What's a reasonable guesstimate of how much less efficient a tandem prop (eg Dornier style) is than two tractors

- assuming that you have counterrotating props and either 1.5~2 prop dia or one prop dia between the props? Why? ...

From : Don Stackhouse

Tandem props that rotate in opposite directions, like the ones you describe, are one variation of the general category of "counter-rotating" propellers;i.e.: multiple propellers rotating in opposite directions on shafts that are in line with each other.

This is not to be confused with "contra-rotating", which is where you have props turning in opposite directions on shafts that are NOT in line with each other, such as on the Lockheed P-38 Lightning or the Piper Twin Comanche CR.

Besides cancelling out torque effects, counter-rotating props can be more efficient in some cases, particularly in applications that have a high "disk loading"; i.e.: a lot of power going into relatively small diameter props. Propellers cause the air in the slipstream to swirl. Propellers that are absorbing a lot of power cause a lot of swirl. The energy involved in making the air swirl represents an efficiency loss, perhaps as much as 15% or more in very high power applications. By mounting a second prop rotating in the opposite direction directly behind the first prop (the closer behind it the better), it is possible to get the swirls of the two props to cancel each other, thereby recovering most of this lost efficiency. The stator vanes in a well designed ducted fan unit perform a similar function, straightening out the swirl from the impeller, at least at the speed, RPM and power setting they were optimized for.

The bad news is that the swirl energy in a typical model airplane propeller is not enough to represent a significant amount of efficiency loss. There just isn't any significant power in the swirl of our slipstreams to be worth the trouble to try to recover.

The down side of counter-rotating props is that you're running one of your two props as a pusher, and both props are using essentially the same mass flow. A conventional side-by-side twin arrangement would be feeding each prop its own airflow, for twice the total mass flow and probably much cleaner airflow into both props. In a counter-rotating arrangement, the rear prop (just like any pusher) has to eat all the efficiency-stealing disturbed airflow from everything in front of it. A tandem arrangement is likely to be especially bad in this regard, since it not only has the turbulence of the forward prop to deal with, but also all the various airframe parts in front of it as well. Like any pusher, the magnitude of this loss depends on how clever the airframe designer is.

The pusher props on the Piaggio P-180 Avanti are one example of pusher props that work well, with relatively small prop efficiency losses due to the pusher arrangement. Of course Piaggio spent over 2006 hours in Boeing's wind tunnel getting the props, motors and airframe to all get along with each other.

However, a poor design can easily cost 15-20% or more in lost prop efficiency. In the case of your tandem arrangement, the rear prop in a pylon-mounting (such as the recent Claudius Dornier flying boats) will at the very least have to contend with the disturbances of the nacelle and pylon plus any distortions from the top of the fuselage and wing. An arrangement like the Dornier Do335 Pfeil ("Arrow") or the Cessna Skymaster will have the turbulence from the fuselage, wing roots, landing gear, tail, etc. messing up its inflow, which could easily end up being even more detrimental than the pylon-mounted arrangement. Once again, it all depends on how well the airframe designers did their homework.

The rear prop will also (like almost any pusher) be subjected to higher vibration levels as a direct result of the disturbed airflow, which can hurt the fatigue life of the airframe, motor and gearbox as well as the prop itself. The chopping-up of this disturbed airflow by the aft prop will also result in increased noise. For example, the Lear Fan (which was powered by a pair of turbine engines) had a sound very similar to the P-51 Mustang's Merlin V-12 recip engine (somewhere I have some tape recordings of it taking off, very exciting to hear but definitely not quiet!). There are other hazards for rear-mounted props as well, and in general they tend to lead significantly shorter lives than forward-mounted props.

In case you haven't figured it out yet, I don't like pusher props for most applications. There are a few exceptions, but in the vast majority of applications they are a losing proposition at best. The full-scale propeller company I used to work for had more total experience with pusher applications than probably anyone else in the business, and our typical first response when someone approached us with a new pusher application was to try to talk them out of it!

Getting back to your question: A prop makes thrust by grabbing air from in front and accelerating it out the back at a higher speed than it came in. Thus, the rear prop in a tandem arrangement will see a higher airspeed than the front prop (because of the accelerating the front prop did to the air when making its share of the thrust), and therefore will need either more pitch or more RPM. How much more depends on the particular installation and flight condition. At low speeds and high power settings the differences would be greater, while in high speed cruise the difference might be fairly small. The pitch increase required in the aft prop will be related to the efficiency of the forward prop.

Also, the slipstream contracts in accordance with Bernoulli's theorem, so that the rear prop can be slightly smaller than the forward prop. For example, on the General Electric Unducted Fan (one of the propfan projects that were active in the 80's), the rear blades were several inches smaller in diameter than the front row of blades.

The swirl does tend to dissipate as it flows aft, so the most effective recovery of energy from the swirl will be when the aft prop is as close as possible behind the forward prop. However, this generally requires a rather complicated gearbox. A tandem arrangement using two separate engines in between the two props does lose some of the energy recovery, but it can eliminate the cost, weight, maintenance, failure modes, etc. of the gearbox. In the case of our models, it really isn't a concern, since we don't have any significant possibility of energy recovery from the swirl to begin with. It's going to be a losing proposition overall pretty much regardless of how you approach it.

The whole reason for having counter-rotating props on full scale applications is because they're trying to absorb more power in too small a prop disk. We simply don't have those kinds of power loadings in our models. Usually about the only good reason to do it on a model is in the case of a scale model of a full-scale aircraft that had counter-rotating props.

Don Stackhouse
DJ Aerotech