Build Log Marston 3D Fish

[TugBoat]

Well, I have searched RCGroups for a build thread for the "3D Fish" and I couldn't find one.

So here we go.

What is a "3D Fish" I hear you ask - surely all fish are 3D! At least the ones I eat for dinner are... I mean, who has ever heard of a 2D fish?

Well I am glad that we have sorted that out:



For full details see: http://www.pteroworks.com/other.htm

Brief statistics are:

39" span
12 -14 oz. rtf
wing area: 290 sq.in.
wing loading: 6.7 oz./sq.ft
fully laser cut from balsa and ply - no sticks!
carbon wing spar included
easy to build
fully aerobatic
Hovers with AXI 2208 outrunner motor

Building experience is recommended for this kit.

Now this should be a fairly straight forward project, however, it is always useful to see how other people approach a simple project... after all there is always the possibility of making it more complex...

The kit arrived well packed in a sturdy box and consists of:

- a nice bundle of balsa and ply, all wrapped in plastic so that the bits don't escape
- a carbon tube (main wing spar) and a carbon reinforcing strip
- complete plans
- three A4 pages of instructions

The instructions are basically a printout of the images from the construction photos http://www.pteroworks.com/3d_fish_build/index.htm with a small amount of explanatory text under each image.

[TugBoat]

Joining the wing to the body

This is left to the creativity of the builder. As this thread progresses I will show some creativity and we will see what happens.

The landing gear

The 3D Fish has been built with a single wheel and also with two wheels. The design is actually for a two wheel landing gear.

For the single wheel, the "post" for the wheel extends only to the wing, so as not to block the hole in brace "A" for the battery to slide into. After the wing is mounted, drill a hole in the front meaty part of the wing which acts as a nice anchor for the post.

[TugBoat]

Wing after step 1: 16.5 gms (post #5)
Wing after step 6: 52.2 gms (post #9)
Wing ready to cover: 52.8gms (post #27)

2 Ailerons with doublers: 12.5 gms (post #10)

Two body sides and four braces 31.5 gms (post #13)
Vertical stab with CF reinforcement 2.5gms (post #14)
Body with braces and vertical stab 34.6gms (post#17)
Body with spine 41.0gms (post #18)
Body with rear balsa + motor mount 44.0gms (post #23)
Body with complete wing mounts 47.6 gms (post #27)
Body with wheel mounts, inc. bolts for wing mounting 52.2 tms (post#31)

Horizontal stab and elevator 10gms (post #15)
Rudder 3.5gms (post #15)

Ready to assemble (what you could call the ARF stage):

Wing covered with 2 ailerons: 86gms (post#30)
Horizontal stab, elevator and rudder covered: 18gms (post #30)
Body covered: 61gms (post #37)

Airframe assembled & covered including servos: 214 gms (post #39)

Final flying weight: 472 gms (overweight...)

[TugBoat]

General

The plan at this stage is not to cover the construction in detail. So it goes without saying that:

- I will be sanding the parts as I get them out of the laser cut sheets
- I will be weighing the parts as I go in an attempt to get the wings as evenly balanced as possible

Step 1: inner wing pieces

So tonight I pulled the parts for the inner wing pieces and put them on the plans. One thing is immediately obvious:

The left and right wings as show on my plans are not exactly the same size. The trailing edge parts are laser cut exactly the same, however, the left wing on the plans is as per parts, and the right wing is slightly larger.

This does not really worry me, because I don't normally build on the plans anyway...

The next interesting thing about this part of the construction is that the parts have to be glued in the correct order to ensure that the parts are aligned, at the same time in an order that permits the parts to fit together.

At this stage it has got a bit late, so I will think about it overnight...

Note: I have passed the information concerning the slightly different wing sizes shown on the plans back to Ron and he is looking into the issue. This isn't really a major issue as you will see as you follow this thread.

[TugBoat]

Step 1: continued

Order of construction all sorted out.

There is a bit of sanding and alignment required, as indicated with the photos. Also it helps to have the leading edge and body pieces available at this stage.

This is an interesting wing construction, there are only a few more steps.

At this stage I can't work out exactly how the wings are attached to the body, but I suppose it will become clear as I plod along.

[TugBoat]

Step 2: Lay out the ribs and leading edges

Not too many parts here are there?

All the laser slots on the balsa are spot on, the ply outer rib (#6) needed a slight file to get the slot to be a smooth fit onto the leading edge.

[TugBoat]

Step 3 + 4: Assemble, do not glue

So, put the spar into the centre section, drop on the ribs in the correct order and then fit the leading edge.

The only trick here is that you need to leave rib 6 off until after the spar is in the slot on the leading edge.

