The purpose of this glossary is to make the text comprehensible to readers new to the subject and give an indication of the meaning of the word in the context used. For more precise definitions, and the meaning of the word in a wider context, the reader is referred to the bibliography.

Words shown in bold type are found elsewhere in the glossary.

AEROELASTIC Refers to the interaction between the flow and the deflections of the airframe caused by the flow.

ASPECT-RATIO is approximately equal to the ratio of span to chord. Geometric aspect ratio is defined as span2 /area. Effective aspect ratio is defined as follows. The actual induced drag in free air (away from the ground) has the same value as that predicted for an elliptically loaded wing of the same area as the effective aspect ratio. Effective aspect ratio will usually be from 0.9 to 1 times geometric aspect ratio.

BOUNDARY LAYER Region close to the surface past which fluid is flowing where velocity calculated according to inviscid flow theory will not apply. In this region, viscosity is of great importance. Adjacent to the surface, velocity is zero. It has been found that generally the flow pattern in this zone is one of two distinct types: laminar or turbulent. In laminar flow each particle proceeds downstream, hence the boundary layer is similar to "laminates" of material sliding over one another. In turbulent flow, the direction of each particle will not be so orderly. This absorbs more energy and hence creates more drag. Laminar flow over a substantial part of the surface can be achieved with an appropriate aerofoil accurately constructed.

CAMBER of aerofoil-section. For a single-surface foil (hang-glider, kite), positive camber entails a convex upper surface and a concave lower surface. A foil of finite thickness can be considered, geometrically at least, as a cambered single-surface foil clad with a symmetrical foil.

CANARD Smaller horizontal surface forward of mainplane.

CHORD Distance from leading-edge to trailing-edge of wing or other surface, or line between these two points.

COEFFICIENT Don`t be frightened by this word. It just means "number". Into many formulas derived by the theoreticians, there needs to be a number, derived by practical people, so as to make the answer come right.

COMPOSITE Form of construction, where fibres are impregnated with a liquid, which then sets hard. E.g. Fibreglass. Word is sometimes used to refer to any bonded construction as in the Mosquito aeroplane.

DIFFERENTIAL ailerons. Different amount of movement. Typically one moves up more then the other moves down. On ordinary ailerons, with equal amounts of movement,, the down-aileron causes more lift on that side, which causes more induced drag which causes a yaw in the opposite direction to the required turn. Differential ailerons tend to compensate for this effect.

DIHEDRAL Angle of each semi-span to horizontal.

DRAG COEFFICIENT similar idea to Lift Coefficient.

For a bluff body, the drag will be of the order of the dynamic pressure,q, times the frontal area.
For streamlined bodies the drag will be appreciably less, and indeed needs to be to enable flight.

The formula for drag is   D = Cd x q x S

Beware when using Drag Coefficient in this formula that you use the appropriate value for the area S. Sometimes Cd is based on frontal area, sometimes on plan area.  Indeed, some analysts base the drag-coefficient of a fuselage on the area of the wing. The values of Cd have been found, by experiment, for various shapes. Hence, drag can be predicted.

ELEVATOR Horizontal control-surface. Enables pitch control.

FAIRING Aircraft component whose function is to provide a smooth exterior shape.

FLIGHT ENVELOPE The range of speeds and maximum g forces for which the plane is designed. A graph of speed plotted against maximum g at each speed. This typically looks rather like an envelope with the flap open.

FLOW The motion of the air relative to the plane. Not quite the same as airspeed which is speed relative to a point far enough away from the path of the plane as to be in undisturbed air. Hence an air speed indicator, unless carefully positioned and calibrated, will at best give a rough guide.

g This is the magnitude of the acceleration due to gravity on the earth's surface. Aeronauts and astronauts use this value as a unit. So, if you experience 3g you feel three times as heavy, and the structure supporting you is subjected to three times as much load. This could happen for instance at the bottom of a loop in an aerobatic aeroplane if vertical acceleration were 2g. This would be vectorially additive to 1g from gravity.

GROUND EFFECT Phrase of many meanings, all referring to the effect of the proximity of the ground on the vehicle. In the context of human-powered-flight there are are at least five effects of the closeness of the ground.

