Reference · 66 terms

Glossary

Short, precise definitions of the words and symbols used in this module, each linked to the lesson where it is taught. Filter the list as you type, jump to a letter, or look a symbol up in Symbols at a glance.

Showing all 66 terms

A

Adverse yaw: The tendency of an aircraft to yaw away from the direction of a roll, because the rising wing, with more lift, also has more drag. The Wrights met it with wing warping in 1901–02 and cured it with a movable rudder linked to the warping.; See: Lesson 2Related: Wing warping, Aileron
Aileron: A hinged surface near the wing tip; ailerons deflect in opposite directions to roll the aircraft. They replaced wing warping as wings became stiffer, from about 1909.; See: Lesson 3Related: Wing warping, Adverse yaw
Area rule: Near Mach 1, wave drag depends on how the total cross-sectional area of the aircraft varies along its length, so the fuselage is narrowed where the wing is. Published by Richard Whitcomb of NACA in 1952; the Convair F-102 could not pass Mach 1 until it was applied.; See: Lesson 5Related: Drag-divergence Mach number \(M_{dd}\), Swept wing
Aspect ratio \(A\): Span squared over wing area, \(A = b^2/S\) (\(b/c\) for a rectangular wing). A high aspect ratio reduces induced drag, \(\CL^2/(\pi A e)\), but needs a heavier wing structure. For a biplane with two equal wings, \(A = b^2/(S/2)\).; See: Lesson 1Related: Induced drag, Oswald efficiency factor \(e\), Maximum lift-to-drag ratio \(\LDmax\)

B

Biplane: An aircraft with two wings, one above the other, joined by struts and bracing wires into a deep, light box truss. Dominant until the early 1930s; its price was the drag of struts and wires and a low effective aspect ratio.; See: Lesson 3Related: Bracing wires, Cantilever wing, Monoplane
Bracing wires (braced structure): External struts and wires that carry wing loads as a truss. Light and stiff with thin airfoils, but with high drag: Loftin\'s First World War aircraft have \(\CDz \approx 0.032\) to \(0.077\).; See: Lesson 3Related: Biplane, Cantilever wing
Breguet range equation: The range in cruise at constant speed, \(L/D\) and \(C\): \(R = (V/C)(L/D)\ln(W_{i-1}/W_i)\) for a jet (Raymer). Range is the product of a propulsive, an aerodynamic and a structural factor.; See: Lesson 6Related: Specific fuel consumption \(C\), Range factor, Maximum lift-to-drag ratio \(\LDmax\)
Bypass ratio: In a turbofan, the mass flow of air passing around the core through the fan, divided by the flow through the core. Higher bypass ratios give higher propulsive efficiency and lower fuel consumption; they grew from about 5 (1969) to more than 10.; See: Lesson 6Related: Turbofan, Specific fuel consumption \(C\)

C

Canard: A horizontal control or lifting surface ahead of the wing. The Wright Flyer had a canard elevator; by 1914 almost all aircraft had their tail behind instead.; See: Lesson 2, Lesson 3Related: Tractor configuration
Cantilever wing: A wing carried by internal spars alone, with no external bracing. It needs a thick airfoil for spar depth; Junkers (1915) and Fokker showed the way, and it became standard by the 1930s.; See: Lesson 3, Lesson 4Related: Bracing wires, Stressed-skin construction
Compressibility: The change in air density with pressure that becomes important near the speed of sound. Above the critical Mach number, shock waves form on the wing and the drag rises steeply.; See: Lesson 5Related: Critical Mach number \(M_{\text{crit}}\), Drag-divergence Mach number \(M_{dd}\)
Conceptual design: The first phase of design: what the aircraft will look like, weigh and cost, and whether it can meet the requirements. Many configurations, quick methods and trade studies. It commits most of the eventual cost.; See: Lesson 7Related: Preliminary design, Detail design, Design wheel
Critical Mach number \(M_{\text{crit}}\): The free-stream Mach number at which the flow somewhere on the aircraft first reaches the speed of sound. A little above it, shocks form; the drag-divergence Mach number is slightly higher.; See: Lesson 5Related: Drag-divergence Mach number \(M_{dd}\), Compressibility

