Airtest – Piper Cheyenne 400LS

The Cheyenne 400LS had its roots way back in 1977 when Piper introduced the experimental Cheyenne Ill at the NBAA Convention in Houston, Texas. However, at the time the aircraft bore very little resemblance to today’s machine. The 1977 version was a Navajo Chieftain-sized, T-tailed Cheyenne powered by a pair of Pratt & Whitney PT6A-41 turboprops mounted on 60-inch longer wings.

Flight tests revealed a VMCA that exceeded 100kts and a deep stall problem when the flaps were fully extended. Piper subsequently spent the next three years developing the Cheyenne Ill configuration. They lengthened the fuselage three feet just forward of the cabin door, increased the height of the tail by 20 inches and moved the engines forward by 14 inches. The increased wetted surface aft of the wings reduced the VMCA to 97 KIAS and the movement of the engines kept the C of G within acceptable limits. The deep stall tendency was eliminated by a combination of stretching the aft h. ,1selage, modifying the inboard portion of the flaps and increasing the size of the horizontal stabiliser by 40%. All this developmental work eventually paid off by producing an aircraft with adequate performance and very pleasant handling characteristics which is nowadays what the Cheyenne Ill is recognised for.

It was also in the late nineteen seventies that Piper management were debating the wisdom of adding a light executive jet to their already excellent line-up of aircraft. Management felt that the technology already existed, although it had not been applied, to produce a 400mph turboprop. The cost of developing a new jet from scratch would be very high and the risks in an already crowded market even higher. Whereas, the development cost of a high-performance turboprop from an existing model would be considerably less and yet the aircraft could actually compete with the light jet on the average 300nm to 500nm business flight.

Very appealing to the management was the fact that the turboprop would have access to far more airports than any jet because of its reduced runway requirements and this advantage would be even more evident on wet and icy runways. Additionally, because a propeller has a higher propulsive efficiency than a jet, it would offer better climb performance thus getting the aircraft to altitude sooner and once there, burn at least 20% less fuel whilst delivering a similar block time over distances up to 500nm. And if needed the proposed turboprop could also cruise at FL410 above most weather and take advantage of the winter jetstreams.

Overall it looked as though developing a high-performance turboprop would be preferable to developing a light jet and when Garrett announced its plans to build a version of the venerable TPE331 series capable of 1,600shp, Piper were convinced they had to build the Cheyenne IV. Management gave the go ahead for the project in 1979 and based the new concept on the yet-to-be-certificated but well-developed Cheyenne Ill series airframe and systems. The final design was frozen and announced to the world in September 1982 and the Piper Cheyenne IV (PA42- 1000, redesignated 400LS in late 1984) received its Type Certificate on July 13, 1984.

Since that time the Cheyenne 400LS has broken numerous world records, these include sea level climb to 3,000 metres (9,842. Sft) in 1 minute 47. 7 seconds, 6,000 metres (19,685ft) in 3 minutes 42. 3 seconds, 9,000 metres (29,527. 6ft) in 6 minutes 34. 5 seconds and 12,000 metres (39,370ft) in 11 minutes 8. 4 seconds. And just to cap all this off the Cheyenne 400LS broke the previous USA transcontinental record (San Francisco to New York – 2,575. 5 miles) with a time of 6 hours 38 minutes and 28 seconds bettering the old record set in February 1984 by a Cessna Conquest 11, by 43 minutes 32 seconds.

At first glance, the Cheyenne 400LS appears to be a Cheyenne Ill with a pair of Allisons fresh out of a Lockheed Electra installed in place of the Pratt & Whitney. But this resemblance is cursory, for whilst the external dimensions are common to the Cheyenne Ill, the 400LS is a far more refined aircraft with a profile drag some 14 per cent lower than its antecedent. This was achieved by contructing the 400LS with a thicker aluminium skin allowing flush riveting and producing an overall smoother skin less prone to ripple under air loads. The engine nacelles contribute to lower drag by being snug around the Garrett engines, incorporating. ng a flush-mounted oil cooler intake and dispensing with exhaust stacks that protrude into the air stream.

Certainly the most conspicuous difference between the Cheyenne 400LS and Ill is the powerplant installation and the 400LS derives the bulk of its performance advantage from the powerplant combination of Garrett and Dowty Rotol. Let us look at the engine first. The Garrett TPE331-14-801 is rated at 1,645shp with the Cheyenne installation being flat rated to 1,000shp which can be maintained to 22,000fton a ISA day or up to 50°C at sea level. Even at 41,000ft the engine still produces 50 per cent torque. The -14 engine is actually a 22 per cent scale up of the earlier TPE331 series.

This engine, like the -10 and subsequent engines, has been modulated to allow access to various sections, including the hot section, without entirely removing the engine from the airframe. An added bonus is that the -14 does not have a designated TBO, currently being ‘on condition’ with an initial Hot Section Inspection _ at 3,000 hrs. The engine has a growth potential of 1,900 Within the same frame size and the gearbox is stressed to 1,250hp.

