Why have propeller engines never been mounted on the tail in production transport aircraft?

Question

Recently, Embraer teased this concept.

Unlike jet engines, I have never heard of a production airliner or executive aircraft that mounted propellers on the tail like this.

From the source:

the new Embraer turboprop will have rear fuselage-mounted engines. This, it says, will provide a quiet cabin and allow jetbridge compatibility.

from the comments section

Well the nice thing about rear mounted engines, turbofan or turboprop is that it free's designers to be much more creative with wing design. The classic example of this is the VC-10, although a commercial flop with rear mounted quad engines had great aerodynamics

It will certainly be more efficient as it keeps the wing clean, not to mention whatever powerplant and propeller combination they use will be cutting edge

Why were these advantages not considered relevant earlier?

Answer 1

There are two really big issues with tail mounted powerplants with propellers; weight distribution and Foreign Object Damage.

Tail mounted engines move the empty C of G aft. This forces you to put the disposable load with a forward bias to compensate, hence the long forward fuselage necessary to put the loaded C of G in the required range. Of course, when passengers get off, the C of G moves aft, and to keep the tail from tipping back while the plane is unloaded, you have to move the gear farther aft than optimal for takeoff. The elevator surface has to be larger than it might normally be in order to lever the nose up on take off with the fulcrum of the tire contact point farther aft than it really needs to be. With so much fuselage forward of the loaded C of G, you might have to use larger stabilizing surfaces to have adequate tail volume for good yaw and pitch stability.

Jets with tail mounted engines live with these problems to gain the huge advantage they provide, the ability to have the fuselage close to the ground to make it easy to get in sitting on a ramp, an important feature for corporate airplanes. For an airliner that uses jet bridges for boarding, this feature isn't important.

Then there is the brutal environment for the poor propellers. Every bit of rain, slush, stones, rubber fragments, and whatever other garbage is on the runway will be going through the propellers, and they will get dinged up and eroded pretty fast (anyone that's owned a seaplane understands the mess that large volumes of water makes of propeller blades).

For an airplane with propellers, back at the tail is about the worst place you can put them if propeller service life is important.

Answer 2

You can make a case that this was the original way to build airplanes as the Wright Flyer had a forward elevator and rear-ish mounted props extending off the back of the wings. Other than that there have been many designs to incorporate rear propellors.

The Bede BD-5 has a rear prop. Burt rutan built various designs with rear mounted piston engines/props. The Piaggio P.180 Avanti is pretty close to what you describe. The Otto aviation 500 has a rear prop but is only now being tested. The Dornier Do 335 had both a front and back prop

Answer 3

As stated in comments and Dave's answer, it has been done, but I'm guessing you were thinking about mass produced (successful) commercial passenger planes. Here are a couple of reasons:

The propellers on a passenger plane need to be quite big, so they would, for clearance reasons, need quite long and thus heavy pylons. Weight in the tail of a plane is generally not a good thing. The more modern fan-like propellers are smaller, and this somewhat mitigates the pylon problem.

Mounting propeller engines on wings is easy, as clearance from fuselage is not a problem, no extra structures are needed and the engine nacelle/fairing often provides a convinient space for landing gears. One advantage also is, that you have "blown flaps", but I think you are correct in that having undisturbed airflow over the wing would be beneficial.

Answer 4

https://en.wikipedia.org/wiki/Propfan#1970s%E2%80%931980s

That Wikipedia page has a good analysis of propfans a.k.a. unducted fans. See the MD80 testbed and reference to various designs (e.g. Boeing 7J7 slated to be the 737 replacement) with rear-mounted propfans. While not coming to fruition evidently it was considered to be a possible alternative to underwing engines (ducted or unducted).

Answer 5

Fuel efficiency is very important for transport bottom line profit, which is why wing mounts are generally seen. A number of ex-passenger liners such as the Boeing 767 are re-purposed as cargo converts.

The Embraer concept moves them back for cabin noise mitigation, but rear mounts give back a little efficiency because a father rearward CG shortens the tail torque level arm, therefor requiring larger tail volume.

For cargo carriers, the best place for the turboprop pylons would be near the nose of the aircraft (if the pilots could stand the noise), a bit like a hammerhead shark, or on the wings. The nose mount would again require a larger horizontal stabilizer (because of increased area ahead of CG). Interestingly, the rear mounts do increase stability a bit. (Aerodynamic effects cancel CG effects for tail sizing). One can see why engine nacelles have been generally mounted on the wings, closer to CG, for a long time.

Also of note, the Embraer design features a biplane horizontal stabilizer, why not mid-mount a combined pylon/stabilizer for better aerodynamic efficiency. This would also alleviate debris damage and rotation issues with the props.

Over many flights, fuel cost is the deciding factor in design for cargo aircraft.

Answer 6

Since mounting the engines up high makes them more difficult for mechanics to service and inspect, you would want to have a good reason for putting them there. It sounds like Embraer's reason is sound (and that fact that their entire line and assembly and design process is based on aft-mounted engines).

I cannot cite any evidence for this hearsay claim, but... years ago I read the comment from a former Boeing employee who said that when they inspected the Boeing 727-100 that was used in 1986 as a test bed for GE's UDF (un-ducted fan), they found that the vibration from the UDF had damaged the structure of the aircraft.

When comparing wing-mounted prop engines to aft fuselage-mounted engines it occurs to me that you have to run fuel lines from the wing tanks to the aft fuselage (weight) and, in the event of a prop failure/separation there is a higher probability of the prop penetrating the fuselage, and possibly even the fuel line. Not to mention the (remote) possibility of a failed prop destroying the rudder, tail, elevators or horizontal stabilizer.

Of course catastrophic prop failures are extremely rare (or less), and (to my knowledge) have never once been a problem for aircraft like the Beech Starship or Piaggio Avanti.