Pendular action in a chinook ?
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Interesting 
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
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
, 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
out of me.... well a little at least.
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...
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
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
, 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
out of me.... well a little at least. 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...
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Originally Posted by HELOFAN
I will not dig too deep
I will not dig too deep
I will not dig too deep
I will not dig too deep
I will not dig too deep
I will not dig too deep
I will not dig too deep
Originally Posted by Matthew Parsons
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.
Originally Posted by 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?
Mart
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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.
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.
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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
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
Last edited by Graviman; 10th Mar 2007 at 10:54.
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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.
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.
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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.
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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.
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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
----
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
