Embry-Riddle has a Cessna 337 as a test bed for a Swiss developed rotary engine package, amongst other things. There was an article in last months AOPA mag; here's the Mistral site, this is the mill they're testing.

Because the rotary engine has inherent problems as far as longevity/durability. They also don't make a whole lot of torque (but plenty of HP), which is kind of the opposite of what you want. Big massive pistons create lots of torque...

I don't know about inside, but outside the plane pushers are usually noisier, and they tend to sound really weird. I think that's because they are operating in disturbed flow which isn't the same all the way around.

I seem to recall Peter Garrison covered the topic of pusher props vs. conventional in a recent column. On a push me pull you aircraft like the Cessna Skymaster, that rear prop can keep flow attached over more abrupt transitions. I seem to recall that in the column, it seemed that which one was better depended to a large extent on the details of the individual aircraft. If you could get symmetrical flow into the pusher prop, and a modified prop design, it would probably be a lot more efficient. But that doesn't usually happen. There are some neat articles on the CAFE web site of how much drag you can get rid of with axisymmetric fuselages with axisymmetric flow and a pusher prop. So just get rid of your wings. Might make sense for a dirigible or submarine.

A tractor prop is going to increase airspeed over the fuselage, and, as I recall, is supposed to create bands of turbulent flow, though I understand that on something like the Nemesis, the flow is laminar in between those bands.

My understanding is that the Long Eze, or perhaps it was the Varieze, was wind tunnel tested, and the prop was very innefficient, but the rest of the airframe was very low drag.

The Piaggo Avanti, a pusher, is supposed to be efficient, but it sure sounds loud and weird.

Canards, like many other configurations, can be quite stable. However, on a stable aircraft the rear surface must be more lightly loaded than the front. So you probably need more wing area to support a given weight, and the canard has to work very hard.

An alternative is the three surface airplane such as the Piaggo Avanti. This can have, I recall reading, lower trim drag than some others. Or at least the induced part of the trim drag can be lower. If it's done right.

I think wankels would show up on airplanes a lot more often if they were on all our cars and we'd been developing them for 100 years.

Canards were the design of choice for the early aviation pioneers, the Wright Flyer, Samuel Langley's Aerodrome the AEA Silverwing to name a few. By the end of the the first world war conventional aircraft design became the defacto standard. Only Burt Rutan and a few other designers have re-adopted this concept. Canards are visually interesting and have the selling point of being unstallable, although some of these "unstallable" designs have been demonstrated to stall and even spin at extreme rear C of Gs.

One drawback of canard designs is their relative lack of upward scalability. While this type of design works beautifully on small aircraft it becomes more and more difficult to achieve as the design gets larger. Recall that in order for the canard design to work properly the canard always must stall before the rear wing. (Conversely, a convention elevator must have enough relative airflow to create the tail balancing force, to rotate the aircraft.) More complex flap requirements on large aircraft makes this requirement more and more difficult to accomplish. Remember the problems Beechcraft/Raytheon had in certifying the Beech Starship; it was only a medium-sized canard aircraft. With smaller aircraft like the VariEze, LongEze, Cozy etc. making the canard stall first is relatively easy to accomplish. But since canard planes don't have flaps the aircraft have higher minimum flying speeds and thus require added runway length to takeoff and land safely. Lower stall speeds directly equate to lower takeoff/landing speeds and more survivability in low-speed crashes.

Another drawback of canards is less inherent directional stability. Just think of a dart: its mass is mostly in the front. That mass, plus the fins at the rear of the dart, make it stable when thrown. This basic principle is the same for most conventionally-designed light aircraft, especially tractor propeller designs with heavy engines, avionics, instruments, seats and occupants in the front third of the aircraft.

Other practical problems exist with canards, like where to locate baggage compartments and fuel tanks, plus the well-known issue of flying canards in rain.

As to the idea of diesel-powered rotary engines, this is much easier to say than accomplish. Everything in aviation is more expensive to purchase, and to build, than in the automotive industry, due to limited market size, the myriad of regulations the engine builders must satisfy and the pervasive influence of the regulators. I also believe there isn't much room for a new suite of radial engine manufacturers, and existing manufacturers like Lycoming and Continental probably already have design patents on proven, certified radial engine designs. And, more basically, why re-invent the wheel? There are many certified and non-certified flat-opposed engines to chose from, for aircraft manufacturers and homebuilders.

In some ways using diesel fuel is more advantageous, as it is denser and therefore has more potential energy available per unit volume. (About 7 pounds per US gallons, versus about 6 for avgas). That could allow airframe manufacturers to install fuel tanks about 15% smaller and somewhat lighter for the same range, or about the same size for about 15 percent more range.

One potential use for diesel radials would be to manufacture them in a variety of sizes to re-engine many existing warbirds, like T-6s and other WW-2 designs, particularly since leaded gasoline probably won't be with us for much more than a few years. Smaller diesel radials might be viable choices for many experimental and kitplane designs, if they could be sold in sufficient numbers to make a reasonable business case for them. They could provide improved fuel economy, just as they have done in some automobiles.

The radial design is ideally suited to air-cooling, but their engine cowls are much more draggy than aircraft with flat-opposed engines so they reduce the top speed of aircraft with radial engines. For example, the P-51D had, I believe, a 1695 HP inline Merlin that was about 50 mph faster than the F6F-5 Hellcat with a 2000 HP, dual spool, 18-cylinder Wright R-2800 radial.

Doug McIntyre
Surrey, British Columbia