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#3213516 - 02/21/11 12:43 PM How does a Chinook yaw?  
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This is actually a question out of personal curiosity but I figured if anyone had the answer, it might also benefit the C-H team when they come to fully modelling the flight characteristics of the playable - and indeed AI - CH-47 Chinook tandem-rotor heavy-lift helicopter.

How does the Chinook go about yawing? Does it pivot around the forward or rear rotor, or does it use a mixture of input on both rotors?

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#3213526 - 02/21/11 01:18 PM Re: How does a Chinook yaw? [Re: Flyboy]  
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Mixture of input on both... When yawing to the left for instance, the forward rotor is tilted so that the rotor disk wants to bank to the left. Same as would happen when you push the stick to the left in a conventional rotor configuration. On the same time, the rear rotor is tilted so that the disk wants to bank to the right. This banking of the two disks in opposite direction results in the aircraft not banking. However, due to the downward airflow of both rotors being directed away from each other, the helo turns around its vertical axis. Finally, because the downward force of the helo is no longer positioned 90 deg. in relation to the ground, the aircraft would lose some lifting power while yawing. To compensate for this, the blades are tilted to produce more lift during yawing.

And as you can imagine, it requires a rather sophisticated FCS. Whereas in a regular helo the pedals control the tail rotor and the collective and cyclic control the main rotor, in a Chinook both 'main' rotors are controlled by all 3 flight controls. I assume there is no mechanical connection between the flight controls and the rotors and that everything is managed by fly-by-wire as I simply cannot come up with a mechanical way of having all 3 flight controls connected to the rotors.

To wrap it up you could say the following:

Stick left/right: both disks tilt left/right
Stick forward/backward: forward disk generates less lift than backward disk/vice versa
Collective up/down: both disks generate more/less lift
Pedals left/right: forward disk tilts left and backward disk tilts right/vice versa and at the same time more lift is produced by both disks

Got it? wink Kinda hard to explain without graphical examples...

Last edited by JayPee; 02/21/11 01:52 PM.
#3213546 - 02/21/11 01:45 PM Re: How does a Chinook yaw? [Re: Flyboy]  
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Well the Chinook came out in the days when digital flight control systems and fly-by-wire (as far as I know) were just a dream - or at least is the case for earlier tandem-rotor helicopters. Look at the CH-21 Shawnee in the Vietnam War (first flight in 1952) for example. These must have used analog controls with 'physical' connections between controls and rotor discs.

#3213548 - 02/21/11 01:49 PM Re: How does a Chinook yaw? [Re: Flyboy]  
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I am not sure whether those types of helos had the same flight control configuration (stick, collective, pedals) and if they did, I do not know how the combination of input is mechanically transfered to the rotor hubs. However, I do know that the theoretical part behind the controls of a Chinook is correct the way I posted it.

Btw, never wondered how a coaxial configuration yaws? smile

Last edited by JayPee; 02/21/11 01:51 PM.
#3213596 - 02/21/11 02:40 PM Re: How does a Chinook yaw? [Re: Flyboy]  
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That pretty much covers it.

Co-axial yaw is though torque differential. Fred will know the precise mechanics over at Hovercontrol dot com.


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#3213672 - 02/21/11 03:59 PM Re: How does a Chinook yaw? [Re: Flyboy]  
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Well I don't care much for how co-axial rotors work as there aren't any military helicopters in current U.S. military service that use that configuration. And apart from anything by Sikorsky I don't like co-axial helicopters all that much.

#3213881 - 02/21/11 07:12 PM Re: How does a Chinook yaw? [Re: Flexman]  
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Originally Posted By: Flexman
That pretty much covers it.

Co-axial yaw is though torque differential. Fred will know the precise mechanics over at Hovercontrol dot com.

I'mma have to pay Fred a visit then at HC's. Now that Flyboy's asking for it, I'm wondering how this mechanical link is set up between the 3 flight controls and both rotors.

#3214050 - 02/21/11 09:00 PM Re: How does a Chinook yaw? [Re: Flyboy]  
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Originally Posted By: Flyboy
Well I don't care much for how co-axial rotors work as there aren't any military helicopters in current U.S. military service that use that configuration. And apart from anything by Sikorsky I don't like co-axial helicopters all that much.


