Schweizer Aircraft's Model 333 light single-turbine helicopter, which is set to debut at HAI. We test flew it
Peter Gray/ELMIRA
Schweizer aircraft's Model 333, certified by the US Federal Aviation Administration in September, has a distinguished lineage. It has evolved from the Hughes Model 269 series (a programme Schweizer took over in 1983) via the Schweizer 330, the New York-based company's first turbine-powered helicopter. The programme has benefited from $25 million in research and development, which resulted in many design changes and technology improvements.
Advances in rotor technology have increased useful load by 28% and speed by 23kt (43km/h) while improving hover performance. The major modifications are a wider chord main rotor blade with cambered aerofoil, a larger main rotor diameter, increased engine power and high skid gear. Yet the 333 has the same empty weight as the 330 - a remarkable achievement. The helicopter's safety features, originally designed to meet military specifications, have been retained and extended.
Emphasis has been placed on maintenance by simplifying the aircraft as much as practicable and by incorporating components designed for high lives and lengthy intervals between overhauls, reducing direct operating costs.
Because the helicopter's Rolls-Royce 250 turboshaft has been derated from 315kW (420shp) to 190kW, not only does the maximum power remain available even when operating hot, high and heavy, but the engine consumes relatively little fuel - 68kg/h (150lb/h) at maximum cruise speed and 53kg/h at best endurance speed.
According to Schweizer, the purchase and operating costs of the 333 are about 30% less than those of most other single-turbine helicopters. The aircraft sells for $604,750 and direct operating costs are calculated at $133 per flying hour. The ratio of maintenance manhours to flying hours is estimated at 0.87:1.
Schweizer, which has built nearly 6,000 fixed- and rotary-wing aircraft to date, markets the Model 333 as a multi-purpose helicopter. The helicopter's predecessors were successful civil and military training aircraft, hence the unique cockpit arrangement whereby the two outboard seats and one of the centre seats can all be fitted simultaneously with flight controls.
The 333 is also aimed at police operations, where a three-seater with plenty of space and weight for equipment is preferred; for powerline patrol where good visibility is important; for tuna spotting, electronic news gathering, aerial photography; and for the private commuter where interior space and comfort is important. Outside, there are plenty of hardpoints for attaching equipment such as forward-looking infra-red sensor, searchlight, camera, spray bars and so on.
Wide nose, short tail
Bearing these criteria in mind, Flight International flew the 333 in New York in December. This was the first Schweizer aircraft I had ever flown, and my initial impression was regarding its shape, particularly the wide nose and short tail. To satisfy all potential customers, the helicopter has a combined cockpit and cabin wide enough to seat four abreast. This is a unique arrangement. Having flight controls at three seats allows an instructor to train two students simultaneously. I tried both the centre seats, which are raised 150mm (6in) and moved back 250mm relative to the outside seats, and found all-round visibility to be excellent.
Schweizer's chief test pilot Carl Olson performed the quick, but thorough pre-flight inspection. All the relevant components, sight glasses and such are easily accessible. The company's aim was to provide a forgiving, simple aircraft in which there is plenty of margin in the flying qualities, redundancy in the systems and multiple loadpaths in the structure.
If the worst should happen and the aircraft land hard and roll over, its design improves chances of survival. The high-set main rotor not only avoids the blades hitting people, but prevents any part of the system penetrating the cabin in a crash.
The tall, energy-absorbing undercarriage has 650mm of stroke, and in the factory I was shown the 400mm of crushable belly structure. The cabin has overhead hardpoints and stringers for rollover protection and the nose compartment has a crashworthy keel to protect the occupants if the aircraft lands heavily nose down. These features obviate the need for impact-attenuating seats, says Schweizer, which argues this approach is a more efficient way of absorbing heavy landing forces.
The fuel cell is crashworthy and separated from the cabin by a titanium/stainless steel firewall. I liked the location of the engine, slung low beneath the fuel cell with excellent access. A baggage compartment can be installed in this area as an optional extra.
A belt drive between the engine and main gearbox, again easily accessible, eliminates the need for reduction gears requiring lubrication, inspection and maintenance. The well-proven tailrotor requires no lubrication. Despite a fairly fast tailrotor speed, which can be a source of severe noise, the 333 has a low fly-over noise level of 81.5dB.
With full fuel and ballast, we were right on the 333's maximum gross weight of 1,157kg. With full fuel, 284kg is available for payload. Despite the high skids, getting into the pilot's front left-hand seat was easy, as there a step halfway up the gear. The seat was pleasingly comfortable and, as usual in a small helicopter, non-adjustable, but designed to take a wide variation of human shapes and sizes.
