Wheels -v-Skids
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The point is that oleos do not CAUSE ground resonance! They are specifically used to prevent ground resonance, so if you are dumb enough to allow them to go mis-serviced, you might get ground resonance.
THe little boy was standing on the corner shouting "No Elephants!" The man asked him why, so he said, "I want to chase the elephants away." The man said, "But there are no elephants anywhere close to here!" to which the boy responded, "See, it works!"
THe little boy was standing on the corner shouting "No Elephants!" The man asked him why, so he said, "I want to chase the elephants away." The man said, "But there are no elephants anywhere close to here!" to which the boy responded, "See, it works!"
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Lot's of interesting stuff being written here! But surely there is no right answer. 'Each to their own' as they say.
I do know one thing....wheels wouldn't have helped, the way the skids did, in my little mishap!
I do know one thing....wheels wouldn't have helped, the way the skids did, in my little mishap!
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To: Nick Lappos
What you said about properly serviced oleo struts preventing ground resonance is not always true. In the icebreaker illustration I provided above the struts were properly serviced and they were doing what they were supposed to do when exposed to the externally applied loads caused by the rolling motion of the ship. The helicopter in question was an S-51, which has a very high CG, and it is mounted on tricycle landing gear. Any externally applied load will cause the helicopter to rock/roll on the struts and even though the wheels are up to pressure there is a rolling motion reacted by the tires. The rotor system has independent flapping and lag hinges similar to the heads on the CH-47. As the rotor is brought up to speed the blades will hang back and not being equally disposed at 120-degrees there will be an imbalance in the rotor system until it is brought up to speed. It was during this transitionary period that the out of balance rotor system and the movement of the helicopter on the struts would interact and exacerbate the situation. Because of this we could not keep the helicopter tied down and the only thing keeping it in place were the chocks on the three wheels. If the condition did not worsen then the pilot would keep it on the deck until released by the ships Captain. If the condition did worsen the pilot would immediately lift off to break contact with the rolling deck.
Elephants can be chased away using the method you described but penguins can not be chased from you mind. For the rest of the long weekend try not to think of penguins.
What you said about properly serviced oleo struts preventing ground resonance is not always true. In the icebreaker illustration I provided above the struts were properly serviced and they were doing what they were supposed to do when exposed to the externally applied loads caused by the rolling motion of the ship. The helicopter in question was an S-51, which has a very high CG, and it is mounted on tricycle landing gear. Any externally applied load will cause the helicopter to rock/roll on the struts and even though the wheels are up to pressure there is a rolling motion reacted by the tires. The rotor system has independent flapping and lag hinges similar to the heads on the CH-47. As the rotor is brought up to speed the blades will hang back and not being equally disposed at 120-degrees there will be an imbalance in the rotor system until it is brought up to speed. It was during this transitionary period that the out of balance rotor system and the movement of the helicopter on the struts would interact and exacerbate the situation. Because of this we could not keep the helicopter tied down and the only thing keeping it in place were the chocks on the three wheels. If the condition did not worsen then the pilot would keep it on the deck until released by the ships Captain. If the condition did worsen the pilot would immediately lift off to break contact with the rolling deck.
Elephants can be chased away using the method you described but penguins can not be chased from you mind. For the rest of the long weekend try not to think of penguins.
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Lu said:
(Nick)What you said about properly serviced oleo struts preventing ground resonance is not always true. ......... Any externally applied load will cause the helicopter to rock/roll on the struts and even though the wheels are up to pressure there is a rolling motion reacted by the tires. .......Because of this we could not keep the helicopter tied down and the only thing keeping it in place were the chocks on the three wheels...
Nick sez:
Few things are ALWAYS true (except gravity.) But the belief that oleos cause ground resonance is ALWAYS incorrect, and also the belief that your scenario has something to do with oleos is mistaken.
The aircraft as a system relies on all the ways it receives forces externally to quell its dynamic response. We have had this go-around before, Lu about dynamic (oscillatory) behavior, so I will only write this once, regardless of how many ways you work it around in the interminable posts to come on this subject.
