2 Day Workshop on Digital Datcom and Simulink

DHA
Suffa University
D e p a r t m e n t o f M e c h a n i c a l E n g i n e e r i n g
Project TitleWorkshop: Digital DATCOM
Dr. Bilal Ahmed Siddiqui

DHA
Suffa University
1. Introduction to Aerodynamics
2. Stability and Control
3. USAF DATCOM
4. Digital DATCOM
5. Enhancements
6. Missile DATCOM
7. Sample Cases
8. Some Practice
9. What’s More?
2
Contents

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Suffa University
Aerodynamics
• Aerodynamics : interplay of air & bodies trying to
move through it
– Air resists some motion…and aids some motion
– Important to understand this interplay to harness it
• Formal study of aerodynamics began 300 years
ago, so it is a relatively young science!
• Informally, we have been harnessing wind since a
long time…….really long

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Suffa University
Ancient Egyptian Aerospace!
https://en.wikipedia.org/wiki/Helicopter_hieroglyphs [1290 BC]

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Suffa University
But Preserved History is different…
• In 1799, Sir George Cayley became the first person to identify the
four aerodynamic forces of flight
• In 1871, Francis Herbert Wenham constructed the first wind
tunnel, allowing precise measurements of aerodynamic forces.
• In 1889, Charles Renard became the first person to predict the
power needed for sustained flight.
• Otto Lilienthal was the first to propose thin, curved airfoils that
would produce high lift and low drag.
• However, interestingly the Wright brothers, mechanics – not
engineers- found most of the initial work flawed and did
something else….a hundred years after Cayley.

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Suffa University
27th of Ramadhan….a Friday
• On Dec 15, 1903 (corresponding to Hijri date
above), Wilbur and Orville Wright made
history after failing to achieve it for 3 years.
• Cycle mechanics, enthusiastic about flight,
they designed airplanes based on
aerodynamic data published by Leinthall and
Langley.
• All attempts were splendid failures, so they
began to doubt the crude theories of their
times.
• They built themselves a windtunnel and
started testing airfoils. To their surprise, they
obtained reliable results and the rest is
history.

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Suffa University
The [W]Right [Bi]Plane

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Suffa University
Forces and Moments in Flight
• Straight level flight means constant velocity and altitude.
• There are four main forces which govern straight level flight
• For level flight, Lift=Weight and Thrust=Drag
• In other words,
𝑇
𝑊
=
𝐿
𝐷
in level flight
• Except weight, all other variables depend
on Aerodynamics.
• Aerodynamics also causes moments in all three axes

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Suffa University
Aerodynamic Moments
• Aerodynamics also causes moments in all three axes.
• Performance of aircraft depends on aerodynamics!

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Suffa University
Source of all Aerodynamic Forces
&Moments
• No matter how complex the body shape and flow, the
aerodynamic forces and moments on the body are due to only
two basic sources:
a) Pressure distribution p over the body surface
b) Shear stress distribution τ over the body surface
• Pressure varies with velocity of air over the surface and acts
normal to it. For incompressible, inviscid flow, it follows Bernoulli
principle.
• Shear stress is due to friction in the boundary layer and acts
tangent to the surface. Typically 𝜏 ≪ 𝑝

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Suffa University
Net Effect of Pressure and Shear
Distribution
• Each body shape and flow condition creates unique p & τ
distribution
• The net effect of the p and τ distributions integrated over
the complete body surface is a resultant aerodynamic force
R and moment M on the body.
• Far ahead of the body, the flow is undisturbed and called
free stream.
• V∞ = free stream velocity=flow velocity far ahead of the
body.

