Wheels -v-Skids
Iconoclast
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To: the chopper
At the time I was working aboard the icebreaker in Greenland our two sister ships both US Navy were operating Bell HTL-5s. The HTL-5 had spring loaded teeter stops that kept the blades in the neutral teeter position until sufficient rotor speed was built up and the stops would disengage allowing the pilot to operate his cyclic. Our HTL-1s did not have this provision. Shortly before our deployment Bell issued a mod order requiring that the springs be removed from the centrifugal clutch. With the springs removed the clutch would immediately engage when the engine started to turn and in a fraction of a second the clutch would be fully engaged and the rotor would be turning over at engine speed minus the gear reduction.
That meant that our pilots and me the mechanic had no experience aboard ship with the helicopter in this modified configuration. The first few times we started we were not breaking ice and the ships’ flight deck was very stable. We did encounter the bouncing due to the turning frequency of the ships’ screws but there was no rocking. With a minimal wind speed the rotor would maintain somewhat of a median position which did not cause any problems during run up.
However when we started breaking ice and the ship was rolling left and right and pitching and heaving at the same time the helicopter would really bounce on the floats and it would also roll quite a bit on the floats. At this same time the blades would be moving all over the place. When the engine was started the blade would immediately start rotating at a high rate of speed. If the blades were teetered too far the physical action would be to move them to their neutral teeter position and this could result in a physical force that would cause the helicopter to roll over. It only took one time for us to realize that the blades had to be maintained in the neutral teeter position during start up. The Navy did not experience this phenomenon due to the spring loaded teeter stops.
To show how fast the blades rotated at start up if you did not pull your hand back after having the blade snatched from between your fingers the second blade hit your hand. Once again, it only took one time to learn the lesson. Or as they say, it doesn’t take me long to learn how to check out a hot horseshoe.
To answer your question about how many Bell 47s on floats rolled over during start up the answer is none but only because we learned from the experience. It was close but it didn't roll over although it could have.
[ 26 August 2001: Message edited by: Lu Zuckerman ]
[ 26 August 2001: Message edited by: Lu Zuckerman ]
At the time I was working aboard the icebreaker in Greenland our two sister ships both US Navy were operating Bell HTL-5s. The HTL-5 had spring loaded teeter stops that kept the blades in the neutral teeter position until sufficient rotor speed was built up and the stops would disengage allowing the pilot to operate his cyclic. Our HTL-1s did not have this provision. Shortly before our deployment Bell issued a mod order requiring that the springs be removed from the centrifugal clutch. With the springs removed the clutch would immediately engage when the engine started to turn and in a fraction of a second the clutch would be fully engaged and the rotor would be turning over at engine speed minus the gear reduction.
That meant that our pilots and me the mechanic had no experience aboard ship with the helicopter in this modified configuration. The first few times we started we were not breaking ice and the ships’ flight deck was very stable. We did encounter the bouncing due to the turning frequency of the ships’ screws but there was no rocking. With a minimal wind speed the rotor would maintain somewhat of a median position which did not cause any problems during run up.
However when we started breaking ice and the ship was rolling left and right and pitching and heaving at the same time the helicopter would really bounce on the floats and it would also roll quite a bit on the floats. At this same time the blades would be moving all over the place. When the engine was started the blade would immediately start rotating at a high rate of speed. If the blades were teetered too far the physical action would be to move them to their neutral teeter position and this could result in a physical force that would cause the helicopter to roll over. It only took one time for us to realize that the blades had to be maintained in the neutral teeter position during start up. The Navy did not experience this phenomenon due to the spring loaded teeter stops.
To show how fast the blades rotated at start up if you did not pull your hand back after having the blade snatched from between your fingers the second blade hit your hand. Once again, it only took one time to learn the lesson. Or as they say, it doesn’t take me long to learn how to check out a hot horseshoe.
To answer your question about how many Bell 47s on floats rolled over during start up the answer is none but only because we learned from the experience. It was close but it didn't roll over although it could have.
[ 26 August 2001: Message edited by: Lu Zuckerman ]
[ 26 August 2001: Message edited by: Lu Zuckerman ]
NR Fairy,
Actually, no - not fair enough. Lu is a bloke who's paid thousands to check details, yet he managed to put his own interpretation on Nick's message by changing 'Army' to 'military'. He tried to make an invalid point based on a factual misrepresentation. Unacceptable.
Actually, no - not fair enough. Lu is a bloke who's paid thousands to check details, yet he managed to put his own interpretation on Nick's message by changing 'Army' to 'military'. He tried to make an invalid point based on a factual misrepresentation. Unacceptable.
