Tandem Rotor Helo Yaw Control

[HTB]

There may be a simple answer to a question that has occupied my waking thoughts for the past few days (might be the combination of cheese and wine the previous evenings); without the obvious tail rotor yaw control, how does something like a Chinook do spot turns? That is, with torque presumably taken care of by contra-rotating fans on top (big assumption), how are aerodynamic forces applied to do the turning?

Layman's terms will be just fine; brain overloads too easily these days.

Ta

[iPodder]

As the title suggests this answer is aimed at purely Cmn(thicko) levels of understanding; before I'm flamed I am a Cmn with plenty of Chinook hours.

Your 'Big Assumption" is correct.

We'll assume that the HP(Handling Pilot) requires the aircraft to spot turn to the left in the 10' hover in these examples(for turns to the right just apply uncommon sense).

To spot turn left the HP applies the correct Yaw Pedal input, as a result this input is fed through the 1st and 2nd stage mixing units and with hydraulic assistance the rotating heads are displaced. The Fwd head simply tilts left laterally and the aft head tilts to the right; as a result, the nose of the aircraft is dragged left and the tail is dragged right simultaneously, thus yawing the aircraft about it's centre.

The advantage of this system is the ease in which the aircraft can be yawed about the nose or tail. This manoeuver often causes issues to inexperienced Pilots on tail rotor aircraft. To yaw about the nose(ie keep the nose above a given point and turn the tail through 360) we simply take the above example and add right cyclic, this tilts the front head right(now level) and adds further right tilt to the aft head causing the yaw to act about the nose. Left cyclic would level the aft head and add more tilt to the front head causing the aircraft to yaw about the tail.

Simple and elegant!!! Standing by for grammar/spelling and tech nibbers.

Rgds iPodder.

[teeteringhead]

Even simpler (dunno about elegance):

Discs tilt in opposite directions! (but it "feels" about the same to the driver, who makes [about] the same control inputs)

[HTB]

Thanks iP - bleedin' obvious with such an elegant explanation; I especially like the turn about nose or tail option:ok:

[pshakey]

.....and there was me thinking the RH pilot drives the front rotor and the LH pilot drives the rear.

All my illusions shattered.:rolleyes:

[davejb]

;)....and in the Hormone (Kamov Ka-25) the copilot sits on the pilot's lap, of course....

[Sailor Vee]

All I can say is thank **** the tilt-rotor is comparatively easy!

[CirrusF]

Whilst on the subject of techy chinook questions, can anybody explain why the undercarriage layout was chosen - ie double-bogey wheels about a third of the way from the front, and singlies at the back? Is it for ground-handling reasons? Or centre of gravity and loading considerations?

I've also wondered how fast can you fly sideways in a Chinook? In my wildest fantasies as an aero-engineer I've often wondered if you could build a chinook with an aerofoil shaped fuselage, then accelerate sideways until you reached a speed where the fuselage produced lift, then transition to a flying wing. A bit uncomfy for the pilots perhaps!

[Sven Sixtoo]

In a Sea King in the Falkland Islands in 1983, I was once overtaken by a Chinook going sideways - about 90 kts I guess. I believe they do it by switching out the autopilot, at which point the back end tries to overtake the front ...

Presumably this is why only the crabs fly chinnies.

Sven

[teeteringhead]

To expand a little on previous post, and keeping it simple assuming HTB is not a rotary driver...

1. Any helicopter can turn in the hover about any point inside the airframe, in front of it or behind it. Turns about points within the aircraft are conventionally called spot turns.

2. These are coordinated by the pilot as a combination of yaw and sideways movement, using appropriate controls, ie pedals for yaw and stick for sideways movement.

3. In different configurations (single rotor, twin rotor, coax, notar) the "appropriate controls" will do their work by different systems of "linkages, levers, push-pull rods and bell-cranks" or their hydraulic, electric, mechanical or computerised equivalents, but with substantially the same results.

4. This is an oversimplification, but my coloured chalks don't work on the computer .....;)

[HTB]

Never assume....beware the hidden past.

Hiller 12E; Whirlwind 7; Wessex 1; Sea King.

Brain just couldn't figure out the tandem thing until the first two lucid explanations; as I said, bleedin' obvious innit.

[Thud_and_Blunder]

Well, MY coloured chalks didn't work on the computer either, so I just used permanent felt-tip pens.

...it looks as if I'll have some explaining (and I don't mean P of F) to do to my taller-than-me teenage son when he gets in from mugging grannies or whatever passes for recreation with "yoofs" these days...

[Tandemrotor]

One of the neatest things about the Chinny, and spot turns, is that if you press the trim button whilst engaged in such a turn, the A/P will then give you a constant rate of turn, without any further input from the pilot! Making everything look very neat, particularly in an airshow, whilst turning the tail through the wind!

Clever man that Mr Boeing!

[chinook240]

Tandem,

The clever man was actually Frank Piasecki, the founder of the Vertol Company, which originally made the Chinook - check out the Chinook yaw pedals and you'll see a "V" symbol there! Frank died earlier this year, but I was fortunate to meet the great man at a meeting in Philadelphia many years ago. http://www.nytimes.com/2008/02/15/business/15piasecki.html?_r=1&oref=slogin

[Gericault]

So in a vaguely similar vein, can someone explain to a pointy jet bloke how the NOTAR bit works in the event of engine failure? How is yaw managed with effectively no hot air to the rear control system? Must make for interesting auto rotation?

[sycamore]

Doesn`t use hot air,try `google`-notar

[BarbiesBoyfriend]

Geri

The fan thats pushing the air out the NOTARs tailboom is coupled to the main rotor.

So as long as you keep the disc going- you keep your yaw control.

Of course if the rotor stops you lose yaw control, but tha'td be the least of your worries.

[RODF3]

So in a vaguely similar vein, can someone explain to a pointy jet bloke how the NOTAR bit works in the event of engine failure? How is yaw managed with effectively no hot air to the rear control system? Must make for interesting auto rotation?

You only need a tail rotor(air for the NOTAR) to counter the torque reaction from the engines driving the main rotors. If the engines fail, there is no torque and therefore no need for a counter-reaction.

[sycamore]

The torque comes from the transmission,and even if the engine(s)stop,there is transmission `drag`,which will yaw the a/c in the direction of the rotors,hence it is still necessary to use the feet,to balance the residual turning....

[Tandemrotor]

Dave

Thanks for the reminder. But as you know, it's a long time since I last saw any Chinook yaw pedals! :)