  1. One is beneficial and has the effect of reducing induced drag. This effect varies with the ratio of height to span and the observed effects are as predicted by classical theory, (Glauert 1948) but this is not the total effect because -
  2. There is another effect which will vary with the ratio of height to the wing chord and -
  3. Close to the ground one is flying in air which has small-scale turbulence due to the wind blowing over the ground, or waves, and -
  4. The plane is operating in the region of wind-shear. Perkins exploited the second effect beneficially on the Reluctant Phoenix which flew at about one wing thickness from the ground. It can also be negative, and racing-cars use it to keep them on the ground. At usual flying heights this effect will be smaller than the others. The third effect will increase drag in a way not yet fully understood.
  5. Another effect is that, when close to the ground, the pilot will need to concentrate on not hitting it, thereby increasing the workload. The total effect is that for each combination of pilot, aircraft, wind and terrain there will be an optimum height at which it is easiest to fly.

HPA Human powered aircraft.

HPF Human powered flight.

HPV Human powered vehicle. In this context, land sea or air.

I BEAM Beam with cross-section like letter "I". Resists bending-loads, with an efficient use of material. This shape is not rigid in torsion.

INDUCED DRAG typically accounts for about half the total drag of a wing. Induced drag is minimised with a large wing-span (most importantly), elliptic distribution of lift along the span, carefully designed tips or flying in formation as birds can be seen to do, so that the flock effectively approximates to one wing of much larger span than each bird. If a wing is not of infinite span and if it is generating lift then it also generates trailing vortices. The direction of these is such as to produce a downwards velocity component at the wing,(downwash). Hence the wing is effectively operating in a region of sinking air, and so is "flying uphill". The power required to climb this "hill" can be expressed as speed times induced-drag.

INVISCID FLOW THEORY is an application of the Laplace equation and is used, with corrections, to predict the flow around any body immersed in a fluid flow, e.g. an aeroplane wing. The theory assumes that the fluid is not viscous. In reality, air does have some viscosity and this is particularly manifested close to the surface. Engineers are able to predict the flow around a wing by using inviscid flow theory for everywhere except very close to the surface where boundary layer theory is used, and another correction to the theory is to recognise that air cannot flow around a sharp trailing edge.

KING-POST exterior structural post for supporting wires.

LAMINAR FLOW see boundary layer.

LATERAL CONTROL Steering of the craft during turning, or indeed selecting not to turn & keeping straight and level.

LIFT COEFFICIENT ( see "coefficient", then come back here )

Dynamic pressure is the pressure that you feel when facing a wind. Wing area is the area of the planform of the wing, (or wings for a   biplane)). Hence it is reasonable to expect that the lift from a wing will be the dynamic pressure,q, times the wing area,S. Lift coefficient is merely the number that you need to multiply this reasonable explanation by to get the value of the actual lift. If the lift is exactly equal to  q x S then we say that the lift coefficient, Cl, = 1 In general, we say that   Lift = Cl x q x S All this formula seems to say is that the Lift will be something-we-don`t-know times something we can calculate. Luckily, experiments have been done of aerofoil sections so that we do know the lift coefficient at various angles of attack and various Reynolds Numbers. Hence, we can predict the lift of a proposed wing.

LIFT MOMENT It has been found with most symmetrical aerofoil sections, that the centre of pressure will be at a distance of one quarter of the chord from the leading edge, regardless of the angle of incidence to the airflow. Hence the moment produced by the lift will be one quarter times the lift times the chord, if taken with reference to the leading edge, or itwill be zero if taken with reference to the quarter chord point. Thus an all moving symmetrical tail-surface hinged at the quarter chord point will be easy to move. However, with a cambered or asymmetric section the centre of pressure moves when the incidence is changed, hence the lift moment will have a real and varying value which can have considerable magnitude. The designer may strive in designing the foil-section and the positioning of the spar to arrange that the lift-moment about the spar is small, but allowance must be made for all incidences and speeds. see Lippisch.

LONGITUDINAL CONTROL Enabling the pilot to select nose-up or down. (The German word for elevator literally translates as "high-rudder"), see "Tail-volume-ratio". Also affected by power input.

LAYSHAFT In a transmission system, this shaft receives drive from one medium (e.g. chain) and passes it on via another.

MELINEX ICI trade name for polyester film. See "Mylar". A synthetic plastic sheet. It does appear to be the ideal material for the covering of HPA. However, Kimura considered (Nihon 1977), that a better shape can be achieved by 1/64th inch (0.8 mm) thick styrene-paper or by "ganpi-shi" (a sort of Japanese paper).

MONOCOQUE The material which forms the profile shape also has the function of providing all of the rigidity and carrying all applied loads. Not quite the same as "Stressed Skin".

MYLAR DuPont trade name for polyester film. See "Melinex".

OPTIMISATION Design procedure of considering various values of various relevant sizes and other parameters until the combination is found which is best or optimum.

ORNITHOPTER An aircraft with flapping wings.

