# Modelling and Control of an Urban Air Mobility Vehicle Subject to Empirically-Developed Urban Airflow Disturbances

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## Abstract

**:**

## 1. Introduction

## 2. Background

- Less than 0.315 $\mathrm{m}/{\mathrm{s}}^{2}$, not uncomfortable;
- 0.315 to 0.63 $\mathrm{m}/{\mathrm{s}}^{2}$, a little uncomfortable;
- 0.5 to 1 $\mathrm{m}/{\mathrm{s}}^{2}$, fairly uncomfortable;
- 0.8 to 1.6 $\mathrm{m}/{\mathrm{s}}^{2}$, uncomfortable;
- 1.25 to 2.5 $\mathrm{m}/{\mathrm{s}}^{2}$, very uncomfortable;
- Greater than 2.5 $\mathrm{m}/{\mathrm{s}}^{2}$, extremely uncomfortable.

## 3. Model, Disturbance, and Controller Development

#### 3.1. Selection of the UAT Platform for Generic UAT Flight Dynamics Model

#### 3.2. Geometric and Inertial Parameter Estimation

#### 3.3. Aerodynamic Parameter Estimation

#### 3.3.1. Lift-Curve Slope Coefficient Estimation

#### 3.3.2. Longitudinal Static Analysis

#### 3.3.3. Estimation of Stability Derivatives

#### 3.4. State-Space Model

#### Stability Analysis

#### 3.5. Urban Airflow Disturbance

#### 3.6. Inner-Loop Controllers

## 4. Results

## 5. Discussion

## 6. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Abbreviations

AAM | Advanced Air Mobility |

ADRC | Active Disturbance Rejection Control |

ESO | Extended State Observer |

IMUs | Inertial Measurement Units |

PSD | Power Spectral Density |

RPAS | Remotely Piloted Aircraft Systems |

SAS | Stability Augmentation System |

STOL | Short Take-off and Landing |

UAM | Urban Air Mobility |

UAT | Urban Air Taxi |

VTOL | Vertical Take-off and Landing |

## Appendix A

#### Appendix A.1

#### Appendix A.2

#### Appendix A.3

${\mathit{C}}_{\mathit{x}}$ | ${\mathit{C}}_{\mathit{z}}$ | ${\mathit{C}}_{\mathit{m}}$ | |
---|---|---|---|

u | −0.5028 | 0 | 0 |

$\alpha $ | 0.8752 | −6.8307 | −1.0246 |

q | 0 | −25.7177 | −56.7287 |

$\dot{\alpha}$ | 0 | 0 | 0 |

${\mathit{C}}_{\mathit{y}}$ | ${\mathit{C}}_{\mathit{l}}$ | ${\mathit{C}}_{\mathit{n}}$ | |
---|---|---|---|

$\beta $ | −0.4264 | −0.0587 | 0.1623 |

p | −0.1173 | −1.1936 | −0.4050 |

r | 0.3247 | 0.9255 | −0.1236 |

#### Appendix A.4

Longitudinal | Lateral | ||||
---|---|---|---|---|---|

Elevator | Aileron | Rudder | |||

${C}_{{x}_{\delta e}}$ | 0 | ${C}_{{Y}_{\delta a}}$ | 0 | ${C}_{{Y}_{\delta r}}$ | −0.8273 |

${C}_{{z}_{\delta e}}$ | −1.1270 | ${C}_{{L}_{\delta a}}$ | 0.6788 | ${C}_{{L}_{\delta r}}$ | −0.1138 |

${C}_{{m}_{\delta e}}$ | 2.3385 | ${C}_{{N}_{\delta a}}$ | 0 | ${C}_{{N}_{\delta r}}$ | 0.3150 |

## Appendix B. Nomeclature

Notation | Description |

Aircraft Model Symbols | |

${\mathrm{x}}_{\mathrm{B}}$ | body frame x direction |

${\mathrm{y}}_{\mathrm{B}}$ | body frame y direction |

${\mathrm{z}}_{\mathrm{B}}$ | body frame z direction |

${\alpha}_{wb}$ | wing-body angle of attack |

$\overline{\mathrm{c}}$ | mean aerodynamic chord |

h | percent location of the centre of gravity |

${\mathrm{h}}_{\mathrm{nw}}$ | percent location of the main wing neutral point |

${\mathrm{h}}_{\mathrm{nwb}}$ | percent location of the wing-body neutral point |

