Pendular action in a chinook ?

[HELOFAN]

So the question is posed, Does a chinook get pendular action ?

I imagine it would sideways but does the rear disc counteract the pendular action that the front disc is creating and visa-versa?

:confused:


Thanks HF

[Granny]

Yes it does, although the term "pendular action" is proberly not the correct description. If you are a severe with fwd and aft cyclic inputs you can enter Dash override where the cyclic actually drives in your hands. Basically the aft head is driving fwd at the same time fwd is driving aft and then it reverses ,it can become very severe and if you chase it with cyclic inputs it will become worse. Not an entirely pleasant experience. uhoh:

[Agaricus bisporus]

Pendular action???

I've had an ATPL(H) for 20 years and never heard of it! Please enlighten us...

[HELOFAN]

OK I'll bite.
I will assume your know it under something else & not pulling the piss.

Because the mass of the aircraft is supported or suspended if you like by a single point ( rotorhead assy ), the mass is able to swing ( oscillate ) in either longitudinally ( forward & backward ) or laterally ( left & right ) just like a pendulum does.
( you can put a small rock on a piece of string & swing it around if you are old enough now cause you were too poor to afford rocks and string when you were a kid like I was :} )

DONT TAKE OUT SOMEONES EYE !! The fun stops there

Now I know that I stated single point & I can understand that possible the Chinook wont oscillate forwards or backwards as easy, or possibly it might be even easier to get, as you now have two big rotordiscs that are ALIGNED longitudinally and you may get disc blowback on the rear that works for or against what the front disc is is doing and visa versa.

Now I was thinking laterally now that the two discs are now parrallel to each other that it would most likely get it like any single rotor helicopter.
But never having flown a chinook I dont know, I can imagine that your control inputs have to be just as carefull to avoid things like that.

Ok so thats basic Pedular action for today, tomorrow we are going to cover coning & coriolis effect.
Remember kids study !.

Also one last question for the chinook drivers, are any of the controls reversed at anytime compared to a single disc helicopter ?
As in for flareing , taking off with nose low attitude etc etc ?
I mean do do you do opposite movements as you would for a single masted heli for the same reaction ?

Am I clear or is this as good as mud on a dark night?

Thanks all

HF

[Matthew Parsons]

I haven't heard that particular term, but I've experienced the same in single rotor as well as tandem rotor helicopters. The load is free to swing, doing so will shift the balance in forces. Improper use of the cyclic will exacerbate the oscillations.

The Chinook has a few features which minimize this. One is that it is heavy, so it would take a heavier load to upset the force balance, to the same degree. Of course the Chinook does lift large loads, but then the other feature comes into play: the multi hook system. By rigging the load properly and using more than one attachment point, you can limit how much the forces will come out of balance.

There was an exciting event when a Canadian Chinook was slinging a CF101 Voodoo to a museum. The Voodoo started oscillating fore and aft, divergently. The load was released when it looked like the voodoo was going to fly directly in front of the Chinook (it diverged quickly). Voodoo splashed into some water, and the man fishing got quite wet. Granted, the load oscillations initiated due to aerodynamic forces on the Voodoo, but once the oscillations are started, its just a load under a helicopter.

[HELOFAN]

I think I have been misunderstood.

I am not talking about it carrying a load, I am taking about the Heli itself being the load being supported by the rotor system.

Hmm

[jab]

HELOFAN
There is some misunderstanding because the term "pendular action" is virtually unknown outside the US, as far as I know. I have only ever seen it in the Rotorcraft Flying Handbook, page 3-2 in fact.
The helicopter does act like a pendulum as it is suspended from the rotor head but I view this as purely theoretical as I have never seen or felt anything which could be called "pendular action", especially not in forward flight. I believe I understand what you are asking but in practice, any tendency for the fuselage to swing like a pendulum is normally only seen and felt when over-controlling and students soon learn what not to do. Am I on the right track here?

[HELOFAN]

Its fairly easy to feel.
Just sit in a hover and push say left cyclic as the Helo moves to the left put in right cyclic , the helo will will climb a little and slow down then move to the right & visa versa. ( its swinging under the rotorhead )
Forward and back is the same.

If you watch new students hovering its fairly pronounced as their control inputs are large and exaggerated.

You can see the rotor disc move, then the helo move and so on.

Its quiet easy for this swingin gaction to be made fairly obvious & I imagine that it is forgotten about from higher time pilots because their inputs are so controlled & slight.

I am stunned that good folk outside the US have not heard this term.

I have heard of it outside the US & all helo's are principally the same no ?

