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Advancing Blade 

The portion of the rotor disc in which the rotation of the blade is moving in the direction of the aircraft’s travel. 


Hydrodynamics and aerodynamics are both branches of fluid dynamics, which is the study of fluids in motion. The fundamental laws governing the movements of gases, such as air, and liquids, such as water, are identical. Although many liquids are almost incompressible.

The equations representing these natural laws are, however, so complex that although formulated over a hundred years ago, they cannot be easily solved to account for all situations and conditions. The equations which describe in a general fashion the motion of fluids were developed in 1820 and subsequently perfected by G.G. Stokes.

At the beginning of the present century, aerodynamics was introduced with the possibility of flight in air. It started with the same assumption as hydrodynamics with the added assumption of incompressibility replacing what was a fact for water. 


A surface designed to produce lift from the movement of air over it. Ideally, it should present the greatest amount of lift with the least amount of drag. 


Anodizing is the deposition of a thin film of synthetic oxide on a light metal, such as aluminium, to prevent the further access of air to the surface, preserving the lustre and preventing corrosion. The article is made to anode in 3% solution of chromic acid at about 104 degrees F. The voltage is gradually increased to a maximum of 50 volts and the process may take an hour. Sulphuric and oxalic acid processed are also used, and the anodic film may be dyed various colours. 


A heavier-than-air flying machine which derives its lift from a rotor system mounted above the machine, with blades rotating more or less horizontally. 


A flight condition made possible by the vertical or horizontal movement of air through the rotor system. 

Blade Loading 

The load placed on the rotor blades of a gyroplane, determined by dividing the gross weight of the craft by the total combined areas of all blades (not the disc area, but the blade area). 


Rotor blades that are equal in weight will balance each other. Unbalanced rotor blades may cause control stick shake and instability when in flight. 


Sideward tilt of the aircraft in flight. When correctly executed, the bank compensates for centrifugal force, and the passengers will be pressed straight down in their seats. 


The maximum altitude to which a gyroplane can climb. Because of thin air, the engine decreases in power, or the rotor blades no longer provide lift to climb. 

Centre of Gravity 

Called CG. A point where the resultant of all weight forces will hang evenly from this point Usually at or very near to the main mast. 

Centre of Pressure 

An imaginary point on the cord lines of all the rotor blades where all the aerodynamic forces of the airfoil surfaces are concentrated. 

Centrifugal Force 

The force caused by the rotation of an object with mass. 


A straight line between the exact center of the leading and trailing edge of the rotor blade. 

Chordwise Balance 

A term that refers to the mass balance of an airfoil. It is designed to be in the center of lift. 


Forces acting on a rotor blade when its tip speed approaches the speed of sound. 


The blades tend to bend upward in flight, when they are lifting the weight of the aircraft. Referred to as the coning angle. 

Cruise Speed 

An airspeed that usually results in the best fuel economy and is usually between 112 and 2/3 of full power. 

Density Altitude 

Pressure altitude calculated from air temperature, altitude and humidity. 


The area swept by the blades of the rotor. 

Disc Loading 

The gross weight of the gyroplane divided by the rotor disc area. The greater the disc loading, the gyroplane’s sinking speed will increase, and its glide-angle will become steeper. 

Dissymmetry of Lift 

The unequal lift across the rotor disc, caused from the advancing blade creating more lift than the retreating blade. 

Dynamic Roll Over 

A roll over on the ground caused by violent rotor flapping. Caused by insufficient rotor rpm combined with excessive ground speed. 


Flying with the wind direction. Flying downwind near the ground is dangerous, true airspeed is lower than groundspeed and the aircraft’s lift is less. 


The maximum length of time a gyroplane can stay aloft on its fuel supply. 


The up and down motion of the rotor blade on its hinge. Without flapping, a gyroplane would roll over on its side during flight because of the unequal lift of the rotor disc. 


A landing maneuver performed near the ground to slow the gyroplane’s rate of descent and air speed. The gyroplane is in a nose-high attitude during the execution of this maneuver. 


A self-induced oscillating motion of improperly designed rotor blades. 

Ground Effect 

Also known as ground cushion, A beneficial increase in lift near the ground. Readily apparent when the rotor height is one-half of the rotor’s diameter over the ground. Less engine power is required due to the air being thrust downward to meet the ground. This denser air is partially trapped beneath the rotor disc. 

Ground Vortex 

The horizontal whirlwind thatforms at the forward edge of the rotor wake when the gyroplane flies at low speeds close to the ground. 


An aircraft whose lift is developed by the rotor blade system using the principles of autorotation.

Horsepower Loading 

The ratio of gross weight to horsepower, obtained by dividing the total weight by the engine’s horsepower rating. 

