Are Propfans The Great White Hope?

According to Airbus Industrie (or at least, one of their executives, who had best remain unnamed), propfans are nothing more than “a load of bananas whirling round. ”

Airbus’ outward scepticism falls into perspective if one notes that it has pinned its fortunes on the Airbus A320 150-seater, now under development and due to enter airline service in 1988.

It will largely be powered by what is probably the ultimate development of conventional turbofan jet engines, the IAE2500. This gives Airbus a major incentive to publicly maximise the · advantages of known engine technology while deriding propfans as a new-fangled idea unlikely to reach commercially acceptable standards of reliability and low noise level for at least a decade.

By contrast, Airbus’ arch-rival, Boeing, and to a lesser degree, McDonnell Douglas, has every reason to dangle propfans temptingly before airline managements as a near reality, something akin to the second (or third) coming in aircraft propulsion.

They have nothing to match the A320 except their warmedover derivative designs like the 737-300 and MD-80 series. Their argument is that the A320 can offer only marginal performance, economy and comfort benefits over such existing aircraft, but that if airlines are prepared to wait until 1992, they can gain considerable improvements in efficiency from major new advances in technology.

So just what are propfans and what makes them so desirable?

What is their state of development, what technological problems remain to be solved? When are they really likely to enter service?

And what do things like UDF, UBE, EHB and UBH really mean?

Propfan is a portmanteau term coined to describe a propulsion concept that combines the characteristics of a turboprop with some of those of a turbofan. (Turboprops use a gas turbine turboshaft engine to drive a conventional propellor, in much the same manner as a piston engine drives a propellor. In turbofan “jets”, which power most airliners today, the central gas turbine engine drives a large number of turbine-like blades that extend beyond the central gas generator section and drive “bypass” air around it. All this is contained inside a large engine nacelle).

The term “propfan” was initially applied to a high-speed propellor with multiple thin, swept, variable-pitch blades 80 per cent efficient at Mach 0. 8, driven by a turboshaft engine, via a gearbox. Having two contra-rotating propellors makes no fundamental difference, so the term propfan still applies. The fans are often referred to as “propulsors” since they actually produce thrust, to distinguish them from the turbine engine, or “gas generator” which provides the power.

The essential difference of fans from propellors is that they can remain aerodynamically efficient at higher rotation speeds when conventional propellors would lose efficiency and “quit flying” (in other words, they stop producing thrust to pull the aircraft through the air).

Propfans’ advanced design and materials allow them to operate at higher speeds and manufacturers claim they will move aircraft along at much the same speed as jets, but will consume substantially less fuel. (At the recent Paris air show, the Russians displayed a contra-rotating 16-blader of 4. 6m diameter which they described as a propfan. US engineers claimed however, that it was nothing more than an advanced conventional propellor efficient up to Mach 0. 7, rather than the Mach 0. 8-0. 85 possible with propfan. )

General Electric, which appears to have a development lead in the field and is predicting engine certification by 1992 evolved its unducted fan (UDF) from the single rotation geared propfan, moving first to contrarotation to gain efficiency, then to a pusher configuration to limit cabin noise. As propfan diameter was reduced to ease installation, rotation speeds went up until GE realised that propellor RPM was approaching that at which a contra-rotating power turbine gas generator could be made to operate efficiently.

The result was a gearless propfan with the blades mounted directly on to a free power turbine. The gearless unducted fan is therefore a sub-species of the propfan. UDF, however, is a GE trademark.

Pratt and Whitney is a little more conservative than GE, claiming that the technology for propfans coutd be in place by 1987 with engine certification possible by 1992.

It is also working on a propfan alternative, the ducted prop, which is similar aerodynamically to a turbofan, but mechanically resembles a propfan. Pratt and Whitney refers to it as a turbofan with a higher bypass ratio than current engines.

It has some gearing, but not a gearbox in the conventional sense, and the fan itself will probably have variable pitch.

Probably most suited to larger applications, such as on an· advanced 747, its fuel-burn and efficiency would probably not quite match a propfan but lighter and smaller new-technology nacelles would mean less noise and fewer containment and mounting problems.

Seeking a blanket term to describe both geared propfans and gearless unducted fans, Boeing coined the term ultra-bypass engine (UBE). McDonnell Douglas has gone for ultra-high bypass (UHB).

Instead of clearing things up, this has added to the confusion by grouping propfan/UDF with ultra-high-bypass-ratio turbofans, also known as ultra-bypass-fans (UBFs), such as Pratt and Whitney’s geared turbofan.

Essentially, the difference is this – geared propfans and gearless unducted fans will have bypass ratios of around 40-1 and no ducts. Ultra bypass turbofans will have bypass ratios of 20-1 and ducts.

