The AH-1W SuperCobra is the basis of the Bell/ GEC-Marconi Venom Cobra, a candidate for the British Army attack helicopter
Peter Gray/DALLAS
My first pull-up in to the hover in the Bell AH-1W SuperCobra combat helicopter was adequate, but not completely satisfactory, so I landed and tried again. In every helicopter one has not flown before, the first pull-up presents several challenges. You have to try to outguess the aircraft's attitude in both pitch and roll and try to keep it straight as it breaks ground, so as not to lurch sideways, forwards, or backwards, or yaw.
The front cockpit of the SuperCobra provides additional challenges because the floor-mounted cyclic stick and collective lever are absent.
There is a 150mm stalk (or "stubby") on a shelf on the right-hand cockpit wall, instead of the cyclic stick, and a second 150mm stick, with two twist-grip throttles, representing the collective lever on a similar shelf on the left-hand side. This is moved fore and aft, instead of up and down, which is more usual. Both shelves are padded, which allows the pilot's arms to be comfortable and relaxed.
This arrangement makes room for the weapon-sighting and firing systems, which take centre place in the front-gunner's cockpit. The cyclic stubby has no trim-release switch or "coolie-hat" trim to help centralise it.
INITIAL TAKE-OFFS
Watched by Bell's chief production test pilot, John Honaker, in the rear cockpit, I pulled the collective side-stick slowly towards me, lightly grasping both throttles - my left arm was balanced on the cushioned shelf. I waited expectantly, with my right hand poised around the cyclic sidearm.
As we lifted off, we drifted slightly to the left, but the heading and height were good, so I was not too unhappy with the results. Despite the strangeness of the controls and their feel, I was able to contain the subsequent hover to within a metre or two, with satisfactory pitch, roll, height and heading control, which was just as well, since we were on the main apron at Bell's Fort Worth, Texas plant surrounded by expensive machinery.
I was not satisfied with my first attempt, so I performed my first landing and had another go. With the advantage of hindsight, I was able to achieve a near vertical take-off on my second attempt because of the AH-1W's good handling qualities and design.
We used 75% of the available power, giving us a healthy power margin, which is vital for nap-of-the-earth low flying - 50ft (20m) and below. Although our weight was 660kg less than the maximum operating weight of 6,700kg, at 1,500ft density altitude, the two General Electric 1,210kW (1,625shp) T700-401 engines still had plenty of power in hand at gross maximum weight up to altitude. We also had extremely impressive single-engine performance, as I was soon to find out.
Because vibration-free performance is so important in attack helicopters, to allow accurate target sighting, tracking and weapons firing, I assessed it carefully throughout the whole flight envelope. Two-bladed machines are renowned for high vibration levels, compared with their multi-bladed equivalents. The SuperCobra is remarkably smooth in the hover and all hovering manoeuvres. Bell has developed strut-and-weight mechanical vibration suppression system called Vibrex. The canopy transparency is divided by the thinnest of struts, giving superb all-round visibility. This, too, is important for the job to be done.
The Cobra's many predecessors date back to 1962. For helicopter enthusiasts, it ranks as one of the world's great helicopter designs. Many modifications have been made since then and there are many more to come. The AH-1W is one of the latest versions. GEC-Marconi is offering a variant of it to the British Army in the form of the Cobra Venom.
PRE-FLIGHT CONSIDERATIONS
After a thorough ground briefing which included a week's groundschool in 90min, Honaker showed me round the aircraft. Approaching from the front, I noted the typically thin Cobra fuselage, the non-reflective, laser-sight-resistant paint (both designed to avoid detection), the three-barrelled 20mm cannon under the nose, the sighting system on the nose, the two 3.25m-span stub wings, with one empty TOW launcher installed on each, and the cable cutters - no matter which way the wire goes, up or down, the cutters will deal with it. I noted, too, the pressure-refuelling cap. After a mission, the aircraft can be checked, refuelled and re-armed in about 15min.
Unlike the Atlas Rooivalk, McDonnell Douglas Apache and Eurocopter Tiger, the other attack helicopters on offer for the British Army contract, the Cobra has a skid landing gear.
Bell showed me a video of a test flight where the pilot deliberately attempted to break the aircraft by smashing it progressively harder on to the ground with each successive landing. At one stage, the belly of the fuselage touched the ground, but sprang back up. The aircraft survived his attempts.
Skids will often cope successfully with uneven surfaces on which a wheeled undercarriage would have difficulties. I have done many a running landing, skidding over large holes, hillocks and even the occasional narrow ditch in skid-equipped helicopters.
