Wheels -v-Skids
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Wheels -v-Skids
Goood Morning Rotorheads,
Apart from the ground handling benefit, what advanage or disadvantage is there from fitting Helis with retractable wheels, when they seem to be an added weight and technical complication compared with Skids, and, which do you consider the most safe and practical option, Wheels or Skids?
Apart from the ground handling benefit, what advanage or disadvantage is there from fitting Helis with retractable wheels, when they seem to be an added weight and technical complication compared with Skids, and, which do you consider the most safe and practical option, Wheels or Skids?
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Can't be too much of a technical complication otherwise the plank drivers wouldn't be able to work out how the lever works. Up and down wheels have been around for a long time I don't think there is anything too complicated about it.
Wheels come with oleos makes landings softer when done right, saves your butt when done wrong or when the go meter stops ticking over.
When tucked away reduces drag with all the flow on benefit that entails.
Speed brake when too hot.
With park brake on is very secure and stable on slopes, rigs and the like.
Makes OEI landings less exciting at the bottom. Or the ride of your life if they don't come down.
Maintain directional control thru differential braking when the T/R stops doing its thing.
Bumpy when flat spotted after poor use of differential braking or after leaving park brake on during run on landing.
Inflated to very high pressure (200psi
) so will hurt ones toe when kicked.
Wheels come with oleos makes landings softer when done right, saves your butt when done wrong or when the go meter stops ticking over.
When tucked away reduces drag with all the flow on benefit that entails.
Speed brake when too hot.
With park brake on is very secure and stable on slopes, rigs and the like.
Makes OEI landings less exciting at the bottom. Or the ride of your life if they don't come down.
Maintain directional control thru differential braking when the T/R stops doing its thing.
Bumpy when flat spotted after poor use of differential braking or after leaving park brake on during run on landing.
Inflated to very high pressure (200psi
) so will hurt ones toe when kicked.
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200psi said most of it very well and very tersely (Lu, take a lesson!), let me add:
For the S-76, the retractable wheels gain you 6 knots at best range speed, enough to save the extra weight of the gear (about 90 pounds)on every full tank of gas.
Wheels are better for slope and rough field landings, as it is easier to find stable solutions for landing with 3 points on the same plane (wheels), rather than two parallel lines on that plane (skids).
Skids have advantages, such as cheaper to design and build, easier to maintain, lighter.
Generally, skids are not as safe as wheels, because in most landing and ground handling cases skids do not help the situation a bit, where wheels contribute positively.
For the S-76, the retractable wheels gain you 6 knots at best range speed, enough to save the extra weight of the gear (about 90 pounds)on every full tank of gas.
Wheels are better for slope and rough field landings, as it is easier to find stable solutions for landing with 3 points on the same plane (wheels), rather than two parallel lines on that plane (skids).
Skids have advantages, such as cheaper to design and build, easier to maintain, lighter.
Generally, skids are not as safe as wheels, because in most landing and ground handling cases skids do not help the situation a bit, where wheels contribute positively.
"Just a pilot"
Wheels are great if the surface you're putting them on is firm.
Skids spread the weight a little (surprisingly little) more on unimproved surfaces. Skid gear can allow you to find a level landing point if the surface is really rough. Skids more prone to catch on obstructions. No brakes to worry about.
Skids spread the weight a little (surprisingly little) more on unimproved surfaces. Skid gear can allow you to find a level landing point if the surface is really rough. Skids more prone to catch on obstructions. No brakes to worry about.
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Devil 49 said:
Skid gear can allow you to find a level landing point if the surface is really rough.
Nick sez:
Nope! On rough ground, it is usually hard to find stability with skids, unless the ground is also soft, where the skids can sink in a bit. That is because the skids form two parallel lines that are in one flat plane, and they must retain that relationship even on rough ground, where such planes don't exist. Usually, skids end up with one or both balancing or teetering on firm rough ground, because the long skid's straight line can't conform to the rough or rocky ground, so much of the skid doesn't touch. A wheeled helo can almost always find stability with the three wheels all touching firmly even on the roughest ground. Three wheels simply find their own plane and allow stable touchdowns anywhere, albeit with a sloped aircraft.
