Essential Formulae

Note: All symbols have their usual meanings as used in the text. These equations may be made available for use on exams with the permission of the instructor.


  • Equation of state:

    \[ p = \rho R T \]


  • Hydrostatic pressure in a stagnant fluid:

    \[ \nabla p = \rho \vec{f_b} -\rho g\vec{k} \]


  • Hydrostatic pressure in scalar form:

    \begin{eqnarray*} \frac{\partial p}{\partial x} & = & \rho f_x \nonumber \\[18pt] \frac{\partial p}{\partial y} & = & \rho f_y \nonumber \\[18pt] \frac{\partial p}{\partial z} & = & \rho f_z -\rho g \nonumber \end{eqnarray*}


  • Hydrostatic equation:

    \[ \frac{dp}{dz} = -\rho g \]


  • Special solutions to hydrostatic equation:
    • For a constant density fluid:

    \[ p + \rho g z = \mbox{constant} \]

    • Pressure change with a linear thermal gradient with temperature lapse B between two heights z_1 and z_2:

    \[ \frac{p_2}{p_1} = \bigg( \frac{T_2}{T_1} \bigg)^{-g/RB} = \bigg( \frac{T_1 - B (z_2 - z_1)}{T_1} \bigg)^{-g/RB} \]

    • Pressure change with an isothermal temperature T_0 between two heights z_1 and z_2:

    \[ \frac{p_2}{p_1} = \exp\bigg( \frac{ -g (z_2 - z_1)}{R T_0}\bigg) \]


  • Specific gravity SG of a liquid = \rho/\rho_{H_2 0}

  • Specific weight \gamma of a liquid = \rho g

  • Convert temperature T to Kelvin K from Centigrade or Celsius ^{\circ}\mbox{C}:

    \[ \mbox{K} = \, ^{\circ}\mbox{C} + 273.15 \]


  • Convert temperature T to Rankine R from Fahrenheit ^{\circ}\mbox{F}:

    \[ \mbox{R} = \, ^{\circ}\mbox{F} + 459.67 \]


  • Speed of sound:

    \[ a = \sqrt{ \gamma R T} \]


  • Mach number:

    \[ M = \frac{V}{a} \]


  • Reynolds number based on length l:

    \[ Re = \frac{\rho V l}{\mu} \]


  • Kinematic viscosity = \mu/\rho

  • Shear stress in a Newtonian fluid:

    \[ \tau=\mu \left( \frac{du}{dy} \right) \]


  • Equations of a streamline:

    \begin{gather*} d\vec{l} \times \vec{V} = \vec{0} \\[12pt] \frac{dy}{dz}=\frac{v}{w},~~\frac{dz}{dx}=\frac{w}{u},~~\frac{dy}{dx}=\frac{v}{u} \end{gather*}


  • Continuity equation for a fluid:

    \[ \frac{\partial}{\partial t}\iiint_{\cal{V}} \rho d {\cal{V}} + \oiint_S \rho \, \vec{V} \cdot d\vec{S} = 0 \]


  • Momentum equation for a fluid:

    \[ \vec{F} = \iiint_{\cal{V}} \rho \vec{f} _b d{\cal{V}} -\oiint_S p dS + \vec{F}_{\mu} = \frac{\partial}{\partial t}\iiint_{\cal{V}} \rho \, \vec{V} d {\cal{V}} + \oiint_S (\rho \, \vec{V} \cdot d\vec{S}) \vec{V} \]


  • Energy equation for a fluid:

(1)   \begin{eqnarray*} \iiint_{\cal{V}} \dot{q} \rho d{\cal{V}} + \dot{Q}_{\mu} -\oiint_{S} (p d\vec{S}) \cdot \vec{V} + \iiint_{\cal{V}} ( \rho \vec{f}_b d{\cal{V}}) \cdot \vec{V} + \dot{W}_{\mu} + \dot{W}_{\rm mech} = && \nonumber \\ \frac{\partial}{\partial t} \iiint_{\cal{V}} \rho \left( e + \frac{V^2}{2} + g z\right) d{\cal{V}} + \oiint_{S} (\rho \, \vec{V} \cdot d\vec{S}) \left( e + \frac{V^2}{2} + g z\right) & \nonumber \end{eqnarray*}


  • Energy equation for single-stream system:

    \[ w_{\rm mech} = \frac{V_2^2 - V_1^2}{2} + g ( z_2 - z_1 ) + \left( \frac{p_2}{\rho_2} - \frac{p_1}{\rho_1} \right) \]


  • Bernoulli equation:

    \[ p + \frac{1}{2} \rho V^2 + \rho g z = \mbox{constant} \]


  • Hydraulic diameter of a pipe:

    \begin{equation*} D_h = \frac{4 \times \mbox{Area~of~cross-section}}{\mbox{Perimeter}} = \frac{4 A_c}{p} \end{equation*}


  • Pressure drop \Delta p_L for a pipe flow along a uniform diameter pipe of length L:

    \begin{equation*} \Delta p_L = \frac{1}{2} \rho V_{\rm avg}^2 \, f \left( \frac{L}{D} \right) \end{equation*}


  • Head loss for a pipe flow:

    \[ h_L = \frac{\Delta p}{\rho \, g} \]


  • Pumping power for a pipe flow:

    \[ P = \dot{Q} \, \Delta p \]


  • Area of a wing:

    \begin{equation*} S = 2 \int_{0}^{s} c \, dy = 2 \int_{0}^{b/2} c \, dy \end{equation*}