There is plenty of adjustment, however, on the pedals and I was able to find a comfortable position. The aircraft we were flying was equipped with the small, visual flight rules instrument panel with map case, but had two radios, navigation box and GPS.
There is plenty of space in the cockpit for the black boxes associated with optional mission equipment. Sadly, the N1, torque, turbine temperature and NR gauges are not lined at, say, 85% for maximum continuous and 100% for maximum power, so the pilot must remember all the obscure limit figures.
I started the well-proven 250-C20W engine. The start was hot, quick and easy to control with a comfortable, twist-grip throttle. My first take-off to the hover was uneventful, with crisp controls and no wander. Despite the large vertical distance between the tail rotor and main rotor hubs, the hover attitude was fairly level. Les Schweizer, the design brain of the company, told me this was because of the canted tail rotor, downwards pointing engine exhaust on the right hand side and low centre of gravity.
Torque limited
I could see at a glance we were using only 56% torque, well below the 62.5% maximum continuous level, but with the ambient temperature of –2°C (18°F) our density altitude was-1,000ft. Even so, because the engine is downrated considerably, available power will always be adequate in the normal operating environment. As a result of its downrating, the engine is always torque limited, never limited by N1 or, more importantly, temperature. This helps reduce hot section wear and tear.
Although all-round visibility was excellent, I could not see the skids. This will be a disadvantage when landing on a small platform, such as in seismic exploration operations where there is sometimes only 1m between the skid and the edge of the helipad.
While we were still close to maximum weight, I asked Olson to demonstrate an engine failure and engine-off landing from the hover. He chopped the throttle and the aircraft sank gracefully. Little effort was required on the cyclic and pedals as I followed him through on the controls, just a controlled pull on the collective. We touched down with NR in hand.
Fast sideways and backwards flight produced no surprises, the handling being crisp and nicely precise. As expected, there was no tendency for the nose to tuck down during fast rearwards flight. This is important when the helicopter is manoeuvring in strong cross or tail winds. Fast spot turns in both directions showed the same crisp handling.
We climbed out at the best rate of climb speed of 45kt. Levelling and settling at 2,500ft pressure altitude (zero density altitude) at maximum continuous power produced an indicated (and true) airspeed of 97kt. This compared with a brochure speed of 102kt on a standard 15°C day, but our ambient air temperature was –9°C.
The ride was comfortable and remarkably smooth, bearing in mind there is no sophisticated anti-vibration equipment or hydraulic flight control system to absorb and dampen vibrations. I felt only a slight vibration through the controls, which did not transmit itself to the airframe or the passengers. This is a remarkable achievement and indicates a well-designed rotor system, good dampers and balance. I did notice a little side-to-side yawing in straight and level flight, but not enough to need pilot input. I soon learned to ignore this and found the control forces surprisingly low and comfortable for a helicopter with no hydraulic assistance.
With the aircraft trimmed for straight and level flight, removing hands and feet from the controls produced a slow rising of the nose to about 45¼ and a slight "wing over"; a sequence that repeated itself. We let the aircraft go for several cycles. There was no tendency for the attitude changes to become more extreme, again indicative of a good rotor design.
While still heavy, we went into a slight dive to achieve the VNE of 110kt and carry out some turns in both directions. The VNE remains 110kt up to 3000ft density altitude, then decreases at 6.6kt per 1,000ft. At weights of 1,044kg and less, the VNE increases to 120kt for altitudes up to 4,000ft. Apart from a slight increase in the vibration felt through the cyclic, the ride remained smooth. This helicopter is remarkable in that there is no maximum speed penalty with the doors off.
Levelling off, we executed a steep turn from one direction to the other at 85kt. Handling was crisp and exact. There is no specified bank angle limit, but when the rotor is approaching its aerodynamic limits vibration through the cyclic increases - a useful warning signal. Visibility into the steep turns was excellent in both directions, but there is a blind spot into medium turns caused by the hardpoints and rollover protection struts. I was more than happy to accept the blind spot and move my head around it.
Vortex ring test
Vortex ring, or settling with power, can catch out even experienced helicopter pilots. Some forget it exists, and a few helicopters will drop into the condition easily and quickly, so I like to evaluate the condition in every machine I flight test. Unless the pilot recognises early what is happening he may be left with no cyclic control, the whole disc having stalled. He will then need a lot of height to lower the collective lever to eliminate the vortices around the main rotor and overcome its stalled condition before he can get some forward speed and stop the rapid descent. Some helicopters give ample warning and, provided the pilot can interpret the signs, will allow full cyclic control and sometimes, but rarely, allow the pilot to raise the collective to stop the rate of descent before fully eliminating the vortices.
