Of course, while moving the air over the object was similar to flying through the air, it wasn’t identical, so one of the first concepts Dorothy had to master was the Reynolds number, a bit of mathematical jujitsu that measured how closely the performance of a wind tunnel came to mimicking actual flight. Mastery of the Reynolds number, and using that knowledge to build wind tunnels that successfully simulated real-world conditions, was the key to the NACA’s success. Running the tunnels during the war presented yet another logistical challenge, as the local power company rationed electricity. The NACA nuts ran their giant turbines into the wee hours if necessary, engineers pressing the machines for answers to their research questions like night owls on the hunt for mice. Residents who lived near Langley complained about the sleep-disrupting roar of the tunnels.