${\overline{l}}_{c}$ | distance between the main wing and canard aerodynamic centres |

${l}_{c}$ | distance between centre of gravity and canard aerodynamic centre |

${\delta}_{e}$ | elevator deflection angle |

${\delta}_{f}$ | flap deflection angle |

b | span |

S | planform area |

${I}_{{x}_{p}}$ | mass moment of inertia about x principal axis |

${I}_{{y}_{p}}$ | mass moment of inertia about y principal axis |

${I}_{{z}_{p}}$ | mass moment of inertia about z principal axis |

m | mass |

H | body height |

W | body width |

L | body length |

${a}_{\infty}$ | two-dimensional lift-curve slope |

$\frac{t}{c}$ | wing thickness ratio |

${\Lambda}_{LE}$ | sweepback angle of the leading edge |

a | three-dimensional lift-curve slope |

$AR$ | aspect ratio |

${\left({a}_{\infty}\right)}_{wb}$ | two-dimensional lift-curve slope of wing-body |

${\left({a}_{\infty}\right)}_{c}$ | two-dimensional lift-curve slope of canard |

${\left({a}_{\infty}\right)}_{F}$ | two-dimensional lift-curve slope of vertical tail surface |

${a}_{wb}$ | three-dimensional lift-curve slope of wing-body |

${a}_{c}$ | three-dimensional lift-curve slope of canard |

${a}_{F}$ | three-dimensional lift-curve slope of wing-body vertical tail surface |

${C}_{{L}_{trim}}$ | trimmed lift coefficient |

W | aircraft weight |

$\rho $ | air density |

V | airspeed |

${C}_{{m}_{0}}$ | aerodynamic moment coefficient of the aircraft at zero lift |

${C}_{{L}_{\alpha}}$ | overall lift coefficient with respect to change in angle of attack |

${C}_{{m}_{\alpha}}$ | overall moment coefficient with respect to change in angle of attack |

${C}_{{L}_{{\delta}_{e}}}$ | change in the lift coefficient due to elevator deflection |

${C}_{{m}_{{\delta}_{e}}}$ | change the moment coefficient due to elevator deflection |

${\alpha}_{trim}$ | trimmed angle of attack |

${\delta}_{{e}_{trim}}$ | trimmed elevator deflection angle |

${S}_{c}$ | planform area of the canard |

h | location of the centre of gravity as a percentage of the mean aerodynamic |

chord of the wing | |

${h}_{n}$ | location of the overall UAT neutral point as a percentage of the mean |

aerodynamic chord of the wing | |

${a}_{e}$ | elevator effectiveness coefficient |

${\overline{V}}_{H}$ | horizontal tail (canard) volume ratio relative to the aerodynamic centres of |

the canard and wing-body | |

${\delta}_{e}$ | elevator deflection angle |

${\delta}_{f}$ | flap deflection angle |

$\alpha $ | angle of attack relative to the zero-lift line |

${\alpha}_{wb}$ | angle of attack relative to the body x-axis |

${K}_{n}$ | longitudinal static margin |

${X}_{u}$ | x direction force with respect to forward air speed |

${X}_{w}$ | x direction force with respect to vertical air speed |

g | gravitational acceleration constant |

${\theta}_{0}$ | trimmed pitch angle |

${Z}_{u}$ | z direction force with respect to forward air speed |

${Z}_{w}$ | z direction force with respect to vertical air speed |

${Z}_{\dot{w}}$ | z direction force with respect to rate of change of vertical air speed |

${Z}_{q}$ | z direction force with respect to pitch rate |

${u}_{0}$ | trimmed airspeed |

${I}_{y}$ | mass moment of inertia about body y axis |

${M}_{u}$ | pitching moment with respect to forward air speed |

${M}_{\dot{w}}$ | pitching moment with respect to rate of change of vertical air speed |

${M}_{w}$ | pitching moment with respect to vertical air speed |

${M}_{q}$ | pitching moment with respect to pitch rate |

$\Delta {X}_{c}$ | change in x direction force with respect to control surface deflections |

