The MD 902 Explorer

The market versatility
Ken Armstrong
October 17, 2007
By Ken Armstrong
md902In 1987 McDonnell Douglas Helicopter Company (MD) invited 177 major helicopter operators to help create the perfect helicopter for their worldwide needs.  Representing 26% of the helicopter fleet leadership, these businessmen specified the new design should be a twin that elevated the state of the art for safety, reliability, performance and low operating costs – while maintaining or improving on simplicity. A tall order. Nonetheless, these goals have been achieved with novel techniques and engineering.

During recent test/evaluation flights the question arose: “Where does this helicopter fit in the marketplace and which operators would benefit most from its strengths?” My first flight in the MD 900 series a few years ago confirmed what the marketplace already knows – this is a great helicopter for EMS, hoisting, overwater and police operations. My latest flight evaluation turns up more of the Explorer’s strengths.

The 902 would also make an excellent executive transport and is very competent in the utility category as well. My flight in the LUH (light utility helicopter) competition prototype revealed a helicopter that led in technological advances and had arguably the safest twin turbine on the market. It’s not just the incredibly crashworthy structure, impact-absorbing seats and undercarriage that maximize protection from occupant injury, but a host of other design considerations. Advanced technology also provides the ability to meet Category A performance requirements for OEI (one engine inoperative) operations that allow the machine to fly away from potential accidents that could claim competitors. Moreover, the easily read instrumentation when coupled with the Chelton flight instrumentation system virtually eliminates pilot workload. It should be noted the Chelton system is optional and while it provides many useful cues for VFR flying, it’s greatest strength is during IFR operations when it really excels at allowing a pilot to picture his position and be very situationally aware.

Psychologists determined long ago during bio-feedback testing that aviators with reduced stress levels had more highly tuned senses to assess outside stimuli and were more effective in their aircraft handling.  The fact that the Explorer was so easy to fly made my broad spectrum testing easier to assess and quantify, and this characteristic also implies greater safety margins for occupants of this helicopter type.

Historically Speaking
In the beginning, there was the MD 900 and it was technologically grand – but overpriced by $1 million in the competitive marketplace. Nonetheless, the Explorers were very popular in Europe with their safety-oriented JAR-OPS 3 requirements and since the series provided the only light twin that met the lifting and noise restrictions as well as FAA and JAA certification for single-pilot Category A IFR operations, the Explorer excelled overseas. However, sales languished in North America where service/support issues arose. These problems are consistently evaporating under the guidance and capital investment of the new owner, Lynn Tilton, with the result that MD is poised to gain market share.  Moreover, MD has held the line on acquisition/operating costs and even significantly reduced the prices, making the Explorer a bargain – especially considering the low operating costs that compete with fixed-wing twin turboprops.

Into The Future
This helicopter is actually an optimized cabin with the helicopter components built around the enclosure. Although the same size as the JetRanger (and shorter than the 407 or Astar), the interior dimensions are huge with a flat floor, high ceiling and 124 cubic feet for occupants – plus another 48.7 cubic feet for baggage (accessible through the cabin or a large, hinged door under the tail boom).

The helicopter is rather tall, placing the blades well above the domain of occupants. The NOTAR anti-torque system not only produces whisper-quiet operation but also eliminates the ability of feckless passengers walking into a ‘shredder-like’ tail rotor. The cockpit is roomy, well laid out and visibility is exceptional. The seats move forward and up or down and aft to accommodate virtually any crew height. For police, EMS, executive and other operations, the left seat occupant has a virtually unrestricted view as the low-browed, smallish instrument panel utilizes an Integrated Instrument Display System (IIDS, pronounced ‘idzz’) that eliminates the need for dozens of instruments cluttering a large panel. The IIDS also allows the pilot to display only the mission essential instrument readings in the liquid crystal display. Nonetheless, any malfunction such as an engine spool down would immediately present itself as an audible warning with automatic data readouts for the pilot’s attention.

From the safety standpoint, cabin occupants sit in vertically stroking seat supports that absorb impact energy (11-13 g’s) and the Explorer pilot seats can dissipate 30 g’s of impact at 30 feet per second without causing occupant injuries.  That should pretty much eliminate anyone’s rough landings whether due to judgment or vortexes.

Engineering Advances
The 902 Explorer is an enhancement (Category A version) of the 900 model with advanced Pratt and Whitney 207E engines offering 11% performance enhancement and a higher gross of 6,500 pounds.
During the walkaround one learns the 900 series are tall helicopters, yet easy to preflight as visual gauges ensure all fluid levels. Heavy-duty panels fold down for technicians (or pilots) to have a detailed look at the “on condition” transmission and 11,000-hour rotor blades.

The five-bladed 34-foot diameter main rotors are bearingless, composite and feature 50% fewer parts to minimize maintenance. Each main rotor blade weighs only 35 pounds.  Titanium (2 strips per blade = 1 titanium and 1 nickel) leading edge strips minimize errosion from rain and sand while swept tips reduce drag and noise.

In the case of Category A operations, the MD 902 can carry full gross weights up to 2,800 feet at standard temperature or 28 degrees C at S.L. (13 degrees C above standard temperature) before payload must be subtracted for guaranteed single-engine performance.

