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Old Nov 10, 2009, 06:49 PM
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eatond's Avatar
United States, MO, St Charles
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1/13 scale Midi F-18 with bifurcated exhaust duct and retracts

With the lessons learned from the last two years of playing with the F-15 (mini fan F-15 , midi fan F-15) I've been working on a midi powered version of a legacy F-18 with retracts. I've spent the last couple of weeks working on the CAD stuff. I've still got a lot of work to do but am far enough along to start posting.

The specs for this bird are:
Length: 51.8.0 inches / 131.57 cm
Wing Span: 37.25 inches / 94.6 cm
Fan: Wemotech Midi Fan
Motor: TBD
ESC: Phoenix HV-85
Batteries: XPS 8S 4000
Scale: 1/13

My goal is to get grass field operation off my club's field where the grass is usually cut pretty short.

The other goal is to do the design completely in Rhino. In the past I did most of the work in Vector XT which is the CAD/CAM program I use. For the hard things I’d use Rhino but I was never comfortable enough with Rhino to commit the entire design to it.

Dan Eaton
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Old Nov 10, 2009, 08:20 PM
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The 3D model

Unlike the F-15 I'm not going to develop a 3D model for this bird. Somewhere along the way I acquired a reasonably accurate 3D model of an F-18B/D two seater. I haven't a clue where I came up with the model so don't ask me for a pointer.

I like the look of the two seater and it's something a little different from all the commercialy available Hornets. I'm planning to go with the Navy Test Pilot School paint scheme to give me a bird I can fly on those days I don't dare put the lo-visability scheme on my F-15 in the air.

Here's a couple of shots of the 3D model and one of the full size.

Dan Eaton
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Old Nov 10, 2009, 09:24 PM
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Good luck Dan! I spent most of the year updating my highly modified plans for the enlargemet of Kevin's F-18 that was an EJF short kit. Hopefully the plans will get published in a magazine, the jet columnist for that mag will be getting a hold of a short kit soon, the parts are getting laser cut for me.

If you want to do grass field ops, I'd suggest a 15 degree angle for the gear behind the cg, mine is about 30 degrees and it makes for long takeoffs due the the large amount of downforce on the tail, a higher speed needs to be obtained. I tried ROG on grass with my Haoye conversion a while back and it couldn't get enough speed, but it was not freshly mowed (I had large tires 2 1/2" in the front and 3" mains I think but the grass was long in spots). With the 15 degree angle it may sit on it's butt with no battery in it but you're worried about flying!

I bet your's will look even better with that cad model.


My favorite setup is the Mega 22/30/2 (with HPI heatsink) in the Midi on 6S 5000 (2 x 3s 6000 taped end to end and running in series). Pulls about 74 amps HOC and 1600 watts. But if you have 8S 4000 packs already I guess you need a different motor. Maybe the 22/30/3 Mega.
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Old Nov 11, 2009, 09:01 PM
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Hi Ed, Thanks. Good luck with that magazine article.

That main gear is going to take some thinking. I need to get it all figured out before I can do the ducting. Luckily, I have a local resource who's pretty good on F-18's.

What airfoil and how much wash out did you use on your F-18? From The Incomplete Guide to Airfoil Usage I got the root airfoil was a NACA 65A005 mod and the tip was a NACA 65A003.5 mod. I found another reference that said the washout was around 4 degrees. Keven has used a TLAR airfoil on his Hornets which looks pretty close to the NACA 65A005. I used the real airfoils with success on the F-15's so unless any body has a different idea I'll probably use the real ones on this bird.

I'm not sure what I'll do for a motor yet. I originally used the Mega 22/30/3 in the midi F-15 but when I rebuilt it I remotored and went with a Medusa 036-060-1300V2. Nice motor and I've been happy with it in the F-15 and would love to use it in the F-18 except of course they had to go and get out of the hobby business. I guess I'll have to try an Arc or else go balls to the walls and put a Nue in it.

Dan Eaton
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Last edited by eatond; Nov 12, 2009 at 09:53 PM.
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Old Nov 11, 2009, 09:56 PM
2014 EDF JET JAM We be Jamming
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St. Louis Intl, Missouri, United States
Joined Jan 1997
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Remember my Super Hornet made its first grass takeoff at your field . Don't over build it....I will remind you of this every 5-6 post to this thread. Set your target weight NOW so that you can start trimming the fat before the first cut.
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Last edited by Kevin Cox; Nov 12, 2009 at 05:31 PM.
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Old Nov 12, 2009, 09:32 PM
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I hear ya Kevin. Guess that means I can't do full hydraulics for the landing gear huh.

One place I will save weight is the formers. On the F-15 I ended up with 1/16 ply formers. On another thread someone suggested substituting 1/16 contest grade balsa covered with cabon veil on both sides for the plywood. They didn't have any numbers on exactly how much weight that saves so I bought some carbon tissue and tried a little experiment. I made a 6" x 6" square of contest grade balsa covered by the tissue and another square the same size out of the plywood. The plywood weighed .9 oz, the composite square weighed .3 oz.

