Slowed Rotor/Compound Technology- Why Isn't There More Research?
Carter Aviation Technologies has successfully demonstrated stable, slowed rotor flight. The CarterCopter "achieved a Mu of 1 at a flight speed of 170 mph and a rotor rpm of 107. The flight was stable and extremely smooth, and the pilots reported there were no vibrational indicators that they were even in a rotary-wing aircraft." (CarterCopter Technology Demonstrator Flight Test Data and Analysis). Data from the flight indicates that the slowed-rotor/compound concept does offer the potential for efficient, high-speed flight, in an aircraft capable of performing vertical flight like a helicopter. Yet there has been little funding into research on this concept, despite the huge promise that it shows.
Okay, I guess I should start off with the disclaimer that I'm not impartial in this. I work for Carter Aviation Technologies, and our main goal is developing a slowed rotor/compound aircraft. Let me also put in the disclaimer that all of this is my own personal feelings, and does not in any way reflect the position of the company. Moving on...
What is "slowed rotor/compound technology," you may ask. Basically, it's an aircraft where you put a rotor on the top (like a helicopter) for slow speed flight, and put wings on it for high speed flight. To help reduce drag in high speed flight, you slow the rotor down to an rpm much lower than where it operates at slow speed flight. And because the wings only need to support the aircraft at high speed, they can be much smaller than conventional wings, and don't need the complex, heavy high lift devices (flaps & slats) of conventional airplanes. The end result is an aircraft that can takeoff and land vertically (with good efficiency), but fly much faster and more efficiently than helicopters.
So, this concept seems straightforward enough. You'd think someone would have tried it before. Actually, people have. The most notable attempt (up until the CarterCopter) was the McDonnel XV-1. However, there are several technical issues associated with slowing the rotor, which that program never completely solved (for an overview of problems associated with high speed rotor flight, visit Rotorcraft Speed Limitations on my site, or the Carter FAQ or Carter Papers and Reports). In hindsight, it seems likely that the XV-1 engineers would have eventually solved all of the technical issues, given enough time and money, but the program was cancelled before they had the chance.
On June 17th, 2005, the CarterCopter reached mu-1. Basically, that's reaching the point where the rotor has slowed down so much that at the 9:00 position, all of the air going over the retreating blade is going in the reverse direction of the way it usually goes over the blade (if you don't follow that, read the explanation on the Carter FAQ). That's much slower than any conventional rotorcraft could spin the rotor, and it reduces the drag quite a bit (if you're wondering why not just stop the rotor if slowing it down reduces drag, then read this entry on the Carter FAQ, but basically the increased structural weight required without the benefit of centrifugal force offsets the drag savings). No other rotorcraft has ever achieved stable mu-1 flight. The only aircraft that ever even got close was the XV-1 (I guess one could argue that technically, the Herrick HV2A did operate briefly at extremely high mu ratios, as it transitioned from a stopped rotor to a spinning rotor in flight, but it was only a transitional condition, not a continuous flight condition, plus the pilot had to be extremely careful to keep the rotor at zero lift during the transition, and it wasn't exactly a practical approach). Unfortunately for Carter, on the very next flight, at a much lower mu ratio, for a completely unrelated reason, there was a failure, probably in the prop drive pulley, that further caused damage to the controls, so the pilots didn't have full control and came down in the middle of a bunch of mesquite trees in a farmer's field, tearing up the aircraft, and preventing any future flights to expand the envelope to higher mu ratios.
However, despite the accident, the CarterCopter did fly at mu-1, and there is data from that flight to calculate the performance of the aircraft. Granted, since this was the first time the aircraft flew at mu-1, and with the cautious approach Carter took to flight testing, the aircraft was only at mu-1 for about a second and a half before the pilots backed out. But it's not like mu-1 is some magical number, where the drag of the aircraft is going to instantly drop off, or stability is all of a sudden going to change. Like most things, it's a gradual transition, and the CarterCopter flew at a mu ratio greater than 0.9 continuously for over 20 seconds. So, there's a fair amount of data on high-mu (slow rotor) flight. And this data shows that the CarterCopter was operating very efficiently when flying at high mu ratios.
