Tern (oz7694)

About this Plan

Tern. Radio control sport model for miniature 2 function RC and 0.5cc engines. This is a high wing design, but the plan also includes a 2nd sheet showing alternate fuselage layout with low-wing and an open cockpit.

Quote: "THIS IS THE LATEST design in a series in which the requirement was for a small model with a low capacity motor that could be flown in a small field. An 0.5 cu in engine, the Allbon Dart, was chosen in preference to a small glow motor for the following reasons:

1. Ease in operation (no glow battery required).
2. Very low noise levels compared with high revving glows.
3. It will turn a fairly large prop, which gives a better performance at low airspeeds.
4. Finally, the Dart is the best little IC motor I have ever owned, and seems to go on forever.

The disadvantages are:

1. They are messy, so a well sealed model is needed.
2. Power output is lower than an equivalent glow motor.

These shortcomings are easily remedied, however: the answer to 1 is to cover model with film and for 2, to build light.

Before constructing the Tern, I experimented with a number of small models using aileron elevator controls. A variety of wing sections were tried, with thickness ranging from 10 - 22% symmetrical. They all flew fairly well but the aileron response left as lot to be desired. At low speeds or on fast turns funny things started to happen, eg tip stalls. At high speeds powered with glows they worked but it was not what I was looking for.

About this time, I was looking for a model that would fit into a box on the back of a motor hike, instead of filling the back of an estate car. Even a 35 in (890mm) wing was too big, small two piece aileron wings are fiddly, so it was hack to rudder/elevator. The latest is the Tern and it fits my needs almost perfectly. It fits in the bike box and flys on the lawn at home. It even raised £40 for the RSPCA flying off the back lawn during a garden party! People paid 20p to guess how long it would stay in the air on 6cc of fuel.

It will fly like a free flight sports model - in fact it did so when I forgot to switch the Rx on. On a good day it's not averse to a good thermal, will slope soar, has been used as an R/C chuck glider but it really comes into its own as a low level Barnstormer, performing rolls, loops, pylon turns, 20ft circuits around the operator from a maximum height of 40ft to 3ft - or less if you keep your nerve!

The main requirement is build it light and it will turn tight. Anything over 11.5 oz (325 grams) complete with radio is heavy.

Construction: The model is straightforward to build but when choosing the wood, think in free flight aeromodelling terms.

Wings: Make a ply or hard balsa template for cutting out the ribs. I band saw them out of 3 in block and slice them. Use hard balsa for these, say 7-lb density. Main spars, 9-10 lb balsa, straight grain. Aux spars, 8-9 lb balsa, straight grain. Leading edge, 6 lb balsa, straight grain.

If a 2-piece wing is required, use paper tubes for the 1/8 in alloy welding rod wires. The 5 mm washout is a must, so build it in during construction. Don't worry if it's not quite right, the trailing edge section of the wing is flexible so adjust it when the film covering is shrunk..."

Direct submission to Outerzone.

Update 13/05/2016: article pages, text & pics added, thanks to RFJ.

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