THE KAMAN SEASPRITE was first flown in 1959 and was developed and sold to the US Navy as a single-engined (General Electric T-58-powered) search-and-rescue (SAR) aircraft. It was converted to a twin-engine helicopter in 1965, and has undergone steady development since then.
Over 250 aircraft have been built to date. The version I flew - the AH-2G Super Seasprite - is the latest variant, modified from earlier "F" models in 1993 with new avionics, mission electronics and GE T700 engines. It also fits on to smaller decks, the tail wheel having been moved in from the end of the tailboom by some 2.4m, giving an undercarriage footprint of 5.1 x 3.5m.
The US Navy (largely reserves) now uses the SH-2G for ship-based anti-submarine warfare (ASW) - the Seasprite seeks and attacks - anti-ship surveillance and targeting. The ASW or other equipment can be quickly removed to allow the aircraft to be used for secondary tasks such as SAR, delivering underslung cargo, medical rescue, personnel transfer, surveillance and reconnaissance and, with newly developed equipment, mine hunting. There is capacity for add-on capabilities, such as machine guns in the cabin.
EXTERNAL INSPECTION
As Kaman chief test pilot Gary Kochert and I approached the Navy-owned aircraft, I could not help noticing that the once smoothly designed exterior is now littered with many, but inevitable, protrusions - mirrors for both cockpit occupants to view the rear end; external stores - fuel tanks, torpedoes, missiles (the aircraft was recently cleared for firing the AGM-65 Maverick); provisions for flares; smoke and sonobuoys; external folding rescue hoist; underslung load hook; an attachment for a crane to lift off the blades, gearboxes, engines and everything else on the roof; flotation equipment and so on.
We opened the waterproof nose doors, where most of the avionics and the single battery are kept. The doors carry covered-in avionics equipment - an intelligent use of space. There is plenty of room for more equipment here.
ATTENTION TO DETAIL
All the doors and some windows are jettisonable and are attached to the airframe by cables to prevent them entering the rotors - an example of the good attention to detail which is obvious throughout the design.
The undercarriage retracts forwards and, as expected, accepts enormous landing loads - the Navy requires demonstration of a 720ft/min (3.66m/s) descent rate, which we also accomplished during our flight. The fuel system is self-managing and can be gravity- or pres- sure-refuelled. It is not crashworthy. The Seasprite can also be refuelled while in the hover. Alongside the refuelling ports are two panels, behind which are indicators for the oil levels of the various gearboxes and other components - more useful attention to detail.
Main- and tail-rotor blades can be folded easily, for fitting the aircraft into small spaces such as on board a ship.
We went up top. The engine cowlings are designed to serve as work platforms and are stressed for the weight of two heavy technicians and toolboxes. The engines are spaced widely apart, as on an attack helicopter. Their small size belies the power they produce - nearly 1,490kW (2,000shp) each. The Super Seasprite, however, uses only 1,280kW for 2.5min when powered by one engine, 1,260kW for single-engine power for 30min and 1,070kW for maximum continuous power at sea level and 15°C. This means that, at lower altitudes and temperatures, there is a lot of power in reserve, while full power is available at higher altitudes and temperatures. Not using an engine's maximum design power also increases its life and time between overhauls. Impressive single-engine power is available, as I was to discover after our first take-off at near maximum weight. Parts of the engine can be changed independently without changing the whole unit, although a whole engine change will take only 2h. The engines are self-monitoring. There is no specific overhaul schedule - condition is the only criterion.
I was impressed by the accessibility of everything on top, which is as it should be when the helicopter is based offshore and being maintained by the users. We saw the auxiliary power unit (APU), the sole purpose of which is to provide air to start the engines. This eliminates the need for two large batteries and heavy cables which would be required to start the engines electrically. It also eliminates the tremendous surges which occur in the electrical system when using batteries and aircraft generators to start large engines. The APU gives the aircraft total independence from any outside power source.
I noted the six gearboxes - one at the front of each engine driving into a central combining box which connects into the main box, plus the usual two tail-rotor gearboxes. Every box is monitored by at least two chip-detector lights in the cockpit, with a burn-off function. There is no auxiliary lubrication system for the main gearbox. Kaman assures me that the gearboxes are damage-tolerant, and that the nose and combining boxes have been test-run for 30min with no oil.
Much of the design, operating parameters, limitations and component functions were dictated to Kaman by the US Navy, although the design is flexible enough to allow other clients to dictate similarly - for example, for the provision of glass cockpits.
This dictation is demonstrated by the flight manual, which contains over 800 pages. The contents are sometimes modest about the actual capabilities of the aircraft. For example, the single-engine power-available chart shows less than is actually available, and the maximum weight is restricted to 6,000kg with internal loads. The manual is informative and, as well as the usual data, contains sections on mountain flying, SAR patterns, crew co-operation and other subjects.
