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Old Sep 29, 2007, 02:31 PM
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New Product
1.15 m rotor gas coax.

Walkeras upcoming model LM8.

Main rotor dia. 1150mm.
All up weight 3300g.
Drive system 50A engine.

This thing is BIG!

Looks cool, can't wait to see a video.
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Old Sep 29, 2007, 03:17 PM
I see dumb people
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I'm not sure about that. Why does a coax need a tail rotor? Smells fishy to me.

Thanks,
David
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Old Sep 29, 2007, 03:20 PM
I see dumb people
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Unless the rotors don't provide yaw and just always spin at constant RPMs.
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Old Sep 29, 2007, 04:24 PM
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Quote:
Originally Posted by tkn0spdr
Unless the rotors don't provide yaw and just always spin at constant RPMs.
That's probably it. Single motor to drive both sets of fixed pitch blades with a fixed gear and the belt-driven tail to control the yaw.

Not something I would want to buy, but interesting stuff...
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Old Sep 29, 2007, 05:23 PM
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Doesn't this kinda defeat the purpose of a coaxial design? I mean, eliminating the tail rotor is what all alternative designs are all about (even though the tail rotor is the most popular to this day).

I mean, what, they cancel "most" of the torque with the coaxial design, and leave the tail rotor for yaw corrections? Then this would be a hybrid design, not a pure coaxial.
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Old Sep 29, 2007, 05:53 PM
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It would be extremely inefficient to run a tail rotor with a coaxial design.
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Old Sep 29, 2007, 06:24 PM
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Quote:
Originally Posted by bas10an
Walkeras upcoming model LM8.

Main rotor dia. 1150mm.
All up weight 3300g.
Drive system 50A engine.

This thing is BIG!

Looks cool, can't wait to see a video.
do both of the main rotors spin the same way because that would be the only reason i think there would be a need to have a tail rotor
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Old Sep 29, 2007, 06:55 PM
Lou
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In this design a tail motor would be a good idea, actually in the lama it would be a good idea.

With a single motor contra, the blades would fly ( spin ) at the same or near same rate. That means that you would have no way to correct any slight unwanted tail movement. This movement can be caused by wind, mechanics or what ever.

Add to this a tail rotor that has a constant speed, variable pitch prop and you can adjust the tail. For the most part the tail rotor prop pitch would be adjusted by the servo which would be receiving its instructions based on information generated by a servo. This servo would be a part of a 4n1, 3n1 or it could be a separate servo.

Why is this advantageous? One reason would be that with the main rotors spinning at a near constant rate that when a turn is induced there would be less loss of altitude.

Why you ask? Using the Lama as an example, when you induce a left or right turn, the gyro kicks in and detects a change in tail position. The gyro then induces the necessary voltage change to increase/decrease the power to the two motors for the main rotors. This change in voltage is what reduces the speed of the motor, the loss of rpm/speed is what causes the heli to lose altitude. Of course, the act of turning will also cause some loss of altitude, but when you drop the voltage to a motor, the altitude loss is increased quite a bit.

With both motors spinning constantly because only one motor is being used, then when a turn is induced and the tail position change is detected by the gryo, the correction is made by changing the pitch on the tail rotor. Because the mains are spinning constantly, less altitude is loss during the turn. The corrections are made by the tail rotor, not the main rotors.

This type of heli is what I have posted serveral times about. Single motor main rotors and a tail motor. Put an electic motor in this babe and they will have a winner; add linkage and an upper swash and this heli would be an Olympic champ.
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Old Sep 29, 2007, 07:28 PM
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Even with two fixed-speed rotors, there would still be altitude loss upon rolling, also because one of the rotors would change cyclic pitch, while the other can't. You can also yaw without losing height just by increasing the speed of a motor while decreasing the other's, thus cancelling excessive altitude losses.

The "correct" way of eliminating the height loss upon yawing would be to have a proper double CP system and a differential collective pitch contol on both upper and lower rotors like e.g. on the Kamov helicopter, too bad that it's very complex to implement economically on a scale model.

A hybrix coax-tail rotor design is surely simpler to design and build, but it seems to me that it just brings the WORST of two worlds together: all inherent weaknesses of a tail rotor, plus the inherent limitations in speed and maneuverability of a FP coaxial rotor system with only one swashplate.

They probably chose this solution because a coax gas heli with different rotor speeds would be very hard to implement -again, the "correct" solution being a double differential CP control -.
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Last edited by Velktron; Sep 29, 2007 at 07:50 PM.
Old Sep 29, 2007, 08:04 PM
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Imagine the Toilet Bowl
OUCH!
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Old Sep 29, 2007, 09:27 PM
Lou
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Velktron, I agree with what you are saying. I am just pointing out that 'one' of the reasons for altitude loss is the power reduction induced onto the mains as a result of the gyro altering voltage to compensate for tail movement.

I took my Lama and did a program mix on my Optic 6 so that every time in commanded a left/right turn power was increased. This did reduce the altitude loss to near zero but the heli became a bit more effort to have to fly.

My take is that if you did the same thing with this new Walkera, that with it being a single motor driven contra that you could reduce the loss from a turn quite a bit. Of course that is just theory .... I don't have one yet to try this with.

By the way .... any idea why this thread was moved? I would think that regardless of power system, it is still a multi-blade heli. Seems to me that the multi-blade forum was appropriate.
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Old Sep 29, 2007, 11:26 PM
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The equally contra rotating blades will produce an extremely stable flying platform with virtually no yaw. This now needs the tail rotor to induce yaw, no gains but you lose the gyro, Now there is no need for a gyro,


It is in fact simpicity in in motion.
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Old Sep 30, 2007, 05:34 AM
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Why has this thread been moved here to F/S wanted?
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Old Sep 30, 2007, 08:31 AM
Lou
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Don't know why it was moved, ask a mod to move it back to the multi-rotor forum.
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Old Sep 30, 2007, 12:38 PM
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Quote:
Originally Posted by HELIRLC
The equally contra rotating blades will produce an extremely stable flying platform with virtually no yaw. This now needs the tail rotor to induce yaw, no gains but you lose the gyro, Now there is no need for a gyro,
Even with mechanically linked contra-rotating motors with fixed relative speed, that still isn't a 100% guarantee of having 0% torque, so a gyro or at least an elementary trim control would be required.

Don't forget, mechanical systems are always imperfect, blades do not produde exactly 100% opposite torque, the (fixed) pitch on both blades isn't exactly the same etc. plus the lower rotors can change cylic pitch while the upper ones can't, thus any cyclic input would automatically disrupt the torque balancing.

Don't forget, real coaxial helicopters yaw by changing torque at constant rotor speed, by altering collective pitch, and messing around with one of the rotors is guaranteed to disrupt torque. So, all in all, you'd still need a gyro.

I also would like to know how they plan to implement the tail rotor drive system: since it has only trim/yaw corrections to perform, while the bulk of the torque would be canceled out by the coax system, it can be relatively less powerful that the tail rotor of a single-rotor heli.

From the included pics, it appears to be a continuously engaged CP tail rotor system (perhaps with lower speed). This means that it's as complex and delicate as a traditional single-rotor helicopter, although it requires less power for yaw control and anti-torque balancing.
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