As far as far as a rigid system , I think that they are prone to it as well, why not?
It might not be as pronounced but I would imagine they still get it.

Blades flap so I would think so ..hmm :suspect:
The Helicopter Pilot Manual by Jeppesen metions it as does
The Rotorcraft Flying handbook.

Some other books do & some dont.

Joint Hookes Effect touches on it.

Hmm:suspect:

[jab]

OK, thanks, I see what you are aiming at. Presume a Chinook cannot get it fore / aft as you mentioned for the obvious reasons.

[Matthew Parsons]

Completely misunderstood, sorry. Terminology is not the same around the world, that's one of the reasons for organisations like ICAO. Of course, physics doesn't change across political boundaries so we've all seen that effect.

To be more general, you're talking about stability. Ideally, when a helicopter is deflected from where the pilot wants it to be, the helicopter will correct itself. That's one of the design goals. Positive static stability means that the initial tendency is to do just what I said.

For example, you want to be at zero degree pitch attitude. Bit of turbulence bumps your nose up, your nose starts moving back down. If it passes through zero, then drops below, slows down, goes up, passes through zero again, slows down, etc. then you have oscillatory motion. Sometimes oscillatory motion dampens out so you eventually stabilize, sometimes it just keeps moving up and down, never getting any worse, sometimes it keeps moving up and down but with greater changes each time, that's called divergent oscillation. If you add some cyclic input into this motion, that could make the oscillations better or make them worse. Making them worse is normally called a pilot induced oscillation or pilot involved oscillation (PIO either way). Making them better is our job as pilots.

The tendency for the type of oscillation is called dynamic stability. Positive dynamic stability is where the oscillations get smaller. Negative is where they get bigger.

Why the stability is positive or negative is dependant on many factors. Airspeed is a big one.

"Pendular motion" sounds like a lateral stability or a longitudinal stability mode that has positive static and can have negative dynamic stability.

Tandem rotor helicopters do tend to have a problem with longitudinal static stability. To fix this, designers have added devices like the DASH (differential air speed hold?) on the Chinook, the LCT (longitudinal cyclic trim) on early H46's, and horizontal stabilizers on earlier types like the "flying banana".

The lateral stability of tandem rotors isn't much different from single main rotor helicopters.

So the short answer is yes. You see that "pendular motion" on a Chinook.



The answer for your other question can be a bit difficult to understand. Forward cyclic always pitches the nose down, aft cylic pitches nose up, etc. The controls don't reverse. The trimmed control positions, however, do reverse in some conditions and that is one of the things that the devices I mentioned above do for the pilot. From ~60 to Vne we like the cyclic to move progressively forward. If you have negative longitudinal static stability then the cyclic will move aft when you're trimmed at faster speeds. Of course, this is without the devices that fix the problem so many Chinook drivers might not have seen this.

The other place where some may claim that controls are reversed is during aggressive manoeuvring where suddenly you want to push the stick forward even though you're demanding a higher pitch rate. This is similiar to the trimmed positions above, and perhaps more difficult to understand without a picture or flight. More aft cyclic will still give you more pitch rate, so the controls aren't reversed, but to maintain a higher pitch rate you might find that you need to hold the cyclic further aft. It's called negative manoeuvre stability and can occur on any helicopter type.

I hope this answers your question. Or maybe I still haven't understood what you mean by pendular motion and this was all a waster of time. :)


Cheers,
Matthew.

[HELOFAN]

Thanks Mathew.

You have answered my question on Pendular action very well.

Thanks again.

On the controls reversed question, I was told by an old Ex Vet buddy that flew UH1's and did some time in Chinooks that the forward and aft ( from what I remember of the conversation which was years ago....and I may have been inverted due to the number of drinks consumed that weekend ) inputs on the chinook are reversed , being that a flare ( aft cyclic produced a nose down attitude and a for cyclic input created a nose up result ) and did create quite a scare for the pilot that was transitioning from single mast type helos.

This also may have been whilst there was little or no airspeed or out of ETL... I just cant remember the whole story.

Now saying that , I am not sure if that was something that was from back in the day designs or I misunderstood that statement or he was just leading me up the garden path for a bit of fun as good mates do , though I dont believe he was having a lend to be honest, though thats when mates usually do get you dont they !.

Cheers

HF

[Graviman]

Matthew,

Shawn Coyle in a thread a while back, and SASless below, neatly describe the speed trim system on the Chinook. There is an electric powered actuator trimming longitudinal cyclic for each rotor, to level the fuselage in forward flight. Using an air speed sensor, forward and aft heads both tilt as a function of forward airspeed.

