AE 202: Aerospace Flight Mechanics
Aerospace Flight Mechanics
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AE 202: Aerospace Flight Mechanics
Homework Assignment 4 - Solutions
Spring 2021
Problems
1. [20 points] You are helping to design the landing system for a 1000-kg rover that will explore
the surface of Mars. Your team is considering using only a parachute for descent and landing,
i.e. no rocket-powered descent stage. Assume the parachute is a disk-gap-band supersonic
parachute similar to that used on previous Mars landers. Use the data in Figure 1 and Table 1
to answer the following:
(a) What diameter parachute is required to land the rover on Mars with a touchdown veloc-
ity of 0.5 m/s?
Begin with force balance:
FD =W
1
2
ρV 2CDA = mg
Equation above is rearranged to solve for the area of a parachute. The velocity will be
the terminal velocity, gravity is Mar’s gravity, CD is chosen from the table at subsonic
speed, density is taken as the sea level density.
A =
2mgM
ρV 2terminalCD
A =
2(1000 kg)(3.71 m/s2)
(0.02 kg/m3)(.5 m/s)2(.67)
A = 2.215 x 106 m2
D =
√
4A
pi
D =
√
4 (3.305 x 106 m2)
pi
1
The parachute diameter for a terminal velocity of 0.5 m/s is:
D = 1679.3 m
(b) How many g’s of acceleration would the above parachute initially generate if inflated at
Mach 1.7 at an altitude of 7200 m?
The equation for g-forces is given in the presentation as:
Fg =
a
gE
Recall that the gravity is always referenced back to Earth’s gravity. The acceleration
is found using F = ma, Newton’s Second Law, where F is the force of drag at the
specified conditions.
FD =
1
2
ρV 2CDA
a =
ρV 2CDA
2m
Using the graph, CD is found to be 0.775 at M = 1.7. The velocoty must be found
using the Mach equation, where the temperature, ratio of specific heats, and specific gas
constant for Mars is given.
V =Ma
V =M
√
γMarsRMarsT
V = (1.7)
√
(1.3)(188.9
J
kg −K)(210 K)
V = 386.05 m/s
Finally the density at 7200 m is calculated using the exponential atmosphere model with
the Martian atmosphere constants.
ρ = ρ0e
−h
H
ρ = (0.02 kg/m3)e
−7.2 km
11.1 km
ρ = .010455 kg/m3
Plugging all the above values into the g-force equation:
2
a =
ρV 2CDA
2m
a =
(.010455 kg/m3)(386.05 m/s)2(.775)(3.305 x 106 m2)
2(1000 kg)
a = 1337389.8 m/s2
Fg =
a
gE
Fg =
1995511.9 m/s2
9.81 m/s2
Fg = 136329.2 g
(c) Discuss your results.
This is clearly an absurd amount of g’s. This comes from the very small terminal veloc-
ity that was chosen to be reached at h = 0 in part (a). This resulted in a extremely large
parachute area, which translated to high g forces. This should indicate why the recent
martian langers, with large payloads, chosen to use the sky crane system because they
could not reach a safe terminal velocity with parachutes alone.
Table 1: Mars Properties
Volumetric mean radius 3389.5 km
Surface gravitational acceleration 3.71 m/s2
Surface atmospheric density 0.02 kg/m3
Atmospheric scale height 11.1 km
Average atmospheric temperature 210 K
Specific gas constant 188.92 J/(kg K)
Ratio of specific heats 1.3
3
0.4
0.5
0.5
0.6
0.6
0.7
0.7
0.8
0.8
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Dr
ag
C
oe
ffi
ci
et
Mach Number
Figure 1: Disk-gap-band parachute drag coefficient.
