Reference
Every term here is one the sheets actually use — the list was taken from their own text rather than from a textbook index, so nothing is defined that you will not meet. Each entry says what the thing is and, where it matters, what it does to the aeroplane. The sheet it belongs to is linked underneath.
This page is the words. Nomenclature is the symbols and the sign conventions, and Methods is the formulae themselves.
Terms
A
- Advance ratio
- How far a propeller moves forward in one revolution, divided by its diameter, written J. It is the single number that decides propeller efficiency, and it is why the same propeller is efficient in cruise and wasteful in a climb.Used on Sheet 08
- Adverse yaw
- A rolling aeroplane yaws the wrong way. The down-going aileron makes more drag than the up-going one, so the nose swings away from the turn. Rudder, differential throw or a Frise hinge are the cures.Used on Sheet 11
- Aerodynamic centre
- The point on a chord about which the pitching moment does not change with angle of attack. For most sections it sits at a quarter of the chord back from the leading edge, and it is why the quarter-chord appears everywhere.Used on Sheet 03
- All-up weight
- Everything that leaves the ground: airframe, battery, radio, payload, the lot. Not the airframe weight, and not the advertised weight. Almost every number on this site scales with it.Used on Sheet 05
- Angle of attack
- The angle between the wing chord and the air actually arriving, which is not the angle between the wing and the ground and not the attitude on the transmitter. A wing stalls at an angle of attack, never at a speed.Used on Sheet 01
- Aspect ratio
- Span squared over area, written AR. A long thin wing has a high one. It sets induced drag, so it sets glide ratio and it sets how much of your power goes into making lift rather than going forward.Used on Sheet 01
B
- Balance point
- Where the aeroplane balances, which is where the centre of gravity is. The single most consequential number you will set, and the one most often set by holding the model on two fingers and hoping.Used on Sheet 03
- Bubble, laminar separation
- At model Reynolds numbers the flow leaves the surface, sits above it, and reattaches further back. That bubble decides the polar, moves with angle of attack and surface finish, and is why published data measured at full scale does not describe your model.Used on Sheet 02
C
- C rate
- Current as a multiple of the pack's capacity per hour. A 2200 mAh pack at 22 A is drawing 10C. It is a shorthand for how hard a pack is being worked, not a unit of anything.Used on Sheet 09
- Camber
- The curvature of a section's mean line. More camber means more lift at a given angle and more nose-down pitching moment, which the tail then has to carry.Used on Sheet 02
- Centre of gravity
- Where the aeroplane's weight acts. Its position relative to the neutral point is what makes the aircraft stable or not, and that distance is the static margin.Used on Sheet 03
- Chord
- The straight line from a wing's leading edge to its trailing edge, and the length of it. Every Reynolds number and every coefficient on a wing refers to a chord.Used on Sheet 01
D
- Density altitude
- The altitude at which the standard atmosphere has the density you actually have. A hot afternoon at 500 m can fly like 2,000 m, and the aeroplane does not care which of the two it is.Used on the Reynolds tool
- Depth of discharge
- How far down a pack is taken before you land, as a percentage of its nameplate. Landing with 20% left is a depth of discharge of 80% — and that is your reserve. Taking another bite out of the same energy for a separate reserve counts the same margin twice.Used on Sheet 09, Sheet 16
- Dihedral
- The upward angle of the wings seen from the front. It makes an aeroplane roll level after a sideslip. Negative dihedral is anhedral, and it does the opposite.Used on Sheet 12
- Downwash
- The wing throws air downwards, and the tail flies in that deflected flow. It is why a tail is less effective than its area suggests, and it is folded into the neutral point.Used on Sheet 03
- Drag polar
- The relation between lift and drag, usually written CD = CD0 + k·CL². Two numbers, parasite and induced, and almost every performance answer on this site comes out of it.Used on Sheet 04
- Dutch roll
- A rolling and yawing wallow, where the aeroplane rocks from wingtip to wingtip while the nose swings the other way. Too much dihedral effect against too little fin.Used on Sheet 12
- Dynamic pressure
- Half the density times speed squared, written q. The pressure the air has by virtue of moving. Every aerodynamic force is q times an area times a coefficient.Used on the Reynolds tool
F
- Figure of merit
- How good a real propeller is against the ideal actuator disc at producing static thrust. A compiled number, because it depends on blade shape in ways a disc model cannot see.Used on Sheet 08
- Fineness ratio
- Length over maximum width, for a fuselage or a pod. It sets the form factor that turns skin friction into drag, and a stubby body pays for it.Used on Sheet 07
- Flare
- The pitch-up just before touchdown that trades speed for a gentle arrival. Not modelled on this site: the landing distances assume it happens correctly.Used on Sheet 13
- Form factor
- A multiplier that turns flat-plate skin friction into the drag of a real three-dimensional shape. Raymer's relations are the ones used here.Used on Sheet 07
G
- Ground effect
- Within about a wingspan of the ground, the wing's induced drag falls because the ground interferes with its trailing vortices. It is why a model floats down the strip when you expected it to land.Used on Sheet 13
