Boomerang1

Hi guys!,

I started this build thread on a specialsist scale site but the owner of the site changed the direction that the site was heading so I'll continue it here. Most of the early posts are just reposted here and I'll just continue with the new posts. As the posts by others are not my own (legal situation?) they will not be reposted. - John.

Boomerang1

Hi guys, please be gentle with me as this post is my first and the model is a first for me to, my first turbine model. First some details. I'm in my 40's, have flown models since I was a kid, love scale aircraft and get the greatest pleasure from this hobby from the challenge of scratch building models - 3 view in one end of the building machine, flying scale model out the other end. Do I build failures - oh yes - but very nice ones! Will this one be a flop? - only one way to find out! First of all, someone to thank, Phillip Treweek of Kiwi Aircraft images. His website provided much of the photographical information needed to scratch build a model or document one built from a kit. Take a look at http://www.kiwiaircraftimages.com/aviation.html the site is a real treat for scale modellers. The following pic is from his site, just to let you know what an Aermacci MB339 looks like.

abufletcher

I'm really looking forward to this! I don't think I've seen a single other jet build here and yet, why not? They're scale models too!

Boomerang1

Interestingly, scratch built jets are as rare as rocking horse manure! In the RC jets section it's mostly fibreglass ARF's from Asia. I guess the thinking is if you can afford a turbine you can afford a ready built plane to house it. Even 'real' kits for turbines are rare. - John.

Boomerang1

Why an MB339? It all comes down to what you want the model to do, just like the full size when they write a specification.

Firstly, if you are going to all that trouble to build a turbine trainer you may as well make it a scale model. While I'm a reasonable pilot I didn't want a missile for my first model, the MB339 has nice big wings, good moments. My club has a grass strip so low take off & landing speeds, short undercarriage legs and practicallity are important. Finally being a 'normal' looking model I can use building methods with which I am familiar. If you think about it the only unusual thing about the model will be the engine.

One more thing. This will NOT be a rivet for rivet super detailed scale model, just a simple first jet that I can take to scale rallys. If scale accuracy has to compromised for simplicity so be it. Next, the way I scale up drawings using Corel Draw. John.

As it turned out the Macchi was started before I had the engine & retracts so some mistakes were made designing the model around components I was unfamiliar with arranged in a layout with which I had no experience. In the end I DID end up designing & scratch building a simpler trainer, details later. - John,

abufletcher

How about posting things in reverse? Show us where you've gotten to and THEN show us how you got there.

Boomerang1

The model is nowhere near complete so it won't take long to catch up to where I'm at.

I use 3 computer programs to go from 3 view to full size drawing. To scan & crop I use 'Ulead Photo Express' a simple program that came with my Canon scanner. Corel Photo Paint will also do this but being a very powerful program basic tasks are a little complicated. After scanning with Ulead in grayscale (smaller files) I crop the 3 view into seperate parts and save as seperate files, eg sideview, top view, bottom view and front view. If needed you can re open the images & trim them further, for example I needed the front view of the fuselage but not the wings & tail so I opened the front view, cropped off the bits I didn't need & saved it as yet another file. That's all I use Ulead for.

Corel Photo Paint (part of Corel Draw) is used just to tidy up the individual images. Using the eraser tool from the toolbar on the left hand side I remove any writing, arrows, any bits on the 3 view I do not want to use and thats all for Corel Photo Paint.

Obviously before enlarging parts of your 3 view you have to know what side you want the finished model to be. In Australia the Large Scale class of models start at 2 Metres wingspan for a monoplane so this will be size of the Macchi. l'll start with the fuselage side view. Simple maths tell me that if the model is 2 m span it will be 2032 mm long & about 700 mm from ground level to the top of the fin. To draw this you would need a piece of paper, say, 2150 mm long & 800 mm high. In Corel Draw (my version is 10, previously 8) when you start a new drawing the paper size defaults to A4 size on the top toolbar along with the dimensions of the paper to the right. Change the dimensions to 2150 wide by 800 high & hit enter. The paper is now described as 'custom', the paper on the screen changes shape and size to what you have entered and we are ready to go. - John.

Boomerang1

First IMPORT the side view onto your drawing, in my case it was just a spek! Draw a line across custom paper 2032 mm long, hairline thickness is fine, leaving a little space each end of the paper, remember your custom paper is 2150 mm long. Then draw a vertical line each end of the horizontal line, like a wide 'H'.

Zoom in on the side view & select it with the pick tool. Grab the drawing by the CORNER & stretch it until it is so big it reaches from one vertical line to the other, 2032 mm long. You are now the proud owner of a full size side view of the model, save it to disk. In the property bar on the right hand side you can see the percentage that you have stretched the drawing, THIS IS
IMPORTANT. Note this down somewhere because everything you import from your original drawing must be stretched by this percentage to remain in scale with everything else. - John.