4
2. [20 points] Compute the equilibrium altitude of a zero-pressure weather balloon (relative to
standard atmosphere) if the payload mass is 15 kg, the maximum volume of the ballon is
800 m3, and:
(a) the lift gas is helium, RHe ≈ 2077.1 J/(kg K)
(b) the lift gas is hydrogen, RH2 ≈ 4124.2 J/(kg K)
(c) the lift gas is methane, RCH4 ≈ 518.28 J/(kg K)
Start with a balance of forces (buoyancy and gravity),
∑
F = 0
Fb + Fg = 0
ρairVdispg −mg = 0
Then, assume Vdisp = Vgas = V and, for zero-pressure balloon, Pgas = Patm and Tgas =
Tatm, so with ideal gas law P = ρRT , can write
mp =
V Patm
Tatm
(
1
Rair
− 1
Rgas
)
Solve for P/T ratio,
Patm
Tatm
=
mp
V
(
1
Rair
− 1
Rgas
)−1
and combine with ideal gas law,
Patm
Tatm
= ρatmRair
ρatmRair =
mp
V
(
1
Rair
− 1
Rgas
)−1
ρatm =
mp
V
(
1− Rair
Rgas
)−1
then can calculate density for each lift gas
(a) helium:
ρatm =
15 kg
800 m3
(
1−
287.058 Jkg K
2077.1 Jkg K
)−1
ρatm = 0.02176 kg/m
3
5
(b) hydrogen:
ρatm =
15 kg
800 m3
(
1−
287.058 Jkg K
4124.2 Jkg K
)−1
ρatm = 0.02015 kg/m
3
(c) methane:
ρatm =
15 kg
800 m3
(
1−
287.058 Jkg K
518.28 Jkg K
)−1
ρatm = 0.042028 kg/m
3
Finally, can look up densities in standard atmosphere table and match with altitude
(geopotential) to find the equilibrium altitude for the zero-pressure weather balloon.
(a) If the lift gas is helium, the equilibrium altitude is
h(ρ) ≈ 28, 800 m
(b) If the lift gas is hydrogen, the equilibrium altitude is
h(ρ) ≈ 29, 150 m
(c) If the lift gas is methane, the equilibrium altitude is
h(ρ) ≈ 24, 700 m
6
3. [20 points] Consider a super-pressure balloon designed to operate in the Venus atmosphere. If
the required payload mass is 100 kg, 10 kg of lift gas is used, and the balloon envelope has an
areal mass of 0.75 kg/m2, what is the required volume of the balloon if the desired operational
altitude is 60 km? Assume an exponential atmosphere (ρsurface = 65 kg/m3, H = 15.9 km).
Starting with Archimedes’ Principle,
Fb + Fg = 0
ρairVdispg −mg = 0
ρairVdispg −
(
mp +mgas +me
)
g = 0
ρairVdisp −
(
mp +mgas +me
)
= 0
where me is the mass of the envelope of the balloon. Assuming the balloon is a sphere, can
describe mass using areal mass (ρA) and surface area (S),
mb = ρAS
where surface area is
S = 4pir2
Since we want to find required volume, can write mass of envelope as a function of volume,
V =
4
3
pir3
r =
(
3V
4pi
) 1
3
S = 4pi
(
3V
4pi
) 2
3
me = ρA4pi
(
3V
4pi
) 2
3
Going back to the balance of forces of buoyancy and gravity, can write polynomial with
volume as the only unknown. Assume Vdisp = Vgas = V (i.e. neglect thickness of balloon
envelope),
ρairV −mp −mgas − ρA4pi
(
3V
4pi
) 2
3
= 0
7
Can calculate density of the air using exponential atmosphere model,
ρair(h) = ρSLe
(
−h
H
)
ρair(h) = (65 kg/m
3)e
(
−60 km
15.9 km
)
ρair = 1.493 kg/m
3
Now, using MATLAB root solver, can find volume
(
1.493 kg/m3
)
V − (100 kg)− (10 kg)− (0.75 kg/m2)(4pi)(3V
4pi
) 2
3
= 0
∴ the required volume of the balloon is
V = 138.8 m3
8
4. [20 points] A wing with a rectangular planform mounted in a low-speed subsonic wind tunnel.
The wing model completely spans the test section so that it may be considered an infinite
wing. If the wing has a NACA 4412 airfoil section and a chord of 0.4 m, calculate the lift,
drag, and moment about the quarter-chord per unit span when the freestream air pressure
is 98 kPa, the freestream temperature is 280 K, the freestream velocity is 38 m/s, and the
viscosity is 1.7894× 10−5 kg/(m s) at the following angles of attack:
(a) 5 deg
(b) 16 deg
(c) -2 deg
You can find the required data for the NACA 4412 airfoil on http://airfoiltools.com/search.