Induced Power 

The power associated with developing rotor thrust from the movement of air passing the rotor blade. 

Induced Velocity 

The downward air velocity generated in the process of developing rotor thrust. 

Laminar Flow &
Turbulent Flow 

The air flow immediately against the rotor blade (boundary layer) is of most importance in the efficiency of the blade. Two kinds of fluid flow are possible – laminar and turbulent. In laminar flow, the fluid moves as a series of sheets or lamina, sliding one over the next where there is a difference of speed between them (velocity gradient).

In turbulent flow, particles of fluid (air) can move in any direction – only the mean velocity and direction being defined. This flow can be caused by the overall shape of the object (rotor blade) or disturbances in the surface polish have a noticeable effect on efficiency. 

Laminar Flow Blades 

Laminar Flow Blades are fibreglass blades which cause unusually low drag in the rotorcraft operation. During World War II when experimental planes began to approach the speed of sound, laminar flow wing sections contributed to their success. In the beginning development stages, thin gauges of sheet metal were used in forming the ”skin”. It was determined that the slightest buckle forming in the ”skin”, caused by a counter sunk rivet, could nullify the advantages of the blade section. With the advent of fibreglass, it was determined that the blade could carry as much load as aluminium-alloy sheets, but had a much greater resistance to damage and guaranteed a smoother surface. Continuing development of these materials has resulted in stronger bonding methods and resins which in turn have led to the laminar-flow rotor system currently being offered for rotorcraft. 


The tip movement needed to adjust the rotor blades in a two-bladed system to get the weight of the whole rotor exactly equal on each side of the main shaft laterally. 


The recommended safe duration (in hours or measurable wear on the component) of any part on an aircraft. This length of time is determined by either fatigue or the operational wear on the part. 

Load Factor 

The ratio of rotor thrust to gross weight of the gyroplane. Mast – The main structural member of the gyroplane. The mast is the vertical assembly that connects the rotor blades to the airframe. 

Parasite Power 

The power used to overcome the drag of all non-lifting components of the rotorcraft. 


The in-plane alignment of all rotor blades so they perfectly balance each other. 


The time it takes for an oscillating system to complete one full cycle. 


Pilot induced Oscillation – Caused by delays in the human reaction time. Also known as porpoising, which is caused by over control and inexperience. 


The angle between a blade’s chord line and a plane perpendicular to the rotor bearing. 

Power Loading 

The ratio of the gross weight to the horsepower rating of the rotorcraft’s engine. 

Pull Out 

A flight manoeuvre at the bottom of a dive or descent. 

Push Over 

A flight manoeuvre at the top of a climb or a nose-down dive from level flight. 

Radius of Action 

The maximum distance a gyro-plane can fly down and return to without refuelling. 


The maximum distance a gyroplane can fly without landing or refuelling. 


A fail-safe design which provides a secondary standby structural member. 

Retreating Blade 

This blade is on the opposite side of the advancing blade. It travels with the wind created by the forward motion of the rotorcraft. 


Tilt of the gyroplane along its longitudinal axis. Controlled by lateral movements of the joystick. 


The lift-producing airfoil system of a rotorcraft. Rotor blade refers to a single blade only. 

Service Ceiling 

The altitude at which the rotorcraft still maintains the potential to climb at 100 feet per minute. 

Side Force 

The force on the side of a rotorcraft due to a side slip. 


The controlled flight of a rotorcraft in a direction not in line with its fore and aft axis. 

Solidity Ratio 

The portion of the rotor disc which is filled by the rotor blades. A ratio of the total blade area to the total disc area. 


The main load-carrying member of the rotor blade’s structure. It carries the centrifugal force as well as loads from the root attachments to the tip of the blade. 

Speed Stability 

The tendency of the rotorcraft to pitch up or down when the forward speed is changed. 


The separation of the airflow from the surface of the airfoil or any other component. The resulting loss of lift is a stall. 

Static Stability 

The tendency of a rotorcraft to return to its original flight condition after a disturbance. 

Teetering Rotor 

A two-bladed rotor with a single horizontal hinge for flapping. 

Thickness Ratio 

The ratio of maximum thickness of the airfoil to the chord length of the airfoil. 


The rotor force perpendicular to the tip path plane. 

Tip Path Plane 

The plane in which the tip paths travel when the blades are rotating. 

Tip Speed 

The airspeed at the top of the rotor blade in flight. 

Tip Stall 

The stall condition of the retreating blade which occurs at high forward speeds (approx. 150mph). 


A rotary force. A reaction to the turning effort supplied by the engine is an example. 


Rigging the rotor so that each blade passes through the same slot of air. 


Turning of the gyroplane to the right or left by changing the direction of the airflow over the tail surface through the use of the foot petals.