HPA

This is another refugee build from HPA2, I'm still working on it so I thought I might as well document it.

It started out as a really simple rendition of Gordon Whitehead's Bristol Scout all-sheet design that was a free plan in the December 1975 edition of Aeromodeller; the original had a D.C. Dart and I considered building one at the time because I'd managed to acquire a D.C. Dart but it was already installed in a Luton Minor and in any case, there was a lot of wood in the design and pocket money would only stretch so far.

For those of you not familiar with the design (see Outerzone (https://outerzone.co.uk/plan_details.asp?ID=2527)), it's a really simple rendition with banded-on wings and undercarriage, and is definitely cartoon scale. However, it's quite a well-drawn cartoon.

The chosen power set-up is a KP 02 which will (I think) probably be OK unless a truly vast amount of nose weight is required. The "parts" picture shows the original all-sheet wings that have been ditched (see below) because I didn't like the job I'd done with the wing-joining tape (PVA and gauze bandage, looked hideous).

I ditched the wings because I messed them up and was going to just replace them using more balsa from the stockpile, but started thinking (always fatal!) that now might be a good time to try doing some "proper" scale-like wings, so I had a go at drawing some using the Windsock Datafile drawings - it didn't take long, pic #1 shows an abridged version. I did have to employ the services of the FF Scale Brains Trust (Chris and Bill - thanks chaps) to work out how to affix the struts whilst minimising the possibility of a strut going through the covering in a crash.

Struts are in the process of being constructed and I dare to hope that we'll be able to have a classic Aeromodeller-style bare-bone picture by about the middle of next week.

Looks good Andy, and much better with built up wings

Just a few bits to tidy up some of the details before covering; wire undercarriage is held on with rubber bands (it's very sport-scale), wheels are balsa with card cones as per the plan. These should be OK as I don't anticipate any take-offs.

The wire clips for the interplane struts were aligned using the jig in pic #1, which seems to have worked OK.

Interplane struts are from 1/4" x 1/8" spruce with brass fittings, which were made from flattened 3/32" brass tube because it seemed to be a lot easier than cutting 1/8" wide strips of 1/32" brass sheet.

...and the beast is finally assembled.

I'm quite pleased with it, actually.

Weight of all the parts including the power system and battery is about 6 1/4 oz (178 g), to which I'll need to add a pilot (donated by a mate, so I won't have to carve my own), a windshield and a lewis gun, so I'm hopeful that it'll be about the same weight as the original (~9 oz).

(just remembered, I need to add mounts for the underwing-tip skids)

Covering will be Doculam for the wings with lightweight Modelspan over the top, everything else will be Modelspan apart from the cowl, which will be glassed. I think I have some lightweight Modelspan, otherwise it'll be EzeTissue with looks rather like Modelspan to my untutored eye.

Current plans are to airbrush it in the colours of Scout C 1611, but that may change if I stumble across a more attractive scheme that hasn't got a Lewis gun.

And I've also just realised that the original plan was to take the lower wing off to replace the battery - I'm expecting to get a fair few flights on one charge, but if I'm out for the day then it'll probably need a replacement battery at some point.

The trouble is that taking the lower wing off is now a bit of a Kerfuffle because of the way the struts are attached, so I'm going to need a hatch somewhere in the nose area.

WOW....beautiful details.  Hopefully some flying videos when done?

Quote from: gravitywell on Jan 23, 2026, 12:54 AMWOW....beautiful details.  Hopefully some flying videos when done?

Thank you kindly. It's possible that an old black-and-white kinematograph of a Bristol Scout over the Western Front might be unearthed at some point.

Anyway, back to the plot which is taking ages, far longer than would have been the case if I'd stuck with all-sheet wings. The attached image shows the wings covered in 16 Mil Doculam, which has turned out quite nicely; it's the first time I've used it, turns out if it's treated like slightly fragile Solarfilm, everything works Ok. It'll stick to balsa at 70 degrees Celsius, but can be taken off quite easily. It seems to need 95-100 degrees C to stick properly, and shrinks at about 115-120 C. Stubborn wrinkles might need about 130 degrees, but the iron starts to stick to the plastic at about 135-140.

