Helicopter rudder pedals

[Dr Jekyll]

I appreciate that on most helicopters the rudder pedals control yaw through the tail rotor, but how do they work on twin rotor machines?

[Bravo73]

What, helicopters now have rudders? :confused:


:E



Sorry, I couldn't help myself. FYI, the pedals in a helicopter control the anti-torque device (ie the tail rotor in most designs).

But when you say 'twin rotor', do you mean a tandem machine (like a Chinook) or a co-axial design (like a Kamov)?


Edited to add:

Just did a quick bit of Googling and this site should answer all of your questions, Dr Jeckyll:

Helicopter Yaw Control Methods (http://www.aerospaceweb.org/question/helicopters/q0034.shtml)


HTH

[tegwin]

I believe that in a Chinook the "rudder peddles" change the pitch between the front and rear rotors thus changing the direction of the torque reaction...

For an object to be at rest, for every action, there must be an equal and opposite reaction....so by changing the torque of the rotors differentially you are upsetting this balance...

[Fareastdriver]

They mix the collective or cyclic inputs to get the same effect.

[tegwin]

Its quite an old design...So is it electronic or mechanical mixing?...anyone actually know?

[Hover Bovver]

Bravo 73,
Notars have rudder pedals:)

[Bravo73]

HB,

If I was feeling pedantic right now, I would argue that point with you. But I'm not, so I'll let you off this time... :p


;)

[Hover Bovver]

Or should that be pedalantic :)

[Good Vibs]

:ok:Come On SASless, where are you when one needs you?
As an experienced RVN CH-47 PIC you must know.

[SASless]

Well....since I was called out on this topic....my immediate answer is by means of "PFM".


As it was two generations ago I herded Chinooks about the sky...my technical grasp of the topic is a bit dim.


All those excuses being made....I will try to do my best to explain it in very simple terms (operative word in my case is "simple").


Each rotor head on the Chinook has two actuators, one being called the
"Swiveling Actuator" and the other the "Pivoting Actuator". The actuators work together to provide differential cyclic and collective pitch for the head. There is no "collective actuator" as in single rotor helicopters.


Yaw control is very similar in result as in making a cyclic movement in a single rotor helicopter....except the rotor heads are "tilted" in opposite directions by means of input through the "Yaw Control Pedals".


As Torque is countered by the contra-rotating rotors, there is no "Yaw" as typical of single rotor helicopters.


Collective Sticks are known as "Thrust Levers" in the Chinook.


One of the final tests for a Chinook Instructor qualification used to be having to explain the movements of the flight controls and the resulting actuator movements produced while hovering....the tough part was when the maneuver being discussed was a turn about a point with a fixed radius measured from the Aft Head while encountering a ten knot wind.
I doubt anyone got past the first ten seconds of the explanation before getting totally flumoxed.




This is a photo of a slightly used set of Chinook Yaw Control Pedals....
http://i243.photobucket.com/albums/ff209/rechappell/chinookfootwellviewoutside.jpg


For a bit of perspective....


http://i243.photobucket.com/albums/ff209/rechappell/chinookclaimtofamejune1970-1.jpg

[helicoptertestpilot]

Well, in helicopters like Kamovs with contra-rotating rotors, a mechanism called CDPCM (Collective Differential Pitch Control Mechanism) is used, in addition to LARGE rudders with slats. when the pilot applies rudder, CDPCM causes the pitch angle of both upper and lower rotors to change appropriately so as to provide the required yaw. Needless to say, if in forward flight with rudders effective, the pedal deflection necessary is reduced. The Control Law calculation and the gearing required for the above is actually amazing and hats off to Russian engineering. Notwithstanding, while taxiing in such helicopters, one sometimes needs to come up on collective significantly to effect a sharp turn.

[170']

Large Rudders...with zippo for scale...

http://i232.photobucket.com/albums/ee281/KA32A/Tailfeatherswithzippoforscale.jpg

[alouette3]

Helicoptertestpilot:
Since you have some knowledge about the mechanism of the KA 32,maybe you can help here:
I have heard thet the KA is a b---- to fly on instruments.Roll into a turn, apply rudder to center the ball and voila!the helicopter climbs( or descends --depending on which rudder has been applied).True or false?
Alt3

[PT6ER]

Bravo

on the drawings for the Ch54 / S64, good old Igor's boys refer to the "rotary rudder"

So they are "pedals, rotary rudder"

It also has "alighting gear".........

[SASless]

Kiinda like going wings level in an Alouette III !

[Graviman]

SASless, did you ever figure what took out the pedal? You've told the story on Rotorheads before, but actually seeing the damage is pretty scary...

[Bravo73]

Well, there you go - every day's a training day! :O

I would still reason that rudders belong on boats and aeroplanes. But I guess that the Kamov is the exception to that rule...

[SASless]

Most likely something like this....educated guess was something along the lines of .51 Caliber. It makes one very noisy entrance as it passes through....particularly when it hits the pedals you got yer number nines stuck on.

Scary? Trust me.... It is possible to pee and poop simultaneously!

What bothers me to this day is the guy at the other end did not even know me....and he tried to kill me. I could understand it if he actually knew me.



http://svsm.org/albums/12mm_dshk_naval/IMGP0035.jpg:eek:

[Graviman]

I guess the aircraft and so anyone in it becomes a target. Impressed by your getting the machine down. Pilots with combat experiences earn my respect anyday.

