Discussion
A propeller of \( 3.4 \ \mathrm{m} \) diameter turns at \( 1200 \ \mathrm{rpm} \). Its tip speed due to rotation is approximately
Answer: D
Correct answer: 214 m/s
The rotational tip speed is
\[ V_{tip} = \pi D N = \pi \times 3.4 \times \frac{1200}{60} = \pi \times 3.4 \times 20 \approx 214 \text{ m/s} \]The blade also meets the forward flight speed, so the resultant relative velocity at the tip is the vector sum of the two, and it is that resultant which must stay clear of compressibility effects. This is why propeller speed is limited and why a large diameter propeller must be geared down so far.
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