Chinook & other tandem rotors discussions
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Boslandrew,
I only just spotted your very informative post. Apologies - put it down to quick lunch time posting...
Interesting that Chinook had no peculiarities in handling. Still the idea of VRS in the rear rotor is not pleasant - i guess you had to roll the machine sideways to recover. Prouty commented that they climbed best flying slightly sideways, since this increased the rotor area to flow. I must admit to having tried to work out if there was any reason other than blade clearance for the 5 or 6 degree difference in rotor g/box mounting angles.
I'm not suprised about the vibration being the main constraint though. There are lots of opportunities for the tip vortices to impinge other rotors and structure. The other vibration source would be drivetrain eigenmodes, which i imagine are not insignificant (massive rotors and flexible drivelines). I'm also slightly suprised that they haven't followed the trend and moved to 4 blade hingeless rotors.
Still, good bit of versatile design - it clearly delivers.
Brett,
Thanks for the driveline details. I'll have to do some googling to come up with weights and materials. I imagine the guy that did the original layout (based on Piasecki's concept drawings no doubt) lost a lot of sleep about getting the stress calcs right - i know i do!
From Boslandrew's comments i would imagine that the cost of maintaining such a complicated powertrain is a disadvantage.
Do the electric trimmers bias the forward thrust in any way, or are they just there to compensate for flapback?
Mart
I only just spotted your very informative post. Apologies - put it down to quick lunch time posting...
Interesting that Chinook had no peculiarities in handling. Still the idea of VRS in the rear rotor is not pleasant - i guess you had to roll the machine sideways to recover. Prouty commented that they climbed best flying slightly sideways, since this increased the rotor area to flow. I must admit to having tried to work out if there was any reason other than blade clearance for the 5 or 6 degree difference in rotor g/box mounting angles.
I'm not suprised about the vibration being the main constraint though. There are lots of opportunities for the tip vortices to impinge other rotors and structure. The other vibration source would be drivetrain eigenmodes, which i imagine are not insignificant (massive rotors and flexible drivelines). I'm also slightly suprised that they haven't followed the trend and moved to 4 blade hingeless rotors.
Still, good bit of versatile design - it clearly delivers.
Brett,
Thanks for the driveline details. I'll have to do some googling to come up with weights and materials. I imagine the guy that did the original layout (based on Piasecki's concept drawings no doubt) lost a lot of sleep about getting the stress calcs right - i know i do!
From Boslandrew's comments i would imagine that the cost of maintaining such a complicated powertrain is a disadvantage.Do the electric trimmers bias the forward thrust in any way, or are they just there to compensate for flapback?
Mart
Last edited by Graviman; 27th Oct 2006 at 21:35.
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A couple of points -
The whole Chinook flight control system is very complex. There is collective and lateral tilt on the swashplates, but no fore-aft tilt through the mechanical flight controls. The fore-aft tilt is done only through the Longitudinal Cyclic Trims (LCT) which program as a function of airspeed and density altitude. They must be extending by 60 knots, and the Vne with the aft one not extended is 100 KIAS. The reason for this is the aft shaft from the transmission to the rotorhead would be under way too much flapback beyond 100 KIAS. In fact, the alternate way to tell the aft LCT isn't programming is that 100 KIAS, it got quite smooth as the aft head was now not intermeshing with the forward head.
The LCT system is how the fuselage stays level in all flight attitudes - an interesting system.
As for the system being more efficient with some yaw, the Canadian Air Force version of the CH-46 had a gauge for most efficient yaw angle to be used following an engine failure. Only instance I know of this being measured and used. The yaw angle came from the AFCS which measured and controlled yaw with differential sideslip from the things on the front that look like static ports.
Confused yet?
The whole Chinook flight control system is very complex. There is collective and lateral tilt on the swashplates, but no fore-aft tilt through the mechanical flight controls. The fore-aft tilt is done only through the Longitudinal Cyclic Trims (LCT) which program as a function of airspeed and density altitude. They must be extending by 60 knots, and the Vne with the aft one not extended is 100 KIAS. The reason for this is the aft shaft from the transmission to the rotorhead would be under way too much flapback beyond 100 KIAS. In fact, the alternate way to tell the aft LCT isn't programming is that 100 KIAS, it got quite smooth as the aft head was now not intermeshing with the forward head.
The LCT system is how the fuselage stays level in all flight attitudes - an interesting system.