I installed myself in the front-gunner's cockpit. The seat is not adjustable, so I had to manage with not being able to see over the drooped nose. The pedals are adjustable so, with both my arms placed on the side cushions, I felt quite comfortable and could reach everything. The cockpits have an environmental-control system to enhance comfort. Since the aircraft is designed for worldwide operations, from desert heat in Saudi Arabia to snow in Turkey, this is important, as excessive heat or cold are major contributors to crew fatigue.
The all round view is superb from the front cockpit, although the forward view from the rear cockpit, is restricted by the head-up display and high instrument panels. A T-shaped group of basic flight, engine and rotor instruments occupies the right-hand side of the instrument panel. The weapons sight, a handgrip with all the weapon switches and other related weapons-management equipment occupy the left-hand side. There is a cluster of warning lights and a master caution light.
The layout is fairly simple and uncluttered. In contrast, I consider the rear-pilot's cockpit to be cluttered. As well as flight instruments, weapon-management controls, caution lights, radios and navigation aids, there are circuit breakers, various other switches and miscellaneous equipment. In the heat of battle, the occupant of this cockpit may well be overloaded at times. Plans are afoot to equip both cockpits with multi-function displays.
Engines and rotor start are straightforward and similar to that of other Bells - crank the starter, get 12% compressor speed, then open the throttle to the ground-idle position. The engine acceleration rate and temperature peak are modest. The two on-board batteries allow a start, a 10min charge from the aircraft generators and a complete shutdown, followed by another battery start.
After Honaker quickly completed the many post-start checks, I leaned forward, putting my eyes to the weapon sight, and practised placing the crosshairs on to a small vehicle straight ahead, and zooming in and out. This was for operations later in the flight. The view and accuracy are outstanding.
BEGINNING THE FLIGHT
After my second, more satisfactory, take-off to the hover, I settled down to a prolonged hover, to accustom myself to the rather different controls and their feel. I was quickly able to adapt my technique accordingly. Having progressed to a satisfactory hover, gentle sideways and backwards flight and spot turns (the quick stuff was to come later) I pulled up to an out-of-ground effect (OGE) hover at about 100ft and repeated the same manoeuvres.
The all-round visibility is superb and vibration levels are minimal. Our OGE power requirement gave us plenty of power in hand. I had both hydraulic systems and the stability and control-augmentation system (SCAS) working for me - handling the aircraft "raw" was to come later. I found the lack of a trim system for my cyclic control stick was inconvenient, so I had to work harder than normal to achieve satisfactory results. Bearing in mind the stiffness of the cyclic, I expected that, towards the end of a 1.5h flight, my right hand would suffer from writer's cramp and my left from tennis elbow. Happily, as I learned to relax and handed over control occasionally to Honaker, this did not happen.
I pulled full power of 100% torque (Bell 212 pilots will recognise the torque gauge) and we accelerated rapidly and shot up to 2,000ft. Leveling off and reducing to maximum continuous power, allowing the aircraft to stabilise in straight-and-level flight, gave an indicated airspeed of 142kt (260km/h) and a true airspeed of 150kt. We were at 3,500ft density altitude, at a weight of 5,970kg, 700kg below our maximum operating weight. Lacking a vertical-speed indicator, I had to ask Honaker when we were level. Vibration levels were low, a remarkable achievement for any two-bladed helicopter at this speed.
While we were still fairly heavy, I went for the never-exceed speed (Vne). Because we had a couple of launchers on the wings, this was reduced to 170kt from the "clean"-configuration figure of 190kt. Both Vnes are good up to 4,000ft density altitude, before starting to reduce by 5kt/1,000ft. The Vnes are also outstanding for a two-bladed helicopter.
Bearing in mind the negative g limit of +0.5, I dived down gently to ground level, but managed only 165kt, so a good opportunity presented itself to explore the zoom-up capability and have another go. I pulled full power and we went to 3,000ft quickly. The extra height and an encouraging forward nudge on the cyclic from Honaker in the back seat produced 170kt.
I was impressed, by the smoothness of the flight and the lack, of any significant vibration levels. There was no noticeable increase as I carried out turns in both directions. Balance control was excellent throughout, requiring little work on the pedals. There is no mixing unit to alter the pitch on the tail rotor as the collective lever is moved. The SCAS does this for you.
I handed over control to Honaker for him to demonstrate the aircraft's agility, a mandatory requirement for a combat helicopter. I specified the inclusion of a push over from the cruise to low level - a critical manoeuvre for a two-bladed helicopter with a teetering, semi-rigid, main-rotor system. The SuperCobra's rotor power is high, so one has to be careful not to overdo the negative g.