So going with the composite will only weight 1/3 as much if I use the regular ply. Of course, there's other things to consider like toughness. It's no fun to be constantly glueing formers back together while you're building because they break if you look at them the wrong way. I cut a couple of test formers and other than at the stringer notch weak points they held up pretty good. Not as good as regular ply but close enough. Once they are glued into the stringers they should be fine.

The other concern I had was the carbon dust. Cutting regular ply in my CNC mill churns up a fine dust. I didn't want to do the same thing with the carbon. Since the balsa is sealed by the epoxy bonding the tissue to it I found I could just wipe it down with a wet cloth before cutting and that took care of the dust issue.

I was hoping to start construction tonight but the Z axis on my mill shot craps again. Probably be this weekend before I can get anything else cut.

Dan Eaton
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Old Nov 19, 2009, 04:43 PM
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I got the Z axis on my mill rebuilt over the weekend and got most of the formers for the nose cut out yesterday. The delay actually worked in my favor because I had to move the servo from it's original location on to the top of the plate the nose gear retract was attached to. That just wasn't working for me and then I saw what Tam did to his gear in another thread which gave me the idea for what I did in the photo.

Next up is to get the longerons cut out so I can get all the formers in the right place. Should have that done this weekend.

Dan Eaton
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Old Nov 19, 2009, 05:02 PM
Big gov never Works
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Skunk Water, Rhode Island
Joined Jul 2002
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Hey, cool Dan! You snuck this one under my radar. I have been making glassed balsa parts for yrs, in my 1/2A scale stuff. This was before carbon availability. And it does work.

Fuzz
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Old Nov 21, 2009, 12:10 PM
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Hi Fuzz,
Good to hear you got faith in this glassed balsa. So far, so good. I'm working on getting the remaining longerons cut out today. Seems like things are going slow but I'm doing the CAD work, making plans, and assembling the plane all at the same time.

Dan Eaton
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Old Nov 21, 2009, 12:18 PM
Big gov never Works
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Dan, I was doing the composite parts thing in the late 70's/early 80's. I got the idea from doing full size boat work. Then I went to a Fly-in at Winter Haven, Fla in the early 80's. There was Bob Violett using the same thing in his jets. He later went to using end grain balsa and carbon cloth. Called it Magna-Lite. Great minds think alike, I guess.LOL

But was used for yrs in boating. You can bolt to the stuff and weight loss is very high. Light planes bounce better...

I'm sure you will succeed in your design!

Fuzz
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Old Nov 25, 2009, 10:22 AM
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I got the front end parts put together. I created a jig from foam to support the side longerons since I know they are suppose to be straight. Once I had the pieces fitted together I got out the laser level and shot it down the centerline. With all the formers were lined up I pinned it down to the foam and then started hitting the joints with CA. Those composite 1/16 formers are still a little delicate for my taste at this stage but once the sheeting is on things should firm up nicely.

Dan Eaton
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Old Nov 25, 2009, 10:41 AM
Fast Jets for Fast times
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St. Peters MO.
Joined Nov 2007
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Dan,

I really like your foam build support jig. I guess parts could be lightly spot glued to keep in place and removed later as well. You might be able to try that glass melamine in some areas of high stress. Great Cad work and ideas.
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Old Nov 25, 2009, 03:10 PM
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Nice Jig you have there..... the framing looks good.

Gene
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Old Nov 25, 2009, 03:18 PM
DELTAS RULE
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tehachapi, CA
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looking great so far! i love scratch building. sure gives you a sense of accomplishment when its done. i dont do as much as i used to, but i still enjoy it.
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Old Nov 26, 2009, 01:14 AM
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United States, MO, St Charles
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Quote:
You might be able to try that glass melamine in some areas of high stress. Great Cad work and ideas.
Hi Jeff,
Thanks, and thanks again for that glass. I'm sure I'm going to be burning up some cutters on it.

Quote:
Nice Jig you have there..... the framing looks good.
Hi Gene,
Thanks. Check out the nose cone. I think you'll find that process interesting.

Quote:
looking great so far! i love scratch building. sure gives you a sense of accomplishment when its done.
Hi CN,
Thanks. Yep, one of my daughter's boy friends was commenting how he could never build one of these cause he just didn't have the patience. I told him I was the same way. I just don't have the patience to wait for someone else to make me exactly what I want or the patience to wait for the 12th of never to be able to afford it. It's nice to be in control.

In the mean time, I got part of the left side sheeted and the left LEX sheeted. You can't really tell it from the picture but I'm really happy how well the LEX turned out. It really captures that cobra hood look on the real bird.
I also made up a mockup of the canopy to get a better feel for things.

Since I started sheeting I also have to think about getting the nose cone done. I chopped the nose cone off the 3D model, split it in half, and then generated the g-code for each half to feed to my mill. I had this chunk of machinable wax sitting around so I turned the mill loose on that. The first photo shows the end of the roughing pass where most of the block has been chewed away. The second shot things at the end of the finishing pass.

Not sure what caused those ledges but they'll be easy enough to fix. I took a file and smoothed out the scallops left from the milling and other than the ledges it looks pretty good. I'll probably get the other half machined Friday. I've been inspired to give mold making / fibreglassing a try by the Super Bandit thread so I bought a 50 pound bag of Hydrocal yesterday and figure this nose cone is a good place to learn.

Dan Eaton
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