- Ground loop
- A taildragger swapping ends. The centre of gravity is behind the main wheels, so any yaw it picks up is amplified rather than damped. A property of the layout, not of the pilot.Used on Sheet 15
- Gust, sharp-edged
- A vertical gust modelled as arriving instantly rather than building up. Conservative, standard, and often the case that sizes the spar rather than any manoeuvre you would fly.Used on Sheet 10
H
- Hinge moment
- The twisting load the air puts on a control surface about its hinge. It goes as the square of speed and the square of surface chord, which is why the two decisions people make casually are the two that matter.Used on Sheet 11
- Hysteresis
- A section that stalls at one angle and does not unstall until well below it. Common at model Reynolds numbers, unpleasant when it happens, and the reason some stall notes on Sheet 02 are warnings rather than descriptions.Used on Sheet 02
I
- Incidence
- The angle at which a wing or tail is built into the airframe, measured against a datum. Fixed by the builder; angle of attack is what the air sees and changes in flight.Used on Sheet 03
- Induced drag
- The drag that comes with making lift. It falls as speed rises and as span rises, and it is the half of the polar that dominates slow flight.Used on Sheet 04
L
- Laminar flow
- Smooth, layered flow. Cheaper in friction than turbulent flow and much readier to separate, which at model Reynolds numbers is usually the deciding fact.Used on the Reynolds tool
- Lift coefficient
- Lift divided by dynamic pressure and area, written CL. A shape-and-attitude number with the size and the speed taken out, so it can be compared between aircraft.Used on Sheet 01
- Load factor
- Lift divided by weight, written n and quoted in g. One in level flight, two in a 60-degree bank, negative when inverted. The V-n envelope is a map of it against speed.Used on Sheet 10
- Loiter
- Flying to stay up rather than to get anywhere, at the minimum-power speed. Slower than the speed that goes furthest, and about a quarter slower on every aeroplane.Used on Sheet 16
M
- Mean aerodynamic chord
- The chord of the rectangular wing that would behave like your tapered one, written MAC. Balance points and tail volumes are quoted against it, not against the root chord.Used on Sheet 03
N
- Minimum sink
- The slowest an aeroplane can lose height, which is not the same as its best glide: best glide goes furthest, minimum sink stays up longest. It happens at a lower speed, and on a draggy short-span model the speed it wants is often below the stall, in which case you cannot have it.Used on Sheet 14, Sheet 16
- Neutral point
- The centre of gravity position at which the aeroplane is neutrally stable in pitch. Put the real centre of gravity ahead of it and the aircraft is stable; behind it and it is not.Used on Sheet 03
- Nose-over
- A taildragger pitching forward far enough to put the propeller in the ground. The angle available is set by how far the main wheels are ahead of the centre of gravity, over its height.Used on Sheet 15
O
- Oswald efficiency
- How close a real wing gets to the elliptical ideal for induced drag, written e. Taper, washout and planform all move it, and it is a compiled figure here rather than a computed one.Used on Sheet 04
P
- Parasite drag
- Everything that is not induced drag: skin friction, form, interference, and the wheels hanging in the breeze. Written CD0, and it rises with the square of speed.Used on Sheet 07
- Pitch speed
- The speed at which a propeller would advance if it were a screw in a solid, with no slip. The aeroplane never reaches it, and thrust falls to nothing as it is approached.Used on Sheet 08
- Planform
- The shape of the wing seen from above: span, chords, taper and sweep. It is what Sheet 01 draws.Used on Sheet 01
R
- Reflex
- An upturned trailing edge, which gives a section a nose-up pitching moment. A flying wing has nothing to trim against and must have it; a tailed aeroplane does not need it.Used on Sheet 03
- Reynolds number
- Density times speed times length over viscosity. It says whether the flow is dominated by inertia or by viscosity, and it is not a property of an aeroplane — every part of one has its own.Used on the Reynolds tool
- Roll helix angle
- The corkscrew a rolling aeroplane flies, written pb/2V. It is the honest measure of roll performance, because roll rate on its own means nothing without the span and speed it happened at.Used on Sheet 11
- Rolling resistance
- The friction of a wheel on the ground, as a fraction of the weight it carries. For model wheels on grass it is far higher than the figures quoted for cars, because a small wheel in long grass is ploughing rather than rolling.Used on Sheet 13
S
- Sag
- The drop in pack voltage under load, caused by current flowing through the pack's own internal resistance. It is why a pack that reads 12.6 V shows 11.2 V in a climb, and the missing volts become heat inside the cells.Used on Sheet 09
- Separation
- Flow leaving the surface instead of following it. Costly in drag, and at the point where it spreads over the whole upper surface it is the stall.Used on Sheet 02
- Sideslip
- Flying slightly sideways, with the airflow arriving from one side. Weathercock stability and dihedral effect are both defined by what the aeroplane does about it.Used on Sheet 12
- Slip, propeller
- The gap between pitch speed and the speed actually achieved, as a percentage. Some slip is necessary — a propeller with none would be producing no thrust.Used on Sheet 08
- Span efficiency
- See Oswald efficiency.