One question i didn't ask at the time was why the front mast is at a more forward inclination than the rear mast. I imagine this is to do with airflow through the rotors, since the downwards induced velocity will be higher at the rear rotor. I also imagine that this mast angle difference is the main reason for any negative longitudinal cyclic trim, since speed would produce a nosedown moment (or require higher pitch on the front rotor). Do you know any examples of a single rotor design suffering from negative longitudinal cyclic trim BTW?

Couldn't quite follow the negative manoeuvre stability discussion though. Any chance you could explain this again? Sorry.

Mart

[SASless]

Are you referring to the speed trim system....that levels the fuselage in forward flight by means of an air speed sensor and electric powered actuators that tilt both the forward and aft heads as forward airspeed varies?

Forward and Aft cyclic movement varies pitch angles for each head...adding pitch in one and decreasing pitch in the other.

Thrust (collective lever for kids who fly skids)....increases or decreases pitch in both heads.

The only unusual flight characteristics I recall from flying the Chinnok stem from it being a tandem rotor helicopter.

When making takeoffs with no power margin (yes....one can overload a Chinook)...keeping the aft rotor in clean air by either side slipping or adding right pedal as ETL is reached will allow one to carry heavier loads.

Another interesting experience is "falling through" on landing. Arrive a bit too fast and heavy with a strong deceleration can wind up with you having to apply forward cyclic to level the aircraft which only adds to the sink rate caused by a lack of power. Add in a tail wind component and it gets un-boring quickly. Again....keeping the aft head in clean air prevents a variation in lift the two heads are making.

The one very nice thing about the Chinook is the 144 inch allowable CG range. CG problems are fairly hard to acheive due to the tandem rotor configuration.

[Matthew Parsons]

Graviman,

A negative longitudinal cyclic trim example would be hard to come by as certification requirements tend to lead designers to eliminate that feature. Under certain flight conditions, most helicopters could demonstrate this. I've seen neutral in a few.

Manoeuvre stability took a bit for me to get my head around until one quick flight. An increase in rotor thrust will normally create a pitch up moment. To eliminate this you need forward cyclic. If you manoeuvre aggressively, you may increase power and pull aft cyclic to generate a pitch rate, the higher the pitch rate, the more aft cyclic. However, once the pitch moment from the rotor dominates, you'll find that your cyclic will have to be forward to maintain a higher pitch rate. When that happens, you have negative manoeuvre stability. It makes it difficult to fly the helicopter with any precision, and could cause the helicopter to "dig in" and suddenly create more pitch rate than you wanted.

As far as the inherent tilt to the rotors of a tandem, I could only guess.

Matthew.

[Agaricus bisporus]

Well, thanks for all that guys! I am still not sure why pendular Action is considered that important, I am sure nothing remotely like that was included in my CPL syllabus.

As far as Chinook control inputs are concerned I was lucky enough to fly it with very few hours at the time (under 300) and found it flew just like any other helo control wise, tho there were some differences in technique, take off & landing were a wee bit different - a bit like sloping ground technique because of the nose up attitude in the hover. Two wheel taxiing was fun, and autos were flare flare flare with no check at all, and a long long nose up ground run. Glorious toy!

[NickLappos]

I wish we had this thread when we discussed the tail rotor effect on lateral bank in a hover! Here is why:

The sling load is a big percentage of the mass of the aircraft system (the aircraft + load). This means that the CG of the aircraft + load system is BELOW the aircraft, and therefore the aircraft is far above the CG. If the load starts to swing, the aircraft does also, since the swing is really a rotation of the aircraft + load system, and a load swing leftward is a concurrent aircraft swing rightward! As we must remind ourselves, the CG is where the forces and moments react (Thus the reference to the tail rotor effect on bank, where some ppruners painted force arrows all over the aircraft, as if the forces moved themselves like arrows! They act at the CG, guys!).

In some helos, this swinging about a point below can excite the AFCS or the rotors to "pump" a bit. This is a natural dynamic response of the aircraft. Look at the feet of a Cobra's transmission to see the plastic covers on the dampers that stop this motion (called "pylon rock").

A Chinook has a similar mode between its two rotors, and this is easily compensated for , as discussed below.

If you get the length of the line at exactly the right length, you can create a circumstance where the load bounces vertically at a frequency that excites the airframe, and the helo can take itself apart! There is a notorious movie on the internet of an Israeli 53 with a load on the Saini that lost its tail as the load bounced.

[HELOFAN]

Thanks for the replies folks.

