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Sep 16, 2020, 09:26 AM
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DominicM's Avatar
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British Airways Boeing CH-47 Chinook


The first of two flights of this Chinook I filmed at an awesome scale helicopter event. Unfortunatley on this flight it was later discovered one of the rear blades have been installed the wrong way around. Still flew OK-ish but I guess had the potential for disaster given the noticeable vibration that you can see especially on the slow mo. Flight two soon.

WOW !!! BOEING CH-47 CHINOOK TANDEM ROTOR RC ELECTRIC HELICOPTER (5 min 33 sec)
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Sep 16, 2020, 04:00 PM
www.scotiarc.co.uk - OMP Helis
Smoggie's Avatar
Wow, they were lucky, the blade was vibrating like crazy. Would have been a shame to loose a model as special as that to something so daft.
Sep 16, 2020, 10:15 PM
rc user
pdooley's Avatar
that thing was vibrating like a $29 HK450 kit.
indeed lucky it didn't come apart.
Sep 17, 2020, 01:46 AM
Registered User
Atomic Skull's Avatar
That's a Model 234 not a CH-47.
Nov 03, 2020, 10:36 AM
Registered User
How does a flight controller work on a heli that has no tail and two swashplates? Maybe two, each rotor with its own?
Nov 04, 2020, 05:25 PM
Registered User
Atomic Skull's Avatar
Quote:
Originally Posted by HeliFlyer14
How does a flight controller work on a heli that has no tail and two swashplates? Maybe two, each rotor with its own?
It has to support 4 servos and a custom swashplate mixer.

First understand that on a tandem there is no elevator cyclic axis as such. Tandem helis only need aileron cyclic (Current CH-47's do have elevator trim but the first generation CH-47 had only aileron cyclic) Elevator is by collective difference between the front and rear rotor, aileron cyclic works as normal and yaw is by opposing aileron cyclic.

On the 3DIGI you set up a 4 servo swashplate in an "X" arrangement, the top two legs of the X are the front swashplate servps and the bottom two legs are the rear swashplate servos.

So in the case of the 3DIGI, you set up a custom "X" shaped 4 servo swashplate and then mix in the yaw channel output from the rudder channel into the aileron servos so it creates aileron cyclic in opposite directions at the front and back. When you input forward elevator on the stick the servos at the front move down and the ones at the back move up and when you input aileron the two servos on each side move together. When you input rudder the two rotors push in opposite directions at the front and back rotating the aircraft.To the FBL it just looks like one big rotor (you also need to disable piro compensation but a tandem doesn't need that anyway)

So the swashplate on a tandem only has the aileron axis. RC tandem models usually use "Y" links to connect a three servo swashplate up to two servos so that it has collective and aileron but no elevator.

Early full scale CH-47s only had aileron but later revisions added "longitudinal trim actuators" which is basically elevator cyclic but it has nothing to do with directional control. It is used to compensate for the CH-47's tendency to hover nose high by bringing the nose down and also to bring the nose up during fast forward flight in order to reduce drag. This is handled by the avionics and the pilot does not control it directly.

Elevator cyclic actually doesn't work for directional control on a tandem, it won't cause the machine to move forward it'll just bring the nose up or down and cause it to drift foward or back. That's why they use differential collective for elevator control instead of elevator cyclic.
Last edited by Atomic Skull; Nov 04, 2020 at 05:38 PM.
Nov 04, 2020, 06:19 PM
Registered User
Hey, thanks for the great explanation. Maybe someday I'll build up a scale model and refer back to this for setting up the controller.
Nov 05, 2020, 12:35 PM
rrg
rrg
Registered User
rrg's Avatar
Wow tracking get ya every time.....
May 29, 2021, 11:16 PM
Registered User
hookpilot's Avatar
Quote:
Originally Posted by Atomic Skull
It has to support 4 servos and a custom swashplate mixer.

First understand that on a tandem there is no elevator cyclic axis as such. Tandem helis only need aileron cyclic (Current CH-47's do have elevator trim but the first generation CH-47 had only aileron cyclic) Elevator is by collective difference between the front and rear rotor, aileron cyclic works as normal and yaw is by opposing aileron cyclic.

On the 3DIGI you set up a 4 servo swashplate in an "X" arrangement, the top two legs of the X are the front swashplate servps and the bottom two legs are the rear swashplate servos.

So in the case of the 3DIGI, you set up a custom "X" shaped 4 servo swashplate and then mix in the yaw channel output from the rudder channel into the aileron servos so it creates aileron cyclic in opposite directions at the front and back. When you input forward elevator on the stick the servos at the front move down and the ones at the back move up and when you input aileron the two servos on each side move together. When you input rudder the two rotors push in opposite directions at the front and back rotating the aircraft.To the FBL it just looks like one big rotor (you also need to disable piro compensation but a tandem doesn't need that anyway)

So the swashplate on a tandem only has the aileron axis. RC tandem models usually use "Y" links to connect a three servo swashplate up to two servos so that it has collective and aileron but no elevator.

Early full scale CH-47s only had aileron but later revisions added "longitudinal trim actuators" which is basically elevator cyclic but it has nothing to do with directional control. It is used to compensate for the CH-47's tendency to hover nose high by bringing the nose down and also to bring the nose up during fast forward flight in order to reduce drag. This is handled by the avionics and the pilot does not control it directly.

