Why are push-propellers so rare, yet they are still around?

Question

Jet engines are by their very nature push-engines, however, most propeller airplanes use pull-engines. Is there an inherent advantage to using pull-propellers except for the increased airflow over the fuselage and tail (with its rudder and elevator)?

Twins generally have their engines on the wings, and the tail is no longer directly behind it, does that mean the choice of a pull-engine is not as advantageous?

If there isn't an inherent disadvantage, why are pusher configurations so rare? If there is one, why do they exist at all? Disregarding designs where the choice is obvious, like powered parachutes where you simply don't want a propeller in your face.

The Convair B36 is one notable multi-engine aircraft with engines in pusher configuration, as is the Piaggio Avanti. And the Cessna Skymaster is a push/pull configuration (If you get a multi-engine rating in a Skymaster, your ticket will be limited to multi-engine aircraft with in-line thrust). Single engine aircraft are even more uncommon, and pretty much all I could find except the Lake Buccaneer are all kit-planes (e.g. Velocity, Rutan), ultralights (Quad City), military, or experimental.

Answer 1

There are many disadvantages, they seem to outweigh the advantages.

Here are two:

• A pusher prop is working in a disturbed airflow, causing increased vibration and noise
  If the propeller is fitted behind a wing, each propeller blade is passing through the separated boundary flow twice each rotation. These cycles create additional noise and lower the efficiency of the propeller. The vibration makes the propeller blades more susceptible to metal fatigue.

• Propeller clearance at takeoff
  Due to the pitch up at take-off, the propeller gets close to the ground. Therefore the diameter needs to be reduced (loss of efficiency) or the landing gear struts need to be made longer (added weight). Since the propeller is behind the landing gear, it is susceptible to debris kicked up from the gear, increasing the need for added blade protection (increased weight, loss of efficiency)

Wikipedia has a list of additional disadvantages.

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Edit:

Your statement that

Jet engines are by their very nature push-engines

is not entirely true.

In turbofan engines, most of the thrust is generated by the fan and compressor stages. Even in a pure jet engine, a lot of thrust is generated by the compressor. Therefore the shaft of a jet engine is tension loaded, just like a propeller shaft in puller configuration.

Source: Rolls Royce - The Jet Engine

Answer 2

The pusher design is more efficient, because the suction forward of the prop reduces flow separation, and the accelerated flow behind it is not streaming around the fuselage (or wing), where it would create additional friction drag. In case of the Do-335 (see picture below, source), the single-engine top speed was 30 km/h higher with the rear engine running than with the front engine (both were DB-603s with identical power rating).

On the other hand, the puller prop will help to maneuver the plane on the ground (this is a big benefit for taildraggers - note how many two-engined, taildragger airplanes have an H-tail (two rudders as endplates of the stabilizer). They were placed in the prop wake and this gave much better directional control at low speed on the ground. Also, the prop wash helps to increase the lift from flaps.

The main disadvantage for a single-engined aircraft, the reduced tail clearance, has already been mentioned. If you cannot really rotate, takeoffs and landings are high-speed affairs. More disadvantages are the pilot risking to hit the propeller in an emergency bail-out and increased stability of the aircraft. When developing the Saab J 21 (pictured below), Saab also developed their own ejection seats to allow safe egress.

A pusher propeller influences stability much as an additional tail, but without control surfaces. Especially for a fighter aircraft, this is the opposite of what you want. THAT is why almost all high-powered, singe-engined aircraft have their propeller in the front: Maneuverability!

The stabilizing effect increases in proportion to the propeller surface area and the thrust, of course. Since a regular airplane needs to have basic stability with the engine running at idle, any additional stability change due to propeller placement comes on top. At full power and with the long lever arm of a single pusher prop on a central fuselage (think LearAvia Learfan), the aircraft becomes stiff as a brick. A two-boom layout (think Saab J 21, pictured below, source) is better, but creates additional friction and interference drag, so the advantage of the pusher arrangement is reduced. Note, however, that studies of a tractor engine variant (Saab J 23) showed inferior performance to the pusher design.

