Aeromaster Project Diary


As I entered the usual 'what to build this winter' phase, I thought it might be a good idea to consult my fellow aeromodellers for ideas and inspiration. So I put out a message on our club's group mail list. I was keen not to constrain their thinking with too many restrictions but the one thing I did specify was that I didn't want a scale project. So what sort of response did I get? Well, there was certainly no shortage of ideas and the suggestions came flooding in: Blohm & Voss Wiking, Minimoa, Custer Channel Wing, Caspian Sea Monster,

 

So, I re-iterated the 'no scale' constraint and waited for the next wave: Lancaster, Halifax, Slip Wing Hurricane, Optica, Spruce Goose, Miles Student. . . As one of the contributors to the debate admitted, it seems that it was just too tempting to offer up the subjects that they themselves had always dreamed of building rather than pay any heed to my one and only requirement. So, much as I admired the breadth of ideas, it seemed that if I truly wanted a non-scale project, I was on my own.

 

I became acutely aware some years back that I have a number of models which, although they fly well enough, require just that extra bit of effort (and/or car space) to operate and so are in danger of becoming Hangar Queens. To combat this, I introduced my 'everything has to fly every year or it goes on the disposals list' rule. This has worked well, but it does make me very wary of adding to this collection of rarely flown specimens. Accordingly this Winter's project had to be a viable everyday flyer, but still sufficiently different from anything else in the fleet.

 

Seeing a fellow modeller enjoying himself throwing a Wots Wot around the sky, it occurred to me that I don't have a sport aerobatic biplane and, after a bit of internet research, the Aeromaster soon came to the fore as a likely candidate. In truth, I had reached this point before putting out the call for other ideas and, by the end of that consultation, the decision was pretty much made.

 

Plans & Planning

When I say the decision was made, it wasn't so simple! A quick search on the Outerzone site threw up a number of plans. As well as the original Aeromaster, there is the Aeromaster Too, the Super Aeromaster and a re-worked version called the Aeromistress. Essentially the Aeromaster Too provided more options on wing shape - various permutations of swept/straight, 48in/52in span for the upper and lower wing are shown. Another area of difference is in the wing fixings, particularly the upper wing and its supporting cabane structure. The original had banded on wings top and bottom but the later variants show a number of different approaches. The main visually distinguishing feature of the Super Aeromaster is a pseudo-radial cowl ln the front end.

 

You will be unsurprised to hear that, by the time I had made my choices on these various aspects, none of the plans actually reflected my permutation of option selections! In the end I had the Super Aeromaster plans printed off, on the basis that I prefered the cabane arrangement shown on that particular plan. I don't intend to do the faux radial front end but, since mine will be electric powered, the alternative 'upright engine in the fresh air' nose is equally inappropriate so I'll be doing my own thing in that area.

 

One of my other self-imposed rules is that Winter builds can't be started until Spetember. Of course 2020 was the Covid 19 and lockdown year (I really hope I don't have to come back later to edit that to read 'the first Covid 19 year'!). Once the UK lockdown began to be eased we were able to get out and enjoy some fine flying weather but this came to an abrupt end at the beginning of August when a prolonged spell of wet and windy weather set in. I have to admit that this resulted in me straining at the leash somewhat to get started on the Aeromaster. However, whilst I did spend quite a bit of time ordering bits and pieces and getting a wood order together, I'm very proud of myself for not picking up the scalpel and glue pot before the due date!

September 2020
The general build plan for this model is to tackle the lower wing first, followed by enough of the fuselage to sort out the wing fixing. Then I intend to build enough of the upper wing to be able to sort out the cabane structure and the upper wing attachment method.

So the appropriate section of the plan was duly cut out, spread out on the building board and given a good dose of looking at. Considering that this is a parallel chord wing, I was somewhat surprised to see no fewer than nine rib section drawings for this wing variant alone - there are only nine rib stations in each half panel! The numbering of the ribs is also somewhat eccentric - R1 is the tip rib, R7 is the root rib then, working outwards we have two R3s, R4, R5 and three R6s. Attentive readers will have spotted that R2 doesn't put in an appearance at all!

A quick peek at the upper wing plan reveals that R2 is used in the central section, so that at least clears up that mystery so it was then a case of playing 'spot the difference' between all the separate drawings, after all, one of the benefits of a parallel chord wing is that the wing ribs can usually be cut out in bulk.