Your shipboard example proves my point, because you had the aircraft tied down. The tiedowns change the oscillatory response of the whole system, and make ground resonance more likely. All Navy folks know this, we even stencil it on the side of the aircraft next to the tie down rings!
The oleos PREVENT ground resonance, virtually ALL the time, that's why they are used. They are really shock absorbers, just like a car's (which prevent the car from bouncing uncontrollably when a bump is hit). Of course if the oleo is broken or impropoerly serviced it won't do its job - if you are improperly serviced, you won't do your job either.
IF THE AIRCRAFT IS TIED DOWN, ground resonance might occur. This has NOTHING to do with oleos, Lu, it has to do with a crew preventing the oleos from doing their job.
Gosh, I wish you could stay on topic, and not use unserviced parts or tied down scenarios to confuse the group. Of course, I wish I'd win the lottery so I could buy my own S-76, but that's a pipe dream, too.
(Nick)What you said about properly serviced oleo struts preventing ground resonance is not always true. ......... Any externally applied load will cause the helicopter to rock/roll on the struts and even though the wheels are up to pressure there is a rolling motion reacted by the tires. .......Because of this we could not keep the helicopter tied down and the only thing keeping it in place were the chocks on the three wheels...
Nick sez:
Few things are ALWAYS true (except gravity.) But the belief that oleos cause ground resonance is ALWAYS incorrect, and also the belief that your scenario has something to do with oleos is mistaken.
The aircraft as a system relies on all the ways it receives forces externally to quell its dynamic response. We have had this go-around before, Lu about dynamic (oscillatory) behavior, so I will only write this once, regardless of how many ways you work it around in the interminable posts to come on this subject.
Your shipboard example proves my point, because you had the aircraft tied down. The tiedowns change the oscillatory response of the whole system, and make ground resonance more likely. All Navy folks know this, we even stencil it on the side of the aircraft next to the tie down rings!
The oleos PREVENT ground resonance, virtually ALL the time, that's why they are used. They are really shock absorbers, just like a car's (which prevent the car from bouncing uncontrollably when a bump is hit). Of course if the oleo is broken or impropoerly serviced it won't do its job - if you are improperly serviced, you won't do your job either.
IF THE AIRCRAFT IS TIED DOWN, ground resonance might occur. This has NOTHING to do with oleos, Lu, it has to do with a crew preventing the oleos from doing their job.
Gosh, I wish you could stay on topic, and not use unserviced parts or tied down scenarios to confuse the group. Of course, I wish I'd win the lottery so I could buy my own S-76, but that's a pipe dream, too.
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To: Nick Lappos
Because the helicopter was moving on the struts due to the pitching, heaving and rolling motion of the flight deck we could not tie the helicopter down to the deck. As I stated in a previous post it doesn’t take me long to check out a hot horseshoe (Learning by experience). Originally we kept the helicopters tied down until they were ready to lift off but the one time we did it and the ship was rolling and pitching and heaving we almost lost the helicopter when the pilot was forced to lift off. Luckily, I was able to release one of the tie downs and my assistant mechanic hit the cable release on the other and the pilot was able to free himself from the moving deck. He came up hard when the cable extended to its’ limits and if he tried to move up or sideways any further he would have crashed on the flight deck.
Because the helicopter was moving on the struts due to the pitching, heaving and rolling motion of the flight deck we could not tie the helicopter down to the deck. As I stated in a previous post it doesn’t take me long to check out a hot horseshoe (Learning by experience). Originally we kept the helicopters tied down until they were ready to lift off but the one time we did it and the ship was rolling and pitching and heaving we almost lost the helicopter when the pilot was forced to lift off. Luckily, I was able to release one of the tie downs and my assistant mechanic hit the cable release on the other and the pilot was able to free himself from the moving deck. He came up hard when the cable extended to its’ limits and if he tried to move up or sideways any further he would have crashed on the flight deck.
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tacr2man:
Not sure you'll get any better response than the three pages already posted on this thread. Between Mr. Lappos, Mr. Zukerman and the rest, you've got a lot to digest here already!
Not sure you'll get any better response than the three pages already posted on this thread. Between Mr. Lappos, Mr. Zukerman and the rest, you've got a lot to digest here already!
