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Helicopter Rotor Changing  
User currently offlineDon81603 From Canada, joined Jul 2005, 1185 posts, RR: 0
Posted (8 years 5 months 3 days 10 hours ago) and read 2481 times:

I did a search, and found nothing about this. I have witnessed the air ambulance in Thunder Bay (not sure of the type, sorry) take off. After bring the rotors up to speed, it's clear to see the rotors lower towards the fuselage before liftoff. Why does this happen? Any help would be appreciated.


Eagles may soar, but weasels don't get sucked into jet engines.
17 replies: All unread, jump to last
 
User currently offlineSfomb67 From United States of America, joined Dec 2005, 417 posts, RR: 0
Reply 1, posted (8 years 5 months 3 days 8 hours ago) and read 2471 times:

Not exactly sure what you're seeing, but I did work on helicopters 38 years ago in the air force, so I'll give it a shot. After the engine(s) is started and the rotors engaged, to lift off, the pilot slowly pulls up on the collective stick which increases the pitch of the blades equally, all the way around their path. At this point, the blades will tend to rise up at their tips, as they push the air down. This will create a slight arc in the length of the blades (equally), the tips being maybe 2' - 3' higher, than where they attach to the rotorhead. Sort of like a wingtip flexing up at the tip during takeoff. As the helicopter rises, the amount of flex decreases, untill landing, when it will again reach the maximum amount of upward flex at the tips as the pilot tries to slow his descent at touchdown. I would guess this is due to the blades flexing as pitch in the blades is increased to create lift, and as a result of centrifigual force of the blades rotating.
Hope this helps.



Not as easy as originally perceived
User currently offlineAmtrosie From United States of America, joined Jan 2005, 274 posts, RR: 0
Reply 2, posted (8 years 5 months 2 days 23 hours ago) and read 2426 times:

The entire rotor head does "tilt" forward via the swashplate. The rotor head is actually tilting in the direction of travel, which is controlled with the cyclic.

User currently offlineDon81603 From Canada, joined Jul 2005, 1185 posts, RR: 0
Reply 3, posted (8 years 5 months 2 days 20 hours ago) and read 2397 times:

Thanks guys, but the entire rotor assembly lowers towards the top of the fuse (about 9 inches to a foot).

I'm trying to get some more info on the make/model of the bird (even a picture, if possible)



Eagles may soar, but weasels don't get sucked into jet engines.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 4, posted (8 years 5 months 2 days 20 hours ago) and read 2394 times:

Quoting Don81603 (Reply 3):
towards the top of the fuse

Front or back?

Ahead of the main rotor mast or behind it?



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 5, posted (8 years 5 months 2 days 20 hours ago) and read 2391 times:

I just re-read your question:

Quoting Don81603 (Reply 3):
but the entire rotor assembly lowers towards the top of the fuse

Are you saying that the entire main rotor mast SHORTENS? The shaft that drives the rotor, and all the linkage associated with it gets shorter?

I'd sure like to see what type aircraft that is.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineScarletHarlot From Canada, joined Jul 2003, 4673 posts, RR: 56
Reply 6, posted (8 years 5 months 2 days 19 hours ago) and read 2386 times:

Ha, here is a thread I can help with! I saw Bandage One, etc. fly over my house many, many times in Thunder Bay. One of my neighbours piloted it. He would run the siren as he passed over our house so his wife would know to come down to the airport and pick him up.


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Photo © Jason Hick
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Photo © Jason Hick




But that was when I ruled the world
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 7, posted (8 years 5 months 2 days 18 hours ago) and read 2374 times:

Well, assuming it is indeed an S-76 we are talking about, I'm pretty sure there is no mast-shortening going on. Beyond that, I am not real familiar with their flight control system in specific terms. I can offer some generalities and some speculation.

Picture the entire rotating set of main rotor blades as a disc. As has been mentioned, the pilot has two controls over the blades in that disc, and, in effect, the disc itself.

Collective the lever on the pilot's left, which includes a twist-grip throttle changes the pitch of all blades equally and (rather automatically these days) increases engine power at the same time. Pulling collective increases the overall amount of lift being produced by the rotor disc.

Cyclic the 'stick' being held in the pilot's right hand angles a swashplate, usually on the main rotor mast to translate that angle to the entire rotor disc at onc. The swashplate is the place where the pilots simple push-pull/ fore/aft left/right cyclic control inputs get converted to rotating push-pull throughputs to the blades themselves. Push forward on the cyclic and through rotating and non-rotating linkage, the main rotor disc angles forward.

The significance of being able to do this is that the net lift generated by the main rotors is perpendicular to that disc. Angle it forward and the helicopter will not only be lifted but pulled forward. Collective pitch increases (with accompanying power increases) will lift the thing in the air. It will then move off in any direction you angle the main rotor disc.

If I am sitting there idling on the ground with the main rotor spinning, I may choose to angle the disc in one direction or another. So long as I do not increase collective ("pull pitch") there is no inherent harm in doing this. If I have people boarding from the right side, I may want to angle it left to increase the distance off the ground of the right side of the rotor disc. In the absence of a specific need like that, I may want to keep it pretty much horizontal to maximize the ground clearance all the way around.