INSIDE THE CABIN
A look inside the cabin reveals a sophisticated sensor operator/weapons station with the latest naval equipment, plus a large, nearly-empty ,tail-cone area with lots of space available for additional equipment. Half the cost of this aircraft is probably accounted for by the avionics. There is accommodation for cabin configurations to allow transport of passengers, litters and internal cargo.
I asked Kaman for a maximum-weight (6,140kg) aircraft, to explore the single-engine and other performance parameters. We obtained 5,900kg.
The Super Seasprite is designed to be a single-pilot aircraft. The other cockpit seat is intended for a tactical co-ordinator. There is a full set of dual flying controls, however.
This was my first experience of flying a helicopter with an aerodynamic servo-tab rotor-control system. Charles Kaman, founder of the company and now chairman and chief executive, designed and developed servo-flap control of helicopter main rotors in the 1940s. The blade angle of attack is controlled by trailing-edge tabs, the pilot moving the tabs with his collective-pitch lever and cyclic pitch stick through a light control linkage. The resulting light control-forces also allow use of an automatic or pilot-operated in-flight blade-tracking system using small electric actuators.
COMFORTABLE COCKPIT
Ease of access through the large sliding doors is good and the cockpit is comfortable.
The layout is sensible and ergonomically pleasing, with well-marked, easy-to-read, instruments grouped together. Temperature and pressure gauges can be rotated so that all needles are in line, making a small loss of pressure or increase of temperature easy to spot before the condition becomes serious.
As well as the usual warning caution and advisory lights, there are two warning sounds for low-level flight, with the undercarriage raised. All-round visibility is good. The aircraft has adequate avionics, with plenty of space available for additional extras.
The circuit breakers are sensibly laid out and there is a copy of the layout in the checklist as nice touch to facilitate finding the one sought - such a requirement nearly always happens when flying at low level in bad weather at night.
Rather than go through the normal checklist, Kochert demonstrated the emergency-sortie start-up technique. He started the APU by using the small single battery, simultaneously pressing engine-starter buttons on the engine-condition levers, known to civilians as speed-select levers, running both levers up to "fly", checking that the warning, caution and advisory lights were out and switching on the auto-stabilisation equipment (ASE). We were ready to go in about 2min.
STRAIGHTFORWARD HOVER
The day was cold (4°C), with very little wind, and we were at a density altitude of -1,500ft (-460m). The take-off to the hover was straightforward, and I could achieve an accurate, steady-state, hover. Kaman has tilted the main-rotor drive-shaft forwards and to the left, giving a level hover attitude. While we were at our heaviest, Kochert immediately put us on one engine. The rotor RPM drooped by 2%, but the functioning engine quickly restored it and we remained in the hover with all parameters below the 30min power limit, with plenty of power in hand. To prove the point, we moved into forward flight on one engine.
To show how manoeuvrable the aircraft is with the servo-flap rotor-control system, we levelled at 40kt (75km/h) and went quickly from 45° of bank one way to the other. I noted a slight one-per-revolution vibration. As we accelerated to 70kt, then to 115kt, the rotor action became smoother. Considering that there is no anti-vibration system installed, the ride is remarkably smooth. Kochert used the manual in-flight rotor-tracking system to put one blade out of track, producing a one-per-rev thump, then engaged the auto-track to correct it. This it did in just a few seconds. We were left with a slight one-per-rev vibration, caused, I was told, by a heavy blade. The autotrack had taken out about half of the vibration.
While still fairly heavy, we went to maximum continuous power in straight-and-level flight and reached a healthy 133kt true airspeed at a fuel flow of 470kg/h according to the flight manual, the same as a Sikorsky S-61N with much older and smaller engines (you would expect to achieve only about 110kt in an S-61N). Normal fuel-flow is 320-410kg/h.
The ASE was stretched out at 90kt and we flew hands-off (trim is still available, unlike in some helicopters). The Super Seasprite wallowed off slowly by itself after many seconds of straight-and-level flight. We tried the same at nearly zero airspeed, with almost the same result - an impressive demonstration of good stability characteristics for such an aircraft. During normal operations the pilot may not even notice an ASE failure, and so there is a warning light.
Any other helicopter of this size would normally be uncontrollable with no hydraulic assistance to the rotors, and so would require both a primary and a fully independent auxiliary system. The Super Seasprite has only one system, which Kochert now switched off. I felt a vibration through the cyclic only. The controls were slightly stiffer, but nothing unacceptable. The aircraft was still quite stable. Kaman tells me that the Seasprite has been landed in this condition on ships at night with no problems. I flew an approach and had no trouble coming to an accurate hover over my target, having taken the precaution of doing a flat, gradually decelerating approach to avoid any rapid lever movements. I was agreeably impressed by the docile handling for such a large aircraft - the servo-flap system had served me well.