Type “4412” in the “Text search” box and click “Search.” Click “Airfoil details” for the
NACA 4412 in the search results. Under “Polars for NACA 4412”, click “details” for the
appropriate Reynolds Number (use Ncrit = 9 for wind tunnels). You can then scroll through
the airfoil data or download a text file.
First, calculate freestream density using ideal gas law
P∞ = ρ∞RT∞
ρ∞ =
P∞
RT∞
ρ∞ =
98× 103 Pa(
287.058 Jkg K
)(
280 K
)
ρ∞ = 1.2193 kg/m3
Then find dynamic pressure,
q∞ =
1
2
ρ∞v2∞
q∞ =
1
2
(1.2193 kg/m3)(38 m/s)2
q∞ = 880.31 Pa
Next, calculate Reynolds number,
Re =
ρ∞v∞c
µ∞
Re =
(1.2193 kg/m3)(38 m/s)(0.4 m)
1.7894× 10−5 kg/(m s)
Re = 1, 035, 701.4
9
Use Reynolds number to find required data for the NACA 4412 airfoil,
α,◦ Cl Cd Cm
5 1.0254 0.00797 -0.0998
16 1.6698 0.04827 -0.0447
-2 0.2625 0.00715 -0.1038
Table 2: NACA 4412 airfoil data forRe = 1035701,Ncrit = 9
Using the data in Table 2 , can calculate lift, drag, and moment about the quarter-chord pre
unit span using the following expressions,
L = q∞SrefCl
D = q∞SrefCd
M c
4
= q∞SrefCmc
Since we have an infinite wing,
Sref = c(1)
Sref = (0.4 m)(1 m)
Sref = 0.4 m
2
Sample calculation for α = 5◦,
L = q∞SrefCl
L = (880.31 Pa)(0.4 m2)(1.0254)
L = 361.07 N per unit span
D = q∞SrefCd
D = (880.31 Pa)(0.4 m2)(0.00797)
D = 2.81 N per unit span
M c
4
= q∞SrefCmc
M c
4
= (880.31 Pa)(0.4 m2)(1.0254)(0.4 m)
M c
4
= −14.06 N m per unit span
10
(a) For an angle of attack α = 5◦, the lift, drag, and moment about the quarter-chord
are
L = 361.07 N per unit span
D = 2.81 N per unit span
M c
4
= −14.06 N m per unit span
(b) For an angle of attack α = 16◦, the lift, drag, and moment about the quarter-chord
are
L = 587.98 N per unit span
D = 17.0 N per unit span
M c
4
= −6.30 N m per unit span
(c) For an angle of attack α = −2◦, the lift, drag, and moment about the quarter-chord
are
L = 92.43 N per unit span
D = 2.52 N per unit span
M c
4
= −14.62 N m per unit span
11
5. [20 points] This problem requires the use of 3 data files containing inviscid CP values for a
NACA 2412 airfoil. The data is stored in comma-separated value (CSV) text files.
Filename Angle of attack
naca2412i_1.csv 0 deg
naca2412i_2.csv 5 deg
naca2412i_3.csv -5 deg
(a) For each angle of attack, co-plot CP versus x/c with an inverted vertical axis (i.e. neg-
ative values above horizontal axis) for the upper and lower surfaces.
(b) Write a computer program/script to approximate cl for each angle of attack with CP
data in the files using the method discussed in class. To help you check your program,
cl ≈ 0.256 for a NACA 2412 airfoil at zero angle of attack.
(c) Create a plot of cl as a function of α using the computed cl values. What is the zero-lift
angle of attack for this airfoil?
(d) Discuss your results.
(e) Include your computer program/script as text in your assignment.
(a) Figures 2-4 shown below contain plots for CP vs x/c for the upper and lower surfaces
for various angles of attack.
Figure 2: NACA 2412 airfoil CP vs x/c, α = 0◦
12
Figure 3: NACA 2412 airfoil CP vs x/c, α = 5◦
Figure 4: NACA 2412 airfoil CP vs x/c, α = −5◦
(b) To approximate Cl, need to use the following expression,
Cl =
1
c
∫ c
0
(
CP,l − CP,u
)
dx
13
Finding the area between the data sets in Figures 2-4 results in the following
lift coefficients,
Cl(α = 0
◦) = 0.2564
Cl(α = 5
◦) = 0.8294
Cl(α = −5◦) = −0.3181
(c) These lift coefficients are shown below as blue diamonds in Figure 5.
