Reference · 37 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 37 terms

A

Affordable empty-weight fraction: The empty-weight fraction a design can have and still carry its crew, payload and fuel: \(1 - \Wf/\Wo - (W_{\text{crew}} + W_{\text{payload}})/\Wo\). The sizing solution is where it meets the statistical trend.; See: Lesson 1Related: Sizing equation, Empty-weight trend

B

Breguet endurance equation: The loiter fraction \(W_i/W_{i-1} = \exp[-EC/(L/D)]\), with \(E\) in hours.; See: Lesson 5Related: Loiter, Breguet range equation
Breguet range equation: The cruise fraction \(W_i/W_{i-1} = \exp[-RC/(V\,L/D)]\), with \(R\) in km, \(V\) in km/h and \(C\) in 1/h.; See: Lesson 5Related: Cruise and loiter \(L/D\), Breguet endurance equation

C

Class of aircraft: The group of earlier aircraft behind a statistical trend: jet transport, jet fighter, general aviation single, and so on. Choose it by mission, engine type and size, and stay inside the weights of the data.; See: Lesson 2Related: Empty-weight trend, Technology factor \(K_{\text{tech}}\)
Combat fraction: The weight fraction of a combat segment at full thrust for time \(t\) (h): \(1 - C(\TW)t\), because the engines burn \(CT\) of fuel weight per hour.; See: Lesson 5Related: Segment weight fraction \(W_i/W_{i-1}\), Thrust-specific fuel consumption \(C\)
Crew and payload weight: The numerator of the sizing equation, from the requirements: crew at a standard mass (90 kg each here) and payload: passengers with baggage (100 kg each here), cargo, weapons.; See: Lesson 1Related: Sizing equation
Cruise and loiter \(L/D\): Jets cruise at \(0.866\,\LDmax\) and loiter at \(\LDmax\); propeller aircraft cruise at \(\LDmax\) and loiter at \(0.866\,\LDmax\), because a propeller's \(C\) grows with speed.; See: Lesson 3, Lesson 4Related: Maximum lift-to-drag ratio \(\LDmax\), Equivalent \(C\) of a propeller aircraft

E

Empty weight \(\We\): Structure, engines, landing gear, fixed equipment and avionics: everything not crew, payload or fuel.; See: Lesson 1Related: Empty-weight fraction \(\We/\Wo\), Takeoff gross weight \(\Wo\)
Empty-weight fraction \(\We/\Wo\): The share of the takeoff weight that is the aircraft itself, typically 0.4 to 0.65. In the first estimate it comes from a statistical trend.; See: Lesson 2Related: Empty-weight trend, Empty weight \(\We\)
Empty-weight trend: \(\We/\Wo = A\Wo^{\,C}K_{vs}\), fitted to a class of aircraft. \(A\) depends on the unit of \(\Wo\): \(A_{\text{kg}} = A_{\text{lb}}(0.4536)^{-C}\).; See: Lesson 2Related: Class of aircraft, Empty-weight fraction \(\We/\Wo\), Variable-sweep factor \(K_{vs}\)
Equivalent \(C\) of a propeller aircraft: \(C = c_PgV/(1000\,\eta_p)\) with \(c_P\) in kg/(kW h) and \(V\) in m/s. It grows with speed; \(V/C = 3600\,\eta_p/(c_Pg)\) km does not.; See: Lesson 4Related: Power-specific fuel consumption \(c_P\), Propeller efficiency \(\eta_p\), Thrust-specific fuel consumption \(C\)
Equivalent skin-friction coefficient \(C_{fe}\): A drag coefficient per unit wetted area that lumps skin friction with form, interference and miscellaneous drag, measured on existing aircraft of each type: about 0.0030 for transports and 0.0055 for light single-engine aircraft.; See: Lesson 3Related: Zero-lift drag coefficient \(\CDz\), Wetted aspect ratio \(A_{\text{wet}}\)

F

Fuel fraction \(\Wf/\Wo\): \(\Wf/\Wo = 1.06(1 - W_x/W_0)\): the fuel burned on the mission plus 6% for reserve and trapped fuel.; See: Lesson 5Related: Mission weight fraction \(W_x/W_0\), Reserve and trapped-fuel allowance

