The thrust force from a pusher propeller is applied to a rather robust section of the airframe, so additional structural strengthening is minimal. Pusher propeller driven aircraft also tend to exhibit a slight stabilizing tendency in pitch and yaw in comparison to a tractor configuration. The pusher propeller has an aerodynamic advantage over a tractor in that it can reduce skin friction drag because the part of the aircraft in front of the propeller flies in undisturbed air. A shorter fuselage can also be accommodated because the propeller inflow allows for a steeper fuselage contour without flow separation.

There are also disadvantages to the pusher propeller configuration. The propeller is in the disturbed wake of the fuselage so it will typically suffer from reduced efficiency. Also, the pusher propeller is vulnerable to damage by objects kicked up by the main landing gear wheels. The main landing gear has a wide track in order to alleviate this to a degree.

A pusher propeller tends to be noisier than a tractor propeller, but by the use of a duct and a multi-bladed propeller with a low rotational speed, it is possible to alleviate some of the additional noise. In addition to the external noise reduction gained by ducting the propeller, cabin interior noise is reduced by the use of the pusher configuration. An added safety advantage of the ducted propeller is that it is very difficult for people outside of the aircraft to be inadvertently injured by a spinning propeller that is shrouded by a duct and is not readily accessible.

The disadvantages of the ducted propeller are increased weight and complexity. Also, if duct is used a reduced propeller rotational speed is desired. This is compatible with the undersize diameter of the propeller and the resultant lower tip speed.

Unlike a tractor configuration, they don't have that big fan in front of them, pusher aircraft generally suffer from poor cooling on the ground.

Instead of locating the long shaft to the tractor propeller inside a strong tapered tube, which is part of the fuselage, consider that this strong tapered tube is the shaft. It turns with the propeller and the bag of it is attached to the body of the fuselage by stronger-than-otherwise radial bearings

Simple question, but as usual no simple answer. For the types of disk loadings we see in models, it's likely to be less efficient than two independently mounted tractors.

A propeller imparts a swirl to the air passing through it. It takes energy to accelerate the air into this rotation, and therefore represents an efficiency loss. The idea behind a contra-rotating propeller system (i.e.: two props rotating in opposite directions on the same axis; "counter-rotating" means two props rotating in opposite directions on different axes, such as on the Lockheed P-38) is to have the swirl from one prop cancel the swirl from the other, eliminating the rotation in the slipstream.

Supposedly this can provide efficiency improvements of as much as 15%. The problem with this concept is that there must be that much efficiency loss already in the basic design, that is available for recovery. This is only true in props with extremely high disc loadings (i.e.: massive amounts of horsepower being forced into a relatively small diameter prop), such as the "propfans" that NASA was experimenting with back in the 1980's. Those were trying to absorb 12,000 horsepower or more in a prop only about 10 to 12 feet in diameter, less than the diameter of the 4500 horsepower props on a C-130 Hercules! When you have that much power going into such a relatively small prop, there is lots of swirl, and therefore a lot of energy that can be recovered by the second prop. That's also why those props use so many blades; more disc loading requires more blades to absorb the power.

At the disk loadings typical of our models, there is very little swirl by comparison, and I doubt that you could expect more than a percent or so of recovery from it at best, IF you did everything exactly right (a virtual impossibility in the real world).

About the only case where swirl in models is a significant factor worth doing something about is in ducted fans. In that particular case we have a lot of power going into a very small prop, so there is lots of swirl. To combat this, we put stator vanes in the duct behind the prop to straighten out that swirl. Those stator vanes are nothing more than the special case of a contra-rotating propeller system, with the second prop in the system designed to run at an RPM of exactly zero. Even so, those stators must be designed and optimized very carefully, or the energy losses due to their own drag will be greater than the energy revered from the swirl, resulting in a net loss. This is the exact same problem most energy recovery devices (such as winglets) face, that of delivering a benefit that exceeds their cost.

The other benefit of a contra-rotating system is that it can cancel out the torque and P-factor effects of a large engine. This is one of the main reasons for its use in planes such as the later Rolls Royce "Griffon" engined versions of the Spitfire, the Bugatti racer, or the Fairey Gannet. Unfortunately, that also requires the use of a fairly complex gearbox, and gearbox-driven props of any kind have a long history of nothing but trouble. The British seem to have had the best luck with them (the gear drive that combined the two crankshafts into the single propshaft on the Napier "Sabre" 3000 to 5000 horsepower H-24 engine was particularly ingenious, and very successful), but other than those successes, the propeller gearbox has historically been the ruin of many airplanes. Gearbox problems were on of the biggest factors that kept the Northrop XB-35 flying wing bomber from being ready before the end of WW II.