D

Damage tolerance: The design principle that cracks will occur and must be found by inspection before they grow to a critical size. It followed fail-safe design, which followed the Comet accidents of 1954.; See: Lesson 5Related: Fail-safe structure, Metal fatigue
Delta wing: A triangular wing with a highly swept leading edge, used for supersonic aircraft such as the Convair F-106 (60° sweep) and Concorde. It keeps the leading edge behind the Mach cone.; See: Lesson 5Related: Swept wing, Mach cone
Design wheel: Raymer\'s picture of design as a cycle: requirements, design concept, design analysis, sizing and trade studies, and back to the requirements. Each turn refines the design.; See: Lesson 7Related: Conceptual design, Trade study
Detail design: The phase in which every part of a fixed configuration is designed for manufacture, followed by tooling, production and testing.; See: Lesson 7Related: Preliminary design, Conceptual design
Drag area \(f\) (equivalent flat-plate area): The zero-lift drag coefficient times the reference area, \(f = \CDz S\), in m²: the area of a flat plate with the same zero-lift drag. Zero-lift drag is \(D_0 = qf\) and its power \(P_0 = \tfrac12\rho V^3 f\).; See: Lesson 1, Lesson 3Related: Zero-lift drag coefficient \(\CDz\), Zero-lift drag power \(P_0\)
Drag polar: The relation between drag and lift coefficients. The parabolic drag polar used in conceptual design is \(\CD = \CDz + K\CL^2\) with \(K = 1/(\pi A e)\).; See: Lesson 1, Lesson 3Related: Zero-lift drag coefficient \(\CDz\), Induced drag, Oswald efficiency factor \(e\)
Drag-divergence Mach number \(M_{dd}\): The Mach number at which compressibility makes the drag rise steeply. About 0.7 for the thick straight wings of the 1940s; sweep and supercritical airfoils raise it.; See: Lesson 5Related: Critical Mach number \(M_{\text{crit}}\), Swept wing, Supercritical airfoil

E

Empty weight \(\We\): The weight of the aircraft itself: structure, engines, landing gear, systems and fixed equipment, excluding crew, payload and fuel.; See: Lesson 1Related: Empty-weight fraction \(\We/\Wo\), Takeoff gross weight \(\Wo\)
Empty-weight fraction \(\We/\Wo\): The fraction of the takeoff weight that is the aircraft itself. Raymer correlates it with \(\Wo\) for each class of aircraft as \(\We/\Wo = A\,\Wo^{\,C}\); a fit to Loftin\'s jet transports gives \(A = 1.117\), \(C = -0.0698\) (kg).; See: Lesson 1, Lesson 7Related: Empty weight \(\We\), Sizing equation

F

Fail-safe structure: A structure with more than one load path, so that the failure of one member does not cause the loss of the aircraft.; See: Lesson 5Related: Damage tolerance, Metal fatigue
Flaps: Hinged or sliding trailing-edge surfaces deflected for takeoff and landing to raise \(\CLmax\). Split and Fowler flaps of the 1930s let wing loadings rise without raising landing speeds.; See: Lesson 4Related: Maximum lift coefficient \(\CLmax\), Stall speed \(V_s\), Wing loading \(\WS\)
Fly-by-wire: Flight controls in which the pilot\'s inputs go to computers that move the control surfaces electrically. It allows relaxed static stability (F-16, 1974) and envelope protection (Airbus A320, 1987).; See: Lesson 6Related: Relaxed static stability
Fuel fraction \(\Wf/\Wo\): The fuel weight as a fraction of takeoff weight, found from the mission segments (for cruise, from the Breguet equation).; See: Lesson 7Related: Sizing equation, Breguet range equation

I

Induced drag: Drag due to lift, caused by the trailing vortices of a finite wing: \(C_{D_i} = \CL^2/(\pi A e)\). It falls with span and aspect ratio. At \(\LDmax\) it equals the zero-lift drag.; See: Lesson 3Related: Lifting-line theory, Aspect ratio \(A\), Oswald efficiency factor \(e\)