So, I traced one of the rib drawings then offered the tracing up to each of the other drawings in turn. Sure enough the ribs were all identical in outline. Furthermore all have the mainspar and rear spar cutouts. The outer ribs will clearly be cut later to separate off the ailerons but the only other differences seemed to relate to the installation of the aileron servo and bellcranks. This of course was not relevant as I planned to fit separate aileron servos in each wing panel.

All ribs are shown on the plan as 3/32in balsa but I opted to use 1/16in for the outer ones and 3/32in for the inboard ones.

So at last, a couple of 1/16in ply templates were cut out and I set about making a couple of batches of ribs.

A set of spars were stripped from firm sheets of 6mm and 3mm balsa and a dry assembly showed that everything fitted pretty well. Before any assembly took place though, R4 underwent some work to accommodate the aileron servo and holes were cut for a wiring tube in the two R3s. With the servo in this location, the lead is just long enough to reach through the top skin inside the fuselage without the need for any extensions.

Some 1/4 square balsa offcuts were pinned to the board to support the ribs then the lower spar, ribs and upper spar were fitted in one operation, along with the 1/16in balsa shear webs which served to keep the ribs nice and vertical. The root rib (R7 remember!) was omitted at this time becaue this lower wing has dihedral so the root ribs will need to be cut to accommodate the ply joining braces.

The underside trailing edge sheeting was tackled next, followed by the wing tips. I opted to cut a 3/32in wide strip out of the tip rib doublers to form a rebate for the wing tip to locate in (see below). This made it easier to ensure that the wing tip plate was correctly positioned when it was glued in, so all I had to do was prop the wing halves up vertically to make sure the tip plates didn't droop while the glue dried.

Once the tip plates are in position, they are braced top and bottom with extensions to the main and rear spars.

Eagle-eyed viewers will have spotted that the underside leading edge sheeting has also been fitted in these last two pictures (I forgot to take any photos). This of course is all part of the plan to do as much work as possible on the two separate panels before joining them together.

However, the moment inevitably arrived when it seemed pointless to put off the grand joining process any longer, so the three ply dihedral braces were cut out and dry fitted. A quick check showed that the dihedral angle was spot on.

Okay, I've cheated again inasmuch as the above two pics were taken before fitting the underside sheeting and wingtips, but in the intertests of narrative continuity, they really should lead into the next two. The first of these shows the 1/16in ply braces being fitted to the false leading edge and to the rear spars. A piece of 1/4in balsa was then cut to fit between the upper and lower mainspars before fitting the final 1/16in ply brace shown in the second pic.

The next step was to install two more pieces of ply for the wing fixing bolts to bear on. 1/16in ply was used at the rear, followed by a piece of 1/8in ply up against the leading edge brace. The front portions of the central rib and rib doublers were slotted to fit round the plate and installed at the same time to help keep everything lined up.

The spaces above these ply plates were built up with balsa and then sanded back ready for fitting the leading edge and trailing edge 1/16in balsa sheeting.

Having marked up the aileron for separation, it looked like the tip rib could be a bit prone to damage so I thought it prudent to fit the sheeting to the wing tips before cutting the ailerons free. The ailerons were then cut away, faced and hinged. The leading edges strips were added too. They have been planed back roughly but final sanding will be delayed as long as possible to minimise the scope for hagar rash!

The last job of the month was making up the aileron pushrods.

I had hoped to have completed the lower wing this month but, as you can see from the pic above right, there's still the centre section sheeting to be done, plus a few more cap strips. However, hopefully within a week or two I'll be able to make a start on the fuselage - at which point a whole raft of decisions will have to be made!

October 2020
So, first job of the month was to fit the centre section sheeting and cap strips, first to the underside of the wing and then the top, remembering to fish out the servo leads first.

With the lower wing more or less complete, attention turned to the fuselage. The plan shows a 'Box-Lok' construction incorporating some thick balsa interlocking plates around the wing fixing area. I decided to go a more traditional route and cut out a pair of 1/32in ply doublers.

Holes were cut in the doublers to accept 3mm ply plates for the front and rear wing bolts and a 6mm ply undercarriage mount. I'm using an off-the-shelf fibreglass undercarriage which meant that the mounting plate had to be re-positioned to bring the wheels into the right position.

These ply parts were dry assembled and offered up to the lower wing to check the fit.

