Laminar flow airfoils were developed in the 1930s by shaping to maintain a favourable pressure gradient to prevent them becoming turbulent. Their low-drag wind tunnel results led to them being used on aircraft such as the P-51 and B-24 but maintaining laminar flow required low levels of surface roughness and waviness not routinely found in service. Krag states that tests on the P-51 airfoil done in the high speed DVL wind tunnel in Berlin showed the laminar flow effect completely disappeared at real flight Reynolds numbers. Implementing laminar flow in high-Reynolds-number applications generally requires very smooth, wave-free surfaces, which can be difficult to produce and maintain.