weight estimation of airplanes
DESCRIPTION
The present document provides some weight definitions for airplanes and describes Roskam's methodology to estimate their maximum takeoff weight and basic operating weight.TRANSCRIPT
2
Content
3
Guidance
4
Reasoning
Aircraft weight, and its accurate prediction, is critical as it affects all aspects of performance, manufacturing costs, selling price and all other items.
Designer must keep weight to a minimum as far as practically possible.
Preliminary estimates possible for take-off weight, empty weight and fuel weight using basic requirement, specification (assumed mission profile) and initial configuration selection.
Glossary
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AFM: Aircraft flight manual
MTOW: Maximum takeoff weight
MEW: Manufacturer’s empty weight
MZFW: Maximum zero-fuel weight
MLW: Maximum landing weight
BOW: Basic operating weight
FAR: Federal Aviation Regulation
L/D: Lift-to-drag ratio
Some Tasks in the Conceptual Design
6
Preliminary
sizing
(We,Wto,Wf)
Sensitivity study (Wto to Wpl,
We, R, S.F.C(Cj), and L/D)
Estimating
T/W, W/S
Configuration
selection
Design of cockpit and
the fuselage
Design of the
wing
Landing gear design
Cost prediction
Selection Integration
of the Propulsion
system
Design of stabilizers
and control
surfaces
Estimation of cg
variation and
airplane inertias
This course material is concerned with
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Preliminary
sizing
(We,Wto,Wf)
Sensitivity study (Wto to Wpl,
We, R, S.F.C(Cj), and L/D)
Estimating
T/W, W/S
Configuration
selection
Design of cockpit and
the fuselage
Design of the
wing
Landing gear design
Cost prediction
Selection Integration
of the Propulsion
system
Design of stabilizers
and control
surfaces
Estimation of cg
variation and
airplane inertias
8
Manufacturer’s Empty Weight:
Weight of the structure, powerplant, furnishings, systems and other items of
equipment that are an integral part of a particular aircraft configuration. It is
essentially a “dry” weight, including only those fluids contained in closed
systems.
Includes:
- airframe, systems
- closed system fluids
- seats, seat belts
- seller-furnished emergency equipment
- fire extinguishers
Does not include:
- galley structure, ovens, inserts, etc.
- escape slides
- life rafts, life vests
- portable oxygen bottles
- fluids like engine oil, trapped fuel, potable water
Standard Items:
Equipment and system fluids which are not considered an integral
part of a particular aircraft configuration, are not included in the
MEW, but which do not normally vary for aircraft of the same type.
Standard items may include, but are not limited to:
- unusable fuel, oil, and engine injection fluids
- unusable drinking and washing water
- first aid kits, flashlights, megaphone, etc
- emergency oxygen equipment
- galley/bar structure, inserts, ovens, etc.
- electronic equipment required by the operator
Operational Items:
Personnel, equipment and supplies necessary for a particular
operation but not included in the Basic Empty Weight. These items
may vary for a particular aircraft and may include, but are not
limited to:
- flight and cabin crew plus their baggage
- manuals and navigation equipment
- removable service equipment:
cabin (blankets, pillows, literature, etc.)
galley (food, beverages, etc.)
- usable drinking and washing water
- toilet fluid and chemical
- life rafts, life vests, emergency transmitters
- cargo containers, pallets, and/or cargo tiedown equipment if used.
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Weight Definitions • disposable load = payload + useable fuel (+any necessary ballast)
Where
Payload = the revenue earning load
Maximum ramp weight: MTOW + start, taxi, and run-up fuel
Maximum ramp weight is that approved for ground maneuver
Maximum landing weight: maximum weight approved for touchdown
Maximum zero fuel weight: Maximum weight allowed before usable fuel must
be loaded in defined sections of the aircraft. Any weight added above the MZFW
must be only due to fuel.
• APS weight (aircraft prepared for service), which is the same as the basic empty
weight, i.e. fully equipped operational, without crew, usable fuel or payload (the
load that generates revenue, income).
• AUW, Wo The all-up (gross) weight is the maximum weight at which flight
requirements must be met.
Maximum to take-off weight = gross (all-up) weight = MTOW
= operating empty weight + disposable load
in which operating empty weight and disposable load are built up as follow
Basic empty weight = Manufacture’s weight + standard items
Operating empty weight = basic empty weight + operational items
(From an equipment standpoint, the airplane is ready for operation.)