Not completely true. The K-Max was used for a while, albeit not an officially purchased military helicopter, it was used with the Navy off of Navy ships.

Quote:
I'mma have to pay Fred a visit then at HC's. Now that Flyboy's asking for it, I'm wondering how this mechanical link is set up between the 3 flight controls and both rotors.


The same way they're all connected in every other helo. Push tubes, swash plates and pitch-change mechanisms. Up until a year or two ago, there hasn't been a fly-by-wire helo. Sikorsky's H-60 prototype was the first (unless there was a test bed aircraft before that that I'm missing, which is possible). There are fly-by-wire components on helos (like the stab on the -60), but the AFCS systems on production aircraft are still an electro-hydro-mechanical input to the actual controls.

#3214099 - 02/21/11 09:26 PM Re: How does a Chinook yaw? [Re: Flyboy]  
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Then perhaps I did not express myself clearly enough. What I meant to say is that I have difficulty understanding how a direct mechanical translation from flight controls to the swashplates will work. I am aware that collective input will simply shift the swashplates up or down along the rotorshaft whereas cyclic input will tilt the swashplates to the front, the rear, left and right.

The first 'layer' of controls would be the control rods connected to the fixed swashplate that make it tilt. The second 'layer' of controls would be the system that simply moves this entire unit of rods and swashplates (being the first layer) up and down, regardless whether the position of the swashplates is tilted and therefore maintaining the amount and direction of tilt given by the first layer of controls.

Now if the pedals would also directly be connected to the swashplates, how would this third 'layer' work? Because this would mean that 3 independent input devices all act upon the swashplates at the same time... For some reason I'm having a very hard time visualizing how this direct mechanical translation from control input to rotors would work. Hence I was merely assuming that flight control inputs is measured by a computer and subsequently the computer itself calculates how much it should change the height of the control rods.

http://www.pilotfriend.com/training/flight_training/rotary/tandem.htm
And they nicely stated:
Quote:
This is accomplished by a complicated system that requires a lot of linkages, a lot of control tubes, and a pilot who trusts his maintenance crew to make sure it all was put together properly.
No sh1t!? Pretty explanatory... rolleyes

Last edited by JayPee; 02/21/11 09:41 PM.
#3214292 - 02/22/11 12:21 AM Re: How does a Chinook yaw? [Re: Flyboy]  
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Don't think of it as 3 (or 2 for conventional helos) layers that add control inputs to each other, but as "layers" that adjust each control input. I don't know tandem rotors systems as I have no experience with them, but with conventional systems, if I put 50% collective in, then move the cyclic x amount to the right, the control mixing doesn't move the blades x amount MORE to make the control input, it adjusts the swashplate to have the desired effect, which may mean it takes OUT control input from the original collective position.

Another example is if you have a 4 bladed rotor system w/ one of the blades at the 12 'o clock position. If I move the cyclic fore and aft, the 12 'o clock blade won't move. But if I move the cyclic fore and aft AND the collective up and down, the blade will move. Basically, it has "Thermos technology." It just "knows" (and has the appropriate mixing).

#3214322 - 02/22/11 12:46 AM Re: How does a Chinook yaw? [Re: Flyboy]  
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gatordev - that second part of your post looks interesting. You seem to be talking about a similar phenomenon that I noticed in Longbow 2 (sorry to go slightly off-topic). Could you look over my post and see if you can add anything to it, if it is indeed like what you mention?

http://simhq.com/forum/ubbthreads.php/topics/3144670/LB2_main_rotor_bug.html

I know so far in that topic it appears from the replies of other's that how the rotors act is a game bug, but I'm not so sure. I mean, if it was a bug, why would the rotors of the Black Hawk behave differently to the Longbow's and why do they act in the way that they do at all? Quite a complex bug I'd say!

#3214640 - 02/22/11 09:51 AM Re: How does a Chinook yaw? [Re: Flyboy]  
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Flyboy, after reviewing your link above, I would say that Bushmaster78FS is more or less correct, in that the LB2 rotor dynamics are a bug, or just a byproduct of limited computer programming of the time, limited aerodynamic understanding, or a combination of both.

