Would connecting propeller blades with a continuous ring reduce induced drag?

Question

Turbine engines are covered, which of course is to contain the process (just like a super/turbocharged engine is as soon as the air enters the intake). But it got me thinking, doesn't this also reduce - or even completely remove - the induced drag around the tips of the fan blades?

Would it be possible to have a similar setup for a standard prop, it doesn't even have to be stationary, it could be a ring connecting the tips of the prop, spinning with it, like an infinite winglet. It has the added safety benefit that it'll be visible when the prop is spinning. And I figure if the ring is strong enough to maintain its circumference, the load on the prop should be marginal since it's spinning around its own center of mass.

Is the induced drag on the prop not large enough to warrant any thought, or would such a prop-winglet-ring (I'm sure there's a real name for it, anyone know what I'm talking about?) cause other disruptions of the airflow? Or perhaps there are other reasons, like it would simply be too hard to solve for constant-speed props?

Answer 1

What you're talking about does exist, they are called Q-Tip Propellers.

Remember that a propeller blade is just an airfoil - like a wing - and the basic aerodynamics are no different than a wing. But the rotation of the blade creates more phenomena than a wing, in particular the helicoidal vortex one sees behind the prop and causes all sorts of propeller effects.

In theory, nothing would prevent us from having winglets at the prop tips : the advantages would be

1. Making the prop more efficient by reducing the induced drag (same as a winglet on a wing )
2. Reducing noise
3. Keeping the propeller tip speed subsonic by decreasing its length

The big problem is in aerodynamic stresses, and as far as I know there have been some quite spectacular failures during testing, so the solution is now to give a greater sweep to the tips (see that as equivalent to the 777 wing tip compared to the 787 for instance). Try and find articles on the Hartzell Q-tip.

As naval propellers are wider and capable of deealing with bigger torque stresses, modern ones do have winglets. You could find some pictures on the web.

Answer 2

As others have pointed out, fitting the ring to the prop will greatly increase the stress on the blades. The same effect can be had with a nicely fitted shroud.

There has indeed been a plane which used this concept, the RFB FanTrainer (see picture below). To reduce weight and wetted area, the prop diameter was much smaller than with a regular propeller, so the overall efficiency was not better. However, the smaller rotating inertias did produce a more turbine-like effect (less precession), so the concept was used for a basic trainer for future jet pilots.

In the end, the FanTrainer enjoyed only limited success and was discontinued after 50 had been built. The design was too lightweight to support all the desires of air forces for a basic trainer, and the private market at that time was shrinking and full of older planes which served the cost-conscious customers equally well. It did, however, offer almost jet-like characteristics for a uniquely low price per flight hour.

In general, if you want to shroud the propeller for better efficiency, you need to accept the higher surface area of the shroud, which will quickly add more drag than you are ever likely to save by preventing flow around the prop tips.

What could be saved by shrouding the prop? Induced drag would be the same, since this comes from lift creation. The classical theory for minimum induced loss propellers by A. Betz and L. Prandtl requires an elliptic lift distribution over the propeller disc, such that lift smoothy tapers off at the tips. Artificially increasing it would only help if this could reduce blade chord at the tips - since the tips see the highest dynamic pressure, this could indeed translate into less friction drag. However, this gain is small when compared to the massive increase in friction drag of a shroud.

At high speeds the induced losses are small, and other factors become dominant. Note that turbofans and highly loaded propellers are not designed for minimum induced loss, but for maximum thrust with a given diameter. A shrouded propeller can enjoy a higher disc loading, so you get the same thrust with smaller blades and lower tip speeds, which will help in high speed efficiency. Smaller blades translate into less friction losses on the prop, and lower tip speeds translate into higher cruise speed before Mach losses begin to bite.

Thus, at high speed a shroud can be helpful when it is not too large. Turbofan engines suffer from this dilemma. They could have much higher bypass ratios than today, but this would mean huge nacelles, and the increased nacelle drag would offset the gains from the increased bypass ratio. Actively laminarising nacelle flow is the way forward here, but so far the practical implementation has yet to happen.

Answer 3

A ducted fan comes close to what you're describing, although the ring around the propeller is stationary instead of attached to and spinning with the propeller.

The main advantage of a ducted fan is higher efficiency due to reduced propeller blade tip losses (essentially induced drag) but this efficiency advantage is lost at higher speeds and/or lower thrust demand.

In "normal" aircraft, the drawbacks of a ducted fan outweigh the efficiency gains. Ducted fans are mainly used in airships and VTOL aircraft like the infamous Bell X-22. They are also used in most jet model airplanes.

Answer 4

Q-Tips and Ducted fans are the big ones to solve the problems you're thinking of.

Your ring idea would be very difficult to implement for a number of reasons, weight being primary. A metal ring all the way around the propeller would add a significant amount of weight to the aircraft, which would likely cancel out any efficiency gains you get from stabilizing the airflow. In addition, the tips of a propeller are already experiencing several thousand G's at normal operating RPMs. This is acceptable because the prop gets continuously lighter as you approach the tips. But if you were to attach a ring of metal weighing a few dozen pounds, the forces would be astronomical, and your prop would very quickly fail.

A second issue is that in order to have efficient propellers, we rotate the blades slightly to change the angle at which they bite into the air. These are called constant speed propellers, and they're already somewhat complicated. If you go and add a second pivoting point to the prop tips so that they can move inside the ring, you're just adding a bunch of bearings, grease, weight, and another failure point.

Finally, balancing the ring would likely be a difficult task. First your ring would have to be manufactured to very precise tolerances which would be quite expensive. The slightest nick or dent in the ring (which happens frequently to propellers) will cause it to become unbalanced and would at minimum require work, and at most could cause the whole propeller to shake itself apart. This is already a minor concern for props, but when you put your heavy disk out on a long arm from it's fulcrum, then subject it to incredibly high G forces, you're just amplifying any faults it may have.

Answer 5

For historical perspective do some research on the Culver Channel Wing produced in 1952-53. This twin engine (pusher) plane has two ducts that do not completely encircle the prop but are part of the wing. This lead to extremely short take off capabilities because the airflow over the wing was not tied to forward ground speed. I would even go as far to say that it was an early step in ducted fan VTOL capabilities.

This article by Doug Robertson posted in 2005 on airport-data.com contains some beautiful pictures, and an what appears to be a well researched narrative history of the aircraft.