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Old 01-20-2008, 03:51 PM   #1
Gra55h0pper
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Default Tandem theory on forward/aft cyclic differential

I was wondering what the theory is on having differential between the forward/aft cyclic (i.e. in the elevator plane) between the front and the rear rotor?

I see three possibilities:
1. both rotorwashes are pointed straight down
schematic side view: | |

2. the rotorwashes diverge
schematic side view: / \

3. the rotorwashes converge
schematic side view: \ /

I currently have my Twinn-Rexx setup according to #1.

Looking at #2 however, it seems that this gives less potential for the rotorwashes to start interacting causing what's called "rotor vortex interference".

However, when looking at pictures of the full-size chinook, it seems #3 may be the case (???). See the attached pic and the drawing in which the red-lines indicate the differential (not really in cyclic but actually in the angle of the rotor shaft. I guess the effect is the same though(?)).

I don't understand what would be the benefits of this setup though? Any ideas/references? Did anybody experiment with this on the Twinn-Rexx?

H0pper.
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Old 01-20-2008, 09:33 PM   #2
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On a full size 'hook, the forward shaft is canted forward 9*, and the aft is canted forward 3* from the centerline of the aircraft. You are correct -- they do converge.

I have two guesses as to why this might be. First is forward flight efficiency. Well, at least that's why they are both canted forward. Maybe the front does more pulling, while the aft does more lifting. That could keep it more stable, too. A wagon or boat pulled from the front is more stable than one pushed from behind.

My second idea is hover stability. Imagine a pencil with a string tied to each end. If you lift the pencil by holding both strings straight up, you can move the pencil back and forth pretty easy. But if you lift the pencil by pulling the strings outward, the pencil can't move as much. Perhaps it's the same with helicopters and rotorwash?

Since I'm taking wild guesses, I'm just going to start making stuff up now ....

Front rotor vortex interference --- As the helicopter increased forward airspeed, there is a point where the front rotor downwash starts to affect the rear downwash. Maybe the aft rotor needs to make up for this by having a more downward component of thrust than the front? If this were the case, I would expect it to need more than 6*, though.
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Old 01-20-2008, 10:04 PM   #3
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The full scale CH47 has 9 deg tilt forward on the front rotor and 4 deg. tilt forward on the rear rotor. This info is from the Boeing CH47 blueprint here: http://www.tech-mp.com/Twinn_Rexx_do...7D_Drawing.jpg

I have chatted with numerous CH47 pilots over the years and they all said the front rotor is tilted forward more for ground handling purposes.

The Twinn Rexx has both rotors tilted 5 deg forward relative to the body. They are parallel to each other for practical reasons (weight & complexity) and since it dosen't seem to hurt the flying quality at all.


Forward flight would not be possible without DCP. With just elevator cyclic, the heli could hover fine but would not be able to break free of the FRVI (front rotor vortex interence). DCP allows it to get up and go


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Old 01-20-2008, 10:05 PM   #4
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On a related topic ... If you don't already know this, you may find it interesting:

The Twinn Rexx is set up with the cyclic stick controling the elevator funtion of the swashplates. Differential collective pitch is slaved off of this, and only in a small amount. Full scale Chinooks don't use "elevator" -- they are purely DCP.

However, full size 'hooks have a function called LCT - Longitudinal Cyclic Trim. There is an actuator on each swashplate that will tilt the swashplate forward and aft, depending on the flight condition. Early chinooks hovered very nose high, and flew nose low. Nose low forward flight is very inefficient, due to the large surface area being dragged through the air. The LCTs trim for this. Ar a hover, the rotor disks tilt back, more parrallel with the airframe, to have a level hover. As the aircraft nears cruising speed, the LCT's trim the disks forward, so they are pulling forward, rather than up. When the aircraft is on the ground, the LCTs trim the rotors partway forward, to aid in ground taxiing.

One of the many things I want to experiment with the Twinn Rexx is to have the cyclic stick control only DCP and set the elevator to either the Gear or Flight Mode switch. I think I know how to do it, too. I'll leave it to Joe to say why not to.
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Old 01-20-2008, 10:14 PM   #5
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Red face

Quote:
Originally Posted by joe@tech-mp View Post
The full scale CH47 has ... 4 deg. tilt forward on the rear rotor. This info is from the Boeing CH47 blueprint here: http://www.tech-mp.com/Twinn_Rexx_do...7D_Drawing.jpg
Ooops :redface:
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Old 01-20-2008, 10:21 PM   #6
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chinookmark,

Both myself and Walt Ferar experimented with different ratios of cyclic elevator and DCP years ago with the Dragonfly Pro tandem. We found that the heli would fly ok with just DCP but had pitch-up tendancies in certain flight attitudes and was harder to fly in windy conditions. When we blended in the cyclic, which caused the rotor discs to follow the direction of flight, it made a huge difference. We found that a blend of DCP and cyclic elevator worked best for our 2 blade flybar heads.