After some vigorous and spectacular manoeuvring by Honaker, over and down we went. His push over was constant, but cautious, as we finished up in a steep dive and high rate of descent. I felt my weight come off the seat. Vibration levels throughout were still benign.
I now handled the aircraft "raw" in the cruise with the SCAS and number-one hydraulic system off - the worst condition. This hydraulic system provides power to the tail-rotor pitch-control mechanism and also absorbs its feedback forces. Both these functions now had to be carried out by the pilot. The SCAS provides stability in pitch, roll and yaw, providing a hands- and feet-off-the-controls capability, when you get the opportunity. There are no problems in handling the aircraft in this degraded state, even using the stubby. Tail-rotor forces were not excessive, so I elected to attempt my first approach to the hover in this condition.
SINGLE-ENGINE PERFORMANCE
After restoring the SCAS and hydraulics and while we were at 3,000ft in a high-speed cruise, I asked Honaker to put us on one engine. My intent, as handling pilot, was to take no immediate action and see what happened. There was hardly any loss of rotor RPM, which is the first consideration after any loss of power, and our speed settled to a healthy 125kt at single-engine maximum continuous power.
An audio warning, in the form of a female voice, told us that something was amiss and required our attention. The same audio warning is used for low-rotor RPM, single- and double-engine fires and to signal that the rotor brake is on.
To demonstrate how good this aircraft is on one engine, we came to a hover at 3,000ft and performed a spot turn to the left, the most power-absorbing condition. After relighting our second engine, we did a 180° auto-rotation to ground level. We settled at the minimum rate-of-descent speed of 70kt and achieved a rate of descent of 1,900ft/min (10m/s), which is normal for an aircraft of this weight, size, shape and rotor - a mere 19kt vertical speed. Rotor RPM stabilised gently within the wide limits, even during the turn, when one often has to apply some collective pitch.
The flare effects at the bottom from the big blades (14.6m long by 0.8m wide) were pronounced, adequately arresting our forward speed and rate of descent, giving us a momentary increase in rotor RPM and thus lift for the actual touch-down and time to accomplish a well-controlled engines-off landing, had we needed to.
We returned to the cruise and next checked for any rotor droop when lowering and raising the collective lever rapidly. There is an anticipator on the collective, which immediately signals both fuel-control units on the engines as soon as the pilot moves the lever. This starts a fuel-flow adjustment before the rest of the power-turbine/main-rotor RPM controls can react. So droop is insignificant throughout the whole power range.
There are over-torque and over-temperature limits and limiters. These allow a pilot faced with a difficult situation to pull excessive torque and temperature. The limits are generous and no damage is done. The pilot is warned through the head-up display on his helmet that these limits are being approached. Honaker demonstrated this by pulling to 110% torque - the engines were trimmed gently back. He gave a similar demonstration with the exhaust-gas temperature.
I asked Honaker to switch off one, then both generators. At that stage, we had more than 20min of battery life left before all electrical power would be lost. Even then, the aircraft would still be flyable and safe for day visual flight.
In the event of a malfunction of a fuel-control unit, the engine can be changed to manual throttle, not by a switch like on some other Bell aircraft, but by rolling open the throttle, then backing it off. The engine in manual throttle is then adjusted to a setting just below that of the "good" engine, which then does all the automatic controlling of RPM and power. I tried this and much preferred this system of selection, where it is impossible to make a mistake by selecting the wrong switch or omitting to close the throttle before selection, which usually burns out the engine.
VIBRATION SUPPRESSION
On our way to Bell's satellite airfield, Honaker switched off the vibration-suppression system. We were cruising at high power and speed. The increase in vibration made my teeth chatter slightly. When switched back on, all was sweet and smooth again - a good demonstration of the system's effectiveness.
We joined the circuit at the airfield and I flew the aircraft "raw" down the approach and in to the hover. I warned Honaker that, if I found directional control hard at the bottom while pulling in power and applying left pedal, I would call for his help on the pedals. I had no problem, however, and found that the SuperCobra was relatively easy to fly in this condition. There was a slight lag between applying pedal and the resultant action, but this was not excessive. I soon learned to anticipate this. The pedal forces required were not excessive, so I felt confident enough to do a spot turn in each direction.
The SuperCobra is slightly unstable in pitch and roll, but easy to contain. The aircraft was getting to be a bit of a handful at this stage, with no SCAS, no hydraulic boost to the tail-rotor control, a stiff stubby and delayed pedal reaction, but, even so, after some practice, a pilot should be able to hover and put it down quite safely. To my surprise, Honaker told me that the aircraft is manageable with no hydraulics. This is unusual for any helicopter of this size and must reflect the good control systems and well-balanced rotors.
