Lesson objective to discuss another - PowerPoint PPT Presentation

1 / 22
About This Presentation
Title:

Lesson objective to discuss another

Description:

The atmosphere in which it flies - Varies with altitude (primary) and weather (secondary) ... No - it can't fly at subsonic speeds at 70Kft at this wing loading. ... – PowerPoint PPT presentation

Number of Views:30
Avg rating:3.0/5.0
Slides: 23
Provided by: armand5
Category:

less

Transcript and Presenter's Notes

Title: 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
17-1
2
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
Chapter 6
If you dont understand both environments, you
cant design for them
17-2
3
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)

17-3
4
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
17-4
5
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
17-5
6
Key definitions
17-6
7
q - the alternate form
17-7
8
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

17-8
9
Spread sheet equations
Approximate 1962 U.S. Standard Atmosphere
(17.9)
(17.8)
(17.10)
(17.11)
(17.13)
(17.12)
17-9
10
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)

17-10
11
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 needs 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 ClqSref (See RayAD Chapter
    5.3)
  • ?W/S ? wing loading Clq or q (W/Sref)/Cl
    (17.16)

17-11
12
Calculations - contd
  • 3. contd
  • Combining (17.14) and (17.16) we find
  • M sqrt((W/Sref)/(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?

17-12
13
Calculations - contd
  • 4. contd
  • From (17.16)
  • ? (W/Sref)/(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 Equations (17.8) through (17.13) are
programmed in class SpreadSheet
ASE261.StdAtmosphere.xls and are also
incorporated as worksheets in the sizing
spreadsheets that will be introduced later
17-13
14
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

17-14
15
Global Hawk Example
http//www.fas.org/irp/program/collect/global_hawk
.htm
17-15
16
Global Hawk Assessment
  • Takeoff weight 25,600 lbs, wing area (Sref) 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/Sref) would be
  • - W/Sref (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 or Clcr) be?
  • Answer Cl-cruise (W/Sref)/q 0.71
  • At mid-mission (loiter with 50 fuel) the wing
    loading would be
  • - W/Sref (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 (Cllo) 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?
17-16
17
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!
17-17
18
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

17-18
19
Example problem
  • Chart 15-38 assumed a cruise speed (Vcr) of 180
    Kts at an altitude of 27 Kft for our TBProp WAS
    concept
  • - Then in Chapter 16 (chart 16-14) we calculated
    a required Cl of 0.805 to fly at LoDmax
  • Using SpreadSheet ASE261.StdAtmosphere.xls we
    calculate optimum cruise at
  • Mach (Mcr) 180/597 0.302
  • Dynamic pressure (qcr) 45.8 psf
  • The wing loading associated with this flight
    condition is
  • W/Sref qcr?Cl_at_LoDmax 36.87
  • Since this value can be maintained at only one
    flight weight (fuel) condition, by definition,
    speed (or altitude) must vary in order to cruise
    at LoDmax
  • If loiter takes place at a slower speed (which it
    normally does), the associated wing loading will
    be even lower

17-19
20
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 ASE261.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

17-20
21
Homework (individual)
  • 5. Select an AE261 spreadsheet program (ICProp,
    TBProp or TBFan as appropriate) and make a copy
    of the standard atmosphere worksheet.
  • - Using Homework problem 4 (chart 16-15) as an
    input, use the worksheet to calculate the dynamic
    pressure (qcr) required to fly your air vehicle
    concept at your assumed cruise speed (Vcr)
  • - Use the output to calculate the associated wing
    loading for cruise at LoDmax
  • Compare your result with RayAD Table 5.5 and
    assess the reasonableness of your calculation

17-21
22
Intermission
17-22
Write a Comment
User Comments (0)
About PowerShow.com