In any case, torque and P-factor are generally not significant issues on models. However, the asymmetric thrust in the case of a failed engine on a twin (yes, even electrics can have those), can be a significant issue. The "centerline thrust" of a contra-rotating twin arrangement can solve this. This was one of the biggest reasons for this layout on the Cessna 337 and the Rutan "Defiant".

For a typical un-ducted contra-rotating propeller system, one of those two props is a pusher prop, and therefore you have all the problems and efficiency losses inherent in a pusher prop, which can be considerable on any size airplane. The myth of pusher efficiency assumes that by putting the prop at the back end of the airplane so the rest of the airframe in line with the prop does not feel the increased speed of its slipstream, you save on airframe drag. In actual practice this may be true, although in the vast majority of cases the savings from this are microscopic. If we convert that airframe drag savings into its equivalent in terms of propeller efficiency, we're looking at typical differences on the order of a small fraction of one percent. Recent wind tunnel studies by NASA even show that the majority of the flow behind a propeller tends to be laminar, not turbulent.

Meanwhile, putting the pusher prop at the back, so the airplane does not have to fly through that prop's slipstream, means that the prop now has to fly through all the disturbed airflow coming off the airframe. The efficiency losses from that are typically at least 2-5%, and can be well in excess of 15% in some cases, not to mention the increase in vibrational stresses and noise, the added FOD ("Foreign Object Damage") of stuff coming off the airframe, rocks kicked up by the wheels on takeoff, etc.. Keep in mind that on a propeller driven aircraft, only a very small percentage of the airframe is actually immersed in the propeller slipstream, and therefore only a small percentage of the total airframe drag is affected by the slipstream. Meanwhile, essentially all of the thrust comes from the propeller, so anything you do that hurts the propeller's ability to do its job will have big effects on thrust and efficiency.

In addition, pusher props are usually restricted in diameter because of ground clearance problems. This tends to force additional efficiency losses. Diameter is probably the single most important factor in the efficiency of most propellers, and even a small restriction on it can have big effects. This is especially true at high power and low speed, such as takeoff and climb, although less so at high speeds. This is one of the major reasons the Prescott Pusher (among others) was such a disaster. Try comparing its takeoff performance with conventional tractor aircraft in the same power and payload class and you'll see what I mean.

A pylon-mounted arrangement like you're considering doesn't have ground clearance issues, but has restrictions due to the height of the pylon. All that thrust way above the C/G and the hull tends to shove the nose down, especially on takeoff. I know of at least one amphibian with a pylon-mounted engine that has been unable to accept larger engines, because any significant power increase beyond the plane's current engine tends to make the plane want to become a submarine when you open the throttle for takeoff.

With a pylon-mounted arrangement, the forward prop sees some disturbed inflow due to the flow next to the fuselage and wing, but the aft engine also sees the disturbances from the forward engine and nacelle, as well as the pylon and any external bracing.

The net result of all of this is usually little or no measurable benefits from reducing airframe drag, but quite significant losses due to these other factors, for an overall net loss. Even the possibility of recovering swirl energy, as in the case of a contra-rotating propeller system, usually does not start to see measurable benefits until you get into the sorts of horsepowers typical of turboprops and very large piston engines. I used to be an engineer for a propeller company that happened to have more experience with pusher installations than probably anyone else in the business (Voyager was one of those). Our usual first reaction when someone approached us with a new pusher application was to try to talk them out of it.

There are a number of aircraft designers (including Rutan) who have at some time in their careers been a big proponent of pusher designs. In general, they are airplane designers, not propeller designers, and tend to overestimate the benefits to the airframe of a pusher arrangement while badly underestimating the detrimental effects on the prop. There is a tendency to think of props as these mystical devices that you just bolt to the engine to make thrust, with little thought given to the prop's own needs and idiosyncrasies. To really get a decent working relationship between a pusher prop and the airframe usually takes an incredible amount of work. Piaggio came up with one of the better pusher designs (from an aerodynamic standpoint) in their P-180 "Avante", but it took a huge amount of engineering effort including over 2000 very expensive hours in Boeing's wind tunnel to achieve it.