I have heard of marketing hype that discusses how much better skids are on rough terrain, and that is simply untrue. The US Army, with 7,000 helicopters in its inventory, and missions all over the world no longer will buy a skidded helo.
Devil 49 is right, skids don't conform to the surface, and the touchdown pressure on the ground is actually higher than wheels (more tendency to sink in and create dynamic rollover problems in skids). The touchdown pressure of the typical Bell 212/412 is about 170 psi. The typical tire pressure of a wheeled helo is 125 to 140 psi (the S-76 is the exception, at 165 psi for its tiny wheels). The psi of the landing gear directly measures the sinkability on soft terrain. That's why the HUMMER has a tire pressure adjustment system, and can go down to 10 psi if needed on mus or sand.
Skid gear can allow you to find a level landing point if the surface is really rough.
Nick sez:
Nope! On rough ground, it is usually hard to find stability with skids, unless the ground is also soft, where the skids can sink in a bit. That is because the skids form two parallel lines that are in one flat plane, and they must retain that relationship even on rough ground, where such planes don't exist. Usually, skids end up with one or both balancing or teetering on firm rough ground, because the long skid's straight line can't conform to the rough or rocky ground, so much of the skid doesn't touch. A wheeled helo can almost always find stability with the three wheels all touching firmly even on the roughest ground. Three wheels simply find their own plane and allow stable touchdowns anywhere, albeit with a sloped aircraft.
I have heard of marketing hype that discusses how much better skids are on rough terrain, and that is simply untrue. The US Army, with 7,000 helicopters in its inventory, and missions all over the world no longer will buy a skidded helo.
Devil 49 is right, skids don't conform to the surface, and the touchdown pressure on the ground is actually higher than wheels (more tendency to sink in and create dynamic rollover problems in skids). The touchdown pressure of the typical Bell 212/412 is about 170 psi. The typical tire pressure of a wheeled helo is 125 to 140 psi (the S-76 is the exception, at 165 psi for its tiny wheels). The psi of the landing gear directly measures the sinkability on soft terrain. That's why the HUMMER has a tire pressure adjustment system, and can go down to 10 psi if needed on mus or sand.
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Every helicopter that I ever maintained had conventional wheeled landing gear. The only exception was the HTL-1 (Bell-47) that came with wheels and we changed those for floats when we went aboard ship. In many cases when landing on a small flight deck with the ship rolling while breaking ice the pilots had to land athwartship. This made the ground handling of the helicopter extremely dangerous and many times we almost lost it over the side. This particular helicopter (HO3-S) had tricycle landing gear and there were no direct acting brakes other than the parking brakes so the instant it landed we had to attach tiedowns from the landing gear to the deck. Another major problem with this landing gear setup was the possibility of getting into ground resonance during start up as the helicopter would be moving on the shock struts and the three blade system would go out of balance during start up. The problem of movement during landing was somewhat solved by placing a latticework of heavy lines across the deck. This however caused another problem and that was moving the helicopter to make room for the next one to come in for a landing. The other one was the HTL-1 on floats. To facilitate ground handling of this helicopter we used a hydraulic jacking system mounted on a dolly. The dolly had small wheels and these too would get caught up in the latticework of the lines.
The egg crate concept came a bit later and this was even worse as the helicopter could not be moved once the wheels were inside the individual boxes of the egg crate.
The Canadian Navy eventually solved the problem when they invented the beartrap system. A system similar to the beartrap was installed on the DD-963 destroyer series to facilitate both landing of the helicopter by winchdown and then translating the trapped helicopter into the hangar.
So it boils down to what works best for the application. Regarding the US Military not buying skid mounted helicopters I believe they are still purchasing the latest models of the AH1 series with skids. I think it could have been better stated that any new contract build helicopters would incorporate conventional wheeled landing gear. The last three major contracts for Army helicopters include the Apache, The Blackhawk and the Comanche and all of them have conventional landing gear.
The egg crate concept came a bit later and this was even worse as the helicopter could not be moved once the wheels were inside the individual boxes of the egg crate.
