Forced transition as a design and robustness variable for the lifting surfaces of solar stratospheric platforms

Abstract Forcing boundary-layer transition is the standard remedy for the laminar separation bubble that dominates the drag of low-Reynolds lifting surfaces, and on a solar stratospheric platform it is normally discussed for the wing. Here the forced-transition location is treated as an explicit design variable inside a coupled environment–sizing–mission closure, the trip is charged for its own excrescence drag, and the result is resolved by component. Section polars for five low-Reynolds airfoils are generated over chord Reynolds numbers \(\:3\times\:{10}^{4}\le\:R{e}_{c}\le\:{10}^{6}\) with the transition location and the amplification factor \(\:{n}_{\text{c}\text{r}\text{i}\text{t}}\) as free axes; the trip height is sized from the roughness Reynolds number criterion. Three results follow. First, charging the trip for its own drag moves the profitability threshold from \(\:R{e}_{c}\approx\:3.5\times\:{10}^{5}\) to \(\:\approx\:2.0\times\:{10}^{5}\). Second, the platform straddles that threshold: the wing, at \(\:R{e}_{c}=1.97\times\:{10}^{5}\), loses from a trip at every height considered, while the empennage, at \(\:1.08\times\:{10}^{5}\), gains throughout — tripping both surfaces costs 1.8% in gross mass, the empennage alone saves 4.6%, the wing alone costs 6.6%. Third, the benefit grows as the disturbance environment quiets, from 1.5% at \(\:{n}_{\text{c}\text{r}\text{i}\text{t}}=7\) to 8.7% at \(\:{n}_{\text{c}\text{r}\text{i}\text{t}}=13\), so transition control is worth most precisely where transition prediction is least verifiable; against wind-tunnel data, tripping reduces the sensitivity of predicted drag to the amplification factor eighteen-fold. The design rule is therefore component-wise, not chordwise: trip the surfaces below the bubble threshold and leave those above it clean.