What are reciprocating engines for aircraft called heat engines?

Your question isn't clear. Are you asking WHY reciprocating engines for aircraft are called heat engines, or are you asking whether or not they are considered heat engines? Or are you asking something else entirely? Heat engines include many types of engines, and probably all of them. The term simply describes any type of engine that uses heat to generate power.

Reciprocating engines for aircraft are simply piston-driven (reciprocating) internal combustion engines. They usually burn higher octane low-lead gasoline. The higher octane helps the engines burn fuel more efficiently, which helps high up in the air where the varying oxygen content and turbulence may diminish the less-efficient lower-octane fuels.

Not all aircraft use reciprocating engines. Some other aircraft engine types are jet engines and turbo-prop engines. Both of these run on jet fuel. They differ in that jet engines completely eliminate props (propellers) and work by creating thrust from pressurized burning fuel. Turbo props are a sort of hybrid, where jet fuel powers the props, which in turn create movement in the same way that reciprocating engines do.