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Multi Rotor Helicopters - Tandem Multi Rotor Helicopters - Tandem


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Old 02-14-2008, 12:48 PM   #1
Wenlock
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Default Strange tandem behaviour

Iíve just finished building a ďpod-and-boomĒ type tandem heli made from two Sceadu Evo 50 chassis linked by an alu/CF lattice frame. Iím using the Tech-MP TH-2 mixer, and I have standard 2-bladed Evo heads with flybars. At the moment I have no gyros on board.

Iíve just put the beast through its first couple of flights with training gear fitted, to try to get it dialled in, and Iíve come across some confusing flight characteristics (well, itís confusing for someone whoís used to helis with one big fan on top and a little one at the back). I wonder whether any of the Tandem Gurus could shed some light on what seems to be happening?

If I pull up into a hover and put in some rudder, the heli slowly starts to yaw, and the rate of yaw builds up nicely if I hold the stick in the same position. Once the heli is turning, I can centre the stick and it tends to just keep on turning. I have to put in some opposite rudder to stop it from turning. Is this normal?

In slow forward flight, or facing into a breeze, it will suddenly and without warning pitch hard up or hard down. It requires a quick and heavy input on the stick to bring it level again. I think this is something to do with having the rotor heads at the same level, IIRC this is why full-sizers have the heads at different levels, to prevent the forward head causing turbulence for the rear head to fly into. I donít have the option to raise the rear head, so I guess I might need a gyro on the elevator axis.

The thing that has me really scratching my head is this: If I pull up into a hover and put in some right rudder, the heli turns to the right and the nose drops; if I put in left rudder it turns to the left and the tail drops. Iíve managed to more or less null this out using a rudder-to-elevator Tx mix, but it seems weird, I canít figure out why this should happen. I have to use a lot of rudder to elevator mix to stop it (~50%). Anyone have any idea why this happens?
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Old 02-14-2008, 02:02 PM   #2
JKos
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This will be a total guess, but, hey, why not... I can think of two ways to produce yaw with tandem rotors. One is differential torque (by changing relative collective) and the other with differential aileron (opposite aileron inputs on each rotor). It sounds like you may have some differential torque going on.

When you give rudder inputs, what do the swashplates do? Look carefully for any change in collective. What options do you have in the software for the mixer?

- John
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Old 02-14-2008, 04:42 PM   #3
joe@tech-mp
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Wenlock,

Both the yaw axis and pitch axis require a gyro.

I would recommend the GY240. Mount them as close to the CG as possible.

Setup the pitch axis gyro on the DCP circuit for best results. See our Twinn Rexx documents for more info on setup http://www.tech-mp.com/Twinn_Rexx_support.htm


Joe

www.techmodelproducts.com
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Old 02-18-2008, 03:05 AM   #4
Wenlock
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Originally Posted by JKos View Post
This will be a total guess, but, hey, why not... I can think of two ways to produce yaw with tandem rotors. One is differential torque (by changing relative collective) and the other with differential aileron (opposite aileron inputs on each rotor). It sounds like you may have some differential torque going on.

When you give rudder inputs, what do the swashplates do? Look carefully for any change in collective. What options do you have in the software for the mixer?

- John

I thought it might be differential torque, but the swashes just tilt when I put in some rudder - one side up, the other down by the same amount. No change in collective.
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Old 02-18-2008, 03:32 AM   #5
Wenlock
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Originally Posted by joe@tech-mp View Post
Wenlock,

Both the yaw axis and pitch axis require a gyro.

I would recommend the GY240. Mount them as close to the CG as possible.

Setup the pitch axis gyro on the DCP circuit for best results. See our Twinn Rexx documents for more info on setup http://www.tech-mp.com/Twinn_Rexx_support.htm

Joe

www.techmodelproducts.com
Hi Joe,

Yep, I think gyros would be a good idea! I just wanted to see which axes really needed to have gyros, and I think I have my answer now! I must say, I was expecting the bird to have a bit more natural stability than it actually has.

I have one GY240 sitting on the shelf, but can't get another one, no retailers in the UK seem to have any at the moment. Do you think I'd be ok using a 401 on the DCP input, seeing as that needs to be HH? I have a 401 on the shelf and a spare channel for the gyro gain input.

Thanks for the link to the Twin Rexx docs, I sometimes think I should have built one of these babies and made life a bit easier!

Last edited by Wenlock; 02-18-2008 at 03:41 AM.. Reason: Bad formatting
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Old 02-18-2008, 08:43 AM   #6
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Wenlock,

I would put the GY401 on Yaw and the GY240 on the DCP. Many are using this setup with good results so far.


Joe

www.techmodelproducts.com
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Old 02-18-2008, 11:18 PM   #7
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Quote:
Originally Posted by Wenlock View Post
If I pull up into a hover and put in some rudder, the heli slowly starts to yaw, and the rate of yaw builds up nicely if I hold the stick in the same position. Once the heli is turning, I can centre the stick and it tends to just keep on turning. I have to put in some opposite rudder to stop it from turning. Is this normal?
I don't feel like I can comment on most of your conserns but I think I can offer an opinion on this one. I've got one of the old Hirobo Vertol 30 size birds. I haven't flown it in some time but I do remember noting the same thing you're talking about here and this is my take on it.

With a conventional single rotor / tail rotor design, if you nose it over gently from a hover position it will start to accelerate slowly and pick up speed as it goes. If you reverse the cyclic it will take it time to slow back to a hover. Since a yaw command in this case, is basically a differencial cyclic command (right on the front swashplate, left on the rear and vice versa), you're experiencing basically the same thing. A yaw command starts the turn and as long as you hold the command, the faster the yaw rate becomes. Reversing the control imput, especially after you've built up some momentum, takes a bit of time for the yaw rate to decelerate and stop. It will never be as crisp as a conventional tal rotor machine in terms of how fast it will start and stop but it works pretty good nontheless once you get used to it.

FWIW!
Good luck & Fly Safe,
Steve R.
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Old 02-25-2008, 02:28 AM   #8
Wenlock
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Originally Posted by Intrepid175 View Post
With a conventional single rotor / tail rotor design, if you nose it over gently from a hover position it will start to accelerate slowly and pick up speed as it goes. If you reverse the cyclic it will take it time to slow back to a hover. Since a yaw command in this case, is basically a differencial cyclic command (right on the front swashplate, left on the rear and vice versa), you're experiencing basically the same thing. A yaw command starts the turn and as long as you hold the command, the faster the yaw rate becomes. Reversing the control imput, especially after you've built up some momentum, takes a bit of time for the yaw rate to decelerate and stop. It will never be as crisp as a conventional tal rotor machine in terms of how fast it will start and stop but it works pretty good nontheless once you get used to it.

Thanks for your reply, Steve. Yes, I guess this makes sense, once the model is turning there's no reason it should stop until you do something to oppose the rotation. I think I was a bit surprised by how much input was required to get it to stop turning, but I suppose a twin-rotor machine has a lot more rotational inertia than a single because most of the mass is so far from the CoG. Once it gets turning, it just wants to keep going.

There's an awful lot to learn about these twin-rotor machines!
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Old 02-26-2008, 05:37 PM   #9
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Wenlock, To my big surprise, you can fly it without gyro! Your flying skill is too great!

Adding 2 gyro save your brain energy to enjoy the control much better.
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