Figure 5: NACA 2412 airfoil Cl vs α with linear curve fit
The zero-lift angle of attack for this airfoil occurs at
α = −2.23◦
(d) Discussion of answer:
• The line produced is linear, matching what was discussed in theory during class.
• The camber of the airfoil produces positive lift even at zero angle of attack.
• The zero-lift angle of attack is negative because of the airfoil camber.
• Small changes of angle of attack result in large changes in lift, and these change
only occur in a small range of angles.
• An angle of attack of α = −5◦ results in a negative lift coefficient, which would
result in a lift vector that is pointed downwards.
14
(e) MATLAB code attached.
1 %{
2 Andrew Koehler
3 AE 202: Aerospace Flight Mechanics
4 Teaching Assistant | Spring 2020
5 Homework Assignment 3
6 Question 4
7 %}
8 %% setup
9 clear; close all; clc;
10
11 %% Given
12 % NACA 2412 airfoil
13 alpha = [0; 5; −5]; % angles of attack
14
15 %% Load data
16 naca_2412_0deg = csvread('naca2412i_1.csv',1,0);
17 naca_2412_5deg = csvread('naca2412i_2.csv',1,0);
18 naca_2412_neg_5deg = csvread('naca2412i_3.csv',1,0);
19
20 upper_0deg.xc = naca_2412_0deg(:,1);
21 upper_0deg.cp = naca_2412_0deg(:,2);
22 lower_0deg.xc = naca_2412_0deg(:,3);
23 lower_0deg.cp = naca_2412_0deg(:,4);
24
25 upper_5deg.xc = naca_2412_5deg(:,1);
26 upper_5deg.cp = naca_2412_5deg(:,2);
27 lower_5deg.xc = naca_2412_5deg(:,3);
28 lower_5deg.cp = naca_2412_5deg(:,4);
29
30 upper_neg_5deg.xc = naca_2412_neg_5deg(:,1);
31 upper_neg_5deg.cp = naca_2412_neg_5deg(:,2);
32 lower_neg_5deg.xc = naca_2412_neg_5deg(:,3);
33 lower_neg_5deg.cp = naca_2412_neg_5deg(:,4);
34
35 % get rid end of lower arrays which don't have airfoil data
36 lower_0deg.xc = lower_0deg.xc(1:78);
37 lower_0deg.cp = lower_0deg.cp(1:78);
38
39 lower_5deg.xc = lower_5deg.xc(1:78);
40 lower_5deg.cp = lower_5deg.cp(1:78);
41
42 lower_neg_5deg.xc = lower_neg_5deg.xc(1:78);
43 lower_neg_5deg.cp = lower_neg_5deg.cp(1:78);
44
45 %% Calculations
46 % Find Cl
47 Cl = zeros(3,1);
48 % without linear interpolation:
49 % Cl(1) = trapz(lower_0deg.xc,lower_0deg.cp) − trapz(upper_0deg.xc
,upper_0deg.cp);
50 % Cl(2) = trapz(lower_5deg.xc,lower_5deg.cp) − trapz(upper_5deg.xc
,upper_5deg.cp);
15
51 % Cl(3) = trapz(lower_neg_5deg.xc,lower_neg_5deg.cp) − trapz(
upper_neg_5deg.xc,upper_neg_5deg.cp);
52 %
53 n = 100;
54 x = linspace(1/n,1,n);
55
56 lower_0deg_interp = interp1(lower_0deg.xc,lower_0deg.cp,x);
57 upper_0deg_interp = interp1(upper_0deg.xc,upper_0deg.cp,x);
58
59 lower_5deg_interp = interp1(lower_5deg.xc,lower_5deg.cp,x);
60 upper_5deg_interp = interp1(upper_5deg.xc,upper_5deg.cp,x);
61
62 lower_neg_5deg_interp = interp1(lower_neg_5deg.xc,lower_neg_5deg.
cp,x);
63 upper_neg_5deg_interp = interp1(upper_neg_5deg.xc,upper_neg_5deg.