If you want hard data on that: There is an old NACA report (NACA TN 2585) on this by John L. Crigler and Jean Gilman, called Propellers in Pitch and Yaw.

Answer 3

Having been reading up on 1930s and 1940s aircraft, I can think of a few major problems that dogged the design back then.

Pushers had advantages of pushing the wings and control surfaces through undisrupted air and that gave enough advantages that the a lot of pusher designs were put forward. A pusher would have superior cruising speed and better wing loading because it flew through undisturbed air.

The most successful was probably the XB-42 mixmaster which would have been deployed had the war gone on. (As it was, it it eventually deployed as America's first jet bomber.)

The B-36 used pushers precisely to give its wings a clear airflow. It was actually designed in 1942.

But they had big tradeoffs.

A major problem with pusher designs of that era was cooling the engine on the ground.

With a tractor (puller) you have this giant fan running blowing air over the plane and the engine (air cooled radial) and/or radiator (water cooled.) With a pusher, the prop just blew air behind the plane, leaving a powerful engine with nothing but passive cooling.

During a crash, a tractor engine in front served as a crumple zone for the rest of the plane, particularly the cockpit and could plow through obstacles making the plane come to a halt slower. Conversely, with a pusher, the engine was behind the pilot and not only didn't offer any protection but tended to tear loose and ram through the rest of the fuselage like a pile driver.

The pilot could not see and therefore visually inspected the engine e.g. if you start loosing oil in a tractor, you know right away. If the engine is behind you, you might not notice until you see the gauge.

Center of thrust was more difficult to balance and that made center of gravity tricky as well.

Still, with all these disadvantages, XB-42 and B-36 showed that the advantages could be come out on top with enough good engineering. And in the end, jets work at least half by pushing. If jets had be delayed a few years or the war had started back in the mid-30s, we probably would have seen more pusher designs in service.

With jets, the military need for pushers disappeared and in civilian aircraft, there is seldom much need for an increase in performance that justifies fighting all the tradeoffs.

Answer 4

Why are push-propellers so rare

Other answers have covered this.

yet they are still around?

A front mounted propeller limits the field of view of the pilot or payload.

Consider a surveillance drone or UAV. There's no pilot whose view would be obscured but there is almost certainly a great advantage for the radar, optical and other forward-looking systems.

Surveillance missions can benefit from a relatively slow-moving, low-altitude platform. This favours the use of propellors rather than jets and the pusher arrangement can be beneficial as noted above.

MQ-9 Reaper

Elbit Hermes

Sagem Sperwer

Answer 5

Just some additional info and thoughts on this old thread. There are still a few successful pusher aircraft, even a few being built today. The Piaggio Avanti is quite successful and very fast, high cruise is about 400 knots, and there is the above mentioned CBA-123 in development now. Like all engineering design, pusher aircraft are a compromise. You typically lose some propellor efficiency, from the disturbed airflow, but gain some aerodynamic efficiency from the slower airflow over the wings and fuselage. Prop clearance can be an issue, as is FOD of the props from debris picked up by the wheels being thrown into the props.

Noise is a mixed bag. The internal noise is often lower, because the props are not blowing onto the windscreen or fuselage, but external noise can be an issue due to the disturbed flow to the props. The Piaggio Avanti is supposedly one of quietest cabins flying but it is quite noisy when it flies by.

I had a pressurized skymaster for a number of years, and I really liked it, but it was very noisy both inside and out. But the skymaster was very quiet inside with just the rear engine running so it wasn't the pusher prop that was the issue. Performance was slightly better on the rear engine alone. The "wives tales" about taking off on one engine are just that. Like all piston twins the performance on one engine was marginal, and there is no way that a proficient pilot wouldn't notice an engine failure. I did fly the plane on one engine a number of times, both during simulated engine failures on takeoff and one actual engine failure in flight, and while the handling was docile the performance was unimpressive (though a P-337 has somewhat better single engine climb rate than most piston twins).