L

Lift equation of 1900: \(L = kV^2Sc_l\), with \(L\) in lb, \(V\) in mph and \(S\) in ft², used by Lilienthal and the Wrights. Equivalent to the modern \(L = \tfrac12\rho V^2 S\CL\) with \(\CL = (k/0.002557)\,c_l\).; See: Lesson 2Related: Smeaton coefficient \(k\), Lilienthal coefficient \(c_l\)
Lifting-line theory: Ludwig Prandtl\'s theory of the finite wing (1918–1919), which models it as a line of bound vortices shedding a trailing vortex sheet. It gives the induced drag \(\CL^2/(\pi A e)\), with \(e = 1\) for an elliptic lift distribution.; See: Lesson 3Related: Induced drag, Oswald efficiency factor \(e\)
Lilienthal coefficient \(c_l\): The lift of a wing as a fraction of the force on a flat plate square to the same wind, from Lilienthal\'s tables (1889). The modern lift coefficient is about \(1.28\,c_l\).; See: Lesson 2Related: Lift equation of 1900, Smeaton coefficient \(k\)

M

Mach cone: The cone of disturbances behind a point in supersonic flow, of half-angle \(\mu = \arcsin(1/M)\). A leading edge swept more than \(90^\circ - \mu\) lies inside it and behaves as subsonic.; See: Lesson 5Related: Delta wing, Swept wing
Mach number \(M\): Flight speed divided by the local speed of sound, \(M = V/a\), with \(a = \sqrt{\gamma R T}\).; See: Lesson 5Related: Standard atmosphere, Critical Mach number \(M_{\text{crit}}\)
Maximum lift coefficient \(\CLmax\): The highest lift coefficient a wing reaches before it stalls. It sets the stall speed for a given wing loading. Flaps raised it from about 1.3 to about 2 on the aircraft of the 1930s.; See: Lesson 4Related: Stall speed \(V_s\), Flaps, Wing loading \(\WS\)
Maximum lift-to-drag ratio \(\LDmax\): The best ratio of lift to drag: \(\LDmax = \tfrac12\sqrt{\pi A e/\CDz}\), at \(\CL^* = \sqrt{\pi A e\,\CDz}\) where induced drag equals zero-lift drag. About 8 for First World War aircraft, 14–17 for the best propeller aircraft, 18–20 for jet transports.; See: Lesson 1Related: Drag polar, Breguet range equation
Metal fatigue: The growth of cracks under repeated loads well below the static strength. Repeated cabin pressurization grew a fatigue crack from a cut-out corner in the Comet, causing the 1954 accidents.; See: Lesson 5Related: Fail-safe structure, Damage tolerance
Monoplane: An aircraft with a single wing. Early monoplanes were braced; the clean cantilever monoplane with a smooth skin became the standard layout by the mid-1930s.; See: Lesson 3, Lesson 4Related: Cantilever wing, Biplane

N

NACA (National Advisory Committee for Aeronautics): The US government aeronautics research agency founded in 1915 and absorbed into NASA in 1958. Its wind tunnels and reports (cowlings, airfoils, the area rule) underpinned much of the design progress in this module.; See: Lesson 4Related: NACA cowling, Area rule
NACA cowling: A shaped ring around a radial engine, developed by Fred Weick at NACA Langley (1928), that sharply reduced the engine\'s drag and improved its cooling.; See: Lesson 4Related: NACA, Zero-lift drag coefficient \(\CDz\)

O

Oswald efficiency factor \(e\): The factor in \(K = 1/(\pi A e)\) that accounts for a non-elliptic lift distribution and the rest of the aircraft: about 0.7 to 0.85. Loftin used 0.70 for the First World War aircraft and 0.75 for later ones.; See: Lesson 1, Lesson 3Related: Induced drag, Drag polar

P

Power loading \(W/P\): Weight per unit engine power for a propeller aircraft, in kg/kW (or lb/hp). Lower is more powerful: the Wright Flyer had about 38 kg/kW, a P-51D about 4 kg/kW.; See: Lesson 1Related: Thrust-to-weight ratio \(\TW\), Wing loading \(\WS\)
Preliminary design: The phase after conceptual design, in which the configuration is frozen in its main features and analyzed and tested in depth by specialists; the outer shape is defined precisely (lofting).; See: Lesson 7Related: Conceptual design, Detail design
Propeller efficiency \(\eta_p\): Thrust power over shaft power, \(\eta_p = TV/P\). Propellers lose efficiency as their tips approach the speed of sound, one reason piston aircraft were limited to about 750 km/h.; See: Lesson 3, Lesson 5Related: Turbojet

