A major factor in the continued domination of turbofan engines for economic airline propulsion, is the ability to increase bypass ratios. However, increasing fan diameters to go to ever higher bypass ratios increases nacelle weight, aerodynamic drag and duct losses. By eliminating the need for a fan duct, open rotor engines can effectively in-crease bypass ratios, resulting in a significant savings in fuel consumption.

Open rotor engines have been under intermittent development since the 1980s, sub-ject to rising and falling of fuel prices. Technical challenges that continue in their de-ployment include noise reduction, airframe integration and protection from propel-ler/fan blade failure.

## article

The open rotor aircraft engine, with its eye-catching two rows of contrarotating scimitar-like propeller blades, has been under intermittent study and development since the 1980s. Holding the promise of the speed and performance of a turbofan, it is designed to provide the fuel economy of a turboprop engine.

The open rotor is essentially a turbojet engine that drives twin exterior nacelle mounted propeller/fans at the rear of the engine. Since its conception in the 1970s at United Technologies Hamilton Standard Division (in conjunction with NASA), it has had a miscellany of names: Propfan, unducted fan (UDF), advanced turboprop and more recently, contra-rotating open rotor (CROR), or open rotor for short. (Some use counter in place of contra, while in propeller terminology the latter refers to same axis rotation only, and the former denotes different rotational axes.)

## Propulsion Efficiency

To better understand open rotor engines, we need to look at the propulsion efficiency of jet engines in general. The first commercial jetliners were powered by turbojets - jet engines in which all thrust was provided by gases that went through the engine from inlet to exhaust nozzle, exiting in a single high velocity jet. The resulting momentum increase provides the thrust force necessary for flight, but kinetic energy is “wasted” in the exiting jet.

A more efficient design is the turbofan jet engine, so-named for a ducted fan mounted in the front. Air drawn into the fan is divided, with some flowing out of the fan into the jet engine itself and the remainder bypassing the engine. The lower velocity bypassed air and the higher velocity engine air combine downstream to produce thrust with a larger mass flow at an average velocity lower than the high velocity jet flow.

With a large frontal area, the commercial aircraft turbofan is designed to produce peak thrust at takeoff, with most of thrust produced by air drawn in by the fan and bypassing the jet engine core itself. Bypass ratios - the mass of fan air for every unit mass of air through the engine - currently can be as high as 9:1, as in General Electric's 100,000 pound thrust GE90 engine that is used on Boeing 777s, and 12:1 in Pratt & Whitney's PW1400G 30,000 pounds thrust geared fan engine for the Airbus 320.

For subsonic flight, the propulsive efficiency, ηp, of a turbofan is higher than that of a turbojet. This efficiency is defined as the useful propulsive power (the product of thrust and flight velocity, Vo) divided by jet power (rate of change of the kinetic energy of gases through the engine). This simplifies to [1],
$ηp=2Ve/Vo+1$
(1)

where Ve is a suitable average of the lower velocity bypass air and the higher velocity jet exhaust.

In a straight forward and elegant way, Equ. (1) shows that to maximize propulsion efficiency, an engine must produce thrust by moving air through it with as little a change in velocity as possible. Thus, a turbojet engine with a higher value of Ve/Vo , has a lower propulsion efficiency, than a same-thrust turbofan engine with its lower velocity ratio. Also, for turbofans we see from Equ. (1), that the higher the bypass ratio, the greater is propulsion efficiency.

## Why an Open Rotor

As we have seen, a major factor in the continued domination of turbofan engines for economic airlines propulsion is the ability to increase bypass ratios (see Equ. (1)). Currently, these are as high as 12:1 in the Pratt & Whitney geared fan engines with Rolls-Royce planning a new geared fan engine at 15:1.

However, increasing fan diameters to go even higher in bypass ratios increases nacelle weight, aerodynamic drag and duct losses, countering the efficiency gains predicted by Equ. (1).

Open rotor engines can significantly increase effective bypass ratios to more than 30:1, to yield high values of propulsion efficiency (Equ. (1)). By eliminating the need for a fan duct, studies show that open rotor engines can have a 25-35% fuel consumption saving, compared to most current turbofan engines in service, and a 10-20% saving for the newer turbofans, recently introduced into the market.