$\Delta {Z}_{c}$ | change in z direction force with respect to control surface deflections |

$\Delta {M}_{c}$ | change in pitching moment with respect to control surface deflections |

${Y}_{v}$ | y direction force with respect to lateral air speed |

${Y}_{p}$ | y direction force with respect to roll rate |

${Y}_{r}$ | y direction force with respect to yaw rate |

${L}_{v}$ | rolling moment with respect to lateral air speed |

${L}_{p}$ | rolling moment with respect to roll rate |

${L}_{r}$ | rolling moment with respect to yaw rate |

${N}_{v}$ | yawing moment with respect to lateral air speed |

${N}_{p}$ | yawing moment with respect to roll rate |

${N}_{r}$ | yawing moment with respect to yaw rate |

${I}_{x}^{\prime}$ | mass moment of inertia with respect to x direction stability axis |

${I}_{z}^{\prime}$ | mass moment of inertia with respect to z direction stability axis |

${I}_{zx}^{\prime}$ | mass product of inertia with respect to x-z direction stability axes |

$\dot{x}$ | rate of change of state vector |

x | state vector |

u | control input vector |

A | state matrix |

B | control matrix |

$A*$ | disturbance effect state matrix |

$\lambda $ | eigenvalues |

I | identity matrix |

$\zeta $ | damping ratio |

${\omega}_{n}$ | natural frequency |

Urban Airflow Disturbance Symbols | |

${\mathrm{I}}_{\mathrm{uu}}$ | turbulence intensity in u airspeed direction |

${\mathrm{I}}_{\mathrm{vv}}$ | turbulence intensity in v airspeed direction |

${\mathrm{I}}_{\mathrm{ww}}$ | turbulence intensity in w airspeed direction |

${A}_{a,j}$ | Fourier series magnitude at location a, for frequency component j |

${S}_{aa,j}$ | PSD amplitude at location a, for frequency component j |

$\Delta f$ | frequency spacing |

${v}_{a}$ | time domain wind disturbance signal at location a |

N | maximum number of frequency components |

${\omega}_{a,j}$ | the N frequency components that are evenly spaced at a fixed |

frequency spacing | |

t | time |

${\Psi}_{a,j}$ | phase angle for each frequency |

Controller Symbols | |

e | feedback signal error |

${k}_{1}$ | PID proportional gain |

${k}_{0}$ | PID integral gain |

${k}_{2}$ | PID derivative gain |

$\tau $ | PID integration parameter |

${u}_{0}$ | ADRC ideal control signal |

${h}_{0},{r}_{0}$ | ADRC controller tuning parameters |

${e}_{1}$ | ADRC feedback error |

${e}_{2}$ | ADRC feedback error derivative |

$a,d,{s}_{a},{a}_{0},y,{a}_{1},{a}_{2},{s}_{y}$ | ADRC $fhan$ function parameters |

${z}_{1}$, ${z}_{2}$, ${z}_{3}$ | ADRC state estimates |

${\beta}_{1}$, ${\beta}_{2}$, ${\beta}_{3}$ | ADRC observer gains |

$\alpha ,\delta $ | ADRC $fal$ function tuning parameters |

${b}_{0}$ | ADRC controller parameter |

h | ADRC ESO tuning parameter |

$v,{v}_{1},{v}_{2}$ | ADRC set point signals |

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**Figure 1.**Health guidance caution zones from ISO 2631-1. Reproduced from [36].

**Figure 2.**Bell Nexus 4EX [42].

**Figure 3.**Generic UAT layout, where ${\mathrm{x}}_{\mathrm{B}}$, ${\mathrm{y}}_{\mathrm{B}}$, and ${\mathrm{z}}_{\mathrm{B}}$ are the body frame x, y, and z directions, ${\alpha}_{wb}$ is the wing-body angle of attack, $\overline{\mathrm{c}}$ is the mean aerodynamic chord, h is the percent location of the centre of gravity, ${\mathrm{h}}_{\mathrm{nw}}$ is the percent location of the main wing neutral point, ${\mathrm{h}}_{\mathrm{nwb}}$ is the percent location of the wing-body neutral point, ${\overline{l}}_{c}$ is the distance between the main wing and canard aerodynamic centres, ${l}_{c}$ is the distance between centre of gravity and canard aerodynamic centre, ${\delta}_{e}$ is the elevator deflection angle, and ${\delta}_{f}$ is the flap deflection angle.