Iconoclast
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Thud_And_Blunder
I understood what Nick stated about the US Army with their 7000 plus helicopters not placing a contract for helicopters on skids. The point I made was that it should have referenced the release of a new build contract and not just they were not buying skid-equipped helicopters. The three major contracts released by the Army were the Apache, Blackhawk and the Comanche, all of which have wheeled landing gear. I believe that during both the Apache and Blackhawk programs the Army was still purchasing Bell Cobras either for their use of for the National Guard or, for foreign military sales. The Marines did continue to purchase variants of the Cobra as Nick had indicated.
Here is a related trivia question: What was the last contract released to Bell from the military to build a helicopter from the ground up?
I understood what Nick stated about the US Army with their 7000 plus helicopters not placing a contract for helicopters on skids. The point I made was that it should have referenced the release of a new build contract and not just they were not buying skid-equipped helicopters. The three major contracts released by the Army were the Apache, Blackhawk and the Comanche, all of which have wheeled landing gear. I believe that during both the Apache and Blackhawk programs the Army was still purchasing Bell Cobras either for their use of for the National Guard or, for foreign military sales. The Marines did continue to purchase variants of the Cobra as Nick had indicated.
Here is a related trivia question: What was the last contract released to Bell from the military to build a helicopter from the ground up?
Give me skids for operating to jungle platforms and training, but wheels for nearly everything else, especially one that has not been mentioned - dust landings. Not really an option on skids. Wheels definitely for ship landings. The smaller the machine, the more practical are skids. I didn't think you could go past the non retractable gear on the UH-60 for being able to take punishment - something like 10G capable before requiring a heavy landing inspection (I am really stretching the memory here, can you help Nick?)
Lu: you were saying that: >>As the rotor system built up speed the blades would be displaced in such a way as to cause a severe imbalance<<
and: >> 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<<.
I thought that the rotor system was always in (or near to) static balance, and was irrespective of revolution speed (RRPM). Vibrations on the other hand are significantly affected by factors such as airspeed, loading, and in particular, RRPM. The effect of RRPM was because resonance is the manifestation of a harmonic, and a harmonic is excited or damped by relative natural frequencies which are determined (in part) by revolution frequency (RPM). That is why when encountering ground resonance, the thing the pilot can change is RRPM to try and avoid it (he cant change other fixed factors such as the shaft mass/lengths/lead lag dampers/oleos, etc), or he can attempt take off to eliminate the effect of the landing gear frequency excitation. Remember that oleos are present on skid type machines and also make them susceptible to ground resonance when improperly maintained.
Lastly, we used to hold the blades of B205 & B212 during start up when windy to reduce the possibility of blade sail and subsequent mast bump during low RRPM states. Never thought of it for ground resonance, how did it help?
Lu: you were saying that: >>As the rotor system built up speed the blades would be displaced in such a way as to cause a severe imbalance<<
and: >> 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<<.
I thought that the rotor system was always in (or near to) static balance, and was irrespective of revolution speed (RRPM). Vibrations on the other hand are significantly affected by factors such as airspeed, loading, and in particular, RRPM. The effect of RRPM was because resonance is the manifestation of a harmonic, and a harmonic is excited or damped by relative natural frequencies which are determined (in part) by revolution frequency (RPM). That is why when encountering ground resonance, the thing the pilot can change is RRPM to try and avoid it (he cant change other fixed factors such as the shaft mass/lengths/lead lag dampers/oleos, etc), or he can attempt take off to eliminate the effect of the landing gear frequency excitation. Remember that oleos are present on skid type machines and also make them susceptible to ground resonance when improperly maintained.
Lastly, we used to hold the blades of B205 & B212 during start up when windy to reduce the possibility of blade sail and subsequent mast bump during low RRPM states. Never thought of it for ground resonance, how did it help?
Just Dropped In
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Hi peeps!
I reckon alot depends on personal preference & what you're doing with the Heli.
In the Navy & on the North Sea, wheels were essential! Some of my deck landings would have resulted in a very bad back if I'd had skids!
I now fly Police & have to say that I believe skids are the way to go! Alot of the landings we do are no-where near a prepared surface, as such I like the fact that the skids instantly transmit (through me seat) what's happening underneath.
I'm no boffin, so I don't know the facts & figures but I've never sunk yet with skids but have sunk with wheels. I accept the arguement about bear feet (thingies) on wheels alleviating that problem, but then they're hanging down creating drag etc anyway.
But hey, each to their own!
I reckon alot depends on personal preference & what you're doing with the Heli.
In the Navy & on the North Sea, wheels were essential! Some of my deck landings would have resulted in a very bad back if I'd had skids!
I now fly Police & have to say that I believe skids are the way to go! Alot of the landings we do are no-where near a prepared surface, as such I like the fact that the skids instantly transmit (through me seat) what's happening underneath.