The never-exceed speed (Vne) placard gave our limit as 129kt, which is Navy-imposed. We went to this speed and rapidly failed an engine, doing nothing else but sitting back and observing. Rotor RPM drooped by a modest 2% temporarily, and our speed dropped to 120kt. Torque settled at 80%, 9% below maximum. All the other parameters were below those 30min limits, a very satisfactory result.
ASE STABILITY
Because the aircraft is designed for single-pilot operation, I was interested in its stability with the ASE on. The nose was raised sharply and steeply and bank applied, with the cyclic then being released. The helicopter recovered quickly to its original attitude.
Although the aircraft has conservative bank-angle limits, with Kaman's chief test pilot on board we selected the barometric-altitude system hold and went happily to 60°. The system held our height to within 50ft as we quickly reversed the direction of roll. There was a large lake ahead so we engaged the radar-altitude hold and programmed it to 150ft and 80kt. At 250ft, I heard the audio warning. The aircraft levelled off nicely. We flew over a small island - the system took us up and back down again.
The Super Seasprite has an automatic hover-hold, but no automatic-approach mode: we accomplished this manually by winding down the radar-altitude hold and the speed. There is also a deviation bar to help keep on track. Using this system, we drove the aircraft down hands-off to less than 20kt and 80ft, feeding in some deliberate upsets, from which it recovered well.
After climbing up again, although the Vne was restricted to 129kt at our weight and altitude, we dived down to 160ft - the vibration levels remained benign.
HEALTHY WARNING
Kochert took over for our arrival back at the base. We did several low, fast, beat-ups at 145kt, hard pull-ups and wing-overs at up to 90° of bank. During one banked pull-up, the stick shook, reminding us that we were approaching blade stall - a healthy, natural, warning provided by the servo tabs.
The rotor disc is quite small for the weight to be supported, so, on attempting an autorotation, I was not surprised by our rate of descent of 2,600ft/min at our best glide angle and 85kt. The flare had plenty of bite, and restoration of engine power was undramatic, despite the fact that such modern engines take a while to spool up to the higher operating RPM.
I went to 41kt sideways and 33kt backwards, which I read from the groundspeed indicator. At 15kt sideways, there was a slight heading instability, but we accelerated through it, and the heading stabilised steadily at 90° to the direction of movement. Backwards flight was uneventful.
The US Navy required a tough undercarriage. Kochert planted the aircraft at about 700ft/min. We landed on three points with no bounce. The tailwheel, unlike some, is strong enough to take such treatment (and worse, including tailwheel-first landings), although, because of its inboard position, care has to be taken to avoid banging the end of the tail cone.
After Kochert had demonstrated some exceedingly fast turns on the spot, during which I felt the sideways G force, I had no difficulty in achieving accurate, but slower, turns on the spot. Landing on a 9° slope produced no difficulties.
I flew a steep approach on to the H. Although the forward visibility is good, I kicked the nose slightly sideways so that I could keep the target in view to the side of the instrument panel all the way down, flying the approach with slightly crossed controls. I came to a neat hover exactly over the circle.
Manual throttle was selected by taking the engine-condition lever from the "fly" position to "lockout" then immediately back to a detent at about half-way. I flew a circuit and performed a landing. The RPM of the engine in manual throttle was kept slightly lower than that of the governed engine, resulting in this engine doing all the RPM controlling to maintain constant rotor RPM - a standard technique on most helicopters. Few speed-select movements were required. To help, there is a mechanical connection between the manual throttle and the lever - more good design detail. The speed selects, which the pilot can easily remove from the lever to make any adjustments, are on the centre console by the pilot's left hand. They can be repositioned overhead should the client wish.
CONCLUSIONS
One of the big advantages of developing an older and more mature aircraft is that all known defects and problem areas can be eliminated. Kaman receives copies of all the USN's relevant incidents and accidents reports, and has acted accordingly when necessary. So, in 1996, the Super Seasprite is an almost vice-free and safe aircraft, with lots of relevant features for the job to be done, some of them unique. The aircraft and its predecessors have been flown over 1 million hours to date.
A fully equipped aircraft weighs about 4,300kg, leaving 1,800kg for crew, fuel and expendables. Since engine-off landings have not been certificated above a weight of 5,600kg, the last 500kg of the maximum take-off weight has to be carried on the external load fittings: for example, extra fuel, weapons and the like may be accommodated. From a handling and systems-management point of view, the aircraft is easily managed by a single pilot, but, no doubt, prudent operators will put a pilot in the left-hand seat. I was also impressed by the servo-flap system, which has achieved its inventor's criteria.
Source: Flight International