The fuselage sides were then cut out (I opted for some fairly hard 1/8in balsa rather than the 3/16in shown on the plan), and the ply doublers were glued on. Former positions were marked on the insides and 6mm x 3mm longerons glued in place.

I'm keeping my options open for former positions around the wing leading edge and battery stowage area.

I've recently acquired a SLEC building jig so, to be able to make best use of that, a few formers were built up out of 3mm balsa, leaving off the upper section, so that the fuselage could be constructed upside down, flat on the board.

Just one more dry fit before clearing up to make way for the jig . .

With the jig set up, the fuselage sides and formers were glued together, then the tail was pulled in and the rear formers added.

The jig seemed to work pretty well. However I did cut out a couple of rectangles of foam board which have been dropped between the sides to help keep everything square and, as you can see from the picture, I found that rubber bands across the L brackets helped to pull everything tight while the glue set. Others have commented that the jig would be improved by the use of metal (perhaps aluminium?) for the brackets instead of the plastic. All down to keeping it affordable, I guess.

After a bit of careful measuring and drilling, T-nuts were fitted to the wing mounting plates and undercarriage mount, then the model could be assembled for its first 'bench fly'.

This can give an illusory impression of completeness (especially for a biplane!) but it nonetheless is always a good motivator for pressing on with the build. It was also good to see that the undercarriage position looks reasonably sensible.

A quick bit of measuring confirmed that the wing sits nice and square on the fuselage, so the jig seems to be doing its job so far.

While the jig was in place, I popped the fuselage back in it and fitted the underside stringers. I'd tried to be clever by leaving the formers uncut to ensure that the stringer slots would line up. However, cutting the slots in situ proved to be a very tedious exercise. I think next time, I'd trim the formers back so that the stringers can simply be laid on top then, once they are aligned, filler pieces could be fitted in between them.

Anyway, the next job is to sort out the cabane structure, for which we need the upper wing. . .

Whereas the lower wing is straight with a bit of dihedral, the upper wing is flat but swept back. There are numerous ideas around on how to attach the upper wing to the cabane wires, some easier to build than others, some more of a fiddle to rig at the field. I changed my mind on this many times and, as I finally approached the decision point, my favoured option was to build the wing in three pieces so that the central section could be left permanently attached to the cabane. At the field the outer panels would then simply plug on. I even got as far as looking out some 'steel blade in brass box' joiner material, as often used to attach glider wings.

Having drawn up this three piece solution, it all looked eminently practical but, for some reason, I opted to sleep on it once more before finally commiting to it. To my surprise, I woke up having completely gone off the idea - I'm still not really sure why! The Super Aeromaster plan which I am basing my model on uses transverse cabane wires which locate into grooved hardwood blocks running the full width of the centre section of the underside of the wing. Rigging then involves screwing on retaining plates across the wires. As you can see from the first pic below, my modification of this approach has the majority of this underside groove fabricated from balsa, with a 1/16in ply backing. Two short gooved hardwood sections are retained but the idea is that these will stay permanently attached to the cabane wires. Each block has a 5mm threaded insert fitted so that the wing can be bolted on from above, retained by nylon bolts through 3mm ply plates as mocked up in the second picture.

This was all a bit of a fiddle to do but the only really tricky bit was drilling the hardwood blocks at the right angle for the threaded inserts. The idea is that the undersurface of the blocks should more or less conform with the contour of the wing, whilst the insert itself is vertical, ready to receive the nylon bolts from above.

After a bit more fiddling around, the underside could eventually be sheeted in. The first pic below shows how the wing finished up, the second pic shows how the cabane wire, with its attachment blocks, fits into the wing. Those plastic plates holding the blocks to the wire may be replaced by aluminium ones at some point.

Upper sheeting was applied next. Since there are no ply braces joining the wing panels, the joints in the sheeting were staggered in order to spread the bending loads a bit. The recesses for the nylon bolt heads were then opened up and hardened off with cyano.

There's a deceptive amount of work in the aeromaster wingtips. Once they were done, the leading edges could be added, and that was the upper wing effectively complete.

I think the progress made this month is testament to the rubbish flying weather we have had this month. As I write this, it looks as if we are likely to go into a second Coronavirus lockdown in the next few days. At this rate, this 'Winter' build could be finished by Christmas!

I think I'll tackle the tail surfaces next, then work my way forwards via the rear turltle deck and cabane area. Hopefully at that point I'll be able to get some idea of how this model is going to balance before committing to the battery and motor mounting arrangements.