Higher RPM. That being said, it isn't that much higher. A turboprop prop maxes out under 2000rpm. A turbofan fan spins at around 3000rpm.

Not quite that much. The GE36 UDF had a fan blade tip speed of 780 feet per second (238 meters per second) at cruise and 850 fps (259 mps) at takeoff (

UDF design report, p. 141).

- Since the length the blade tip travels in one revolution is the fan circumference, you have
**(blade tip distance traveled per one revolution) = 2 * pi * (fan radius) = pi * (fan diameter)**. - Multiplying by rotational speed, you get
**(blade tip speed) = pi * (fan diameter) * (revolutions per minute, or rpm)**. - Converting to seconds results in
**(blade tip speed in fps or mps) = pi * (fan diameter in ft or m) * (rpm) / (60 seconds per minute)**. - Solving for rotational speed yields the equation
**(rpm) = (60 / pi) * (blade tip speed in fps or mps) / (fan diameter in ft or m)**.

The source for the blade tip speeds also lists a diameter of 11.67 feet for the GE36, so you get a calculated rotational speed of 1,277 rpm at cruise and 1,391 rpm at takeoff.

You need the cruise speed to calculate the helical (3-dimensional) speed, though.

Page 17 of the UDF engine test report lists the maximum cruise speed at Mach 0.80 at an altitude of 35,000 feet. The

Aviation Calculator website converts that into 778 fps, so the cruise speed is about the same as the rotational speed. Applying vector geometry means the helical speed is the square root of (780 fps ^ 2 + 778 fps ^ 2), or 1,102 fps (which is Mach 1.13 if you run that number through the website).

Also,

page 163 of the design report has the heaviest blades weighing 22.5 pounds, if you want to do some kinetic energy calculations.

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What set GE's UDF apart from the other open rotor proposals is that it was a gearless direct-drive setup. The contra-rotating propulsors were driven by free-power turbines (unattached to either high-pressure or low-pressure compressor), but one was attached to a regular shaft, while the other was attached to a rotating drum. The regular shaft had turbine blades representing one-half of each power-extraction stage attached to it, as expected. The rotating drum had a propulsor attached to its exterior wall, but its other half of turbine blades for each stage were attached to the interior wall of the drum. Since both halves of each turbine stage were rotating parts going in opposite directions, there wasn't a traditional non-rotational stator element as in normal turbines, and the effective rotational speed of the turbine was twice of the speed of the contra-rotating propulsors (so 2 * 1,277 = 2,554 rpm). Since the effective turbine rpm is fairly close to a comparable turbofan's fan rpm, the free-power turbine on the UDF can extract power using as few as (or almost as few as) the number of stages in the turbofan's power turbine.

Also, since it had a contra-rotating fan rotor setup as free-power turbines, the GE36 UDF was a three-shaft architecture (plus a rotating drum). When the contra-rotating propulsors are behind the engine core, like the GE36 was, the two compressor shafts are coaxial with each other, but then the propulsor shaft+drum don't have to be coaxial with the compressor shafts. (The drum propulsor might even be coplanar with one of the turbine stages.) GE claimed that the UDF could be designed with the propulsors in front of the engine core, like how turbofans and most turboprops are normally configured, but I think I read that they had trouble with that setup. The propulsor shaft would then have to be coaxial with the compressor shafts, and the drum would have to extend forward of the engine core to attach to its propulsor, but still extend aft of the core to attach to the turbine blades.

The more recent open rotor proposals have all included gearboxes instead of the 1980s UDF design. I don't think GE will need to reprise the direct-drive design, either. It has the 7,500-horsepower GE38/T408 turboshaft engine in service now. With a contra-rotating open rotor, the gearing requirement is supposedly two gearboxes supporting half of what the total horsepower requirement is. 15,000 horsepower might be enough to power the open rotor engines of a narrowbody plane family.

IMO, the more promising application of the contra-rotating direct-drive design might be for ducted engines. Rolls Royce (and probably Pratt & Whitney) have gearboxes powerful enough to handle widebody engines, but GE is a question mark. The GE9x produces 105,000 pounds of thrust using a 134-inch fan. If GE wanted to do an equivalent contra-rotating ducted fan with the same fan tip speed and the same number of turbine stages (ignoring the larger beneficial increase of bypass ratio), it would double the fan diameter, which would halve the rpm. But the effective turbine rpm would be the same. Somehow I doubt the airframers would be willing to fit a 268-inch fan diameter engine onto their planes, so the fan diameter increase would be smaller, which would still lower the rpm from the original GE9x, but the effective turbine rpm would now beat the GE9x. So the UDF design -- essentially a 2:1 ratio invisible gearbox -- could still potentially reduce the number of turbine stages in ducted engines, just like real gearboxes do.