Slipstream Measurements of Small-Scale Propellers at Low Reynolds Numbers

The continuing growth in the use of small UAVs has required the need to more fully understand the propellers that power them. Part of this understanding is the behavior of the propeller slipstream. Using a 7-hole probe, the slipstreams of several small-scale propellers (diameters of 4.2, 5, and 9 in) were measured in both static (V∞ = 0) and advancing-flow (V∞ \u3e 0) conditions at several locations downstream. For static conditions, as the slipstream expanded downstream, the maximum values of the axial and swirl velocities decreased. The general shape of the static slipstream was also found to be nearly the same for the propellers even though their planforms were different. During advancing-flow conditions, a contraction in the slipstream occurred by 0.5 diameters behind the propeller. Beyond that location, the size of the slipstream was relatively constant up to 3 diameters downstream (furthest distance measured). For advancing-flow slipstreams, the shape of the axial velocity distribution was observed to be dependent on the planform shape of the propeller. The static slipstream of a propeller-wing configuration showed that the slipstream portions above and below the wing moved away from each other towards opposite wing tips. However, the maximum axial and swirl velocities in the propeller-wing slipstream did not diminish compared with the isolated propeller slipstream