So we tried to get vortex ring in the 333 at a safe height. The aircraft was reluctant to enter, but eventually, after trying various combinations of low airspeed and power, it happened. At nearly zero airspeed and 45% torque, normally enough power for an out of ground effect (OGE) hover, the rate of descent increased, although not too rapidly, to 700ft/min (7.6m/s) and the heading yawed from side to side. These are classic symptoms and the point where the unsuspecting pilot should suspect. Forward movement of the cyclic stick ended the yawing and Olson was able immediately to pull as much collective as required to stop the rate of descent. I asked him to go from low to maximum power on the lever as quickly as he dared while I observed rotor droop. There was none that I could detect. Then, when he bottomed the lever from high power, there was just a momentary 2% NR/N2 increase. This level of performance is comforting to the pilot who may need to lower or raise the lever quickly to get out of an unsafe condition.
Next followed an amazing demonstration. To check rotor droop after a sudden loss of power, I always ask for a throttle chop while in the cruise and measure how much time there is before the collective has to be lowered. Olson chopped, and I counted. Six seconds later he gently lowered the lever, no rush. I asked for a repeat demonstration to do a more accurate count. After six seconds, at a rotor speed of 360RPM, Olson gently lowered the lever, the nose pitching slightly down and the NR recovered to within normal limits. The normal minimum RPM limit, power off, is 390.
Others should not try this themselves, but always observe the minimum of 390RPM. I was flying with Schweizer's chief test pilot, who was involved from the beginning in the 333's development and in testing to establish the limits and a buffer zone. But it is comforting to know that, if a pilot is really slow to apprehend what is happening in the event of a sudden loss of power, he will probably survive. The low-NR audio warning should eventually wake him up.
Excellent visibility
We diverted to an unmanned airstrip and I carried out a steep approach to a spot on the runway. It was accurate, arriving over the spot, but untidy. Olson said we were downwind. I did another into wind and was happy. The forward and downward visibility so important in such a manoeuvre was excellent. Likewise, in a vertical climb to 100ft and back. The 333 would be excellent for vertical reference operations where the pilot leans over the side to see the load carried beneath the helicopter on a long line.
We then did a number of autorotations, power recoveries to the hover and engine off landings to the runway. Each was initiated by a throttle chop. A straight-in autorotation at 60kt showed a rate of descent (RoD) of 1,700ft/min. This is higher than the published 1,400ft/min and was due, said Olson, to us flying at 60kt instead of the minimum RoD speed of 52kt, as well as to our lighter weight.
The flare had bite, increasing the NR slowing the rate of descent and giving us time to level and then restore the power while coming to the hover. Because the engine is downrated and the rotor is light, the response is immediate - similar or better to that of a piston-powered helicopter. This is an excellent characteristic for a training aircraft. We then did another autorotation, this time engine-off to the ground. There was plenty of time at the bottom to level, then judge when to pull in the collective to soften the landing. We did another at the autorotation VNE of 90kt. The rate of descent was 2,900ft/min - a quick way to get down in a hurry. A zero airspeed autorotation gave 2,000ft/min and we took 500ft to get back to 60kt. So this certifies the top of the height-velocity curve of 500ft. A 360° autorotation at 45° of bank gave us 3,000ft/min and we lost 1,000ft. Throughout these manoeuvres, control of NR was easy and, quite frankly, best left alone. You can easily get into pilot-induced NR fluctuations by overcontrolling light rotors
The helicopter is good at landing on a slope. The limits are an impressive 15° for sideways landings and 10° for nose up/down. We tried all four directions.
Finally I invited Olsen to throw the 333 around. He did some impressive wing-overs at low level and rapid turn reversals at high speed, well below the treetops nearby. This demonstrated once again the aircraft's agility and control response.
Crisp handling
Pilots will appreciate the crisp handling with low stick forces, the stability and forgiving characteristics of the Schweizer 333. Other good points are the many safety features, engine power available, rapid acceleration times and all-round visibility.
But pilots may wish the main gearbox allowed more of the engine power to be used. The slight side-to-side yawing might become annoying on a long trip, although I soon learned to disregard it. It would be an advantage sometimes to be able to see the skid gear when landing. And pilots will not appreciate having to learn all the torque and other limit figures.
Operators will appreciate the helicopter's multi-purpose flexibility, good payload and low acquisition and running costs. The 333's inherent safety features and reliability are also likely to impress them. Technicians will welcome the helicopter's simplicity and low ratio of maintenance hours to flying hours. Accessibility has been an important design feature. Passengers will enjoy the visibility, comfort, smooth ride and low noise, but they may wish for more baggage space. People on the ground should appreciate the Schweizer 333's low noise signature.
Source: Flight International
