Aircraft Basics

Very useful for viva / debriefing / interview section:

• The aircraft's resistance to the airflow (drag) depends on the shape of the fuselage and flying surfaces. 
• The uppermost profile has a lower angle of attack than the lowest one. When the air flows evenly through the surface is called a laminar flow. A too high angle of attack causes turbulence on the upper surface, which dramatically increases the air resistance (drag), this may cause the flow to separate from the upper surface resulting in an abrupt reduction in lift, known as stall. 
• Camber is a measure of the curvature of the airfoil (high camber means high curvature). 
• Aspect Ratio is a measure of how long and slender a wing is from tip to tip. The Aspect Ratio of a wing is defined to be the square of the span divided by the wing area and is given the symbol AR. 
• Wing Dihedral refers to the angle of wing panels as seen in the aircraft's front view. Dihedral is added to the wings for roll stability; a wing with some Dihedral will naturally return to its original position if it is subject to a briefly slight roll displacement. 
• A negative Dihedral angle is called Anhedral. 
• Forces in Flight: Gravity, Lift, Thrust and Drag. 
• Lift is the force generated in order to overcome the weight, which makes the aircraft fly. This force is obtained by the motion of the aircraft through the air. Lift force is therefore dependent on the density of the air r, the airspeed V, the type of airfoil and on the wing’s area. 
• Lift Force = 0.5 * r * V2 * Wing's Lift Coefficient * Wing Area 
• The wing's lift coefficient is a dimensionless number that depends on the airfoil type, the wings aspect ratio (AR), Reynolds Number and is proportional to the angle of attack (AoA) before reaching the stall angle. 
• Thrust is the force generated by some kind of propulsion system. The magnitude of the thrust depends on many factors associated with the propulsion system used:- type of engine - number of engines - throttle setting - speed 
• Drag is the aerodynamic force that opposes an aircraft's motion through the air. Drag is generated by every part of the aircraft (even the engines). 
• Form drag is another source of drag. This one depends on the shape of the aircraft. 
• Induced drag is a sort of drag caused by the wing's generation of lift. Long wing with a small chord (high AR) has low induced drag, whereas a short wing with a large chord (low AR) has high-induced drag. 
• There's also the Interference drag, which is generated by the mixing of streamlines between one or more components, it accounts for 5 to 10% of the drag on an airplane. 
• An aircraft's stability is expressed in relation to each axis: lateral stability (stability in roll), directional stability (stability in yaw) and longitudinal stability (stability in pitch). 
• Stability may be defined as follows:
  - Positive stability: tends to return to original condition after a disturbance.
  - Negative stability: tends to increase the disturbance.
  - Neutral stability: remains at the new condition. 
• Static stability is proportional to the stabiliser area and the tail moment. 
• Dynamic stability is also proportional to the stabiliser area but increases with the square of the tail moment, which means that you get four times the dynamic stability if you double the tail arm length. 
• Lateral stability is achieved through dihedral, sweepback, keel effect and proper distribution of weight. 
• Longitudinal stability depends on the location of the centre of gravity, the stabiliser area and how far the stabiliser is placed from the main wing. 
• A tail-heavy aircraft will be more unstable and susceptible to stall at low speed e. g. during the landing approach. 
• A nose-heavy aircraft will be more difficult to takeoff from the ground and to gain altitude and will tend to drop its nose when the throttle is reduced. It also requires higher speed in order to land safely. 
• The angle between the wing chord line and the stabiliser chord line is called the Longitudinal Dihedral (LD) or decalage. 
• It has been found both experimentally and theoretically that, if the aerodynamic force is applied at a location 1/4 from the leading edge of a rectangular wing at subsonic speed, the magnitude of the aerodynamic moment remains nearly constant even when the angle of attack changes. This location is called the wing's Aerodynamic Centre AC 
• In order to obtain a good Longitudinal Stability the Centre of Gravity CG should be close to the main wings' Aerodynamic Centre AC. 
• For conventional designs (with main wing and horizontal stab) the CG location range is usually between 28% and 33% from the leading edge of the main wing's MAC, which means between about 5% and 15% ahead of the aircraft's Neutral Point NP. This is called the Static Margin, which is expressed as a percentage of MAC. When the static margin is zero (CG coincident with NP) the aircraft is considered "neutrally stable".
