War veteran and peacetime workhorse, Bell's UH-1 gets a new lease of life with the Huey II upgrade
Peter Gray/FORT WORTH
Bell Heliopter Textron has come up with a solution to the problem of improving medium helicopter fleets in an era of tight defence budgets: upgrade existing UH-1 Hueys for a fraction of the cost of a new helicopter and end up with a zero-timed airframe with better performance, lower operating costs, lower maintenance requirements and better reliability.
After decades of UH-1 operation, the US military had done little to enhance the venerable helicopter, so in 1990 Bell took matters into its own hands by initiating the Huey II upgrade programme. The upgrade was designed and tested over the next six years, with the first delivery in 1998. The Colombian air force has installed eight upgrade kits, the Colombian National Police 15 with an additional 10 delivered, and the Phillippine air force one. The upgrade retains all the original design specifications such as a crashworthy fuel system, and uses proven production parts.
The main improvements are:
Replacing the main rotor hub and blades with the bigger, more efficient Bell 212 parts. These blades improve ballistic tolerance and reduce the power required to hover, and decrease cruise power to give greater speed and range. This and other improvements give the aircraft better hover ceilings and payload, both in and out of ground effect (IGE/OGE). Never-exceed speed (VNE) has been increased to 130kt (240km/h), but only at gross weights of 3,400kg (7,450lb) and below. At maximum gross weight, VNE is a modest 111kt. The new blades have increased the g capability by 11% - an improvement that was evident during steep turns. They also give more inertia in autorotation, an important characteristic in single-engine helicopters, and generate less noise. Replacing the pusher tail rotor with a tractor unit which gives more tail-rotor power, allowing flight in stronger crosswinds. More power is also available when operating hot, high and heavy - a contrast to previous Huey models, in which available pedal and, therefore, tail-rotor power have been in short supply when operating in these conditions. Increasing the maximum gross weight by 450kg to 4,770kg and the external-load maximum gross weight to 5,090kg. The belly hook is stressed to take 1,820kg. Incorporating the Bell 212 drive train and tailboom to take the extra power. Modifying the Honeywell (formerly Lycoming) T53-13B engine to the proven -703 standard to increase take-off power by 28%, from 1,050kW (1,400shp) to 1,340kW, and maximum continuous to 1,120kW with no weight or fuel consumption increase. The Huey II will outperform the 212 when hot, high and heavy, says Bell. Upgrading the transmission and increasing its ratings to accept the increased engine power and drive the more powerful tail rotor. The transmission is torque-limited to 960kW for take-off and 845kW maximum continuous, offering a much higher payload. The time between overhauls (TBO) has leapt from 1,100h to 6,000h - one of several TBO/retirement life (TBR) increases aimed at reducing operating costs and increasing availability. Bell calculates Huey II direct operating costs at $360/h, compared with the UH-1H at $650/h. The need to incorporate several previous safety bulletins has been eliminated, further reducing costs and increasing safety. Improvements to the oil cooling system, with better filtration resulting in longer TBOs. Incorporating a more powerful 103 bar (1,500lb/in²) hydraulic system to contain the higher dynamic loads and feedback - a modest pressure rating compared with other similar-sized helicopters. Using the proven and reliable Kaflex engine-to-gearbox drive shaft. Several main structural components such as the pylon support, tailboom attachment and lift beam have been strengthened to support the extra weight and power. This has increased the empty weight by only 160kg, less than the increase of maximum gross weight. With full fuel, the Huey II has a payload of 1,380kg compared with the Huey-1H's 1,090kg. Replacing the tail rotor control cables with control rods.These and other modifications come in the form of a kit, which can be loaded into a freight aircraft. The upgrade starts with removal of the main rotor blades, pylon assembly, main drive shaft, engine and tailboom. Total manhours are estimated at 2,250 - about 90 days - 80% of the operator's current tools are used. It requires:
four mechanics; three sheet metal technicians; two hydraulic technicians; one electrician; four painters; one pilot.Maintenance hours per flying hour have been dramatically reduced by about 50%. No daily inspection is required: 15h, 45h and 105h inspections take between 3 and 5.5 manhours. The most time-consuming are the 195h and 24-month inspections, taking 28 and 24 manhours, respectively. The overall average is 2.3-2.5 manhours per flying hour compared with the UH-1H's 7h.
All the changed parts carry the full Bell warranty and the manufacturer can supply all subsequent spare parts and customer support, even though it is a military machine.
Total costs, including kit purchase price ($1.2 million), engine modifications ($300,000) and labour come out at between $1.6-1.8 million, compared with its nearest competitor, Bell's own 412 at $5 million. Bell has calculated that, at the rate of 40 flying hours a month, the Huey II upgrade will pay for itself within five years. No overhauls are required for five years.
Flight International evaluated the Huey II at Bell's Fort Worth, Texas, facility. Alongside me in the cockpit was test pilot Sam Bowyer.
Engine start was the usual relaxed, slow acceleration, low temperature affair typical of this engine, using both hands and gripping the cyclic stick between one's knees, despite the friction being on. The collective lever downlock was in place. My first hover was stable with the force trim off. Interpretation of power being used and available was adequate - although it is important to know the numbers, despite the instrument markings.
I turned 90° out of wind, putting the 20kt of wind on my right and requiring more tail and engine power, and landed. Plenty of both were available. Downwind was equally easy. Some helicopters twitch badly when hovering in a 20kt tailwind, but the Huey II remained calm. The landing and subsequent take-off were uneventful. Another hover and landing with the wind on the left was also uneventful.
