Load Lifting by Design Not Default 

By By Andrew Drwiega, International Bureau Chief | March 1, 2015

A Chinook lifts underslung a 7,540 pound 777 howitzer during an exercise at altitude taking place around Fort Carson, Colo. in August 2014. Photo courtesy of Boeing

Carrying an underslung load has been, and will continue to be, one of the mainstays of military logistical movement, particularly when any force is conducting an expeditionary campaign. For the U.S. Army and the U.S. Marine Corps (USMC), it is a logistical backbone that has been delivering cargo into remote areas, or from base to base, since the Vietnam War when Chinooks were used to lift artillery onto firebases, often hill tops.

The USMC mission profile includes the need to “assure littoral access by bridging the difficult seam between operations at sea and on land.” This requirement is the driving force behind the Corps’ haste to get the CH-53K (Kilo) King Stallion. However, the Initial Operating Capability is still four years away, currently scheduled for the third quarter of 2019. This will be when the first squadron (HMH-366) deploys with the first four of their new CH-53Ks.


In the meantime, the Corps will be reliant on keeping its fleet of around 149 CH-53E (Echo) heavy lifters available to support the full spectrum of operations.

The role of using the CH-53 to transfer underslung cargo is a central one. Although the figure most quoted for the new Kilo is the capacity to lift 27,000 pounds underslung (over three times that of the Echo), that is over 110 nautical miles (NM) in high/hot conditions (which to the Corps means 103 degrees Fahrenheit at sea level and 91.5 degrees Fahrenheit at 3,000 feet).

The actual total weight that the Kilo can lift is a good deal more, according to Col. Hank Vanderborght, the H-53 program manager for Naval Air Systems Command (NAVAIR). “The center hook is actually rated to 36,000 pounds, while the fore and aft hooks are rated for 25,000 pounds each.” The load is taken up through the underbelly of aircraft so that the weight is evenly distributed though the airframe.

Dr. Mike Torok, vice president, 53K programs at Sikorsky, explained that in designing the all-composite airframe, titanium frames were placed under the gearbox to strengthen the load lifting capacity. “You want to have the primary load centered as much as possible beneath the main rotor,” stated Torok. He explained that the critical technologies that drive the CH-53K program are the rotor system, split torque transmission and fly-by-wire controls: “The balance is state of the art, and the challenge is integration.”

“It is important to design an airframe to manage the dynamic responses and resonances in the airframe … so there is nothing that can interfere with the loads that we can take. The flight control system is also designed to ensure that we have good stability when carrying those loads.”

Pilots returning from Afghanistan have reported that the Digital Advanced Flight Control System (DAFCS) installed in the CH-47F makes the task of underslung load carrying much easier. Photo courtesy of Boeing

Vanderborght reemphasizes the importance of three basic aspects of the Kilo when it comes to lifting heavy loads. “The fourth generation rotor blades with anhedral tips give them much better aerodynamic efficiency; the lighter split torque gearbox can handle more shaft horsepower; and the G38 Bravo engine, while being the same size as the Echo engine, puts out twice the horse power which we have already verified in testing.” The engine test cells already have more than 5,000 hours on them already he revealed.

The Kilo is crucially a fly-by-wire system instead of the mechanical system found in the Echo. “Fly-by-wire will come into its own,” said Torok. “There is a lot of automated capability in the aircraft. The controls allow precision movement through automation to make maneuvering close to the ground much easier.” This is crucial when the expeditionary role of the USMC is considered.

The triple hook allows the Kilo to take a considerable amount of logistics and deliver them quickly to three individual sites with only a quick release on arrival. The design requirement has nevertheless stipulated that it can house two 10,000-pound Air Force 463 Lima cargo pallets internally. “An Air Force C-17 can come into a forward operating base and unload pallets directly onto a 53K quickly using floor rollers, which can then fly straight to austere landing zones,” explained Vanderborght. “Right now we have to spend time, energy and people to break those down before re-shipping the contents.”

The power of the Kilo and the downwash from its five-bladed rotorhead should not cause any problems when delivering underslung loads. However, one problem that is sometimes experienced during the delivery phase is that the underslung load can interfere with the radar altimeter (RADALT) on the helicopter. With some loads, it can give a false reading from the load rather than the ground, but visual cues from the aircrew ensure that the load is delivered safely.

Operationally, the USMC has faced the same brownout (descent into dust) challenge experienced by every other helicopter operator, particularly in Afghanistan. “Brownout has been a big problem,” explained Vanderborght. “With the Echo, it has been all about the pilot’s skill in acquiring the ground and safely before releasing the external load, a task that is especially difficult at night. It takes an entire crew in the Echo to do that.”

However, the Kilo’s integrated avionics management (IAM) system means that the pilot can fly the aircraft to a point and assume the perfect hover, notes Torok. “With the flight director, you can dial the aircraft down to the ground … which translates into a greater level of safety.”