When you factor in something like gyroscopic precession, it is explained like this. Logically, if you push the stick forward to create differential lift on the backside of the rotor system to pitch forward, than the rotor blades on the rear half of the rotor disk would be pitched more than the blades on the front half. Due to gyro precession, the cyclic blade pitch must be MECHANICALLY applied 90 degrees before the desired AERODYNAMIC application. Therefore, when the pilot pushes the stick forward, the blades on the left side of the rotor disk (counter-clockwise rotation) will increase pitch angle, causing the actual up-flap of the rotor blades when they reach the rear half. To solve this, a mechanical offset of 90 degrees must be applied to the swashplate and flight controls. If you look at photos of the upper flight controls and non-moving/lower swashplate of the Apache or Blackhawk, you will see that they are in fact offset 45 degrees. The pitch horns coming off each individual rotor blade pitch housing add a second 45 degree offset, resulting in a combined 90 degree mechanical offset.

The pilot still moves the cyclic in the direction of desired movement, but the helicopter's flight controls are designed to make the required mechanical inputs for the desired aerodynamic effect. I've never flown tandem rotor helos, but I would imagine that even decades ago, engineers were able to design a system of control rods, bellcranks, and mixing units that produce the required mechanical inputs for all possible flight control inputs by the pilot. Computerized flight control systems such as the modern day Chinook's AFCS make inputs into the flight controls to make the pilot's job easier, increase the helo's stability, and if advanced enough, can provide hold modes or autopilot.

In response to gaterdev's comment on the K-max, it actually used an "intermeshing rotor system" that had two individual two-bladed rotor systems offset horizontally and angled away from each other with intermeshing blade paths.

I would have to say that DCS:BS has the most realistically behaving rotor system of any helo sim I've played. But honestly, as long as the aircraft in a sim handles like the real thing, I don't really care too much how the rotor system behaves aerodynamically when in an external view, provided it's not doing something outrageous, lol.


Afterburners are for wussies...hang around the battlefield and dodge tracers like a man.
#3214740 - 02/22/11 01:18 PM Re: How does a Chinook yaw? [Re: Flyboy]  
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Thanks for looking at my link, Raptor9, and for your input. Input, geddit? Cyclic input? Oh, nevermind!

#3215387 - 02/22/11 10:15 PM Re: How does a Chinook yaw? [Re: Flyboy]  
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Source US Army FM 3-04.203 Fundamentals of Flight; page 1-33, para 1-81, 1-82:

Tandem-Rotor Helicopters
1-81. Heading control is accomplished in tandem-rotor helicopters by differential lateral tilting of the rotor
disks. When the directional pedal (right or left) is applied, the forward rotor disk tilts in the same
direction and the aft rotor disk tilts in the opposite direction. The result is a hovering turn around a
vertical axis, midway between the rotors.
1-82. Heading control in forward flight is accomplished by coordinated use of lateral cyclic tilt on both
rotors for roll control and differential cyclic tilt on the rotors for yaw control. Only small changes in
pedal trim are required for changes in longitudinal speed trim or during descents, climbs, and
autorotations.


How to react to incoming 30mm:
Jump up 20 feet and spread yourself out over a wide area
#3215499 - 02/22/11 11:35 PM Re: How does a Chinook yaw? [Re: Flyboy]  
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@Flyboy,

There's so much wrong with the LB2 flight model that the blade movement is only the beginning.

@Raptor,

Good catch. You're right, they aren't coaxial and therefore backs up the statement there aren't coaxial rotor systems in service for the U.S. I guess I got too smart for myself. Thanks for the correction.

#3215526 - 02/23/11 12:11 AM Re: How does a Chinook yaw? [Re: gatordev]  
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Originally Posted By: gatordev
There's so much wrong with the LB2 flight model that the blade movement is only the beginning.


Care to elaborate?

#3215594 - 02/23/11 01:30 AM Re: How does a Chinook yaw? [Re: Flyboy]  
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If you go back to the archives of SimHQ under my username (same username, but different "user" before the "great server crash of SimHQ") you can read some thoughts. A big one is pulling into a flare (nose up) and losing turns in game. Push the nose forward (again, in game) and the turns come back. The exact opposite happens in real life. Another one was how it handled a free-power turbine and autos.

It was a great systems sim for it's time. It's been way too long since I've played it to remember the intricacies of the hellfire in game, but it did have a lot of the right elements, at least for the laser versions. But it was far from "the FLIGHT sim."

