While hover taxying on the Bell 412 during a particularly rainy day, we suddenly got caught in a heavy monsoon downpour and could hardly see the ground below us. Among other things that were going on in my mind at that time, I found myself wishing for a wheeled helicopter so I could have ground taxied. That spurred me to think more deeply about ‘Wheel versus Skids’ – Who is better off?
Being an ex-naval aviator, I must admit that a major chunk of my experience is on wheeled helicopters. It is easy to be biased in favour of wheels if you are from the navy! But in this discussion, I hope to reserve my sentiments and deal with both types with an even hand.
One of the important considerations during design phase of a light helicopter is the type of landing gear – designed to allow absorption of the vertical energy of landing by elastic deformation during normal landings and by plastic deformation in case of hard landings. The two main types are wheel type, which incorporates main wheels and a nose or tail wheel, and the skid type. There are further variations such as retractable, non-retractable, high skid, low skid, skies, floats etc. However, this discussion is limited to the two main types and light helicopters only. Skid-type landing gear incorporates two skids, with forward and aft cross tubes, on either side of the helicopter. The cross tubes are designed as a supporting structure to transfer forces and moments between skids and fuselage and may incorporate pivot mechanisms to reduce ground resonance. The forward end of skids are usually bent upwards to facilitate run-on landings and close-to-ground handling. Wheel landing gear, quite simply, has wheels with hydraulic oleos & struts to attenuate landing loads.
Simplicity & Weight. To start with, skids are design-wise simpler and lighter as compared to wheel landing gear that require brakes and a hydraulic system, if they have to be retractable. As an example, the difference in basic weight between the skid version and fixed wheel version of the Hindustan Aeronautics Limited 5.5-ton Advanced Light Helicopter (Dhruv) is almost 250 kg. Most light, single-engined helicopters thus have skids (e.g, Robinson R22, R44, R66, Schweiser 300, Bell 407 etc). There are exceptions such as the venerable Aérospatiale Alouette III which has served worldwide with great success with its tricycle wheel landing gear and a simple yet efficient braking system.For design simplicity, ease of maintenance and weight saving, skids can be considered superior to wheels. But ahem, there is a rider! Skid and cross tube lives are based on number of landings & ‘on-condition’. Helicopters that see a large number of landings per flying hour (e.g, helicopters rig-hopping during offshore ‘production’ sorties) may have their skids and cross tubes coming up more frequently for checks and replacement thereby diminishing the maintenance payoffs somewhat.
Speed. Helicopters that are optimised for high speed can ill afford to have permanent, drag creating structures like skids or non-retractable wheels. ‘Fast & light’ helicopters with retractable wheels exchange speed and fuel consumption for the simplicity and convenience of skids. As a pilot flying Bell 412 (cruise speed 120 kn), I am quite used to, though not happy, being overtaken by the faster Airbus AS365N3s or Sikorsky S76s (cruise speed 140-150 kn) with retractable wheel landing gear. But these ‘faster’ aircraft have to contend with landing gear and its ancillaries impacting their ‘weight to payload’ ratio. In heavier helicopters like AW139 (6.8tons MTOW), this drawback is compensated by the impracticality of skids and the faster speeds, better performance and improved fuel consumption that a ‘cleaned-up’ aircraft offers. As an example, on the wheel-variant of ALH (Dhruv), there is about a 3% difference in fuel consumption between landing gear extended and retracted. But the wheeled variant is about 250 kg heavier as compared to the skid variant. Such is the world of helicopters – an eternal story of contradictions and trade-offs!
Ground Handling. Helicopters with skids are not readily manoeuvrable once landed. Strap-on wheels or ‘Dolly’ has to be attached for mobility and it needs a towing device for ground movement. Handling ground movements on a busy tarmac full of light, skid-fitted helicopters with limited number of helicarriers or tow carts may be an apprehension for those unfamiliar with skid-fitted helicopters. However, this factor is mitigated by the large number of very efficient single-person towing solutions that are available today (see http://www.helitowcart.com/ for an example). In any case, even light helicopters with wheel landing gear require a towing device over 2-3 tons. So nobody wins here and both types are highly comparable in this aspect!
Ship deck & Offshore operations. Ship decks are seldom calm at sea. Landing on a surface with 6-DOF motion using a skid-fitted helicopter may complicate an already complicated evolution, don’t you think? What about metal to metal contact?? Won’t frequent landings exacerbate wear and tear? Valid concerns for a naval aviator!
Firstly, wheels or oleos are no insurance against a poor ship-deck landing. If you are riding on the oleos or wheels to save you from that awful landing, be advised there are other hazards like ground resonance or dynamic rollover working against you!
Most offshore platforms and even some ships use an anti-skid net on the heli deck. While these are meant to prevent wheeled helicopters from skidding, usage of such nets may pose a hazard for skid-fitted helicopters while providing no real benefit to the pilot (apart from visual cueing of relative movement while operating close to deck). Athwartship landings may sometimes be necessitated due to winds, sea state or operational reasons. Realigning a skid-fitted helicopter and moving it to & from the hangar can be tricky in the dynamic environment onboard a ship. Some wheeled helicopters like the ALH (Dhruv) and naval Lynx have ‘toe-in, toe-out’ main wheels with nose wheels that can castor upto 90 degrees. This allows the helicopter to be pivoted into wind for a relative wind take off without the ship having to alter course. Such luxuries are not yet available for helicopters with skids. In any event, striking down a helicopter into the ship’s hangar is an essential & time-critical activity for ship’s crew. Anybody who has experienced the ease of pushing the Alouette IIIB into hangar while the ship’s Commanding Officer is breathing down your neck to ‘expedite’ knows only too well the advantages that wheels offer in this regard. In rare cases, ship decks could even have protrusions or traversing rails running proud of the deck which may impede operation or ground manoeuvring of light, skid-fitted helicopters. All these factors ostensibly make a sound argument in favour of wheeled landing gear for naval use. But, is it enough reason to totally rule out operation of better performing skid-fitted light helicopters from ship decks? I wonder.