I did many rough field operations in my Skymaster and was lucky to never have damaged the rear prop but there is little doubt it could be an issue. I have not heard of issues of prop damage on Piaggio Avantis but they aren't likely to be used on unpaved runways. So, like all airplanes, the designer needs to consider a host of issues, including the intended use of the aircraft, before deciding whether to use a tractor or pusher configuration. I suspect that much of the reason for the lack of pushers is the inherently conservative attitude of aircraft designers, not the inherent issues with pusher design.

Answer 6

For single engine aircraft at least, having a pusher prop makes egress in flight (a.k.a. bailing out or ejecting) much more dangerous. During WW1 I believe (when pusher props were far more common) more than a few pilots were seriously injured or killed by their props when having to jump out of a burning aircraft.

For this reason the Germans fitted (or planned to) an ejection seat in their Do.335 push/pull design, and Fokker planned to do the same in his D.XXIII.

The B-36 was large enough that pusher props would be far away from the fuselage, getting rid of that problem.

An advantage I can think of of having the props in the front that stems from the increased airflow is that you get free extra cooling of the engines.

Having the engine in a single engine design in the rear with a pusher prop, you end up with a more complex air inlet and tail design than simply letting the engine suck in air that it bites into already. You need ducts and stuff, adding weight and complexity.

Answer 7

Does a pusher propeller run in disturbed airflow, producing more vibration and noise? Yes.

Does a pusher engine have more cooling issues? Yes if it is a piston engine (turboprops are cooled by their own internal airflow).

However there is one thing in favor of the pusher configuration which needs a better evaluation: A pusher propeller picks up airflow that is already decelerated by the wing and fuselage to generate thrust, while a puller propeller picks up undisturbed air to blow it onto the fuselage and wings, which in turn decelerate the airflow. This difference probably means that a pusher engine may be somehow more efficient at higher speeds. A twin pusher, shown in the following photo, was developed here in my country in the 80's called CBA-123 (two prototypes were built) and the experience with that airplane points in this direction.

Answer 8

Pusher props offer a real advantage to small amphibians like the Lake Buccaneer and the Republic See Bee where pilots need to be able to jump out and catch mooring buoys, lines or jetties without fear of striking a spinning prop blade.

Answer 9

One category of aircraft that almost universally uses pusher props is the autogyro. Typically the engine is mounted in the centre of the craft, with the cockpit immediately forward, the rotor above, and the tail somehow mounted behind the prop - which requires an often spindly-looking strut wending its way past the propeller disc.

The same is true of some other types of ultralight aircraft, with a small engine often being strapped directly to the pilot's back, or possibly the back of his seat. In most cases that's the only place it can be mounted without starting to build an actual fuselage.

In both cases, there is some sort of guard which would strike the ground before the propeller itself if sufficient pitch were attained, and the pitch required for this is much greater than most aircraft need at takeoff. A propeller strike is therefore unlikely.

The major advantage of a puller-prop configuration, from an aerodynamic point of view, is that it adds forward airflow over the wings and tail in the most crucial phases of flight, when the aircraft as a whole is going rather slowly. Before the jet era, long tarmac runways were somewhat uncommon, so being able to take off from a short and/or rough field was advantageous. For fighters, the extra few seconds of controllability while manoeuvring in the vertical could easily mean the difference between victory and death.

This was of particular importance for carrier-based aircraft, which during WW2 did not typically have the benefit of a catapult to accelerate them off the deck. The carrier could generate a significant headwind to help out, by steaming upwind as fast as it could, but a heavily loaded fighter or light bomber still needed as much of a safety margin as it could get. The B-25s used in the Doolittle Raid were a truly exceptional case; the crews needed special training in order to take off from something as small as a carrier, and one of them only managed it by floating in the "ground effect" just above the waves after leaving the deck. If they'd been pushers, they probably wouldn't have made it.

Also relevant to carriers is the ordeal of landing on a deck. The universal configuration for any non-VTOL carrier aircraft is a hook, coupled to a heavy-duty shock absorber in the tail, which engages with a series of heavy cables laid across the flight deck. To make that work, the plane basically needs to fly itself into the ship in a nose-up attitude, with none of the flare you'd usually employ when landing on a proper airfield. A rear-mounted propeller would routinely strike the deck in that case, becoming a major hazard to both the aircraft itself and the flight deck crew.

