For Starters – Part Two

For Starters – Part Two

There’s no denying that ARTF models now dominate the R/C scene, and for most new fliers such a model will be their first experience of a fully-fledged flying machine. The novice has a steep hill to climb in mastering his first model; hand-in-hand with learning to fly the aircraft is the requirement to understand what makes the model tick. Experienced pilots often take this sort of information for granted, but it’s important for the novice to possess this understanding. In this instalment, then, we’ll look at the essential parts of an aircraft, the function of the main components and the basic theory of flight.

ESSENTIALS
Within any group of novice model pilots there’ll often be a wide range of aviation knowledge and expertise. Some might be licensed to fly full-size aircraft whilst others may already have had some previous R/C flying experience, returning to the hobby after a prolonged break. But without doubt, most novices arrive at the patch with little or no knowledge of the subject. So, for those entering the world of model aviation for the first time, let’s examine the bits that make an aircraft do what it does:

The essential parts of our aeroplane are:

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  • Fuselage
  • Wings
  • Tail
  • Engine
  • Undercarriage
    The term ‘airframe’ is sometimes used when referring to an aircraft. The airframe is the complete structure less the engine, instruments and ancillary equipment such as radio gear or fuel tanks. Let’s take a look at each of the main parts in turn.

    Fuselage
    The fuselage of a typical ARTF trainer houses the radio equipment and engine, and also provides the specific angles required for both the wing and tail. The fuselage is generally made from two identical wooden sides made from either balsa sheet, lightweight plywood or from many pieces of balsa wood in an open structure based around two long members (aptly named ‘longerons’). The cross members inside the fuselage are known as ‘formers’. Former no.1 (F1) is generally referred to as the ‘bulkhead’ or firewall, and this holds the engine mount. The top and bottom of the fuselage ‘box’ are usually made from simple balsa sheet, but with the grain running across the fuselage rather than lengthwise. When your model is rigged (assembled) for flight it’s likely that it will only be made up of two large components: the fuselage (with the tail mounted) and the wing.

    Wing
    Your R/C trainer will undoubtedly be a monoplane, meaning that it has just one wing. Biplanes (aeroplanes with two wings) are still popular in aeromodelling but don’t make for a simplistic learning tool for the novice.

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    Model aircraft wing construction can take many forms. The wings of most modern ARTF models are either manufactured from a foam core that’s veneered (skinned over) with thin wood, or are built in a more traditional way from many pieces of balsa stuck together to form an open structure. The main components that provide the necessary strength in a ‘built-up’ wing like this are called ‘spars’. These are beams that run the full length of the wing and carry the bulk of the aircraft’s load whilst providing stiffness, helping prevent the wing from twisting or distorting. The aerofoil-shaped pieces that separate the upper and lower wing surfaces are called ‘wing ribs’; these also provide a surface for attaching the covering to. Your trainer will also have a ‘high wing’ configuration as opposed to ‘low’ or ‘shoulder wing’, which affords it a more simplistic construction and also better flight stability.

    Some high-wing monoplanes employ wing bracing struts, though this practice is more common on scale / semi-scale models that are based on full-size aircraft. One end of each strut is normally attached to a bracket on the lower part of the fuselage whilst the other end is attached to the wing at approximately the mid-point. The purpose of wing struts is to prevent the wings from folding during flight by transferring part of the wing load back to the fuselage.

    As well as being the primary flying surface of the model, the wing also houses various controls:

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  • Ailerons – movable surfaces that provide control of the aircraft in the rolling axis. They’re normally hinged towards the back of the wing and form part (or in some cases all) of the wing’s trailing edge.
  • Flaps – control surfaces also sat at the wing’s trailing edge, between the ailerons and the fuselage. They’re rarely used on R/C trainer aircraft but are frequently incorporated on scale models. Functionally, flaps provide better performance on take-off and permit steeper landing approach angles and lower landing approach speeds. Flaps provide both lift and drag functions, affecting the performance of the wing depending on the amount they’re deflected.
    Tail section
    The tail section of the model (also known as the ’empennage’) consists of the fixed vertical stabiliser (more commonly referred to as the ‘fin’) with a hinged rudder, and the horizontal stabiliser (more commonly, ‘tailplane’) with its associated elevator control.