Am just about to start covering with tissue over the top - I found what claims to be lighteight Modelspan in a cupboard so it's been pre-shrunk on a frame, so we'll see how that goes. Wish me luck...

Very nice builtmup wings. Did you have any problems with the colvering warping the struture? I look forward to finding out hownyou get on with painting.

Quote from: Piecost on Jan 31, 2026, 06:02 PMVery nice built up wings. Did you have any problems with the covering warping the struture?

The Doculam tends to warp in a bit of washout, I think because of the tip shape and the way the stuff has been stretched, but this is easily fixed by twisting the wing the other way and briefly running a hot iron over it.

Um. Well... that's not gone well.

Exhibit A shows the tissue on Doculam, which has reacted to the (non-shrinking) dope by slackening off, which has made a bit of a mess. I did manage to clean it off (mostly) and re-shrink the Doculam, it's not perfect as there's a bit of dope residue but it's on the under surface of the lower wing so it's not really noticeable.

Plan B is to forget about ordinary dope and use EzeDope instead, which I am assured will work. I can't do it today because it needs to be thinned with distilled water (de-ionised water will do, apparently) otherwise you tend to get chalky deposits on the surface. I'm sure it'll work...

Oh that's disappointing. Maybe the doculam was not sufficiently tightened so the tissue has shrunk more then the doculam? I've not used the the thinner doculam only the the thicker stuff but I go a lot hotter to shrink the stuff even with Mylar maybe more like 170-180, but then as I say I've not used the thinner doculam. Hope the Ezedope works for you

Remain calm, all is well (this is, of course, an "Animal House" joke)...

I am beyond pleased to report that tissue over Doculam with EzeDope does appear to work as advertised, although the stuff gets everywhere. The attached picture shows the bottom surface of the lower wing, both sides covered in tissue over Duculam with 50/50 EzeDope, and it they both look pretty good; you'd never know that there had previously been a c*ck-up.

A few notes for those of you who might be tempted to have a go;

• Distilled water is required for thinning EzeDope, otherwise you tend to get a chalky deposit on the dried surface. I used Halford's battery top-up water, I'm told that de-ionised water will also work.
• The technique is to lay the tissue on the surface and then spray it with water - it needs to be properly wet. Then after it's smoothed out, apply the thinned EzeDope. It looks, smells and feels very similar to other water-based varnishes, so other products might well work.
• The tissue takes several minutes to expand to it's final size when applied, so it's best to leave it a good five minutes before smoothing it out.
• I couldn't cut the wet tissue with a razor blade so ended up sanding off the excess with 320 grade sandpaper, which does seem to have worked Ok.

I'll probably get back to this on Wednesday, so we'll see how many coats of varnish are required to get a good surface for paint.

Not a great deal of progress to report, but the wings are now double-covered with 16 mil Doculam and Lightweight Modelspan, applied with 3 coats of thinned EzeDope; they look Ok, actually.

The tail surfaces are covered with Lightweight modelspan (three thinned coats). I think in future I'd be inclined to use Asuka tissue instead of modelspan, it's lighter, the finish is better and it doesn't soak up anything like as much dope.

I tried assembling everything and changing the battery over the weekend and it didn't go well - the only way to do it was to take the bottom wing off, and even then it was a hell of a job to get the battery out and put the new one in. I am, therefore, re-jigging the battery arrangement; the thinking is that the lower front fuselage where the speed controller goes can be a relatively large hatch, the battery slides in to a balsa pocket and is a reasonably tight fit, and the hatch is held on by the undercarriage retention bands.

