Sound Off

Not all ducted fans are duds

I read Bill Kellys Duct Dreams in the May 1997 issue of Aviation Consumer with interest and sympathy. His disappointment in the performance of the Auriga Phoenix is similar to many tales of dashed ducted-fan expectations I have heard over time.

I think that much of that disappointment is based on an imperfect understanding of ducted-fan design. So, the question is whether a ducted-fan aircraft could be competitive with a conventional design?

While I am not familiar with the Auriga design, I have had the good fortune to be associated with Rhein-Flugzeugbau, GMBH (RFB) in Germany during the development of the Fanstar ducted-fan program during the 1980s.

I was chairman and senior partner of Fanstar Partners, a U.S. investment group. Along with RFB and the government of Quebec, we were involved in putting the FANSTAR program together.

I was also one of RFBs demonstration pilots and demonstrated the Fanliner (Fanstar 01) for the two years it was in this country. Also, I flew some of the different models of the Fantrainer to gain familiarity with the design and to evaluate the aircraft as a basis for the Fanstar.

RFB has long been a pioneer in ducted-fan technology, and along with Dowty-Rotol in Great Britain, has developed a successful technology. During the 1960s and 70s, RFB had designed and built several prototype ducted-fan aircraft, gradually improving the design of the fan.

This program was capped in the late 70s, when RFB and Grumman-American collaborated on the design of the rotary-engine powered two-seat Fanliner.

With a radical, bug-like (almost) all-glass forward fuselage, the Fanliner demonstrated forcefully that ducted-fan technology was a practical and efficient alternative to the conventional tractor propeller configuration. The Fanliners 150 HP NSU rotary engine provided smooth power and low vibration. However, events proved that the engine was just too small to be acceptable. It had problems with high density altitude conditions and with flight above 5000 feet on warm days.

The fix was a 200 HP turbocharged version of the engine. But the program was canceled by Grumman-American before the swap could be made. Since RFB was busy developing their ducted-fan Fantrainer military trainer, the Fanliner program was put aside.

The Fantrainer, however, was successfully marketed to the Thai military and is still in use there. About 80 were produced altogether. Several examples of more powerful models have been built and eventually a pure jet version formed the basis for the Rockwell Fan Ranger JPATS candidate.

Dr. Hanno Fischer, the technical director of RFB during the developmental years, was the impetus for the ducted-fan research. After working through hundreds of possible designs, he learned that the successful ducted-fan was not ducted at all. Rather, he found the best design was a shrouded propulsor or fan-in-a-ring.

The fan-in-a-ring provided the level of thrust required because it could freely move as much air through the ring as possible. It allowed the fan to be designed as a pure-thrust generator, because the ring did away with a significant percentage of tip losses.

Of course, tip clearances within the ring were critical. Fan blade creep was a consideration and clearance tolerances had to be compromised slightly to accommodate this. But, all-in-all, the Hanno Fischer fan-in-a-ring worked, and worked well.

The fan-in-a-ring is a thrust generator, just like a jet engine. It must be operated in a like manner. In the Fanliner, the NSU engine was capable of a maximum of 6500 RPM. A reduction gearbox of almost a 2-to-1 ratio drove the fan. On take-off, the throttle was full forward. In order for the fan to generate enough thrust to fly, the Fanliner had to gather speed.

At 70 MPH, the fan was developing enough thrust to rotate. It was leveled at about 100 feet and allowed to accelerate to 120 MPH. It could then climb at about 750 FPM on a standard day. Up and cruising, the Fanliner could see 150 MPH on 150 HP. With the projected 200 HP turbomotor, performance would have been significantly increased.

In flight, the Fanliner was operated between 5500 and 6000 RPM. Below 5000 RPM, thrust fell off. In the pattern, power could be reduced to 5000 RPM to slow the aircraft to 100 MPH on downwind and 80 MPH on final. It was wise to carry at least 5000 RPM until flare and touchdown.

The Fanliner was a smooth, low vibration aircraft with delightful handling and incredible visibility. It was not a short field aircraft, but it could be operated out of a 2750-foot runway without anxiety.

The Fanstar 200T (FL-4A) was designed to transfer the RFB fan-in-a-ring technology to a four-place retractable general aviation aircraft. The design was based almost 65 percent on Fantrainer components.

It was powered by a Continental Voyager TSIO 550 of 350 HP. It was designed to carry four FAA folks plus 200 pounds of baggage. It also carried a maximum of 120 gallons of fuel in internal tanks and could carry an additional 50 gallons in two external 25-gallon slipper tanks. All-up weight for take-off, with external fuel included, was 4,100 pounds.

The Fanstars maximum cruise speed at 20,000 feet was 230 KTS and 80 percent power yielded 212 KTS. At 10,000 feet, max cruise was 208 knots and 80 percent was 192 knots. Average rate-of-climb to 20,000 feet was 1250 FPM. Both take-off roll, and landing roll were under 1000 feet. Finally, maximum range at 80 percent power was 1350 stature miles with internal fuel and 1900 miles with internal/external fuel.

Obviously, the Fanstar was designed to have outstanding performance and long range to meet the needs of emerging markets in the Pacific Rim. The cabin had very generous dimensions with four large individual seats and was air-conditioned.

Flight in the Fanstar was quiet, with low vibration and low noise-and the aircraft was to be equipped with radar as standard and EFIS systems were to be available.

As is so often the case, the Fanstar program was discontinued due to a lack of funding, not because of performance considerations.

I think the Fanstars performance answers the question about whether a ducted-fan aircraft can be competitive with everything else in its class. The answer is yes!

-by Bill Rice