steve1mac

I’m starting this thread to show you how I overcame some of the problems with this build and to explain a few modifications that will make field assembly and operation of this plane easier and more enjoyable.Before assembly began, I went over the whole fuselage with epoxy and CA to make sure all joints were secure. Added some where I felt it needed it. CAUTION: make sure you do NOT let glue get into the landing gear mount assembly. This is a tight fit and some day you might want to remove the main gear for replacement. I also applied a thin coat of 30 min. epoxy over the entire inside the front and rear cowlings. This is something I do with all my planes. It adds substantial strength to the cowl and keeps it very clean inside. When these engines spit and sputter fuel it always soaks into the fiberglass. A real pain to clean! With the epoxy coating most fuel just runs off. And wiping out the cowl is a breeze.

I also put larger wheels on this plane. The kit comes with 2 1/2” foam wheels. I went with Du-Bro 3” rubber wheels for a better look and smoother landings. Believe me! I can use all the help I can get!

When installing the side windows, don’t separate the front pair. They fit right in while still connected. Just trim away some of the edge molding. When trimming the covering away from the window openings, leave enough extra covering to wrap around the wood and attach to the inside of the fuselage. I used a white paint marker to cover bare wood in the window corners. I also kept every scrap of covering that I cut from this plane to use as patches.

I would like to know what brand of covering is used on this plane. Does anyone know?

steve1mac

After reading some previous threads about balancing this model I found a great space for two 4 or 5 cell batteries. At the front / top of the fuse, just behind the firewall, there is a cavity large enough to hold two 5 cell batteries. I cut a piece of 1/8” plywood to fit under the X braces and attached it with 30 min. epoxy, leaving the plane upside down to cure. A small piece of wood, at the front of the cockpit, needs to be removed to access this new space. Batteries wrapped with felt fit perfectly. Save the wood that was removed from the canopy as it can be returned to its place to secure the batteries.
I installed a Sullivan Skywriter and 8oz. smoke fluid tank into the cockpit area to further move the balance forward. And I glued and screwed thin plywood to the back side of the central bulkhead just behind the cockpit to cover the opening and to create a mounting area for my glow driver batteries. I am using two single cell 1.2 volt batteries. One for each 4 stroke engine. Using only one servo for the elevator, and adding 1.5oz. of lead to the front engine mount, this plane’s balance is dead on.

steve1mac

In previous threads I learned that it is difficult to assemble the plane at the field. The nylon bolts that attach the TE (trailing edge) of the main wing are under the air scoop on the rear cowl. I solved this problem by building new, wider, mounts inside the fuse. This places the nylon bolts outside the scoop. Too bad you can see these bolts but it sure beats having to remove the spinner, prop and cowl every time you want to get at these bolts! I added plywood and mahogany to the outer skin of the fuse to reinforce the area where the new wing mounts attach. To make this wing modification complete I epoxied the dowel rods provided with the kit for the wing’s LE (leading edge). Then cut off, drilled and tapped them for ¼ x 20 nylon bolts. New TE bolt holes were drilled into the wing and wood was cut out from the under side to access the wing cavity. I then inserted and epoxied as much plywood as I could into this space to give the new holes some strength. Now wing attachment is simply to slide the TE under the scoop and drop the LE down to the fuse. Add four nylon bolts and a few screws for the canopy and you’re done!

Wing tubes are cut to almost perfect length to fit inside the wings. With the tail booms temporarily in place, see how well the wing parts seam together. I had to trim about 1/16” off the end of one tube to get it right. The instructions make absolutely no reference as to how the tubes are attached. There is a pilot hole under the central wing for the larger tube. I used this hole and also scouted out where the smaller tube was sitting. I was confused with most of the hardware that came with the kit so I drilled and tapped these wings and tubes for 6-32 machine screws. I did this for the wing extensions and the central wing section as well, so each tube is held in place with screws at each end (8 screws total for wing tube assembly). Thread locker is used for the wing tube screws. For storage and transportation I leave the tubes in the wing extensions and just add four screws to the central wing section at the field. If a hard landing causes the wings tubes to bend just replace them with 5/8” and 3/8” aluminum or brass tubing.

I also trimmed away some wood at the top of the rear bulkhead. It was right in the path of the air flowing in through the scoop to cool the rear engine.

BTW: If you make this wing mounting mod with 1/4 x 20 blind nuts do not use the nylon bolts that come with the kit. They are NOT 1/4 x 20.

steve1mac

There are no control rod holes through the firewalls so you are on your own as far as lining up your hardware. The servos are close to the fuel tanks so pre-fit everything in that area before you bend and cut to length your control wire for throttles and steering wheel.