Spent part of the available time today test-fitting the electrics to verify that the new hatch (photo 1) functions as planned; it seems to, and I'm now confident that the battery can be changed without fuss and that everything will fit. The front part of the hatch is held on by the undercarriage, and I think the best way of fixing the back edge is probably just a bit of tape.

The engine plate needed some relieving at the corners in order to be able to shove the cables and connectors somewhere out of the way, and the battery is now just a push-fit into a battery box that has replaced the disastrous velcro fastening arrangement (very difficult to remove the battery once attached). Some of you may detect a slight wonkiness to the battery box that I didn't notice until everything was in place, I'm sure it won't make any difference in the great scheme of things.

Just need to tidy up the fuselage, cover the fuselage add a few (a very few!) details and then start the painting.

Finally got the fuselage and cowl covered, that's all the covering done. I must say, though, that lightweight Modelspan wasn't as nice to use as I remembered.

I did the cowl with tissue and sanding sealer in the end, partly because it looked to be a bit easier than faffing around with epoxy or multiple coats of acrylic varnish.

Paint (Vallejo Model Air Aged White) is arriving on Friday, so I'm just going to spray most things with the base colour. Lots of people recommended Xtracolor Doped Natural Fabric, but it's an enamel paint and and I'm more comfortable with acrylics these days. We'll see how it goes, just hope I manage to avoid fingerprints in the paint.

...and of course, I forgot all about fairings to the cabane struts and undercarriage; I'll now have to wait a day or two for the sanding sealer to finish gassing-off, so we're probably looking at Monday for painting.

I'm wondering if I've done the right thing with the paint choice; acrylics are so much easier to use and Model Air paint is pre-thinned and is supposed to be useable straight from the bottle at relatively low pressure (15-20 psi), so the amount of faffing about should be minimised. Against that, there may be a tendency for the airbrush tip to dry out, and they might be a bit fragile - the advice is to wait at least 3 days before masking. Also, I'm not using primer because I want a see through the wings when done, so that might cause problems. I hope not, but we shall see.

The attached picture shows where we are with the airbrushing, the colour looks - from memory - pretty close to the colour of the various natural doped linen aircraft at the Shuttleworth collection, so I'm quite pleased with it. the paint isn't perfect but it's sport scale, so I'm not worrying about it.

The Vallejo Model Air paint is supposed to be ready to use straight from the bottle, but that didn't work with my airbrush set-up (Iwata TR2, 0.5 mm needle, 20 psi). The paint needed about 10% Airbrush Flow Improver and I also added a little less than 10% thinner, but I think it could have done with a little more.

I now need to leave it for another couple of days before (carefully!) masking for the next colours.

Thats a real nice bit of work on the cockpit coaming.  Looks perfect.

Quote from: gravitywell on Mar 01, 2026, 08:30 AMThats a real nice bit of work on the cockpit coaming.  Looks perfect.

Glad it looks Ok in pictures. Up close, it's not perfect by any means but I'm trying to convince myself that it'll do.

Anyway, this is where we are after another week - feels like a lot longer though! I've also airbrushed some white decal sheet for cutting roundels

I'm not very happy with the wheels, the masking hasn't worked very well. I might have one more go with a brush and then leave it be - it's a sport scale test bed and won't be winning anything, so as long as it looks OK from six feet away, I'll take a sensible pill and declare myself happy with it.

As it stands the task list looks like this:

• Clean up the remaining painting issues.
• Cut the decals with an Olfa circle cutter and apply.
• Draw on a tasteful amount of detail, where appropriate.
• Paint the pilot.
• Draw & print the serial numbers onto clear decal sheet and apply.
• Wait an extra day and spray on some gloss or semi-gloss varnish.
• (Almost forgot the windscreen.)
• Assemble and await calm weather.

Very nice job, good to see you fixed the covering problem.

If it's ready do please bring it to the next Trinity 'cos it'd be nice to see it in real life.