What is best for 3 blade heads at this point is TBD

Joe

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Old 01-20-2008, 10:34 PM   #7
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Joe, Chinookmark - That's very interesting stuff! Didn't know the full size chinook effectively doesn't use elevator control, but only uses DCP. Also the info on LCT is very interesting.

Note that on the Twinn-Rexx, one thing I've found is that indeed DCP is way more effective than elevator. The manual calls for intitially mixing DCP being 35% of elevator (CH5 ATV is set to 50/50 and mixing gain is set at 70/70). I've increased my DCP to 60% of elevator and find that gives a more stable helicopter and virtually eliminates FRVI.

Guess all this means we're just going to *have to* do more experimenting on the Chinook...
Here's my plan (need some daylight though...):
1. see what difference it makes when giving the front rotor a bit of forward cyclic, and the rear rotor a bit of aft cyclic, effectively having the rotorwashes converge a bit. I plan to give both elevator linkages one turn each. Following Chinookmark's theory, this may make forward flight more efficient and potentially make the hover more stable...
2. as Chinookmark suggests, put DCP only on the Tx elevator stick and put elevator on another switch or dial. Doing this might enable us to change the hover attitude (nose-high, horizontal or tail-high) using elevator. I guess the theory for a nose-high hover would be to give it a little bit of aft DCP (i.e. increase the CP on the front rotor) and then compensate for the resulting drift backwards by giving it forward cyclic on the elevator. Hmmm, not sure how I'm going to set this up on my DX7 without having a dial for the elevator...
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Old 01-20-2008, 11:35 PM   #8
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this week ill take pic's of the aft and fwd pylon so u can see how the acual servos and the Lct is

but to tell you each rotor sys has 2 servos pivioting and swiveling
it other words the aile and pitch servo
they move together to give collective trust

and as u said dcp for fwd and aft movement with the lct

i have lost of learning matirail on how a ch-47 operates
so if needed
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Old 01-22-2008, 07:24 PM   #9
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this it the 1:1 Ch47D rotor and control system
most these pics are a chinook that is in for a 400hrs phase maint

the black item were thr RC elev would be is the LCT this is the left side of the fwd head

right side fwd head

fwd head

aft rotor left side the swashplate bearing seal removed for inspection

left side aft rotor

aft rotor right side

fwd rotor head (aft is just a mirror image)

second stage mixing (its how the aft head gets controlled)
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Old 01-22-2008, 07:57 PM   #10
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cont...............

the steel controls are for tha aft rotor (Pitch, roll, and thrust go through these controls)

control closet behind pilots

this is what drives the big beast. thier is 4 servos roll, pitch, yaw, thrust
they are acuated by the pilot and the advance flight control computers
thier called ILCA (intergrated lower control acuator)

first stage mixing

fwd head installed

aft swash with acuator blocks (keeps acuators extended)

50,000lbs max gross capable of 180 kts 37 troops (or standing room only)
and yes it is acrobatical (for the brave)
26000lbs external load (capable of slingloading its self "havent seen it but in theory it can")
fastest Army" helicopter
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Old 01-22-2008, 08:15 PM   #11
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Quote:
Originally Posted by chinookmark View Post
On a full size 'hook, the forward shaft is canted forward 9*, and the aft is canted forward 3* from the centerline of the aircraft. You are correct -- they do converge.

I have two guesses as to why this might be. First is forward flight efficiency. Well, at least that's why they are both canted forward. Maybe the front does more pulling, while the aft does more lifting. That could keep it more stable, too. A wagon or boat pulled from the front is more stable than one pushed from behind.

My second idea is hover stability. Imagine a pencil with a string tied to each end. If you lift the pencil by holding both strings straight up, you can move the pencil back and forth pretty easy. But if you lift the pencil by pulling the strings outward, the pencil can't move as much. Perhaps it's the same with helicopters and rotorwash?

Since I'm taking wild guesses, I'm just going to start making stuff up now ....

