PROFILE WING STUNTERS
(Reproduced from Airborne magazine September 1983)
(Author: Frank Coombs)

Introduction and Aims

While I feel all Control Line beginners should ideally start with at least a 6cc (.35ci) engine. I have a soft spot for the smaller motors. Unfortunately the smaller aircraft suffer by flying poorly and have lacked line tension unless the engine run was perfect. The larger engines and their planes were steadier to fly and maintained their line tension even when they were off tune, giving confidence to their fliers. So came the challenge to improve the lot of beginners and sports fliers with small motors. The planes had to be simple, for lazy builders (like me).

The original model in this series, the Black Beast, (mini Gieseke-Nobler) was built in less than one week and has proved to be a charmed airplane. It has given me the best expenditure-of-effort-to-f lying-enjoyment ratio in my 25 years of flying experience. While my personal flying skill is a little lacking. Russell Bond (5th place at the last Nationals in CL Aerobatics) flies the full stunt schedule with my Black Beast, much to the amazement of many who have seen it. Considering the utter simplicity, flat plate wing section and small diesel power, this reaction is not surprising.

Frank Coombs and his mini Gieseke Nobler "Black Beast"

The control system is a straight copy of that used by Bob Baron. The quote from FLYING MODELS magazine (February 1981) "The last improvement sought was a greater mechanical advantage in the control system, in both decrease physical effort at the handle, and insure adequate power to move the controls in low tension manoeuvres, such as the overhead eight and the top two corners of the hourglass. Solutions to these problems were found by Bill Netzeband using the results of his detailed analysis of control line aerodynamics. that were confirmed by flying a series of' ;A planes during the last year".

Also, "The last problem, that of reducing pushrod loads and control handle effort was again solved by Bill with his trusty T-59 calculator. Basically this consists of using a four inch bellcrank and control horns that are at least one inch long. In a manoeuvre such as the overhead eight, only about two G is available to actuate the controls With a system with pushrod loads that are too high. the airplane is simply incapable of generating the necessary forces to move the control surface sufficiently REGARDLESS OF WHAT THE PILOT DOES. This is why many ships have trouble doing a tight overhead eight or a good hourglass".

PLEASE FABRICATE THE CONTROL SYSTEM AS SHOWN. DON'T SUBSTITUTE A SMALLER COMMERCIAL BELLCRANK.

About the wing

By all means change the outlines to your favourite shape Copy a known model by any of the top stunt designers, or of course, use your own design. (Stick to a proven stunt design if you want to be certain of success. Suitable subjects appear almost monthly in the American aeromodelling magazines ID).

It's worth remembering that most, if not all, 2.5cc stunt models have been too small in area and far too short. Long fuselages are inherently correct for small stunt models. Wing loading must be low This doesn't mean making the structure weak, but rather making the plane large enough.

10.5 oz/square foot (32 grams/square decimetre) or less works. Use the chart below to calculate your target weight.

As done in other stunt articles, this is wing area including flaps but not including stabilizer and elevator area.

In this day of proper stunters having 18% - 25% thick airfoils, our approximately 2.3% seems ridiculous, yet 2.5cc motors don't seem to have the pulling power to work with thick sections. Around 10% thick may be optimum: however, we haven't bothered to find out as we are having too much fun with our current models!

Engine and prop considerations

It is most important that you use an upright or inverted engine as the engine tune is far better than an engine mounted in the more usual sidewinder style for profile models. Use a mild amount of out-thrust. Best propellers tried to date are Master Airscrew 8x4 nylon, Esaki 8x5 wood.

THE LARGE BELLCRANK, THE WING FLAPS, LOW WING LOADING, UPRIGHT OR INVERTED MOUNTED MOTOR AND THE FUEL TANK AS SHOWN ARE WHAT MAKE THESE MODELS SUCCESSFUL.

Before thinking of simplifying these features or cutting corners remember: If it's worth doing, DO IT RIGHT. Having said that, be my guest and further refine these features. Glow 2.5cc motors all need muffler pressure. Connect the muffler pressure outlet with silicone tubing to the fuel tank breather pipe. Depending on what engine you wish to use, check to see if your muffler clears the fuel tank with the engine mounted in an inverted position. For example: The Fuji 15 clears the fuel tank. The OS 15 can only just fit if the fuel tank is dimpled with a hammer and resoldered. The Thunder Tiger Mk.3 and maybe others can't possibly fit.

Motors without adequate muffler clearance will need to be mounted in an upright position. DO use the muffler. You need the muffler pressure to make the fuel system work with glo motors. Diesels used so far are PAW 2.5D. TAIPAN Series 71 and SUPER TIGRE G20D. It is a pity they can not tolerate effective mufflers without overheating. All diesels however ran perfectly adequately without muffler pressure. Wish ENYA would make an Aluminium-Chrome 2.5cc diesel. Their 11CX converted to diesel with a muffler is excellent. Many thanks to Dick Sarpolus whose many magazine articles have always insisted that FLAT PLATE WINGS CAN STUNT.