cp,x);
64
65 Cl(1) = trapz(x,lower_0deg_interp) − trapz(x,upper_0deg_interp);
66 Cl(2) = trapz(x,lower_5deg_interp) − trapz(x,upper_5deg_interp);
67 Cl(3) = trapz(x,lower_neg_5deg_interp) − trapz(x,
upper_neg_5deg_interp);
68
69 %% Plots
70 font = 18;
71 figure_size = [350 150 1200 750];
72 colors = [0 0.4470 0.7410; 0.8500 0.3250 0.0980; 0.9290 0.6940
0.1250; 0.4940 0.1840 0.5560; 0.4660 0.6740 0.1880; 0.3010
0.7450 0.9330; 0.6350 0.0780 0.1840];
73 set(groot,'defaulttextinterpreter','latex');
74 set(groot,'defaultAxesTickLabelInterpreter','latex');
75 set(groot,'defaultLegendInterpreter','latex');
76 mk_sz = 8;
77
78 %% figure 1
79 fig1 = figure(1);
80 plot(upper_0deg.xc,upper_0deg.cp,'Color',colors(1,:),'LineStyle','
−','Marker','o','MarkerSize',mk_sz,'Linewidth',2)
81 hold on
82 plot(lower_0deg.xc,lower_0deg.cp,'Color',colors(2,:),'LineStyle','
−','Marker','x','MarkerSize',mk_sz,'Linewidth',2)
83 hold on
84 grid on
85 set(gca,'FontSize', font)
86 set(gcf, 'Position', figure_size);
87 xlabel('$x/c, nd$')
88 ylabel('$C_{P}, nd$')
89 %title('$C_{P}$ vs $x/c$, $\alpha = 0^\circ$')
90 leg1 = legend('$C_{P}$ Upper','$C_{P}$ Lower','Location','
northeast');
91 leg1.FontSize = font;
92 set(gca, 'YDir','reverse')
93
94 %% figure 2
95 fig2 = figure(2);
16
96 plot(upper_5deg.xc,upper_5deg.cp,'Color',colors(1,:),'LineStyle','
−','Marker','o','MarkerSize',mk_sz,'Linewidth',2)
97 hold on
98 plot(lower_5deg.xc,lower_5deg.cp,'Color',colors(2,:),'LineStyle','
−','Marker','x','MarkerSize',mk_sz,'Linewidth',2)
99 hold on
100 grid on
101 set(gca,'FontSize', font)
102 set(gcf, 'Position', figure_size);
103 xlabel('$x/c, nd$')
104 ylabel('$C_{P}, nd$')
105 %title('$C_{P}$ vs $x/c$, $\alpha = 5^\circ$')
106 leg2 = legend('$C_{P}$ Upper','$C_{P}$ Lower','Location','
northeast');
107 leg2.FontSize = font;
108 set(gca, 'YDir','reverse')
109
110 %% figure 3
111 fig3 = figure(3);
112 plot(upper_neg_5deg.xc,upper_neg_5deg.cp,'Color',colors(1,:),'
LineStyle','−','Marker','o','MarkerSize',mk_sz,'Linewidth',2)
113 hold on
114 plot(lower_neg_5deg.xc,lower_neg_5deg.cp,'Color',colors(2,:),'
LineStyle','−','Marker','x','MarkerSize',mk_sz,'Linewidth',2)
115 hold on
116 grid on
117 set(gca,'FontSize', font)
118 set(gcf, 'Position', figure_size);
119 xlabel('$x/c, nd$')
120 ylabel('$C_{P}, nd$')
121 %title('$C_{P}$ vs $x/c$, $\alpha = −5^\circ$')
122 leg3 = legend('$C_{P}$ Upper','$C_{P}$ Lower','Location','
northeast');
123 leg3.FontSize = font;
124 set(gca, 'YDir','reverse')
125
126 %% figure 4
127 x1 = linspace(alpha(3), alpha(2))';
128 p1 = polyfit(alpha,Cl,1);
129 zero_lift_alpha = roots(p1);
130 y1 = polyval(p1,x1);
131
132 fig4 = figure(4);
133 scatter(alpha,Cl,150,colors(1,:),'d','filled')
134 hold on
135 plot(x1,y1,'Color',colors(2,:),'Linewidth',2)
136 grid on
137 set(gca,'FontSize', font)
138 set(gcf, 'Position', figure_size);
139 xlabel('$\alpha, ^\circ$')
140 ylabel('$C_{l}, nd$')
141 %title('$C_{l}$ vs $\alpha$')