Quote:
Originally Posted by Graham Boak
Your basic equation. V2/V1 = SQRT(W2/W1) is actually inverted. the greater the weight, the greater the drag, the lower the speed. I assume this is a simple typo. The equation is a concatenation of three relationships.
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There seems to be a typo but the relation itself just shows that at given Cl (or AoA) the weight and speed relation stays constant. That also means that the formula can't be used to determine speed change in the case where the Cl changes.
The way I calculate the speed change is very basic stuff. I assume that at any steady flying condition drag equals thrust ie:
D = T
Drag being:
D = Cd * p * V^2 * 0,5 * A
Where Cd is drag coefficient, p is density, V is speed and A is reference area (wing area).
And Cd being:
Cd = Cd0 + Cdi
ie total drag coefficient is zero lift coefficient plus induced drag coefficient the later being:
Cdi = Cl^2 / (pii * AR * e)
where Cl is lift coefficient, AR aspect ratio and e efficiency factor. The lift coefficient is:
Cl = L / (A * 0,5 * r * V^2)
where L is lift force (9,81 * weight in this case using SI).
And the thrust is:
T = (n*W) / V
Where n is efficiency and W is engine power.
I use spreadsheets for iterations, in the Typhoon example I used Cd0 value 0,019, wing area 25,83m2, AR 6,2, 80% prop efficiency, e value 0,8, density 1,225 kg/m3 and weights 4800kg and 5300kg.