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Components of Aerodynamic Force R
• Let distance between leading and trailing edges be “chord”=c
• “Angle of attack” is the angle between 𝑉∞ and c
• R can be resolved into two sets of components: either wrt 𝑉∞ or c
•
𝐿 = 𝑙𝑖𝑓𝑡 = 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡 𝑝𝑒𝑟𝑝𝑒𝑛𝑑𝑖𝑐𝑢𝑙𝑎𝑟 𝑡𝑜 𝑉∞
𝐷 = 𝑑𝑟𝑎𝑔 = 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡 𝑝𝑎𝑟𝑎𝑙𝑙𝑒𝑙 𝑡𝑜 𝑉∞
•
𝑁 = 𝑛𝑜𝑟𝑚𝑎𝑙 𝑓𝑜𝑟𝑐𝑒 = 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡 𝑝𝑒𝑟𝑝𝑒𝑛𝑑𝑖𝑐𝑢𝑙𝑎𝑟 𝑡𝑜 𝑐
𝐴 = 𝑎𝑥𝑖𝑎𝑙 𝑓𝑜𝑟𝑐𝑒 = 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡 𝑝𝑎𝑟𝑎𝑙𝑙𝑒𝑙 𝑡𝑜 𝑐
• But, {L,D} and {N,A} are related through 𝛼
𝐿 = 𝑁 cos𝛼 − 𝐴 𝑠𝑖𝑛α
𝐷 = 𝑁 𝑠𝑖𝑛𝛼 + 𝐴 𝑐𝑜𝑠𝛼

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Suffa University
Source of {N,A} and {L,D}
• Pressure p, shear τ , surface slope θ are
functions of path length s.
• For a unit span l=1, the forces are
𝑁 = −
𝐿𝐸
𝑇𝐸
𝑝 𝑢 𝑐𝑜𝑠𝜃 + 𝜏 𝑢 𝑠𝑖𝑛𝜃 𝑑𝑠 𝑢
+
𝐿𝐸
𝑇𝐸
𝑝𝑙 𝑐𝑜𝑠𝜃 − 𝜏𝑙 𝑠𝑖𝑛𝜃 𝑑𝑠𝑙
𝐴 =
𝐿𝐸
𝑇𝐸
−𝑝 𝑢 𝑠𝑖𝑛𝜃 + 𝜏 𝑢 𝑐𝑜𝑠𝜃 𝑑𝑠 𝑢
+
𝐿𝐸
𝑇𝐸
𝑝𝑙 𝑠𝑖𝑛𝜃 + 𝜏𝑙 𝑐𝑜𝑠𝜃 𝑑𝑠𝑙

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Source of Aerodynamic Moment
• The aerodynamic moment exerted on the body depends on the point about
which moments are taken.
• Moments that tend to increase α (pitch up) are positive, and moments that tend
to decrease α (pitch down) are negative.
• Moment about the leading edge is simply the forces x moment arms.
𝑀𝐿𝐸 =
𝐿𝐸
𝑇𝐸
𝑝 𝑢 𝑐𝑜𝑠𝜃 + 𝜏 𝑢 𝑠𝑖𝑛𝜃 𝑥 − 𝑝 𝑢 𝑠𝑖𝑛𝜃 − 𝜏 𝑢 𝑐𝑜𝑠𝜃 𝑦 𝑑𝑠 𝑢
+
𝐿𝐸
𝑇𝐸
−𝑝𝑙 𝑐𝑜𝑠𝜃 + 𝜏𝑙 𝑠𝑖𝑛𝜃 𝑥 + 𝑝𝑙 𝑠𝑖𝑛𝜃 + 𝜏𝑙 𝑐𝑜𝑠𝜃 𝑦 𝑑𝑠𝑙
• In equations above, x, y and θ are known functions of s for given shape.
• A major goal of aerodynamics is to calculate p(s) and τ(s) for a given body shape
and freestream conditions (𝑉∞ and α) aerodynamic forces/moments

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Suffa University
Getting rid of the
Dimensions…convenience
• It will become clear later that it is of benefit to non-dimensionalize forces
and moments.
• Let 𝜌∞ be the free stream air density and S and l be reference area and
reference length respectively.
• Dynamic pressure, 𝑄∞ =
1
2
𝜌∞ 𝑉∞
2
• Lift coefficient, CL =
L
Q∞S
• Drag coefficient, CD =
D
Q∞S
• Lift coefficient, CL =
M
Q∞S𝑙
• Axial and normal force coefficients are similarly defined.
Coefficients makes the math
manageable. An aircraft with a 50m2
wing area and weight of 10,000kg at sea
level cruise will have a lift coefficient of
0.3 at a speed of 100m/s, rather than a
lift of 9.8x104 N.