The tail rotor, although appearing like a larger version of that on the Bell 212, is of relatively new design and modern materials. To help offload some of the yaw, the vertical fin is offset and has an angled trailing edge. The AH-1W is limited to 35kt sideways and 30kt back-wards flight. This is to allow such manoeuvres at maximum operating weight at altitude and still give the pilot a 10% control margin in hand. We were by now fairly light, so I took it to an estimated 45-50kt sideways flight to the left and needed full right pedal.
At the same speed to the right, I still had some left pedal available. The handling was impressive. There was not even a twitch from the large (3m long by 0.3m wide) two-bladed tail rotor as we hurtled sideways down the runway. High-speed back-wards flight was also viceless. Honaker was later to demonstrate this to me somewhat dramatically. Aircraft attitude, handling, visibility and vibration levels, were satisfactory when hovering crosswind and downwind in the 12kt breeze.
Next came the examination of the business end of the SuperCobra's performance and flying qualities - nap-of-the-earth flying at 50ft and below from low speed to maximum speed, target sighting, tracking and shooting. Honaker reminded me that it is prudent to keep positive g on at all times, then the aircraft can be thrown around nimbly, with impressive rotor power available to the pilot.
We ducked around trees, flew through narrow gaps and climbed over bushes. I made a quick stop, by kicking the aircraft sideways with the pedals to see where I was going, observe the obstructions ahead and protect the tail rotor. It worked well and, because of the high drag, we stopped quickly. We carried out a dash, keeping low and among the trees and bushes. Honaker knows every branch in this area. We slowed down to 40kt and eventually finished in the hover in the undergrowth.
There was a tempting white dome on the horizon 2,100m away, right on the nose. Honaker took control of the aircraft and invited me to use the sight to put the crosshairs on to the dome, zoom in to get a close-up view, hold it in the sight, arm the cannon, then, pull the trigger. He could see what was going on in his head-up display. I did as I was told.
After some trial and error, I got it all together and pulled the trigger. Honaker assured me that, if the cannon had been loaded, there would have been a direct hit on the dome, which had loomed large in the sight. When he told me to look up (I had been concentrating head down in the sight), he had sneaked the aircraft's heading around 110° and, although the cannon was still pointing directly at the target, it was now also pointing 110° left of the aircraft's nose. This demonstrated the cannon's range of movement of 110° left and right of the nose. It can also be moved 20° up and 50° down.
Pilots have reputedly put missiles through car windows at over 2,000m with this system. I was shown a night sight on one aircraft. If I were using other weapons, the pilot would have control of the cannon to help protect us. He has a sight glass on his helmet, which is coupled to the cannon turret.
Having carried out a single-engine OGE hover at the beginning of the flight, it was pointless trying a single-engine approach and landing. To emphasise the SuperCobra's single-engine performance, Honaker shut down one engine again. I came to the hover, climbed to 1,000ft and headed for base.
THE RETURN TO BASE
After relighting (another benign restart of a hot engine), Honaker took charge of our arrival back at base. He established with the control tower that there were no other aircraft around, a necessary precaution for what was about to happen. We arrived downwind, fast and low.
We slowed down a little and turned 180¡ to have us hurtling backwards down the runway. As I followed him through on the controls, I noted that Honaker pulled a lot of power to keep the positive g going, and up and backwards we went at 2,000ft/min. The nose was constantly being lowered until we finished up probably vertical. We dived down, accelerating rapidly, skimmed along the short runway at high speed, very low, zoomed up into a wing over, going over the vertical on the way round and recovered to the hover.
This was an impressive demonstration of what can be done, when you know how, with a two-bladed semi-rigid, teetering rotor.
CONCLUSIONS
A combat attack helicopter is called upon for many tasks. The challenge to the helicopter manufacturers has been to provide a multi-role vehicle, which will do everything required of it in all sorts of weather and terrain.
The challenge far exceeds that of its fixed-wing counterparts. The helicopter is required, to be capable of taking off vertically, perhaps from a confined unprepared area, such as a clearing among the trees, at high density altitude with a full load of weapons and fuel, be flown long distances, seek the target, with or without outside help, without being detected, destroy it and return to base. Some manufacturers have achieved those objectives.
Considering that helicopters have been in regular service for just over five decades, the end product - the result of research, development and actual battle experience - is awesome. Bell is one of the manufacturers which have risen to the challenge with the SuperCobra
Source: Flight International
