Why are multiple engines different?

Mxsmanic wrote:
Don't you adjust props and deal with landing gear in single-engine
aircraft, too? Or do I need a multiengine certification just to have
retractable gear??

depends. Basic trainers (single engine) have fixed landing gear,
fixed pitch props. It makes them cheaper and simpler for initial
training (there is enough already to worry about before adding
extra goodies); to add retractable gear / variable pitch props
you need a 'complex' endorsement; it is not a license or certificate
or rating; it consists in additional training from an instructor (see
14 CFR 61.31(e) for details) who then endorses the logbook, it's a
one time thing. There are similar endorsements required to fly
'high performance' aircraft (engine with more than 200hp),
tailwheels aircraft and for some high altitude operations.

Now a multi- can be complex or not (rare but it exists),
high performance or not (note that it is not the sum of the
power of the engines that count, whether or not it has any
engine with more than 200hp -- i.e., you could have an
aicraft with ten 200hp engines which would still not qualify
as 'high performance' :-) ), tailwheel or not, pressurised
or not, so do single engines. It is orthogonal if you
like.

--Sylvain