PARASOL WING Wing above and clear of fuselage, but connected to it as a parasol is to its handle.

PITCH Rotation about lateral axis.

REYNOLDS NUMBER When studying the flow of water through pipes Reynolds discovered that the nature of the flow pattern depends on the radius of the pipe, the speed of the fluid and the viscosity of the fluid. If one multiplies the radius by the speed and then divides the product bythe value of the viscosity, one arrives at a number. It might appear that such a number would only be of interest to the more statistically minded plumber, but not so! Reynolds' important discovery was that if there were two different sets of conditions, then providing this number was the same for each, the two flow patterns would be the same. Furthermore, the same principle applies to all forms of flow, to any fluid and to any shape that the fluid is flowing inside or outside of, provided that the shape is the same in the two sets of conditions under comparison. Hence if we know how the flow around a shape behaved on test when the Reynolds number was, say one million (typical for HPA), and we know that on a proposed aircraft the wing will be operating at the same Reynolds number, (which may have been derived from different "ingredient" dimensions to those of the test), then we can predict the flow around our wing.

The viscosity of air is such that the Reynolds number for a wing will be

N x the speed in ft/sec x the chord in feet.

N is 6993 at 0 C ( 32 F), & 6289 at 15 C ( 59 F ); or 753 & 677 if using metre units. For water the values are 52083 at 0 degrees Celsius and 81301 at 15 degrees; 5606 or 8751 if using metre units. Hence one can expect a hydrofoil to behave differently at different water-temperatures, whereas for air the effect of variation in temperature is not so marked ( Glauert 1948).

ROLL Rotation about longitudinal axis.

SEPARATION BUBBLE Sometimes there may be observed by a riverbank a small region where the water is swirling rather than proceeding downstream. A similar situation can occur close to the surface of a wing, when the main flow has separated, leaving the bubble inside to swirl. The main flow will then re-attach further aft. Because of the reattachment, these bubbles are not easy to detect, but they will increase the drag, and an increase in incidence may cause the bubble to "burst", i.e. the flow suddenly ceases to reattach, causing sudden changes in the flow pattern which typically will be a loss of lift. One method of avoiding these bubbles is to induce turbulent flow at a point forward of where the bubble would otherwise form. With things like this to watch out for, the design of an aerofoil is certainly no trivial task, but HPA designers have learnt to do it for themselves. If you don't want to borrow someone else's aerofoil, you can maybe borrow their computer program to tailor a shape suitable for your particular aeroplane.

SPAN (of aerofoil), distance from right tip to left tip.

STORED ENERGY in this context means energy stored by the crew, prior to flight, into some device. Theoretically, and as allowed by competition rules, this device can be of any nature. Historically three types have been used, the bunjee, the battery or twisted rubber. The bunjee, a length of stretched rubber fixed to stakes on the ground and to the aircraft nose, just aids take-off, like a catapult. The electric battery is charged by pedalling a dynamo while on the ground. The energy in the battery may be tapped at any stage of the flight. Wound up rubber as well known for model-aircraft was used by Wayne Bleisner and on the Stork series.

STRESSED SKIN The material which forms the profile shape also has the function of providing some of the rigidity, and carrying some of the applied loads. Not quite the same as "monocoque".

TAIL BOOM On most engined aircraft the rudder and other tail surfaces are fixed to the rear of the fuselage. However with a short fuselage or "pod" such as is sufficient to house one person this would mean that the tail surfaces would be too close to be effective. Hence on some HPF a tube, typically made of aluminium alloy or carbon extends aft from the pod for the mounting of the tail surfaces, and sometimes a tailwheel.

TAIL VOLUME RATIO A measure of the horizontal tail effective size and is defined as :-

(horzl tail area x tail arm)/(wing area x mean chord) (horzl tail area x tail arm) will have the units of area x length, namely the units of a volume. Similarly (wing area x mean chord) will have the units of a volume, hence "volume ratio". Tail arm is usually measured from aircraft centre of gravity to quarter chord point of tail. Sherwin suggests a value of 0.2 (Sherwin 1971). The Daedalus was able to get away with a small tail by dint of a very small pitching moment of inertia by concentrating all the fuselage weight close together. Bliesner suggests a static stability margin of 0.5. When flying, some pilots prefer to hold speed constant, others to hold attitude constant.

TORSION Torque. Combination of forces tending to twist.

WASH-IN, WASH-OUT Indicates that a wing is built twisted. With wash-out the tip will have a lower angle of attack than the root. With wash-in, a higher angle than the root (see Condor).

WIND-SHEAR Effect of wind variation with height from ground.

YAW Rotation about vertical axis

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