The intake is continuously heated, not only to prevent, but to maintain its shape accurately, contributing to its excellent 107 per cent intake ram recovery. In this installation, the -14’s are controlled by an ‘Integrated Engine Computer’ located under each pilot’s seat. These units set the fuel flow, control the start sequence, supervise limitations, operate the engine instruments and while in flight record the engine operating parameters. These parameters can be recorded about 20 times before a ground engineer extracts the information by plugging his portable interrogator into the system. The information is then passed on to Garrett for analysis. The same interrogator can also be used to adjust various engine settings without the need for a ground run.

Garrett have gone a long way to reduce the maintenance costs of their new engine. As mentioned earlier the engine is built in modules, this allows an HSI to be done on the wing. Tooling to carry out inspections and overhauls will cost only about 20% of the tooling to cover earlier TPE331s. The engine is covered by the ‘Maintenance Service Plan’ (MSP) at a cost of about $184 per hour for both engines. This will give operators a known quantity to budget for engine maintenance and offer peace of mind in the knowledge that all engine-related problems, including loaner engines and airfreighting of your own engine, are covered by Garrett.

The enormous power developed by the Garrett engines would be of little use if the propeller technology did not keep up and Dowty Rotol have certainly kept up their end of the partnership. However, during early flight tests Piper were rather disappointed with the Cheyenne IV’s performance. The aircraft was some 20kts slower than early estimations had indicated. Subsequent test showed that the props were producing efficiencies of 0. 867 at 0. 52 Mach and 0. 845 at 0. 55 Mach, which although quite high, were not in the order of 0. 888 at 0. 52 Mach which Piper had based their performance estimates upon.

Dowty Rot traced the shortfall in prop efficiency to drag associated with the cuffs surrounding the inboard portions of the propeller blades near the hub, it appears that due to insufficient streamlining the blades were subject to compressibility at the high forward speeds of the 400LS. Dowty-Rotol subsequently came up with a new cuff that had a greater fineness ratio and less drag. These props now offered the anticipated prop efficiencies resulting in 0. 870 at 0. 52 Mach and 0. 862 at 0. 54 Mach.

A close look at the blades will reveal the unusual shapes deployed bX Dowty Rotol to obtain this extremely high to high ram air recovery mentioned earlier. The wide chord four-bladed composite propeller of 106 inches diameter is geared to turn at a slow 1,540rpm at 100 percent. At 350kts the blades slice through large portions of the air and still maintain a tip speed of less than 0. 9 Mach.

The Cheyenne 400LS has completely conventional mechanically actuated ailerons, elevators and rudder. Quite surprising for an aircraft with the capability of FL410 and the high installed power. Piper really did their homework, through good design they eliminated the need for ‘fix it’ items such as vortex generators, bob weights and down springs. There is a conventional stall warning horn triggered by the angle of attack that will operate 12kts above the normal stall speed. However, as the power-to-weight ratio approaches 1:1 the deck angle in a full power stall could reach an unreasonable 45°, Piper have therefore installed a stick pusher to lower the nose before reaching an excessive deck angle or a speed much below the normal stalling speed of 87kts. The stick pusher system is electronic and only operates when the angle of attack is undesirably high, the power levers are open and the speed reduces towards 80kts.

The flaps are electronically actuated and have a 10° and 30° pre-select for approach and landing. The electronic flap control system will stop the flap motor if flap asymmetry reaches 5°.

The undercarriage is hydraulically actuated and operated; it is basically the same as the Cheyenne Ill system beefed up to accommodate the 400LS’s higher ramp and landing weights. The main gear doors are canted inward at the front to enable aerodynamic pressure to assist with undercarriage retraction. Wheel brakes are foot powered through the rudder pedals as is the nosewheel steering. The emergency undercarriage system is a standard Piper via a hand pump.

The electrical system was completely redesigned for the latest Cheyenne. It consists of a dual bus arrangement powered by two 300A starter generators and two 40 amp hour batteries which offer the advantage of two independent electrical systems lined by a cross-tie control. The system is designed to provide redundancy so that any one battery and generator can supply the entire electrical system. The clearly marked and conveniently located circuit breaker panels make the search for that elusive circuit breaker an orderly task. The buses are broken down to left and right main buses, an essential bus, a non-essential bus, two avionics buses and an emergency avionics bus.

Whilst the 400LS will lift 3,8191bs of fuel (the same as the maximum fuel load on the IIIA) it differs in that there are no locker tanks, due to the Garrett engines and the portion outboard of the nacelle is thus an integral tank. Each wing has a tip tank, an integral tank and two bag tanks in each wing root. All four tanks are interconnected and filled through a single gravity filler in the tip tank. Electrical pumps are needed only for starting and emergency operations. The engine’s mechanical pumps supply fuel pressure once the Garretts are fired up.

Pressurisation is supplied by both the low pressure compressor and the high-pressure compressor. With its 7. 5 psi differential, the 400LS can maintain a 10,000ft cabin at FL410. At this stage there would be a considerable draw on the high pressure compressor, however below FL200 the low pressure compressor can cope with the environmental systems bleed air demands. The selecting from low to high pressure is automatic and the pilot need only adjust temperature during the climb or descent.