I'm no boffin, so I don't know the facts & figures but I've never sunk yet with skids but have sunk with wheels. I accept the arguement about bear feet (thingies) on wheels alleviating that problem, but then they're hanging down creating drag etc anyway.
But hey, each to their own!
Hi Roofus, don't get too complacent. We had a lynx land on a slop at an exercise location in Germany many years ago. When the crew returned the downhill skid had sunk putting the aircraft past the limits for sloping ground. It took a long time to dig out under the uphill skid to put it back in limits!
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Mighty Gem,
What about the London HEMS machine (AS365)which landed on the top of a public park hill only to be found at the bottom of said hill when the crew returned to it with the patient. The brake accumulator does bleed off after a wile you know. Skids don't have that problem.
What about the London HEMS machine (AS365)which landed on the top of a public park hill only to be found at the bottom of said hill when the crew returned to it with the patient. The brake accumulator does bleed off after a wile you know. Skids don't have that problem.
Iconoclast
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To: helmet fire
There were actually two different problems described and both problems had a common thread. That thread was the movement of the ship during the breaking of ice. Under the icebreaking condition the ship was made to heel left and right by shifting water ballast and at the same time water was pumped forward and aft. This imparted a rolling motion. If you can imagine a disc that is spinning and then loses momentum and it starts to roll on its’ edges until it finally stops. It is this final rolling motion that that best describes the action of the ship. This motion is further exacerbated by the vertical movement of the aft end of the ship (where the flight deck is located) caused by the screws and the major vibration imparted during the ice breaking process.
In the first instance the helicopter is mounted on tricycle landing gear and it is disposed over the lateral axis of the flight deck. The movement of the ship causes the helicopter to pitch and roll on the oleo struts and some sideward movement on the tires (tyres). When the rotor is brought up to speed the blades will due to inertia hang back on the dampers. With the blades not being in a 120-degree relationship with each other the system will be out of balance. The design of this helicopter S-51 (HO3-S) placed the cg very high. This imbalance would further exacerbate the rolling motion caused by the ship and the helicopter could possibly go into ground resonance. That is why I stated that the pilots had to be prepared to lift off as soon as rotor rpm had built up.
In the case of the Bell HTL-1 the helicopter could be aligned with either axis of the ship. Under icebreaking conditions the helicopter would bounce on the floats and would also shift position on the deck caused by the constantly changing side loads on the floats. It did not move from one position to another, it just moved from side to side as the floats rolled in relation to the flight deck.
Ground resonance although not a major problem was still possible. The first time we fired the helicopter up under icebreaking conditions the blade was free to teeter. With the springs removed from the centrifugal clutch as soon as the engine started the rotor would start rotating. The first time under these conditions the blades were at their maximum teeter angle and when they started to rotate they tried to position themselves at the neutral teeter position and the forces generated almost tipped the helicopter over. After that, we held the blades in the mid position during start up. Mast bumping was not a problem with this rotor system as they had Sprague cables that limited the rotor movement in relation to the teeter axis.
There were actually two different problems described and both problems had a common thread. That thread was the movement of the ship during the breaking of ice. Under the icebreaking condition the ship was made to heel left and right by shifting water ballast and at the same time water was pumped forward and aft. This imparted a rolling motion. If you can imagine a disc that is spinning and then loses momentum and it starts to roll on its’ edges until it finally stops. It is this final rolling motion that that best describes the action of the ship. This motion is further exacerbated by the vertical movement of the aft end of the ship (where the flight deck is located) caused by the screws and the major vibration imparted during the ice breaking process.
In the first instance the helicopter is mounted on tricycle landing gear and it is disposed over the lateral axis of the flight deck. The movement of the ship causes the helicopter to pitch and roll on the oleo struts and some sideward movement on the tires (tyres). When the rotor is brought up to speed the blades will due to inertia hang back on the dampers. With the blades not being in a 120-degree relationship with each other the system will be out of balance. The design of this helicopter S-51 (HO3-S) placed the cg very high. This imbalance would further exacerbate the rolling motion caused by the ship and the helicopter could possibly go into ground resonance. That is why I stated that the pilots had to be prepared to lift off as soon as rotor rpm had built up.
In the case of the Bell HTL-1 the helicopter could be aligned with either axis of the ship. Under icebreaking conditions the helicopter would bounce on the floats and would also shift position on the deck caused by the constantly changing side loads on the floats. It did not move from one position to another, it just moved from side to side as the floats rolled in relation to the flight deck.
