Building a DUELIST MK 2 From Plans!
 

After building many kits over the years I have decided to try a Twin. I have two nice O.S. 40SF engines sitting around and decided a while back to focus on the Duelist as a good twin to start. After looking at the Pica Kit and all the odd thrust and incidence angles inherent in the MkI design I decided I'd purchase the M.A.N. plans and build the MkII instead. This will be my first plans build. It's designed with 0,0,0 and reportedly flies just as well as the PICA version, only faster! It also flies better inverted.

The plans came after about a month(!) rolled in a sturdy tube with a photocopy of the original build article that was essentially some comments about the history of Dave Platt's design and, though interesting, not real helpful to a builder. The first thing I noticed was that while the plans were very well inked and professional looking, all the rib and former templates were nested atop one another. The only way I could think to make templates was old-fashioned tracings - ugh, what a pain...

Next I noticed there was no materials take-off so it took several hours to make the list, then convert it into something I could actually buy from a wood supplier. I ordered my wood from Balsa USA and it arrived promptly, well packed, accurate, and most important, the quality was excellent.

RE: Building a DUELIST MK 2 From Plans!
 

While awaiting my wood to arrive I set about the dreaded task of making tracings of all the ribs and fuselage formers. I traced using a mechanical pencil for obvious reasons and it didn't take as long as I thought it would. However, when I went to make photocopies on my fax machine the pencil didn't get picked up so I had to darken all the lines with a pen.[:@]

Ok, with that problem licked I ran the originals through the fax machine and they photocopied beautifully! Then I checked them against the originals in front of a strong light and discovered large distortion in the top to bottom direction on the copies.[:@]

Ok, took 'em to Kinko's who have a bit fancier copier and voilla, problem solved!

Next I cut out each rib and former and arranged them onto my sheetwood in the most efficient configurations I could think of. While holding them on the wood upside down, I transferred the photocopied image by dipping my finger in lacquer thinner and rubbing the image. Finally, a technique that worked great the first time I tried it.. It even worked great on plywood.

RE: Building a DUELIST MK 2 From Plans!
 

I decided to start with the fuselage first. Using a sharp Exacto knife while watching TV I managed to cut out all my bulkheads in short order. The 1/8 ply bulkheads were cut using a scroll saw. Some folks swear by their rotary sanding tables for this operation but I find it takes too much material away when sanding balsa shapes and creates more work and clean-up than its worth.

Next I laid out my pushrod arrangement through the bulkheads. In the past I've kinda fudged this, often waiting till the fuse was framed to install flexible plastic rods. This time I plan to use carbon graphite rods for better precision and less slop. Carbon doesn't like abrupt bends so I drilled holes where they will intersect each bulkhead. I ended up having them cross near the tail so I could keep them inside the fuselage as far back as possible.

Next, I mounted each bulkhead on a straight board. (Kinda brings new meaning to the term Building Board.) The tough part was keeping the bulkheads straight while gluing them down to the board. After laying out a centerline on the board with a station line for each bulkhead I drew a vertical centerline down each bulkhead and used a laser-level to verify. I had just enough room to slip a triangle behind each bulkhead to get that third and final dimension straight. Hey, this just might work....

RE: Building a DUELIST MK 2 From Plans!
 

It's nice to see a plan build.. I'll be watching.. I've had this one on my list for awhile..

RE: Building a DUELIST MK 2 From Plans!
 

Hi Sscherin,
Yes the Duelist is quite an airplane. It reminds me of the pattern ships of the '70s converted to a twin. Other than the rather involved fuselage layout she builds much like a Kaos, Dirty Birdy, etc. I love the shape of the fuse and consider it worth the extra effort to have such a streamlined shape.

In order to complicate things as much as possible I've decided to go with pneumatic retracts, probably Spring-Airs.

The one nit I have with the looks are the engine nacelles. Platt designed them as basically a plywood shoe box glued to the outside of the wing. He did it to make things easier on the builder I guess. After studying some Royal plans of the B25 and other multi's I think I might have come up with something a bit more graceful. We'll see.

One thing everyone agrees on: This is a fast airplane!

RE: Building a DUELIST MK 2 From Plans!
 

Here is a picture of a Pica DUELIST I found on the internet. The builder was certainly going for speed with this rig. Note the rear-facing exhaust engines and tuned pipes!

