Aero Engineering 315
Lesson 26
Fighter Design Project Introduction
Performance—
Power Required and
Power Available
Fighter Design Project
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Sign-up next lesson (two per team)
Worth 100 points plus 5 bonus points
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Mission
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Can skip design iterations and perform analysis on
baseline for 85% credit
Hi-Lo-Hi
20 minutes loiter
Land with required reserve
Armament
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3 x JDAM (Mk-82 variant)
2 x AIM-120Cs
Internal carriage
Fighter Design Project (cont’d)
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Objectives
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Design aircraft to meet following
requirements:
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While
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Range, endurance (fuel required and volume
of fuel required)
Max sustained G, min rate of climb, payload
Minimizing weight/size
So that you
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SAVE MONEY
Fighter Design words of wisdom
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All info and template is on k: drive
Read “How To” guide
Input your performance calculations first
DON’T CHANGE THE DESIGN UNTIL YOU VERIFY
THE CALCULATIONS
Do your hand calculations next on the baseline
design
Save your baseline—then start making design
changes
 Track your changes so you can remember
what you did right
 Update your new Mach numbers after a
design change
Spreadsheet
Additional thrust
example: T-38
At Sea Level:
1. What is (L/D)MAX for 8,000 lb?
2. What is the Parasite Drag at
(L/D)MAX for 8,000 lb?
3. For W=8,000 lb, what Mach
range can I achieve using
reverse command on the
throttle (i.e. back side of
curve)?
Power Req’d & Available objectives
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Sketch power required (PR) & power available curves
for a turbojet aircraft
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State the slope of a power available curve (TA )
Find min drag point on power required curve
From a T-38 chart find PR, min PR, excess power, max
excess power, max and min Mach
Know relationship between induced & parasite drag at
min PR
From a drag polar calculate velocity for min PR
Sketch TR and PR changes with wt, alt & configuration
Know how parameters & velocities change with wt, alt
& config
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Min drag, (L/D)max, and min PR
Thrust vs. Power vs. a Pickup
What is Power?
 Power
and Work
Power = Work / Time
 For
and
Work = Force x Distance
therefore
Power = Force x Distance/Time
or
Power = Force x Velocity
aircraft
PA = TAV
PR = TRV = DV
Power Required and Power Available versus Velocity
PR
P
Note: curve steepens at
high velocities
PA (DRY)
Slope = rise/run = PA/V = TA
VMAX
PRMIN
VMIN
VPR
MIN
V
The slope of a line from the origin to any point on the
PR curve is rise/run or PR/V = TR. So the point where
the slope is minimized is the point of TRMIN or DMIN.
P
VPR
MIN
PR for a given V
< VTR
MIN
If this slope = TR,
where is TR,MIN ?
VPR
MIN
Slope = (PR/V)MIN
= TRMIN = DMIN
VTR
MIN
= VDMIN
V
T-38 Power Chart from
Supplemental Data
Sea Level
PR for M= 0.8
= 2.15x106 ft-lbs/sec
Min PR, W=8000 lbs
= 2.5x105 ft-lbs/sec
At 0.325 Mach
Min Power Required using Drag Polar
TR = ½ r V2 S CD,0 + 2 kW2 / (r V2 S)
PR = TR x V = ½ r V3 S CD,0 + 2 kW2 / (r V S)
dPR /dV = 3 ( ½ r V2 S CD,0) - 2 kW2 / (r V2 S) = 0
So, at min PR : 3 x Parasite Drag = Drag due to Lift
Thus: 3CD,0 = CD,i
or
3CD,0= k CL2
so: CD = CD,0 + CD,i = 4 CD,0 = 4 k CL2/3
Solving for CL:CL = (3CD,0 /k)1/2
At Minimum PR
Example: T-37 again
Using CD = 0.02 + 0.057CL2 (from whole aircraft
lesson), S = 184 ft2 and W = 6,000 lb @ SL
Find V @ PRMIN
CL = (3CD,0 /k)1/2 (at PRmin )
L = 6,000 lbs = (3CD,0 /k)1/2 ½ r V2 S
so V = (12,000 / (3CD,0 /k)1/2 r S ) 1/2
CL = (3•0.02/0.057)½ = 1.026
V = 163.5 ft/s
Excess Power
P
PA (DRY)
PX = Excess Power
= PA – PR (@ V1)
PX
PR
V1
V
Maximum Excess Power
P
PA (DRY)
Line parallel to PA and
tangent to PR
PXMAX
PR
VPX
MAX
V
Configuration Changes
“Dirty configuration” – increases CDo
1
TR  r V 2 S C D ,0
2

kW 2
1
rV 2 S
2
T
PA
P
TR
TA
PR
V
V
Weight Changes
Increasing Weight
1
TR  r V 2 S C D ,0
2

kW 2
1
rV 2 S
2
P
T
PA
TR
TA
PR
V
V
Altitude Changes
Increasing Altitude
1
TR  r V 2 S C D ,0 
(Decreasing r)
2
T
kW 2
1
rV 2 S
2
TR
TA
r
TA = TSL r
SL
Less Parasite Drag
More Drag
from Lift
V
Altitude Changes
Increasing Altitude
(Decreasing r)
P
PA
PR
V
Parameter Changes
CD0
Dmin
VDmin
Vmax
Vmin
PRmin
VPRmin
L/D max
T & PA
W
h
TAS
A little
A little
TAS
TAS
Next Lesson (27)…
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Prior to class
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Read text 5.7 – 5.8
Complete homework #29, 30
In Class
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Discuss gliding flight, climbing flight,
ceilings