Some basic terms related to Aircraft - Part 9

  • Let us now apply what we understood about the aeroplanes as a whole, things are going to become interesting.
  • Now it is where the practical men comes, namely those who want to build and fly aeroplanes.
  • We will begin to see how this is going to lead to build aeroplanes.
13. Straight and Level Flight
  • The flight of an aeroplane may be divided into two parts:
           1- Straight and level flight
           2- Manoeuvres
  • When considering straight and level flight we shall include only that rare condition in which the aeroplane is moving forward at constant air speed and neither loosing nor gaining height.
  • Under the second condition – manoeuvres – we shall include take off, landing, climbing, gliding, turning, looping, and spinning – all these, and anything else an aeroplane will do when it is not in level flight comes under manoeuvres.
14. The Four Forces acting on an aeroplane.
  • The forces must be balanced, they must be in equilibrium , if we have to maintain a condition of steady level flight.
  • To maintain a constant height the two vertical forces, lift and weight, must balance, must be equal.
  • The lift of the wings must be equal to the weight of whole aeroplane.
  • Now the two horizontal, forces thrust and drag, are equal – thrust is equal to drag.
15. Thrust
  • All we need to know here is that there are several methods of providing this force to an aeroplane.
  • It all depend upon the principles of pushing air backwards with the object of causing a reaction, or thrust, in the opposite or forward direction.
16. Balance of Aeroplane
  • To return to earth – and to balance the aeroplane – the equality of lift and weight – maintains a constant height.
  • Equality of - thrust and drag – maintains a constant speed.
  • The position of four forces, their lines of action, must also be considered.
  • If the lift is far back and the weight is far forward, the aeroplane will tend to turn on its nose – nose heavy.
  • If the thrust high and drag low the opposite effect ( nose will go up).
  • Now it is the job of the designer and the builder to see that the aeroplane is neither nose-heavy nor tail-heavy in level flight.
  • The lift comes mainly from wing, so the position of wing will determine the line of action of lift.
  • The tail plane, the fuselage and other parts effects will be very small.
  • We must remember that the CP is well forward of the centre of wing and it moves with the angle change.
  • The line of action of weight is, of course, through centre of gravity.
  • It is easy to measure the CG of a finished aeroplane.
  • The line of action of thrust must be along the propeller shaft or centre line of the aeroplane.
  • The line of action of the drag will depend upon the positions and relative values of all the separate parts, most difficult of all to measure.
17. The Tail Plane
  • The tail plane or stabilizer is to provide a force of such a size and in such a direction as to correct any out-of-balance effects of the four main forces.
  • The tail plane force need only be small to be effective, because, being situated at the end of the fuselage, it has great leverage.
  • At the rear of the tail plane are hinged the elevators, with which the pilot can upset the balance of the aircraft at will and thus maneuver the machine into any desired position.
18. Stability of Aeroplane
  • Having ensured that the aeroplane will properly balanced in normal flight, our next job is to make it stable.
  • A very stable aeroplane resists every change in flight path, and it may positively refuse to perform certain useful manoeuvers such as spin or steep side slip.
  • Too much stability is not advisable.
19. Degrees of Stability
  • If an aeroplane is disturbed from its path, it is very rarely that it will return to it without some oscillation about its original position.
  • There are at least five ways in which it may behave.
  • Whatever may be the type of stability it should be inherent, due to features incorporated in the design of the aircraft, such as dihedral angle, automatic slots or latest an autopilot.
20. Rolling, Pitching and Yawing
  • Motion round the lateral axis is called pitching.
  • The control or stability so far this motion is concerned is called longitudinal control or stability.
  • Motion round the longitudinal axis is called rolling.
  • The corresponding control or stability is called lateral.
  • Motion round the normal axis is called yawing, and the control or stability is called directional.
21. Longitudinal Stability
  • In order to get longitudinal stability the CG of whole aircraft must be well forward.
  • A forward position of the CG helps longitudinal stability.
  • The area of tail plain contributes to this.
  • The length of fuselage ( from trailing edge of wing to leading edge of tail plane) also contributes to this.
22. Lateral Stability
  • The lateral dihedral angle is by far the most common and effective means to obtain lateral stability.

23. Directional Stability
  • It is only necessary to look at a view of the side elevation of an aeroplane to see what surfaces will have most effect on directional stability.
  • First the Fin and Rudder.
  • Next the fuselage.
24. Control
  • The standard system of control is by means of control surfaces hinged at the rear of the tail plane (for longitudinal control)(Elevator),
  • Along the trailing edges of wings (for lateral control)(Ailrons),
  • And at the rear of the fin (for directional control)(Rudder).
  • A movement of the control surface will cause a force due to the defection of air flow and changes the attitude of aircraft.
  • The effectiveness of control surface will depend upon its area, the distance from the axis around which it is intended to turn the aeroplane,
  • And the velocity of air over the control surface ( speed of the aircraft).
25. Landing.
  • The art of landing is to transfer the aeroplane from the medium in which it has been flying – namely the air –as gently as possible to the ground.
  • When very close to the ground the forward velocity-relative to air-is reduced to minimum at which flight is possible.
  • This is effected by increasing the angle of attack (by raising the elevators), so that the speed falls, the lift is kept equal to the weight by increase in angle.
26. Wing Loading
  • The weight per square foot of wing area.
  • In actual figures, wing loading may very from 1 or 2lb/sq ft on gliders to 7 or 8lb/sq ft on light aeroplanes and 20,30,40,50,100 or more on high speed aircrafts.
  • As the wing loading goes up, so does the landing speed.
27. STOL and VTOL
  • STOL- Short Take Off and Landing Aircraft.
  • Auto gyro – gyroplane.
  • The aircraft officially a rotorcraft, not an aeroplane- is propelled through the air in usual way by engine and propeller.
With this I conclude the explanations about some basic terms related to Aircrafts. Understanding the basic terms is sufficient for a scratch builder to read a plan and start building his own Aircrafts. If you want to know more about aerodynamics and design aspect of a RC-Aircraft you can refer books and web sites which explains how to design RC-Aircrafts.
Those who want some more deep understanding about the above mentioned basics about Aircrafts can refer the book "Flight Without Formulae" (How and why an Aeroplane flies, explained in simple language) by A.C.Kermode. For more information about various calculations involved in RC Airplane design visit the web site -

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References : A ‘Without Formulae” Book, Flight Without Formulae (How and why an Aeroplane flies, explained in simple language) by A.C.Kermode, Author of “Mechanics of Flight” and “The Aeroplane Structure”, and co-other of “Hydrofoils” and