# Iteration 2: Weight and Airfoil

## Contents

# Iteration 2: Weight and Airfoil#

```
from unyt import km, m, mm, g, kg, hr, s, degree, radian
import numpy as np
from math import pi as π
```

```
ρ = 1.225*kg/m**3 # air density
μinf = 1.81e-5*kg/(m*s) # viscosity of air
```

Goals:

Decrease aspect ratio

## Initial Design#

```
b = 2850*mm # wingspan
c = 230*mm # chord
```

```
S = b*c # wing area
AR = b**2/S
```

```
AR
```

```
unyt_quantity(12.39130435, '(dimensionless)')
```

Better, although I may have overdone it

## Weight Estimate#

Weight taken from Freecad model > Parts spreadsheet.

Freecad model not saved :(

```
W = 3700*g # weight
```

## Airfoil Selection#

See also: https://youtu.be/kAXN3MlQxxc

```
V = 45*km/hr # cruise speed
```

Randomly selected, see https://rcmodelhub.com/how-fast-do-rc-planes-go/#0-typical-speeds-of-popular-rc-planes-

```
CL = (2*W)/(ρ*V**2*S)
CL.to('s**2/m')
```

```
unyt_quantity(0.05897975, 's**2/m')
```

```
Re = (ρ*V*c)/μinf
round(Re.to_value(), -3)
```

```
195000.0
```

As per: http://airfoiltools.com/airfoil/details?airfoil=clarky-il

Clark Y chosen because:

No need for inverted flight

Long term goal is efficiency

Relatively easy foam manufacturing

From bottom left up: Blue = 50,000 Orange = 100,000 Green = 200,000

```
# rough estimations
α0 = -3.75*degree
Clα = 1.05
e = 0.8
```

```
CLα = Clα/(1+(Clα/(π*e*AR)))
CLα
```

```
unyt_quantity(1.01575315, '(dimensionless)')
```

```
CL_unitless = CL.to_value('s**2/m')
CL_unitless
```

```
0.05897974750540949
```

```
α = α0+((CL_unitless/CLα)*radian)
α
```

```
unyt_quantity(-0.42311827, 'degree')
```

### Notes#

I previously used a lift curve slope (Clα) of 0.083 (instead of 1.05), because I misread the Cl v Alpha graph. This resulted in an α of 0.667 or 38°. My conclusion was:

Oops, angle of attack for cruise is uncomfortably close to aerodynamic stall. It should ideally be about 5° (see Cl/Cd v Alpha below).

We can increase CL by:

Increasing air speed

Increasing wing area

Decreasing weight is not a possibility

As α is now well below 10° (the point of aerodynamic stall for the Clark Y), this is not really applicable anymore. In fact, I have since actually changed my weight estimate downward because I had a feeling I was overestimating the wing weight (3744 -> 2340).

Since the cruise angle of attack is now actually on the low side, I’ll lower the airspeed in the next iteration. This will hopefully get us closer to 5°. Not exactly 5, but at least something higher than 0.