G

Growth factor: The increase in \(\Wo\) per kilogram added, found by resizing. With the fractions fixed it is \(1/(1 - \Wf/\Wo - \We/\Wo)\), a little more than the resized value because \(\We/\Wo\) falls with weight.; See: Lesson 7Related: Sensitivity

H

Historical segment fractions: Typical fractions for segments with little fuel and uncertain details: warmup and takeoff 0.970, climb 0.985, landing 0.995.; See: Lesson 5Related: Segment weight fraction \(W_i/W_{i-1}\)

I

Iteration: Solving the sizing equation by repeating: guess \(\Wo\), evaluate \(\We/\Wo\), compute a new \(\Wo\). It converges in a handful of steps because the trend changes slowly with weight.; See: Lesson 1, Lesson 6Related: Sizing equation, Affordable empty-weight fraction

L

Loiter: A segment flown for time: a reserve hold or time on station. Its fraction comes from the endurance equation.; See: Lesson 5Related: Breguet endurance equation

M

Maximum lift-to-drag ratio \(\LDmax\): \(\tfrac12\sqrt{\pi A e/\CDz} = \tfrac12\sqrt{\pi e A_{\text{wet}}/C_{fe}}\): from the sketch's span and wetted area.; See: Lesson 3Related: Wetted aspect ratio \(A_{\text{wet}}\), Cruise and loiter \(L/D\)
Mission weight fraction \(W_x/W_0\): The product of the segment fractions, \(\prod W_i/W_{i-1}\): the weight at the end of the mission as a fraction of \(W_0\).; See: Lesson 5Related: Segment weight fraction \(W_i/W_{i-1}\), Fuel fraction \(\Wf/\Wo\)

O

Oswald factor \(e\): The span efficiency of the drag polar, about 0.75 to 0.85 for a first estimate; Module 9 estimates it properly.; See: Lesson 3Related: Maximum lift-to-drag ratio \(\LDmax\)

P

Payload drop: Weight released in flight. After a drop the weights are not fixed fractions of \(\Wo\), so the mission is followed segment by segment and \(\Wf = 1.06(\Wo - W_{\text{drop}} - W_x)\), iterating on \(\Wo\).; See: Lesson 6Related: Iteration, Mission weight fraction \(W_x/W_0\)
Power-specific fuel consumption \(c_P\): Fuel mass per unit shaft power per hour of a piston engine or turboprop, in kg/(kW h) or lb/(hp h): \(1\ \text{lb/(hp h)} = 0.6083\ \text{kg/(kW h)}\). Called the brake-specific fuel consumption (BSFC) for piston engines; turboprop data may be per equivalent shaft power (ESFC). Typical cruise values 0.4 (piston) and 0.5 (turboprop) lb/(hp h).; See: Lesson 4Related: Equivalent \(C\) of a propeller aircraft, Propeller efficiency \(\eta_p\)
Propeller efficiency \(\eta_p\): Thrust power over shaft power, \(TV/P\): about 0.8 in cruise for a variable-pitch propeller or turboprop.; See: Lesson 4Related: Equivalent \(C\) of a propeller aircraft, Power-specific fuel consumption \(c_P\)

R

Range wall: The condition in which the affordable curve stays below the trend everywhere: the sizing does not close, and the requirement, class or technology must change.; See: Lesson 1Related: Affordable empty-weight fraction, Sizing equation
Reserve and trapped-fuel allowance: A factor of 1.06 on the fuel burned, covering reserves and fuel that cannot be used.; See: Lesson 5Related: Fuel fraction \(\Wf/\Wo\)

