Chinook & other tandem rotors discussions
 

I am asking any of your ex mil Pro's out there, in simple terms how do you fly a Chinook, I am always in awe of these "flying buildings " whenever I see one, but yet I can never detect any cyclic movement to the rear blade's, is it all done at the front set, can you help?
My regards

Nothin to do with the green ones mate, this is the story according to the late lamented BV234!

Flying controls operate as any other helo.

Rotor articulation is a bit different.

There is no fore and aft (cyclic) movement in either disk. (there cant be, theyd strike each other) Aircraft pitch is controlled by differential collective between the disks.

Roll is produced by lateral (cyclic) in both disks simultaneously, and yaw by differential lateral cyclic, if you follow my terminology.

Actually there is a slight fore and aft input by a systen called LCT (Longitudinal cyclic trim) that tilts the gearboxes fore and aft to reduce excessive deck angle at speed, but it is not part of the primary flight controls.Conventional flight controls produce these rather unusual inputs via a very complex mechanical mixing unit in the broom cupboard behing the P1.

Due to the dynamics the Chinook develops maximum translational lift in sideways flight to the left, hence the antics of the logging machines plugung sideways on the end of theit long cables - gives them greatly increased lift capacity.

Chinook is extremely docile in handling, yet extremely maneuverable when light. Autorotation is of the thistledown type, rotor inertia is massive and the beggar wont come down fast even if you want it too. Engine off landing is merely a continuous flare to a running landing on the rear wheels which requires quite a lot of runway. I dont know if the military use other techniques.

Take off requires progressive fwd movement on the cyclic as the nose rises to prevent it scuttling off backwards, and vv for landing. There are no torque effects. On icy surfaces it is semi-hover taxiied on the back wheels - groung handling is not helped by rather delicate landing gear, fronts fixed, aft portside castoring and aft stbd only steerable.

What a fabulous toy!

Flying an ILS backwards was fun too (in the sim). Lift off on the landing datum, translate backwards and climb up the slope on instruments. 60Kts sideways and backwards rings a bell??



[This message has been edited by Fr O'Blivien (edited 21 April 2001).]

FO'B, thank you for that explanation, I admire all fellow pilots be they pro's or ppls, but to stand and watch one of these double eggbeaters fly and land is something quite different, more than any other craft, I still find it fasinating to watch them fly! my regards

I had the opportunity last year to spend an hour in a Chinook simulator, with a standards guy in the left hand seat, and another standards guy acting as crewman.

After some initial collective pumping, it felt pretty straightforward to fly - mind you that was probably the AFCS, rather than me !! I got to try some general wazzing, an auto, formation flight and some sling load pickups and drops - one to take the aircraft weight to just under max. All good fun - now, where can I get my hands on the money required to buy that simulator mouldering up in Scotland ?

Cannot bettr Fr O'Blivien for the tech bit, but the mil version flew just the same - actually even more manoeuvrable 'cos it was lighter. Fighter affil in the wocker was awesome, even 2 A10's had difficulty, and power margins impressive. On leaving the fleet to go to Gazelles was told to expect sports car - ugh! Chinook was faster, more fun and could lift more than almost anything else we have in the mil (I don't know about Merlin). It could even fly for ages with the overload tanks on. Odiham - Nimes or 7 hours over the Atlantic (Air India SAR), and the seat was even comfortable.

Yes I miss it - great aircraft, shame about the reputation.


Pertama

Otherwise known as the Boeing Body Bag.... I have only declined a cabbie in two helicopters to date, this one and an Mi8 (Hip)

Wimp!

Went to Chinooks after a joyful few years flying UH-1H with the strap-on offensive bits ..... have to say I love the Huey, but adore the chook ..

She's a joy to fly (as long as one doesn't think too much about the tech), bags of power, can haul a sh**load of coffee home from PNG and is faster than a Blackhawk (much to my brethren's chagrin at times)

Just a pain to pre-flight is all .....

