Cyril Carr's plan build article - 20/1/12
It was 12 year-old Matthew Evans who gave me the inspiration for the model presented here. You see, I was asked to help Matthew learn about designing, building and flying a radio controlled aircraft. He had won a grant called the ‘John Cooper Scholarship’ at Wareham Middle School, which helps young students develop ideas that could help them in a future career. Incidentally, Matthew’s ambition is to be an aircraft engineer or aviator - so, a worthy cause if ever there was one!
We decided that a simple electric (pusher) glider would be the best solution, and the outcome was the yellow model you see in a photo here. It proved to be a very easy aircraft to build and fly, and as a result, I decided to develop the idea just that little bit further.
Hence, we move on to Skylark. The unnamed yellow aircraft was of entirely balsa construction with an enclosed cockpit, whereas Skylark has an open ‘office’ and 1/32” ply fuselage sides, which gives a wood-stained antique aircraft effect. A further bonus of the open cockpit is that NiCad access becomes easy, and it allows the opportunity to add some 2D ‘character’ pilots.
Antique Solartex was used to cover the wings on my model, but Philip Beard, who built another version in order to ‘prove’ the plan, covered his in red transparent film. Philip used 1/16” balsa for the ribs instead of the 1/8” shown on the plan; furthermore, notice that the wing spars are 3/16” sq. spruce - a modification made after the prototype broke its wing during a loop.
ONE FOR ALL...
Skylark is suitable for a wide range of pilots with differing abilities, from the junior flier who is just starting out to an experienced modeller desiring of something relaxing to fly on a quiet summer evening. It’s so easy to control that you’ll find it’ll only need the occasional nudge in the right direction and yet, if you fancy, the model will perform very simple aerobatics.
This latter facet, coupled with a shallow glide path, makes it a darn good all-rounder, and a half decent thermal soarer too. Remove the motor, place an Rx NiCad in the cockpit, and Skylark can easily be flown off the slope. One further modification I intend to try is increasing the wingspan by two or three panels, to see how it affects the glide performance.
As an increasing number of people are discovering these days, 400-size electric models can give a tremendous amount of pleasure for a relatively small outlay. Building materials, a motor, the speed controller and flight packs are less expensive that the i.c. alternative, and offer many other more obvious advantages. Better still, this model can be built and made ready to fly in less than three days. I’ve intentionally kept construction very simple, yet the pusher configuration affords maximum crash protection for the motor, shaft and propeller.
THAT BUILDING LARK
Since the fuselage sides are parallel, the model can be built on a flat board. Cut the two sides from 1/32” ply, and make sure that the best surfaces are facing outwards. Mark the positions of the various formers, and use balsa cement or cyano to glue the various parts: nose block, formers FI to F7, top surfaces TI to T7, and lower surfaces B1 / B2, onto the first side. Note that B1 is laid cross-grain, and that all hatches are cut later.
Install the servo tray, plus three snake outer tubes - one for rudder, one for elevator, and one for the aerial. With this complete, add the second side to ‘box’ the whole lot in, then use weights to hold it all down. Whilst the tail is a simple sheet affair, do note that the fin slots into the tailplane to assist with alignment.
Attach the two skids, cut the NiCad, Rx and servo access hatches, then roll and glue the motor tube from two layers of 1/64” ply. The motor will be pushed in from the rear, and held in place with either sticky-back plastic or Diamond tape at a later date.
RIBS AND SPARS
Bearing in mind my earlier incident, I’ve found it best to use 3/16 spruce for the top and bottom main spars. The extra weight has not spoilt the flight performance, and it will at least prevent your wing from breaking should you loop the model with any enthusiasm. Decide whether you wish to fix the wing with rubber bands or dowel and screw.
Pin the trailing edge and lower spars onto the board, glue all the main ribs into position, then add the top spar and shaped leading edge section. When dry, remove from the board, and sheet the centre-section with 1/16” balsa.
The outer wing panels are next to be built, followed by fabrication of the ply dihedral braces (W3 and W4). Cut slots in the ribs as required, and glue the tip panels into position at 20° dihedral. Wing tip blocks and sub ribs can now be fixed, the latter helping immensely to give the wing that ‘old time vintage’ look.
SPIT AND POLISH
Although I used teak wood stain for the fuselage, and vintage Solartex for my wings, the flying surfaces also look nice in transparent film covering. It really doesn’t matter what you use, but if you do go for the glossy stuff, you’ll find that Diamond tape makes a good hinging medium - otherwise, you may as well opt for Mylar.
As for the running gear, I’ve found that the 6 volt 400 motor is best with a 6 x 3” prop and a 7-cell pack of 500AR or 600AE cells. Rondo or Jeti controllers with BEC can also be used to good effect.
Skylark has forgiving characteristics, and a very good rate of climb; indeed, you’ll soon be at altitude, and able to switch off for a gentle and very flat glide. As mentioned previously, the model will perform very nice loops and barrel rolls. It has an excellent high speed spin, with an easy recovery that you must neutralise quickly so that you don't enter another in the opposite direction. In terms of duration, look forward to flight times of 10-12 minutes. All-in-all, I've found that Skylark gives an excellent return for very little building effort, and it doesn't leave a large gap in your wallet. So, have a break from those complicated, expensive and high performance machines, and start some serious Skylarking about.
Designed by: Cyril Carr
Wing area: 180sq. in.
All-up weight: 17 - 22oz.
Wing loading: 17 oz. per sq. ft. at 22oz. AUW
Motor: 6V Speed 400
NiCads: 7-cell 500AR or 600AE
Control functions: Rudder, elevator, throttle
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