A beginners guide to PSS - Part 1

A Power Scale Soarer is a flying model of a full-size aircraft that had some form of motive power (e.g. Spitfire, Mustang, etc.), flown as a glider. PSS seems to have maintained a constant following over the years, there always seems to be someone at the local club who has a PSS model tucked away in the attic but they seem to be more common in the hilly and windy north than in the genteel and civilised south.

Fancy a go at this?..

 

Quite a few people have expressed an interest in PSS, but have probably been put off by the apparently complicated construction and the impression that PSS models need lots of wind.

I have to say that there may be some truth in this if you're looking at building a large multi-engined bomber or an F-104 Starfighter (yes, it has been done), but it doesn't have to be that way.

Above: Andrews Hawk Flies well in next to no wind and will thermal too... Right : This Phantom has a deceptively generous wing area....

My favourite slope model is a reasonably scale PSS Hawk that will fly in next to nothing and will thermal very nicely, thank you. True, the wing is enlarged quite a bit but it's not noticeable in the air and the model is very smooth and neutral to fly.

On one memorable occasion at a PSS event in 2001, it out-soared most if not all sport models present and collected an embarrassing amount of silverware.

Most models of full-size aircraft can be flown off the slope provided that you're prepared to accept a certain amount of extra wing area for the more challenging subjects, but no-one notices because everybody does it (that's what I tell myself, anyway).

Walter Audley and Steve Griffiths with thier Vulcans...

Derek Price here with his beautiful Learjet...

Given a big enough slope and enough wind, you can probably fly anything but if you want to fly at your local slope in all weathers, you really do need to keep everything light. If you aim for a wing loading of between 10.5 and 13 oz/sq ft, you can't go far wrong.

The best PSS models are usually built from plans, simply because the owners have been able to select their own wood (a lot of the stuff in kits these days seems to be more suitable for building kitchen furniture than model gliders) and if they're really keen, they would have corrected some of the scale inaccuracies, if any. However, most people seem to want to build from kits wherever possible so I thought it might be instructive to look at some of the available offerings and discuss their potential usefulness as PSS models.

Even this AT-6 Type would fly as a PSS model if kept light enough...

Or how about Ron Collins F-14...

As a general guide, we're looking for two things:

a) A light wing loading; granted, we don't have to cart around the engine, fuel tank and engine-mount, and we may be able to get rid of some of the local strengthening that is usually there to soak up vibration, but it will still have to balance in the same place so a typical WW2 fighter *will* require some ballast.

A few seconds work with the calculator will reveal whether a kit has potential as a PSS model. For a 1/12 to 1/10 scale model (44 to 50 inches span), take the weight in ounces less about 8 oz for an I/C model or the weight of the battery (say about 14oz for seven cells) for an electric kit, and divide it by the wing area in square feet. If the number is around 14 oz/sq ft or less, you're laughing. Larger models can be a bit heavier, but try not to go much above about 16 oz/sq ft unless you have a big hill.

b) A reasonable airfoil section; most power models seem to be outfitted with an enormously thick semi-symmetrical section, presumably to limit the terminal velocity in vertical dives, or maybe to make the wing strong enough to withstand an untidy arrival at full bore. Whatever the reason, big, fat airfoils will make life difficult on the slope because they will limit the amount of speed that can be generated for aerobatics.

If your kit is afflicted with one of these then it can probably still be made to fly, but it won't have the performance that a glider-type wing section will confer. Most electric models don't have an abundance of power and therefore have much better wings; the old Balsacraft range mostly had Selig S3021 which is perfect for PSS.

Modeltech have an ARTC P-51 kit that is available from YT International, Daren has a review elsewhere on Bumpy Green and I've flown one of these for the past few years.

Modeltech P51 For PSS or Electric power...

RBC models version of this aircraft...

It's designed as a PSS model, it's light and flies well but it does have a few minor foibles that can be dealt with by building in about three degrees of washout each wing, or (preferably) about two degrees washout with the wing incidence reduced by one degree (raised 5 mm at the root trailing edge) - the latter solution will require some internal surgery and some re-sculpting of the radiator scoop but the model will look and fly better.

The wing bolt area needs to be reinforced with 1/32" ply or glass cloth, the radio gear including the elevator servo needs to be stuffed in the nose, which could really do with some cosmetic work with some extra wood, a sanding block and some filler to make it more Mustang-like. Having said all that, it's probably the easiest and quickest way into PSS. Mine has even won the odd prize.

Balsacraft are, I believe dead and gone for the moment but they deserve a mention here because all their kits built up into excellent PSS models with good airfoil sections. Provided they're kept light at the back end (perhaps best to stay away from the Solartex), they will all fly extremely well. Hopefully someone will start the production line up again.

RBC Kits are designed for electric power and are imported into the UK by The Electric Aeroplane Company. They look to be excellent candidates for PSS; I have the Spitfire kit (un-built) which looks very accurate but the fuselage construction is very light indeed (1/16" sheet), so a moderate amount of care might be required during the landing phase, although a glass/epoxy finish will strengthen it quite a bit. It has a reasonable wing section and should build into a very light model, although it is almost certain to need some nose weight.

Spitfire from RBC Kits...

A4 Skyhawk from the same stable...

The other models in the range (currently P-51D, Curtiss Hawk 75, AT6 Texan, Beaufighter, Tipsy Nipper, Skyray and A-4 Skyhawk) are apparently designed to the same high standard

.JR Models are also imported by The Electric Aeroplane Company and have a number of speed 400 twins (Mosquito, P-38 Lightning, B-25 Mitchell) that look very promising indeed as lightweight PSS models (provided that my arithmetic is correct!), and have reasonable airfoil sections. They have glass fuselages and foam wings and therefore cost more than conventional balsa kits, but they shouldn't take long to build.

JR Models B-25...

Simon Cockers F-16...

Cambrian Models have a series of fun-fighters (Spitfire, Mustang, Me109E and Hurricane) designed for 0.15 to 0.25 power at about 42" span. I've seen the Mustang and Hurricane fly in a reasonable amount of wind but I've no idea what they're like in typical conditions - I'd guess the wing loading to be just over 14 oz/sq ft for PSS.

Cambrian models do have a Spitfire and Mustang in a slightly larger size (55"), both these look very accurate and the Spitfire can be made to fly very well in reasonable lift. If the wing section is suitable, both these could be good candidates.

Flair Products have a Hunter and a Lightning for PSS or I.C. power. Both these have a fair amount of wing incidence but will fly as designed, given a big enough hill. The lightning needs a good blow and some reflex on the ailerons, as well as some optimisation of the c.g. position.

The Hunter is not particularly good when built as the designer intended but I've seen two examples that have been transformed by reducing the wing incidence to zero (or nearly zero) degrees. If you can build light and want a jet, the Hunter is a good choice provided that the wing incidence is reduced.

In Part two I would like to discuss the finer points of Building and finishing PSS models. I sincerely hope that this article has inspired some of you to have a go at this area of slope soaring and that you will come back and follow up this introduction by reading my next article.

See you then...

Article by Andy Blackburn for Bumpy Green Model Aerodrome
1st March 2002