QuoteI'll take a sensible pill and declare myself happy with it.

Now that is definitely the way to go - it looks great to my untrained scale eye :)

John

Looks great Andy

QuoteIf it's ready do please bring it to the next Trinity 'cos it'd be nice to see it in real life.

+1 to lurk's comment too

Well, that was a bit of a kerfuffle.

I couldn't get any Sunnyscopa decal sheet so had to use what was available from Amazon, which was Hayes waterslide decal paper. This stuff hasn't got a layer of film to print on, it's just paper and either clear or white adhesive and needs spraying with 3 coats of "acrylic lacquer" before it can be printed on. Since I was going to cut-out some circles and rings using an Olfa circle cutter I thought that the matt Tamiya acrylic paint would probably be Ok if it was sprayed straight on to the adhesive layer...

Unfortunately, that didn't work because the cured acrylic paint just splits when attempting to apply the decal - the layer of acrylic paint has insufficient structural integrity.

I next tried Tamiya clear spray acrylic lacquer which just about worked Ok for the white circle background, but it didn't take the acrylic paint for the red and blue parts of the roundel - bit of a mess, really.

Back to the model shop again and the last roll of the dice before ordering some Xtracolor Enamels (which are known to work) was a couple of cans - red and blue - of humbrol acrylic spray lacquer; the colours aren't quite right but by this stage I'm past caring.

Thankfully, this seems to have worked and has produced some smooth, gloss painted decal sheet. Actually applying the roundels was relatively straight forward, I do need to get some Tamiya acrylic varnish on them tomorrow to stop them lifting.

More kerfuffle - I have just discovered that the built-up stabiliser has warped in about three different modes at once!

B*gger!

(can I say that?)

I really want to get this thing done so I've located some 5.5-6 lb sheet balsa and I'll just make as new all-sheet stabiliser as on the original plan, because it's relatively quick. I will, however, make the elevators adjustable.

Quote from: AndyB...the built-up stabiliser has warped in about three different modes at once!

Yus, werl, s'probably yer fractal folding innit? Or mebbe's sunnink quantum. Defin-ately not yore fault. :)

Here, have this consolatory cup of hot sweet tea and a digestive biscuit.

These things are sent to try us. Or so I used to be told...

Finally got a new tailplane made from 6 lb balsa, doped and covered in Esaki with hinges (4 x 0.5 mm aluminium sheet), it unfortunately weighs about 12 grams even before it's been painted, which is double what the built-up one weighs. That's about an ounce of noseweight, right there  :( .

On the other hand, it does seem to be flat rather than looking like a large Pringle.

Managed to find time (in between indoor flying and prep for same) to airbrush the solid sheet tailplane a doped linen colour. The tailplane is probably slightly rushed so isn't as smoothly finished as normal, but Time Is Of The Essence because the Oxford MFC "Bring a New Model" evening meeting is on Wednesday and I want to get it into a flyable state by then.

I'm currently going through my job list with a red pen and removing anything that isn't 100% necessary, but I'm telling myself that this is just a modern expression of the "Coates Doctrine" (as championed by the late great Eric Coates) where we ascertain whether the model will in fact commit aviation in an acceptable manner before we expend too much effort on the final details...  8)

I think that's a great idea... and the Scout is looking very nice BTW

What a neat effort Andy. good luck with it.

Ok, so it's sort of finished, for a very flexible value of "finished".

It's in a suitable state to try and commit aviation - it's missing a pilot, windscreen and rear fuselage serials but if it's going to pile in at the first attempt then I won't have wasted any extra time adding the details.

It's also rather heavier than hoped at 9.57 oz, including 1.7 oz of nose weight. I think it'll probably fly with the KP02, but the power system will probably be working quite hard.

All I need to do now is to find some long grass and a calm morning to do some test glides...
 

Looks great Andy, nice model

Well good luck with the trimming Andy. hope you find plenty of KK grass.

John

Nice.