Add a whistle to your warbird or fighter jet. Simulates the sound when the plane is diving (caused by air passing over the gun barrels). Similar in design to a Nerf football with whistle sound. Also had luck adding them in the exhaust stream of an EDF.
Needs 3/8 inch hole for mount in clean airflow (not behind an obstacle). Cylinder needs to be perpendicular to airflow, and only the head needs to protrude from the plane. Semicircle faces forward. Plenty loud enough for electric R/C plane. Pitch can be changed by changing the length of the cylindrical portion.
QTY 4 included