• However, for conventional designs the static margin should be between 5% and 15% of the MAC ahead of the NP. 
• Stall is an undesirable phenomenon in which the aircraft wings produce an increased air resistance and decreased lift, which may cause an aircraft to crash. 
• The stall occurs when the airflow separates from the upper wing surface. It happens when a plane is under too great an Angle of Attack (AoA). 
• Geometric Angle of Attack is the angle between the airfoil chord line and the direction of flight. The Angle of Attack is also known as Alpha. 
• The angle of attack measured relative to zero coefficient of lift is called the Absolute Angle of Attack (Absolute AoA). 
• There's also the Pitch Angle, which is measured with respect to the horizon. 
• Thus, stall may occur during take-off or landing, just when the airspeed is low: To keep altitude at low airspeed, the wing's lift coefficient has to increase, and if a non-experienced pilot tries to lift the aircraft's nose at a too low airspeed, it may exceed the critical angle of attack and stall occurs. If you're flying near the stall speed and make a steep turn, the aircraft will stall. 
• Stalls may also occur at high airspeeds. If at max airspeed and full throttle the pilot suddenly applies excessive up elevator, the aircraft will rotate upwards, however, due to aircraft's inertia, it may continue flying in the same direction but with the wings at an angle of attack that may exceed the stall angle. 
• However, a flat bottom high wing with dihedral is more sensitive to crosswind gusts, so the first flights should be done during calm weather. 
• A well-rounded leading edge is therefore preferable, as it better conveys the airflow onto the upper wing surface allowing higher angle of attack at low speed. 
• The wing loading should not be greater than about 60g/sq.dm (19-oz/sq. ft). Wing loading is the aircraft's weight divided by the wing area. 
• Engine - provides the power to rotate the propeller.
• Propeller - (also Prop) is attached to the engine's shaft to convert rotational motion into thrust and speed, which depends on the Prop's diameter, pitch and the Engine's power.
• Spinner - streamlined part that covers the end of the Prop shaft.
• Fin - (also Vertical Stabiliser) provides directional stability (stability in yaw).
• Rudder - moveable part fitted to the Fin's trailing edge, is used to change the aircraft's direction.
• Stabiliser - (also Horizontal Stabiliser or Stab) provides longitudinal stability (stability in pitch).
• Elevator - moveable part fitted to the Horizontal Stabiliser's trailing edge, is used to make the aircraft climb or dive.
• Ailerons - movable parts on both sides of the wing, are used to make the aircraft roll about its fore - aft axis. When one aileron moves up the other moves down.
• Wing - provides the aircraft's main lifting force. 
• Check the CG location with empty fuel tank by supporting the model with your fingertips underneath the wings. Find the position where the fuselage gets level or its nose is pointed slightly downwards. 
• The relation between lift and drag is called the Lift to Drag ratio (L/D) and is obtained by dividing the Lift Coefficient by the Drag Coefficient. 
• With high wings the dihedral angle is typically between 3 to 6 degrees. Dihedral should be lower when using ailerons (up to 3 deg). Although not strictly needed, a washout angle between 3 to 5 degrees is advisable in order to improve stall characteristics. 
• The ideal incidence and motor thrust angles are usually found by trial and error. Initially, one may start with 2 to 3 degrees down and right thrust. 
• Flat bottom wings may need more down thrust than symmetrical and/or semi- symmetrical ones. 
• Landing gear placement on a tail dragger should have the axle coincident with the leading edge of the wing, whereas on a tricycle the main gear should be slightly aft of the CG balance point in order to get easier take-offs. 
• Flat bottom wings (high cambered airfoils) are mainly used in slow and relatively light powered models. They have high lift coefficient but also high pitching moment, so a relatively longer tail moment or larger stab area may be needed in order to achieve a good longitudinal stability (stability in pitch). 
• Stall speed (m/s) = [2*Weight / (Clmax*1.225*Wing Area)] 0.5 
• Some unit conversions:
  1ft = 0.3048m
  1in = 2.54cm
  1lb = 16oz = 0.4536kg
  1oz = 28.35g
  1sq. ft = 144 sq. in 
• Tricycle gear aircraft are easier to land because the attitude required to land on the main gear is the same as that required in the flare, and they are less vulnerable to crosswinds. As a result, the majority of modern aircraft are fitted with tricycle gear.