As for the system being more efficient with some yaw, the Canadian Air Force version of the CH-46 had a gauge for most efficient yaw angle to be used following an engine failure. Only instance I know of this being measured and used. The yaw angle came from the AFCS which measured and controlled yaw with differential sideslip from the things on the front that look like static ports.
Confused yet?
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Shawn,
Thanks for detailing the LCT. By extending do you mean that the LCT is out of trim by 60 KIAS, to get the forward thrust component while maintaining level attitude ?
A wealth of wisdom - one copy of C&C winging it's way to me as we type...
Interesting fact - Dave, you might like to take a note of that.
Mart
Thanks for detailing the LCT. By extending do you mean that the LCT is out of trim by 60 KIAS, to get the forward thrust component while maintaining level attitude ?
A wealth of wisdom - one copy of C&C winging it's way to me as we type...
Originally Posted by Shawn Coyle
In fact, the alternate way to tell the aft LCT isn't programming is that 100 KIAS, it got quite smooth as the aft head was now not intermeshing with the forward head.
Mart
Last edited by Graviman; 28th Oct 2006 at 10:05.
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A lot of the vibration stemmed from general interaction between the two rotors not helped by the three blade system. If the blades were tracked and balanced properly the vibration could be minimised. However, speed and weight affected vibration enormously. We had a graph produced showing vibration against weight at VNO and it was a gentle slope up to 45000 lbs but then curved sharply up between 45000 and 48500, the civilian MAUW.
As regards increased performance from yawing the aircraft slightly, the BAH aircraft were operating to Public Transport standards so Performance 'A' standards were required which did not allow the technique. However, operating empty on one occasion for underslung work, we made a take-off into wind but with the aircraft yawed thirty degrees to starboard putting the rear disc in clearer air. There were no handling problems and it went up like a rocket.
As regards increased performance from yawing the aircraft slightly, the BAH aircraft were operating to Public Transport standards so Performance 'A' standards were required which did not allow the technique. However, operating empty on one occasion for underslung work, we made a take-off into wind but with the aircraft yawed thirty degrees to starboard putting the rear disc in clearer air. There were no handling problems and it went up like a rocket.
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Boslandrew,
Interesting that vibration increased non-linearly above 20.4 tonnes MAUW. It may just be that the orginal design was pushed up from this, with parts beefed up as required. The higher disk loading probably pushed blade pitch away from ideal, using more dependancy on vortex shedding to provide retreating blade lift. Basically a stalled wing will briefly increase lift as the circulation breaks away - in a helicopter this can be used every revolution. It would explain why vibration increased with speed too.
Did you find that having two rotor sets on the one aircraft worked out more expensive to operate? I realise a machine with a MAUW just shy of 22 tonnes would be harder to achieve with one rotor, although the obvious example is the MIL26 (payload ~ 20 tonnes). I just wondered how often the synchronising driveshaft and nose/combining/forward&aft gearboxes needed overhauling. My experience (albeit not with helicopters
) it that complexity normally introduces cost, both investment and operation.
Mart
Interesting that vibration increased non-linearly above 20.4 tonnes MAUW. It may just be that the orginal design was pushed up from this, with parts beefed up as required. The higher disk loading probably pushed blade pitch away from ideal, using more dependancy on vortex shedding to provide retreating blade lift. Basically a stalled wing will briefly increase lift as the circulation breaks away - in a helicopter this can be used every revolution. It would explain why vibration increased with speed too.
Did you find that having two rotor sets on the one aircraft worked out more expensive to operate? I realise a machine with a MAUW just shy of 22 tonnes would be harder to achieve with one rotor, although the obvious example is the MIL26 (payload ~ 20 tonnes). I just wondered how often the synchronising driveshaft and nose/combining/forward&aft gearboxes needed overhauling. My experience (albeit not with helicopters
) it that complexity normally introduces cost, both investment and operation.Mart
Avoid imitations
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Vibration? Ask a Chinook pilot a question about it and he will nod a reply.
He will keep on nodding, even after he's given his answer.
The day after that, too.....
He will keep on nodding, even after he's given his answer.
The day after that, too.....
Avoid imitations
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They're not that bad....
...can't take the banter?
...can't take the banter?
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I sometimes think that what this site needs is a Forum just for intolerant comments and the sort of remark that people wouldn't dream of making other than from the safety and anonymity of the Internet. This thread has hitherto been very informative and friendly. Couldn't we keep it that way?