Cruise Mach numbers, M, of airliners range from 0.7 to 0.9. For the higher end of this range, propeller losses increase markedly (shocks forming in tip regions) so open rotor engines are typically being designed to run at a maximum of about M=0.78, taking full advantage of swept scimitar shaped blades to mitigate compressibility effects [2].

## Open Rotor Features

Gas turbine aerodynamicists strive to have engine gas path flows move axially wherever possible, to minimize losses and maximize axial momentum changes. Thus, open rotor engines have two rows of pitch controlled, contra-rotating propeller/fan blades, with the second row taking out the swirl from first, so that its exit flow is in a near axial direction. (This two row configuration also means that the aft row chops through blade wakes from the first row, which can generate siren-like noise.)

The propeller/fan blades are rotated by the turbine of the turbojet, by either direct drive or by a gearbox. Direct drive is gotten from contra-rotating, statorless turbine stages, which means both rows contra-rotate at the same speed. A gearbox adds weight, but allows for the flexibility of control of different speeds for each row.

A comparison of turbofan and open rotor (propfan) engines has been given by Hendricks and Tong [3]. The comparison to similar thrust sized engines is summarized in Table 1, taken from [3].

One can note in the table, the predicted reduced fuel consumption of the 13.8 foot diameter propeller/fan open rotor engine, compared to the much smaller diameter turbofans. Also, the open rotor weight is highest but an aircraft so powered, will require less fuel to complete its mission. This leads the authors [3] to deduce that overall aircraft takeoff gross weight would be similar between turbofan and open rotor powered aircraft. (However, it should be noted, aircraft weight might be added if fuselage hardening is required for safe open rotor operation.)

## Open Rotor PROGRESS

With its Hamilton Standard inception and the Arab oil embargo induced fuel price hikes of the 1970s, open rotor projects commenced in earnest in the 1980s.

A Pratt & Whitney/Allison 578-DX open rotor engine was flight tested on a McDonnell Douglas MD-80 passenger aircraft in 1989. It had propeller/fan blades of diameter 11.6 ft., driven by a 13:1 gearbox. With falling fuel prices, the program ended, with a team member stating [4], “The operation was successful, but the patient died.”

Starting in 1986, General Electric Aviation tested their GE36 open rotor engine (trade named Unducted Fan, or UDF) on both MD-80 and Boeing 727 aircraft. With ll.6 ft. diameter propeller/fan blades, it had a direct turbine drive. The GE36 demonstrated a 15% fuel burn reduction, compared to contemporary turbofans [2], based on some 281 hours of flight testing. Faced with a market where fuel prices were dropping, the program was ended in 1989. In the words of a GE manager, “The signs were there the year before, that at 65 cents per gallon, the fuel price was too low to justify the UDF. If fuel were at a buck or so a gallon they'd be clamoring.”

In this new century, as fuel prices rose dramatically in the early 2000s, the price of a barrel of oil continues to be a deciding factor for the fate of open rotor engines. Added to this, is the consideration of climate change and reduction of greenhouse gas emissions by aircraft engines.

Recent development efforts include Rolls-Royce, which has a prototype gear-driven open rotor engine, the RB3011, under development and aimed at a market entry by 2020. In France, Safran Aircraft Engines has completed ground testing of an open rotor engine, as part of the European Clean Sky 2 research program.

The Safran open rotor engine has a gearbox, with carbon fiber 15 ft diameter propeller/fan blades. Recently plans for flight testing the engine on an Airbus aircraft have been halted, with reports Airbus interest in the fuel-saving engine have been put on hold.

## Open Rotor Challenges

Van Zante [2] has outlined three technical challenges that remain to be addressed for successful open rotor propulsion:

1. Noise is a problem. Additional noise reduction beyond what has been accomplished to date, is necessary.

2. Airframe integration, with open rotor propeller/fan blade diameters of 12-15 ft. is a challenge, for both wing and tail mounting locations.

3. Protecting passengers in the event of a propeller/ fan blade failure (e.g., caused by a bird strike) needs to be addressed.

Of course, underlying everything, is the price of a barrel of oil!

## References

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Hendricks, Eric S. and Tong, Michael T., 2012, Performance and Weight Estimates for an Advanced Open Rotor Engine”, 48th J. Prop. Conf., AIAA-2012-3911, NASA/TM-2012-217710.44
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