Anyone who wishes to fly the Chinook in Oz ... apply for a job, we need drivers!!!



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Park it, tie it down, lock it, lets go to the pub!!!!!

The A models were a hoot. The tail was pointed as apposed to blocked, called them pin tails, also had no stabilizing "strakes" so, if the SAS came off in a turn the aircraft would fight you tooth and nail to swap ends. If the Longitudinal Cyclic Trim (LCT) fails it means the automatic forward tilt of the rotor system, not transmissions, doesn't happen which carries with it an airspeed limit or malfunction procedure depending when they fail. This aircraft was ment to work hard. I too spent many years in UH-1D & H aircraft and the Hook. Two really great aircraft.

CHINOOK? I love it to bits! I joined the team in 1982 and thoroughly loved my 5 years on the best pilots' machine in the world. I've lifted another CH47, carried 11700 Kg underslung and enjoyed two-wheel taxy games with the best. Its a beast in turbulence; a delight in straight & level (can you do 160 Kts without a dive?) and I hated my family being a passenger in it!!!!!!!!! This machine has attitude.

Sure would enjoy another posting to Hueys after this !!!!

Life is good, even if we do bitch about Timor, Bougainville and Risk Managment !@

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Park it, tie it down, lock it, lets go to the pub!!!!!

Chinook,

Heartily concur - had 2.5 years on single Hueys starting in '82, did Chinooks from '89 to '98 and have been enjoying twin Hueys since then. Best of all worlds, eh?

PS I think that while you're Long Looking, there should be a Stn Master at Odiham who thoroughly enjoyed his exchange tour in Oz. Good timing on your part, I reckon. If you go there with the same attitude as the 2 crewmen who came through around 1997 you'll have a whale of a time; they were good ambassadors for all things Antipodean.

I've just started reading a book called "Chinook", and according to that it's actually the crewman who flies the aircraft.

Not only that, but apparently the best way to pass your aircrew training course is to do no study, drink Pi55 heavily and steal the exam papers!

I can hardly wait for the next in the series, "Orderly Room Clerk".

To: Fr. O’Blivien

You stated:

“There is no fore and aft (cyclic) movement in either disk. (There can’t be, they’d strike each other) Aircraft pitch is controlled by differential collective between the disks”.

You further stated,

“Actually there is a slight fore and aft input by a system called LCT (Longitudinal cyclic trim) that tilts the gearboxes fore and aft to reduce excessive deck angle at speed, but it is not part of the primary flight controls. Conventional flight controls produce these rather unusual inputs via a very complex mechanical mixing unit in the broom cupboard behind the P1”.


Actually, there is a forward cyclic component but it is not introduced by the forward tilting of the gearboxes. An electromechanical actuator that is attached to an arm extension of the swash plates introduces forward cyclic. It does not actuate until the helicopter reaches a forward speed in excess of 60 Knots (approx.). Since the blades intermesh, they can’t contact each other when cyclic is input is made by the automatic system. The pilot does not have direct control of this system and therefore the pilot can’t perform a cyclic flare while flying forward. He can introduce a cyclic flare while flying sideways and because of this, autorotatations must be entered into while flying sideways.


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The Cat

[This message has been edited by Lu Zuckerman (edited 30 April 2001).]

Hey Lu

This is humor, right?

"The pilot does not have direct control of this system and therefore the pilot can’t perform a cyclic flare while flying forward. He can introduce a cyclic flare while flying sideways and because of this, autorotatations must be entered into while flying sideways."

This is a link to the accident report for C-FHFH, a BV234, which crashed in Canada in 1997.

Just thought it might be of interest to any of you Chinookies.

http://www.tsb.gc.ca/ENG/reports/air.../ea97p0303.htm

Thanks for the info on the Chinook crash. I wasn't on the mission but our CH113 (Canadian Forces BV107) was the helicopter on scene for the rescue (wasn't possible).

We were all very interested in why this crashed. Similiar flight control systems in the two helicopters.