#3215632 - 02/23/11 02:39 AM Re: How does a Chinook yaw? [Re: Flyboy]  
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Originally Posted By: gatordev
If you go back to the archives of SimHQ under my username (same username, but different "user" before the "great server crash of SimHQ") you can read some thoughts. A big one is pulling into a flare (nose up) and losing turns in game. Push the nose forward (again, in game) and the turns come back. The exact opposite happens in real life. Another one was how it handled a free-power turbine and autos.

It was a great systems sim for it's time. It's been way too long since I've played it to remember the intricacies of the hellfire in game, but it did have a lot of the right elements, at least for the laser versions. But it was far from "the FLIGHT sim."


The archives only store old topics of the EEAH/EECH forums, there's nothing for LB2. And a search for your username is useless, as the search database is broken.

What do you mean by 'turns'? And by 'free-power turbine' do you mean the APU? What are 'autos'? And what has the Hellfire got to do with anything, we're talking about rotors (and now flight model it seems) here!? smile

#3215811 - 02/23/11 09:08 AM Re: How does a Chinook yaw? [Re: spike_knock]  
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Originally Posted By: spike_knock
Source US Army FM 3-04.203 Fundamentals of Flight; page 1-33, para 1-81, 1-82:

Tandem-Rotor Helicopters
1-81. Heading control is accomplished in tandem-rotor helicopters by differential lateral tilting of the rotor
disks. When the directional pedal (right or left) is applied, the forward rotor disk tilts in the same
direction and the aft rotor disk tilts in the opposite direction. The result is a hovering turn around a
vertical axis, midway between the rotors.
1-82. Heading control in forward flight is accomplished by coordinated use of lateral cyclic tilt on both
rotors for roll control and differential cyclic tilt on the rotors for yaw control. Only small changes in
pedal trim are required for changes in longitudinal speed trim or during descents, climbs, and
autorotations.
Thanks for letting me know which manual to look for! Didn't know such documentation was available on the net!

#3215849 - 02/23/11 11:23 AM Re: How does a Chinook yaw? [Re: Flyboy]  
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Originally Posted By: Flyboy


The archives only store old topics of the EEAH/EECH forums, there's nothing for LB2. And a search for your username is useless, as the search database is broken.

What do you mean by 'turns'? And by 'free-power turbine' do you mean the APU? What are 'autos'? And what has the Hellfire got to do with anything, we're talking about rotors (and now flight model it seems) here!? smile


I didn't realize the site didn't go back anymore. "Turns" is a slang for Nr or rotor speed. As a helo pilot, you always want to keep your "turns" up. An auto is an auto-rotation, when you drive the rotor system w/ the induced air flowing up through the blades rather than driving them w/ an engine. The cost of that is that you lose altitude very quickly. At the bottom of an auto, you recover by pitching the nose back which will increase Nr due to the increased flow through the rotor system (this will happen even if you're operating in powered flight...nose back, Nr increases). For heavier helos with low-inertia heads (like the -60, for example), at the bottom of the auto, you have to eventually rock the nose forward so that you "land" (it will be a very firm landing) level. Rocking the nose forward reduces Nr. Again, this will happen in powered flight as well.

LB2 models this the exact opposite way, what is pretty maddening when you're trying to quick stop in-game.

A free-power turbine is the type of "jet" engine helos (and ships and turbo-props) use. The compressor turbine drives the compressor blades but is not connected to the power turbine, which drives the transmission and makes the rotors go. I seem to recall (and again, it's been a long time) that when you lost both motors in LB2 and had to auto, you had to hit a key which was supposed to "unlatch" the motors from the rotor system (it was different from the rotor brake, in game). Wasn't the rotor brake the "R" key and the "unlatch" function was "Control-R?" Something like that. At least that's how I remember it, so correct me if I'm wrong. Regardless, that's not what happens in modern, turbine-powered American helos. In real life, the rotor system will uncouple from the engines as the engines spool down and stop making power.

The Hellfire comment was in relation to how LB2 got a lot wrong in the flight model portion, but how they modeled a lot of the mission systems very well for the day.

All that said, LB2 still provided me plenty of entertainment back in the day, and in the end, that's all that matters, right?

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