Editing Flat-four engine (section)

== Use in aircraft ==
[[File:ULPower UL260i.jpg|thumb|[[ULPower UL260i]] aircraft engine]]

[[Light aircraft]] commonly use flat-four engines with displacements up to {{convert|390|CID|L|1|abbr=on|order=flip}} from manufacturers such as [[Rotax]], [[Lycoming Engines]], [[Teledyne Continental Motors|Continental Motors]] and [[Franklin Engine Company]].

For [[radio-controlled aircraft]], flat-four engines with displacements of {{convert|40|-|50|cc|cuin|1|abbr=on}} are produced by companies such as [[O.S. Engines]].

A notable recent flat-four aero-engine is the [[Flathead engine|side-valve]] [[Belgium|Belgian]] [[D-Motor LF26]].<ref>A modular variant of the LF26 is the 4-litre [[flat-six]] [[D-Motor LF39]]</ref> Although the side-valve format has long been abandoned for most automotive applications because its combustion chamber is a bar to high engine [[rpm]], the  massively over-square (1.295:1) D-Motor is a very simple, low-revving, compact, reliable lightweight aero-engine (without the heavy complication of [[ohv]] valve-gear)<ref>Note: A low-revving  engine is eminently suitable to drive a propeller, which is typically most effective at about 2,000 rpm. If a valve in a side-valve engine should stick open, it cannot hit the piston and cause engine failure; the engine will continue with just that cylinder inoperative. This is an important safety feature for aero-engines.</ref>