Question

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

• A. $25.55m/s$
• B. $63.55m/s$
• C. $13.25m/s$
• 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 $v_2=63.55m/s$