R

Range factor: The product \((V/C)(L/D)\) in the Breguet equation, in km: it combines the propulsive and aerodynamic efficiency of a cruising aircraft.; See: Lesson 6Related: Breguet range equation, Specific fuel consumption \(C\)
Relaxed static stability: Designing an aircraft to be neutrally stable or unstable, with a computer providing the stability. It allows a smaller tail and less trim drag. (The Wright Flyer was unstable in pitch too, with the pilot as the stabilizer.); See: Lesson 6Related: Fly-by-wire
Rotary engine: An early air-cooled engine whose cylinders spin with the propeller around a fixed crankshaft. Light for its power, but with strong gyroscopic effects; common in First World War fighters.; See: Lesson 3Related: Tractor configuration

S

Simple sweep theory: The idea that on a long swept wing only the flow component normal to the leading edge matters: \(M_n = M\cos\Lambda\). Real wings gain somewhat less than the theory predicts.; See: Lesson 5Related: Swept wing, Drag-divergence Mach number \(M_{dd}\)
Sizing equation (first weight estimate): Raymer\'s first estimate of takeoff weight, \(\Wo = (W_{\text{crew}} + W_{\text{payload}})/(1 - \Wf/\Wo - \We/\Wo)\), solved by iteration because \(\We/\Wo\) depends on \(\Wo\). Its small denominator amplifies every error: the weight snowball.; See: Lesson 7Related: Empty-weight fraction \(\We/\Wo\), Fuel fraction \(\Wf/\Wo\), Takeoff gross weight \(\Wo\)
Smeaton coefficient \(k\): The pressure on a flat plate square to a 1 mph wind, in lb/(ft²·mph²). The traditional value 0.005 was about 50% too high; the Wrights found about 0.0033 in 1901 (modern 0.00326).; See: Lesson 2Related: Lift equation of 1900, Lilienthal coefficient \(c_l\)
Specific fuel consumption \(C\) (thrust-specific): Fuel used per unit thrust per unit time, by weight, in 1/h (lb/(lb·h)). Typical cruise values for sizing: about 0.9 (turbojet), 0.8 (low-bypass turbofan), 0.5 (high-bypass turbofan).; See: Lesson 6, Lesson 6Related: Breguet range equation, Turbofan
Stall speed \(V_s\): The lowest steady flight speed, at \(\CLmax\): \(V_s = \sqrt{2(\WS)/(\rho\CLmax)}\). It sets the landing and takeoff speeds.; See: Lesson 4Related: Maximum lift coefficient \(\CLmax\), Wing loading \(\WS\), Flaps
Standard atmosphere (ISA): The reference atmosphere used for performance: \(T = 288.15 - 0.0065h\) K up to 11 km and 216.65 K from 11 to 20 km; \(\rho_0 = 1.225\ \text{kg/m}^3\) and \(a_0 = 340.3\ \text{m/s}\) at sea level.; See: Lesson 5Related: Mach number \(M\)
Stressed-skin construction (semi-monocoque): A structure whose smooth metal skin, with frames and stringers, carries the loads itself. It replaced the fabric-covered truss in the early 1930s, saving weight and drag.; See: Lesson 4Related: Cantilever wing
Supercritical airfoil: An airfoil with a flatter upper surface that keeps the supersonic region weak, delaying the drag rise (Richard Whitcomb, NASA, late 1960s). The gain can be spent on a thicker wing, less sweep or a higher cruise Mach number.; See: Lesson 6Related: Drag-divergence Mach number \(M_{dd}\), Winglet
Swept wing: A wing whose leading edge is angled back (sweep \(\Lambda\)), proposed by Busemann (1935) and R. T. Jones (1945), to delay compressibility drag. Jet transports use 25–37.5°.; See: Lesson 5Related: Simple sweep theory, Delta wing

T

Takeoff gross weight \(\Wo\): The total weight at the start of the design mission: \(\Wo = W_{\text{crew}} + W_{\text{payload}} + \Wf + \We\) (Raymer).; See: Lesson 1Related: Empty weight \(\We\), Sizing equation
Thrust-to-weight ratio \(\TW\): Engine thrust over weight, dimensionless. With wing loading, the main parameter of a conceptual design: about 0.25 for jet transports, 0.35 for the first swept-wing fighters, above 1 for the F-15 and F-16.; See: Lesson 1, Lesson 5Related: Wing loading \(\WS\), Power loading \(W/P\)
Tractor configuration: A propeller at the front of the aircraft, pulling. Standard since Blériot\'s monoplane of 1909; the Wright Flyer was a pusher.; See: Lesson 3Related: Canard
Trade study: A systematic variation of design parameters (wing loading, thrust-to-weight ratio, aspect ratio, sweep) to find the combination that best meets the requirements.; See: Lesson 7Related: Design wheel, Conceptual design
Turbofan: A jet engine that drives a large fan, passing most of its air around the core. High-bypass turbofans (C-5A 1968, 747 1969) cut fuel consumption and noise.; See: Lesson 6Related: Bypass ratio, Specific fuel consumption \(C\), Turbojet
Turbojet: A gas-turbine engine that produces thrust by accelerating all its air through the core to a high speed. Its thrust stays roughly constant with speed, so its useful power grows with speed. First flight: Heinkel He 178, 1939.; See: Lesson 5Related: Turbofan, Propeller efficiency \(\eta_p\)