The maximum allowable weights that can legally be used by a
given airline are listed in the AFM, and Weight and Balance
Manual; these are called the airplane’s Certified Weight Limits:
• Maximum weights chosen by the airline
• Some airlines refer to these as the “purchased weights”
• Certified weight limits are often below the structural limits
• Airlines may buy a certified weight below structural capability
because many of the airport operating fees are based on the airplane's
AFM maximum allowable weight value. Typically the purchase price
is a function of the certified weight bought
The maximum allowable Operational Takeoff Weight may be
limited to a weight which is lower than the Certified Maximum
Weight by the most restrictive of the following requirements:
• Airplane performance requirements for a given altitude and
temperature:
- Takeoff field length available
- Tire speed and brake energy limits
- Minimum climb requirements
- Obstacle clearance requirements
• Noise requirements
• Tire pressure limits
• Runway loading requirements
• Center of gravity limitations
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Weight Definitions
Take-off weight (WTO) – (Roskam method)
WTO = WOE + WF + WPL
where:
WOE (or WOWE ) = operating weight empty
WF = fuel weight
WPL = payload weight
Note that other methods (e.g. Raymer) use slightly different
terminology but same principles.
(1)
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Weight Definitions
Operating weight empty may be further broken down
into:
WOE = WE + Wtfo + Wcrew
where:
WE = empty weight
Wtfo = trapped (unusable) fuel weight
Wcrew = crew weight
(2)
17
Weight Definitions
• Empty weight sometimes further broken down
into:
WE = WME + WFEQ
where:
WME = manufacturer’s empty weight
WFEQ = fixed equipment weight
(includes avionics, radar, air- conditioning, APU, etc.)
(3)
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Weight Figures for Transport Aircraft Aircraft MTOW (tones) MLW(tones) Basic Operating
Weight (tones) BOW/MTOW
Jet Airliners/Transports
Airbus A319 75.5 62.5 40.6 0.537
Airbus A380 560 386 276.8 0.494
ERJ-145LR 22 19.3 12.114 0.550
Embraer 170ER 37.2 32.8 20.94 0.563
Embraer 190LR 50.3 43 27.72 0.551
Boeing 747-400ER 412.769 295.742 180.985 0.438
Boeing 767-400ER 204.117 158.758 103.1 0.505
Boeing 777-200 (HGW, GE
Engines) 286.9 206.35 137.05 0.478
Boeing 777-200LR 347.452 223.168 145.15 0.418
Boeing 777-300ER 351.534 251.3 167.83 0.477
Boeing 727-200ADV 95.1 73.1 45.72 0.480
Boeing 757-200 115.65 95.25 62.10 0.537
Boeing 737-900 79.15 66.36 42.56 0.536
Boeing 787-8 219.539 167.829 114.532 0.522
Business Jets
Cessna Citation X 16.14 14.425 9.73 0.603
Dassault Falcon 50 EX 18.498 16.2 9.888 0.535
Embraer Legacy 600 22.50 18.5 13.675 0.600
Cessna Encore 7.634 6.895 4.763 0.624
Gulfstream G350 32.160 29.937 19.368 0.602
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Weight Figures for Transport Aircraft (cont.)
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Weight Figures for Fighter Aircraft
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Overview
All textbooks use similar methods whereby comparisons made with existing aircraft.
In Roskam (Vol.1, p.19-30), aircraft classified into one of 12 types and empirical relationship found for log WE against log WTO.
Categories are: – (1) homebuilt props, (2) single-engine props, (3) twin-
engine props, (4) agricultural, (5) business jets, (6) regional turboprops, (7) transport jets, (8) military trainers, (9) fighters, (10) military patrol, bombers & transports, (11) flying boats, (12) supersonic cruise.
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Overview (Cont.)
Most aircraft of reasonably conventional design can be assumed to fit into one of the 12 categories.
New correlations may be made for new categories (e.g. UAVs).
Account may also be made for effects of modern technology (e.g. new materials) – method presented in Roskam Vol.1, p.18.
Raymer method uses Table 3.1 & Fig 3.1 (p.13).
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Roskam’s Empty Weight Estimation Method
Category 7 Category 8
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Raymer’s Empty Weight Fraction Estimation Equation
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Process begins with guess of take-off weight.
Payload weight determined from specification.
Fuel required to complete specified mission then
calculated as fraction of guessed take-off weight.
Tentative value of empty weight then found
using:
WE(tent) = WTO(guess) – WPL - Wcrew - WF - Wtfo (4)
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Values of WTO and WE compared with appropriate
correlation graph.
Improved guesses then made and process iterated
until convergence.
Note that convergence will not occur if specification is
too demanding.
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Initial Guess of Take-off Weight
Good starting point is to use existing aircraft with similar
role and payload-range capability.
