Lesson objective to discuss another

1

• Lesson objective - to discuss another
• UAV Operating Environment
• The atmosphere

Expectations - You will understand how this
environment drives UAV design and operations
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Why is it important
A UAV operates in two important environments 1.
Physical - The atmosphere in which it flies -
Varies with altitude (primary) and weather
(secondary) 2. Functional - The rules for
how it operates - Originally established for
manned aircraft - Now imposed on unmanned
aircraft
This lesson
Lesson 6
If you dont understand both environments, you
cant design for them
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Definitions

• Indicated air speed (IAS) - the speed sensed by
  the aircraft (proportional to dynamic
  pressure)
• Equivalent air speed (EAS) - indicated air speed
  corrected for instrument and installation
  errors and compressibility effects
  (important when M gt 0.5)
• True air speed (TAS) - the actual speed through
  the air
• MSL Altitude - Altitude above mean sea level (the
  altitude used for pilotage and air traffic
  control)
• AGL - Altitude above ground level
• Indicated altitude - The altitude sensed by the
  aircraft
• Pressure altitude - Indicated altitude corrected
  to sea level standard conditions
• Geometric altitude - Actual altitude (measured -
  RF, etc)

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The atmosphere
Defined by internationally agreed models on how
pressure, temperature and density vary with
altitude above mean sea level (MSL). Used widely
for - Altimeter calibration - Air traffic
control - Aircraft design - Performance
computation - Etc. Models describe a Standard
Day and provide variations for other conditions
- Hot day - Tropical day - Cold day -
Polar day
For simplicity we will use the Standard Day only
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Standard atmosphere
Sea level standard (SLS) conditions (in British
units) Static pressure (P0) - 2116.22 lb/ft2
(psf) or...
- 29.92 in. Hg Temperature (T0)
- 518.67 degR or...
- 59.0
degF Assumptions - - No moisture - Obeys
perfect gas law
For simplicity (mine) we will use English units
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Key definitions
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q - the alternate form
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Atmosphere characteristics

• Sea level - 36089 ft
• (troposphere)
• - Exponential ambient pressure decrease
• - Linear ambient temperature decrease
• 36089 - 65617 ft (stratosphere)
• - Logarithmic ambient pressure decrease
• - Constant ambient temperature
• 65617 - 104987 ft
• - Exponential ambient pressure decrease
• - Linear ambient temperature increase

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Spread sheet equations
Approximate 1962 U.S. Standard Atmosphere
(17.9)
(17.8)
(17.10)
(17.11)
(17.13)
(17.12)
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Typical calculations

• Throughout this course we will use the standard
  atmosphere for performance, aerodynamic and
  propulsion calculations. Some examples
• 1. A UAV is flying at M 0.75 and h 65 Kft.
  What is the dynamic pressure?
• - From (17.4a) q 1481.35?M2
• - From (17.10) ? .05567
• - ? q 1481.350.055670.752 46.4 psf
• 2. A UAV is flying at KTAS 350 Kts. What Mach
  will it fly at h 30, 50 and 70Kft? How about
  KEAS?
• - From (17.4-7) KTAS Mc M661.5sqrt (?)
  (17.14)
• and KEAS KTASsqrt (?) M661.5sqrt (?)
  (17.15)

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Calculations - contd

• 2. contd
• From (17.15) M KTAS/661.5sqrt (?)
• ?M_at_h30Kft 350/(661.5sqrt (.7938)) 0.59
• M_at_h50Kft 350/(661.5sqrt (.7519)) 0.61
• M_at_h70Kft 350/(661.5sqrt (.7565)) 0.61
• From (17.16) KEAS M661.5sqrt (?)
• ?M_at_h30Kft 0.59661.5sqrt (.2970)) 213 KEAS
• M_at_h50Kft 0.61661.5sqrt (.1145)) 136.5
  KEAS
• M_at_h70Kft 0.61661.5sqrt (.0438)) 84.4 KEAS
  
• 3. A UAV with a wing loading of 60 psf wants to
  loiter at a Cl of 0.8 for L/Dmax. What M will it
  fly at 30/50/70 Kft?
• At loiter L W ClqS (See RayAD
  Chapter 5.3)
• ?W/S ? wing loading Clq or q (W/S)/Cl
  (17.16)

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Calculations - contd

• 3. contd
• Combining (17.14) and (17.16) we find
• M sqrt((W/S)/(Cl1481.35?)) (17.17)
• ?M_at_h30Kft sqrt(60/(0.81481.350.2970))
  0.41
• M_at_h50Kft sqrt(60/(0.81481.350.1145))
  0.66
• M_at_h70Kft sqrt(60/(0.81481.350.0438))
  1.07
• - Is there something wrong at 70 Kft?
• No - it cant fly at subsonic speeds at 70Kft at
  this wing loading. It either needs a bigger wing
  or has to lose weight (burn fuel - and a lot)
• 4. Assume the UAV in problem 3 has a loiter Mach
  number of 0.6. What will its loiter altitude be?

