A BRIEF ACHER CASE STUDY
SKIDS VERSES WHEELS IN THE OFFSHORE ENVIRONMENT AND SPECIFICALLY IN THE SHIPPING MARKET.
The original development of skids was as a result of the limited power available from piston/turbine engines used in helicopters during their development stages. Skids were light and simple structures that allowed the weight saved to be converted into internal disposable load. Modern powerful turbine engines have done away with this weight saving requirement.
Skids are now fitted to very few helicopters in the commercial and military market. In fact almost the only helicopters still fitted with skids are found in the light helicopter range and a few exceptions in the medium helicopter range. They are light, cheap and require little maintenance. Those in the medium range are usually twenty or more year old designs.
Skids. A set of skids hanging out in the air stream all the time cause drag. This both slows the helicopter down and increases fuel consumption.
Wheels. A helicopter with retractable wheels offers a ‘clean’ profile.
Skids. If a helicopter is to be moved on skids a special set of ‘dolly wheels’ are required and must be specifically fitted for this. The helicopter must be ‘manhandled’ to its location in the hanger or anywhere else it has to be moved to. It is also unable to ground taxi in a confined area. (On the apron at an airport) It has to hover taxi causing a strong downwash of air and the consequent potential blowing over or damage to surroundings.
Wheels. On wheels the helicopter is simply pushed or pulled with a small tug to where ever it is required. If required, the helicopter on wheels can ground taxi with very little down wash of air within a confined area.
Skids. Single engine reject landings require the helicopter is skidded onto the runway damaging both the skid strips and the runway.
Wheels. The helicopter is run onto the runway and brought to a stop gently with brakes.
Skids. These offer very little traction on ships sloping decks. The possibility of slipping is always very real. The possibility of landing on small deck protrusions is always there. (Especially at night) This does cause expensive damage to the skids. Slipping on deck causes scratching of the deck paint. This has to be repaired before rust starts. A nuisance for the ship.
Wheels. Offer very good traction. Small protrusions offer no hazard. No scratching of deck paint. No sea-saw effect sometimes encountered with skids on an uneven deck welds.
Centre of Gravity and Hoist Location.
Skids. Because the skids protrude some distance from the side of the helicopter this means anybody being hoisted must be hoisted outside of the skids. This is some distance from the centreline of the helicopter. This in turn causes a large lateral centre of gravity moment tending to reduce the disposable load on the hoist. Some light French helicopters overcome this problem of lateral moment and direction of rotation of the main rotor system by fitting the hoist on the opposite side of the helicopter to the helicopter pilot. (A most unsatisfactory arrangement for ship service activity)
Wheels. No obstructions are offered by wheels. The marine pilot is hoisted up/down close to the door. Does not have to be pulled in. There are few centre of gravity limitations and the hoist is fitted on the same side as the helicopter pilot.
(Note. This is a very important consideration)
Sloping ground. (Decks)
Skids. Helicopters fitted with skids have reduced limitations with sloping ground landings. This also applies to deck landings and a vessel rolling. (which is the same as sloping ground landings.)
Wheels. Because of the wheel traction and oleo struts (suspension) a greater slope can be accommodated. This varies from helicopter to helicopter. Wheels offer greater slope (roll) capability.
Unprepared landing areas.
Skids. Far more likely to suffer damage with large ‘footprint’
Wheels. Tricycle undercarriage offers three point contact with pneumatic oleo struts (suspension) for unprepared or uneven ground.
Skids. Emergency floatation is generally attached to the skids. Increases profile drag and in an emergency the helicopter sits higher out of the water. (More unstable)
Wheels. Emergency floatation attached to the side of the helicopter. Quick release/removal and in an emergency, helicopter sits lower in the water. More stable.
There are VERY few maritime applications, (if any) either civil or military, where skids are called for in preference to wheels. Generally speaking, all offshore helicopters are fitted with wheels and generally, these are specifically called for in the tender and technical processes.
Some operators do use helicopters on skids for offshore ship service operations. One suspects that they are compelled, or patriotically influenced. Helicopter operators have to make special arrangements to accommodate the limitations imposed.
There are some onshore applications where a high ground clearance is of benefit. Here high skids are specified. These high skids cannot however then be used offshore as they make hoisting difficult or severely limit the hoist load with centre of gravity limitations.
A medium size old Bell 212 (fitted with skids) was originally used in the early days of Acher ship service activity. Whilst an excellent and robust machine, this helicopter was fitted with pieces of car tyre strapped to the skid bottoms in an attempt to offer better traction and reduce deck damage.
In conclusion, there is ‘no contest’. Wheels are far better than skids in almost all maritime environments.
Acher Aviation (Pty) Ltd
28 March 2001