Aircraft Performance

Lecture 8:
Aircraft Performance and Design

2023
Aircraft Performance
 The Philosophy of Aircraft Design
• Airplane design is both an art and a science. In that respect it is
di cult to learn by reading a book; rather, it must be experienced and
practiced.

• Airplane design is the intellectual engineering process of creating on

paper (or on a computer screen) a ying machine to

(1) meet certain speci cations and requirements established by

potential users (or as perceived by the manufacturer) and/or

(2) pioneer innovative, new ideas and technology.

• The design process is indeed an intellectual activity, but a rather

special one that is tempered by good intuition developed via
experience, by attention paid to successful airplane designs that
have been used in the past, and by (generally proprietary) design
procedures and databases (handbooks, etc.) that are a part of every
airplane manufacturer.
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Phases of Aircraft Design
• Conceptual design
• Preliminary design
• Detail Design
 Conceptual Design
• The design process starts with a set of speci cations
(requirements) for a new airplane, or much less frequently as the
response to the desire to implement some pioneering, innovative
new ideas and technology.

• Goal: shape, size, weight, and performance of the new design

• Product: layout (on paper or on a computer screen) of the airplane
con guration.

• The layout may slightly changed during the second phase,

preliminary design

• Fundamental aspects such as the shape of the wings (swept back,

swept forward, or straight), the location of the wings relative to the
fuselage, the shape and location of the horizontal and vertical tail,
the use of a canard surface or not, engine size and placement, etc.
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Conceptual Design
Questions to answer

• Can the design meet the speci cations?

• Is the design optimized, that is, is it the best design that
meets the speci cations?

• These questions are answered during the conceptual design by

using tools primarily from aerodynamics, propulsion, and ight
performance

• Structural and control system considerations are not dealt with in

any detail. However, they are not totally absent.
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 Preliminary Design
• In the preliminary design phase, only minor changes are made to the
con guration layout

• Indeed, if major changes were demanded during this phase, the

conceptual design process would have been seriously awed to begin
with.

• Serious structural and control system analysis and design take place.
• Wind tunnel testing will be carried out, and major computational uid
dynamic (CFD) calculations of the complete ow eld over the airplane
con guration will be made.

• It is possible that the wind tunnel tests and/or the CFD calculations will
uncover some undesirable aerodynamic interference, or some unexpected
stability problems, which will promote changes to the con guration layout.

• At the end of the preliminary design phase, the airplane con guration is
frozen and precisely de ned.
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Detail Design
• For detail design, the airplane is now simply a machine to be
fabricated.

• The precise design of each individual rib, spar, and section of

skin now takes place.

• The size, number, and location of fasteners (rivets, welded joints,

etc.) are determined.

• Manufacturing tools and jigs are designed.

• At this stage, ight simulators for the airplane are developed.
• At the end of this phase, the aircraft is ready to be fabricated.
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 THE SEVEN INTELLECTUAL PIVOT
POINTS FOR CONCEPTUAL DESIGN
1. Requirements • Make a selection of the airfoil
section.
2. Weight of the airplane - rst estimate
• Determine the wing geometry
3. Critical performance parameters (aspect ratio, sweep angle, taper
a. Maximum lift coe cient CL, max ratio, twist, incidence angle relative
b. Lift-to-drag ratio L /D to the fuselage, dihedral, vertical
c. Wing loading W/S location on the fuselage, wing-tip
d. Thrust-to-weight ratio T/W shape, etc.)

• Choose the geometry and

Iterate

4. Con guration layout - shape and size of the

airplane on a drawing (or computer screen) arrangement of the tail. Would a
canard be more useful?
5. Better weight estimate

No 6. Performance analysis - does the design

• Decide what speci c power plants
are to be used. What are the size,
meet or exceed requirements? number, and placement of the
Yes engines?
7. Optimization - is it the best design?
• Decide what high-lift devices will be
necessary.
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Requirements
 Requirements
• Range
• Takeo distance
• Stalling velocity
• Endurance (usually important for reconnaissance airplanes)
• Maximum velocity
• Rate of climb
• For dog ghting combat aircraft, maximum turn rate and sometimes minimum turn radius
• Maximum load factor.
• Service ceiling
• Cost
• Reliability and maintainability
• Maximum size (so that the airplane will t inside standard hangers and/or be able to t in
a standard gate at airline terminals)
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Requirements
Example
Beechcraft Musketeer
Cessna 172
Cessna 177
Weight Estimation
 1st Weight Estimation
No airplane can get o the ground unless it can produce a lift greater than its weight.
And no airplane design process can "get o the ground" without a rst estimate of the gross takeo weight.

• Initial weight of the aircraft:

W0 = We + Wc + Wp + Wf

where We: empty weight; Wc: crew weight; Wp: payload weight
(passenger + luggage); Wf: fuel weight

• Rearrange the weight function to obtain:

Wc + Wp
W0 =
1 − Wf /W0 − We /W0
• We /W0 can be obtained from historical data of the other aircrafts
• Wf /W0 can be obtained from maximum range estimation
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Estimation of We /W0
We /W0 indeed is a function of W0. Here we assume We /W0 is a xed value.

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Estimation of Wf /W0

• Breguet’s formula for range of a propeller driven engine:

SFC CD ( Wfinal )
η CL Winitial
R= ln

Wf = Winitial − Wfinal

• Adding some margin for takeo , landing and loitering

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Estimation of Maximum L/D

• At this stage, we have not had aerodynamic details for

maximum L/D

• An estimation can be based on historical data, obtained by

Loftin:
Estimation of Engine Parameters

• Some assumptions to be made:

Current engines: SFC is from 0.4 to 0.7 lb of fuel per hp per
hour (1 hp = 550 ft.lb/s)

Propeller e ciency η = 0.85 for a variable-pitch propeller

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Critical Performance
Parameters
Maximum Lift Coefficient CL,max

• Choose an airfoil
• Estimate the maximum cl,max

• Assumption: CL,max ≈ 0.9cl,max for AR >5

• Other aerodynamic assumptions (because of our limited
resources):

Zero-lift drag coe cient CD,0 from 0.027 to 0.032 (historical

data).

Oswald e ciency e = 0.6 for low-wing general aircraft.

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Wing Loading W/S

• Wing loading W/S is determined from:

Landing distance constraint

Vstall constraint
Thrust to Weight Ratio T/W

• Thrust to weight ratio T/W is determined from:

Takeo distance

Rate of climb

Maximum velocity
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Constraint Diagram
Configurations
Configuration layout
• Propulsion: number of engines, position: tractor or pusher
• Wing: aspect ratio, wing sweep, taper ratio, airfoil variation along
the span, geometric twist

• Wing position: high-wing, mid-wing or low-wing

• Fuselage: large enough for crew, passengers, luggage, engine
mount, fuel

• Center of gravity: for longitudinal stability

• Propeller: size, thickness, pro le
• Stabilizers: horizontal tail, vertical tail
• Landing gears
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Performance Analysis
When you design it ... think about how you
would feel if you had to fly it! Safety first!
Sign on the wall of the design office at
Douglas Aircraft Company, 1932
Performance Analysis
• Thrust Required curve
• Power Required curve
• Power Available
• Maximum velocity
• Cruising speed
• Stalling speed
• Rate of climb
• Takeo distance
• Landing distance
• Flight ceiling
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