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Saturday, March 1, 2008

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X-49A Numbers

When I saw the X-49A cover, I eagerly looked forward to learning more from your writer, Steve Colby, about the Piasecki Aircraft Corp. design ("Beneficial Bird," December 2007, page 46). I’ve been fascinated with it since I first saw an artist’s rendering of a compound UH-60 so many years ago and was intrigued even more after reading of the AH-56’s 3,000-shp engine transmitting 2,250 shp to the pusher prop in high-speed cruise and only 750 to the main rotor.

As a pilot, I’m a numbers guy and just can’t make sense of some of your numbers. Could you please clarify some points?

You say the ducted propeller is designed to absorb 80 percent of installed horsepower to provide thrust. The given figure, 3,400 shp, has nothing to do with H-60 engine horsepower. It is, rather, the main gearbox input limitation on the -60L and SH-60s. The engines produce more than the gearbox can handle to give the aircraft more than just ISA-@-sea-level performance, just like any other helicopter.

Using the transmission rating number as a guide, does this mean you are referencing the total installed horsepower — 3,400+600 for the Rolls-Royce C-250-30 supplemental power unit (SPU) — and 3,200 shp (80 percent of 4,000) goes to the ducted propeller? If so, this seems to disagree with the number from the new, wider gear-faced tail-rotor output pinion that has been upgraded to handle "well over three times the original’s 600-shp rating." These two figures — 1,800+ vs. 3,200 — are two very different numbers. Which one is it?

Also, is the need for 600 shp more (from the SPU) at the pusher prop specifically for added anti-torque thrust during hot/high conditions and/or while hovering?

David Wagner

Era Helicopters

El Paso, Texas

Steve Colby replies: Mr. Wagner is absolutely correct about gearbox limits vs. installed power. Let’s look at Piasecki’s planned design phases.

Phase 1 uses just two standard T700 engines, with output to the main rotor and the ducted propeller through the upgraded tail-rotor driveshaft power path (via the new combining/reduction gearbox). The main gearbox can transmit about 1,800 shp to the ducted propeller. It is limited by the new gear-face design. (The old gear’s limit is 600 shp, hence the 3X-power claim). The 3,400-shp input module limit and 1800-shp output gear limits are why the current aircraft won’t get much above 180 KTAS (which is also a U.S. Navy administrative limit on Phase 1 tests).

Phase 2 would add 600 shp via a SPU, which inputs directly into the combining gearbox, then to the ducted propeller for anti-torque or thrust (as required by flight regime).

Phase 3 would put a third CT7-series engine in place of the Rolls SPU, which would add roughly 1,700 shp.

The Speedhawk employs a dual (actually triple) power-path design.

In Phase 2, the X-49A would use all 3,400 shp, through the input modules, to produce lift in normal hover modes or lift and thrust in forward flight. It would not waste installed "input module horsepower" on anti-torque.

The combining gearbox would be a secondary power path that transmits SPU horsepower directly to the ducted propeller for anti-torque, bypassing the input module torque limits. This 600 shp would bring total installed horsepower to 4,000; it would be used for anti-torque in hover and for thrust in forward flight.

The main gearbox pinion can direct 1,800 shp (its face-gear limit) to the ducted propeller or absorb SPU horsepower into the main gearbox as essentially a third input module. This means in Phase 2 the SPU can augment main engine power for high/hot hover and provide additional thrust for anti-torque.

As it is in Phase 1, Phase 2 horsepower to the ducted propeller would be far less than what that aft unit can handle. Mr. Wagner astutely observes that our numbers were presented with a leap from Phase 1 to Phase 3.

Piasecki designed the ducted propeller for Phase 3 installed power levels. Roughly (since actual figures are proprietary), that would be 3,400 (from the input modules) plus 1,700 (from the third CT7), or 5,100 shp. Of that, 80 percent (roughly 4,080 shp) would go to the ducted propeller (with the X-49A cruising at roughly 235 KTAS) and 20 percent (roughly 1,020) would go for lift/roll.

Piasecki actually plans to run a lot more of the third engine’s power (compared to the original SPU) both to the ducted propeller and back into the main gearbox via the ducted propeller output shaft. This could add main-rotor power through a third main-gearbox load path for high/hot performance that wouldn’t be limited by the 3,400-shp torque limit of the input modules. This is how they can claim that their design improves hover performance as well as forward speed. Some hover performance would be lost with the downwash effect on the wing surfaces, but this is more than offset by the available power and auxiliary power path.

Thanks for your questions. It tells me my audience is actually scrutinizing what I write.

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