RC airliner design - Ian Monty
                                                     

                                                           

The purpose of this article is to share with you some of the model design techniques used in EDFmodel's kits. A considerable amount of research and development has taken place.

Background

In my youth, much frustration occurred at the airfield with many models returning home in pieces. Much hard work was destroyed after a flying accident because the model was not built with part replacement or impact resistance in mind. Nose cones which were lovingly carved, painted and attended too with decals would have to be re-worked even with a small accident.

"I believe more energy in the model workshop should be directed toward developing better assembly technologies for the construction of electric models. One answer is designs that are modular which offer improved model impact resistance with ease of maintenance. One of the most demoralizing feelings any modeller can have is to return from the flying field with a handful of pieces that was once a model. Either the model is totally destroyed or a large amount of rework is required. I do not believe the challenge in scale modelling is to spend large amounts of energy building a model and then putting all the effort on the line for a win or bust flight. Rather I believe that the challenge is to actually fly the model and keep it flying. If an accident occurs a repair should not take so much time and effort that it destroys the modellers enthusiasm".

Build dislikes

- wood in the construction of models (inconsistent mechanical properties and the need for finishing)

- chemically messy techniques

- rigid construction that produces little impact resistance

- finishing techniques with variable results.

Build likes

- the same part replacement approach to building that is clearly evident with RC cars & helicopters

- a simple quality surface finish

- assembly techniques with maintenance & impact resistance in mind (fasteners, double sided tapes, velcro & aerosol glues)

- alternative materials -a model design that is strong and light weight.

Fuselage design

The fuselage shells are an intrinsic part of the EDFmodel construction design -a simpler and light weight fuselage that has sufficient flexibility while allowing for easy maintenance.

One design answer is to build the fuselage starting with a chassis. The design of the chassis must carefully meet minimum tensional and bending moment requirements. This is particularly important when discussing the relationship between the tail plane and wing. The design of the chassis does not have to include any redundant strength and so the opportunity exists to minimise model weight. Components are designed so that should any part fail it can be disassembled and replaced.

Elevator, fin, wing mounts, undercarriage (U/C), servos and battery mounts all are fixed to a chassis.

The thin vacuum molded plastic fuselage shells are joined with the use of a joiner strip. The fuselage shells are loosely assembled to the formers with plastic pins. Upon impact the shell can move slightly to absorb some forces. An analogy might be the same way our skin slightly moves over our body tissues. The fuselage shell should be easily disassembled from the chassis formers. Decals are applied on the outer fuselage surface.

From an impact resistance point of view this design produces a good result. When an accident occurs impact energy is distributed throughout the chassis, fuselage shell members either fly free or bend to accommodate forces. Obviously not all parts will survive all the time, but in general it is possible to recover a model from a misadventure and rebuild it. In some cases, at the air field.

The important distinction between EDFmodels' design approach and traditional fuselage construction methods is that the fuselage shell carries absolutely no loading what-so-ever. This is the function of the chassis. Ignoring this principle will result in frustrated efforts to achieve model rigidity and considerably reduced model impact resistance.
 

Wing design

One center wing box, two inner wings and two outer wings complete the wing design. Cutting foam swept back wings with taper and washout is by definition best achieved with a CNC foam cutter. The Clark-Y section works well with the thicker aerofoil tending to have relatively blunt leading edges which makes the stalling characteristics quite benign. This aerofoil is easier  to fly because it provides a broader range of model air speed. Washout is -3 degrees, dihedral 4 degrees. A carbon rod spar is used  on the upper wing surface to join at the wing center box. The surface of the upper wing is covered with plastic. Vinyl completes the lower wing covering.

Engines & cowlings

Lower wing mounted engines are vulnerable to ground strike. A quality cowling design would offer good scale, impact resistance and ease of maintenance. The engine mounting design has to allow the engine to break free in an accident. If it does not serious damage to the engine pylon mounted in the wing will occur.

Intake lips, cowlings and tail pipes are molded from impact resistent plastic. Self adhesive tape and fasteners are used to secure components together.
  
Cowlings are painted on their inner surfaces with a polycarbonate spray paint such as the Tamiya PS colour range.

U/C design

Without doubt there is nothing more frustrating at the airfield than wasting time repairing U/C. It is always a good construction strategy to invest a little extra weight carried by the model in a solid set of U/C. Multiple foam wheels, wire legs and carbon fibre provide a good solution. Models track well on the ground with multiple wheels.

Nose & tail cones

Molding nose cones from impact resistant plastic produces a strong and light weight answer. Generally speaking they survive well. It takes a direct head-on impact to destroy them. Part replacement at the field is easily achieved.

The all-moving tail plane design produces effective responses in the air. A rudder is not required as the dihedral produces quite a stable model.

In the air

For the reasonably confident modeller the airliner configuration should present few problems. Rotation and landing on a well trimmed grass or hard runway is easily achieved.

- never attempt to rotate to soon otherwise a wing will drop. It is much better to allow excess ground speed on take-off.

- when turning on the down wind leg always be ready with a little extra power.

- the all moving tail plane is powerful and rotation will occur with little up elevator. Never rotate with a high angle of attack but rather a shallow rotation. Be ready to push the nose down just after take-off to avoid  high rates of climb.

What does it all mean?  Many common parts can be used to make many airliners.

(c) Copyright 1996-2008. This web site represents the original modelling design work of EDFmodel.com. All rights reserved. No portion of this publication maybe reproduced without the express permission of EDFmodel.com.