The Canadian Navy eventually solved the problem when they invented the beartrap system. A system similar to the beartrap was installed on the DD-963 destroyer series to facilitate both landing of the helicopter by winchdown and then translating the trapped helicopter into the hangar.
So it boils down to what works best for the application. Regarding the US Military not buying skid mounted helicopters I believe they are still purchasing the latest models of the AH1 series with skids. I think it could have been better stated that any new contract build helicopters would incorporate conventional wheeled landing gear. The last three major contracts for Army helicopters include the Apache, The Blackhawk and the Comanche and all of them have conventional landing gear.
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Too bad Lu can't read.
The statement was "The US Army, with 7,000 helicopters in its inventory, and missions all over the world no longer will buy a skidded helo."
The AH-1 W/Z that Lu refers to is being bought by the US Marines.
The ground resonance Lu refers to is quelled by the oleos, which don't care if they are mounted on wheels or skids.
The statement was "The US Army, with 7,000 helicopters in its inventory, and missions all over the world no longer will buy a skidded helo."
The AH-1 W/Z that Lu refers to is being bought by the US Marines.
The ground resonance Lu refers to is quelled by the oleos, which don't care if they are mounted on wheels or skids.
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To: Nick I believe that you might be a bit wrong on what the oleos do and can’t do. If the oleos are not correctly inflated they can set up a resonant system that will allow the helicopter to move on the oleos and if during start up the three bladed system will be out of balance until it comes up to speed and centrifugal forces build up. This same situation can exist with properly inflated struts coupled with an external excitation source. A similar situation can develop if a damper is out of tune and this could result in ground resonance or even air resonance. The case I was describing the helicopter was aligned with the lateral axis of the ship. Because the ship was an icebreaker it would be rolling excessively to the left and then to the right. If it were actually be breaking the ice while rolling, the ship would also have an excitation from the bow to the stern. This combined rolling and pitching motion would cause the helicopter to rock on the struts. As the rotor system built up speed the blades would be displaced in such a way as to cause a severe imbalance. Because of the very high CG of the helicopter the rocking and pitching motion and the rocking on the struts would couple up with the imbalance of the rotor system. Another problem was that the dampers had cold soaked over night and their timing was off a bit. This problem would solve itself as soon as leading and lagging would take place but until that time the blades may not be in the same place due to the inertial loads created during startup. During this period the pilots would have to be prepared to lift off as soon as the rotors came up to speed or, sure as hell there would be a very major problem on the flight deck.
We had a similar problem with the float mounted Bell. When the helicopter was being started the flight mechanic would have to hold the blade tip in his fingertips in order to keep the blades at there mid point of teeter. The same rocking and rolling motion would apply to this helicopter on floats as it would rock back and forth and at the same time, it was bouncing on the floats. If this wasn’t done and the blades were free to teeter during start up the blade disc would whip so fast and so hard as to tip the helicopter over
We had a similar problem with the float mounted Bell. When the helicopter was being started the flight mechanic would have to hold the blade tip in his fingertips in order to keep the blades at there mid point of teeter. The same rocking and rolling motion would apply to this helicopter on floats as it would rock back and forth and at the same time, it was bouncing on the floats. If this wasn’t done and the blades were free to teeter during start up the blade disc would whip so fast and so hard as to tip the helicopter over
Approaching nine years of non-stop EMS work, so a fair proportion of real rough ground work - give me skids every time please!! Quite right that three wheels might be more stable once settled - however as you are lowering yourself gently among the surrounding boulders the wheels have a charming habit of finding the lowest spots to settle into while the aircraft skin is likely to find the highest pointy rock. Skids let you feel the 'average high spots'and if it does not feel OK you just have another go!
Also wheels sink, i've found no better combination than skids and anti-sink plates.
Also wheels sink, i've found no better combination than skids and anti-sink plates.
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Good points, Tees, although some really nice belly clearance is also part of the right mix. 12 inches of clearance is tight, and 18+ inches is about right for rocky work. Often, wheeled helos are skimpy on belly clearance, probably a bias toward paved ramp activities worked into their assumed mission needs,
Regarding sinking, anti-sink plates on skids work very well, as do bear pads on wheels (just don't retract them!!)