An accurate initial guess will accelerate the iteration
process.
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Payload Weight & Crew WPL is generally given in the specification and
will be made up of:
passengers & baggage; cargo; military loads (e.g.
ammunition, bombs, missiles, external stores, etc.).
Typical values given in Roskam Vol.1 p8.
Specific values for some items (e.g. weapons)
may be found elsewhere.
Prof. Bento S. de Mattos
30
Mission Fuel Weight • This is the sum of the fuel used and the reserve
fuel.
WF = WF(used) + WF(res)
• Calculated by ‘fuel fraction’ method.
– compares aircraft weights at start and end of particular mission phases.
– difference is fuel used during that phase (assuming no payload drop).
– overall fraction is product of individual phase fractions.
(5)
Prof. Bento S. de Mattos
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1. Start & warm-up 2. Taxi 3. Take off 4. Climb 5. Cruise 6. Loiter 7. Descend 8. Taxi
• Fuel fractions for fuel-intensive phases (e.g. 4, 5 & 6 above)
calculated analytically.
• Non fuel-intensive fuel fractions based on experience and
obtained from Roskam (Vol I, p12), Raymer, etc.
civil jet transport
Reference: Roskam Vol. I - Table 2.1 Prof. Bento S. de Mattos
33
• Using Roskam’s method/data for a transport jet
(Vol.I, Table 2.1):
W1/WTO = 0.99
W2/W1 = 0.99
W3/W2 = 0.995
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For piston-prop a/c:
For jet a/c:
where:
Ecl = climb time (hrs), L/D = lift/drag ratio, cj is sfc for jet a/c
(lb/hr/lb), cp is sfc for prop a/c (lb/hr/hp), Vcl = climb speed
(mph), p = prop efficiency, W3 & W4 = a/c weight at start and
end of climb phase.
3
4
1lncl
clj cl
WLE
c D W
3
4
1375 ln
p
cl
clcl p cl
WLE
V c D W
(6a)
(6b)
Prof. Bento S. de Mattos
35
• Initial estimates of L/D, cj or cp, p and Vcl
made from Roskam or Raymer databases for
appropriate a/c category.
• Alternatively, use
approximations, e.g. from
Roskam Vol.1, Table 2.1
(W4/W3=0.98 for jet
transport, 0.96 to 0.9 for
fighters).
Prof. Bento S. de Mattos
36
Phase 5 (cruise)
• Weight fraction calculated using Breguet range
equations.
• For prop a/c:
• For jet a/c:
• These give the range in miles.
(7a)
(7b)
4
5
1375 ln
p
cr
clcl p cr
L WR
V c D W
4
5
lncr
clj cr
V L WR
c D W
37
• For jet a/c, range maximised by flying at 1.32 x
minimum drag speed and minimising sfc.
– wing operates at about 86.7% of maximum
L/D value.
– cruise-climbing can also extend range.
• For prop a/c, range maximised by flying at
minimum drag speed and sfc.
– wing operates at maximum L/D value.
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Initial Estimates of Lift/Drag Ratio (L/D)
• Using Roskam (Table 2.2 – selected values):
cruise loiter
Homebuilt & single-engine 8 - 10 10 - 12
Business jets 10 – 12 12 - 14
Regional turboprops 11 – 13 14 – 16
Transport jets 13 – 15 14 - 18
Military trainers 8 – 10 10 - 14
Fighters 4 – 7 6 – 9
Military patrol, bombers & transports 13 – 15 14 – 18
Supersonic cruise 4 - 6 7 – 9
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Jet Airplane Airplane fitted with propeller
1ln i
fj
L WR
Wc D
ln i
fj
V L WE
Wc D
ln i
fj
V L WR
Wc D
1ln i
fj
L WE
Wc D
In order to obtain a better estimation for the L/D ratio we shall
consider the Breguet equations for range (R) and endurance (E):
(6b) (6a)
(7a) (7b)
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2
0D D LC C kC
Considering that he TSFC does not vary with speed and that the
drag polar can be written as
After inserting into the preceding Breguet equations the above
drag polar, we obtain the L/D ratio for maximum range and
maximum endurance for a jet airplane deriving the resulting
equations and equaling them to zero:
max range 0
1 3
4 D
L A e
D C
max endurance 0
1
2 D
L A e
D C
with 1
kAe
(8a) (8b)
(9a) (9b)
41
with
42
Specific Fuel Consumption Jet aircraft - Initial estimates of cj (lb/hr/lb)
• Using Raymer (Table 3.3):
• Roskam Vol.1 Table 2.2 (p.14) gives a/c
category-specific values (see next slide).
