5 ms·
> the range-optimal speed is where the parasitic drag is equal to the lift-induced drag Not quite true -- range-optimal speed is where the sum of those terms
by gnramires 6y ago
> the range-optimal speed is where the parasitic drag is equal to the lift-induced drag
Not quite true -- range-optimal speed is where the sum of those terms is minimal. With some assumptions, this is where the derivative is 0, dDrag/dv = 0, and since derivative is linear, this means: the range-optimal speed is where the (infinitesimal) increase of parasitic drag (with speed) is equal to the decrease of lift-induced drag (in other words, opposite derivatives).
- mliben 6y agoUsing the simple drag polar approach, D = A*v^2 + B/v^2 (D is total drag, first term is parasitic drag, second term is lift-induced drag) dD/dv = 0 where v = (B/A)^(1/4) Plug in v = (B/A)^(1/4) D = sqrt(AB) + sqrt(AB), aka dD/dv = 0 exactly when parasitic drag is equal to the lift-induced drag
- gnramires 6y agoI stand happily corrected :)