Block 2 CH-47F To Tackle Payload Shortfalls

By Graham Warwick
Source: Aviation Week & Space Technology

While the D-to-F upgrade improved the airframe, cockpit and flight controls, it added empty weight. Other modifications, such as armor, engine inlet particle separators and exhaust infrared suppressors, as well as added survivability equipment such as laser missile jammers, have also increased weight. Block 2 will seek to “buy back” lost payload by both boosting lift and trimming weight, Boeing says.

“We are working with Tradoc [Army Training and Doctrine Command] on the requirements for the future Chinook and the lifting capacity needed after 2020,” says Lt. Col. Joe Hoecherl, CH-47 modernization product manager. “We are also working with Boeing, doing initial development with the focus on payload, primarily JLTV.” The upgrade is expected to be fiscally constrained, so trade studies are under way to identify potential changes and determine what will be affordable.

Because of budget limitations, the Block 2 is expected to involve remanufacturing existing F-model Chinooks, a process some airframes already have gone through twice, having been built as CH-47As then rebuilt first as D models, later as Fs. The D-to-F upgrade has modernized the airframe and avionics, so the Block 2 program is focusing on the dynamic system and other areas of the aircraft.

One of the first pieces of Block 2 is the advanced Chinook rotor blade (ACRB), which is already under development and planned to be cut into production during the second multiyear contract, in late fiscal 2016. Drawing on experience from the canceled Boeing/Sikorsky RAH-66 Comanche, the ACRB has new airfoil sections, increased twist and a redesigned tip. This increases lifting capacity by more than 1,900 lb.

Blade length is unchanged and no detrimental loads are introduced into the rotor hub or controls, says Boeing, allowing the hub to remain unchanged and enabling existing Chinooks to be upgraded, which is important as there will be approximately 400 aircraft in the field by the time the new blade becomes available.

It is expected the engines will be upgraded, with Honeywell's T55-715 turboshaft offering 20% more power than the 4,900-shp -714A now powering the CH-47F. “We are looking closely at increasing the power capability,” says Hoecherl. This would require modifications to the combiner and nose gearboxes and, unlike the ACRB, would come at the cost of extra weight. So Boeing is looking for modifications that will remove weight from the Chinook.

Foremost among these is a new fuel system derived from that used in the special-operations MH-47G and Canadian CH-147F variants. Instead of three tanks per side—forward, center and aft in the fuselage sponsons, each with their associated pumps and plumbing—there would be a single tank. “There are two main and four auxiliary tanks, so there is potential to consolidate them as eliminate pumps and plumbing,” he says.

While the MH-47G and CH-147F have “fat” sponsons housing extended-range tanks with twice the fuel capacity, Boeing is investigating a “skinny” single-tank solution for standard Chinooks—with the potential that eliminating the extra tank hardware could allow the sponsons to be slimmed down to reduce download from the rotors and so increase lifting capacity.

Another expected element of Block 2 is the active parallel actuator system (APAS), a next-generation version of the CH-47F's digital advanced flight-control system. APAS would provide tactile cueing for pilots and improve rotor torque management. On today's Chinook, pilots do not know the exact split of torque, and therefore lift, between the fore and aft rotors so a safety margin is built in. APAS would reduce that margin and allow more of the rotor system's performance to be used.

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