In a test experiment on a model aeroplane in wind tunnel, the flow...

Applying Bernoulli's principle on upper & lower surfaces, we get

${P}_{1} + \dfrac{1}{2}ρ{\left({v}_{1}\right)}^{2} + ρg{h}_{1} = {P}_{2} + \dfrac{1}{2}ρ{\left({v}_{2}\right)}^{2} + ρg{h}_{2}$

Since, height difference between upper and lower surface is negligible.

$⇒ {P}_{2} - {P}_{1} = \dfrac{1}{2}ρ{\left({v}_{1}\right)}^{2} - \dfrac{1}{2}ρ{\left({v}_{2}\right)}^{2}$

Therefore, upthrust acting due to pressure difference:

$⇒ \dfrac{F}{A} = \dfrac{1}{2}ρ\left({v}_{1}^{2} - {v}_{2}^{2}\right) ⇒ F = \dfrac{1}{2} \times 1 . 2 \times \left({70}^{2} - {65}^{2}\right) \times 2$

$= 810N$