For Lu, I assume that the "autorotations must be entered sideways" was a joke. Although fore and aft cyclic works differently than in tail rotor helicopters, it works the same from a practical point of view. Including autorotation.

Matthew.

To: heedm

The pilot of a CH-47 has absolutely no fore and aft cyclic input. As I had indicated previously the cyclic is input automatically when the aircraft reaches approximately 60 Knots.** The pilot has no part in it. The cyclic that is input is forward cyclic and this forward cyclic will be removed when the aircraft reaches a speed below 60 Knots. In a conventional helicopter the pilot can pull the cyclic back to flare to arrest his speed and at the same time increase rotor speed. This can’t be done on a CH-47 if you enter an autorotation going forward. Cyclic input can only be made when the aircraft is flying sideways and the reason for that is the pilot introduced left or right cyclic.

**On the CH-47 the highest contributor to maintenance action and overhaul cost are the two rotorheads. Because the rotorheads are mainly in full collective in a hover and a reduced collective on the Fwd. Rotor and an increased collective on the rear rotor when flying forward the rotor heads are 90% scrapped during overhaul due to the high stress levels. Boeing is developing a new elastomeric rotorhead and I will guarantee you that it won’t last as long as the rotor system it replaces


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The Cat

Lu,

Tell me again how I can't reduce speed or increase RRPM autorotating a CH-47D going forward?

Sounds like just semantics to me.

I stand to be corrected, but I think Lu's saying that cyclic pitch changes can't be made on the rotors in the fore and aft sense, and therefore it would technically be wrong to say that pitch controlling is done 'cyclically'.

However, in any helicopter I'd call that stick between my legs (now, now) the cyclic, and I'd be extremely surprised if a Chinook pilot can't pull back on that and make the aircraft flare, in autorotation or not.

If you can pull the nose up, you can change the relative airflow into the disc and therefore get flare effect.

Right or wrong?

To: Lu
Either you're joking around or are much too compulsively pedantic. If it's the former...you got me. You're right that fore and aft movement of that control does not result in any cyclic changes.

If I assume the latter, then I must point out that the basis of your argument is the definition of the word cyclic. A valid argument, but when you referred to "the cyclic" you were using a noun form of the word cyclic. I could find no dictionary definition for cyclic the noun. In common usage it is the pitch and bank control in a helicopter.

Since that control moved aft causes pitching up and increases Nr during autorotation, it behaves the same as a cyclic control to the user. If it looks like a duck....

Until Oxford adds cyclic the noun this argument could keep going in circles. Cyclically.

Matthew.

[This message has been edited by heedm (edited 02 May 2001).]

Regarding the pilot’s ability to introduce a cyclic flare by pulling back on the cyclic that result in an increase in rotor RPM you may be correct. However in order to do so he must severely increase the collective pitch in the forward rotor system while at the same time decreasing pitch in the rear rotor. This will change the angle of attack in relation to the relative wind causing an increase in rotor RPM by changing the attitude of the airframe but at the same time the rotors are being subjected to a severe drag penalty because of the increased pitch in the rotor heads. The only power being supplied to the rotor system is that of the autorotative forces however there are anti autorotative forces in action coupled with the increase in drag. If all of this were true, it would seem to me that if the pilot went straight in on an autorotation he would have only one chance.

However, if he entered into an autorotation going sideways he would be in complete control and he could make several cyclic flares during his descent with out the drag penalties associated with increased collective.

That's my theory but I could be totally wrong.


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The Cat

Lu, you gotta quit doing this. Stop making wild statements that lean on your previously posted "I'm an engineer" credentials and then backing off (I could be wrong) when challenged.

The Chinook was designed to respond to flight control inputs as other helicopters respond. When the pilot applies aft cyclic differential collective pitch produces a reduction in aft rotor pitch and a corresponding increase in forward rotor pitch, the nose pitches up (flare) and, the speed of the aircraft decreases. When the speed decrease is sensed in the AFCS computer the Longitudinal Cyclic Trim (LCT) actuator is adjusted to apply the equivalent of aft cyclic for rotor system tilt as necessary to remain coordinated to the current airspeed.
The aircraft flares in auto just like any other helicopter. If you flare and don't reduce power in cruise, the aircraft will climb, just like any other helicopter.

