spacer) to clear the plate but did not check to make sure they lock....yet!

Finished all the hinging yesterday, had to trim about 3/32" from the rear edge of the fins to allow adequate throw for the rudders (about 5/8" each direction), and maybe 3/16" on the bottom of the wing for the flaps.

The old trick of using some 150 grit sandpaper wrapped around pieces of dowels of varying thickness was very handy for cleaning up the mating faces of the wings and fins, allowing a better fit of the control surfaces with less rubbing.

Barry

RE: Skymaster F-18 build thread
 

I think I'm going to have to get one of the new F/A-18E super hornets when they're ready, although a Blue Angel F/A-18C sure has charm too.

RE: Skymaster F-18 build thread
 

I also think a BA F-18 is neat. That's why I've got one coming! Can hardly wait to get it flying and get a jet going again.

RE: Skymaster F-18 build thread
 

WELCOME BACK TONY!!!!!!!!! will you be at Fresno??

Ralph

RE: Skymaster F-18 build thread
 

I am gonna repaint my BA F18 with rivets and decal from Aeroloft. Just couldn't live with the seam lines. The cockpit I got from Tam looks good!
David