Airtest – McDonnell Douglas Helicopters MD 520N

Concept development began in 1975 and it was six years later in 1981 that the concept demonstrator first flew. Five years later in 1986 the prototype demonstrator had its first flight and it was three years after that in 1989 that the first production prototype flew. Finally, production and first deliveries took place in 1991 with the Phoenix Police Department being the first organisation to take delivery of the new aircraft. McDonnell Douglas Helicopters claims that the MD 520N fitted with the Notar system provides unparalleled safety, significantly reduced external noise levels over other helicopters, improved handling and consequent reduced pilot workload, and finally increased payload and performance allied with easier maintenance. These were the major points I had in mind throughout the airtest.

MD 520N design features

During our walkround of the aircraft, Lindsay Snell of Austcopters Pty Ltd pointed out the main features of the 520N with the tail boom and anti torque control obviously the main points of discussion.

The most significant difference to any conventional helicopter is of course the lack of a tail rotor, which is visually obviated to a certain extent by the fixed horizontal stabilizer and the two vertical fins. The left hand fin is linked to the anti torque pedals and moves through a 30 degree range. The right hand fin is linked to a servo actuated stability augmentation system (SAS) and moves through a 15 degree range. There is no pilot input to this fin.

The constant section tail boom is constructed of graphite and kevlar and has two thin control slots running the length of the right hand side. The slots are not obtrusive and would most likely be missed by the casual observer.

At the rear of the boom is the thruster section which is a large square section sleeve valve which is directly connected to the anti torque pedals. Movement of the pedals rotates the outer sleeve valve varying the size of the opening and hence providing lateral control.

Mounted at the rear of the cabin is a 53cm fan with 13 blades, each connected to the anti torque pedals through a pitch change plate and individual links. The fan is driven through a separate gearbox which is in turn driven by the main transmission. This arrangement is peculiar to the 520N to limit differences to the 500E which has the same basic fuselage structure, apart from the tail boom. Subsequent follow on new design aircraft would have the fan drive integral with the main gearbox.

The fan provides low pressure air to the two tailboom slots and to the direct jet thruster and sufficient pressure is developed to ensure effective control should the boom be damaged. MDH claims that the boom could be holed by up to a tenth of a square metre and still function satisfactorily.

Clearly the first of the claims as to safety almost goes without saying. Although we didn’t have the opportunity to put the tail into a clump of bushes, there are ample photographs in overseas reports showing the aircraft hovering in just such conditions.

A significant maintenance advantage over other helicopters is that the tail boom can be removed by two people in under five minutes for access to the fan. Another advantage is that fan blade balancing is a simple procedure compared to conventional tail rotors.

The MD 520N is powered by an Allison 250-C20R engine rated at 450shp, but derated to 425shp for maximum takeoff power which is 50shp more than the 250-C20B of the MD 500E The 250-C20R evidently runs hotter than the 250-C20B which has led to some overseas claims that 520N’s performance is not that much greater than the 500E. The problem is that pilots have not been pulling the extra power due to the higher operating temperatures.

Although the 520N is capable of operating at higher altitudes at a higher gross weight than the 500E, brochure figures indicate that the latter aircraft is capable of maintaining a higher cruise speed (132 v 122kts/245 v 226km/h) for a better endurance (2. 7 v 2. 2 hours). This can be attributed to the higher AUW of the 520N (1520v 1360kgs) and the larger rotor system, engine, fan assembly, and step up transmission.

However interestingly, overseas pilot reports indicate that it is difficult to detect the difference.

Finally, the higher skid gear fitted to the demonstration model significantly increases the height of the lower fuselage above the ground, from 0. 4m to 0. 7m making it look very stalky.

Flying the MD 520N

It was a bright but blustery day, with the wind gusting around 15kts (28km/h), ideal conditions in which to see if the much vaunted Notar was as good as the glossy brochures and publicity blurbs purported.

After our walkround I was invited to take the left hand seat, which is the command seat in the 520N as the fifth seat (in the executive version), when fitted, is located between the two front seats, requiring the removal of the righthand collective. After start up and the usual checks which were all fairly straight forward, Lindsay lifted it into the air and immediately invited me to have a go.

I must admit that I was a little apprehensive about how I would perform in the much vaunted MD 520N as I had not, apart from a brief encounter with the R22 a year earlier, done any serious helicopter flying for over twelve years. In addition I was a generation behind having done most of my helicopter flying in the Sycamore, Wessex and finally UH-1B Iroquois. I needn’t have worried, the 520N is so easy to fly that I felt comfortable right from the start. The aircraft is solid with none of the twitchiness of smaller machines, enabling me to slip back into helicopter mode without any difficulty. The controls, which are all manually actuated without any form of hydraulic boost, have just about ideal breakout forces. The cyclic, however, can develop quite heavy control loads and is best flown on the trim, which is fast enough to match pilot input demand. Having suffered the slowness of the Wessex trim which was always several aircraft lengths behind what you wanted, it was a pleasure to have a control that responded immediately to a touch of your thumb. Vibration was practi- cally non existent, with none of the usual “stir the pudding” motion of your right hand with lesser rotor systems.

