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I'm wondering what advantage/disadvantage, if any, does mounting an engine above the fuselage give? This question comes up after seeing the Macchi M.33 below: A Black-and-White image of an Italian Macchi M.33 floating on the water using its buoyant fuselage, with the engine visibly mounted above the fuselage on struts.

The engine mounted in such a way does look interesting, but I'm wondering if it might make the plane want to dip down slightly. Any help would be appreciated.

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    $\begingroup$ I suppose you mean, other than the obvious one of keeping the propeller out the water and spray? $\endgroup$ May 28 at 16:29
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    $\begingroup$ Clearly, yes. But I'm talking about benefits to flight. $\endgroup$
    – Ginger
    May 28 at 16:32
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    $\begingroup$ Given that you only see this configuration on seaplanes you may want to ask what the drawbacks are. $\endgroup$
    – GdD
    May 28 at 18:22
  • $\begingroup$ @GdD The Boeing 727 wasn't a seaplane, and it had one of its engines in a similar position. $\endgroup$
    – nick012000
    May 29 at 3:59
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    $\begingroup$ Not really @nick012000, on the 727 it was far closer to axis of the airplane. $\endgroup$
    – GdD
    May 29 at 11:32
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It's strictly to stay clear of spray in seaplanes, or because the engine was tacked on as an afterthought for some reason (like a motorglider conversion) and that was the only place to put it.

There aren't any aerodynamic benefits, or you would see the configuration on landplanes.

You've increased the frontal area by having a big pod sticking out. The high thrust line wants to pitch the airplane over, increasing tail trimming forces required to compensate, and hence drag. There there are the undesirable handling qualities, mainly a strong pitch down tendency when you add power quickly, and worse, a pitch up tendency when you remove power quickly, which can get the unawares into quite a lot of trouble.

So to summarize, sticking an engine there is only done when there's nowhere else to put it.

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  • $\begingroup$ "There aren't any aerodynamic benefits, or you would see the configuration on landplanes." The Boeing 727 has one engine like this. $\endgroup$
    – nick012000
    May 29 at 4:00
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    $\begingroup$ No the engine is in the tail cone. The intake is an S duct. The DC-10 is closer to your point, but the engine is right on top of the fuselage, not sticking way up on a pylon in order to clear a propeller. Irrelevant for the purpose of the question. $\endgroup$
    – John K
    May 29 at 4:23
  • $\begingroup$ It's still mounted above the fuselage, which is what the question's about. It's not as far above the fuselage, proportionally, but it's still above it. $\endgroup$
    – nick012000
    May 29 at 4:24
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    $\begingroup$ “There aren't any aerodynamic benefits, or you would see the configuration on landplanes.“ Non-sequitur. It could conceivable be that there are benefits, but only small ones that are outweighed by disadvantages of safety, maintenance etc.. $\endgroup$ May 29 at 10:02
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    $\begingroup$ @nick012000: WRT the 727, DC-10, and similar, the problem that tail-mounted engine is addressing is where the heck are you supposed to stick a 3rd engine? Regulations meant you had to have at least 3 for over-water flights, 4 got you into size & cost territory. So you traded off the disadvantages of things like S-ducts and more complicated tail structure for the advantages of the 3rd engine. $\endgroup$
    – jamesqf
    May 29 at 17:31
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Many light-sport airplanes and ultralight airplanes have a high-wing configuration with a high-mounted pusher engine behind the pilot. Here the engine is above most or all of what one would normally define as the "fuselage", including the tail boom. One advantage of this configuration is that it gives the pilot a more unobstructed view in the forward direction.

Examples:

Challenger ultralight

Bailey-Moyes Dragonfly

Sportflight Talon

MX Quicksilver

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    $\begingroup$ These configurations have the wings as main drag source in line with thrust, and hence avoid the the pitch-down torque of the M.33 in the picture. Plus, design criteria in Ultralights are really different to "normal planes" (pilot positioning affects cg a lot, focus on forgiving flight behavior over aerodynamic efficiency .. ) $\endgroup$ May 29 at 15:40
  • $\begingroup$ Again, these aircraft have engines SLIGHTLY high, not way up on a pylon. $\endgroup$
    – John K
    May 31 at 0:26
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The main purpose is, of course, avoid that the propeller hits the water (or ground).

In the other hand the thrust is applied above the CG point of the aircraft. This introduce a considerable moment arm that, translated in manovrability, means an unwished tendence of the plane to dive when the power is suddenly increased.

So we add a little pitch angle up on the engine pod. Because the prop is forward of the CG, this helps counter-act the nose down torque of the high mount when thrust is applied.

The advantage is to counter-act the tendency of a plane to climb when thrust is applied. Many aircraft have down thrust angle even when the motor is mounted in front of the fuselage near the center of gravity, rather than above it.

In design, this plane is essentially a low wing PBY Catalina. Amazingly, it was bested in the 1925 Schneider Cup races by two biplanes: the Gloster IIIA and the Curtiss RC3-2 (flown by none other than Jimmy Dolittle). It could not be raced at full throttle due to wing flutter issues.

Macchi would go on to set a speed record for all piston aircraft with the M.C. 72 at 440 mph in 1934, a record that would stand for 5 years.

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No need for the usual floats.

Floats obviously add both weight and drag so it is beneficial to fly without them. With the fuselage sailing like a ship, they are not required. There are some under the wings but tiny. They do not support the full weight of the aircraft.

But floats also keep the aircraft in expected orientation, raised above the water, same as it would be raised above the ground. Without them, the traditional place for the engine is just too low.

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