Regarding sinking, anti-sink plates on skids work very well, as do bear pads on wheels (just don't retract them!!)
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Nick
Our tyre pressure is around the 195psi mark, is there differing pressures between the A/A+/A++/B/C model range.
Lu you must live in a 'what if' world of poor planetary alignment. Improper oleo pressures, damper alignment and ice bergs crikey why make a simple question into an unecessary diatribe. Take some oestrogen and have a good lie down. I believe we have maintenance procedures to keep things humming along.
Our tyre pressure is around the 195psi mark, is there differing pressures between the A/A+/A++/B/C model range.
Lu you must live in a 'what if' world of poor planetary alignment. Improper oleo pressures, damper alignment and ice bergs crikey why make a simple question into an unecessary diatribe. Take some oestrogen and have a good lie down. I believe we have maintenance procedures to keep things humming along.
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One more place where wheels prevail is when you have to taxi a 21,000 lb. helicopter near Cessna 172's, R22's, people, buildings, etc. If we had to hover up to the in ground tanks, we'd be breaking things.
As far as rocks and belly clearance, we have a crew member lie down and look underneath helicopter just prior to setting down to ensure clearance. Best of both worlds.
Matthew.
As far as rocks and belly clearance, we have a crew member lie down and look underneath helicopter just prior to setting down to ensure clearance. Best of both worlds.
Matthew.
Have operated both configurations, and find it's horses for courses. The Chinook certainly wouldn't be comfortable during a running landing at 60kts on skids; the Wessex could never have been towed into our cunningly-camouflaged field hides on skids; we couldn't have shut-down at half the Jebel Akhdar (Oman) landing sites if the footprint had been as large as a wheeled heli, and the same goes here in the Borneo jungle (Blackhawk restricted to large LPs only - should've bought Seahawk ) while B212 lands lever-fully-down on the tiniest pinnacle or arete.
Overall, unless it's for jungle - give me wheels any day. Even if the Chinook are only inflated to 88psi...
) while B212 lands lever-fully-down on the tiniest pinnacle or arete.Overall, unless it's for jungle - give me wheels any day. Even if the Chinook are only inflated to 88psi...
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Personally I've had to dive on the front of a B206 skid to prevent it toppling off the back of the truck. Wheels don't have that prob.
Wheels are easier to forget to select down. A guy in OZ even managed to cancel the 200ft warning before removing the aerials on the underside. Twice.
Skids bend. Once someone lands on uneven terrain or with the aircraft weight in the middle of the skid there is a really good chance it will stay bent in the middle. This generates stress and stress makes cracks and cracks make....
It also appears that the result of uneven load distribution through a skid would stress the airframe.
Another thought.
Why has no-one designed a manually retractable skid gear for a light turbine? Something to fold away and clean up the drag.
Race ya to the patent shop...
Wheels are easier to forget to select down. A guy in OZ even managed to cancel the 200ft warning before removing the aerials on the underside. Twice.
Skids bend. Once someone lands on uneven terrain or with the aircraft weight in the middle of the skid there is a really good chance it will stay bent in the middle. This generates stress and stress makes cracks and cracks make....
It also appears that the result of uneven load distribution through a skid would stress the airframe.
Another thought.
Why has no-one designed a manually retractable skid gear for a light turbine? Something to fold away and clean up the drag.
Race ya to the patent shop...
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Imust admit that for operations I prefer retractable undercarriage but we also carry a crewman at all times. For training though I have to go for skids with a low belly. This increases the "1 foot bubble". the upshot of this is to make landing much more difficult for a student. They tend to slide of the bubble and overcontrol but when they do get it right you know that they can actually land a helicopter and not just plonk it on the oleos from 2-3 feet.
"Just a pilot"
In spite of learned and persuasive argument, I'll take skids. They have a slight advantage in landing surface selection, but are quicker if the surface is a consideration. Like horseshoes and hand grenades, close is good enough for skids. I can locate within a foot on approach with no delay, and if it's softer than expected or not level, a quick slide forward will frequently get me level and stable.