    The fin is the fixed vertical airfoil located on or near to the horizontal stabiliser, and is used to provide directional stability. The rudder is the control surface that’s hinged to the fin to provide directional control in the ‘yaw’ axis. Note that whilst the fin provides directional stability (much like the dorsal fin on a fish), it’s the rudder that provides directional control.

    The tailplane is the fixed airfoil mounted horizontally on the tail section that provides longitudinal stability of the aircraft, whilst the elevator is a moveable control surface hinged to the trailing edge of the tailplane that controls the model in the ‘pitch’ axis.

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    Undercarriage
    There are two primary undercarriage (‘u/c’) configurations: a three-wheeled combination incorporating a nose wheel (a.k.a. ‘trike’) and a system incorporating two wheels at the front and either a small wheel or simple wire skid to the rear (often referred to as a ‘tail dragger’). Trainers often feature a steerable nose wheel that works in conjunction with the rudder to aid ground handling; tail draggers are generally a little more difficult to control on the floor, especially if the tail wheel is allowed to rotate or ‘castor’.

    A trainer’s main u/c (to which the front wheels are attached) is usually fixed and made of either piano wire (tempered steel spring wire) or an aluminium alloy plate (duralumin or ‘dural’). Retractable undercarriages are often installed on some aerobatic aircraft as well as on many scale models, mimicking the retractable undercarriage of the full-size aircraft. Right then, with the basic airframe components covered we now need to look at how they all co-exist in the air, with the addition of a fundamental element: lift.

    FLIGHT ACHIEVEMENT
    Aerofoil section
    In order for your model to be capable of flight it has to generate ‘lift’: a force resulting from its movement through the air. Lift is generated chiefly by the wing, which is so designed as to enhance the effect of this process. The wing has an aerofoil section, the purpose of which is to produce lift (you can see this aerofoil section by looking at the end of the wing). The rounder, blunt end is known as the ‘leading edge’ (l.e.), as this is the first part that breaks the airflow. Likewise, the ‘trailing edge’ (t.e.) is the thinner edge at the back of the wing.

    Lift is generated by a pressure differential caused by the shape of the wing, requiring the air to flow faster over its top than underneath. This creates lower pressure above the wing, causing the higher pressure underneath to push upwards and so generate lift. The aerofoil section helps generate lift more efficiently. There are three general shapes of aerofoil used for basic model aircraft:

  • Flat bottom – used primarily on trainers and slow-flying aircraft. Sometimes referred to as a ‘Clark-Y’ section, which is a popular version of this type.
  • Semi-symmetrical – most commonly used on the type of model you might progress to after your trainer. These types of aerofoil possess more of a curve on the upper part of the section than below.
  • Symmetrical – used on more advanced aerobatic aircraft, as it’s especially suited to inverted flight. It generates equal lift on both sides of the aerofoil but is set at a slightly nose-high angle to help the model fly. This angle is known as the ‘angle of incidence’ (not to be confused with the ‘angle of attack’, which is the angle at which the wing is forced to meet the oncoming airflow).
    So, lift is generated by the wing in the following ways:

  • As the aircraft moves through the air a high pressure air mass is created under the wing, which in turn tends to lift the wing.
  • A low pressure area is created on top of the wing, which tends to pull the wing upward. Since the air travelling over the upper surface of the wing has a greater distance to travel than the air passing under it, the air above the wing is less dense and thus at a lower pressure. Consequently, the wing is drawn upward into the area of low pressure.
  • The downward flow of air at the wing’s t.e. also produces a degree of lift The airflow over the upper surface of the wing moves at a much faster speed than the air passing under it, therefore when these two streams of air meet at the t.e. a downward flow of air results, which produces an upward reaction on the wing.
    From this we can see why lifting forces are greater on a flat bottom or semi-symmetrical wing than on a fully symmetrical wing: a flat or semi-symmetrical aerofoil produces lift at 0° angle of incidence, but a fully symmetrical aerofoil produces no lift at 0° and will only produce lift when it’s set at a positive angle of attack, with the nose of the wing raised in relation to the oncoming air.