To: VLift

Please tell me where I went wrong. In a conventional single rotor helicopter when entering an autorotation the pilot will lower collective and adjust his cyclic to establish a specified rate of descent and a specified forward speed in accordance with the weight of the helicopter. In the process of descending the pilot can arrest his forward and downward movement by making a cyclic flare. This change in attitude in relation to the incoming air stream will result in an increase in rotor speed and arrest the forward movement. The pilot can do this several times to increase the inertia of the rotor system and then make his final flare with the attendant increase in rotor speed and then move his cyclic forward and pull and drop collective to make his touchdown.

Now to the CH-47. It is my understanding that the LCT actuators can be operated manually and automatically. It is also my understanding that these actuators are in the normal state when the swash plate is in the neutral position relative to fore and aft travel. I don’t remember if they extend or retract when in operation in order to introduce forward cyclic upon reaching 60 Knots. When the aircraft drops below 60 knots the actuator will return to the normal condition. I do not believe that the actuators will extend or retract beyond the normal position and therefore can’t introduce aft cyclic. Incidentally, the cyclic stick is actually the pitch and roll stick.

If the autorotation speed is in excess of 60 Knots there will be a forward cyclic component in both rotorheads and when the pilot pulls back on the pitch roll stick he will increase the actuator induced cyclic setting by increasing the pitch on the forward rotor and decrease pitch on the rear rotor but any collective input will be modified by the cyclic input.

The CH-47 control system is the most complex of any helicopter and most probably any fixed wing system. It obviously works but based on some of the postings on this thread there are not too many people that have a full understanding of the mechanics of the system and that includes me.

The V-22 has a similar control system to that of the CH-47 but it is not as mechanically complex. The complexity lies in a couple of electronic boxes as the V-22 is totally fly-by-wire.


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The Cat

Vlift is right on the button and the ignorance portrayed on these pages is incredible. The pitch attitude of the CH47 is controlled by DIFFERENTIAL COLLECTIVE PITCH - less on the front and more on the back puts the nose down & so on. Forget the LCTs - they are a distraction. Oh yes, you can autorotate (but only to a firm surface) for a medium speed run on landing without touching the collective. Listen to those who fly it & not the "experts"!

Always pleased to see RAF Chinooks who regularly train and refuel at my airfield. Taxiing backwards on the rear wheels around a 130° bend when the marshallers had cocked up the parking arrangements was most impressive!

My five year old daughter calls them 'Wocca Wocca' helicopters!

With reference to Lu's statement about the pilot being able to flare several times to boost Nr in auto, there seems to be a bit of confusion as well.

As most pilots would know, the first flare when entering auto helps (along with lowering the collective) to regain lost Nr.

Once you're established in steady autorotation there should be no need to flare unless you want to change your speed, or at the bottom when you want to bleed off speed and rate of descent, whilst maintaining Nr by flare effect.

Sometimes in auto you're carrying a bit of collective to stop the Nr overspeeding.

Your statement seemed to indicate that there might be a need to flare a few times on the way down to keep the Nr up, Lu, but as I said, in steady state autorotation there's no need to.

Sorry if I've misinterpreted your post.

Cheers

I had assumed most of you in this discussion had a general understanding of the tandem rotor controls. Not entirely the case, so I thought I'd lay out how things happen and where the LCT comes in. Just to keep things simple I'll refer to the pilot's right hand interface as the stick rather than the cyclic.

Two rotors means two lift vectors. By varying the magnitude of the vectors you can change total lift or you can initiate a moment about the pitch axis. The collective changes total lift and the stick moved fore and aft creates the moment by varying the collective pitch of each rotor differently, or "differential collective pitch".

