I see on propeller-driven airplanes two main designs: Propeller facing forwars… or backwards (I don’t think that a propeller facing one of the sides of the airplane would be of much help… although the boats on Mississippi river have something like that… anyways).
Why do they have those “opposite” configurations? I saw even on big airplanes, such as the Convair NB-36H, engines facing the back of the airplane. This one is quite interesting because it has also jet engines. (did that happen on other planes?), but mostly on small planes. Why??
No, not really.
There are boats (‘sidewheelers’) that have the propellers (paddlewheels, actually) located on the sides, but they direct their propelling force forward (or backwards) along the river.
Small airplanes generally want the engine at the front for balance reasons. Else wise you have to balance the plane with passengers/pilot/luggage all of which are variable so you need to run numbers each flight. With an engine on the nose, the passengers and pilot sit very near the CG, so have far less effect on balance. Still might want to run a weight and balance if old Chumley and his lead brick collection will be in the back seat.
Even tricycle geared planes drag the tail often enough that tree is usually a skid there. Nose is safest place to put a prop and keep it off the ground.
Dirty (turbulent, or with air flowing different directions in the plane of the propellor) airflow ahead of the prop can induce horrible vibration, so mounting the prop at the front insures clean inflow.
For aerodynamic reasons, Lower speed aircraft wings are thickest at about 25-33% back from the leading edge. This makes it easier to put wing mounted engines on the front of the wing, and leaves the TE free for ailerons and flaps.
WAG: Jet/recip hybrids put the props on the back because jet engines are even fussier about needing clean intake airflow than propellors.
Prop-jet hybrids were one of those transitional things in the early part of the jet age due to limitations in engine power. IIRC, the jet engines have better high speed performance, but the old piston engines were better at lower speeds. There were even a few jet-prop fighters in Navy service (a product of the shorter takeoff runs on aircraft carriers; in contrast, land-based jets could take as long as they wanted to get up to speed).
For bombers and transports, adding some jet engines to the mix basically meant the plane could carry a bigger payload (bombs, fuel, or cargo). There were also some air-refueling tankers (including modified B-29 Superfortresses) that had jet engines added so they could keep up with the jet bombers they were refueling. Otherwise, their only option was for both planes to be in a shallow dive during refueling, with the bomber deploying their air brakes and flaps to slow themselves down while the tanker was in a full power dive.
Eventually, various types of engines became powerful enough to make the whole thing unnecessary (turboprops, turbofans, etc.)
Turboprops like that airbus are a whole different deal than the B-36 that had low bypass jet engines and reciprocating radial engines spinning propellors. "Four a’burnin’ an’ six a’turnin’ "
On further thought, the jets put out a rather narrow wash, that can be positioned pretty easy to miss the props. But props put out a huge wash, so it would be harder to keep the jet intakes out of it if the props were in front.