Boomerang1

Printing the full size drawing leaves you with 3 options - use your plotter to print it full size, email the file to a drawing printing company who will use their plotter to print it or, as I did, print it on my A4 printer and stick the pieces together like a jigsaw puzzle.

To do this in Corel Draw click on FILE, PRINT PREVIEW, you will be left with an A4 sheet with a tiny part of the drawing on it. On the bottom right of the screen is a TILED PAGES icon. Click on the icon & this is the result. Click print & wait for the pieces! You may have to trim off the edges if your printer does not print right to the edge. I have printed off side, top view and front views, I cropped the wings & tail of these in Ulead before enlarging & printing to make the drawings more managable & save paper. John.

Boomerang1

When enlarging drawings using this method use the horizontal reference line to line up the individual sheets of paper if printing as 'Tiles'. The drawbacks of this method? The lines on the print become thicker - the Macchi was an extreme case from a very small drawing to quite a large model but still fine to use to build from.

For those that may think modern jet trainers are whimpy aircraft it's interesting to compare the MB 339, a 1976 design about middle of the pack of jet trainers for performance, with the 'mighty' P-51 D Mustang. From the information I dug up the 339 is almost 90 mph faster at altitude (more than 150 mph at sea level!) Initial climb rate is almost twice as fast, it's 1000 lbs heavier, carries twice the bomb load and has greater range without external fuel (yes it can carry external drop tanks) and it's about the same size. With 2 people in air conditioned comfort! That's progress I suppose.

With a full size (model is 1:5.4 scale) drawing of the fuselage top, side & front to work from design of the basic fuselage box can begin. This will be nothing more than a bracket to hold the nosegear, radio, fuel tanks & engine, wing & tail surfaces in alignment. The 'look' of the model will come from the foam cladding.

In theory the model can be flown without the cladding, the only important factor is how much fuel & goop would be spilled on the unprotected wood. This should not be a problem with a turbine for obvious reasons. My B-36 was test flown 'in the bones' before it was clad.

abufletcher

Fascinating!

Boomerang1

To fit the turbine jetpipe and the necessary clearances around it the 339 has a rather narrow rear fuselage and this may have to be enlarged a little to provide enough clearance. Yes, the pipe does get hot but not as much as you would imagine. Cooling air travels past the outside of the pipe. The engine is mounted a small distance in front of the start of the pipe and the speed of the hot air rushing down the pipe sucks cold air into the gap cooling the jet exhaust & increasing effective thrust.

The name of the game here is to make a square section basic fuselage as large as possible to fit in the outline of the 3 view and still leave room for foam on the outside to finish to the scale contours.

The engine, jetpipe, fuel tank and the bit that controls the aircraft (model - RC gear, full size - pilots) are installed in exactly the same place as in the full size aircraft.

Using the front view, side view & top view to determine the maximum size the square section basic fuselage is marked on the full size drawings. In this model it must accomodate the nosegear, engine air inlets, wing mounting tube, wing fixtures, tail bumper skid, tailplane & fin mounting, ventral fin mounting and rear jetpipe mounting.

I usually just draw on the enlarged 3 views but to illustrate the idea I've added it to side view in Corel Draw. John.

Boomerang1

Moved down to the garage & I'm now drawing in the centre box on the plan as shown on the computer screen. The box is nothing special, 6 mm balsa sides with ply doublers about 1/2 way along to support high stress areas like the wing tube & nosegear. A friend in my club built a copy of Rhom air retracts in the Air Force & I plan to use these. I also have to chase up the wing tube, I've been told the local aluminium merchant has T6 alloy tube in various sizes by the metre. This will extend out as far as the main gear.

Maybe you guys can help me. I was working next to an aluminium merchant today & bought a 1 metre length of 32 mm tube with a wall thickness of 1.6 mm, weight is almost 400 grams, cost 5 bucks. This will be more than strong enough for my purposes but it is T5 grade, not T6, is there much of a difference in strength? On checking the plan the tube will have to extend well past the main gear, the Macchi maingear retracts outward like a Spitfire/Me 109 but it still has a reasonably wide track.

After chasing up some other 'normal' aluminium merchants it's become obvious that special grade alloy tubes are only available special aluminium merchants! By this I mean aircraft materials suppliers. I will stick with the T5 tube but to save weight make it shorter and drill some lightening holes in it near the ends where there is less stress. The position of the tube is determined by the front view of the model for the wing position from the bottom of the fuse, where the intakes will line up with the fuselage, where the wing spars will be and where the maingear will retract to.