You're right, Sky Torque, about the vibration. I once flew nearly nine hours out of Aberdeen without a shut-down on a Christmas Special and was still vibrating the next day.
As regards the economics Mart, I have no direct information - the figures were kept confidential. My only memory is that the oil companies said they had to fly the aircraft full to make it pay - rarely did we fly with less than a full load. The servicing cycle for the transmission was no more frequent or demanding than for, say, the S61. It was a well-proven airframe from military service. Having said that with five gear boxes as opposed to two or three it was an expensive aircraft to operate.
You're right, Sky Torque, about the vibration. I once flew nearly nine hours out of Aberdeen without a shut-down on a Christmas Special and was still vibrating the next day.
As regards the economics Mart, I have no direct information - the figures were kept confidential. My only memory is that the oil companies said they had to fly the aircraft full to make it pay - rarely did we fly with less than a full load. The servicing cycle for the transmission was no more frequent or demanding than for, say, the S61. It was a well-proven airframe from military service. Having said that with five gear boxes as opposed to two or three it was an expensive aircraft to operate.
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The Chinook is a relatively old design - remember it started with metal blades and a maximum weight of much less than it has now.
There have been significant, incremental improvements in the machine over the years - fiberglass rotorblades made a huge difference in vibration and noise, hydraulic system completely redesigned for reduced maintenance, and so on.
It keeps getting improved, and I see that yet another refinement has just flown at Boeing's works in Philadelphia.
It would be very unfair to make comparisons of the machine to other helicopters based on the early version - remember it was nearly 25 years ago that it was used in passenger transport. A lot has changed since then.
There have been significant, incremental improvements in the machine over the years - fiberglass rotorblades made a huge difference in vibration and noise, hydraulic system completely redesigned for reduced maintenance, and so on.
It keeps getting improved, and I see that yet another refinement has just flown at Boeing's works in Philadelphia.
It would be very unfair to make comparisons of the machine to other helicopters based on the early version - remember it was nearly 25 years ago that it was used in passenger transport. A lot has changed since then.
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Originally Posted by Boslandrew
Having said that with five gear boxes as opposed to two or three it was an expensive aircraft to operate.
Originally Posted by Shawn Coyle
It would be very unfair to make comparisons of the machine to other helicopters based on the early version - remember it was nearly 25 years ago that it was used in passenger transport. A lot has changed since then.
Mart
Last edited by Graviman; 29th Oct 2006 at 19:17.
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The previous posting was deleted because the helicopter would not vibrate.
If anyone wants to see what the intended effect looks like Click on this link and then click on the 'Vibration Analysis' link.
The information posted on this thread is much appreciated. Thanks.
Dave
If anyone wants to see what the intended effect looks like Click on this link and then click on the 'Vibration Analysis' link.
The information posted on this thread is much appreciated. Thanks.
Dave
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All the major components on the Chinook, like all aircraft, were 'lifed' after which they went off to be re-conditioned. I think the gear-box lives were 1200 or 2400 hours and the engines the same. The BAH Chinooks often flew ten hours a day/five days a week/2500 hrs a year so gear-boxes and engines were probably changed more than once a year. Maintenance was done on an on-going basis, a gear box change here, an engine change there, hydraulic systems and generators, all highly regulated but done to ensure that the aircraft wasn't down for too long at any one time. Major servicings were done at required intervals when the aircraft might be in the shop for a few days.
Daily pre-flight and after flight inspections were carried out on top of that. An engineers inspection might take half an hour but only after any scheduled work had taken place overnight.
Daily pre-flight and after flight inspections were carried out on top of that. An engineers inspection might take half an hour but only after any scheduled work had taken place overnight.
A perfect example of ground resonance!
I think they were just testing to see what happened and do some analysis, AFAIK that chinook was due to be decommissioned anyway.
I think they were just testing to see what happened and do some analysis, AFAIK that chinook was due to be decommissioned anyway.
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Originally Posted by Boslandew
All the major components on the Chinook, like all aircraft, were 'lifed'...
Mart
Last edited by Graviman; 2nd Nov 2006 at 11:43.
Chief Bottle Washer
There's an interesting article on a Chinook repair in field,Widow Maker: stuck between a rock and a hard spot
A couple of photos from the story:
No doubt they all wore Safety Vests, which protected them from falling down the cliff
A couple of photos from the story:
No doubt they all wore Safety Vests, which protected them from falling down the cliff