S

Segment weight fraction \(W_i/W_{i-1}\): The weight at the end of a mission segment divided by the weight at its start: historical, Breguet cruise, endurance or combat.; See: Lesson 5Related: Historical segment fractions, Mission weight fraction \(W_x/W_0\)
Sensitivity: How much \(\Wo\) changes when one input changes, found by resizing. For the business jet, 1% of \(L/D\) or \(C\) is worth about 1% of \(\Wo\), and the empty-weight trend much more.; See: Lesson 7Related: Growth factor, Tornado chart, Trade study
Sizing equation: \(\Wo = (W_{\text{crew}} + W_{\text{payload}})/(1 - \Wf/\Wo - \We/\Wo)\). The denominator, usually 0.1 to 0.3, amplifies every error in the fractions.; See: Lesson 1Related: Iteration, Crew and payload weight

T

Takeoff gross weight \(\Wo\): The weight at the start of the design mission: \(\Wo = W_{\text{crew}} + W_{\text{payload}} + \Wf + \We\). The first number of a design.; See: Lesson 1Related: Sizing equation
Technology factor \(K_{\text{tech}}\): A stated multiplier on the empty-weight trend for technology the data do not represent, such as about 0.9 for an extensively composite airframe.; See: Lesson 2Related: Empty-weight trend, Variable-sweep factor \(K_{vs}\)
Thrust-specific fuel consumption \(C\): Fuel weight per hour per unit thrust, in 1/h; \(C = c_T[\text{mg/(N s)}]/28.33\), where \(c_T\) is the same consumption as fuel mass per unit thrust per second. About 0.9 (turbojet), 0.8 (low-bypass) and 0.5 (high-bypass) in cruise.; See: Lesson 4Related: Equivalent \(C\) of a propeller aircraft, Cruise and loiter \(L/D\)
Tornado chart: A bar chart of the change in \(\Wo\) for the same percentage change in each input, sorted by size: the inputs to get right first are at the top.; See: Lesson 7Related: Sensitivity
Trade study: Resizing over a grid of requirements or design choices, so that each combination's takeoff weight can be compared.; See: Lesson 7Related: Sensitivity

V

Variable-sweep factor \(K_{vs}\): 1.04 on the empty-weight trend for an aircraft with a variable-sweep wing, 1.00 otherwise.; See: Lesson 2Related: Empty-weight trend, Technology factor \(K_{\text{tech}}\)

W

Wetted area \(\Swet\): The area of the aircraft's skin touched by the air. Wings and tails \(\approx S_{\text{exposed}}(1.977 + 0.52\,t/c)\); fuselage \(\approx 3.4(A_{\text{top}} + A_{\text{side}})/2\).; See: Lesson 3Related: Wetted aspect ratio \(A_{\text{wet}}\), Equivalent skin-friction coefficient \(C_{fe}\)
Wetted aspect ratio \(A_{\text{wet}}\): \(A_{\text{wet}} = b^2/\Swet = A/(\Swet/\Sref)\): span against wetted area, the sketch's best predictor of \(\LDmax\).; See: Lesson 3Related: Maximum lift-to-drag ratio \(\LDmax\), Wetted area \(\Swet\)

Z

Zero-lift drag coefficient \(\CDz\): The drag coefficient at zero lift, mostly skin friction at subsonic speed. First estimate: \(\CDz = C_{fe}\Swet/\Sref\).; See: Lesson 3Related: Equivalent skin-friction coefficient \(C_{fe}\), Wetted area \(\Swet\)

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 \(\Wo\)
\(A,\ C,\ K_{vs}\)	Trend coefficient, exponent, variable-sweep factor	Empty-weight trend
\(W_i/W_{i-1},\ W_x/W_0\)	Segment and mission fractions	Mission weight fraction \(W_x/W_0\)
\(\Swet,\ \Sref\)	Wetted and reference area	Wetted area \(\Swet\)
\(A_{\text{wet}},\ C_{fe},\ e\)	Wetted aspect ratio, equivalent skin friction, Oswald factor	Wetted aspect ratio \(A_{\text{wet}}\)
\(C,\ c_T,\ c_P,\ \eta_p\)	Thrust-specific and power-specific fuel consumption, propeller efficiency	Thrust-specific fuel consumption \(C\)
\(R,\ E,\ t\)	Cruise range, loiter time, combat time	Breguet range equation

Statistical values are from D. P. Raymer, Aircraft Design: A Conceptual Approach, Chapter 3; check them against your edition.