Watch this unmanned MQ8B Fire Scout landing on USS McInerney here
A recent comparison that I reviewed belies the arguments in favour of wheeled landing gear for offshore helicopters. Ship motion envelope in an operator’s manual for floater landings on the civil certified Bell 412 is 4 degrees roll & 4 degrees pitch. This is comparable to the ship motion limits in ‘unaided’ (no deck lock) SHOL of most light, ‘wheeled’ helicopters onboard naval ships. So for those of us who like to believe that we require the comfort of wheels and oleos to operate from decks, what does this contrast signify? I would like to hear your views.
Close-to-ground manoeuvring. Skid-fitted helicopters have no option but to air taxi. This generally requires more manoeuvring space and their downwash may affect aircraft in vicinity, especially in small dispersals shared by other light aircraft. Regulatory inter-aircraft clearances required for manoeuvres at hover may be higher than ground taxi clearances due to reasons of controllability and positional scatter. Effect of downwash also needs special precautions to be exercised by other helicopters starting their engines (read rotors) while other helicopters are taxiing in/out. As we move on from light singles and twins to medium and heavier helicopters, this distinction assumes importance. Downdraft from heavier helicopters like the Kamov-28, S92 or the EH101 can seriously affect lighter aircraft or helicopters parked in the vicinity. Apart from strength of skid design, this limits usage of skid landing gear on helicopters with AUW in excess of 5-5.5 tons, beyond which wheels become the only practical option.In the light helicopter category, this aspect does not assume much importance and skids fare as well as wheels.
Ground Clearance. Skid-fitted helicopters usually have better ground clearance as well the option for ‘high skid’ configurations in some cases like the Bell 412. While operating from unprepared areas such as in EMS or ALE, this could be an advantage over wheeled helicopters. That said, the weight of the helicopter is distributed over three ‘points’ in a wheeled configuration versus two skids with their greater length and a ‘linear’ contact area. Remember, it may be easier to put down three wheels on a slope as compared to two parallel tubes of the skid landing gear. But the greater force per unit area exerted by wheels could cause them to sink if ground is soft or slushy. Skids (skies) are also necessary for operating in ice-bound areas such as the Siachen glacier where the Indian Army and Indian Air Force operate Cheetahs (Lama) on a daily basis.
Again, for a ship-borne helicopter, incorporating a deck lock like the Harpoon on a skid-fitted helicopter is presently impractical owing to the same issue of higher ground clearance. I would venture to state, if operation to unprepared or semi-prepared sites are envisaged, skids would be the landing gear of choice for light helicopters.
Run-on Landings. Run-on landings, though rare on helicopters, is still a necessity under some exigencies like OEI, directional control failures, etc. Having done a few training engine-out landings on light singles, I can vouch for the mental comfort provided by wheels as you start down on that one-way ticket once the lever is pulled back. Till I actually did the first few run-on landings on skid-fitted helicopters, I was quite sceptical. Fact is – they do it nearly as well as wheeled helicopters if executed properly (though I personally dislike the way it goes ‘kerrruncchh’). But talk about a run-on landing at 40-60 knots on a skid-fitted helicopter (a piece of cake on wheeled helicopters) and I may start having second thoughts. So will my maintenance chief and the ATC officer! I would also be uncomfortable landing under potential ‘brown out’ conditions with a skid-fitted helicopter. For run-on landings, wheels are the obvious choice, especially with frequent landings like when the helicopter is used for training. But then, the helicopter was never made for run-on landings; or was it?
Ground Resonance. Ground resonance can occur when the spacing between rotor blades of a helicopter is disturbed due to any reason and, simultaneously, the helicopter’s natural frequency is excited by the ‘out of balance’ rotor. Landing gear can contribute to this ‘dance’ if its frequency also matches with the resultant oscillation. However, to the best of my knowledge, skid-fitted helicopters are no less or more prone to those with wheels. In fact, incorrectly maintained oleos & struts in wheeled helicopters can predispose them to ground resonance. If the causative factors are present, ground resonance is a real danger to both types and does not really differentiate between the two. Have a look at this skid-fitted AS350 helicopter shaking itself to pieces!
As you can appreciate, the design of any helicopter is a juxtaposition of several contrasting and often contradictory requirements which the designer must resolve with the ultimate aim of optimising intended roles of the helicopter. For light commercial helicopters, management of ‘useful payload’ or ‘performance’ drives many design considerations. For heavier helicopters with specialised roles such as an attack helicopter or a naval anti-submarine helicopter, some other factors may prevail.
During design iterations therefore, the type of landing gear is finalised by designers in consultation with users or test crew. In the end, to each his own and we go about selecting what fits our requirements the best. I would love to hear from experts in the field with varied experiences on this subject.
Thanks for reading and stay tuned for more!