RE: Building a DUELIST MK 2 From Plans!
 

i'm watching for sure. have fun.

david

RE: Building a DUELIST MK 2 From Plans!
 

Welcome to my thread David. I've been doing some structural stuff over the weekend. I've attached some progress shots of the build. The bones of the fuselage are starting to take shape. Reminds me of a dinosaur skeleton. Although this is my first plan build its going pretty smooth at this point. Not much different than building from a kit. Having each bulkhead perfectly symmetrical at this stage isn't critical but you do have to emphasize overall straightness - I've never built a banana yet and don't plan to start with this plane. I really like this upright building method. It opens up the possibility for much more intricate shapes in the future.

You might have noticed the Titebond wood glue in the background. I rarely use CA glue for anything more than tacking parts together now. The CA is so brittle parts can be snapped off and repositioned. After I'm satisfied things are straight I "roll" a generous fillet of Titebond at every joint with a toothpick. It dries in 15 minutes, makes for a MUCH stronger airframe and the wood glue is $8 a pint (!) at home centers. Heck, they even sell it by the gallon for not much more. If your into giant scale there's some serious savings...

RE: Building a DUELIST MK 2 From Plans!
 

Hi CrateCruncher,

Very nice pics - keep posting! I have the plans myself, and intend to build one of these one day. The pics make my fingers itch - building the fuselage looks simpler than I thought.

I'll keep my eye on this thread!

KrisG

RE: Building a DUELIST MK 2 From Plans!
 

CrateCruncher,

This is perfect, I'm going to build the same plane as you are..... :D

I'm almost finished building an Goldberg Extra 300, as soon as it is finished I'm going to start my DUELIST MK 2 From Plans also.

I'm looking forward to your pictures and advice.....

Thanks
Ulf

RE: Building a DUELIST MK 2 From Plans!
 

I also have the plans for a future build. Watching anxiously!

RE: Building a DUELIST MK 2 From Plans!
 

I'm glad to see such a healthy interest in this subject. I started the thread over in the scratchbuilding forum but didn't get much interest. I usually hang out in the scale and pattern forums but a Duelist really doesn't fit there either. With the production of interesting kits drying up its only a matter of time before kit builders are going to transition to plans building so....

...Onward through the build. I dropped by my LHS yesterday and bought my pushrods. I didn't care for the stuff available so I came up with my own. It consists of a piece of antenna tubing and a 1/16" piece of music wire. I will eventually solder a threaded connector on each end. I may still go with carbon fiber rod to save weight. Either fit. It's important at this stage to get the guide tubes in place and the pushrods moving freely with no resistance. The holes I drilled in the bulkheads earlier were suprisingly accurate. I did elongate 2 holes slightly and it made a noticeable difference because the antenna tubing clearance is much less than pushrod tube. Just eyeball it and look for humps.

RE: Building a DUELIST MK 2 From Plans!
 

Without any build instructions its difficult to know when the best time is to remove the fuselage from the board. The tail still feels a bit flimsy. One step I had been dreading was adding the fuselage bottom. I finally decided to leave the fuse on the board. I kept the wing trailing edge bulkhead and tailpost glued to the board and sawed away everything else in between. I then slipped the fuse bottom in and glued everything with Titebond. Now that wasn't so bad..

RE: Building a DUELIST MK 2 From Plans!
 

CrateCruncher

I was looking at the plans and I was wondering if you shouldn't have installed the fuslage sides first before removing / cutting the tabs on the bulkheads and when that was dry install the bottom last.

You are right, it is very hard trying to figure out what the proper building sequence is....

Keep up the good work!
Ulf

RE: Building a DUELIST MK 2 From Plans!
 

Hi Ulf,
I thought about doing it that way also. In fact it might be easier than the way I chose. The clue I used was the cross-section on the plans which shows the side skins on the outside of the bottom piece. From that I figured the bottom had to go on before the sides. However, if you left the skins unattached from the crutch line down to the bottom piece you could do it your way and still wind up with the same cross-section. In any event I now have my side skins complete and will attempt to skin the fuselage tomorrow - then I think I can safely remove it from "dry dock".
Mike.

RE: Building a DUELIST MK 2 From Plans!
 

Long day in the Duelist Werks. I sat about sorting the wood for my fuselage side skins. I wanted the lightest in the tail and the densest stuff at the wing saddle. Next I pinned and glued the sheets together with thin CA. I used CA because I will soak the skins and wood glue could soften.