Front rotor vortex interference --- As the helicopter increased forward airspeed, there is a point where the front rotor downwash starts to affect the rear downwash. Maybe the aft rotor needs to make up for this by having a more downward component of thrust than the front? If this were the case, I would expect it to need more than 6*, though.
one of the main reasons for the tilt in the fwd rotor is for ground taxi
and also when at a hover both rotor heads are mostly on the same plane
wich imakes the rotor disks almost level with the ground. and yes the aft rotor does provide more lift than the fwd and yes the blades do cross planes
i will ask my maintanace pilots about the rotor wash thing
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Old 02-06-2008, 07:32 PM   #12
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Thanks for the trip down memory lane, I still remember the first time I rigged a "D" Model.....







Saudi, Christmas 1990. Heading out to deliver turkeys to the front line troops. That's me on the end, "Quick Draw McGraw" with the bayonet.
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Old 02-06-2008, 07:40 PM   #13
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Sorry for the OT response...

By the way, any of you guys able to get a -10 manual? Maybe an older one for the "D"?

When I left Ft. Bragg in Sep 1991, I never envisioned the need or want for one 17 years later....
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Old 02-07-2008, 05:21 AM   #14
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Hey Chris,

Send me a PM with an address. No longer in need of a D model -10, flying the F model now, and that pub has no restrictions on distrubution.
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Old 02-09-2008, 12:21 AM   #15
sokal
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hey sir i just would like to say NO FAIR
ME WANT F MODEL
im in the 5/159th avn at eustis VA
im a reserve/ mil tech
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Old 02-09-2008, 06:45 AM   #16
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The F model has some nice features and the new paint looks good but it's still a chinook. VHP seals still blow, transmissions still chip and to add to the maintenance down time there is a butt load of avionic "anomolies" that very few people have any experience with. To be honest, I would like my D models back.
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Old 02-09-2008, 09:41 AM   #17
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All I want is the electric hydraulic pump.
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Old 02-11-2008, 04:25 PM   #18
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yes the PT stick sucks
more so when u have an apu prob and trial and error with eletrical guys
all day
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Old 09-01-2010, 02:17 PM   #19
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Quote:
Originally Posted by Gra55h0pper View Post
Joe, Chinookmark - That's very interesting stuff! Didn't know the full size chinook effectively doesn't use elevator control, but only uses DCP. Also the info on LCT is very interesting.

Note that on the Twinn-Rexx, one thing I've found is that indeed DCP is way more effective than elevator. The manual calls for intitially mixing DCP being 35% of elevator (CH5 ATV is set to 50/50 and mixing gain is set at 70/70). I've increased my DCP to 60% of elevator and find that gives a more stable helicopter and virtually eliminates FRVI.

Guess all this means we're just going to *have to* do more experimenting on the Chinook...
Here's my plan (need some daylight though...):
1. see what difference it makes when giving the front rotor a bit of forward cyclic, and the rear rotor a bit of aft cyclic, effectively having the rotorwashes converge a bit. I plan to give both elevator linkages one turn each. Following Chinookmark's theory, this may make forward flight more efficient and potentially make the hover more stable...
2. as Chinookmark suggests, put DCP only on the Tx elevator stick and put elevator on another switch or dial. Doing this might enable us to change the hover attitude (nose-high, horizontal or tail-high) using elevator. I guess the theory for a nose-high hover would be to give it a little bit of aft DCP (i.e. increase the CP on the front rotor) and then compensate for the resulting drift backwards by giving it forward cyclic on the elevator. Hmmm, not sure how I'm going to set this up on my DX7 without having a dial for the elevator...
Hi Gra55hOpper:

I know this was written 2 years ago... but im trying to solve my tandem rear rotor down reaction and found your post.
Questions: how did you solve it? Im using TH2 on a homemade 500 size kind-of-TwinRex, and a 7 ch JR radio.
Both gyros on rate mode ( feels better) and DCP 20% from elevator internal TH2 setup.
If i configure ch5 for DCP, have to Y wire both gyro gain wires ( same gain for both ).
May be I need another radio...
Appreciatte your comments on pitch, DCP, elevator suggested settings.
Here is a video where the tail down reaction can be seem. ( taken 1 day before striking rear blades)
[ame]http://www.youtube.com/watch?v=KhQBTqwoXPM[/ame]

Jalex
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Old 06-03-2017, 07:43 AM   #20
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Old thread but thought I would add something. I used to work for a company called United Scale Models in Concordville, PA. We built 1/25th scale training models that had all of the control working. The links were color coded so when you moved the cockpit controls you could see how the outputs through the mixing box got mixed.
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