Ground resonance although not a major problem was still possible. The first time we fired the helicopter up under icebreaking conditions the blade was free to teeter. With the springs removed from the centrifugal clutch as soon as the engine started the rotor would start rotating. The first time under these conditions the blades were at their maximum teeter angle and when they started to rotate they tried to position themselves at the neutral teeter position and the forces generated almost tipped the helicopter over. After that, we held the blades in the mid position during start up. Mast bumping was not a problem with this rotor system as they had Sprague cables that limited the rotor movement in relation to the teeter axis.
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"oleos" will contribute to ground resonance. It's a constant consideration on the 269 and has been since 64' when the U.S Army started buying them.
If the Thrust Control Rod was not down in it's proper position in a CH-47 with non-proportional ECLs before going to flight with the ECLs, and a strut was not properly serviced, you could get a real ride till the blades got in sync.
A wheeled aircraft in rough enough terrain can be like a table in a restaurant with one short foot extension. Hard to stuff enough napkins under the wheel.
Skid gear, on the other hand, tend to sit on the high points, when there are enough of them.
Give me wheels to taxi around the airport.
If the Thrust Control Rod was not down in it's proper position in a CH-47 with non-proportional ECLs before going to flight with the ECLs, and a strut was not properly serviced, you could get a real ride till the blades got in sync.
A wheeled aircraft in rough enough terrain can be like a table in a restaurant with one short foot extension. Hard to stuff enough napkins under the wheel.
Skid gear, on the other hand, tend to sit on the high points, when there are enough of them.
Give me wheels to taxi around the airport.
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It would appear that wheels have the majority vote, however what is the preferred configuration, nose or tai-wheel and why?
For my part, a tail wheel wins every time if you are in a hurry and don't want to mince about in a LZ.
For my part, a tail wheel wins every time if you are in a hurry and don't want to mince about in a LZ.
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My five cents - I prefer wheels. Better on rocky ground, and allow flexibility for running landings when OEI. The tailwheel config is the only config I have been exposed to, but it is great for fast approach and landing using aerodynamic braking with the tailwheel pinned (mil tac ops). Oleos with a long stroke can absorb vertical impacts and help save your back at the bottom of an auto. I suppose skids are a fair bit lighter though, which is better for smaller helicopters.
TeeS: agreed, skidded are the best for emergency work. Our terrain lends itself to skids for the foll reasons:
massive ground clearance to take all our extra belly pod equipment. Less chance of smacking it!
Skids find an even datum after settling in sand (beach) unlike wheels where one of them might decide to dig down deeper!!
Better for a more stable line of contact while emplaning/deplaning, whereas a single point of contact on one wheel makes life a little harder to judge!
No worries re ground resonance!
Harder to pick up the helo to store in the hangar with wheels.
Smack a skid (hard) and you've still got a reasonable chance of securing a landing later on, damage one of the wheels and you've possibly got a major incident on your hands??
Apart from that very little in it....sorry NL.
massive ground clearance to take all our extra belly pod equipment. Less chance of smacking it!
Skids find an even datum after settling in sand (beach) unlike wheels where one of them might decide to dig down deeper!!
Better for a more stable line of contact while emplaning/deplaning, whereas a single point of contact on one wheel makes life a little harder to judge!
No worries re ground resonance!
Harder to pick up the helo to store in the hangar with wheels.
Smack a skid (hard) and you've still got a reasonable chance of securing a landing later on, damage one of the wheels and you've possibly got a major incident on your hands??
Apart from that very little in it....sorry NL.
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Vlift said:
"oleos" will contribute to ground resonance. It's a constant consideration on the 269 and has been since 64' when the U.S Army started buying them.
Nick sez:
Gosh, vlift, the oleos prevent ground resonance! If you think they cause it, we'd just remove them!! Please don't try that, you won't get one complete rotor engagement before the parts made terrible noises and scattered across the ramp.
"oleos" will contribute to ground resonance. It's a constant consideration on the 269 and has been since 64' when the U.S Army started buying them.
Nick sez:
Gosh, vlift, the oleos prevent ground resonance! If you think they cause it, we'd just remove them!! Please don't try that, you won't get one complete rotor engagement before the parts made terrible noises and scattered across the ramp.
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Nick, Vlift is correct about the 269, although perhaps he should have said "improperly serviced oleos".
Here's a link to an NTSB report on the destruction of a Schweizer 300C (based on the 269) due to ground resonance, that had improper pressures in the skid dampers.
http://www.ntsb.gov/ntsb/brief.asp?e...07X03924&key=1
Here's a link to an NTSB report on the destruction of a Schweizer 300C (based on the 269) due to ground resonance, that had improper pressures in the skid dampers.
http://www.ntsb.gov/ntsb/brief.asp?e...07X03924&key=1
Just thought of another advantage for skids:
Ever imagined 3 abseilers per side trying to balance on one wheel?
Ever imagined 3 abseilers per side trying to balance on one wheel?