**Figure 4.**Block diagram showing arrangement of state-space matrices, controller, and wind disturbance for simulation development.

**Figure 5.**Final sub-scale CAD model for city model. Reproduced from [51].

**Figure 6.**South wind direction data points. Measurements taken at 105 m full scale above the ground. Reproduced with modifications from [51].

**Figure 7.**West-south-west wind direction data points. Measurements taken at 57 m full scale above the ground. Reproduced with modifications from [51].

**Figure 8.**PSD plot of lateral, v-direction, wind speed variation using data from Reference [51] for point 32.

**Figure 9.**Urban airflow disturbance for stream wise, u-direction, lateral, v-direction, and vertical, w-direction for point 32.

**Figure 10.**ADRC system architecture. Reproduced from [53].

**Figure 15.**Pitch and elevator angle response of the UAT to an urban airflow disturbance generated from point 32.

**Figure 16.**Yaw rate and rudder angle response of the UAT to to an urban airflow disturbance generated from point 32.

**Figure 17.**Bank and aileron angle response of the UAT to to an urban airflow disturbance generated from point 32.

**Figure 18.**Frequency-weighted accelerations for the uncontrolled, PID- and ADRC-controlled UAT operating under south wind direction.

**Figure 19.**Frequency-weighted accelerations for the uncontrolled, PID- and ADRC-controlled UAT operating under west-south-west wind direction.

**Figure 20.**Frequency-weighted accelerations for the uncontrolled, PID- and ADRC-controlled UAT operating under south wind direction at reduced cruise speed.

**Figure 21.**Frequency-weighted accelerations for the uncontrolled, PID- and ADRC-controlled UAT operating under west-south-west wind direction at reduced cruise speed.

**Table 1.**Geometric parameters for main wing and canard, where b is the span and S is the planform area.

Main Wing | Canard | |
---|---|---|

b | 12.33 m | 6.44 m |

$\overline{\mathrm{c}}$ | 1.08 m | 0.45 m |

S | 10.23 ${\mathrm{m}}^{2}$ | 2.90 ${\mathrm{m}}^{2}$ |

Aspect ratio | 11.42 | 14.27 |

Thickness ratio | 0.12 | 0.12 |

${\mathit{I}}_{{\mathit{x}}_{\mathit{p}}}$ | ${\mathit{I}}_{{\mathit{y}}_{\mathit{p}}}$ | ${\mathit{I}}_{{\mathit{z}}_{\mathit{p}}}$ |
---|---|---|

$1.56\phantom{\rule{0.222222em}{0ex}}\times \phantom{\rule{0.222222em}{0ex}}{10}^{4}$$\mathrm{kg}\phantom{\rule{0.166667em}{0ex}}\xb7\phantom{\rule{0.166667em}{0ex}}{\mathrm{m}}^{2}$ | $1.59\phantom{\rule{0.222222em}{0ex}}\times \phantom{\rule{0.222222em}{0ex}}{10}^{4}$$\mathrm{kg}\phantom{\rule{0.166667em}{0ex}}\xb7\phantom{\rule{0.166667em}{0ex}}{\mathrm{m}}^{2}$ | $3.03\phantom{\rule{0.222222em}{0ex}}\times \phantom{\rule{0.222222em}{0ex}}{10}^{4}$$\mathrm{kg}\phantom{\rule{0.166667em}{0ex}}\xb7\phantom{\rule{0.166667em}{0ex}}{\mathrm{m}}^{2}$ |

Wing-Body | Canard | Vertical Tail | |||
---|---|---|---|---|---|

${\left({a}_{\infty}\right)}_{wb}$ | 6.198 1/rad | ${\left({a}_{\infty}\right)}_{c}$ | 6.198 $1/\mathrm{rad}$ | ${\left({a}_{\infty}\right)}_{F}$ | 4.167 $1/\mathrm{rad}$ |

${a}_{wb}$ | 5.285 $1/\mathrm{rad}$ | ${a}_{c}$ | 5.445 $1/\mathrm{rad}$ | ${a}_{F}$ | 2.192 $1/\mathrm{rad}$ |