Whew! the aluminium tube will fit. It starts off at about 40% chord at the root but ends up about 20% where it ends due to the leading edge taper of the wing. The retracts & wheels can only go in one place and this determines the location of the tube more than anything else. I will use the Selig 8036 airfoil, my Top Flite P-47 uses this at the root and it's a sweet flyer. I obtained the airfoil from a program called 'Profili' a free program by Stefano Durone from Italy from the 'The Future is Electric' website hosted by Ken Myers http://members.aol.com/kmyersefo/homepage.htm Thank's both Ken & Stefano for making the program available at a most competitive price! ( note, Ken Myers website seems to have moved but the Profili website is here - http://www.profili2.com/eng/default.htm ) Progress is slow on the project due to the available free time, I run my own business, am raising a family, am the president of my model club and like to go flying. How long to retirement?! - John.

Boomerang1

From the same program I picked the tailplane section - RAF 30, a medium thickness symetrical section with the correct thickness to chord ratio for the model (in normal speak it won't look too thick in the front view!)

I placed tracing paper over the blowwn up side view & drew in a box fuselage to accomodate all the gear. Having never built a turbine model before more thinking is required. I cannot get over how much fuel these things carry. Lots of fuel = lots of weight so the tank MUST be located close to the CG, not in the usual place. Main tank is 50 oz, secondary tank is 20 oz, header tank for fuel feed to fuel pump about 4 oz! The location for these & the other gear is determined on the box fuselage drawing. -
On the box drawing I've determined the shape of the ply doublers for the front of the fuselage and where to cut lightening holes as required. I use 3 mm door skin ply for this, lighter than aircraft ply, strong enough for this purpose & available cheaply in long lengths.

I trace these from the box drawing and transfer the shape to the ply using the tracing paper method. - John.

Boomerang1

It's beginning to sound like I'm talking to myself here but I'll continue.

The engine I intend to use is the Wren MW 54 super sport with around 18 pounds of thrust. I would like to think the Macchi will come in around 20 pounds all up weight without fuel so with thrust/weight approaching 1 to 1 performance should be brisk! I've ordered the fuel tanks, 4 x digital servos, 4 x normal servos and 2 x 'gas fuel tank stoppers' with some tygon fuel tube.

Amazing, I've found the time to actually start construction! I had to join the balsa from 3 pieces to get the required length, around 6 feet. The local had no 48 in sheets in the required thickness so 36 in it was. These will not be seen when the model is clad in foam.

The two main fuel tanks, 50 oz & 20 oz. are shown. These are plumbed in series to a third header tank of about 6 oz. The fuel pickup in the header tank is in the geometric centre of the tank with no air at all so the pump has an uninterupted flow of fuel.

The ply doublers are now glued to the sides and the sides pinned together & sanded to be exactly the same size. While they are pinned together I cut the hole for the wing tube with a 32 mm hole saw so it's in exactly the same place on both sides.
The wing tube will be used to help keep the fuselage in alignment while the formers and top/bottom sheeting are added. - John.

Boomerang1

These are formers 7 & 8 along with the tank floor & the main fuel tank. They are shown in about the correct position relative to the wing tube which is just in front of the CG. The 3 servos (1 each for the elevator halves, one for the rudder) connect to pushrods which run down the spine of the aircraft away from the jetpipe. Also from these servos I hope to connect cables to steer the nosewheel and to operate the Dubro mechanical brake on the noseleg, operated by full down elevator. If I cannot get them to work properly I'll have to resort to 2 additional servos mounted in the nose. (in my 'jet trainer' this turned out to be impractical, it uses a seperate servo for the nosewheel steering & I fitted a Kavan electric brake to the nosewheel powered by the engine battery pack via a microswitch operated by down elevator.)
- John.

Boomerang1

A few more formers done, No's 5,6 & 9. No 10 will be the last of the formers in the parallel section of the box. Engine access is from the bottom along with the wing bolts & wing air & radio connections.

Where does the air get in to the engine? Air enters through intakes just ahead of the wing root & then turns about 45 deg to pass through these holes in the fuselage sides. It then passes the wing tube & then into the engine mounted behind the main fuel tank. I have a few doubts about the ability of the inlet path to feed adequate air to the engine. If there is not enough I'll fit a cheater hole in the hatch on the bottom of the plane.

Two plywood doublers are fitted to help support the fuselage/ tailplane join. I'm making the tailplane fixed, it will still fit in my station wagon or trailer with just the wing off. - John.