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Calculations - contd

• 4. contd
• From (17.16)
• ? (W/S)/(Cl1481.35M2) (17.18)
• ? ? _at_ M0.6 60/(0.81481.350.36) 0.141
• - At what altitude will ? 0.141?
• Answer - From chart 17.7, we can see that the
  altitude is somewhere between 40 and 50 Kft. If
  we solve (17.10) we will find that hlo 45.7
  Kft

Note We will program equations (17.8) through
(17.13). We will use them a lot.
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Implications for design

• At a given air speed, dynamic pressure drops
  about an order of magnitude by 50Kft. It drops
  about another order of magnitude by 100Kft.
• - Generating lift above 65K ft is a major
  challenge
• - Simple aerodynamic analysis show why
• Altitude is also a problem for the engine even
  though drag goes down with q
• - Engine power required to run generators does
  not go down (environmental control requirements
  can go from cooling to heating)
• Above 65K ft. lift and power are major design
  issues
• - This lesson will address the lift issue

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Global Hawk Example
http//www.fas.org/irp/program/collect/global_hawk
.htm
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Global Hawk Assessment

• Takeoff weight 25,600 lbs, wing area (S) 540
  ft2, fuel 14,500 lbs
• Assuming 15 fuel consumption for takeoff and
  climb to initial cruise altitude (50Kft), the
  initial cruise wing loading (W/S) would be
• - W/S (25,600-0.1514,500)/540 43.4 psf
• At a nominal cruise Mach of 0.6, cruise q 61.0
  psf . If L W, what would the cruise lift
  coefficient (Cl-cruise) be?
• Answer Cl-cruise (W/S)/q 0.71
• At mid-mission (loiter with 50 fuel) the wing
  loading would be
• - W/S (25,600-0.514,500)/540 34.0 psf
• at a required loiter altitude of 65Kft.
  Assuming the loiter speed is also Mach 0.6 (q
  29.7 psf), what would Cl-loiter have to be?
• Answer Cl-loiter 34/29.68 1.15 (near or at
  stall speed?)

In order to operate efficiently at high altitude,
Global Hawk has to fly at high cruise and loiter
lift coefficients. What if it had to loiter even
higher - say 70Kft?
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70Kft Assessment

• Option 1 - Loiter at a higher speed
• For Cl-loiter 0.84 and W/S 34.0 psf as before
• - Required q-loiter 34/0.84 40.5 or M 0.8
  (too high for a thick unswept wing!)
• Option 2 - Loiter at a higher Cl
• For M 0.6 (q 23.35) and W/S 34.0 psf as
  before
• - Required Cl-loiter 34/23.35 1.45 (way too
  high!)
• Option 3 - Loiter at a lighter weight
• Assume M 0.6 (q 23.35) and Cl-loiter 0.84
  as before
• - Allowable W/S 23.350.84 19.6 (less than
  empty weight )
• Option 4 - Combination of Options 1-3
• Assume M 0.65 (q 27.4) and Cl-loiter 1.1
• - Allowable W/S 27.41.1 30.1 (35 fuel
  remaining)

A 70Kft requirement would be a significant
challenge!
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Summary - Atmosphere

• Simple atmospheric models in combination with
  basic aerodynamic analysis are invaluable for
  first-order evaluation of air vehicle
  requirements and concepts
• High altitude requirements drive air vehicle
  design
• Very high altitude (gt65Kft) is a difficult (but
  not impossible) design space
• Simple analysis of existing aircraft can provide
  very useful insight for pre-concept and
  conceptual design
• Secondary power at high altitude is a major issue
  that we will address in subsequent sessions

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Expectations

• You should now understand how to model a standard
  atmosphere
• Equations for pressure and temperature are all
  you need to fully characterize the standard day
  physical environment
• You will use these models extensively throughout
  this course (and your career if you plan to work
  in air vehicle design)
• See spreadsheet SSC.StdAtmosphere.xls
• You should also understand the issues associated
  with UAV operating environments
• - Very high altitude will be a major design
  challenge
• - Simple atmospheric and aerodynamic analysis can
  identify the problems

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Homework (extra credit)

• (Individual grade) Use spreadsheet
  ASE261.StdAtmosphere.xls to calculate true
  airspeed at the WAS altitude required for your
  project UAV assuming a wing loading of 35 psf
• - Assess one lift coefficient per team member
• Cl 0.3, 0.9,1.2 or 1.5
• (team grade) Which of the four (4) lift
  coefficients would be most appropriate for your
  WAS UAV at loiter? Explain your answer.

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Intermission
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