${\mathit{\delta}}_{\mathit{e}}$ | ${\mathit{\delta}}_{\mathit{f}}$ | $\mathit{\alpha}$ | ${\mathit{\alpha}}_{\mathbf{wb}}$ | h | ${\mathit{h}}_{\mathit{n}}$ | ${\mathit{K}}_{\mathit{n}}$ |
---|---|---|---|---|---|---|

5.77° | 5.00° | 8.70° | 7.16° | −1.435 | −1.285 | 0.15 |

$\frac{{X}_{u}}{m}$ | $\frac{{X}_{w}}{m}$ | 0 | $-g\mathrm{cos}{\theta}_{0}$ |
---|---|---|---|

$\frac{{Z}_{u}}{m-{Z}_{\dot{w}}}$ | $\frac{{Z}_{w}}{m-{Z}_{\dot{w}}}$ | $\frac{{Z}_{q}+m{u}_{0}}{m-{Z}_{\dot{w}}}$ | $\frac{-mg\mathrm{sin}{\theta}_{0}}{m-{Z}_{\dot{w}}}$ |

$\frac{1}{{I}_{y}}[{M}_{u}+\frac{{M}_{\dot{w}}{Z}_{u}}{m-{Z}_{\dot{w}}}]$ | $\frac{1}{{I}_{y}}[{M}_{w}+\frac{{M}_{\dot{w}}{Z}_{w}}{m-{Z}_{\dot{w}}}]$ | $\frac{1}{{I}_{y}}[{M}_{q}+\frac{{M}_{\dot{w}}({Z}_{q}+m{u}_{0})}{m-{Z}_{\dot{w}}}]$ | $-\frac{{M}_{\dot{w}}mg\mathrm{sin}{\theta}_{0}}{{I}_{y}(m-{Z}_{\dot{w}})}$ |

0 | 0 | 1 | 0 |

$\frac{\Delta {X}_{c}}{m}$ |

$\frac{\Delta {Z}_{c}}{m-{Z}_{\dot{w}}}$ |

$\frac{\Delta {M}_{c}}{{I}_{y}}+\frac{{M}_{\dot{w}}}{{I}_{y}}\frac{\Delta {Z}_{c}}{(m-{Z}_{\dot{w}})}$ |

0 |

$\frac{{Y}_{v}}{m}$ | $\frac{{Y}_{p}}{m}$ | $\left(\frac{{Y}_{r}}{m}-{u}_{o}\right)$ | $g\mathrm{cos}{\theta}_{o}$ |

$\left(\frac{{L}_{v}}{{I}_{x}^{\prime}}+{I}_{zx}^{\prime}{N}_{v}\right)$ | $\left(\frac{{L}_{p}}{{I}_{x}^{\prime}}+{I}_{zx}^{\prime}{N}_{p}\right)$ | $\left(\frac{{L}_{r}}{{I}_{x}^{\prime}}+{I}_{zx}^{\prime}{N}_{r}\right)$ | 0 |

$\left({I}_{zx}^{\prime}{L}_{v}+\frac{{N}_{v}}{{I}_{z}^{\prime}}\right)$ | $\left({I}_{zx}^{\prime}{L}_{p}+\frac{{N}_{p}}{{I}_{z}^{\prime}}\right)$ | $\left({I}_{zx}^{\prime}{L}_{r}+\frac{{N}_{r}}{{I}_{z}^{\prime}}\right)$ | 0 |

0 | 1 | $\mathrm{tan}{\theta}_{0}$ | 0 |

$\frac{\Delta {Y}_{c}}{m}$ |

$\frac{\Delta {L}_{c}}{{I}_{x}^{\prime}}+{I}_{zx}^{\prime}{N}_{c}$ |

${I}_{zx}^{\prime}\Delta {L}_{c}+\frac{\Delta {N}_{c}}{{I}_{z}^{\prime}}$ |

0 |

Longitudinal | Lateral-Unstable | Lateral-Stabilized | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Mode | Eigenvalue | Period [s] | ${\mathbf{t}}_{\mathbf{half}}$ | Mode | Eigenvalue | Period [s] | ${\mathbf{t}}_{\mathbf{half}}$ | Eigenvalue | Period [s] | ${\mathbf{t}}_{\mathbf{half}}$ |

