They are different variations or updates of the same aircraft design. The original Cessna 172 dates back to 1955. The 172N was introduced in 1977, while the 172S was introduced in 1998, with a slightly more powerful engine and a bit more weight carrying capacity. The 172S is also more likely to carry modern avionics like the Garmin G1000 system for navigation.
almost 60,000
yes there are 172s and the grand caravans
'bout 30 Litres/hour (8 gallons)
A Cessna 172s direction would change using it's ailerons or it's rudder on the tail. These would be controlled using the yoke or pedals in the cockpit.
Cessna 172s hold 56 gallons of fuel. At Felts Field in Spokane, 100LL aviation gasoline is $6.27 per gallon as I write, so it would cost $351.12 to take the plane from completely empty tanks to completely full ones.
You are correct in that it is the exact same engine, the Lycoming IO360-L2A as you mentioned. The difference is in the propeller, namely the propeller RPM limits. I don't have the numbers in front of me but prop on the 172S is rated for higher RPM, and therefore 20 more BHP, and as a result, an extra 100 lbs of useful load. Additionally, there is a conversion (STC) available to swap out the prop on the R model with the S model prop (most often done at prop overhaul time) giving it the extra horsepower and useful load of the S model.
That depends on the model and year of the aircraft, and what equipment is installed. Cessna started building 172s in 1955, and has built more than 60,000 individual models of the airplane. Different engines, different radios & navigation equipment, different interiors all effect the weight of an individual airplane. The AVERAGE empty weight (no fuel in tanks) of a Cessna 172R, built from 1996 to 1998, is 1691 pounds.
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"Aftermarket" means a part was made by someone besides the part's original manufacturer and sold by someone outside the manufacturer's distribution channel. Like on your car: if you have a Ford and you need an oil filter, you could buy a Motorcraft filter from a Ford dealer, or you could buy a Fram from Autozone--probably for about the same price, because the parts department manager at the dealership likes selling oil filters and he knows he ain't gonna if they're too high-priced. Now here's the thing: unless you've got a really old airplane whose manufacturer is out of business, there aren't any true aftermarket aircraft spare parts. There's no money in it. Consider the Cessna 172. According to Wikipedia, they have made more Cessna 172s than they have any other model of aircraft--over 43,000 specimens. They still make them. In aviation, 43,000 units is a runaway success but compared to cars it's terrible: it's less than one year's production of the AMC Gremlin, which they should have named the AMC Beta--as in "beta than walking, but not by much." Oh, don't get me wrong--you can go to companies besides the Cessna dealer to buy your parts, but with very few exceptions they're made by the same company Cessna bought them from in the first place.