Humbrol Tiger Moth

[Piecost]

Ribs cut out from reasonably light balsa. Those at the interplane strut locations, four on the top left, are from denser wood and have lighter 1/16" balsa doublers on each side. It is poor structural design since the doublers terminate just short of the trailing edge. They would look less structurally egregious if from 1/32", but the kit did not employ this wood thickness.

Since they they did not prove to be a weak spot on the original model: I have kept them.

The cabine struts are shown slotted into their ribs. I have departed from the plan and made them and the interplane struts from bass. I had a nightmare when repairing the original model in picking broken balsa struts from their rib sockets, so do not want the parts to be easily damaged. I will glue the struts sparingly with balsa cement, so they can be dismantled with thinners if repairs are needed.

I will next tidy up the spar slots and
stack all the ribs together and sand them to a consistent profile.

[Piecost]

The ribs were surrounded in frames made of "Nano Blocks". These are half sized Lego bricks. The photos show before the spar slots were routed and after with 1/16" square inserted. After this; the upper profiles were sanded to be uniform.

[OZPAF]

Your use of Nano Lego blocks for temporary jigs is interesting - nice work.

John

[Spiros]

It's an excellent idea!

Having only adult children anymore, I'm now looking forward to some grandkids to have the excuse for buying Lego again......

Thank you for this tip! Excellent!!

[Piecost]

I bought the Nanoblocks to fit between closely spaced Peanut wing ribs, where Lego won't fit. They do not feel as nicely made as Lego and are very fiddly.

I attach photos of the full chord wing ribs with pre shaped leading and trailibng edges. I had allready sanded the front of the ribs down to near the 1/8" leading edge thickness. The rear if the rib profile is over thick and a poor match to the pre shaped trailing edge. I hope that they sand down Ok.

I also show the shorter center section ribs that go over thw top of the trailing edge. Their fronts need sanding down to the leading edge.

The top wing is assenvled with ribs sandwhiched between Lego blocks to ensure that they are straight.

[Piecost]

I have made a cowling mould that I am happy with, on my third attempt. Note that I am not trying to reproduce an accurate representation of a real Tiger Moth; but to reproduce the kit part.  Frustratingly; I had lost the original piece so used the photograph of the vacuum formed parts from Outer Zone and the side view from my original plan as references.

My first attempt at a mould consisted of carving a single block of MDF using a washer to form the round area behind then propeller. It looked too blocky.

My third attempt turned the propeller region seperately and attached the lower part on completion. I did not perminaly attach them together, but attached them ,touching, to the plinth. It was a mild revalation to realise that I did not have ton faithfully reproduce the difficult concave transition between the two parts of the mould, under the propeller. But, the plastic will do that anyway (if not vacuumed down too hard)

The parts were taped to a blue foam plinth with a slight undercut which transfered to the plastic to mark the edge of part. The plinth was hollow so that the air intake apertures sucked the plastic into dimples under vacuum.

A vacuum box corresponded to the size of 9" plastic desposible party plates. The latter was bulldog clipped onto a embroidery hoop as a support. A heatgun was set to its maximum of 300 degrees C and heat applied to only the middle of the plate. The plate was held in a gloved hand. Annoyingly; the black plastic plates have a textured pattern. But they proved to be a useful temperature indicator. When they disapeared; the plastic was hot enough. The plate was then placed onto the vacuum box, over the mould. The hot air gun could only heat a small part of the plastic so I had to locally heat parts and smooth them down with an oven gloved finger.

The finish of the test piece is not great, but I am satisfied with the shape.

Disapointingly; I was not able to order yellow plastic plates, but found some 0.5mm yellow plasticard online. This is over double the thickness of the plates , so should be tougher and produce a smoother moulding. I plan to employ 0.25mm white plasticard for the top decking.

[Piecost]

I have finished assembling the wings. A change made relative to the plan was adding 1/16" square posts between the upper and lower spars. This should add some resiliance with little weight gain.

I am now procrastinating about sanding them before gluing the panels at the correct dihedral.

That is the last of the woodwork complete.

 I have to finish the last MDF mould for the vacuum form plug for the top of the nose. I dislike carving/sanding MDF; so need to pluck uo coarage to do this job as well.