  • Aspect ratio of a wing:

    \begin{equation*} A\!R = \frac{b^2}{S} = \frac{4s^2}{S} = \frac{4s^2}{2 \displaystyle{\int_{0}^{s} c \, dy}} \end{equation*}


  • Mean aerodynamic chord (MAC) of a wing:

    \begin{equation*} {\rm MAC} = \overline{\overline{c}} = \frac{2 \displaystyle{\int_{0}^{s} c^2 dy}}{S} \end{equation*}


  • Aerodynamic coefficients for an airfoil section:
    • Lift coefficient, C_{l} = \displaystyle{\frac{L/ \mbox{unit~span}}{q_{\infty} c}} = \displaystyle{\frac{L'}{q_{\infty} c}}
    • Drag coefficient, C_{d} = \displaystyle{\frac{D/ \mbox{unit~span}}{q_{\infty} c}} = \displaystyle{\frac{D'}{q_{\infty} c}}
    • Moment coefficient at some point a, C_{m_{a}} = \displaystyle{\frac{M_a/ \mbox{unit~span}}{q_{\infty} c^2}} = \displaystyle{\frac{M'_a}{q_{\infty} c^2}}

  • Aerodynamic coefficients for a finite wing:
    • Lift coefficient, C_{L} = \displaystyle{\frac{L}{q_{\infty} S}}
    • Drag coefficient, C_{D} = \displaystyle{\frac{D}{q_{\infty} S}}
    • Moment coefficient about some point a, C_{M_{a}} = \displaystyle{\frac{M_a}{q_{\infty} S \overline{\overline{c}}}}

  • Induced drag coefficient of a finite wing:

    \begin{equation*} C_{D_{i}} = \frac{(1 + \delta) {C_L}^2}{\pi A\!R} = \frac{ {C_L}^2}{\pi \, A\!R \, e} \end{equation*}


  • Drag coefficient for an airplane:

    \begin{equation*} C_D = C_{D_{0}} + \frac{ {C_L}^2}{\pi \, A\!R \, e} = C_{D_{0}} + K {C_L}^2 \end{equation*}


  • General equations of motion of an airplane:

    \begin{eqnarray*} \mbox{$\parallel$ to flight path:} \quad && \hspace*{-7mm} \left(\frac{W}{g}\right) \frac{d V_{\infty}}{dt} = T \cos \epsilon - D - W \sin \theta \\[18pt] \mbox{$\perp$ to flight path:} \quad && \hspace*{-7mm} \left(\frac{W}{g}\right) \frac{V_{\infty}^2}{r_1} = L \cos \phi + T \sin \epsilon \cos \phi - W \cos \theta \\[18pt] \mbox{Horizontal plane:} \quad && \hspace*{-7mm} \left(\frac{W}{g}\right) \frac{(V_{\infty} \cos \theta)^2}{r_2} = L \sin \phi + T \sin \epsilon \sin \phi \end{eqnarray*}


  • Breguet endurance and range equations for a propeller aircraft:

    \[ E = \frac{ \eta_p}{{\rm BSFC}} \left(\frac{C_L^{3/2}}{C_D}\right) \sqrt{2 \rho S} \left( \frac{1}{\sqrt{W_0 - W_f}} - \frac{1}{\sqrt{W_0} } \right) \]

    \[ R = \frac{\eta_p}{{\rm BSFC}} \left( \frac{C_L}{C_D}\right) \ln \left( \frac{W_0}{W_0-W_f}\right) \]


  • Breguet endurance and range equations for a jet aircraft:

    \[ E = \frac{1}{{\rm TSFC}} \left( \frac{C_L}{C_D} \right) \ln \left(\frac{W_0}{W_0 - W_f} \right) \]

    \[ R = \frac{2}{{\rm TSFC}} \sqrt{ \frac{2}{\rho S}} \left(\frac{C_L^{1/2}}{C_D} \right) \left( W_0^{1/2} - W_1^{1/2} \right) \]


  • Equivalent exit velocity of a rocket in terms of specific impulse:

    \[ V_{\rm eq} = I_{\rm sp} \, g_0 \]


  • Thrust equation for rocket:

    \[ T = \dot{m} \, V_{\rm eq} \]


  • Rocket equation (with gravity term):

    \[ \Delta V = V_{\rm eq} \ln \left( \frac{M_0}{M_b} \right) - g_0 t_b \]


  • Conversion factors:
    • 1 knot = 1.68781 ft/s
    • knot = 0.51444 m/s
    • 1 mph = 0.44704 m/s
    • 1 mph = 1.46668 ft/s
    • 1 ft = 0.3048 m
    • 1 statute mile = 5,280 ft
    • 1 nautical mile = 6,076 ft
    • 1 hp = 550 lb ft s^{-1}

  • Table of ISA Properties at MSL:
Property Symbol SI units USC units
Pressure p_0 1.01325\times10^{5} N m^{-2} 2116.4 lb ft^{-2}
Density \rho_0 1.225 kg m^{-3} 0.002378 slugs ft^{-3}
Temperature T_0 15.0^{\circ}C = 288.15 K 59.0^{\circ}F  = 518.67 R
Dynamic viscosity \mu_0  1.789\times10^{-5} kg m^{-1} s^{-1} 3.737\times10^{-7}slug ft^{-1} s^{-1}
Speed of sound a_0 340.3 m s^{-1} 1116.47 ft s^{-1}
Gas constant R 287.057 J kg^{-1} K^{-1} 1716.59 ft lb slug^{-1}R^{-1}
  • Moody chart for the estimation of the Darcy-Weisbach friction factor:
Click on graph for larger size.

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