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Suffa University
Reference Area and Length
• In these coefficients, the reference area S and
reference length l are chosen to pertain to the
given geometric body shape
• E.g., for an airplane wing, S is the planform
area, and l is the mean chord length c.
• for a sphere, S is the cross-sectional area, and
l is the diameter
• Particular choice of reference area and length
is not critical
• But, when using force and moment coefficient
data, we must always know what reference
quantities the particular data are based upon.

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Parameters which influence Aerodynamics
• By intuition, the resultant aerodynamic force R should depend on
– Freestream velocity 𝑉∞ (faster air  more force and moment)
– Freestream density 𝜌∞ (denser air  more force and moment)
– Freestream viscosity 𝜇∞ (viscosity  shear stress)
– Size of the body (more reference area and length  more force and moment)
– Compressibility of air (density changes if flow speed is comparable to speed of sound 𝑎∞)
– Angle of attack 𝛼
Therefore,
𝐿 = 𝑔𝑙(𝜌∞, 𝑉∞, 𝑆, 𝑐, 𝜇∞, 𝑎∞, 𝛼)
𝐷 = 𝑔 𝑑(𝜌∞, 𝑉∞, 𝑆, 𝑐, 𝜇∞, 𝑎∞, 𝛼)
M= 𝑔 𝑚(𝜌∞, 𝑉∞, 𝑆, 𝑐, 𝜇∞, 𝑎∞, 𝛼)
for some nonlinear functions 𝑔𝑙, 𝑔 𝑑 and 𝑔 𝑚.
• This is not useful as this means a huge combination of parameters needs to be tested or
simulated to find the relationships necessary for designing aerodynamic vehicles and
products.

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Suffa University
Dimensional Analysis
• Fortunately, we can simplify the problem and considerably reduce our time and effort
by first employing the method of dimensional analysis.
• Dimensional analysis is based on the obvious fact that in an equation dealing with the
real physical world, each term must have the same dimensions.
• So it is equivalent to find relationships between dimensionless groups of parameters
rather than the parameters themselves.
• We can show that force and moment coefficients depend on the Reynold and Mach
numbers and flow angles only, i.e.
𝐶𝑙 = 𝑓𝑙(𝑀∞, 𝑅𝑒∞, 𝛼)
𝐶 𝑑 = 𝑓𝑑(𝑀∞, 𝑅𝑒∞, 𝛼)
𝐶 𝑚 = 𝑓𝑚(𝑀∞, 𝑅𝑒∞, 𝛼)
i.e. fl, fd and fm.
• Notice that all parameters are dimensionless!
• Notice that we have reduced the number of parameters from 7 to just 3!
• There may be other “similarity parameters” other than these 3, depending on problem.
𝑅𝑒∞ =
𝜌∞ 𝑉∞ 𝑐
𝜇∞
𝑀∞ =
𝑉∞
𝑎∞

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Suffa University
Flow Similarity
• Consider two different flow fields over two different bodies. By definition, different flows
are dynamically similar if:
1. Streamline patterns are geometrically similar.
2. Distributions of
𝑉
𝑉∞
,
𝑝
𝑝∞
,
𝑇
𝑇∞
etc., throughout the flow field are the same when plotted against
non-dimensional coordinates.
3. Force coefficients are the same.
• If nondimensional pressure (CP) and shear stress distributions (
𝜏
𝑄∞
) over different bodies
are the same, then the force and moment coefficients will be the same.
• In other words, two flows will be dynamically similar if:
1. The bodies and any other solid boundaries are geometrically similar for both
flows.
2. The similarity parameters are the same for both flows.
• This is a key point in the validity of wind-tunnel testing.