On the day that I was to fly the Cheyenne there was a minor industrial dispute and we were unable to uplift any Jet A 1 out of Bankstown. Pacific Aviation’s Chief Pilot, Trevor Frost suggested we fly to Mascot and uplift the required fuel there. (Pacific Aviation is the Australian Distributor for Piper aircraft). Trevor flew the aircraft on the four minute hop from runway 11, Bankstown to runway 07 Mascot while I observed. Our flight plan would take us back to Bankstown via Katoomba, Cowra, Bindook and the 062 localiser. After refuelling I called Sydney Clearance Delivery and requested a 16 Katoomba one departure, for demonstration purposes. Clearance delivery didn’t have our details as yet and asked what height we wanted to climb to. They did a double take when we advised FL410. This will take some getting used to. At our weight of 10,3001bs and a temperature of ISA+ 6, we would use 100% power, a rotate speed of 105kts and this would give us a ground roll of 1,400 feet and a takeoff distance of 2,100ft over a 50 foot obstacle. The accelerate stop distance was thus 3,200 feet.

Starting the Cheyenne 400LS is simplicity itself, the fuel pump is switched on, then the start/enrich button is pressed and the Integrated Engine Computer takes care of the rest. The start cycle is completed at 55% and the engine idles at 65%. Noise levels inside the aircraft on the ground are very low. Nose wheel steering and general ground handling of the 400LS is very pleasant and easy, the big Dowty Rotols give instant speed control and are very predictable. We had only a very short delay before being cleared for takeoff, this was going to be a maximum climb performance demonstration. The power levers were pushed to 100% and almost instantly there was a strong push on my spine as the 400LS rapidly accelerated down the runway. The 105kt rotate speed loomed up and I pulled the nose skyward. A maximum rate climb calls for a 25° nose up attitude, this is a ridiculously steep deck angle and at first I was reluctant to pull the nose that high, but our required speed was 115kts and anything less than 25° deck angle accelerated the aircraft through 140kts.

We had climbed to 800ft within seconds and no sooner had we turned to intercept the 163 Sydney VOR radial than 3,000ft came up, now it was time to make a left hand turn back towards the VOR. As I did the left hand turn I looked out the window to see the runway 34 threshold pass by underneath. The requirement is to cross the VOR at 5,000ft or above, we crossed at 9,300ft and all this was achieved inside the aerodrome boundary. I felt like an astronaut being catapulted out of the atmosphere. Six minutes and nineteen seconds after brakes release we were 10 DME from Sydney and leaving FL200. Ten minutes thirty three seconds after brakes release we were leaving FL300 on our way to FL410.

This type of climb was exhilarating for two pilots, but with the Chairman of the Board in the back, one would use an initial climb speed of 170 KIAS which gives 3,000 feet per minute rate of climb and a still steep 12° to 13° nose up. An 8° nose up attitude will still result in2,500 ft:1et per minute rate of climb and 210 KIAS. We levelled off at FL410 and allowed the Cheyenne to accelerate. At this altitude with the prop rpm down to 96%, we had an IAS of . 135kts considerably down on our maximum permissible (VMO) of 177kts and also well above the 92kt stall speed, giving a large envelope for any reasonably competent pilot to retain control. Our TAS was 296kts with a fuel flow of just 440 lbs/hr. I tried a couple of steep turns as we approached Cowra at FL410. The aircraft handled these as if we were at 5,000 feet. Some people say that the handling of the 400LS is heavy, I must admit though that I liked the handling feel of the big Piper throughout tt. le entire envelope and couldn’t fault it.

During the cruise I went back into the cabin to sample the noise levels and found it was pleasantly quiet. One could sit in the rear seat and hold a normal conversation with a passenger in one of the front seats. The cabin is well appointed, as would be expected in an aircraft of this type. The large windows assist in giving a very roomy and airy feel and ensure the passengers have an excellent view. In fact with a little effort one can even see the contrails. We were now on our return flight to Bankstown and commenced our descent which was flown 10kts short of the barber pole until we obtained 230 KIAS which we held to about 20nm from Sydney. The ailerons had a good responsive feel making the aircraft very stable and easy to set up for the ILS. Changes in configuration, required no real trim changes which helps reduce pilot workload. We carried out our overshoot and tracked for Bankstown where we flew several circuits before we returned to the hangar. The fighter like power to weight ratio of the big Cheyenne makes most emergencies a non event. Even a balked landing, leaving undercarriage down and full flap, resulted in a 2,000 feet per minute rate of climb. A touch and go is precisely that, you land, touch your main wheels, apply power and you are airborne commencing your next circuit. On landing you need only pull the power levers into Ground Idle to exit the runway by the second taxiway at Bankstown.

The Cheyenne 400LS, a light jet with propellers, a true pilot’s aircraft that opens up a whole new market and sets new standards. Several weeks before our flight, ‘Frosty’ had flown the 400LS from Alice Springs to Adelaide at FL390 in an elapsett time of 2n(S 6mins and Adelaide to Sydney at FL390 in 1hr 35mins. Wifti) these speeds and economies and a range of up to 2,240 nautical miles, we should see the 400LS quickly find its way into Australian corporate operations or even ultimately pressed into service with the Government’s VIP squadron.