T-prop single starts life as P Navajo piston twin

 - May 6, 2008, 9:41 AM

Converting a piston twin into a turboprop single may seem like a bizarre idea, but an international team of technicians and engineers in Russia and Switzerland has combined its expertise to produce just such a transmutation. [Beech created the Lightning experimental prototype in the mid-1980s by placing a Garrett TPE331 in the nose of a Baron 58P airframe.–Ed.]

Encouraged by growing market acceptance of single-turboprop executive aircraft designs and aware of the demand for low-cost but reliable air transport in the former Soviet Union, Intracom  in 1998 set about developing an innovative solution. The combination of a proven airframe with a turboprop engine widely used in eastern Europe and Russia was perceived to be an attractive way to bring a new design to market relatively quickly.

Intracom General Machinery, with facilities in Russia, Kazakhstan and Switzerland, launched the GM-17 Viper project with the object of producing a multipurpose aircraft much faster than would have been possible if starting from scratch. The team chose the Piper PA-31P pressurized Navajo as the basis for the Viper and a single 751-shp Walter 601E turboprop as its powerplant. The Czech-built turboprop engine is well known in the CIS, not least because it powers the widely used 19-seat Let L-410 regional airliner.

While a standard Avia Hamilton three-blade propeller is fitted to the Viper prototypes, Intracom managing director Dr. Nik Schmidt told AIN that series-production Vipers will be fitted with a specially designed propeller from the same company. “We looked at a five-blade prop from Avia Hamilton, but the gains in performance and noise levels were insufficient to justify such a change, so new blades in the current configuration are preferred instead.”

Indeed, production Vipers may well be powered by the Walter M601F, which has a 3,000-hr TBO compared with the 1,600-hr TBO of the E model. Schmidt noted that the Pratt & Whitney Canada PT6 could be fitted as an alternative, but this would increase the price of the Viper, which is currently set at $860,000.

Before conversion work began at the Intracom plant at Smolensk, a scale model was tested in a wind tunnel at the Zhukovsky Institute in Moscow. After those initial design tests, three prototypes were produced. The first, without cabin windows, is being used for static tests. A flying prototype first went aloft some nine months ago, and the third aircraft, to be built to production standards, will be used for certification trials starting next year. Final flight tests will be conducted in Switzerland.

Considering the radical nature of the conversion from P Navajo to Viper, remarkably few changes have been made to the original airframe. The basic wing shape remains unaltered following removal of the 425-hp Lycoming TIGO-541 engines, though winglets have been added  and the nose has been lengthened to accommodate the turboprop engine. A small fillet has been added to the horizontal stabilizer and the Viper has a new ventral fin.

However, the interior of the Navajo has been extensively upgraded, complete with new paneling and furnishings, while new hydraulic, heating, pressurization, air conditioning, de-icing and brake systems have been installed. The extended nose includes a 20-cu-ft baggage compartment that, like the existing 22-cu-ft rear baggage compartment, is placarded for 200 lb. Hinged upper and lower panels allow easy access to the engine.

A new instrument panel has been devised for the Viper and the upgraded avionics include items from Garmin and Honeywell Bendix/ King. While the cabin window layout of the Navajo has been retained, with three on the right side and two on the left, Intracom has redesigned the window frames.

Up to eight passengers can be accommodated in the Viper, although Schmidt said some potential customers are interested in using the aircraft for special-mission or monitoring roles.

Detailed design changes that will be applied to production Vipers include a slightly shorter nose, and the Smolensk plant is gearing up to begin production of 32 aircraft converted from the P Navajo.

However, in 2005 the plan calls for production of entirely new-build Vipers for which the company will have to build jigs and tools. New Piper has expressed no interest in becoming involved with the Viper program and indeed may perceive it to be a competitor to its own Meridian turboprop single.

Between 1970 and 1977, some 269 PA-31Ps were produced, so Intracom should have no difficulty in acquiring pre-owned aircraft to produce its initial batch, which will be largely promoted in the CIS market. Supporting the claim of low-cost operation, Schmidt noted that it costs around $514 to top off a Navajo but only $330 for the Viper (developed models will include additional tanks).

The Viper’s mtow is 7,268 lb, and the useful load is 154 lb greater than the P Navajo’s. However, the Viper’s maximum cruise speed is 227 kt, about four knots slower than the PA-31P’s. The Viper has a service ceiling of 23,500 ft and a maximum range of 1,560 nm. Its takeoff distance is 2,400 ft and the landing distance is 1,540 ft.

Schmidt intends the Viper to make its international debut at the AERO trade show and exhibition, to be held in April in Friedrichshafen, Germany. He also aims to secure Russian certification and make first production deliveries in the same year. Although he will not seek orders until certification nears, Schmidt reports strong interest from four or five potential customers.