In his 'Inside the cockpit - 'Messerschmitt Bf 109 E' video, Chris from the 'Military Aviation History' channel describes the dials on the instrument panel of the Deutsches Museum Emil. One small 'clock-like' indicator on the bottom right of the panel is marked "Luftschraube" - 'propeller'. All we see is a clock dial with no numerical markings but divided into hours and minutes. This simple instrument relates to engine management in a 109 by aiding the pilot in selecting propeller pitch. This is the propeller pitch control indicator on early Emils - in early Emils there was no automatic propeller pitch control.
Simply put, the pilot set the pitch manually - degrees of either 'fine' or 'coarse' according to the rpm - by selecting a 'clock time' - the "Luftschraube Uhrzeit " as seen on performance charts. The Luftwaffe fighter pilot would have to learn the rpm and associated 'clock' settings to manage the engine at its most efficient settings and avoid 'over-speeding' the engine.
Below; an extract from a wwiiaircraftperformance Bf 109 engine settings chart for 'Sperrflug' or 'patrol'. From left to right , 'Altitude', 'RPM' 'Manifold pressure' and 'Prop pitch time'. Note the low rpm settings for increased endurance (lower fuel consumption).
Note: the sentences under the 'Sperrflug' table state that when in the 'Sperrflug' regime 'switch off' (ausschalten) the autocontrol and adjust (einstellen) pitch either using the thumb control on the throttle or the lever on the instrument panel. This instruction of course refers to later Bf 109 variants with auto pitch control and/or earlier variants with retro-fitted auto pitch control. The power settings given in the table give very low fuel consumption, perhaps about 100 ltr/h.
The electric VDM system was produced to compete with the American Hamilton standard 'hydramatic' pitch propeller control system and was different from most other propeller hub and pitch control systems - the centre of the hub was hollow so that it could be fitted to engines with centrally mounted engine armament such as the DB 600 series of engines. This resulted in the need for the propeller pitch control system to be remotely located as opposed to being located in the hub as it was in most other systems.
Below; prop pitch control mechanism on the side of the DB 601 engine in the Emil.
Prop pitch control lever on an Emil instrument panel
In early 109s (including the Emil) the control 'switch' was a lever attached to the instrument panel but as it became clear that flying an aircraft in combat and having to move your hand off the throttle (or the stick) to change the prop pitch was not a good idea, a rocker thumb switch was added to the top of the throttle. From this point on all the pilot had to do was move his thumb up or down to increase or decrease the prop pitch.
A decrease in engine rpm means an increase in propeller pitch or coarsening the propeller blades and an increase in engine rpm means a decrease in propeller pitch and a 'fining' of the propeller blades. Thus there are no markings on the face of the 'Luftschraube 'clock'. It is read just like a clock - higher rpm means that the clock needs to rotate clockwise to its maximum reading of 12:30 which is fully fine on the prop pitch ( about 22 degrees). For lower rpm the clock should be rotated all the way round anti-clockwise to 4:30 which indicates the propeller is now fully coarse. (blade pitch about 70 degrees) ..
The "Bf 109 Mecanik" youtube channel explains how this worked in both early (DB 601) and later (DB 605) variants of the Bf 109 in this short video.