Can a pusher configuration sort of like this have its propeller close to the tail (if it were one of those split tails like on a A-10)?
Why did they put the propeller above the cabin and not in the tail?
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1$\begingroup$ A heavy, vibrating engine in the tail is a bad idea. Any frequency neighbourhood will lead to flutter. It is better to place it on a dedicated post. Also, a propeller in the tail stabilizes the aircraft, so it will need bigger control surfaces. $\endgroup$– Peter KämpfJul 8, 2017 at 21:24
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$\begingroup$ Related: aviation.stackexchange.com/questions/30146/… $\endgroup$– Peter KämpfJul 8, 2017 at 21:52
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$\begingroup$ @PeterKämpf - "... a propeller in the tail stabilizes the aircraft ..." did you mean "destabilizes"? $\endgroup$– Kevin FeganJul 9, 2017 at 5:47
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$\begingroup$ @KevinFegan: No, it stabilizes the aircraft. For more, I recommend this NACA report titled "propellers in pitch and yaw" which should prove the point. Or look here. $\endgroup$– Peter KämpfJul 9, 2017 at 6:27
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4 Answers
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Yeah good question. They need it to be out of the way of the spray, so high up, and they position it on top of something that looks like a vertical stabiliser, so why not use the vertical stabiliser, I presume is what you mean. Like this one, but then in a pusher comfiguration
My answer would be that that would position the centre of gravity too far back. The engine is heavy, it's best if it is mounted close to the centre of the plane. The combination of pusher prop and tail mounting would make for a configuration that wants to tip backwards. Placing the wings further back is awkward, it's a small plane and leaves little moment arm for the tail surfaces to work effectively.
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1$\begingroup$ Can you identify that aircraft please? I'm fascinated by it. $\endgroup$– CriggieJul 8, 2017 at 5:41
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3$\begingroup$ @Criggie - Google reverse search says it's a A-90 Orlyonok. $\endgroup$ Jul 8, 2017 at 7:10
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1$\begingroup$ Engine weight produces torque raising the nose, but placing the propeller high produces torque raising the tail. Therefore, combining high and back seems to make sense. $\endgroup$– PereJul 8, 2017 at 8:41
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2$\begingroup$ @Criggie They are fascinating planes aren't they, some of the ekranoplans are just pure science fiction. $\endgroup$– KoyovisJul 8, 2017 at 11:35
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$\begingroup$ @Pere yes get the total cog including engine somewhe around mid frame, and account for nose down moment with increased throttle. Good for anti-stall. $\endgroup$– KoyovisJul 8, 2017 at 11:38
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The heaviest single item in an aircraft is the engine. If you were to move the engine to the tail and make no other changes to this design the aircraft's centre of gravity would move so far aft that the aircraft would be unflyable.
For a stable configuration the centre of gravity needs to be close to the centre of lift. We could achieve that by moving the wing aft as well, so that the weight of the fuselage ahead of the centre of lift balances the weight of the engine behind.
Now, though, you have almost no pitch control from the elevators. We can counter that by moving them forward (where they're called a canard) and now you have this design:
Rutan Vari-eze
Source Wikipedia
However, your aircraft also has to be a boat. Placing a large weight at one end of a boat will make its handling problematic. Better to place the heavy weight in the centre of a boat, and we're back to the original design.
The engine is placed high to keep it clear of the water, and away from possible spray during landing and take-off.
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2$\begingroup$ Heavy weight in the back is preferable for a planing boat. Speedboats have no problem with rear mounted engines. $\endgroup$– KoyovisJul 8, 2017 at 0:09
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2$\begingroup$ Yes, but you're designing a part-time boat - the rest of the time it's an aircraft. And aircraft are awfully picky about the position of their center of gravity relative to the wing. Put the engine all the way in the back and you're going to have to move the wing all the way to the back as well. $\endgroup$ Jul 8, 2017 at 2:37
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2$\begingroup$ +1 for pointing out its a boat, that enters and exits the water at speedboat velocities. $\endgroup$– CriggieJul 8, 2017 at 5:39
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$\begingroup$ Also, if you had a puller in that configuration, the pylon would need to be taller, or the prop would hit the cabin. $\endgroup$– jamesqfJul 8, 2017 at 16:40
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At first sight, placing the propeller in the tail looks attractive, but there have been very few designs which put that into practice. The best known is probably the Trislander derivative of the venerable Britten-Norman Islander utility aircraft.