Long (Phugoid) | −0.0119 ± 0.1772i | 35.5 | 58.2 | Dutch Roll | −0.1126 ± 1.444i | 4.35 | 6.15 | −0.0727 ± 1.393i | 4.5 | 9.5 |

Rolling | −2.322 ± 0i | n/a | 0.3 | −2.468 ± 0i | n/a | 0.28 | ||||

Short (Pecking) | −0.867 ± 0.1356i | 4.6 | 0.8 | Spiral | 0.0613 ± 0i | n/a | 11.3 | −0.0025 ± 0i | n/a | 277.2 |

Flying Quality Characteristic | Level | $\mathit{\zeta}$ | ${\mathit{\omega}}_{\mathit{n}}$ | Time Constant |
---|---|---|---|---|

Short Period Mode | 1 | 0.99 | 0.88 | N/A |

Phugoid Mode | 1 | 0.07 | 0.18 | N/A |

Roll Mode | 1 | N/A | N/A | 0.41 |

Dutch Roll Mode | 2 | 0.05 | 1.39 | N/A |

Point Number | u RMS [m/s] | v RMS [m/s] | w RMS [m/s] |
---|---|---|---|

29 | 1.65 | 2.20 | 1.84 |

31 | 2.66 | 1.67 | 1.70 |

32 | 1.79 | 3.04 | 2.14 |

35 | 1.93 | 3.23 | 2.25 |

7 | 1.23 | 1.32 | 1.17 |

9 | 0.79 | 0.96 | 0.87 |

30 | 1.73 | 1.92 | 1.34 |

33 | 1.48 | 1.13 | 1.36 |

ADRC Parameters | PID Gains | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

Controller Tuning Params | ESO Gains | ESO Function Params | ||||||||||||

Control Channel | ${\mathit{h}}_{\mathbf{0}}$ | ${\mathit{r}}_{\mathbf{0}}$ | ${\mathit{b}}_{\mathbf{0}}$ | $\mathit{c}$ | ${\mathit{\beta}}_{\mathbf{1}}$ | ${\mathit{\beta}}_{\mathbf{2}}$ | ${\mathit{\beta}}_{\mathbf{3}}$ | $\mathit{\alpha}$ | $\mathit{\delta}$ | $\mathit{\alpha}\mathbf{1}$ | $\mathit{\delta}\mathbf{1}$ | ${\mathit{k}}_{\mathbf{1}}$ | ${\mathit{k}}_{\mathbf{2}}$ | ${\mathit{k}}_{\mathbf{0}}$ |

Elevator | 0.01 | 0.01 | 0.1 | 1 | 1 | 5 | 20.10 | 0.5 | 0.1 | 0.25 | 0.1 | 50 | 30 | 20 |

Rudder | 0.0005 | 0.1 | 8 | 1 | 1 | 22.36 | 731.69 | 0.5 | 1 | 0.25 | 1 | 0.5 | 0 | 0 |

Aileron | 0.05 | 0.01 | 8 | 1 | 1 | 2.24 | 2.91 | 0.5 | 0.09 | 0.25 | 0.09 | 2 | 1 | 1 |

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© 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

McKercher, R.G.; Khouli, F.; Wall, A.S.; Larose, G.L.
Modelling and Control of an Urban Air Mobility Vehicle Subject to Empirically-Developed Urban Airflow Disturbances. *Aerospace* **2024**, *11*, 220.
https://doi.org/10.3390/aerospace11030220

**AMA Style**

McKercher RG, Khouli F, Wall AS, Larose GL.
Modelling and Control of an Urban Air Mobility Vehicle Subject to Empirically-Developed Urban Airflow Disturbances. *Aerospace*. 2024; 11(3):220.
https://doi.org/10.3390/aerospace11030220

**Chicago/Turabian Style**

McKercher, Richard G., Fidel Khouli, Alanna S. Wall, and Guy L. Larose.
2024. "Modelling and Control of an Urban Air Mobility Vehicle Subject to Empirically-Developed Urban Airflow Disturbances" *Aerospace* 11, no. 3: 220.
https://doi.org/10.3390/aerospace11030220