I then took the aircraft sideways to the right to 35kt, the worst condition requiring left pedal and, as a result, more engine power as the aircraft tries to yaw to the right. All the older Huey models would have run out of left pedal long before 35kt sideways flight was achieved or, in a hot climate, reached a maximum engine or transmission limitation. There was plenty of left pedal remaining, and the engine was well below any limitation. This is a major improvement which all Huey pilots will appreciate. Sideways flight to the left at 35kt was equally benign.
Backwards flight at high speed, or hovering downwind in a strong breeze, can cause control and attitude problems in older-design helicopters. The nose will tuck down, causing the pilot to pull back on the cyclic stick to correct, leaving him with the nose hard down and the stick hard back, and little reserve to extricate himself from the situation. The Huey II has no such problem. At 30kt the aircraft remained level with the cyclic more-or-less in neutral. Rapid spot turns left and right again demonstrated the more than adequate tail-rotor power. We transitioned to level flight at 1,000ft (300m), calculating the VNE at this density altitude and weight using the placard. It came out at 123kt. This was achieved in level flight using less than maximum continuous power.
Turns in both directions, rolling from one to the other, produced no increase in the already benign vibration level. Handling was nicely balanced and crisp. The same was true for steep turns up to 75° of bank. Like its predecessors, the Huey II's main rotor drive shaft has a 5° forward lean, and so the fuselage stays nicely level during the cruise.
Rotor RPM (NR) is a low 324. I asked Bowyer to chop the throttle while in the cruise to check how long the pilot has before having to lower the lever to contain NR. It was three seconds, which should be adequate for a pilot to ascertain what has happened and lower the lever. There is the usual audio low NR warning and light.
I did not bother checking settling with power/vortex ring criteria since I was already aware of the Huey's reactions; there are early warning signs, increased rate of descent but plenty of engine, main and tail-rotor power to recover in the Huey II. Rapid movement of the lever caused only momentary rotor speed fluctuations, but NR stayed well within limits.
After switching off the generator to simulate a failure, the master caution warning light came on accompanied by the caution light. A reset procedure is available, but plenty of time was left on the battery to provide all the electrical power needed. The Huey II's electrical demands are small. Once trimmed for straight and level flight, the aircraft flies happily, hands and feet off the controls for some time, though the 20kt wind did not help.
At my request, Bowyer switched off the hydraulic system. Cruise flight was easy to contain with only a slight increase in effort to move the controls. If I did not move them, feedback was zero. The flight manual calls for a speed reduction to 60-80kt and a run-on landing. The satellite runway which Bell uses for such manoeuvres was partially flooded so we could not complete the landing. But I flew the aircraft down a shallow approach using minimum control inputs, gradually decelerating, and would have been confident enough to put the aircraft down on this first attempt.
Forward and downward visibility is good, so a steep approach on to a target on the wet runway was easily accomplished. The same was true for a vertical take-off to 100ft and back. With the power available, there was no problem hovering out of ground effect with our complement of passengers and nearly full fuel.
While on the ground, I wound off the throttle to ground idle and, gripping the stick between my knees, selected manual throttle by means of the switch on the centre console fuel panel with my other hand. In the air, one would have to practise doing both with the left hand. A circuit approach to the hover and landing was easy, reverting to the usual piston engine technique of raising the lever and opening the throttle, lowering and closing it again and taking care on the landing not to dump the lever.
We completed the flight with a series of autorotations. Unfortunately, we were not able to do them to the ground because of the sodden runway and the lack of a rescue vehicle. A minimum rate-of-descent autorotation at 65kt gave a modest 1,600ft/s (81m/s). Because transmission limits do not allow all the engine's available power to be used, it will accelerate quickly to achieve a powered recovery to the hover. The flare to the hover had bite and there was plenty of right pedal available for a landing if one had been possible. A best-range autorotation at 85kt showed a small increase of rate of descent to 1,700-1,800ft/s, but the angle of glide was noticeably flatter. The best rate of descent was produced by descending at 60kt and pulling the NR back to minimum.
Back at the base, Bowyer chopped the throttle in the hover. The aircraft sank gracefully and only small inputs were needed on the lever and pedals to achieve a nicely controlled touchdown. The rotor stalls completely at about 60% NR, says Bowyer. This is low indeed.
Conclusions
The Huey II is a well worked-out upgrade and should appeal to current UH-1 fleet owners and those acquiring old Huey airframes.
Pilots will appreciate the increased engine, main and tail-rotor power available, particularly when hot, high and heavy. Handling compared with previous models has been enhanced, Bell says, although I found them similar: forgiving and safe.
Autorotation characteristics were always excellent. One can do a low speed autorotation to the ground, for example for an engine-off forced landing on a small area among hostile terrain. Just aim for the landing area and forget about forward speed, washing off any remaining ground speed at the bottom and cushioning the landing with two quick pulls on the lever. The undercarriage may break, but the crew will step out of the aircraft alive and well.
Although the VNE has been increased, the Huey II is still quite slow at normal operating weights. Pilot transition/conversion time for the upgraded Huey II is minimal.
Pilots and operators will be happy with the enhanced safety levels. Technicians will appreciate the ease of maintenance and reduced workload. The increased TBOs and TBRs will help. The operator will also benefit from the low acquisition/conversion and direct operating costs. They should find the increased payloads, both internal and external, and the greater range, increased availability and reliability most beneficial.
Source: Flight International
