Originally Posted By: BrocLuno
Aircraft engines have somewhat looser clearances than modern automotive engines. They are more akin to air cooled motorcycle engines than to water cooled cars and trucks. Many, many are simple flat fours, twin magnetos and carburetors. Bigger and slicker planes get six's. Really slick planes get fuel injection.
A little plane like a PIPER CUB or a SUPER CUB will not have a pre-heater. May not even have a starter. May have to start it by pulling the prop through
The idea that 20W or 15W is thick is a BITOG thing. 20W and 15W are good to well below zero. The motor will fire and run fine. By the time you have completed ground check, and taxied to the runway, the engine is warm. It's also on full rich and manifold heat is on (warming intake charge off exhaust) so that the motor is ready to go full power.
Once airborne, you can lean it out manually, drop manifold heat and get the EGT (exhaust gas temp) you want with RPM and mixture controls.
The reason they turn slower than car engines is that the props are running at crankshaft speed and you don't want to over-speed the prop. It looses efficiency and gets very noisy. Four and five foot long two-blade props don't need a lot of RPM to have the tips exceed the sound barrier, and then they are not making any thrust ...
All airplanes are about airfoil efficiency (wing and prop). That's how they fly. They use larger displacements to make the torque to turn the prop at the required speed to maximize thrust. Cubic inches is your friend, RPM's are not. Reduction boxes that allow engines to rev are added weight. Cubic inches does not cost much weight. It's the better trade off
Great explanation, great post. Thanks.