The Carter website has a summary of the flight data reduction, including graphs of Lift to Drag vs. Airspeed of the CarterCopter versus various aircraft. Lift to Drag is a good measure of the aerodynamic efficiency of an aircraft, and is used quite often in aerospace engineering. The report on the Carter website lists quite a few reasons why the company thinks the aircraft could perform better than it actually did (notably flow separation causing increased drag), but ignore that for now. Just look at the actual flight data compared to helicopters. The CarterCopter compares quite favorably to those aircraft. At the worst, the CarterCopter is operating at around the same efficiences as those aircraft (effective lift to drags of around 3 to 4), but at higher speeds, the CarterCopter operates at a much better efficiency (effective lift to drag of around 5.5), and the trend from the data shows that the efficiency of the CarterCopter is increasing at the point where the data stops, as opposed to the helicopters which at those speeds have already surpassed their peak efficiency and are on a downward trend. (Note that effective lift to drag is a little different from actual lift to drag - actually a little bit lower - and is explained in detail in the Carter report.) This alone should be enough to warrant further research into slowed rotor/compound aircraft. Here is a rotorcraft with significantly higher efficiency than any conventional rotorcraft.
Now, I'd like to direct the reader to an analysis done by Nick Lappos, comparing tiltrotors to helicopters. The original version is posted here, but if that ever goes offline, another version can be found on the X-Plane Forums. What I think is interesting to note from this report, is that the extra speed of tilt-rotors is offset by their inefficiency in hover. The report looks at an interesting parameter, ton-miles per hour, defined as speed times payload. This is a good parameter to use to show how productive an aircraft is in delivering payload. Comparing tilt rotors to helicopters of comparable size, the helicopters do much better. The report found that the "CH-53E has 1.66 times the transport productivity of the V-22."
The reason why tiltrotors are so inefficient in hover is because of their necessarily high disk-loading. Basically, disk-loading is how much weight the rotor is lifting divided by the area of the rotor disk. It's pretty well known in aerodynamics that it's more efficent to produce a force with air by taking a large bite out of the air and accelerating it a little bit, as opposed to taking a small bite and accelerating it a lot. You can see this in common examples. It's the reasons why helicopters have large rotors, and not just propellers, and it's the reason why modern jetliners have high-bypass engines, as opposed to pure jet engines. There is a good discussion of this on slide 28 of Lappos's report. Helicopters can have big rotors, limited mainly by structural considerations in the rotor itself. Tilt-rotors, because they need two rotors, one mounted on each wing, are limited in a different way. Since you can't have the rotors colliding, the blade radius is limited to being about half of the wing span. The wing span on a tilt rotor is also limited by structural considerations, since in hover the entire weight of the aircraft must be supported all the way out to the wing tips, where the rotors are located, as opposed to conventional airplanes where only the root of the wing needs to carry the entire weight of the aircraft, and the rest of the wing structure is a function of the lift distribution. This forces tilt rotors to have small wing spans, thus causing a high disk loading, and in turn making the aircraft inefficient in hover. There's another factor to consider, as well. Rotors produce lift by accelerating air downwards. The aircraft itself can block this flow, in effect reducing the effective area of the rotor. Because helicopters have such large rotors, and pretty much only the fuselage blocking the downflow, there's not much loss to the effective area of their rotors. As a percentage, tilt-rotors lose much more of the effective area of their rotors by being blocked by the fuselage and the wings, because the rotors are so much smaller to begin with.
One of the things to take away from Lappos's report is that over the long run, helicopters are more efficient that tilt-rotors, because helicopters are so much more efficient at hover because of their larger rotors. So, if a helicopter outperforms tilt-rotors over the long run because of increased hover performance, it would stand to reason that an aircraft with hover performance approaching that of helicopters, but much better high speed performance, would be the best option of all. And that's exactly what the slowed-rotor/compound technology offers. A slowed-rotor aircraft has a large rotor on the top, just like a helicopter. The only difference is that a slowed-rotor/compound aircraft also has wings, which do act somewhat to reduce the effective area of the rotor, but not to anywhere near the same extent as a tilt-rotor.
Carter Aviation Technologies successfully demonstrated that stable flight is possible at high mu ratios (low rotor rpm), and the data from the flight clearly show that slowing the rotor can significantly improve the performance of the aircraft. This technology also shows the potential to be more efficient at transporting cargo than either helicopters or tilt-rotors. For these reasons, there should more funding being spent for additional research on this technology.