To go into forward flight, you use forward stick to pitch the helicopter to an accelerating attitude and increase collective to make up for the loss of the vertical component of total lift (same as all helicopters). As the helicopter is pitched down, forward flight creates drag forces on the fuselage that make the helicopter want to pitch up again, so forward stick must be maintained, and in fact, increased through the speed range.

Once higher speeds are obtained, the drag combined with the higher collective setting on the aft head (from forward stick) stresses the aft pylon shaft (shaft between aft transmission and aft rotor). To reduce that stress, the difference in the magnitudes of the two lift vectors must be reduced. To accomplish this, one or both of the lift vectors are tilted forward, allowing the helicopter to have a less nose down pitch attitude. (should probably draw a picture at this point) This tilting of the lift vector(s) is done by the LCT. An LCT (longitudinal cyclic trim) is a system that controls an actuator that tilts the corresponding swashplate to create cyclic pitch changes that tilt the rotor disk forward.

If in forward flight the engines fail, lowering of the collective and aft stick will reduce blade drag, induce autorotative forces, and with enough aft stick will get the relative airflow moving up into the disk (ie flare). The LCT actuators move relatively slowly and tilt the rotor disk by only a few degrees. In a flare, the stick changes the pitch attitude of the helicopter
(and thus the angle of attack of the rotor disks) by about 20 degrees for a gentle flare rather than a few degrees from the LCT.

One person mentioned that we should listen to the people who fly rather than the "experts". One of the nice things about this system is that to fly them you don't have to learn anything special...they behave the same as tail rotor helicopters. (For the record I fly a tandem rotor helicopter).

I tried to make this clear rather than precise. Feel free to point out where I was less than precise.

Matthew.

[This message has been edited by heedm (edited 03 May 2001).]

The answers and information to my initial question has been A1, and a mighty thanks to all of you , I wonder if you would answer this, I once stood in the entrance to the cockpit of a Brit Army Helio , twin rotor I think it was called a Belvedere, but I plainly remember two huge sticks almost like ski poles, with what seemd to be guarded or shrouded twist grips at the very top of these stick's, is that like the Chinook or does it have a single Cyclic, like say a B206, and a single collective, or am I still way of beam, can a chinook do spot turns like a normal Heli, if so how, I must say all the input to this thread, has been brilliant to read, despite some bad press about the Chinnok, I would ride in one, ( suppose that makes me a bit of a kid)but what the hell,

VfrPilotb :

To the best of my recollection, sopt turns are accomplished by a combination of differential torque and differential cyclic ( i.e. to spot turn right, front blades give a right moving component, rear ones a left moving component ).

Combinations of pedal and left/right cyclic vary the pivot point from the front head through the centre of the aircraft to the rear head.

Have I got it right ?

Had one of a pair divert in today with one engine shut down. Commander elected to land with a 10kt tailwind (nearest runway). I'd appreciate any info on the handling charactersitics/special requirements in similar situations for future reference...strictly layman's terms...I can't understand the physics of your spirally-whirly things...I'm only an ATCO

Matspart3,

For a Chinook it's probably not as much of a problem to land with a tailwind as for a tail rotor helicopter because of the lesser tendency for it weathercock into the wind, therefore easier directional control.

However as a rule I think it's preferable to approach into wind whenever you can:

-less ground speed for a given airspeed, easier to stop the beast and less chance of bending yourself if you contact some fixed object!

-will have translational lift longer, therefore less power required, (or later requirement for high power)

-less likely to put you in a situation where the rotor's ingesting its own vortices, so less chance of vortex ring problems

The only time I'd land with a significant downwind is if I had to due to terrain/landing area considerations, or if I had a time-critical emergency that demanded landing right now - maybe that was the situation in this case?

OK ....

Lu .... here's the empirical data:

straight and level, 100 kias, 100% RRPM ...

Auto entered .....

aft cyclic introduced (pitch 10 deg NU)

shazzam ...

70 kias, RRPM 103% ...

speak about what you KNOW

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Park it, tie it down, lock it, lets go to the pub!!!!!

