SOFT SKILLS: AEROMODELLING

While browsing the web a couple of years ago, I came across aeromodelling websites. I remembered about the basics that I had learnt in college (NCC) as a cadet. Thus, started this passion of building small foam gliders with less expensive materials.

The common 9” diameter FOAM PLATE can be used, which is available in stores everywhere. It is light in weight, water resistant, easy to obtain and inexpensive. For this simple reason I concentrated on designing model airplanes.

Why is it a Great Activity

It is a creative activity and a lot can be learned from doing small adjustments about aerodynamic theory.

Made of easy-to-find materials and inexpensive.
Most people are thrilled to build things that can fly.
Gliders can be flown in small areas and indoors such as hallways, big rooms and as well as outside when there is less wind.
Safe to build and fly only a few tools are needed.

Before building any model a few things have to be understood.

Lift – Lift for the glider comes from the wing. If the wing has a slight curve in it, known as “camber”, the lifting force will be greater. If the curve is more, the wing will generate drag and the glide will not be there. A flat wing will generate lift, but not as good as a wing with camber.

Stability –Gliders needs stability to fly through the air in a level manner. Without stability it will drop or dive straight into the ground.

To keep the glider from tilting too much from side to side, the stability is achieved through “dihedral” angle, where the center of the wing is flat but it angles upward just towards the ends of the wing.

To achieve stability and flight efficiency, the design must be dictated mainly by aerodynamics and not by some particular structural pattern. Let us discuss the basic proportions for an efficient and stable glider by doing and learning.

The most important aspect in designing a flying model is finding out the CENTER OF GRAVITY or CG. There are actually two things for a steady glide - stability, and trim. They are interrelated and depend on the location of the Center Of Gravity, and the angle of the horizontal tail. The center of gravity is adjusted by adding or removing some weight on the nose. Weight can be added to the nose by fixing a pencil eraser with a rubber band and by slicing off some until the CG is achieved. This process of adjusting the CG can be frustrating and discouraging. Once a person gets it, it can be done speedily.

To learn is TO DO and find it out. We will learn with the initial phase of calculating the measurements. Please refer to the diagram for correlation of the terminology.

The starting point is the ‘WING SPAN’ because all other measurements should be proportionate to it. Span is the distance from one wing tip to other. To keep construction simple and cost low, the glider should be with a Wing Span (S), of 16 inches. Less than this will make the flight irregular and adjustment difficult.

The next one is length of ‘WING CHORD’, the distance from the front edge to the rear edge of the wing. An average value is about 1/7 the wing span. Therefore, Wing Chord (C), is 2-1/4 inches. The relation between the Wing Span and Chord is called "Aspect Ratio" represented by:

Span/Average Chord = S/C = 16/2.25 = 7.1

To determine the location of the tail planes, the ‘MOMENT ARM’ (M), must be long enough to provide stability without excessive large tail surfaces. A length equal to 1/2 the wing span is standard, so

Tail Moment Arm, M = 16/2 = 8", the distance from wing-center to the center of the Stabilizer.

For complete longitudinal stability, the Stabilizer size and proportions must be found out. The tail plane is called the ‘STABLIZER’ (S_S). Stabilizer Chord (Cs) is the breadth of the tail plane, that is, the distance from the front edge to the rear edge of the tail plane.

Stabilizer span S_S = (0.4) x S = 6.4".

Stabilizer Chord Cs = (0.85) x C = 1.9"

The basic rule is that the stabilizer area of a glider should be about 30% of the wing area.

Fin proportions are directed by the wing as it causes directional instability, which the fin controls.

FIN HEIGHT (F_H) is (0.43) x S_S, which is F_H = 2.75".

FIN CHORD (C_F) is the same as the stabilizer chord, C_F = (Cs), which is 1.9".

The NOSE LENGTH (N), is the distance from the wing-center to the front-tip of the frame stick, body or ‘FUSELAGE’, and is (0.62) of the Moment Arm,

N = (0.62) M, so N = 5".

DIAGRAMMATIC REPRESENTATION OF ALL PARTS

Therefore, the total frame-stick length or FUSELAGE LENGTH (F_L) is taken as 7/8^th of the wingspan (S), which is 14". 7/8 x 16 = 14".

For lateral stability and quick recovery, the wings are given a dihedral of 1/12 the wing span, so dihedral = 1/12 X (16) = 1.33" (as illustrated below).

Frontal view of the glider

Each wing tip has to be raised 1.33" above a horizontal line passing through the wing chord at the wing’s lower surface at its center as shown above.

The stabilizer is set at 0 degrees on gliders, so it must be parallel to the stick centerline, giving a difference in angle between wing and stabilizer of 1-1/3 degrees, an important factor for longitudinal stability.

These calculations are the result of continuous trial and error methods and by researching the web to find a stable flying glider without any glitches. An EXCEL WORKSHEET is attached here where you can give the wing span value of your choice and the remaining values for other proportions of the glider will be generated automatically.

to download the worksheet, click on the link below.

GLIDER CALCULATIONS

Pictures of flying models designed by me

Rubber band powered glider with 16 inches wing span (propeller made out of soft drink can, wings and fin made out of thermocol plates, fuselage made out of bamboo stick)