Elevator cyclic actually doesn't work for directional control on a tandem, it won't cause the machine to move forward it'll just bring the nose up or down and cause it to drift foward or back. That's why they use differential collective for elevator control instead of elevator cyclic.
All CH-47s had LCTs, (Longitudinal Speed Trims) Not used to compensate for nose high hover attitude. Used to maintain level fuselage attitude to reduce drag at high speed and more importantly to reduce stress on aft vertical shaft at higher airspeeds and blade flapping angles. LCTs can be controlled by the pilot, but as you say, normally left in AUTO mode. What do you mean by elevator cyclic? Not a term used on full size Chinooks. Very interesting aerodynamics in one of these. All models also had to incorporate an extensible link in the flight controls to give the helicopter what is called positive stick gradient. Without it you would find yourself first pushing forward on the cyclic to accelerate, then once at speed you'd have to bring the cyclic back to neutral to maintain a set airspeed. A lot of black boxes were needed to make a Chinook feel like a normal helicopter. Great conversation on the Chinook by the way. If you'd like to really dig into the complexities of the flight control systems on a CH-47D, take a look here:http://www.chinook-helicopter.com/st...t_Controls.pdf
Last edited by hookpilot; May 30, 2021 at 07:21 PM. Reason: add link
May 30, 2021, 03:33 PM
Registered User
Woga65's Avatar
Having built a few dual rotor helicopters during the last couple of month, I can confirm that the flight controller works very much like it works on a quadcopter except for the yaw axis which is controlled by steering the nose and tail into opposite directions.

On my builds there are three servos per swashplate. Please note that those are redundant, they just move up and down with the collective. I kept the elevators servos just because i did not want to modify the swashplates and servo mounts.

INAV powered Micro Variable Pitch Bicopter - testing LIDAR / Terrain Follow Mode (2 min 36 sec)

Micro Variable Pitch Bicopter - The Yaw Question (1 min 49 sec)

INAV powered Twin Rotor Helicopter (1 min 30 sec)
Oct 12, 2021, 07:36 AM
Registered User
The blades are not vibrating. It is the stroboscopic effect of a poorly tracking rotor heads. The 3 blades just not turning at the same plane and the image was frozen by the fast camera shutter under bright sunlight.
Oct 18, 2021, 10:17 PM
Registered User
Atomic Skull's Avatar
Quote:
Originally Posted by hookpilot
What do you mean by elevator cyclic? Not a term used on full size Chinooks.
I mean the tandem equivalent to that which is increasing collective on one disk while decreasing it on the other. I said "elevator cyclic" to make that easier to understand to people who don't know how a tandem differs from a single rotor helicopter. If I said something like "longitudinal collective pitch differencing" people wouldn't know what I meant.
Last edited by Atomic Skull; Nov 24, 2021 at 03:52 AM.
Old Nov 02, 2021, 05:46 AM
BladeRunn
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Nov 17, 2021, 10:18 AM
AeroDan
Quote:
Originally Posted by Atomic Skull View Post
It has to support 4 servos and a custom swashplate mixer.

First understand that on a tandem there is no elevator cyclic axis as such. Tandem helis only need aileron cyclic (Current CH-47's do have elevator trim but the first generation CH-47 had only aileron cyclic) Elevator is by collective difference between the front and rear rotor, aileron cyclic works as normal and yaw is by opposing aileron cyclic.

On the 3DIGI you set up a 4 servo swashplate in an "X" arrangement, the top two legs of the X are the front swashplate servps and the bottom two legs are the rear swashplate servos.

So in the case of the 3DIGI, you set up a custom "X" shaped 4 servo swashplate and then mix in the yaw channel output from the rudder channel into the aileron servos so it creates aileron cyclic in opposite directions at the front and back. When you input forward elevator on the stick the servos at the front move down and the ones at the back move up and when you input aileron the two servos on each side move together. When you input rudder the two rotors push in opposite directions at the front and back rotating the aircraft.To the FBL it just looks like one big rotor (you also need to disable piro compensation but a tandem doesn't need that anyway)

So the swashplate on a tandem only has the aileron axis. RC tandem models usually use "Y" links to connect a three servo swashplate up to two servos so that it has collective and aileron but no elevator.

Early full scale CH-47s only had aileron but later revisions added "longitudinal trim actuators" which is basically elevator cyclic but it has nothing to do with directional control. It is used to compensate for the CH-47's tendency to hover nose high by bringing the nose down and also to bring the nose up during fast forward flight in order to reduce drag. This is handled by the avionics and the pilot does not control it directly.

Elevator cyclic actually doesn't work for directional control on a tandem, it won't cause the machine to move forward it'll just bring the nose up or down and cause it to drift foward or back. That's why they use differential collective for elevator control instead of elevator cyclic.
Wow!!!!!!!! Did you work for/or with Frank N. Piasecki?
Nov 24, 2021, 03:53 AM
Registered User
Atomic Skull's Avatar
Quote:
Originally Posted by Bell47G2
Wow!!!!!!!! Did you work for/or with Frank N. Piasecki?
No just what I've read.


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