Gunsight, K-13 Compensating Sight TBM Avenger indicating RPM for the rotor

indicating RPM for the rotor on the inner scale and rotor for the outer scale with a ratio of 9

Used in post-WWII 4 engine aircraft such as B-50

000 inches

This is a Machmeter

but also on the US Army Air Corps' UC-43 Beech Staggerwing (see photo from flight manual dated 1944)

Gunsight, K-13 Compensating Sight TBM Avenger indicating RPM for the rotorThe K 13 Compensating Gunsight, made by Sperry, is a electrically powered gunsight designed to compensate for altitude and airspeed. Typically mounted on the rear housing of a 50 cal machine gun from a side waist gun position in the aircraft. Used in Boeing B 17 Flying Fortress and Consolidated B 24 Liberator bombers during WW2. It was a late war innovation that used the aircraft altitude and speed with the direct correlation of where the weapon was