Light Fighter (Lightfighter). Radio control sport model, for either electric power with Astro 035 cobalt motor, or glow power with .10 two-stroke engines.
Quote: "Dave's 'Light Fighter' squadron now includes twin, float and biplane versions. Our plan is for the single engine glow and electric versions, as well as a slope soaring glider.
Here's details of a versatile little 40 inch sporster that can be built in a variety of different configurations. Dave Philpotts explains what's deferent about his multi-mode design.
My interest in electric flight began some seven or eight years ago when I decided to take up aeromodelling again after a break of about twenty five years. In those days most of my models were 'fly-by-wire' (lightweight Laystratel) so radio control was to be a new experience. Not being able to fly didn't prevent me from buying a Humming Bird 20 motor, which the man in the model shop said was equivalent to a .20 size glow motor (I was naive enough to believe him). So armed with a motor, a handful of the wrong sort of nicads, some balsa sticks and a few feet of covering, I built myself something which looked like a Blackburn Monoplane. I never got more than an extended glide from that model - nor did anyone who could fly either!
After about two years of messing about, I decided to do the sensible thing and join a club. I went through the protracted experience of learning to fly (and land) using a variety of slope soarers, plus a Tinker Biplane (oz8754) with an OS 10 bolted to what might arguably be called the sharp end. I would never claim to have mastered the art of flying R/C models, but I felt sufficiently confident to believe I was ready for another crack at electric power.
P is for Prototype. By now I was reading the model magazines and seeing what everyone else was doing. I couldn't help but be impressed by what was happening in the scale world, four-engined heavies and the like, but my interest was now leaning towards aerobatics and this type of model, by contrast, seemed to be not nearly so advanced. Most models of this type seemed to follow the same basic layout. They would have fairly high aspect ratio wings, a flat bottomed aerofoil section and a rather simple angular shaped fuselage.
The reasons for this were pretty obvious - everything was done to build a lightweight model that would be efficient enough lug a heavy battery around on very limited power. Whether models had to follow this format, or not, was impossible to determine - such fundamental information as weight and duration was rarely given, even in specialised articles. Faced with a desire to design my own model, I had but two choices:
1. To follow established practice and design a model that I could be sure would fly, or
2. To do my own thing and to design a model which I couldn't be sure wouldn't fly!
I decided to go the second route and set myself three targets:
a) The model should look something like a real aeroplane.
b) It should have a symmetrical wing section which would allow negative manoeuvres.
c) It should be capable of taking off from grass.
The model was called 'Polar Bird', an appropriate sounding name borrowed from a racehorse. Polar Bird did fly and, up to a point, did what it was supposed to do, but with a weight of 48 ozs and 330 sq in of wing, the wing loading was such that it needed to fly fast to fly at all, and if it were turned too tightly or fell out of manoeuvre, stability would only be regained after it had hit the ground. An assessment of the situation suggested that a lighter version was not only desirable, but possible, so it was back to the drawing board for a second attempt. I also decided that the new design would be capable of being adapted for glow power and also slope soaring.
E is for Electric. The new design had the same wing area as 'Polar Bird', but the section was thinner to reduce drag, increase speed and therefore increase the lift. The rudder area was increased to enable stall turns to be performed and the ailerons were enlarged to improve the roll rate when slope soaring. An Astro 035 cobalt motor replaced the Kyosho 360PT and the power for the avionics was tapped from the flight battery. The latter two modifications, plus numerous changes in construction techniques, were to reduce the weight to 38 oz. The complete airframe, fully covered and ready for all the hardware, weighed just 10 ozs, so light compared to its size that it seemed to float upwards of its own accord when I put my hand underneath to lift it. The light weight, in conjunction with its fighter-like appearance, made the name 'Lightfighter' an obvious choice.
It would have been nice to say that the new model was an instant success, but it would not have been the truth. The problem was that the club's flying site is not smooth and the Astro's armature shaft is not hardened. I won't dwell on this matter, but one of them had to go and flying sites are hard to come by! What to replace it with? The 360PT used in Polar Bird had more than sufficient power, but being a 550 size motor the greater weight would have pushed the CG too far forward.
The question then became which of the enormous selection of 540 motors would be sufficiently light and powerful to fly Lightfighter in the manner intended. I would like to say that I carefully analysed all the technical information on 540 size motors and made my decision accordingly. What technical information? Reading through one manufacturer's catalogue, the descriptive text suggested that any of their range of motors would be absolutely ideal for any application I might have in mind!
I had a lucky break. A clubmate (Christchurch & District MFC) offered me a Ripmax 'Spirit 600' to try. This motor had originally been bought for use in a rather angular looking kit-built model, with high aspect ratio, flat-bottomed wings and subsequently rejected on the grounds of insufficient umph. However, I was using an extra (7th) cell and it would have been rude to refuse the offer, so I accepted. I discovered two things. Firstly, that at 5.5 ozs the Spirit was only 0.25 ozs heavier than the Astro - I had expected a bigger difference. Secondly, that if you screw a Spirit 600 onto the front of a Lightfighter and then open the throttle, the prop will cut the tops off the bumps on the runway, mow the occasional clump of grass and still have enough power in reserve to haul the model into the air!
Once airborne, the Lightfighter showed itself to be capable of performing loops, bunts, horizontal eights, cuban eights, stall turns, in fact almost any manoeuvre not requiring brute power, e.g. vertical eights and square loops. The running time was surprisingly long at 6 minutes using a 7 cell pack of Sanyo 1200's. The propeller used was a Graupner 7x4 power prop and the static thrust 17 ozs. You can buy a Spirit 600 from around £10, which is about one fifth the price of a cobalt motor..."
Light Fighter, RCM&E, August 1994.
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