    FACTORS AFFECTING LIFT
    Four main factors influence the lift on an aerofoil. These are:

  • Wing area.
  • Airspeed (the rate of airflow over the wing).
  • Thickness and shape of the wing.
  • Angle of attack of the wing.
    Generally speaking, the greater the wing area, the greater the lift; the greater the airspeed, the greater the lift; the thicker the wing (up to a point), the greater the lift; the greater the angle of attack, the greater the lift.

    Ok so now we know how the wing works and how lift is generated, but we need to look at this in terms of our model aircraft. Just how does it fly?

    FLIGHT
    There are four forces acting on an aircraft in level flight:

  • Lift
  • Weight
  • Thrust
  • Drag
    When an aeroplane takes off, these forces aren’t in balance. At the start of the take-off run the aircraft is moving slowly so the only drag is caused by the wheels passing over and through the grass. However, as the throttle is fully advanced to give maximum thrust the aircraft will accelerate, as there will be more thrust than drag. With the model now at speed the pilot feeds in some ‘up’ elevator and the nose of the aircraft will lift, increasing the angle of attack of the wing in relation to the direction of travel. The increased angle of attack will produce more lift than is required to overcome the weight of the aircraft and the aircraft will take off and begin to accelerate upwards. We now have a situation where the aircraft is accelerating forward (gaining flying speed) because thrust is greater than drag, and it’s accelerating upwards (climbing) because lift is greater than weight. If the attitude of the aircraft isn’t changed then it will continue to accelerate and climb until drag equals thrust, whereupon the aircraft will settle into a state of equilibrium.

    Reducing the thrust (i.e. slowing down the engine) will require an increase in the angle of attack of the wing if straight and level flight is to be maintained, as drag will be greater than thrust. The slower speed will reduce the lift generated by the wing, so more lift will have to be generated to compensate. More ‘up’ elevator will increase the angle of attack, and thus increase the lift of the wing. However, the increased angle of attack also increases drag, with a corresponding reduction in speed. These forces will interact until a new terminal velocity is reached and the forces are once more in equilibrium.

    From this we can conclude that there’s a terminal velocity for each speed during level flight. For each terminal velocity the wing’s angle of attack will be different, greater at slower speeds than at higher speeds.

    STABILITY
    An aircraft in flight is constantly subjected to a variety of forces that tend to disturb it from its normal horizontal flight path. These include such things as rising columns of warm air (thermals), down drafts, gusty winds, etc., which when encountered in flight tend to make the aircraft’s nose rise or fall, a wing to drop or the nose to yaw to the left or right. How the aircraft reacts to these disturbances depends on how stable the thing really is.

    Stability is defined as the tendency of an aircraft, when displaced in flight, to return to its straight and level attitude without any corrective action by the pilot. Stability may be classified as three states:

  • Positive – if disturbed, the aircraft will return to its original straight and level attitude.
  • Neutral – the aircraft will maintain its last attitude without either returning to straight and level flight, or moving further away from it.
  • Negative – once the aircraft is displaced by a disturbance, it will move progressively further away from its last attitude (negative stability = instability).
    Obviously, novice radio control model fliers want to train on an aeroplane that has lots of positive stability; an aircraft that will practically fly hands off, an aircraft that will recover from any unusual position when the pilot lets go of the controls. After becoming proficient on such a model the novice can then move on to an aircraft that has neutral stability, i.e. pattern models or other such aerobatic designs.

    NEXT TIME…
    As promised last time, I’ve included a basic glossary of aerodynamic terms at the end of this article to help you cut through the jargon. If you have any questions, speak to your instructor and discuss the matter with him. This will also help to get you used to the terminology that he might use when he’s teaching you to fly.

    Next time we’ll look at what transmitter controls do, and how they can be used to affect the flight of your model. We’ll also introduce the instructor’s role in the training scheme and provide a pre-flight check list thatll get you ready to take to the air.

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