HPA

This build is a continuation of a project started 40 years ago. My father and I constructed a 480mm (18.9") span Humbrol Tiger Moth.

The kit included nice light balsa for the sheet fuselage and tail, built-up wings and rather thick and heavy vacuum formed fuselage top decking and nose cowling.

We originally built it for rubber power, unpainted with a bright green propeller. I have a fond memory of it flying well for a short straight flight in my front garden. This strong positive memory ensired my motivation for free flight scale decades into the future.

Later; disaster struck when I was admiring (playing) with it on the floor when a large piece of chipboard was knocked over onto the model. On removing it; the plane resembled a swatted fly. The wings were intact, but the top wing was now touching the bottom with no gap. The fuselage was splintered and flat. Luckily; the vacformed parts were robust enough to survive. The undercarrage legs were splayed out wide, completing the swatted look.

This was a point of descision for me; was this  setback going to move me away from aeromodelling and onto something less risky, such as stamp collecting? I showed some strength of character and stayed with the hobby.

My brother, who I accused of knocking over the fateful board, a bureau lid, helped draw up replacement parts.

A new fuselage and tail were mated to the original wings, which were patched where the struts were forced through the tissue. The hardest part of the restoration was in digging out the ends of the broken struts from their recesses.

Unfortunately, this time I badly brush painted the model in unthinned Humbrol enamels. This increased the weight, which did not help the 2nd and last flight of the model.

A brand new Telco CO2 motor was installed with lots of downthrust and maybe some side thrust. A test flight was conducted over the short grass of a football pitch. A sharp left spiral dive into the frozen turf destroyed the inner rib bays of the upper and lower wings. This ended the career of the model. I kept the plan and the parts.

I was recently delighted to find the plan and print out of parts on Outerzone.
https://outerzone.co.uk/plan_details.asp?ID=13059
I was supprised to see the same funky green propeller and blue acetate for the windscreen.

Looking back, I am impressed with the kit. The design was simple, with the complex double curvature parts ready made, in plastic. The top deckings was rather thick and heavy; being.l made from the same plastic as the nose cowl, which the propeller nose bearing fitted to.

The balsa, in my particular kit, was nice and light. Important for the fuselage and empenage. The kit captured the important aspects of the full sized plane without the complexities of a strict scale model.

I decided to remake the kit and get it to fly! I will reproduce the attractive yellow scheme from the box photo and have procured some yellow Asuki tissue.

Annoyingly; I have the complete remains of the original model with the exception of the most complex and time consuming to reproduce part. The nose cowl.

Since I will have to make a form for this I decided to replace the three top decking parts as well.

I have rough cut the vac form moulds. These are the most onerous part of the build so I should concentrate on it. Once complete, it will be plane sailing for the rest.

I will power the model with a restored Telco CO2 motor. The original plan suppliment recomended no right thrust and  5 degrees downthrust.

Can anyone confirm this recondation or suggest more reasonable values to start with?

Good luck with the archaeology!  :)

That doesn't sound right unless there is some very unusual trim that I don't understand. If the wings are at about +3 degrees and the tail zero, the motor should be about 1 degree down and 1 degree (max) right. Top marks for perseverance!

Thanks for the advice Bill. I attach the rather suspect instruction sheet from the kit.  I note that the tank is shown immediately behind the motor, where it will maximally effect the CG as the gas runs out. Whilst we unwisely replicated the downthrust; at least we installed the CO2 tank in the front cockpit, under the CG. We also installed a thick ply plate to screw the filler nozzle; as recommended by Telco. I will not be repeating this.

I have now carved an MDF form of the turtle deck and am about to make the first attempts at plunge forming. I am using disposable plastic plates. They used to be widely avaiable at party shops in a wide range of colours, but are now generally replaced with paper. So, I have to order them on line. Annoyingly, the last ones had a diamond pattern embossed on them. Luckily, this vanishes as the plastic reaches the correct forming temperature. Inhave ordered some yellow plates, which I hope will not stand out from the balsa under the tissue.

I am also slightly baffled as to why the Humbrol CO2 motor is not shown mounted inverted, as we did with the Telco.

Yes there is no need to invert and it doesn't add much to the appearance!

It might be worth comparing with the excellent Stan Cole plan:
https://outerzone.co.uk/plan_details.asp?ID=3003

It shows 1 1/2 degrees down and 1 right, which sounds right to me.

This is a nice project. I wasn't aware of the Humbrol Tiggie. I checked out the other Humbrol plans on Outerzone too, and found the Wasp, which was a very simple stick fuselage rubber job, but with a built up wing. I made one about 50 something years ago after getting the kit as a birthday present. It came with an attractive black and yellow striped sticker saying 'WASP' to stick on the wing. So thanks for bringing back that memory!

My first attempt at plunge forming the turtle deck. The photo shows my third piece of plastic moulding attempt using a rather roughly finished mould.  The mould is straight tapered in side profile, but curved in planform. I am confident that wetted 1/32" balsa could easily be made to fit. But, I want for the construction to be in keeping with the original.

Luckily, the mould just fits into the flat centre of a 9" disposable plastic party plate.  The plastic is sandwhiched between a 4mm MDF frame and thick card with an aperture with less than 1mm clearance to the male mould. Bulldog clips hold the assembly together whilst it is heated and inch or two from the nozzle of a heat gun set to 300 degrees C. I was annoyed when I bought the plates,  over mail order, that they had a diamond pattern and were not smooth. But, this pattern soon disapeared when the plastic was heated near the required temperature.

Several attempts at heating and plunging were needed. I wondered if oven heating might prove more straightforward.

I had earlier constructed a vacuum form box to fit the 9" plates. This seems unessarary so far, but may be needed when tacking the cowling with its concave contours.

