Question

The speed of flow past the lower surface of a wing of an aeroplane is $50\, ms ^{-1}$. What speed of flow over the upper surface will give a dynamic lift of $1000\, Pa$ ? Density of air $=1.3\, kg\, m ^{-3}$.

• A. $25.55\, m / s$
• B. $63.55\, m / s$
• C. $13.25\, m / s$
• D. None

Answer 1

Answer: (B)

Applying Bernoulli's theorem to the lower and upper surface of a wing
$P_{1}+\frac{1}{2} \rho v_{1}^{2}=P_{2}+\frac{1}{2} \rho v_{2}^{2} $
or $P_{1}-P_{2}=\frac{1}{2} \rho\left(v_{2}^{2}-v_{1}^{2}\right)$
$\Rightarrow 1000=\frac{1}{2} \times 1.30\left(v_{2}^{2}-50^{2}\right) \left[\because \text{lift} =P_{1}-P_{2}\right]$
or $v_{2}^{2}-2500=2000 / 1.3$
or $v_{2}=63.55\, m / s$