For these and several other reasons, the front-mounted propellor became conventional in military designs and, by extension, in civilian designs as well. A pusher automatically seems unconventional in that context, and is associated with ultralight and otherwise unusual aircraft, such as autogyros.

The shift in CG associated with moving the engine to the back also encourages major changes in planform even with full-sized aircraft, up to and including canard configuration, which makes such an aircraft appear even more outlandish. A canard aircraft also has different handling characteristics at the edge of its flight envelope, which may put prospective pilots off even further. More recently, the three-surface layout (with both canard and conventional tail) has appeared, carrying at least theoretical advantages from both canard and conventional layouts.

Few aircraft designers are willing to stick their neck out into such an environment; that the Beechcraft Starship is widely regarded as a failure - Beechcraft actually destroyed many Starship airframes in an attempt to avoid the costs of supporting them - would tend to encourage further caution. Burt Rutan (of Scaled Composites) is a notable exception to the rule; he has designed several successful canard and three-surface aircraft with pusher engines.

The Piaggio Avanti, however, is a rare example of an aircraft which takes the pusher's inherent advantages to their logical conclusion. With a three-surface layout and very careful attention to aerodynamic details, it is a turboprop aircraft with very close to corporate-jet performance, yet with significantly lower operating costs and the ability to operate from shorter runways than a jet. Being backed by Ferrari, including ferrying the Scuderia racing team around in an Avanti, can't have hurt its popularity. Perhaps more remarkably, the only significant accident I can find involving an Avanti was the loss of an unmanned drone based on the type - not at all bad for an aircraft type that's been flying hundreds of examples for nearly 30 years.

So the pusher configuration has its own advantages, which most aircraft designers don't really know how to make the most of because jets and front-mounted props have been the standard for so long. But the latter became dominant due to decisive advantages it held in the pre-jet era.

Answer 10

One advantage of the tractor not yet mentioned is that it puts the engine weight in the front. We all know that model gliders need nose weight and the same applies to the real thing. using the engine makes for a more compact, lighter and simpler construction.

The issue of crash safety has also been mentioned. Funnily enough, the original Biggles books were technically a unique historical record by an ex-WWI pilot, Capt. W.E. Johns. Biggles Learns to Fly contains a graphic and shocking exposition on the relative merits of pusher and tractor in a crash.

At the start of WWI many British types by the RAF and Vickers were pushers. Whan newer types like the Sopwith 1 1/2-strutter came along, they proved far more compact and agile - and safer in a crash-landing. The pusher became obsolete overnight. It has remained a niche option ever since.

Answer 11

The same as in Do-335 happened with a twin engine Cessna, the Skymaster, in single engine operation, it was better with the rear engine. This Dornier-335 airplane looks more as a racer than the fighter-bomber it was designed for.

Answer 12

Pushers were tried during WW2 without much success, and this was a time when just about anything was being tried.

The US had two pusher fighters under development during WW2, the Curtiss XP55, and the Northrop XP56. Neither became operational, both had stability issues, although those issues derived more from the radical designs rather than the pusher prop. The XP55 used a canard at the front for elevators and wingtip rudders, while the XP56 was effectively a flying wing with it's short, stubby fuselage.

The XP55's basic layout was later used in the Beechcraft Starship. Sadly, the Starship was not successful, costing more and flying slower than contemporary executive transports like the Cessna Citation and Lear, or even tractor turboprops like the Piper Cheyenne and Beech KingAir.

Japan developed one pusher fighter, the Kyushu J7W Shinden. Its pusher design reflects the intent to power it with a gas turbine engine, the pusher/tailless design to get around the problem of hot exhaust melting the tail. Like the American efforts, it never saw production, although the decaying industrial situation in Japan in 1945 was more responsible for it's failure to enter service.

In the case of the fighter designs, the pusher configuration was just one of several radical departures from conventional design, so it's hard to determine how much the pusher configuration contributed to the problems encountered. Note that the DO335 was a push-pull configuration, not a pure pusher.

What can be said with some degree of certainty is that pusher configurations, definitely in fighter aircraft, were not found to be beneficial when prop fighters were at their peak.