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Suffa University
Wait a Minute…Too much Math 
• Hey, this was supposed to be fun

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So how to solve these equations?
• Obviously, this is pretty darn hard mathematics!
• So how to solve these equalities and inequalities?
• No closed form analytic solution (except simplest cases)
• One way is the wind tunnel!
1/3 scale model of space shuttle in
NASA’s 40-foot-by-80-foot WT
Mercedes-Benz’s Aeroacoustic Wind Tunnel Educational Wind Tunnel

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Suffa University
Computational Fluid Dynamics (CFD)
• An alternate technique is to divide the flow into small
boxes (grid) and solve the full Navier Stokes (or
simplifications) equations at every point numerically.
• This is now possible with high speed computing.
• But it is not really as useful as thought.
• In the end, it is really Colorful Fluid Dynamics. A lot of
colors which may mean something…or nothing.
• Needs a lot of calibration and EXPERTISE.
• Also, not feasible for trade studies and initial design.
• Slow solutions. One design iteration can take days.

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Suffa University
Between Wind Tunnels and CFD
• So, wind tunnels are expensive and time
consuming.
• So is CFD. Both requires experts to interpret
results.
• You really can’t DESIGN your aircraft in WT/CFD.
• Here is where “engineering solutions” come in.
• Ultimately somewhere between an “educated
guess” and JUGAAR!

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Suffa University
Engineering Aerodynamic Softwares
• For preliminary aircraft design, we need quick, somewhat
crude solutions. Ball park estimates will do.
• The USAF saw that need, and decided to compile data on
aerodynamic predictions. Contract: McDonnel Douglas.
• There were some approximate analytic solutions.
• There were some correlations.
• There were half a century of wind/water tunnel tests
• All of this was compiled in two volumes called USAF Stability
and Control Data Compendium (or DATCOM!) in 1978

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What is DATCOM?
• DATCOM is a collection, correlation, codification, and recording of best knowledge,
opinion, and judgment in the area of aerodynamic stability and control prediction
methods.
• Used for
– Conceptual and Preliminary aircraft design
– Evaluate changes resulting from proposed engineering fixes
– For making simulators.
• For any given configuration and flight condition, a complete set of stability and control
derivatives can be determined without resort to outside information.
• Methods range from very simple and easily applied techniques to quite accurate and
thorough procedures.
• The book is intended to be used for preliminary design purposes before WT/CFD.
• It is not easy to sift through two volumes of 3000 pages though!

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Digital DATCOM
• USAF Stability and Control Digital DATCOM is a computer
program that implements the methods contained in
the USAF DATCOM.
• First version in 1978. Implemented in FORTRAN IV.
• It calculates the stability, control and dynamic derivative
characteristics of fixed-wing aircraft.
• It uses text based input and text based output.
• Some GUIs have recently come out, but are not very stable.
• The program was declassified around the year 2000.
http://www.pdas.com/datcomdownload.html

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Suffa University
What can DATCOM do?
• Datcom requires two basic inputs
– Flight Conditions (Altitude, Speed, Re, M, Flow angles)
– Aircraft Geometry (Wings, Tails, Fuselage dimensions etc)
– Optionally propulsion data (jet/propeller can also be input)
• It calculates:
– Lift, Drag, Moments, Center of Pressure, Flow angle derivatives
(stability derivatives)
– Output can be for whole aircraft or components
•CL
•CD
•Cm
•CN
•CA
•CLα
•Cmα
•CYβ
•Cnβ
•Clβ