The resulting part has a thickness of only 0.008", whilst one of the original parts was a rather substantial 0.034". It may prove too delicate to withstand being tissue covered. I might perform a trial of the tissue covering to find out, since I have not used Asuka before; in particular over curved sheet surfaces. I can always double or tripple the skin; if nessesary

Interesting project, I didn't know that Humbrol made stick & tissue models (ok, stick, plastic & tissue..). I like the old silver and maroon paint scheme, even though the paint used didn't help with weight watching  :)

I have started balsa bashing

I recall that the crash of the original model 4 decades ago caused the curved inboard trailing edges of the upper and lower wings to split along the grain. Whilst a cartwheel on frozen, short grass is always going to break the structure; I have ever since  disliked using a single piece of wood over on a curved part where the end grain is exposed.  I love the elegance and superior strength of laminated parts, even though the glue significantly increases the weight. So, I departed from the plan and am employing laminated parts.

I noted that the inside profile of this trailing edge piece "W6" approximated that of a real of tape. So, i cut around the tape onto 2mm depron to make a circular form. A mylar sheet was added tona foamboard base.

I found some nice fibreous 9 lb/ft^3 1/32" balsa and cut it into 2mm strips. They were soaked in boiling water for quater of an hour or so. Each strip was pulled in tension around an aerosol can lid to impart a curve. They were coated in aliphatic glue and pinned around the circular form.

However careful I am in ensuring each strip lies flat onto the base, I routinely get slightly uneven cross sectional profile due to gaps under some of the strips. So, I cut the strips over thickess and will sand the assembly down to 1/16" (1.5mm) thickness when dry.

I should be a able to get a full set of parts from this single lamination.

Bill, thanks for going to the trouble of finding that nice plan showing more moderate thrust angles. I will apply them.

I have cut out the fuselage and empenage from nice, light 1/16" balsa. I plan to inset cross grained inserts into the fin and tailplane to prevent warping. As shown by the masking tape.

I will cover with yellow Asuka tissue. I am not aiming for a perfect finish or to completely fill the wood grain. I have bought some Deluxe Sand n Seal. Does anyone recommend that I apply before or after I apply the tissue?

I will apply a light coating of paint to acieve a uniform colour across the different construction materialz.

Anyun showed the Czech technique of colouring the balsa before covering, in his "Rieseler R III scale 1/25" thread. Much mass is typically added stopping the balsa from showing through the covering; more so than is required to give the general colour.

Does anyone think this sensible to pre-colour the whole fuselage and tail? It kind of defeats the object of weight saving if I paint such a large area.

I am considering a light dusting of Tamiya acrylic from an airbrush. Perhaps, this will help to unify the colour of the balsa/plastic/tissue areas.

Good point. Tamiya paints are naturally thin and provide good coverage. Especially when they're stirred well—and all paints should be stirred, of course. I only use Tamiya paints on my models. Plus, they retain their coverage even when further thinned.
Have fun building your great model.

Peter

Wing trailing edges

I have a clear memory of using too soft a strip for one of the wing trailing edges on the original model. It bend after doping the tissue. This was my introduction to wood selection.

Now, I weigh every sheet of balsa. It is rather overkill for a sports model, but keeping a record of what I used and seeing how it fares on the completed  model, gives me an insite.

I cut a 5mm strip from the edge of a sheet of #7lb/ft3 balsa. I was lucky that I could use the Jim Jones balsa stripper at its maximum width setting.

I had cut from the sheet where it was very opaque when held up to the light. This strip turned out to be  #10lb/ft3 density! The variation within a single sheet is supprising.

I dislike sanding trailing edges on the completed wings as I often damage the ribs and spend much effort repairing the damage.

So, I made a sanding jig consisting of a  length of 1/16" diameter piano wire cyano'd near the edge of a glass shelf from a fridge. It took several attempts to determine the nessesary inset from the edge. An 8mm inset, the width of a Lego brick, yielded the correct chamfer for the strip to match the rear profile of the ribs. The trailing edge of the strip is quite thick: about 1mm. I don't care about achieving a thin trailing edge, but don't want a change in slope at the trailing edge.

Using fresh sandpaper on a hard sanding block made short work of the job. A horrid scratching noise indicated that the sand paper was touching the piano wire and the edge of the glass. Thus, the strip was abraded to the correct profile. The strip was then inverted and a sanding sponge cleaned up the other side of the strip. The yielding surface allowed a little more material to be removed.

I did not induce any noticable bending of the strips due to sanding stresses. If they appear over the next few days then I will soak them in boiling water and weigh them down flat.

I was a bit worried about the stiffness of the sanded sections. So, I performed a simple buckling test by standing each strip vertically on a digital scale and pressing down with my thumb. The reading at which they buckle gave a measure of its stiffness. For indoor duration models; it is critical to employ the combination of the lightest and stiffest woid. But, for this application I only wanted to eliminate the outliers. I found a couple of strips of half the average stiffness and one of double!

So, I cut a strip from a different sheet and and repeated the process untill I found four trailing edges of similar stiffness.

I glued the leading and trailing edges together with the wingtip. I am generally impressed with the design of the kit. However; the straight trailing edge was butt joined to the curved part, as was the outer trailing edge to the tip. I have changed these to long lap joints.I have also added a slot for the lower spar to lock into.

I had pre-shaped the leading edge 1/8" square into a half round profile. I am slightly concerned how well this matches the rub profiles. But, it should have eliminated much sanding after assembly.

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.

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.

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

John

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...... ;D

Thank you for this tip! Excellent!!

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.

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.

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.

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

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.

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?

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

Great!, thanks for the feedback George.

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?

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

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