When I am going to lift off to a hover, I want it pretty much dead level else the thing will roll, or take off in the direction of tilt. When I come to a hover the disc should be level - or angled ever-so-slightly into the wind to hold me stationary. To move off forward I will put the cyclic forward which will angle the whole rotor disc slightly forward, and I will increase power-collective at the same time. This will require inputs to the tail rotor through the anti-torque pedals but that is a whole other discussion. (Think Newton's third law of motion)

So if the movement you are inquiring about is a tilting motion, and if this has not fully explained it, could you please be more specific in your question, using up/down - left/right - forward/aft etc.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineDon81603 From Canada, joined Jul 2005, 1185 posts, RR: 0
Reply 8, posted (8 years 5 months 2 days 15 hours ago) and read 2339 times:

YES! That's the craft, and the entire rotor lowers just before liftoff. Shaft, blades, everything lower about a foot. Next time I get the chance, I'd catch it on video in case someone can help.


Eagles may soar, but weasels don't get sucked into jet engines.
User currently offlineBugdriver From Antigua and Barbuda, joined Aug 2005, 6 posts, RR: 0
Reply 9, posted (8 years 5 months 2 days 15 hours ago) and read 2337 times:

The blades flexing under load is known as "coning" and it is seen in all helicopters. Some helicopters have the rotor head "pre-coned" ie. the Bell 206 models.
I very much doubt that the whole rotor system shortens on the S-76 as has been described. It must be some optical illusion, or something that makes it appear like it does shorten.


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 10, posted (8 years 5 months 2 days 15 hours ago) and read 2332 times:

Quoting Don81603 (Reply 8):
the entire rotor lowers just before liftoff. Shaft, blades, everything lower about a foot.

I'd sure like to see that video because it doesn't look like there is more than four inches clearance between the rotorhead and the top of the cowling in these pictures.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 11, posted (8 years 5 months 2 days 15 hours ago) and read 2327 times:

Quoting Don81603 (Reply 8):
YES! That's the craft, and the entire rotor lowers just before liftoff. Shaft, blades, everything lower about a foot.

Well I sure don't see anything here in the database that supports this.

Here it is shut down:

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Photo © Bill Shull


Running on the ground - no load:

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Photo © Propfreak


Lifting off:

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Photo © Danielsson


Flying:

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Photo © Rogier Westerhuis



So when does this happen? All these seem to show the rotorhead in about the same position relative to the top of the surrounding cowling. There are 583 photos of the S-76 in the database, perhaps you can find some that illustrate the lowering you mention.

Any Sikorsky pilots or mechanics on here fill us in on this?



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 41
Reply 12, posted (8 years 5 months 2 days 14 hours ago) and read 2326 times:

Don81603: Is the helicopter you saw the one here, in Volume 19, Shot 7? Click "Play Demo" (sorry I can't find a direct link)...

http://www.dalco.co.uk/timeimagev19.html


User currently offlineNORTHSEATIGER From United Kingdom, joined Sep 2003, 432 posts, RR: 5
Reply 13, posted (8 years 4 months 1 week 4 hours ago) and read 2115 times:

I think what you are mentioning is specific to the S-76, as if you watch it take off it does so in a certain manner, and I think it id this which you relate to not that the disc of a helicopter moves to the rear fuselage. As already stated most discs will "cone" on application of collective and the moving forward or rearward will be controlled by cyclic input.

Regards NST



T's And P's look good....Rotate
User currently offlineJarheadK5 From United States of America, joined Nov 2005, 216 posts, RR: 1
Reply 14, posted (8 years 4 months 6 days 16 hours ago) and read 2052 times:

Quoting SlamClick (Reply 11):
Any Sikorsky pilots or mechanics on here fill us in on this?

I've worked on Sikorsky CH-53E's and D's, and S-61A/L/N/R's. NONE of them have any means to shorten the main rotor shaft (or mast, depending on who taught you terminology). Nor do the Agusta A109's or A119's I worked on.

Here's the best explanation I can come up with:
The swashplate moves with control inputs (obviously); specifically, in Sikorsky products, the swashplate goes UP when the collective is pulled up. During startup of helicopters with fully-articulated main rotor heads (the S-76 is one of these), when the engines are brought from Ground Idle to the Flight position, the pilot has to pull the collective up a bit (an inch or so depending on the aircraft) to get the rotor blades off the droop stops so they can extend and not limit the blades' movement in-flight. Once the rotor head is near or at 100% Nr, the collective is lowered to the stop. What you may be seeing is the gap between the swashplate and main rotor transmission cowling, which will obviously get bigger when the swashplate goes up.

Hope this helps...



Cleared to Contact
User currently offlineDon81603 From Canada, joined Jul 2005, 1185 posts, RR: 0
Reply 15, posted (8 years 4 months 6 days 15 hours ago) and read 2047 times:

Thanks JarheadK5. I bumped into a guy at work yesterday who is a chopper fanatic, and he told me pretty much the same thing.


Eagles may soar, but weasels don't get sucked into jet engines.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 16, posted (8 years 4 months 6 days 11 hours ago) and read 2003 times:

Quoting JarheadK5 (Reply 14):
What you may be seeing is the gap between the swashplate and main rotor transmission cowling, which will obviously get bigger when the swashplate goes up.

Okay, that does make sense. I long wondered why Bell, for example, chose to use scissors above the swashplate for collective pitch changes when raising the entire swashplate would work as well. Thanks.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1609 posts, RR: 52
Reply 17, posted (8 years 4 months 5 days 17 hours ago) and read 1941 times:
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Quoting JarheadK5 (Reply 14):
I've worked on Sikorsky CH-53E's and D's, and S-61A/L/N/R's. NONE of them have any means to shorten the main rotor shaft (or mast, depending on who taught you terminology). Nor do the Agusta A109's or A119's I worked on.

I don't know of any helicopter that has a means to change the main rotor shaft height while the rotor is operating. The UH-60 was designed such that an insert in the main rotor shaft could be removed, to lower the rotor head for air transport in C-130s.


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