Propfan output is expressed as shaft horsepower (SHP) because there is a turboshaft engine and gearbox involved. UDF output is expressed as thrust simply because there is no gearbox. At sea level with a static engine, one horsepower is broadly equivalent to two pounds of thrust. In cruise, the two are roughly equal – one SHP to one pound of thrust.

A 150-seat airliner powered by two 25,000-pound thrust turbofans, therefore, would require equivalent-sized UDFs or 12,000-13,000 SHP propfans.

GE (and SNECMA of France, which is taking a 35 per cent share in GE’s UDF program), has contracted to flight-test a development UDF engine with Boeing in mid-1986 and expects to do the same with McDonnell Douglas in 1987.

Pratt and Whitney and Allison are developing engines which will be teamed with a geared propfan unit being developed by Hamilton Standard.

GE and Boeing have wind-tunnel tested a two-foot diameter blade propfan model, powered by a 500hp air turbine, at speeds up to Mach 0. 83, and proclaim themselves generally pleased with results on performance and noise levels.

The demonstrator UDF to be ground-tested later this year before next year’s flight tests uses the core of GE’s F404 fighter engine (which powers the RAAF’s F/A-18 Hornets).

For Boeing’s 150-seater 1992 airliner, GE is developing a 22,000-pound thrust engine using essentially the same unducted fan propulsor but driven by an all-new high-power core. The demonstrator will have a fan blade diameter of 11. 7ft and the tip speed of the carbon cloth/glass blades, joined around a titanium spar, will range between 750-800 feet per second.

Full authority digital electronic control (FADEC) will be used to regulate variable pitch blades and synchronise the rotors. The pilot will set the power and FADEC will set the RPM and pitch angle to comply. Reverse thrust will also be possible.

Addition of the second, contra-rotating fan, is claimed to improve efficiency by about seven per cent, as well as allow more power in a single package.

The aft propellor in effect operates like an exit guide vane on a turbo-fan engine to smoothe out the swirling flow of air from a single fan and make it straighter, thus reducing energy losses.

Initial applications for propfan/UDF are pusher configurations, in an effort to locate the noise source well aft of the passenger cabin, as in the Boeing and McDonnell Douglas flight tests.

A geared propfan demonstration as a tractor on the wing of a Gulfstream II is planned for May 1987 by Hamilton Standard and Allison plans to have its contra-rotation pusher propfan engine ready for flight test late in 1987.

Propfan, with its gearbox, can be driven from the front or the rear of a turboshaft engine, whether single rotation or contrarotation. A pusher configuration presents problems, however, with turbulent flow from the wing, nacelle and engine pylon impinging on the thin blades, and hot turbine exhaust.

While this provides some inherent de-icing it creates certain design problems and Allison and Hamilton Standard are working on lobing the exhaust and leading edge of the propellor to spread the exhaust flow and reduce peak temperatures.

Contrary to popular belief, says GE, UDF does not have to be a pusher. It admits the gas generator core has to be ahead of the UDF propulsor, but the whole engine can be mounted forward of the wing.

The core accounts for only about a quarter of the engine weight, according to GE, and since the main engine mount is located at the join between the core and the propulsor, which has a largely hollow centre, a pylon can be passed through the centre of the propulsor to attach to the main engine frame.

The end result locates the blades in a similar position relative to the wing as does propfan, which is useful if propulsor slipstream is to be used to increase wing lift in a military STOL transport.

While gearbox size and complexity may place an upper limit on the size to which geared propfans may be developed, GE has studied UDF engines with thrusts ranging from 5000 pounds to 75,000 pounds, using existing engine core gas generators.

With scaling up of UDF, the reduction in rotation speeds necessary as propulsor diameter increases is offset in the turbine by the increase in diameter so that efficiency remains roughly constant, according to GE.

GE, Allison, Hamilton Standard and Pratt and Whitney say they will have a viable propfan by 1990. Boeing agrees with them.

Rolls Royce says it will be at least 1995 before propfans or UDF will be suitable for civil service and Airbus generally agrees with it. It points out that massive funding, probably of military origin, will be necessary for propfan/UDF development and GE admits UDF will cost at least $1000 million through to production startup.

Boeing is reported to have already signed contracts with suppliers for risk-sharing development of other new technology equipment ranging from fibre-optic databuses to flight guidance computers adaptable to the global positioning system (GPS) navigation and microwave landing systems. It has set itself and its suppliers a target of 1987 by which to come up with feasible and economic answers to its new technology plans. By that time it plans to have completed market studies and be able to offer performance, size and price guarantees for delivery of the new airliner in 1992.

Although Boeing and McDonnell Douglas both currently favour UDF rather than a geared propfan for their 1992 aircraft in the 150-seat range, they are watching propfan developments by Pratt and Whitney, Allison and Rolls Royce, in case UDF encounters problems.

But GE’s general manager, advanced technology operations, Robert Turnbull, is adamant: “We will have the UDF engine certificated by 1990”