cruise loiter
Turbojet 0.9 0.8
Low-bypass turbofan 0.8 0.7
High-bypass turbofan 0.5 0.4
43
Specific Fuel Consumption Jet aircraft - Initial estimates of cj (lb/hr/lb)
• Using Roskam (Table 2.2):
cruise Loiter
Business & transport jets 0.5 - 0.9 0.4 - 0.6
Military trainers 0.5 - 1.0 0.4 - 0.6
Fighters 0.6 - 1.4 0.6 - 0.8
Military patrol, bombers,
transports, flying boats
0.5 – 0.9 0.4 - 0.6
Supersonic cruise 0.7 – 1.5 0.6 - 0.8
44
Specific Fuel Consumption
• Using Raymer (Table 3.4):
• Take propeller efficiency (p) as 0.8 or 0.7 for
fixed-pitch piston-prop in loiter.
cruise loiter
Piston-prop (fixed pitch) 0.4 0.5
Piston-prop (variable
pitch)
0.4 0.5
turboprop 0.5 0.6
45
Specific Fuel Consumption
• Using Roskam (Table 2.2):
Cruise loiter
Single engine 0.5 – 0.7, 0.8 0.5 – 0.7, 0.7
Twin engine 0.5 – 0.7, 0.82 0.5 – 0.7, 0.72
Regional turboprops 0.4 – 0.6, 0.85 0.5 – 0.7, 0.77
Military trainers 0.4 – 0.6, 0.82 0.4 – 0.6, 0.77
Fighters 0.5 – 0.7, 0.82 0.5 – 0.7, 0.77
Military patrol, bombers &
transports
0.4 – 0.7, 0.82 0.5 – 0.7, 0.77
Flying boats, amphibious 0.5 – 0.7, 0.82 0.5 – 0.7, 0.77
Specific Fuel Consumption
Better estimation for
Engine Thrust and
fuel flow
Java code and applet can be obtained @
http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html
Prof. Bento S. de Mattos
47
• Fuel fraction (W6/W5) found from appropriate endurance equation as in Phase 4.
• For jet a/c, best loiter at minimum drag speed (maximum L/D value); for prop a/c at minimum power speed.
W7/W6 = 0.99
W8/W7 = 0.992
48
• Mission fuel used (WF(used))
8 7 6 5 34 2 1
7 6 5 4 3 2 1
ff
TO
W W W W WW W WM
W W W W W W W W (10)
(11) ( ) 1F used ff TOW M W
49
• WF then found from equation (5), by adding
reserve fuel (WF,res).
• This then allows for tentative value for WE(tent) to
be found, from eq. (4).
• This may be plotted with WTO on appropriate a/c
category graph to check agreement with fit.
• If not, then process must be iterated until
satisfactory.
50
• Two other possible mission phases may need
to be considered for certain aircraft:
– manoeuvring
– payload drop
51
• Breguet range equation may be used with
range covered in turn (Rturn) from perimeter
length of a turn (Pturn) multiplied by number
of turns (Nturn).
• For manoeuvre under load factor of n:
turn turn turnR N P
2
22
1turn
VP
g n
(12a)
(12b)
52
Payload Drop
• Treated as separate sortie phase with change in
total weight but no fuel change.
• Fuel fraction taken as 1 but subsequent phases
corrected to allow for payload drop weight change.
• Roskam Vol.1 pp.63-64 gives details.
• e.g. if W5 and W6 are weights before and after
payload drops: 5 34 2 1
5
4 3 2 1
TO
TO
W WW W WW W
W W W W W (13a)
(13b) 6 5 PLW W W
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Worked Example – Jet Transport (Roskam Vol.1, p55)
Specification
• Payload: 150 passengers at 175 lb each & 30 lb
baggage each.
• Crew: 2 pilots and 3 cabin attendants at 175 lb each
and 30 lb baggage each.
• Range: 1500 nm, followed by 1 hour loiter, followed
by 100 nm flight to alternate and descent.
• Altitude: 35,000 ft for design range.
• Cruise speed: Mach number = 0.82 @ 35,000 ft.
54
Worked Example – Jet Transport (Roskam Vol.1, p55)
Specification (Cont.)
• Climb: direct climb to 35,000 ft at max WTO.
• Take-off & landing: FAR 25 field-length of 5,000 ft.
55
Jet Transport Example
• WPL = 150 x (175 + 30) = 30,750 lbs
• Wcrew = 1,025 lbs
• Initial guess of WTO required, so compare with
similar aircraft:
– Boeing 737-300 has range of 1620 nm for payload
mass of 35,000 lbs – WTO = 135,000 lbs.