Its easy to see why you would not get pendular action easily on an aircraft that has a system that compensates for it , but looking at the replies it seems that the aircraft ( Chinook ) is prone to it like any other helicopter just its not as pronounced.

Nick , the rest of your reply seems to be missing ??

Agaricus bisporus, the importance of pendular action with out a load is basically to expose the fact that the helicopter is supported by the rotorhead and with some ( lets say exaggerated or sloppy ) control inputs , the helicopter will swing.

For example: from a hover - if you push forward on the cyclic till the aircraft begins to move forward then neutralise the cyclic, the helo will pitch down, move forward then level then would slightly nose up.

IF you then moved the cyclic back then neutralise the cyclic , the aircraft will pitch nose up, move back and then begin to move back.
This swinging action ( pendular action ) is basically demonstrating the forces in place because the AC is suspended from a point & is free to move like so.

Think of a rock suspended from a piece of string - pull the rock 90 degrees to where you are holding the string and let go of the rock and it swings back and forth.

If this pendular action did not excist then the aircraft would simply move forward, backward & sideways like it was on rails without all 6 axis coming into play.

This can be compounded when the AC is carrying an external load & again can grossly exaggerate/highlight the pendular action.

So control inputs are generally controlled & deliberate.


Thanks again all for your replies & if I am missing something or on the right/wrong track , let me know ..... I am always on a learning curve.
:ok:

HF

[HELOFAN]

That video is nasty.
You can see the tailboom shaking vertically till it tears itself off.
:eek:
It devoloped very quickly too.

I hope the pilot didnt suffer any back injuries from the crash !!
I just watched it through & they drop him very harshly on the stretcher , then drop him whilst they are caryying him on the stretcher OMG !

HF

[Shawn Coyle]

Personally, I think the whole terminology of 'pendular action' as described by the Rotorcraft handbook is incredibly misleading.
It may appear as if the helicopter reacts on a pendulum, but that is really due to pilot-induced oscillations. Nothing in the physics of any helicopter would support the concept of 'pendular action'. If there is something to show how it happens, I've never seen it in any learned publication.
As for a Chinook and pendular action - not a chance (underslung load not withstanding). It is very stable longitudinally, even without the AFCS working, especially in the hover.

Sorry for the short rant, but isn't it time we learned to describe things correctly and in technically valid terms?

[FredFri]

It may appear as if the helicopter reacts on a pendulum, but that is really due to pilot-induced oscillations.+1 ! :E

In fact that's exactly what Helofan says :
For example: from a hover - if you push forward on the cyclic till the aircraft begins to move forward then neutralise the cyclic, the helo will pitch down, move forward then level then would slightly nose up.

IF you then moved the cyclic back then neutralise the cyclic , the aircraft will pitch nose up, move back and then begin to move back.The conclusion is then wrong (in my opinion) :

This swinging action ( pendular action ) is basically demonstrating the forces in place because the AC is suspended from a point & is free to move like so.The AC isn't "free to move", it just responds to the pilot's inputs on the controls.

Its stability in a hover may be questioned but nothing more...

[HELOFAN]

Interesting :suspect:

Not sure what to make of it now. I think when it comes to my check ride I will ask them to support the claim that it exists LOL.. maybe not :E

Shawn, is it possibly the term that is reffered to as pendular actions actuaally the lag reaction from when the rotor disc changes attitude to when the helicopter catches up to it?

I didnt mean to dig this deep into it, it was just a question/discussion I had with my CFI about Chinooks after covering the basic on single masted ships, now its gone a little more than that.

Not than I am going to try and upturn the schools curriculum, it will be more a, just accept the ideology and keep my :oh: , but it does certainly raise an eyebrow as to whether or not the concept of it actually exists.

Not that it changes alot with the physics of the aircraft or how it flies but it does make me wonder as to the how of it a little different.

Thanks again to all for confusing the :mad: out of me.... well a little at least.
:D

:ok:

HF

I will not dig too deep
I will not dig too deep
I will not dig too deep
I will not dig too deep

and so on...:)

[Graviman]

I will not dig too deep
I will not dig too deep
I will not dig too deep
I will not dig too deep


Are you referring to Matthew's comments regarding aggressive manouvring? ;)


Manoeuvre stability took a bit for me to get my head around until one quick flight. An increase in rotor thrust will normally create a pitch up moment. To eliminate this you need forward cyclic. If you manoeuvre aggressively, you may increase power and pull aft cyclic to generate a pitch rate, the higher the pitch rate, the more aft cyclic. However, once the pitch moment from the rotor dominates, you'll find that your cyclic will have to be forward to maintain a higher pitch rate. When that happens, you have negative manoeuvre stability. It makes it difficult to fly the helicopter with any precision, and could cause the helicopter to "dig in" and suddenly create more pitch rate than you wanted.