Boomerang1

Turbines are cool things! (actually quite hot things, you know what I mean!) Shown, from right to left, are the tank for starting gas (just like a retract air tank), solenoid valves for the gas & fuel, the engine control unit (about the size of a reciever), the fuel pump and the engine itself. Not included was a 6 cell 1200 mA or larger nicad to make it all go. Yep, that's all you need, look in some turbine models though & you will see what looks like 10 times as much stuff! Turbine flyers LOVE wires & pipes!

Before you say what a lot of gear to carry around remember it's a full autostart engine. Put the model on the flight line, with the throttle trim forward simply move the throttle stick fully forward then back to idle within one second. The engine should go through the start sequence within a few seconds & settle down to a steady idle. (what I did on the trainer & plan to do on this model is leave the gas tank out & start the engine directly from a camping gas cylinder. Simpler, lighter & one less thing to explode!) These things seem to rely on big numbers! Idle speed 40-45,000 RPM, maximum thrust 17.8 lbs @ 160,000 RPM, exhaust gas velocity 'greater than 800 MPH'. Sounds like fun!

The supplied engine mount is remarkably ordinary compared with everything else, just some bent bits of sheet metal.

How big is it? Here's the basic engine next to an O.S. 120 four stroke. The turbine weighs about 4 oz more than the 120, 2 lb, 1 1/2 oz for the basic engine including the starter motor. With a prop & an alloy mount the four stroke would be heavier. - John.

Boomerang1

The last part of the puzzle has arrived, the jetpipe. I'll be making an outer pipe out of litho. I can now determine the location of the engine mounting plates, the size & position of the former behind the engine and the remaining formers where the fuselage tapers front & rear.

The engine is positioned about 10 mm in front of the pipe and a venturi effect supposedly draws cold air into the pipe with the engine exhaust. This same effect should also draw air from outside the pipe, the inside of the pipe extends past the outer litho sheet tube sucking cool air between the inner & outer tube. there will also be an air gap between the outer pipe & the fuselage.

Two 1/4 inch ply plates will support the engine. These are fitted with blind nuts for the 4mm mounting screws. The plan is to slot these into the fuselage sides. I have made my first mistake, I have cut away the plywood fuselage side doublers where these engine mounting plates are to go. Looks like I will have to make some small support plates. I guess this is a consequence of starting to build before you have all the parts you need to accomodate. - John.

abufletcher

I don't know how I missed these last few "building" posts! I guess it's because I've been traveling. Your Macchi is a very different sort of build so it's interesting and someone surprising to see how it progresses. I'm surprised to see how small the turbice is.

Boomerang1

On the advice from a friend & looking at a couple of other jets I've made up some longer engine mounting plates. These will also slot into the sides but will stretch from the former in front of the engine to the one behind which also mounts the front of the pipe.

Back again although I've done a bit. I have modified and completed most of the fuselage formers and have slotted the fuselage sides to accomodate them. The Wren jetpipe is a single walled type but I wanted a dual wall type to shield the airframe from radiated heat although I have been assured that the single walled pipe does not get really hot. As per the instructions with the pipe I rolled an outer tube from litho plate, seen here over the inner pipe. Air is sucked between the inner & outer pipe by the venturi effect of the jet efflux at the outlet end of the pipe.

Just a couple of nose formers to cut out & I can join the fuse sides permanently and it will start to look something like an aeroplane. The fin spar is part of the second last former. I was originally going to use Rhom Air retracts but the model will be too heavy for these 60 size retracts so I am looking at Robart 630 series retracts. I'll make up my own trailing link legs to suit these retract mechanisms. - John

abufletcher

OK, now, that's starting to look like an airplane! But I still can't get my head around the idea of a turbine surrounded by wood.

camdyson

Hi John,

Just spotted this thread. Please try and make it sound as frustrating and tedious as possible, so my determination to resist jets isn't eroded.. Seriously, nice to see a trad' built jet for a change. Will it be a Roulette?

Merry Christmas,

Cam

abufletcher

No worries there for me. The price will take care of that!

Boomerang1

No Cam, this Macchi is a 339, a later version with the stepped cockpit. The Roulettes used the 326 where the second pilot sits down low behind the guy at the front. Thanks for dropping in. I attended a jet fly in at Sale, not far from you & a couple at Wangaratta, do you go? Hopefully you can be hooked into jet flying.

Turbines & wood? DeHavilland's first jet, the Vampire, was all metal except for the wooden fuselage which housed the pilot & engine. Seriously, fibreglass or wood, if it really catches fire it doesn't make a lot of difference. At least the wood would give off less toxic fumes! They usually catch fire during start up if at all, more on that later.

No, I'm not a machine that scratch builds models super fast, the thread was transferred from another site. It's getting close to catch up stage where the build will be back in 'real time'. For a true scratch build (own designed from just a 3 view) that means slow progress. - John.