That looks like the result of a hydraulic oil fire? I've seen how quickly it catches on diesel exhaust manifolds.

BTW, back on topic, how did you find the yaw response of the Chook? I imagine all that inertia along with differential cyclic control created a noticeable time lag. I guess that flapback kept the rate in check, but did you have to be gentle about applying/reversing pedals?

[SASless]

The Chinook was much easier to control in yaw than any other helicopter I have flown....makes sense in that the Yaw (during power changes) is minimized by the design.

On the ground we had a very small range of travel if all wheels were on the ground...something like an inch either way....but upon lifting the forward gear off the ground and doing a two wheel taxi that limitation went away.

The inertia of the aircraft due to its size and weight aids in cross winds or gusty winds...but the down side is when you let the rate of turn get away from you it takes a Heaps of pedal to get it stopped again.

These fancy photos you see of the Chinook setting on top of a very small ledge or house top makes one think it is really hard to do. From a flight control standpoint it is quite easy really. Finding a reference to use while doing it can be a bit tricky.

The same method used to pick the aircraft up to a hover works for those aft wheel only landings.

Pedals keep the nose straight, the cyclic gets pulled back two inches (used to have a cyclic position indicator in the CH-47 A, B, and C's), then the thrust lever is raised until a 5-7 nose high attitude is reached...brakes can be released at that point and the desired aicraft position is maintained by the thrust lever and pedals....the cyclic rarely needs moving once a balanced position is found. As the folks in back do their thing....thrust lever movements counter the movement from changes in CG caused by loading or unloading.

The one characteristic of the Chinook is both rotorheads want to be in front of the other....and a bitter feud it is too. The original A model's had a sharp ended empennage and later models have a square end which is supposed to give the aeronautical effect of 35 feet of fin. That dampened the yawing of the aircraft when in un-SAS'd flight.

The improvement of yaw control done by the SAS system on the A models is hard to describe to anyone that has not flown the Chinook. On more than one occasion while instructing on the aircraft....we have had the droop stops pounding at a 100 knots in what was rapidly becoming sideways flight as the rear end swung around to one side or the other when the SAS system was turned off. The B-D models are much more stable in yaw than the A model.

[Graviman]

Thanks for all the info, SASless. I took the route of engineer since that is where i am best applied, but i like to understand the Chook handling quirks. I'm starting to gain insight as to just how vital SAS is on large helicopters.

Was there a tendancy to use cyclic to correct for front translation with yaw? I imagine converting to tandem the first thing to get used to is sitting so far ahead of the CofG. Likewise i imagine that it took some discipline not to correct the thrust everytime pitch attitude was changed, like lifting aft wheels off the ground. I imagine getting the feel of the machine took a little time.

[SASless]

An interesting situation in Tandem Rotor helicopters is when one head enters translational lift and the other does not....which frequently happens if a takeoff at max possible weight (MPW) is attempted. MPW being where you exhale to lighten the load while creeping off as smoothly you can for any roughness of control touch results in a settling back to earth.

As the forward head enters ETL....the tendency is for the nose to climb as the forward head is producing more lift than the aft head....an application of forward cyclic cures the pitch up problem but also decreases lift in the aft head...if your rate of acceleration is insufficent to get both heads flying in ETL....major problem.

A very common techique for acheiving MPW performance was to use Yaw pedals to swing the aft head into clean air and combine the sideways rotation of the airframe and the cleaner air to bring the aft head into ETL at the same time as the forward head. Timing was of the essence in that maneuver.

It is great fun to find yourself behind the power/altitude/speed/experience/idea curve with the Mr down from 230 rpm to 190 rpm, gennies dropped off line, and SAS kicked out due to loss of electrics....while holding the thrust level up under your armpit and trying to think up a sound explanation that will satisfy the Accident Review Board.

MPW by the way is the real ACL (Average combat load) for the aircraft as found in reality vice the SOP's.

We asked a lot of the old girls and their held together pretty well considering the abuse they got.

[Shawn Coyle]

SASless:
MPW - is that a certified, approved term???? (he said, tongue in cheek).
I'd certainly like to use it in future work - it really does say it all.
And wouldn't it be nice if the manufacturer gave you some indication of what sideslip angle gave maximum possible thrust when creeping out with everything on the red line???

[TheMonk]

Great link Bravo73. Thank you.

Monk

[TheMonk]

Sasless "What bothers me to this day is the guy at the other end did not even know me....and he tried to kill me. I could understand it if he actually knew me."

I know the feeling Sasless. I was growing up in Laos at the time the B52's were dropping more bombs than WWI and WWII combinded down onto Laos' soil. Those guys didn't know me neither, my dad nor my grandpa. I was only three-four years old at the time and from what I heard Laos has never ever attacked the US. :}

Anyways, grandpa always said that if he ever wanted a new pond, all he had to do was light a small bonfire at the spot that evening and the next morning there'd be a fresh new pond all dug up nice and round for him. I think Nick Lappos knows what I mean by that. :8

Monk

[helicoptertestpilot]

Grat stuff SASless, but when you say aeronautical effect, I'm sure you actually meant aerodynamic effect??!

[Graviman]

SASless, thanks again. I always enjoy reading your posts. A lot of experience there - both good and bad...

TheMonk, it is interesting to hear your viewpoint. In Britain we owe a great debt to aviators who in the 40's kept our country free. That was before my time, but i still like make sure that pilots feel the risks they have taken earned my generations respect.