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Suffa University
Let’s Begin with the Input File
• Remember we are talking about FORTRAN, so data must be entered in the correct column.
– FORTRAN uses Control Cards (objects) and Namelists (functions).
– Namelists start with $ sign. They star after one space.
– Inputs to namelists can be entered one space after a namelist title or beginning with the third space of a new line.
– Control Cards come alone on a line. The start in the first column.
• Easier to edit old files than making one from scratch.
• Begin by first naming the aircraft or “case” to be run.
• This is done by typing CASEID, spacing once, and typing the desired name.
• CASEID is a control card, and thus the “C” should be the first letter on the line and no characters
should follow the case name on the line.
• Next, the system of units for the input may be specified by DIM control card :
– DIM M – kilogram-meter-second
– DIM CM – centimeter-gram-second
– DIM FT – foot-pound-second
– DIM IN – inch-pound-second
• Angles are entered as degrees, always!

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• Next, flight conditions should be entered.
• This is done by calling out the FLTCON namelist. A namelist must be called out
on a new line by spacing once and entering the $ symbol followed by the
namelist title.
• The inputs under the FLTCON namelist includes
– MACH – Mach number
– VINF – Airspeed in units of length (as chosen by the DIM control card) per unit time
– NALPHA – number of angles of attack to be evaluated
– ALSCHD – angles of attack to be evaluated, written sequentially
– GAMMA – flight path angle
– ALT – altitudes to be evaluated
– WT – aircraft weight
• Alternatively, Reynold Number can be entered.

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What does it look like so far?
The “picture” so far

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Geometry….Fuselage
• Next, geometry of the aircraft must be entered.
• Fuselage is defined by the BODY namelist.
• It can be defined by a maximum of 20 longitudinal stations.
– NX – number of stations used to define the body
– X(1) – longitudinal location of station
– R(1) – planform half-width of the fuselage in the spanwise direction
– ZU(1) – location of upper vertical surface of fuselage with respect to an
arbitrary reference plane
– ZL(1) – location of lower vertical surface of fuselage with respect to an
arbitrary reference plane
• Alternatively, a cylindrical fore, mid and aft body is all that is needed

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Namelist BODY

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Geometry….Lifting Surfaces
• Wing, horizontal and vertical tails all have similar inputs.
• Use WGPLNF, VTPLNF, and HTPLNF namelists.
• All the necessary inputs to define a straight tapered planform are as follows:
– SSNPE – exposed semi-span
– SSPN – theoretical semi-span
– CHRDR – root chord
– CHRDTP – tip chord
– SAVSI – sweep angle
– DHDADI – dihedral angle
– TWISTA – twist angle
– CHSTAT – reference chord station for inboard for panel sweep angle
– TYPE – type of wing planform (1.0=straight tapered planform)
• It is also possible to make cranked wings (see manual)

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WGPLNF, VTPLNF, and HTPLNF

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Wing Sections (Airfoils)
• Airfoil can be entered using the NACA control card. (begin column 01)
• NACA 23012 airfoil for a vertical tail is given by:
– NACA-V-5-23012
• V specifies that the airfoil is for the vertical tail. A W, H, or F in the same
place would specify the wing, horizontal tail, or ventral fin airfoil
respectively.
• The 5 specifies the type of airfoil, in this case the 5-digit airfoils.
• Other options are 1, 4, 6, and S for 1-series, 4-digit, 6-series NACA
airfoils, and supersonic airfoils respectively.
• Last input is the airfoil designation.
• You can also enter your own or exotic airfoils (See manual)

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The “Picture” so far…

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Locating the Components on the Fuse
• SYNTHS namelist used to place wings, tail, center of gravity.
– XCG – longitudinal location of center of gravity
– ZCG – vertical location of center of gravity
– XW – longitudinal location of theoretical wing apex
– ZW – vertical location of theoretical wing apex
– XH – longitudinal location of theoretical horizontal tail apex
– ZH – vertical location of theoretical horizontal tail apex
– XV – longitudinal location of theoretical vertical tail apex
– ZV – vertical location of theoretical vertical tail apex
– ALIW – wing incidence angle
– ALIH – horizontal tail incidence angle
•