Since I am using Magnum .91 XL engines I decided to upgrade to 20 oz. fuel tanks for longer flights. I mounted the front tank flat in its place with foam and balsa twigs glued around it for support. The steering pushrod is under the tank and under the X braces holding the tank. I used short pieces of tubing glued in at certain places to help guide the pushrod under the tank and through the firewall. The rear tank is on its side and fits in real snug. This also leaves plenty of room for the rear throttle servo and piano wire. Three servos! That’s all you’ll find in this big fuselage.

On my plane, ailerons and flaps are Y harness hookups. CAUTION: Make sure your ailerons are mirror image (i.e. both servo arms are facing inwards). And your flap servo arms should both be on the same side (i.e. both arms face the left wingtip). It doesn’t matter inwards or outwards, left tip or right tip as long as the aileron servos are opposites and the flap servos are the same.

The plans call for two servos to control the elevator. Well, since I have read about all the weight other builders added to the nose I decided to install just one elevator servo. For better control I installed 2 control horns and a dual servo arm. This mod had a large effect on the balance. It’s a standard Futaba S3004. Maybe under powered (might change it to something stronger). Another option is to use flexible pushrods and put the servos farther forward near the main wing somewhere. If anyone has success with this method let us know.

Since we are talking about the rear of the plane I want to add that I only used the bolts and blind nuts for horizontal stabilizer attachment. I should be able to move the tail booms out about an inch, one at a time, to expose the joint so I can add epoxy. I’ll let you know when it happens.

steve1mac

At Nitroplanes support forum (http://www.rcdiscuss.com/index.php?board=84.0 ) I asked about larger engines (Saito .72’s). I was told it would fly okay but it would be way over scale. Well, that’s what I want! I even went one step farther and installed Magnum .91 XL’s so it should have plenty of power. I bought a fish scale to measure thrust and am playing with different prop setups to get the most out of these engines. For now I’m using APC 14 x 6 props front and back. In reserve I have a Master Airscrew 3 blade pusher 13 x 8 that I might try on the rear engine. I like the look of the 2” conical spinners but try to find a 3 blade pusher prop spinner! More on that later.

I am breaking in these engines with Omega 10% nitro and am getting readings of a combined thrust of 15.5 lbs. on my fish scale. My goal is to get it up to around 18lbs.or more with 15% nitro so I can do vertical with ease! Plane dry weight is 14lbs. 11oz. Loaded with full tanks its around 16lbs. 12oz.

Nothing special about the front engine installation. Make room for plenty of air movement in the cowlings. I think I might have taken a little too much away from the front cowl.
The rear engine looked weird with a forward facing muffler so I made some modifications to the exhaust pipe and muffler. I mounted the muffler on the firewall. But before I did this I prepared the firewall with high heat insulation. I found it at an auto parts store where they call it Tan Exhaust Pipe Wrap. It is made by Design Engineering Inc. Web site: http://www.deicycle.com. It’s like a tight mesh burlap fabric that can withstand temps up to 1500 degrees. I also wrapped the muffler and secured the fabric with wire provided in the package. A copper pipe clamp loosely holds the muffler to the firewall. The insulation should help absorb vibration.

I used 3/8” copper tubing to reroute the exhaust pipe. I bent it with one of those long spring pipe benders. The best way to bend pipe is to pack it with wet sand and use the spring bender. This will help prevent the pipe from collapsing at the tightest bends. 3/8” copper tubing just fits over the exhaust pipe from the engine (Magnum .91 FX) and an additional exhaust pipe going into the muffler. You might have to ream the ends of the copper pipe to make them fit your particular engine. I tried to cut the pipe with a rolling pipe cutter but this caused the pipe ends to collapse too much. A Dremmel or hack saw does a better job. Just make sure you file and clean everything before you bring it near the engine. The 2 junctions were wrapped with rubber exhaust pipe tubing that I purchased at a local hobby shop and are held down with 4 hose clamps - One on each side of the joints. This rubber tubing is also called Thermal Exhaust Sleeve. You can find it a high performance auto stores.

The only visible exhaust component at the rear cowl is the Du-Bro exhaust deflector. Nice clean look!

steve1mac

If you are using a Y cable for throttles it is common practice to adjust your high needle valves to match engine RPM. Although this is the easy method for matched RPM, it can also spoil a good engine by leaning it out too much. REMINDER: A rich mixture not only delivers more lubrication, but the castor and synthetic lube is also carrying away a significant amount heat from the engine as it blows out with the exhaust. This is the main process for internal cooling in 2 and 4 stroke engines.

Even though these engines are identical and were broken in the same way at the same time, there are still significant differences between them. Because the rear engine is behind a firewall and its prop is moving air away from the engine instead of blowing at it, most likely, the rear engine runs at a higher temperature. And because the rear prop is spinning in the wake of the front prop, the rear engine runs with less torque. And in my plane, the rear exhaust configuration is much different than the front which results in increased resistance in the rear exhaust flow. So, higher temps, less torque and restricted exhaust flow, all combine to create quite a different environment for the rear engine.