Don Stackhouse
DJ Aerotech

In general, the term ducted fan is applied to configurations of small disk area, high rpm, and low aspect ratio (long, narrow ducts), while a shrouded propeller has larger diameter, lower disk loading, lower rpm, and a high aspect ratio (short chord) duct. Many varieties of ducts and shrouds are possible, each with its own characteristics, which explains the conflicting opinions and evidence regarding their efficacy.

The following programs are in the computer at C:\helicopter/programs\propeller\;

xx

Engines: Continental IO-360

One of the primary design drivers for the Calvert was to minimize the engine power while working within stall and compressibility constraints. In an effort to keep the fuselage level, minimize shaft tilt, and prevent power losses due to the dwindling propulsive efficiency of the main rotor at high forward speeds (see Figure 6.1(a)), the Calvert uses a propeller that overcomes 80% of the total aircraft drag at cruise velocity. This design required the selection of several key parameters that define the propeller.

Several configurations were studied using standard propeller design charts. These included 4 and 6 bladed single- and 4 and 6 bladed dual-rotating configurations [BH40, Nel44]. A typical tip speed of 230 m/s was chosen for the propeller. A propulsive efficiency of greater than 0.8 (at a thrust of 80% of the total aircraft drag in cruise) was chosen as a design constraint in order to minimize the Calvert's engine power. This choice was made after an exhaustive trade study involving several different configurations, propeller thrusts, propeller weights and cruise power. The propeller diameters required for achieving an efficiency of 0.82 at the same thrust (80% of total drag) for each of these configurations are shown in Figure 4.14.

Yes indeed, Steve. There have been quite a few, I believe, and some early ones too, including a man-powered gyro!

Back in the 80s we designed, built & adapted a large number of ducted-fan systems for light hovercraft and other applications. The reason for using them was often as much to do with intrinsic safety as performance - if you have to guard the thing, using a duct can help recover some performance.

But ducts are not magic!

A good big 'un usually beats a good little 'un, since disc loading is a factor that affects the efficiency of fans, props and rotors. Something like a cube law prevails, so if you double the duct (or prop) area you will find something like 20% more thrust at the design point condition. Theory suggests 25%, but I've never seen it.

Now there _is_ a downside to big props/fans, apart from the weight and the design hassle.

As you improve the efficiency by increasing swept area, so the air speed range over which it stays good reduces.

It gets 'peaky' somewhat like a highly tuned 2-stroke or a Hi-Q tuned circuit. Works best at one speed.

So for gyroplane use I'd recommend that a ducted fan use power loadings similar to that used by successful propellers in the same application.

All the best, Ben.

PS Ken Wallis may have been one of the first with ducts on a gyro, at least he mentioned their early use in his last talk to the BRA at Shipdham. He rejected them because of their increased tendency to pick up rocks and such from the runway. Just like the misnamed "Rock Guard" on a Bensen.

Consider ducting with static fins to counter some of the torque - roll coupling.

The center of the thrust should be higher, in line with the center of mass and drag, to give better stability, particularly on loss of engine or sudden aft cyclic.

The direction of rotation of a pusher prop is the opposite of the majority of propellers (which are for the tractor configuration)..

Consider the validity of using the collective metrology that is used in tail rotors.

A propeller whose blade pitch angle can be changed in flight. A controllable-pitch propeller and a constant speed propeller are similar, the main difference being the control. The pitch of a constant speed propeller is controlled by a governor and the pitch of a controllable-pitch propeller is controlled by a manually actuated valve or switch.

Note that this type of propeller does not incorporate any means of yaw control (lateral cyclic).

Alternative: A possible alternative would be to have the two blades joined along the pitch axis. This way the two blades change pitch together. Of course, this is not appropriate for three blade props and probably cannot be done for four blade props.

The hydraulic system could also be used for the landing gear and the rotor blade pitch controls

The jackshaft from the soft start to the rotor's mid-gearbox could have universal joints at both ends to get the sheave closer to the prop shaft. Similar to the drive shaft in a car. If the jackshaft had a sliding spline then the vibration of the engine would not effect the rotor drive train.

See 'Firewall Forward', Propellers & Spinners, section 12.