– Initial guess of 127,000 lbs seems reasonable.
• Now need to determine a value for WF, using
the fuel fraction method outlined above.
56
Jet Transport Example
As in earlier example, for a transport jet:
W1/WTO = 0.99
W2/W1 = 0.99
W3/W2 = 0.995
57
Jet Transport Example
Phase 4 (climb)
W4/W3 = 0.98
• The climb phase should also be given credit in
the range calculation.
• Assuming a typical climb rate of 2500 ft/min at
a speed at 275 kts then it takes 14 minutes to
climb to 35,000 ft.
• Range covered in this time is approximately
(14/60) x 275 = 64 nm.
58
Jet Transport Example
• Cruise Mach number of 0.82 at altitude of
35,000 ft equates to cruise speed of 473 kts.
• Using eq. (7b):
• Assumptions of L/D = 16 and cj = 0.5 lb/hr/lb
with a range of 1500 – 64 (=1436 nm) yield a
value of:
W5/W4 = 0.909
4
5
lncr
clj cr
V L WR
c D W
59
Phase 6 (loiter)
• Using eq. (6b):
• Assumptions of L/D = 18 and cj = 0.6 lb/hr/lb.
• No range credit assumed for loiter phase.
• Substitution of data into eq. (6b) yields:
W6/W5 = 0.967
3
4
1lncl
clj cl
WLE
c D W
60
Simple Cruise Mission Example
• No credit given for range.
W7/W6 = 0.99
• May be found using eq. (6b) again.
• Cruise will now take place at lower speed and
altitude than optimum – assume cruise speed of
250 kts (FAR 25), L/D of 10 and cj of 0.9 lb/hr/lb.
• Gives: W8/W7 = 0.965
61
Simple Cruise Mission Example
• No credit given for range.
W9/W8 = 0.992
• found from eq. (8) (with additional term for
W9/W8)
= 0.992x0.965x0.99x0.967x0.909x0.98x0.995x0.99x0.99
= 0.796
• Using eq. (9), WF = 0.204 WTO = 25,908 lb
62
Simple Cruise Mission Example
• Using eq. (4):
WE(tent) = WTO(guess) – WPL - Wcrew - WF – Wtfo
WE(tent) = 127,000 – 30,750 – 1,025 – 25,908 - 0
= 69,317 lb
• By comparing with Roskam Vol. 1, Fig. 2.9, it is
seen that there is a good match for these values of
WE and WTO, hence a satisfactory solution has
been reached.
Prof. Bento S. de Mattos
63
• Specification / design requirements often re-
evaluated and refined at this stage, using above
method.
• Examples include:
– Effect of a range increase/decrease on MTO.
– Effect of payload mass change on MTO.
– Effect of using composite materials instead of
aluminium alloys.
• More details and examples in Raymer p.28-31 and
Ch.19.
64
• Essentially Roskam’s version (Vol.1, p.68) of
Raymer’s trade studies detailed above.
• Sensitivity of MTO is investigated with changes to
the following typical set of parameters:
– Empty weight (WE), payload (WPL), range (R),
endurance (E), lift/drag (L/D), specific fuel consumption
(cj or cp) and propeller efficiency (ηp).
• Sensitivity to general parameter y expressed by:
• Regression constants used in equations are relevant
to particular a/c category.
TOW
y
Prof. Bento S. de Mattos
Estimating Cruise Fuel Consumption
Performance
Max operating Mach number 0.83
Max operating altitude 41,000 ft (cabin altitude: 8,000 ft)
Take-off field lenght 6,500 ft (SL / ISA + 15°C / MTOW)
Landing field 5,000 ft (SL / MLW = 90% of MTOW)
Range with max payload 2,200 nm (overall fuel volume for 3,200 nm version)
External noise FAR 36 Stage IV minus 15 db
IPET7 Airliner
67
Estimating Cruise Fuel Consumption
41000 ft
0,150
0,170
0,190
0,210
0,230
0,250
0,270
0,290
0,40 0,50 0,60 0,70 0,80 0,90
Mach
SR
[n
m/k
g]
MTOW 90% MTOW 80% MTOW
Long Range MMO
SR vs. Mach number 41000 ft
0,00
2,00
4,00
6,00
8,00
10,00
12,00
14,00
0,40 0,50 0,60 0,70 0,80 0,90
Mach
M*L
/D
MTOW 90% MTOW 80% MTOW
Mach*L/D vs. Mach number
The number of Mach for maximum specific range (SR) is not the same as that for
maximum M*L/D because sfc increases with speed
IPET7 IPET7
TASSR
Fuel flow
69