OTHER:Intermeshing Configuration - The Synchropter - PRA Classroom
DESIGN CLASSROOM - COLLECTED WORKS - POPULAR ROTORCRAFT ASSN 1974
In the last lesson we studied 2-rotor side-by-side configurations, which achieved notable success in earlier helicopters. Starting with widely separated rotors we advanced by overlapping laterally placed rotors, and today we will go into still closer spaced "intermeshing" rotors.
Figure 1 shows the general configuration of the closest overlapping side-by-side 2-rotor design. The rotors not only mesh like gears, but their axes are inclined away from each other in the bevel gear fashion, which allows the spacing between the hub centers to be as short as 20 percent of the blade radius. This makes the aircraft extremely compact in comparison to other side-by-side set-ups and removes the old bugaboo of excessive weight of the side booms.
Since the blades in this configuration must mesh to miss each other, obviously the rotors must be synchronized and turning in opposite directions. It is from these "musts" that it derived its names of "intermeshing" and "synchropter".
The first one to build a successful intermesher was German inventor Anton Flettner, who previously experimented with autogyros and helicopters of other designs.
So successful was Flettner's latest design FL-282, it was put into quantity production in Germany during WWII and actually saw military service - one of the few to claim that distinction. Even more noteworthy was its amazing lifting efficiency: it was capable of lifting 16 lbs per horsepower - a figure not attained to this day by any of the modern helicopters.
Flettner's machine was also reported to be exceptionally agile in maneuvering and free of maintenance problems in comparison to other helicopters of that day. Mechanically it was simpler than the simplest tail-rotor helicopters.
Its remarkable lifting efficiency can be credited to Flettner's ability to use large rotor diameters, which are more efficient than smaller diameter rotors. This he could do because he had no problem of making room for the tail rotor, nor faced the "blade closure" problem of coaxial rotors. His configuration in so many words was the nearest thing to the coaxial design with all its advantages but without its problem of blade tips striking each other.
The end of the war interrupted its further development and there were no more intermeshers built in Europe.
The name "synchropter" was coined by Kellett Aircraft Corp. in US, which picked up where Flettner left off. Having had considerable experience with autogyros - it was a Kellett autogyro that carried airmail from the roof of Philadelphia Post Office to Camden airport in Eastern Airline colors - Kellett engineers chose to use 3-blades for each rotor rather than 2-blades as in Flettner's original design. This produced a smoother flight, but made it more crowded at the hub, as now they had to intermesh 6 rotor blades instead of 4. It is to their credit that they never had serious in-flight blade collisions (at the roots), although it was at some sacrifice in control amplitudes.
Kellett built two sizes of "synchropters", 2-place and 10-place, both of which were sponsored by the Air Force and carried the designations XR-8 and XR-10. Unfortunately again, the War's end cut their development funds, and the helicopters were scrapped after logging an impressive record of successful flights.
Another American inventor, Charles Kaman, was the last to enter the synchropter field with the original intent to design it specifically as an agricultural crop duster. His models K-180 and K-225, like Flettner's, were designed to carry 2 people and used two blades per rotor. Unlike Flettner's fully articulated blade suspension, Kaman used teetering hub attachment and no feathering pivots. This simplified the rotor head considerably and made it sturdier.
To control the blades in cyclic and collective pitch controls he twisted the entire blade by means of a small controllable flap located at about a three-quarter radius on each blade. These ingenious "servo-flaps", patented by Kaman and shown in Fig. 2, gave the craft inherent flight stability - a feat not achieved by any other helicopter without artificial stabilization. The flaps were controlled by a more-or-less conventional swashplate mechanism, with control rods going through the centers of hollow rotor shafts.
Kaman achieved the distinction of manufacturing the largest number of "synchropter" helicopters and sold them to all US military services: Navy, Army and the Air Force. There were but few civilian sales.
Synchropters are in service to this day. Named HH-43 "Huskies", they are used by the Air Force for fire patrol and short distance shuttle at ICBM guided missile sites and at various air bases. The Navy, too, uses them operationally on aircraft carriers to rescue downed fighter pilots.
Here then is another successful helicopter configuration, which like a comet came with a bright promise, shown brightly for a while, and then went into eclipse. Why? What are the real pros and cons of the intermesher configuration?
On the plus side we already mentioned the advantage of lightweight and compactness of design rivaling that of the coaxial design. Large rotor diameters also permitted low disc loadings, an important ingredient for efficient lifting of loads.
There was also the opportunity to use low tip speeds, which again further improved lifting efficiency of the rotors. Low tip speeds and low disc loading yielded another valuable - although not then appreciated - side effect; the rotors were almost noiseless. Combined with a muffled engine (such as Kaman's K-225 powered by a silenced gas turbine), the craft was so silent, its flight could go undetected as close as 500 feet away.
In addition to having good flight stability in pitch and roll, the last generation of synchropters was also stable in yaw. The absence of yaw oscillations in the intermeshers of course came as a direct result of equal division of torque between the two rotors. The left rotor exactly balanced the right in straight flight, completely eliminating the annoying tail wagging which frustrated many a gunner and photographer in a conventional copter.
Whenever a yaw control was applied to initiate a turn, collective pitches of rotors were changed differentially, aided by some differential fore-and-aft cyclic control inputs. This produced steady yaw moments, which were independent of engine power variations and produced smooth turns without overshooting.
With all these advantages going for the synchropters, one wonders why was their manufacture stopped.
The best way to answer it perhaps would be to say that they were overtaken by more vigorous competitors. There are no basic flaws in the intermeshing configuration itself - it's just that the tail rotor machines were preferred by the customers who were buying the helicopters. The biggest buyers of all, of course were the military services and they forever pushed for higher speeds even at the sacrifice of lifting efficiency.
Synchropters presented too much frontal area and were not easily designed for airspeeds higher than 120 knots. Thus they were outclassed in competitions by tail rotor designs that were capable of 200 and 250-knot top speeds.
With military support withdrawn, the synchropter did not find itself a champion to help it enter into the strictly commercial field where high speeds were of low importance. And so it withered on the vine while still in the prime of life.
In eulogy we must invite the reader to shed a tear or two for its passing. Its vanishing from the aviation scene leaves the amateur homebuilder with the impression that the congenital tail rotor design is the only configuration that is left for him to emulate. And it's too bad, because synchropter configuration, as a flying machine was better suited for amateur rotorcrafters than the tail rotor design. With its low disc loading it had excellent autorotation characteristics, a shallow glide angle, and the capability of a full flare landing with cyclic pitch control only, as in autogyros. It was exceptionally steady and stable in flight, which made pilot training a breeze. In fact, this is why the Navy ordered them in the first place, as trainers - only to decide later that they were "too easy to fly" as a preparatory stage in training for conventional helicopters.
It would be difficult to imagine a helicopter better suited for weekend flying and small, unhurried, flying chores such as aerial photography, agricultural work, sightseeing, etc. Let us hope that some smart designer will pick up where Kaman left off and will give the synchropter its rightful place among the aerial workhorses of tomorrow.
More on the intermeshing configuration.
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Last Revised: February 4, 2007