Next I drew a horizontal line down the length of each skin that would be in the center of the crutch. I also defined 2 points in common with both the skin and the fuselage. This line will help line up the skin later so the saddle is in the right place. I also traced the outline of the wing saddle and made holes for the pushrods. Then it was off to the bathtub for a good 30 minute soak in ammonia water.

Next I sprayed the fuselage with a mist of water to retard the wood glue from drying (an old masons trick). I then smeared on a liberal amount of Titebond on all parts of the fuselage. Then I offered up each side to the fuselage at the same time. Its important to alternate from side to side in the same spot so as not to build in a warp. I pushed long straight pins into my reference points on the skin, then found the same point on the fuselage with the end of the pin. Everything is guaranteed to line up that way. I pinned along the crutch toward the tail, then the crutch toward the nose. I then began folding the top together toward the tail and so-on. Three hours, 200 straight pins and 40 rubber bands later I had a skinned fuselage.

Nothing left to do but trim the excess wood off, kick back and celebrate with a toast.

Note: This could have gone much faster if I had used 3/32" sheet instead of the 1/8" called out on the plan. The pins simply didn't have enough structure to hold onto against the stiffness of the 1/8" so I was constantly tacking with CA. The rubber bands were brought in as unscheduled reinforcements too. If you are planning to fiberglass your fuse like me, I recommend the thinner stuff here.

RE: Building a DUELIST MK 2 From Plans!
 

Mike,

Fuslage is looking really good, I can't believe you got this much done already.....

What engines are you going to use for power?

I'm thinking of using a pair of OS AX .55 engines.

I'm glad you are taking pictures and sharing them with us, because it is going to be a big help for me when I get started.

Ulf

RE: Building a DUELIST MK 2 From Plans!
 

Ulf, I'm planning to use my O.S. 40SF's. Mine make a ton of power for the price and they are reliable as all get out. (Kinda important on a twin!) The .55's would also be a great choice but power usually comes at a price. If you go with retracts the nose gear length is limited by the wing leading edge without major surgery. With the landing gear length maxed out you might have to go with 3 bladed props to attain sufficient ground clearance-especially if flying off grass. One of the reasons I decided to build the MK II was the ground clearance issue. The MK I nacelles in the Pica kit are cut with 2 degrees of down-thrust moving the blade tips precariously close to the pavement.

I can't remember if the .55 is heavier than a typical .40 but the Duelist tends to come out a bit nose heavy because of the long nacelles. Platt made the nacelles really long in order to get a decent fuel load without cutting into the structure of the wing. Those stock 8 oz tanks are going to dry up a bit faster with .55's also.

I'm getting ahead but I plan to shorten the nacelles by about 1 or 2 inches and locate 10 oz round tanks within the rib bays of the wing. In addition to having longer flights I move the fuel mass closer to the balance point and reduce frontal area. The plane will be completely framed by the time I build the nacelles so I can adjust the length to suit the balance situation.

RE: Building a DUELIST MK 2 From Plans!
 

With the framing of the fuselage out of the way I cleared the decks for the stabilizer. Simple stick construction here but very strong and stiff after it's sheeted with 1/16" balsa. I used my hardest balsa for the trailing edge. The angles of the bracing don't have to be perfect but a tight joint at each intersection will ensure a strong stab. The piles of hand-cut ribs in the foreground are a not-too-subtle hint of whats going to take place next in the build sequence.

RE: Building a DUELIST MK 2 From Plans!
 

Mike,

I did check out the OS engine selection that is available today and this is what is available

OS .46 AX ABL
weight - 17.2 oz 489 g
Power - 1.65 hp @ 16,000 rpm
High speed needle on back cover. (I don't like this)
Prop dia. 10.5" to 11"


OS .55 AX ABL
Weight - 18.52 oz 525 g
Power - 1.75 hp @ 16,000 rpm
Hight speed needle on carb, angled back.
Prop dia. 12" to 13"

Both engines use the same engine mount 40 size...

I talked to a couple of friends and they sugested going to a three bladed propeller in order to keep the prop diameter smaller, that would work.
The other problem is the size of the fuel tank of only 8 oz, the OS .55 will empty that fast, I'll have to look into finding someting larger.....

I know there are other engines manufactures that are good but I like OS for realiability and easy to get parts for.....


Mike, are you going to use two servos for throttle or the single servo setup as the plans show?