More ideas and features

Use medium density balsa for the wing; medium to light for all else. although flaps should preferably be stiff quarter grain. While the original Black Beast got away with no centre-wing re-enforcement, we must recommend hardwood stub spars, as the wing bends considerably in manoeuvres and as our newer designs are longer in span and have a wider wing flap chord, this tends to give higher bending loads on the wing centre, hence the re-enforcement. So remember: on your first flight turn a blind eye to the wing bending. Simply ignore it.

DON'T SUBSTITUTE A COMMERCIAL FUEL TANK OR USE ONE IN THE CONVENTIONAL POSITION.

What is presented here is a vast improvement on all others used for small motors seen to date. This tank is a uniflow, forward-feed chicken hopper type and is designed to give a steady peaked run. as we can't afford a two stroke/four stroke run. Any four-stroke running would cause a lack of line tension, so don't test fly a motor that's new and needs running in on a windy day! The needle valve setting must be peaked, not set safely rich. (Frank's tank doesn't suffer from the rich to lean syndrome, making a fully peaked setting quite safe. ID). Tilt the outboard wing down as you fill the tank to allow the chicken hopper to fill and the air to escape.

If you are apprehensive about soldering a fuel tank. ask a modeller accomplished at soldering to teach you. The same applies to fibre-glassing the nose of the plane. While gauze bandage and balsa cement can do an adequate job. ask to be taught, as there may be future projects of yours that can benefit from fibre-glassing.

CONSTRUCTION.

Trim the edges of the 4.8mm (3/16in) balsa wing sheets so they fit together. When gluing them together use straight hardwood sticks (12.7 by 95mm engine bearer wood works well) and large rubber bands as shown in sketch No.1 to keep the wing flat. Use 48in sheets if possible. Otherwise key short pieces onto 36in stock as shown on sheet 4. put the jointed end on the OUTBOARD side when assembling the model.

When the wing is dry trim it to outline, add the wing tips (grain fore and aft) and the two tip flap extensions Slot the wing for the hardwood spars. Trim the spars to length and glue them into place, again using the hardwood strips and rubber bands as before. Make the flap and elevator horns as shown in sketch No.2. Solder the joints well and test them for strength.

Cut the fuselage to outline and cut out the wing and tail slots. It is very important that these are exactly parallel with the engine thrust line. Make small notches at the rear of these cut-outs to give clearance for the flap and elevator horn wires.

Cut the tail and elevators to outline. Round off the wing and tail to section and trial fit into the fuselage. Check their alignment and squareness. When satisfied, remove the wing and tail temporarily, slip the flap and elevator horns into their locations in the fuselage. Now glue the wing and tail into place permanently, checking their alignment and squareness again before the glue sets.

Now cut the wing flaps to outline and carve them and the elevators to section. Slot the root ends for the horn wires. Hinge them onto the wing and tail and glue the horn wires into the flaps and elevators with epoxy glue. Check that both flaps and elevators lie in the same plane with respect to each other when viewed from the rear. Round off and add the fin, dorsal fin and the rudder. Bevel the edge of the rudder to give the correct off-set as shown in the plans.

Mark parallel lines on either side of the fuselage for the engine bearers. Glue the bearers to the fuselage, spacing them apart with scrap balsa to the width of your crankcase. Mount the engine using blind mounting nuts (Kavan 3mm are recommended). For inverted installations most Oriental engines will need 3mm aluminium spacers between the engine lugs and engine bearers in order to line up correctly with the fuel tank Cut away the fuselage for the fuel tank and install it using epoxy glue.

Glue scrap balsa around the nose and carve to a smooth contour. We recommend strengthening the nose with one layer of 2 oz. per square yard fibreglass cloth applied with epoxy or polyester resin.

Make the bellcrank as in sketch No 3. Bend the undercarriage (3mm steel wire) to shape. Cut out all the plywood reinforcement pieces and glue them in place on the wing. When dry, drill the bellcrank mount Install the under-carriage using J bolts or U shaped clamps and screws. Add the wheels. (Our favourites are the new foam type I.M brand of 50mm diameter.) Be careful not to melt the wheel hubs when soldering the retaining washers to the undercarriage legs! A piece of thin cardboard between the wheel and washer when soldering helps.

Due to the likelihood of warps that could be caused by dope on the large flat balsa surfaces, we used clear nitro-cellulose lacquer thinned with lacquer thinners. which are available from industrial paint suppliers. Five coats are used. sanding with 500 grit paper between coats. Two of the diesel powered models used coloured Plasti-Cote. a fast drying lacquer available in spray cans Dupli-Colour is a more expensive alternative that is fuel proof to mild glow fuels, whereas where the former is not. and comes in a multitude of colours.

Carl Goldberg hinge tape using balsa cement is used on our flaps and elevators for hinges. Use any of the popular glues for the model construction.

Install the control system. Metal Kwik-Links are used on the threaded pushrod ends. Pushrods must be straight and rigid enough not to buckle under compressive loads. Solder the Kwik-Links to the pushrod threads for safety after the model has been flight trimmed. Arrange the lead-outs so that the front lead-out connects to the UP line i.e. crossed over between the bellcrank and lead-out guide.

Well. that's it! May your model be as successful as ours are. Why not send us a photo. If it is suitable AIRBORNE will publish it.

Click on Plan image for enlargement.