Final answer:
On an engine with a fixed pitch propeller, the throttle control simultaneously adjusts both manifold pressure and r.p.m. As the throttle is opened or closed, it affects the air pressure within the intake manifold and changes the engine power output, which is reflected in the engine speed.
Step-by-step explanation:
On an engine equipped with a fixed pitch propeller, the throttle control adjusts both the manifold pressure and the revolutions per minute (r.p.m.). Manifold pressure refers to the air pressure within the intake manifold, which is directly affected by the position of the throttle valve. The r.p.m. is controlled by the engine power output which is influenced by the manifold pressure. Therefore, when the throttle is opened to increase power, both manifold pressure and r.p.m. increase.
It is important to note that with fixed pitch propellers, unlike constant speed propellers, the pilot does not have separate control over the propeller blade angle to regulate r.p.m. Therefore, engine r.p.m., and power output, are solely managed by the throttle position, which in turn influences both the engine's speed and the manifold pressure.