Same question for the ailerons, two servos or one?

Thanks
Ulf

RE: Building a DUELIST MK 2 From Plans!
 

Ulf, I'm glad to see you are putting thought into this. People seldom bother to ask these kinds of questions and just go for the biggest engine displacement they can find. I too like the consistency and reliability of O.S. engines but the knock-offs are a great value and testing my loyalty.. I already have a couple of OS.40s in hand so they're free right?

Notice the displacement difference of the engines you mention is 20% yet the power is only 6%! (and thats at 16K rpm, more realistic is 12k so the power there will only be a 4.5% difference) Frankly, I doubt I'd even perceive 5% at full throttle.

There are some really smart guys over on the "glow engine forum" that can help answer fuel consumption and prop questions. The one thing I would ask is "What is the best speed prop for each engine?" I think three bladed props look VERY cool on a DUELIST but the spinners and props are more expensive and prop size/pitch selection ain't great.

I don't like the sloppy throttle bellcrank systems at all on a twin and the torque tube aileron thing was clearly causing flutter headaches for Platt as you see the ailerons were shortenend at the ends and there are a million hinges in each!

I've decided (for now) to use two aileron servos (side-mounted in wing panel at mid-point of aileron span) and two throttle servos (micro servo in each nacelle). I just bought a seven channel DX-7 radio and discovered I can bind an extra channel to my aileron stick. It allows me to play around with differential and maybe even flaperons! The throttles can be on separate channels too allowing me to synchronize rpm by adjusting the servo endpoints. Radio programming is still kinda new to me so things will probably change some as I get closer to that point.

RE: Building a DUELIST MK 2 From Plans!
 

very nice build thread you have started here. we are enjoying it.

david

RE: Building a DUELIST MK 2 From Plans!
 

I loved that plane and have the plans to built it. Just have to get a couple others out of the way first. That thing is FAST with 2 40s and I think 55s would be over kill. It would be easier to find new or used 40s to put in this plane then 55s. I know some people want a rocket, but I can't fly one of those any more. Getten to old for that. I'm really enjoying your build and glad to see someone building it. Keep up the good work.
Ed

RE: Building a DUELIST MK 2 From Plans!
 

Glad to hear folks are enjoying the thread. I've been working on the elevator for the last two nights and finally have something to show for the trouble. I couldn't find any 1/4x3/8 hardwood to join the two elevator halves so I substituted 3/8" 7-ply plywood figuring I could combine a hardpoint for the 4-40 bolt I intend to use as a control horn. (I've had trouble with those triangle shaped nylon horns crushing the balsa underneath and loosening up over time and the holes can elongate. They are also a pain to mount straight and only have 2 or 3 usable adjustment holes. A bolt is easier to install, stronger, infinitely adjustable, and can be easily replaced.)

After epoxying the two elevator halves I was disappointed with the stiffness of the joiner. (A flimsy elevator can induce roll during high-deflection elevator maneuvers.) I decided to fashion a bracket out of a 1/4" strip of brass and epoxied it to the back of the joiner. It's super stiff now! The toothpick joint reinforcements are probably overkill..

I contoured the elevators from 3/8 down to 1/8" to reduce drag (and for looks). Setting the stab/elevator on the plane the horn lines up nicely with the elevator pushrod. Stab is photographed upside down to show the final horn and nylon coupling. I use hinge points exclusively now. I have nothing against the other styles, I just can't seem to ever get my slots aligned. Drilling a hole is much easier for me to do.

It might seem like I'm spending an inordinate amount of time on my elevator but to me it is THE most critical part of the entire airplane. (Even wing bolts have redundancy.) I figure if I spend a little extra time taking care of the tail feathers the plane will fly straighter, stay in trim longer, and will just be a lot more fun to fly.

RE: Building a DUELIST MK 2 From Plans!
 

Mike, I'll probably end up with two OS .46 they are cheaper and more sutiable for the Duelist because of prop diameter and fuel capasity.

I used to have a Super Sportster 40 that I made into a twin engine using two OS .25, it was a great airplane fun to fly, but I used a single servo for the trottles, won't do that again...
My plan for the Duelist is going to be the same as yours, two small servos for throttles and two regular servos for ailerons.


Great job on the tail section.......

Did you sand the elevators down from 3/8" to 1/8"?

It looks like you are ready to start building the wing.