V

Variable sweep: A wing that pivots to change its sweep in flight: spread for takeoff and efficient cruise, swept for high speed (F-111, 1964; F-14).; See: Lesson 5Related: Swept wing

W

Wing loading \(\WS\): Weight per unit wing area, \(\Wo/S\), in kg/m² (multiply by \(g\) for N/m²). It rose about twentyfold between 1915 and 1970, from about 30 to more than 700 kg/m².; See: Lesson 1, Lesson 4Related: Stall speed \(V_s\), Thrust-to-weight ratio \(\TW\)
Wing warping: Rolling an aircraft by twisting its wing tips in opposite directions, the Wrights\' roll control (tested on a kite in 1899). Replaced by ailerons.; See: Lesson 2, Lesson 2Related: Aileron, Adverse yaw
Winglet: A small, near-vertical surface at a wing tip that reduces induced drag without the full weight of a longer span (Whitcomb, 1970s).; See: Lesson 6Related: Induced drag, Supercritical airfoil

Z

Zero-lift drag coefficient \(\CDz\) (parasite drag coefficient): The drag coefficient at zero lift, based on wing area. Loftin\'s lower bound fell from about 0.040 (1920) to 0.021 (early 1930s) and 0.016 (1940).; See: Lesson 1, Lesson 4Related: Drag area \(f\), Drag polar
Zero-lift drag power \(P_0\): The power absorbed by zero-lift drag, \(P_0 = \tfrac12\rho V^3 f\): it grows with the cube of speed.; See: Lesson 3Related: Drag area \(f\)

Symbols at a glance

Met a symbol in a lesson or in another textbook and not sure what it stands for? Find it here, then follow the link to its entry.

Symbols used in this module
Symbol	Meaning	Entry
\(\Wo,\ \We,\ \Wf\)	Takeoff gross, empty and fuel weight	Takeoff gross weight \(\Wo\)
\(\WS\)	Wing loading	Wing loading \(\WS\)
\(W/P,\ \TW\)	Power loading; thrust-to-weight ratio	Power loading \(W/P\)
\(A = b^2/S\)	Aspect ratio (span \(b\), wing area \(S\))	Aspect ratio \(A\)
\(\CL,\ \CD,\ \CDz\)	Lift, drag and zero-lift drag coefficients	Zero-lift drag coefficient \(\CDz\)
\(K,\ e\)	Induced-drag factor \(1/(\pi A e)\); Oswald factor	Oswald efficiency factor \(e\)
\(\LDmax,\ \CL^*\)	Maximum lift-to-drag ratio and its lift coefficient	Maximum lift-to-drag ratio \(\LDmax\)
\(f\)	Drag area \(\CDz S\)	Drag area \(f\)
\(k,\ c_l\)	Smeaton coefficient; Lilienthal coefficient	Smeaton coefficient \(k\)
\(\CLmax,\ V_s\)	Maximum lift coefficient; stall speed	Stall speed \(V_s\)
\(\rho,\ T,\ a\)	Air density, temperature, speed of sound	Standard atmosphere
\(M,\ M_n,\ M_{dd}\)	Mach number, normal Mach number, drag-divergence Mach number	Simple sweep theory
\(\Lambda\)	Sweep angle	Swept wing
\(C,\ \eta_p\)	Specific fuel consumption; propeller efficiency	Specific fuel consumption \(C\)
\(R\)	Range	Breguet range equation

Notation follows Raymer's Aircraft Design: A Conceptual Approach: \(\Wo\) for takeoff gross weight, \(A\) for aspect ratio (some books use \(AR\)), \(C\) for specific fuel consumption (some use \(c_t\) or TSFC).