4' Engine Selection

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• 4. Engine Selection

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Landing Requirements
Equivalent Velocity rV2/2rs.l.sV2/2rs.l.VE2/2
Main assumption there is little acceleration,
landing is a quasi-static process
Wing characteristics that are not yet known
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The landing phase
Rflare
Rflare q /2
q /2 q /2
Touchdown
Stop
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VE in Power Approach and Landing
Flight test shows landing is essentially
quasi-static and our LW assumption is OK
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Lift curve characteristics
FAR Part 25 requirements
CL,max,landing
CL
CL,landingCL,max/1.44
amargin
a0 0 alinear astall
a
Stall margin, amargin, provides protection from
gusts
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Market survey aircraft results
Since FAR dictates that CL,landing,max1.44CL,land
ing we find
VE,landing
VE 17.17(W /S)/CL1/2
Vlanding for given market survey airplane
(W/S)/CLlanding
50lt (W/S)/CL,landing lt60
Typical values for airliners
35lt (W/S)/CL,max,landing lt42
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Landing distance
hland50 ft fland5/3 (FAR Part 91)
Rflare
Flight path
q3o
Rflareq/2
Touchdown Stop
hland
Sair
Srun
Sland
Correlation equation
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Deceleration in the ground run
(W/S)/CL,maxlanding 35 42
0.6
Design region
Uncomfortable
0.4
Normal
a/g
Correlation equation, s1
0.2
Market survey aircraft
0
0 2000 4000 6000
8000 Slanding (feet)
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Wing loading in take-off
Finding the wing loading will give us the wing
area S
1000nm
We know this from weight estimate
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Nominal Landing Weight Trends for Current
Airliners
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Landing W/S relative to landing CL
42sCL,MAX,l 35sCL,MAX,l
120 80 40 0
Operating line
Market survey range
(W/S)landing (lb/ft2)
Design region
Design range
0 1 2
3 sCL,MAX,l
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Trade-off between thrust to weight ratio and wing
loading
Design range
(T/W)TO
CL,max, l increasing (wing area getting too
small)
CL,max,l decreasing (wing area getting too large)
(W/S)TO
The important design space is that of T/W vs. W/S
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Normal take-off trajectory
Continued take-off
V0
VTO
hTO
Sground
Sair
STO
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Calculating the take-off distance
Approximate equation for take-off distance
Bypass ratio mfan/mcore
Rolling friction coefficient approximation
Climb-out angle
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Aspect ratios of current airliners
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Variation of thrust with Mach number for
different bypass ratios
l0
l1
T/Tstatic0.75(5l)/(4l)
l5 to 6
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T-O distance function of T/W W/S
Take-off is a relatively slow, quasi-static
process
Replacing the take-off velocity by the wing
loading to lift coefficient ratio leads to
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Design region for landing and t-o
CL,max,l increasing
CL,max,TO increasing
(T/W)TO
Desired thrust range
(W/S)TO
Note that take-off distance STO is fixed in this
figure
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Design graph typical values
CL,max,TO 2.0 2.25 2.50
CL,max,l 2.5, 2.75, 3.0
(T/W)TO
0.4 0.3 0.2 0.1 0
STO 6500 ft
100 110 120 130
(W/S)TO psf
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Change take-off distance to improve choice of
engines
(T/W)TO
Take-off distance STO increasing
old thrust range
new thrust range
(W/S)TO
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T-O distance choice of engines
CL,max,TO 2.00 2.25 2.50
CL,max,l2.5 2.75 3.0
STO6500 ft
(T/W)TO
0.5 0.4 0.3 0.2 0.1 0
2.00 2.25 2.50
STO7000 ft
100 110 120 130
140 (W/S)TO
To reduce the design thrust range increase
landing distance
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Cruise constraints on design
L
Tcruise
T
D
W
For equilibrium L/D(T/W)-1 L/D(W/D)
Equating these two expressions for L/D yields
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Engine selection design graph
Range of CL,max,l
Range of CL,max,TO
(T/W)TO
Design Region
Range of CD,0, cruise
(W/S)TO
The competing requirements for good take-off,
landing, and cruise performance define a small
design region
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Operations with one engine out
Balanced field length
STO,1EO
Continued take-off
One engine out at
aborted take-off V0
VVcritical
V0
hTO
Scritical
Sstop
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Performance with one engine out
Abort take-off
Normal ground roll up to Vcrit
Continue take-off with one engine out
Reduced thrust and climb-out angle
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Balanced field length
Field too long
Continued take-off with 1 engine out
Distance S
Field too short
ST-O
Balanced field length
Accelerate, decelerate, and stop
VT-O
Vcritical