Some of the high volume of air from the engine-driven NOTAR fan emerges from slots on the tail boom’s right side and the resulting Coanda effect provides approximately 60% of the anti-torque thrust during hover. The remainder is modulated by the pilot’s pedal input controlling the jet thruster in the tail. The fan blades vary their pitch by turn at a constant 5,412 rpm.  They are lightweight but very strong and testing with three broken blades shows the system continues to work well – albeit with some vibration. Linkages to the pedals allow the pilot to vary the size of the thruster nozzle outlet and pitch on the fan blades providing yaw control. In forward flight the adjustable vertical fins are connected to the collective and vary their angle of attack based on power inputs. Large sliding doors on either side of the passenger compartment allow easy access for six or combinations of stretchers, personnel and equipment while large windows provide an expansive view.  Ditto for large doors in the cockpit with their bulging windows that permit the pilot to look aft (even to see if the fuel cap is secure). The large trapezoidal tail boom door facilitates long loads such as stretchers and the copilot’s seat can be removed for very lengthy loads.

How Does It Do The Things It Does?
My MD compatriot is Nick Page who proves to be a delight to fly with. The cockpit computer shows our density altitude as 2,035 feet on the Mesa, Ariz.,  pad under clear skies. Start up requires only a flick of a switch to bring each of the P&W 207E engines up to idle and another flick to bring them up to flight mode with the full FADEC systems aptly handling the monitoring of cool, smooth spool ups.

With an average mission weight of 6,000 pounds we hover easily at ten feet with 70%Q – lots of reserve power. The slightly “different” feel of the NOTAR is quickly learned and I am told of narrow critical wind azimuth of 120-135 degrees whereby a pilot might require full left pedal upon reaching the “demonstrated” sideways flight of 17 knots. During my evaluation this speed is doubled with nary a twitch of trouble. In fact, Nick later has me laterally accelerate enough to cause a departure from
control – from which recovery is immediate. To say the  Explorer’s NOTAR is less bothered by crosswinds than any other tailrotor systems I’ve experienced on 85 helicopter types puts the anti-torque capability into perspective. Coupled with this is the capability of this helicopter to land on a 12-degree slope – in any direction, including nose down!

In the hover, the NOTAR uses as much power as other T/R systems but a set of directionally variable vertical fins starts to offload the fan system’s power draw during the climb and cruise leaving more power for rate of climb and higher cruise speeds. The system’s quirks require no specific additional skills and during my flight the NOTAR system felt perfectly natural. Still, one must be careful to keep the ball centered as any deflection causes an increase in fan pitch thereby using engine power. For instance, at gross weight, a slip with the ball touching the right vertical line will reduce the rate of climb by 200 fpm while slipping in cruise can increase torque significantly. Flying without yaw in level cruising flight is quite important as slight displacement of a pedal will cause the helicopter to sashay somewhat. Although the pilot has no direct control over the vertical stabilizers, they are driven by the yaw rate gyros of the independent vertical stabilizer control system (VSCS) to provide yaw damping and provide balance in roll and yaw during turns.

For added stability the Explorer is normally flown with the VSCS engaged to provide damping in all three axes. Flight with the VSCS off requires a higher work load but is nothing compared to VSCS-off excursions in a helicopter like the Boeing Vertol tandem twins as the Explorer exhibits excellent static stability. Almost any failure of stability or dual hydraulic systems starts with a reduction of airspeed to 100 knots at which point the aircraft is quite stable and easily flown without any of hydraulic or VSCS gear.

In normal flight the aircraft is comfortable to maneuver both in handling and ergonometrically. The unusually shaped cyclic grip turns out to be a brilliant idea as it positions the hand and forearm so they can be more easily supported on a leg. A 75% torque provided an indicated speed of 120 knots and 100% is an approved cruise torque that yields 130 KIAS at 2,000 feet and 18 degrees C. Surprisingly, the Explorer is vibrationally smooth from hover to 130 knots – that’s impressive balance and tracking – or perhaps it just excellent design….

At the maximum autorotational entry speed of 100 knots the left yaw requires a considerable amount of right pedal – partially due to the ‘descheduling’ of the collective interconnect to the VSCS at low power. At  near gross weight and 3,000 ASL under standard conditions our descent rate was 2,300 fpm.
Added bonuses with these greatly ‘de-rated’ Canadian turbines is the capacity for horsepower growth and the ability to shut them down without a cooling phase after which the rotor brake can be imme-
diately applied – providing a very rapid shutdown for medical evacuation and certainly a safer environment for passengers or personnel on the ground. Combined with the quiet rotor systems these capabilities make the Explorers very neighbour-friendly and likely to be well accepted by the general population for flights into built-up areas.

Exploring The Future
Sales are growing with the 902 at a steady rate with operators and clients looking ahead to a time when many of the European regulations regarding noise, CAT A capabilities and safety requirements will mandate the requirement for these helicopters. The low direct operating costs and ease of maintenance are also attracting purchasers as managers realize escalating costs for other machines are eating up their profits.

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