Question

If the speed of air flow past the lower wing of and airplane is 350 ft/s, what speed of flow over the upper wing will produce a lift of 20 lb/ft^2?

Answer 1

Step-by-step explanation:

We shall apply Bernouli's theorem

ρgh + 1/2 ρ v² + P = constant

ρ is density of fluid ( here it is air ) v is velocity of air , P is pressure of air

For h to be constant

1/2 ρ v² + P = constant

1/2 ρ₁ v₁² + P₁ = 1/2 ρ₂ v₂² + P₂

P₂ -P₁ = 1/2 ρ₁ v₁² - 1/2 ρ₂ v₂²

P₂ -P₁ = Pressure difference = 20 x 32

ρ₁ and ρ₂ are same here and is equal to .07967 lb/ft³

Putting the values we get

20 x 32 = 1/2 x .07967 ( v₁² -v₂² )

( v₁² -v₂² ) = 16066

v₂ = 350 ft/s

v₁²-350² = 16066

122500 + 16066 = v₁²

v₁² = 138566

v₁ = 372 ft/s

[ At lower wing velocity will be lower so that pressure will be higher.]