NOTHING SPECIAL EXCEPT NOTAR AND NO LOGS– MD902

The MD902 Explorer was designed by bright minds at MacDonnell Douglas who were then, when they were taken over by Boeing, promptly spun out to private equity who took on it and the other members of the stable (MD500 and MD600) as MD Helicopters Inc.??An aircraft designed to meet the exacting twin engine light helicopter requirements of the time, it went up directly against the EC135 and A109 in the Police and Air Ambulance roles, or parapublic as they can be called.??For a helicopter pilot used to tail rotors, and all that means, the unique configuration of the tail mounted anti torque system is the first thing that catches the eye and something that demands further explanation.

The NOTAR system works by using the coand? effect which is the tendency of a fluid (or gas) jet to stay attached to a convex surface and is used in many aerodynamic applications such as blown flaps and F1 racing cars.??In the NOTAR a large fan mounted in the front portion of the tail accelerates air so that it can be exhausted along a slot in the tail to produce a horizontal force which opposes main rotor torque.??In addition for rotating the helicopter and other yawing motions and large rotating jet thruster is mounted at the very end of the tail.??Apart from the control system NOTAR helicopters are meant to operate just like normal helicopters just without the danger of the rotating blades of the tail or the vulnerabilities of a tail rotor shaft.

Our ETPS exercise in the Explorer was a combined handling and EO turret assessment such that we all flew two sorties, one on the pilots seat and one in the back using the EO system.??Basically we decided to fly the handling part of the sortie just like any other helicopter such that we could make a back to back comparison and note any anomalies invoked by use of the NOTAR system.??The cockpit, modern for the period, featured an electronic engine control and display system and the aircraft started by use of a single switch for each engine such that everything was up and running very quickly, good for an emergency services aircraft.??We were therefore soon airborne and hover taxiing to the departure point.

These initial low speed activities with the aircraft were unremarkable, showing the effectiveness of the NOTAR system, providing good and reasonably precise heading control.???The transition away from the hover was again easy to achieve with a little fishtailing noted as we gained the correct climb speed (Vy) and set ourselves on the way out to look at higher level manoeuvres.??Again all the standard tests were flown and the aircraft did not disappoint,??stability was fine and pretty well matched to to other similar sized and equipped helicopter types.??This was not an aircraft with a very complex autopilot and thus it was well designed to deliver relatively stable flight.

Only when operating at very high angles of bank (more than 45 degrees) and at high rates of descent or autorotation did the aircraft exhibit slightly unusual characteristics.??Here the NOTAR system struggled to give a response to yaw pedal input as good as a normal helicopter, since, unsurprisingly a system reliant on the flow of air downwards started to struggle when the airflow was going in a different direction.??This meant heading control was harder to achieve with a reduction in pilot accuracy and a gentle fishtailing motion noticeable to those sat in the cabin.

But these issues were not a major problem.??It was clear on the second sortie in the cabin that we could happily achieve the required tasks tracking cars and other things on the ground.??We had developed a way to measure the clarity and accuracy of the system -??it was called the sheep leg system – if you could count the legs on a sheep from 2000 ft and 5nm then that was good!??Of the three pilots who flew the aircraft, none of us could say that the NOTAR System made It unsafe or unflyable, in fact other aircraft of a similar size have some very interesting characteristics if you turn off the stability augmentation.

So what did for the NOTAR system and its safety enhancing tail control system – ideal for urban environments where there are lots of people to consider.??Actually it was two distinct points, one was that when MD Helicopters was split out from Boeing it no appreciable logistics support programme for the aircraft. Spares support to aircraft meant to be available 247 to save lives or keep people safe is an important factor for any government or owner.??The other, not unconnected issue was that MD Helicopters never really sold enough airframes, a look on the Helis database says there were 81 MD902s.??In real terms that means spares provision will be expensive, with no volume or scale savings to make and no ability to spin off into other roles.

As we sit here today MD902s are slowly disappearing being replaced by newer aircraft like the AW169 and even by its old nemesis the EC135/H135.??Users have sought larger aircraft and the benefits of NOTAR seem not to be so important.??It’s a technology that will soon likely disappear and one which offers the eVTOL community cause to ponder how logistics can kill a great idea.