- Spanwise lift distribution
- How the lift is shared along the wing, which decides the bending moment at the root rather than the total lift does. Schrenk’s approximation averages an elliptical distribution with one shaped like the planform, and it is why tapering a wing shortens the lever without shortening the wing.Used on Sheet 10
- Spar cap
- The strip of material along the top and bottom of a spar that carries the bending, as tension in one and compression in the other. Its area goes inversely with how deep the spar is, so a deeper spar of the same strength is a lighter one.Used on Sheet 10
- Speed controller
- The electronic speed controller, or ESC: it turns the pack’s steady voltage into the switched three-phase drive a brushless motor needs, at whatever throttle the receiver asks for. Rated in amps continuous and amps burst, and the continuous figure is the one to size against — with headroom, because it sits in the same hot nose as the motor.Used on Sheet 04, Sheet 08, Sheet 09
- Speed to fly
- How fast to push between thermals: faster than best glide, because the height you lose going quickly costs less time than the extra minutes spent gliding slowly. How much faster depends on the climb you expect next — the MacCready setting — and at zero expected climb it reduces to best glide.Used on Sheet 14
- Spiral divergence
- A slow, tightening turn that gets steeper if left alone. Too much fin against too little dihedral. Curable by changing the other one, which is why adding fin makes a wallow worse.Used on Sheet 12
- Stall
- The angle of attack at which the flow separates and lift falls away. It happens at an angle, not at a speed — the stall speed is just the speed at which level flight demands that angle.Used on Sheet 01
- Stall speed
- The slowest speed at which the wing can hold the aeroplane up in level flight. It rises with the square root of weight and with the square root of load factor, so a turn raises it.Used on Sheet 01
- Static margin
- The distance from the centre of gravity to the neutral point, as a fraction of the mean chord. Positive is stable. Five to fifteen per cent is sound for a tailed model; a flying wing runs far less.Used on Sheet 03
- Static thrust
- The thrust a propeller makes standing still. The largest thrust it will ever make, and the number a motor is advertised with, and not the thrust available at flying speed.Used on Sheet 08
- Sweep
- The angle the wing is swept back from square. It adds an effective dihedral that grows as the aeroplane slows, so a swept model is a different aeroplane slow than fast.Used on Sheet 12
T
- Tail volume
- Tail area times tail arm, divided by wing area times wing chord — a dimensionless measure of how much tail you have. It is the number to compare between designs, not tail area on its own.Used on Sheet 03
- Taper ratio
- Tip chord over root chord. It moves the lift distribution towards or away from elliptical, and aggressive taper without washout puts the stall at the tip where it does most harm.Used on Sheet 01
- Thermal
- A column of rising air. Circling in one is how a glider gains height, and the tighter the circle the more sink it costs.Used on Sheet 14
- Tip Mach number
- The speed of the propeller blade tip as a fraction of the speed of sound. Past about 0.85 the tip goes transonic, efficiency collapses and the propeller gets very loud.Used on Sheet 08
- Transition
- Where laminar flow becomes turbulent along a surface. Its position decides the drag, and at model Reynolds numbers it is often decided by a separation bubble rather than by a smooth change.Used on the Reynolds tool
- Turbulator
- A strip, wire or roughness deliberately added to trip the flow turbulent. It costs friction and buys attachment, which at low Reynolds numbers is a trade worth making.Used on Sheet 02
- Turnover angle
- The angle from the vertical at which an aeroplane will tip over sideways on the ground, measured about the line joining a main wheel to the nose or tail wheel. Past 63 degrees it will go over in a brisk turn.Used on Sheet 15
U
- UAV
- Unmanned aerial vehicle: an aircraft flown with nobody on board, whether by a pilot on the ground or by an autopilot. The site uses it and small unmanned aircraft for the same thing — the camera, survey and FPV aeroplanes the sheets size alongside the models — and the arithmetic does not change between them: a wing does not know whether it carries a receiver or a flight controller.Used on the cover
V
- V-n diagram
- The flight envelope: load factor against speed, bounded by what the wing can generate and what the structure will take. Its corners are the manoeuvre and dive speeds.Used on Sheet 10
W
- Washout
- Built-in twist that gives the tip a lower angle of attack than the root, so the root stalls first and the aeroplane drops its nose rather than a wing.Used on Sheet 01
- Weathercock stability
- The tendency to point into the relative wind, made by the fin and spent by the fuselage. Positive is stable, which is the opposite sign convention to dihedral effect.Used on Sheet 12
- Wing loading
- Weight divided by wing area. It sets stall speed and it sets how the aeroplane feels: low is floaty and gust-sensitive, high is fast and unforgiving.Used on Sheet 01