Wheels vs Skids
Just two observations from early Blackhawk/S-70 days:
1. The Army had done considerable study work prior to letting the UTTAS Request for Proposal out, and had concluded that for a helicopter of that size, wheels were the better solution. A Col Bud Patnode was in charge of this work, and he relayed that the discussions often became heated with the " if its on the Huey, its perfect" community.
2. Qualification testing of that aircraft recognized that if one has tires and struts maintained by humans, they will inevitably become improperly serviced at some point. Therefore, mechanical stability ( ground resonance ) testing was done at various CG's and Weights with a matrix of mis-serviced tires and struts. My recollection was that there was no tendency to self induce ground resonance except at full aft CG somewhere about 50% airborne, and with the cyclic almost full aft, just producing light droop stop contact ( brakes had to be on, and all in all, this is a pretty unusual state to be in ) . One had to wait a half minute or so before the ship would start off at 2/3P. ( this was on level concrete, a condition considered more conducive to ground resonance than softer ground ).
Thanks,
John Dixson
1. The Army had done considerable study work prior to letting the UTTAS Request for Proposal out, and had concluded that for a helicopter of that size, wheels were the better solution. A Col Bud Patnode was in charge of this work, and he relayed that the discussions often became heated with the " if its on the Huey, its perfect" community.
2. Qualification testing of that aircraft recognized that if one has tires and struts maintained by humans, they will inevitably become improperly serviced at some point. Therefore, mechanical stability ( ground resonance ) testing was done at various CG's and Weights with a matrix of mis-serviced tires and struts. My recollection was that there was no tendency to self induce ground resonance except at full aft CG somewhere about 50% airborne, and with the cyclic almost full aft, just producing light droop stop contact ( brakes had to be on, and all in all, this is a pretty unusual state to be in ) . One had to wait a half minute or so before the ship would start off at 2/3P. ( this was on level concrete, a condition considered more conducive to ground resonance than softer ground ).
Thanks,
John Dixson
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Lynx Wheels
As long as we're on the subject, one thing I've always wondered: why are the Lynx rear wheels aligned "toe-out"?
I assume it makes for more stable shipboard ops...no sliding on deck etc.
But what about ground handling? Is there a pilot-activated electrical / mechanical device to re-align the wheels straight-parallel for ground taxi, etc?
I assume it makes for more stable shipboard ops...no sliding on deck etc.
But what about ground handling? Is there a pilot-activated electrical / mechanical device to re-align the wheels straight-parallel for ground taxi, etc?
Hi Matari,
the Navy Lynx wheels are turned that way by a mechanical device by man.
The nose wheel can be castored to 90 degrees, allowing the Lynx to turn on the deck around the harpune securing it to the deck.
So starting/landing into wind is possible without turning the ship.
look about 1min10
Running landing is still possible - evenso some rubber stays on the tarmac.
Greetings Flying Bull
P.S.
If you fly with wheeled helicopters, you might have to put the wheels on hard ground to prevent sinking in
the Navy Lynx wheels are turned that way by a mechanical device by man.
The nose wheel can be castored to 90 degrees, allowing the Lynx to turn on the deck around the harpune securing it to the deck.
So starting/landing into wind is possible without turning the ship.
look about 1min10
Running landing is still possible - evenso some rubber stays on the tarmac.
Greetings Flying Bull
P.S.
If you fly with wheeled helicopters, you might have to put the wheels on hard ground to prevent sinking in
Last edited by Senior Pilot; 6th May 2009 at 20:09.
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From a maintainers view, skids - very little maintenance required but a bugger to tow the aircraft. Wheels - easy to tow aircraft but a lot of maintenance required! Just reading the responses so far and maintaining that perfect strut pressure can take a lot of effort. Dealing with punctures in the field can be fun. Replacing tail wheel lock pins because the pilot forgot to unlock during taxi can be stressful. Bleeding breaks, servicing struts, and pumping up tyres - boring. Undercarriage retraction and emergency blown down tests - exciting. Mixing up the pressure line for the brake and undercarriage jack on a Sea King sponson - spectacular!