RAF Chinook practicing underslung load hook-ups.
Photo courtesy of UK MoD

The computer-aided design process also helped in planning and managing the load capability of the Kilo, added Steven Spoldi, lead for handling qualities and flight dynamics with Sikorsky. The Kilo team has been developing simulations over the past six to seven years, which allow underslung loads to be simulated in a motion-based simulator. “The other day I got a chance to fly a simulated external load onto a maritime vessel,” said Spoldi.

One of the roles that the CH-53 has been used for operationally is for the Tactical Recovery of Aircraft and Pilot (TRAP), and the added power of the Kilo will only serve to add potency to this ability—nothing appeals more to the USMC’s CH-53 community than a photo of its rival, Boeing’s CH-47, slung beneath during an airframe recovery.



Chinook Precision Through DAFCS

Randy Rotte, director of business development for cargo helicopter programs at Boeing, is a 26-year U.S. Army aviation veteran who has flown both Boeing CH-47 Chinooks and UH-60 Black Hawks. He retired with the rank of Colonel and commanded aviation units during both Operation Desert Storm and Operation Iraqi Freedom.

As part of Boeing’s comprehensive and continual review of its rotorcraft in military service, company representatives always join with U.S. Army personnel when aviation units are returning from their operational tours.

“One of the outstanding comments that we unanimously get back from the Army’s Chinook community has to do with the positive effect that the Digital Advanced Flight Control System (DAFCS) has on improving control of the helicopter, particularly when transporting and delivering sling loads in degraded visual environments,” stated Rotte.

“When hooking up, it holds the aircraft ‘rock-solid’ over the top of a load and then gradually descends, even in strong crosswinds,” he added. “This gives them a chance to do missions that they may not have been able to do before.”

Every time a load is dropped off, there is a sense of relief. “The hook-up is more difficult than the drop-off, and trying to get hooked up on night vision goggles (NVG) with someone underneath and others talking to you is certainly a challenge in the older aircraft,” explained Rotte. “When you are under NVGs, hovering at 50 feet with the underslung load at around 35 feet and the ground is below that, locating reference points can be a real challenge. DAFCS has changed the challenge of that dramatically for the better.”

As both a Chinook and a Black Hawk pilot, Rotte has a breadth of experience, but stated that the behavior of an underslung load under different helicopter types doesn’t tend to change that much. Load attitude is usually governed by how heavy it is. “The heavier the load, the more stable it is to fly, as they are less susceptible to wind resistance, but then the extra weight has its own challenge in terms of maintaining power margins,” he said. “Lighter loads can be the most difficult, because they can swing back underneath the aircraft if it goes too fast.”

The Chinook also has three points under the fuselage to attach underslung cargo. “There is a manual on how to hook up different loads,” explained Rotte. The heavy loads are usually slung from the single center hook, although at times the weight will be distributed between the fore and aft hooks, which might be the best option to increase the load’s stability in flight. An important point to remember here is that the load cannot rub hard against the sling, which could wear and even lead to a potential break.

“One of the techniques to stop oscillation is to enter into a turn and let centrifugal force help you. But the digital system picks up on that so quickly the pilot often doesn’t need to make the control input as the system has got there before him and done it already.”

The DAFCS contributes significantly towards a quick approach and minimal loiter time, which is a requirement of adhering to one of the main lessons learned from Afghanistan. “Aircrew know that they need to spend less time in hook-up and drop-off [at Forward Operating Bases] so that they don’t stay in the dust cloud too long.” Not only is this bad regarding dust ingestion into every part of the helicopter, but it is also a tactical giveaway, a point also acknowledged by the USMC’s Vanderborght: “In the desert, a large dust cloud basically tells everyone within 40 kilometers where you are, so there is a real need to minimize loiter time.”

Photo courtesy of
Lockheed Martin

Sometimes what was initially reported as a benign landing zone suddenly turns hot. That only emphasizes the advantage of an underslung load over cargo carried internally, “You can pickle the load and escape fairly quickly,” said Rotte. Heavy internal cargo in a ‘hot’ landing zone also makes any maneuvering out of the threat band a lot more difficult.

Looking ahead at Future Vertical Lift (Heavy), part of the Department of Defense’s future plan for rotary-wing lift from the mid-2030s and beyond, Rotte sees the limiting factor specifically in terms of the external load. “You may spend a lot of money to go fast in a new platform, but it is the underslung load that is still going to be a limiting factor.”

Of course, by then designers may have found a way to mitigate this drawback. In 2010, there was an announcement from the Israel Defense Forces (IDF) that it was studying a new cargo container that had been specifically designed to have aerodynamic wings, which would allow it greater stability in flight when being carried externally under a helicopter.

Developed by the Israeli Ministry of Defense’s (MoD’s) Research and Development Directorate (MAFAT) together with the Technological and Logistics Directorate, it was claimed to allow a helicopter to fly at nearly normal speed with a ton of cargo underslung. Neither Vanderboght nor Rotte were aware of this and subsequent attempts to find out more proved unsuccessful.

Related: Military Heavylift News

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