[ZIP.58]

I was following your post again—and—I had to laugh: Lego bricks!
I've been using these for a long time to keep parts straight. Both the "small" ones and the "Duplo" ones. I filled the ones at the very bottom with lead shot and epoxy resin. That way, the heaviest ones are at the bottom and keep everything from shifting or slipping.
I'll be happy to keep following the report on this model.

Peter

[Piecost]

Thanks Peter,

Lead filled bricks are a good idea. On a different build I have employed Lego bricks containing magnets, onto a steel plate building board.

I recently bought some lead shot which was decanted into small baggies. This proved really useful in holding parts to dry.

A fortuitous thing is that the dihedral called for on the plan is the same as the width of a standard Lego brick. Actually; it is 3/4".

At the moment I use butterfly collecting pins. I can't remember the fancy name for them. They are an inch long and much thinner than dress making pins. They seem more precise and their flexibility seems useful.

But, I love the standard pins in combination with those little black, round pin clamps. A brilliant invention.

[Piecost]

Thanks for the useful pin clamp information. It could do with moving to a more findable location.

Back to the model...

CO2 tank location

I am ready to assemble the fuselage and I am thinking about the CO2 tank location.

I note the following examples shown on plans;

1. Humbrol Tiger Moth 19" span (this model)
    Tank immediately behind motor
    very forward of CG
    Protruding out top of nose
    Tank is vertical
    (See post #3)

2. Stan Cole Tiger Moth 18" span
    Tank in front of cabine struts
    Forward of CG
    Approximately 5 degrees from vertical
    Protruding slightly below fuselage

3. Aerographics Tiger Moth 20" span
    Tank half way between motor and front of cabine struts.
    Far forward of CG
    Vertical & fully enclosed.

My original installation:

4. My original installation
     Tank in front cockpit, at CG
     Protruding out of cockpit
     Vertical
     [See photo]

Proposed installations:

5.  Proposed installation with tank fully enclosed     
    Tank at front cabine strut
     Forward of CG
    Tank is fully enclosed
     Tank is 30 degrees from vertical (is this too much?)
     [See photo]

6. Proposed installation with tank vertical and protruding below the fuselage
Tank at front cabine strut
     Forward of CG
     Bottom of tank is exposed
     Tank is vertical
     [See photo]

Questions:

Is the tank being 30 degrees from vertical too much?
Is there an advantage or disadvantage for the tank being forward of the CG?

Option 6 is my favourite so far. What do you think?

[g_kandylakis]

30 degree angle is not too much, perfectly acceptable.

Regarding position relative to the CG, my personal preference is for the tank to be slightly in front of it, so that when it is full it tends to keep the model from excecive unscale climb and when it empties it allows for a more nose up landing apporach.

This is what I did in my PZL 24 and it is a very good flier. This is what I am also thinking for my d.H. 60 Moth Major currently under construction.

Option 5 for me. Please, no exposed tank (scale nut...)

George

PS. Pinclamp posts split and moved to the techniques board

[Piecost]

Great!, thanks for the feedback George.

[Piecost]

Further attempts at Vacuum Forming

This seemed to work best with the 0.25mm HIPS (Plasticard), the 0.5mm yellow did not form well. My attempt at a nose cowl looked like an accident in a custard factory!

In early efforts; the plastic picked up the slightly rough texture of the MDF. So, I gave it a few coats of Sand'n'Seal to smooth it. Unfortunately, this stuck to the hot plastic. 5 coats of mould release wax corrected it. This was fortunate as I did not have any talcom powder and was considering raiding the kitchen and using flour.

The plastic was cut with an inch margin tonthe part and was bulldog clipped into a thin MDF frame. It waz baked in the oven at 180 degrees C for about 5 minutes. When slightly drooping it was taken out and placed over the mould. The  plastic was not floppy enough to drape fully and I employed a hot air gun set to 200 degrees C and a gloved finger to push the plastic around the form. I am disapointed that this is not working better. If I increase the oven temperature then the plastic splits around the clips.

I have just about got parts for the fuselage top decking, but lack a good, complete nose cowl.

Can anyone prove advice?

[dputt7]

If you increase the area (margin) of the sheet it allows more plastic to be drawn and should mold nicer.

[Piecost]

Thanks for the advice, I will give it a go.