Following some initial hovering, I did a circuit to get the feel of the aircraft in forward flight before returning to the helo pad for more manoeuvring in the hover. As the aircraft moves through translational lift in the climb there is a distinct nose up pitch caused by rotor downwash on the horizontal stabiliser. This needs to be counteracted by trimming forward in the hover before departing.

The collective was so smooth that I can’t honestly recall ZK much about it, it just did what I wanted it to do, and the governor was dead beat.

Now to the anti torque control. Firstly, I must say it was good to see a set of pedals that didn’t look as if they would break off if you pushed too hard! On taking over control, the difference between the Notar and a conventional tail rotor system was immediately apparent. There was none of the usual tap dancing around as you accustom yourself to a new machine, control was direct, linear, and required less pedal input.

Perhaps the most convincing demonstration of the value of the Notar system is to lift the aircraft into a hover with your feet off the pedals. As mentioned earlier, the wind was gusting to 15kts (28km/h) and one would have reasonably expected an uncommanded turn to the right to have occurred. Not so; with a couple of oscillations either side of our then heading, the aircraft settled down into wind and stayed there. I confess this was not done without an instinctive input of left foot pressure on the cabin floor on my part! This characteristic of the aircraft is of course a product of the vertical fins which align the aircraft into wind, and the steady output of the anti torque system.

After a few minutes hovering, (at a slightly higher height than I was used to, to prevent recirculation reducing the efficiency of the Coanda slots), I tried several takeoffs and touchdowns. It was very easy to maintain a stable hover after lift off and equally it was a simple matter to put the aircraft down again on it’s high skid gear.

I then did a hover taxi around the square of markers defining the helo pad, firstly maintaining a constant heading around the four sides and then pointing the nose along each leg.

Control in forwards, sideways and backwards flight was straightforward with no significant difference head or tail to wind. Of course it was necessary to compensate for wind, but there was none of the feeling that you were really having to haul back on the cyclic when the wind was up your tail. Pedal input was decidedly less than I would have expected from a conventional tail rotor system with very little shudder as we rotated tail into wind.

Another circuit, which had to be flown fairly wide to maintain clearance on an A320, gave me the opportunity to sample the aircraft in fast cruise. It remained almost vibration free at just under 130kts (240km/h) with very crisp control response in turns.

It was during this circuit that I discovered that the forward edge of the door and its surround created a fairly large blind arc, so much so that I had to consciously lean forward or back to remain sighted on the A320. However no doubt you would get used to this with time.

Heading control on the final stages of the approach was particularly good with no tendency to become skittish (Bell 206B!) as was pulled to achieve a hover.

Following the second circuit it was back for another demonstration by Lindsay of fast manoeuvring from the hover. Sufficient “g” was pulled in some tight turns to activate an over sensitive ELT (causing some understandable consternation on the part of ATC) demonstrating the aircraft’s manoeuvrability. This was followed by a demonstration of fast sideways flight for which the aircraft is cleared to 30kts (56km/h) (this is also the limit for rearwards flight). Plenty of pedal remained in both directions.

Finally, it was with some reluctance that I flew the aircraft back to the flightline helo pad in a fairly fast air taxy and shut it down. As I said at the beginning, the MD 520N is a delight to fly and once experienced, most pilots would be well and truly hooked.

Reduced external noise

As the accompanying diagrams clearly indicate, MDH can rightly claim that the MD 520N is the quietest certified helicopter in the world. Because I was flying the aircraft, I didn’t get an opportunity to personally hear a fly-by but I was assured by witnesses that it was noticeably quieter than any comparable helicopter. However, I did witness it arrive, flying straight towards me as it landed. Rotor and engine noise plus the fan sounded much the same as any modern small helicopter.

Where it does come into its own is above 200ft (60m), and above 1000ft (305m) it sounds like a light fixed wing aircraft. the Phoenix Police Department has discovered that it can’t usually be heard above ambient street noise which is a significant advantage in night time crime detection work.

Conclusions

There is no doubt that MDH is onto a winner with the MD 520N, with the aircraft truly representing a quantum step forwards in terms of greater safety, significant noise reduction, and improved handling.

Now that the Notar is established as a practical concept, it will be interesting to see whether it affects future helicopter design in the same way that retractable undercarriages, constant speed airscrews et al once introduced, had to be adopted by all aircraft manufacturers.

With the MD 520N’s big brother, the twin engine MD Ex-plorer well into its development flying, with VFR certification expected towards the end of this year, it will be interesting to see how other helicopter manufacturers react to the challenge of the Notar concept. Bell is in the process of developing ashrouded tail rotor, which it would be reasonable to assume is an attempt to meet that challenge while Eurocopter have used Fenestron shrouded tail rotors for years.

Should the MD Explorer prove to be as successful as the MD 520N, the next decade of worldwide design will be very interesting indeed.