Drags of Fixed-wing UAVs

1. Force magnitude in order

• form drag>friction drag>Induced drag

2. Friction drag: boundary Layer Formation、Viscous Forces、Laminar and Turbulent Flow、Surface Roughness
3. form drag 由separation產生
4. 有vortex可以產生turbulence（因為氣流速度快，跟機翼前緣比，壓降少），減少form drag

golf ball turbulence
5. 低雷諾數比較不容易有turbulence(<500000)
6. Wing tips減少induced drag

Takeaway: form drag>friction drag>Induced drag

1. Lift to drag ratio L/D

• Induced drag is defined below:
  
  $$\begin{gathered} C_D=C_{D0}+k{C}_L^2 \\ {C}_D=C_{D0}+\frac{C_L^2}{\pi eAR} \\ {C}_D \end{gathered}$$

$$\begin{gathered} =0.173 \\ \frac{L}{D}=\frac{C_L}{C_D}=\frac{1.7}{0.173}=9.78 \end{gathered}$$
The definition of

$e$:

Takeaway: Drag is a function of lift、AR、AOA.

2. For a cambered wing, the minimum drag

$C_{Dmin}$ occurs at some positive lift

$C_{Lmin}$ drag.

3. Formdrag:

Viscous separation drag is also called "form drag" and depends upon the location of the separation point on the body.

4. Energy, Drag, and [[Lift]]

The energy required to produce these vortices is extracted from the wing as a drag force and is proportional to the square of the lift (see Sec. 12.6)

5. [[Laminar flow]] induces less drag

If an airfoil is designed to maintain a lot of laminar flow, it will have substantially less drag, as long as it is operating near its design lift coefficient.

Parasite Drag

$$C_{D_0}=C_{fe}\frac{S_{wet}}{S_{ref}}$$