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Chinook & other tandem rotors discussions

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Chinook & other tandem rotors discussions

Old 16th Sep 2014, 20:19
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Until recently, certainly in RAF use, the 100 kt limit applied to single AFCS flight, but if you switched the 'good' system off ie AFCS out, you could use the VFR speed limit ie 150+ kts. Not sure how this fits with the Yaw divergence of A models, but I thought it was to do with the protection against DASH runaway. On a single system you don't have the protection of the other DASH backing off in the event of a runaway, hence the 100 kt limit. If you have no DASHs to runaway then the speed could be unlimited but very tricky to keep under control. What model are you flying?
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Old 16th Sep 2014, 21:01
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Single AFCS on CH47D

Thanks for both replies, i am flying the CH47D model.

Not sure if i understood correctly but the comments about why there is a 100kts limit with single AFCS operating are

1) 100 Kts is the maximum speed that flight augmentation can be controlled by a single AFCS based on fuselage aerodynamics

2) 100 Kts is the maximum speed that a single DASH actuation motor can compensate for cyclic stick giving enough negative stick to make necessary speed reduction should the need arises since one of the 2 DASH actuaion motors are inoperative?
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Old 16th Sep 2014, 23:59
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Can anyone point me to the accident report for Columbia's 234 crash in Jan 2013? Interested on why the rotors intermeshed 5 minutes after takeoff. A very intolerant design.

The Sultan
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Old 17th Sep 2014, 00:51
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Sultan, you ever heard of Google?

The Report can be found there......how's that for a pointer?
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Old 17th Sep 2014, 08:18
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"100 Kts is the maximum speed that a single DASH actuation motor can compensate for cyclic stick giving enough negative stick to make necessary speed reduction should the need arises since one of the 2 DASH actuaion motors are inoperative?"

Not quite, it's more to do with the speed that the DASH can move, with both operating a single runaway is counteracted by the good DASH running in the opposite direction giving the pilot time to intervene. With one system failed this can't happen, and in a DASH retract at low level, it might not be recoverable at higher speeds.
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Old 18th Sep 2014, 01:25
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BB

So defensive, sound like SASlass when the Chinook is shown to be a flawed design. I did find this on Google before:

https://www.federalregister.gov/arti...boeing-defense

but it is just the clean up actions. I was looking for a report like the AAIB puts out where the facts are detailed.

From the above. The blades voided, probably had water in them, high 1/rev, causing fatigue failure of the aft pylon. Result the same as on a lot of CH-46's with the same result.

The Sultan
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Old 18th Sep 2014, 01:54
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Sultan, Christmas Turkey's have nothing on you.

I dare say Barrack Obama has more credibility than you.

It seems you think your keel hauling here at Rotorheads has been forgotten.

You got run off before, you trying for a Trifecta or something?
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Old 18th Sep 2014, 04:11
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Single AFCS on CH47D

Thanks Chinook240,

I think i kinda get it but could you elaborate on how a DASH runaway in low level and high speeds with one AFCS operational lead to the pilot not being able to have sufficient control of the cyclic to recover? I have never been on any flights where there were training on low level high speed recovery flight.

Thanks for the knowledge
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Old 18th Sep 2014, 04:53
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Positive stick gradient

Hi,

DASH on the CH47D is supposed to provide positive stick gradient to the pilot on controls can anyone define what is positive stick gradient and explain the concept to me pls.

I understand positive stick gradient as the DASH actuators consistently giving the cyclic/pitch axis more movement travel as the speed of the aircraft increases without requiring the pilot to move the cyclic stick forward. In that way even at high speeds, the pilot will not run out of aft stick when the need comes for a quick deceleration?

can someone do me the favour by verifying my understanding?
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Old 18th Sep 2014, 06:40
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Chinook & other tandem rotors discussions

It's not sufficient control movement but rate of control movement that you are guarding against. I don't have access to the manual at the moment but there is a Boeing graph showing stick position with the DASH fully extended and retracted vs speed which would help here.
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Old 18th Sep 2014, 10:54
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In 2001 when I started this thread, it was to find out more about the Chinook for it really is a helicopter that always grabs my attention , I will even now pull over and watch them until they disappear from sight, also I am still trying to blag a ride, older and wiser but still like a big Kid when I see them.

A lot of very interesting posts on here, and obvious good knowledge from you lucky people who have steered them.

Peter R-B formerly due to computer cock up VfrpilotPB
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Old 18th Sep 2014, 11:26
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Positive Stick Gradient means the Cyclic Stick position is directly proportional to the Airspeed....the faster you go the more forward the cyclic stick will be.

It is more a certification issue than a handling issue.
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Old 18th Sep 2014, 17:34
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Positive Stick Gradient

Hi BB,

Thanks for your reply, can i safely assume now that negative stick gradient means there is a chance the crew on flight controls may run out of aft cyclic to retard speed when needed especially since now they will have to pull aft cyclic, bring it back to neutral and pull back again?

Does that define negative stick gradient? and similarly does negative stick gradient also means the characteristic of gaining more speed requires the pilot to push stick fwd, pull back to centralise and then push forward again?

trying to get these basics right
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Old 18th Sep 2014, 19:54
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Doesn't answer your question but a useful link for understanding D model AFCS.

http://usarmyaviation.com/studyguides/index.php?folder=Documents/CH-47%20Chinook%20Specific&download=AFCS.pdf
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Old 18th Sep 2014, 20:16
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hi Chinook240,

I have read these training manuals as well.

Thanks
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Old 19th Sep 2014, 00:38
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The difference in maximum speed with only one AFCS operational is due to pilot intervention time if an AFCS runaway happens. With two systems working, if one fails (let's say a roll actuator suddenly goes to full right roll), the other system can take out part of the effect - the pilot has to be 'warned' of the failure - which, like engine failures, will take some finite time to be recognized and then for appropriate action to be taken. It''s much worse if you only have one system working, and it decides to runaway - the pilot recognition and reaction time will be the same, but there is no other AFCS to counter the effect of the runaway - at higher speeds, the failure could be more than entertaining - hence the lower maximum speed.
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Old 16th Jul 2016, 03:16
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How does a CH47 or other Tandem rotor yaw ?

Can anyone explain this, I understand how a tail rotor equipped aircraft is yawed but what happens when a Chinook Pilot inputs 'rudder' ?
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Old 16th Jul 2016, 03:27
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Differential lateral cyclic. With a right pedal input, in effect the forward rotor tilts to the right and the aft rotor tilts to the left.
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Old 16th Jul 2016, 03:44
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Ok, understood then, correct me if i'm wrong but neither rotor can tilt forward and aft like a 'conventional' helicopter or it would risk having the blades hit each other ?


Interesting that isn't an issue with the rotors tilting left and right.


So when you input forward cyclic the desired affect is achieved by increasing the pitch on the rear rotor and decreasing on the front and vice versa on rearward cyclic ?
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Old 16th Jul 2016, 04:06
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The early models had "Speed Trims" were electric actuators that tilted both Rotors forward as Air Speed increased from 60 knots in order to level the fuselage and reduce drag. As the airspeed decreased the actuators reversed that input.

If the Speed Trims stayed extended....and the aircraft slowed it put excess strain on the Aft Vertical Shaft that connected the Aft Transmission to the Aft Rotor Head.

There were Manual Switches for use in that case that allowed the Pilots to manually retract the Speed Trims.
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