Makes sense now, thanks Matthew.


Are you referring to the speed trim system....that levels the fuselage in forward flight by means of an air speed sensor and electric powered actuators that tilt both the forward and aft heads as forward airspeed varies?


Yup, but i could have been clearer with the post - thanks.

Mart

[Mr Toad]

I've never heard of pendular action despite nearly 3,000 hours in the 234; however we did experience one interesting characteristic.

Over the North Sea it is common to experience severe frontal weather especially around occlusions; no sane pilot flies straight into it and if you can't get round it you must duck under it.

Strong temperature differences lead to severe windshear; the Chinooks ADC's would become seriously confused as the front of the aircraft pitched up or down and the LCT's tried to fight the uncommanded divergence. At 500 feet over a Force 10 the PF is now trying to control the attitude; he ends up in a divergent fugoid which only ceases as you pass through the shear. 44 less happy punters in the back.

Cure was to take pre-emptive action: slightly before encountering the squall line, make a sharp forward cyclic input and release it immediately. The 234 would then sail through with it's arse in butter. No coffee spilled.
It occurred to us that the military (and Boeing Vertol) must have known about this charming characteristic but didn't feel it necessary to tell us.

Please can some kind American enlighten me (you guys invented the phrase I think)- was this "pendular action"? Because it was not like the effects in this thread caused by underslung loads, either with short or long line.

[Graviman]

Interesting, so speed trim system introduces dynamic instability. As rotor pitches back it causes that head to dig in, with divergent phugoid. I imagine front rotor causes most of the effect, hence cured by the sharp cyclic input before hitting the shear.

Maybe an airspeed sensor was not enough, and there should be a gyro to provide longitudinal damping in the system too. Terninology check LCT stands for Longitudinal Cyclic Trim, but not sure what ADC stands for...

Mart

[Mr Toad]

Graviman:

Air Data Computers (2); computing TAT from OAT probe and measured pitot static pressures among other things; sending data to autopilot systems (2) and eventually to the front and rear control surfaces. LCTs would naturally start to motor as the sudden apparent change in airspeed is sensed when the front end passes through the shearline. Meantime each head is doing it's own thing...Funny thing is the LCTs were introduced at the behest of the FAA who wanted the Chinook to fly like a plank above 45 knots. Never did work out that way but nobody had ever flown this aircraft in weather like we all experience in the North Sea.

This is only my guesstimate; I'm happy to be shot down if anybody has a better explanation.

Happy memories:

Staggering out at dawn from ABZ at 110 knots for the Brent, CWP mostly red.
Racing the 332's and 412's home to the pub at 8,000' (CWP now OK).
Kicking out the tail for extra lift, speed and range.
Fulmar FSU: approach and land 90 degrees offwind, collective on the floor.
Laughing at the fog.

Some not so happy memories.

However our remaining 234's are now in good hands with Columbia Helicopters and I hope some of their pilots read this thread and give their opinions. I believe they recently acquired the Type Certificate rights from Boeing too.

[Mr Teatime]

Not too sure about the pendulum. laterally maybe a little but it dampens down quickly. The reason the discs on the Chinook are tilted forward, 9 Deg on the front and 4 on the aft are so that the ac can ground taxy.

[Graviman]

Mr Toad, thanks for explaining about the ADCs. Since system was introduced to trim rotors for a particular speed, i can see how extreme gusts could confuse it. I have been in a microburst, fortunately i was on the ground having just landed a glider - it was one of the most violent atmospheric events i have ever witnessed. Definately that level of wind shear would test any control system, or even confuse it, particularly if it had not been optimised for that condition.

----

Matthew, your post made me read up in Prouty to try to understand the dynamics of what happens when the helo digs in for aggressive manouvring. As i understand it the machine C of G range is generally chosen relative to rotor for machine stability, depending on horizontal stabilser area etc. During a manouvre the rotor flapback is relied upon to get the extra thrust, but this causes the thrust vector to move fowards relative to C of G. In an extreme manouvre the rotor thrust vector will end up ahead of the C of G, so that the pilot needs to check cyclic aft position to keep heli diverging be ever increasing flapback.

At one time a common solution was to put a bob weight in the system to create a forward force on the cyclic with g, but this was only favoured by pilots with fixed wing experience. Modern designs rely on the pilot to keep machine stable. Perhaps this points to keeping C of G at forward of range for any radical manouvreing.

Mart