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Reference Areas and Lengths
• Finally, we may want to put our own reference areas and
lengths (can be left if you want DATCOM to calculate the
same)

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Flaps, Control Surfaces
• Flaps and elevators can be modeled using the SYMFLP and ailerons by ASYFLP
namelists, which output results for symmetrical and asymmetrical flap
deflections, respectively.
– FTYPE – type of flaps (SYMFLP only)
– NDELTA – number of flap deflections to be evaluated
– DELTA(1) – flap deflections listed sequentially (maximum of 9, SYMFLP only)
– CHRDFI – inboard flap chord length
– CHRDFO – outboard flap chord length
– SPANFI – spanwise location of flap inboard panel
– SPANFO – spanwise location of flap outboard panel
– STYPE – control surface type (1.0=flap spoiler, 2.0=plug spoiler, 3.0=spoiler-slot
deflection, 4.0=plain flap aileron, 5.0=all moveable tail, ASYFLP only)
– DELTAL(1) – left flap deflection angles listed sequentially (maximum of 9, ASYFLP only)
– DELTAR(1) – right flap deflection angles

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Control Surfaces

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Some other Options
• You may also want dynamic deratives (with pitch rate, angle
of attack rate etc)
• Use DAMP control card
• DERIV RAD and DERIV DEG output these derivatives in radian
and degree

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Complete File

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Processing the Input file
• Save the file as ***.inp
• Then run datcom.exe and process

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Output
• Let’s see the output in Class.
• You can import the ouput to Matlab using Aerospace Toolbox
• alldata = datcomimport('astdatcom.out', true, 0);
• Plotting Lift Curve Moments
h1 = figure;
for k=1:2
subplot(2,1,k)
plot(data.alpha,permute(data.cl(:,k,:),[1 3 2]))
end

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Plotting the Input Aircraft
• Bill Galbraith of HolyCows sells DATCOM+ for $100 doing this

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Intermission – Equations of Motion

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Equations of Motion
• So, these are 12 nonlinear ODEs.
• There is no analytical solution.
• We need to use numerical integration methods (Adams, RK..)
• Luckily, this is all programmed in Simulink graphical
programming language.

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Simulink Aerospace Toolbox
• Aerospace Blockset™ software extends Simulink® with blocks for
– modeling and simulating
• aircraft,
• spacecraft,
• rocket,
• propulsion systems,
• unmanned airborne vehicles.
– aerospace standards,
– modeling equations of motion
– navigation,
– gain scheduling,
– visualization,
– unit conversion

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So, let us implement the first step

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Next Set Some Aerodynamic Variables
• Use conversions for flow angles (𝛼, 𝛽)
• Use standard atmosphere and gravity models for (𝑔, 𝑀)

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Tidy Things Up a bit
• Create subsystems

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A detailed example
• Lightweight Airplane Simulator Design
– Four seater monoplane: Skyhogg
• We will use Simulink for rapid
– Aircraft design
– Modeling
– Simulation
– Control Design

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Suffa University
Aerodynamics
• The designed geometry is
modeled in Datcom
• Datcom provides
aerodynamic stability and
control derivatives and
coefficients at specified
flight conditions.
• Import in Matlab using
statdyn = datcomimport('SkyHogg.out');

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Let’s get this Aerodynamics in Simulink

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Next also put the Elevator Model

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Equations of Motion Once again

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Actuator and Sensor Models

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Environmental Models

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Trim and Linearize
• Trim and Linearization can be done to find the operating
point at specified speed and altitude, for example.
• You Analysis>Control System Design> Linear Analysis

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Linear System Analysis

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Control System Design
• Dual Loop Control
– Time scale separation

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Altitude Control

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Suffa University
Pizzaz: Integration with FlighGear

2 Day Workshop on Digital Datcom and Simulink

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2 Day Workshop on Digital Datcom and Simulink