At this stage it turns out the spar is 2mm too long - much better a few mm to long that too short!

So that's it for tonight... trim the tube tomorrow.

Everything fits very nicely, and although I have a religious objection to CA glue I think that I will use it here. The plan is once the wing is completely assembled just "glue all the connecting pieces".

[TugBoat]

Step 4: Continued

The tube is now trimmed and the wing is assembled - not yet glued.

Torsional rigidity is provided by the covering, so the wing itself is very simple.

The parts are interlocking nicely.

There was a little sanding required at the central join of the leading edges to permit the #1 ribs to be parallel and the leading edges to sit correctly.

[TugBoat]

Step 5: Glue it all together

Prior to using the glue I used some 1/4" balsa to lift the trailing edge and pins to secure the ribs in place. The balsa ensure that the ribs can sit down on the trailing edge.

Take a few measurements to make sure that the wing is square and symetrical. Double check that all the ribs are seated correctly and their position with respect to the leading edge is the same on both sides.

Use CA to glue it all together. (To permit step 6 to work properly I did not glue the parts indicated with pink lines in the photo.)

I also used some thick CA to glue the ribs to the CF tube.

Step 6: Add the lite ply pieces

Add the lite ply pieces front and rear on the top of the central section.

When all dry do a quick sand of the trailing edge of the wing where a few little bits of rib stick over the edge.

Note: the rear doubler needs to be sanded down to the same profile as the wing (otherwise it will not be able to fit through the body). So my suggestion is that you do some of this sanding before glueing the ply piece into place.

[TugBoat]

Step 7: Glue doublers to bottom of ailerons

I think that it is easier to shape the ailerons at this stage, once the doublers are on it will be more difficult.

So:

- Sand a 45 degree bevel on the hinge line while the aileron is flat

- Sand the top of the aileron so that the trailing edge of the aileron is a bit thinner. The aileron is 1/8" balsa, about one half of that would be better for the trailing edge.

Then glue the doublers onto the bottom of the ailerons - as the instuctions say: "Be sure to align the control horns."

Sand the 45 degree bevel onto the doubler to continue the one on the ailerons.

And that is just about it for the fish wings.

[TugBoat]

Additional stuff - servo mounts

The next bit of work is the body. So I am doing a few extra things first.

I selected the 'outside' of the body parts (the sides with the least burn marks) and then lightly sanded both sides of the parts.

I also like to have a little more to screw the servo mounting screws into than 3/32" balsa. So I cut some of the ply scrap into small 5mm x 10mm bits and glued them on the inside of the body sides at the end of the servo cutouts. This gives the screws something to screw into.

[TugBoat]

Wheel(s)

There are some things about the body brace 'A' part that need to be attended to if you want wheels.

Now this is a confusing part. There are lots of photos that show the 3D Fish with a monocycle undercarriage. So I started on that without really looking at the plans that closely.

However, the next morning when I was paying a bit more attention I noticed a few things:

- the plans show the battery located exactly where the support would be for the monocycle
- the plans also mention wheels
- the plans mention 1mm x 3mm CF reinforcements for the wheels
- there is one laser cut part that clearly has cutouts for two landing gear supports on the edge of the body, not a monocycle in the middle

It is a bit difficult to tell because the instructions don't mention the wheels. Obviously the whole airframe is much cleaner without wheels.

I was only building in provision for the monocycle should I decide to add it later.

So at this stage I am going to leave the monocycle provision in place and also make sure that I have make provision for the bicycle version as well.

This turns out to be easy because there is nothing that needs to be done at this stage for the bicycle version.

Note: see post #2 about the wheels. The design is for two wheels, although construction with a single wheel is possible. In the single wheel case the "post" needs to avoid the hole in brace A - because of the battery.

So really this whole post (and the pictures) are not really much use, because either way there is nothing to do!

[TugBoat]

Step 8: Glue body together

So now we glue the body together.

- Glue the 4 braces to one side
- Glue the other side on
- Glue in the vertical fin

I have not got to the fin yet, I decided to get the body sides glued together and then work out how to get the fin vertical.

The only tricks to this are:

- The front brace ('A') is curved where it glues to the body side, so you need to hold the shape - I used some tape

- I glued the other side onto all four braces at the same time, and used tape to hold it all tight while the glue dried.

[TugBoat]

Reinforce the vertical stab

There is a 1mm x 3mm CF strip supplied to reinforce the vertical stab. This needs to be cut to size and attached.

[TugBoat]

Sand and prepare the tail surfaces

Now looks like a good time to sand and prepare the tail surfaces.

Make sure you bevel the correct side of the rudder and elevator - the control horn slots are precut and have to go on opposite sides of the fish.