A better way to match RPM is to have each engine running on its own channel (throttle – front, AUX – rear). You could mix throttle to AUX and use the mix parameters to match RPM. This way you can have each engine’s high and low needle settings set perfect for that engine and still get pretty close to synchronized RPM.

Or… You can use TwinSync!
Although a plane with a pusher / puller setup does not need to be synchronized, there is some special programming that Bill Wike (the manufacturer of TwinSync) can add to the control chip that improves performance for planes with in-line engines. I personally like the sound of engines running in perfect sync. And using TwinSync will take a lot of hassle out of trying to match your engine’s RPM.

The special programming that Bill Wike can install allows each engine to run at it’s own pace at any throttle position above 90%.

I’ll explain the TwinSync setup I have programmed for my plane:
The throttle channel is connected to the TwinSync and controls both engines. I am also using the TwinSync Glow Driver setup. This circuit connects in tandem with the TwinSync. It is programmed to switch on with an AUX channel (on my plane I am using the Gear channel to switch on glow drivers). You could, instead, set the glow drivers to turn on automatically below 3500 RPM. Synchronization is disabled below 3500 RPM which helps when starting the engines and lets each engine idle at it’s own pace. And as I mentioned before, at full throttle (anything above 90%) synchronization is disabled to allow each engine to run full out for maximum power. It is also programmed to NOT detect a dead stick. Since this plane will still fly with only one engine running there is no need to throttle down both engines to balance thrust like you would do for wing mounted engines.

Download the TwinSync manual at: http://www.troybuiltmodels.com/items/WRCTWINSYNC.html

To get this special programming installed in your TwinSync at no charge mail the device (or, if you are handy with electronics, just send in the program chip) to: Bill Wike

$5.00 must be enclosed for return shipping.

Contact Bill to let him know what you want at:
http://www.rcuniverse.com/forum/m_4700596/tm.htm


This is the end of my opening comments.
I hope this helps some of you during your build of the Skymaster 337 from Nitroplanes.com

I am open for comments and suggestions. Since the Nitroplanes.com plans for this model are so minimal, the only way to get it right is for all of us to plan together! - This Forum Is the Manual –

Steve McCarthy

klazeem

hey steve...how did your build go? hv u flown it yet?...do you have any more pics during your build?...just got my 337 and looking for build tips..especially tailboom, centre wing, and outer wing issues...(for easy transportation to field)...m gonna run mine on twin 46 thunder tigers...might go for 3 blade props but not sure yet....

fritzdecat

Yeah how did it fly ?

Lookslike you did a great job putting ittogether

steve1mac

The plane flies great. Well balanced. Not very agile. Kind of like a pig with wings. I can do Cuban 8"s and rolls but you are moving a lot of weight that hangs below the airfoil. So although these manouvers can be done, it's kind of funny watching this plane flop through it!

After about 20 flights I damaged the front cowl and firewall. I strengthened the firewall and used a cowl from a Decathalon ARF that fit near perfect except the top corners. I tried to get parts from Nitroplanes but got no response. I think they only support planes that they have in stock. Poor communication, no support at all.
Too bad. I really like this plane but i think we are on our own as far as replacement parts.

While fixing the firewall I added a hatch to the top and bottom of the fuse. Since I built support beams from the firewall to the next bulkhead I used these surfaces to attach my access panels. Inside the top hatch I built a battery tray. I now have easy access to the tank, control rods, wiring, fuel tubing. This plane is better than ever!

I will take some pictures and post them soon.

fritzdecat

Pig with wings ..LOL

steve1mac

Hi Wayne,

I really like this plane. However I am not very happy with Nitroplanes support. They have not made this plane for some time now and have not given me any support from the day I purchased.

When I received the plane the canopy was damaged.  I did contact them about the damage and they gave me the runaround and never sent a replacement. So I fixed it with no major problem.

I installed 2 Magnum .91 4 strokes and really like the power. I also installed the TwinSync system, however this plane doesn't need it since the engines are in-line. I just like the sound of perfectly synced engines.

I installed 2 x 26oz. fuel tanks and get as much as 35 minutes of flight.
With all the additions my plane weight is around 16 pounds! That's why I jokingly called it a pig.
I use 3 1/2 inch tires and reinforced the landing gear to accommodate my weak landing skills.
If you follow my build modifications this plane is easy and quick to setup at the field.
Flight is very steady. It likes to land long - use flaps for landing.
I haven't had much problem with the lack of rudder, however a strong crosswind will push her off course or tip her over.