Upon the leading edges of the Junkers Ju87 Stuka's faired main gear legs were mounted the Jericho-Trompete ("Jericho Trumpet") wailing sirens, becoming the propaganda symbol of German air power and the blitzkrieg victories of 1939–1942.
The siren is electrically engaged from the cockpit. The small prop spins in flight anyway but the actual siren engaged by the pilot electrically (solonoid contact) from the cockpit. There's a "brake" or more closely, a clutch, that keeps the siren mechanism from sounding until the drive brakes are released. As soon as the drive brakes are retracted, the Jerico Trumpet disengages. Sometimes the brake didn't reset, and the pilot had to listen to the thing all the way back to base.
Quote from Stuka-Pilot Hans-Ulrich Rudel: His Life Story in Words in Photographs
"The screaming siren of the Junkers Ju 87 dive-bomber was, at least for the early part of the war, synonymous with German blitzkrieg tactics and Nazi prowess. Highly popular with the Propaganda Ministry, the Stukas represented the peak of German military might. It was usually deployed before the army's attack and would soften any known defensive strong points. In the heat of the battle, however, army commanders would radio the Luftwaffe to send Stukas to destroy any unexpected opposition. When air superiority was obtained, the seagull-looking Stukas would harass retreating or trapped Allied troops. The plane was used extensively and almost exclusively to support army operations. The tight and smooth cooperation and communication between army and Luftwaffe commanders were the key elements in the successful campaigns in 1940. The Ju 87 was slow and had a limited range, but it was a surprisingly accurate bomber. The bombs were basically controlled by the pilot until the very last moment. Usually a wing of 30 Stukas would attack a target in 10 groups of three planes. The common practice of a pilot was to begin his dive at 15,000ft at an angle of 60o-90o, turn on the cardboard siren, and dive earthward at up to 350mph. He would drop the bomb at around 2,000ft, and then immediately pull up the nose and overcome a force of four G to resume level flight. The Ju 87B was armed with two forward firing machine-guns, one machine gun in the rear cockpit for defense, four 110lb-bombs under the wings, and one 551lb-bomb under the fuselage in a clutch."
These photos clearly show these wailing sirens.




Photo source: ecpad website.