Britten-Norman Trislander (picture source)
Disadvantages of a tail-mounted propeller engine
To place a heavy mass, part of which is even rotating in a wide band of frequencies, on a long, flexible beam is generally not a good idea.
- The engine mass will reduce the eigenfrequency of the vertical tail, inviting early flutter, and any imbalance in the propeller will provide the initial excitation which flutter needs to develop.
- Placing heavy masses away from the center of gravity increases the moments of inertia and makes the handling of the aircraft needlessly sluggish.
- Doing so in two axes invites a phenomenon known as inertia coupling.
- Next, a propeller acts like a tail surface. Placing it far aft will stabilize the aircraft and make it harder to control; generally such a propeller location must be combined with larger tail surfaces.
- Placing the propeller high above the center of gravity will also result in trim changes whenever power is changed. This is less than welcome if good handling characteristics are desired
Only seaplanes accept a high propeller location to make sure that spray will not hit the propeller. On the Dornier Do-26 the rear propellers could even be jacked up for take-off and landing.
Existing Designs
Recently, however, a few special airplanes have chosen a tail-mounted engine. The common factors were:
- electric propulsion, so vibrations were low
- low power loading, so the stabilisation and trim effects of the propeller were small
- high aerodynamic and structural efficiency, so the extra post and nacelle for the engine were deemed undesirable.
You see two of them together below: Right is the Icaré II, the first practical solar aircraft, and left is the e-Genius, a battery-powered experimental aircraft.
Icaré 2 and e-Genius in formation flight (picture source)
Given the many disadvantages, designers generally try to avoid a rear propeller location. Pushers give better efficiency, so some tried to avoid a conventional layout. Below you see a compromise, where the propeller is mounted aft of the wing, but still close to the center of gravity. The position on the Lake Buccaneer is another such compromise and a good choice for an amphibian. The cruciform tail is even better than a twin tail because the propeller blast will hit all control surfaces, enabling much better low-speed handling on water.
HB-23 Hobbyliner in flight (picture source)
Note that I never mentioned the rear center of gravity location. This is really not much of a factor in this case.
answered Jul 8, 2017 at 22:42
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$\begingroup$ Note that I never mentioned the rear center of gravity location. This is really not much of a factor in this case. Not with the planes with the long tail arm, or with the one with three engines in total. Mounting the engine on a vertical tail-like support causes flutter issues as well. The aircraft in question is short, stubby, light, and takes off from water. Good broad info though as usual. +1'd, working on the Sportsmanship medal which not many people have :) $\endgroup$– KoyovisJul 9, 2017 at 2:28
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1$\begingroup$ @Koyovis: I upvoted many of your answers, but this time I simply was not happy with it. This is no competition, but a quest to answer other peoples' question as good as possible. Moving the wing a little back on the Buccaneer would clear the cabin area of the spar and reduce the tail lever arm only slightly, so it is not a bad solution. The real drawbacks are in handling and dynamic response. $\endgroup$ Jul 9, 2017 at 5:10
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$\begingroup$ Yes no worries, the info in your answers is always worthwhile. I take the question as why a pusher propellor is not mounted on top of the vertical tail - in that case the engine would stick out at the back, not in front of the tail. Plus I reckon that the engine might be a larger fraction of weight of a light plane. $\endgroup$– KoyovisJul 9, 2017 at 8:09
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Several reasons. The tail's height above water depends on the pitch up or down of the aircraft but you don't want the propeller too close to the water(!). You also don't want water spray hitting the propeller (the very high tip speed can quickly be eroded by hitting water spray) so keeping it closer to the nose and higher prevents that. Putting it near the wing means its weight and other forces are transferred more directly to the aircraft's strongest structure, the wing spars, keeping structural (and overall) weight down. The propeller is also aimed at the tail, giving it more control authority at low speeds (near stall, takeoff and landing, etc.).
Some electric aircraft designs do mount the propeller at the top of the vertical stabilizer as they have a lower weight motor instead of a heavy engine and a larger diameter propeller that needs to clear the fuselage and ground. In such aircraft, the heaviest item is not the engine (i.e. motor) but the battery pack which is usually mounted near the wing spar (often in the wings). It also allows the landing gear to be relatively short / light weight / low drag.