Heedm and Chinook, Thank you,

Lu Zuckerman, in your post 2 may 2001 you stated that after autorotation the pilot pulls and pushes collective down to effect touchdown. If you push the collective down to effect touchdown your going to chop your tailboom off, or end up in a spinal ward. You pull the collective till you cant pull any more, straight and level after the flare, and hold collective and freeze the cylic, please tell us thats a typo Lu,

Good posts, but I think I know why all of the confusion. I have been teaching BV-234 and 47 aerodynamics for the last 14 years and one area of confusion is the LCT's. Remember in a single rotor helicopter, cyclic feathering is used to combat dissymetry of lift and to set the required rotor disc pitch. In a tandem rotor helicopter, dissymetry of lift is controlled by blade flapping up to around 60-70KIAS. After that a means of cyclic feathering is needed to combat this tendency and reduce blade stresses imposed by excessive blade flapping since this cannot be accomplished by the pilot. LCT's accomplish this by inducing cyclic feathering, i.e. the input is made throught the swashplate(fwd disc) on the blade out the right door and left door so that the disc will react 90 degrees later, "phase lag", to pitch down or up as required. This is cyclic feathering that a single rotor pilot moves his cyclic to tilt its disc in the direction the pilot wants to go. The 47 pilot cant induce fore and aft cyclic feathering as was stated above, differential collective pitch is what tilts the fuseloge forward or aft and then combined collective (thrust)will keep it in the air. Also remember that the (DASH) Differential Air Speed Hold) is what gives the chinook the same handling characteristics as a single rotor (i.e. pushing forward on the cyclic and increasing thrust to increase speed). Without the dash, the tandem rotor would require the pilot to increase speed in the same manner (cyclic forward) but when he got to the desired speed, the cyclic would actually come back a certain extent to a neutral position which is inconsistent with a single rotor. Just ask anyone who has flown the aircraft without the PSAS or AFCS. The DASH works by increasing or decreasing the length of the cyclic pitch control rod so that the cyclic does not have to go back to the neutral position as stated above.
LCT's are a function of speed and standard pressure so the higher the altitude the earlier the LCT's start to extend. In a 47 around 15,000 feet (or less), in a hover, the LCT's are almost or fully extended.

This is very basic and changed some of the names (i.e. cyclic control rod) just so you could understand the concept. This info is in the Aerodynamics for Navy Flyers, Fudementals of flight (FM 1-203, Army) and Sikorsky Helicopter Aerodynamics. All are great sleeping aids!!! See Ya!
P.S. This is by no means all inclusive, you could go on for hours on this specific subject.

Thankyou all for your replys to my original question, your answers have been extremly well explained, and very informative, now I need to try and get a ride in one! if anyone could suggest how I could achieve this in the UK , I would be grateful, My regards to you all.

Vfrpilotpb
If you get an invitation, please see if they can take two - I'd jump at the chance! :)

http://www.vhfcn.org/pics/anihook.gif

[This message has been edited by Flying Lawyer (edited 15 May 2001).]

FL, will do, by the way I sent an E to you several weeks ago re your earlier comments on Gazelles, seems like it didn't reach you.I will call you thru the Prune site if I get a ride offer! My Regards

Chinook over Hammersmith
 

Just a quickie. On wednesday evening circa 1800 hrs I heard a very pleasant sound, looked out my window and saw a Chinook possibly heading South East, circa 800ft. Have never seen one over London before. Any ideas what it was doing?

Thanks

uuuuurrrrrggghhh?

Not too sure about that. Fly them daily and haven't encountered this technique yet!

Nobody has mentioned the DASH yet which is fixed into the cyclic pitch control run. It fools you into thinking that the controls move EXACTLY like a 'normal'chopper, whereas the fact that the two heads go translational at slightly different times would otherwise give an odd feel. It just does this by expanding and contracting within the control run, all controlled by the AFCS (Auto Flight Control System)

Awesome helicopter, and great when people are shooting at you!!!!!