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Old Jun 25, 2011, 08:33 AM
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Asymmetrical Flybar Travel on the Big Lama - Related to Death Dives?

I just changed the inner shaft/upper head to the Xtreme +15mm. I hadn't noticed this before, but I'm pretty sure its not related to the new head. There is a huge difference in the amount of flybar (seesaw-type) travel between the ball connector UP versus connector DOWN positions See photos. With the connector down it is the connector itself impinging on the blade grip that stops the travel. Disconnecting the connector eliminates the asymmetry. I'm wondering if there are different grips that don't have this problem.

It seems the consensus on the death dive (stall) issue was the flybar reaching the limit of its travel. I've heard that shortening the flybar eliminates the problem, but I'm wondering if fixing the flybar travel asymmetry doesn't also have some benefit.

Russ
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Old Jun 25, 2011, 10:54 AM
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The death dive occurs because the coax heli has reached the limit of it's ability to bank and starts sloughing off air from under the blades. A coax heli has a very long, vertical CoG, with lots of weight at the bottom (battery). It's like a plumb-bob on a long string, it does not want to move from vertical. The death dive occurs because you ae trying to bank more than the CoG will allow and the lower blades have reached the point where they can no longer sustain lift without being able to bank further. You objective is to prevent the heli from reaching this point, by providing more movement authority to counter the CoG, damping head movement further so it can't get to this point, shortening the CoG so it has less authority, or lengthening the CoG so it has more authority.

I think you will find most people address this condition by the latter method... they do a number of things that are likely to cause more blade strikes, which they address by installing a longer inner shaft. The longer inner shaft increases the CoG, giving it more authority.

FYI, all the upper rotor does is damp head movement and make the CoG longer. You can get coax death dive on some self-stabilizing single rotor helis, like the CB180; same physics involved.
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Old Jun 26, 2011, 07:26 AM
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Originally Posted by Balr14 View Post
You can get coax death dive on some self-stabilizing single rotor helis, like the CB180; same physics involved.
I've heard that about the CB180, which I also have. Thanks for the detailed answer.

I'd still like to know if anyone has addressed the flybar issue I mentioned, or if it's simply not an issue for flight. Also if aftermarket grips like skytec, for example, would be any different.

Russ
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Old Jun 26, 2011, 08:31 AM
"whirlydude"
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Russ, have you tried installing another link on the opposite side of the flybar? This way the travel is equal on both sides, and may help the problem. I have did a little bit of experimenting on this, and have found that it worked rather well using 2 links on the Xtreme extended head, but I prefer using only 1 on the Skytec upper rotor head. One nice thing about the Skytec upper rotor head assembly is one doesn't have to deal with the plastic molding on the flybar that seems to get chewed up from the head after being used for a while. Take a look at the pictures and you will see what I talking about. I believe Skytec has a much better design for an upper head assembly.

edit: I forgot to add the the travel is equal on the Skytec rotor head using only 1 ball link rather than 2. It may not look like it in the pics, since the angle was slightly different, but I have checked the travel distance a few times, and it appears to be the same using just the 1 link.
One other thing that I forgot to add is that I went to a longer ball stem on the upper blade grips to help prevent the link from binding on the grips when at full travel.
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Old Jun 26, 2011, 09:22 AM
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@ Whirlydude - Wow, nice heli - is that BL motors I see? You have the complete Skytec set (except maybe the swashplate?) Looks like the stock inner shaft length?

Thanks for the advice about the links - Ill order some more. I think I'm going to try out the Xtreme head for a while before I go all the way (to skytec that is.) I have some on my CX3 and I love it.

Russ
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Old Jun 26, 2011, 09:59 AM
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Quote:
Originally Posted by rampa202 View Post
I've heard that about the CB180, which I also have. Thanks for the detailed answer.

I'd still like to know if anyone has addressed the flybar issue I mentioned, or if it's simply not an issue for flight. Also if aftermarket grips like skytec, for example, would be any different.

Russ
Any self-stabilizing heli with a long CoG and trailing offset flybar is subject to this effect.

Sorry, I thought I had addressed your flybar movement question, indirectly. From what I have observed, using two links on the flybar causes more resistance and restrictions to flybar movement. That would translate into less movement authority for the heli, making it less likely to reach the point where the blades start sloughing off air. Whether or not the ability of the flybar to travel more in one direction than the other, could trigger this condition, is debatable.
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Old Jun 26, 2011, 10:06 AM
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Originally Posted by Balr14 View Post
Sorry, I thought I had addressed your flybar movement question, indirectly. From what I have observed, using two links on the flybar causes more resistance and restrictions to flybar movement. That would translate into less movement authority for the heli, making it less likely to reach the point where the blades start sloughing off air. Whether or not the ability of the flybar to travel more in one direction than the other, could trigger this condition, is debatable.
I wasn't savvy enough to get it. Thanks for your comments.

Russ
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Old Jun 26, 2011, 10:13 AM
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Originally Posted by rampa202 View Post
I wasn't savvy enough to get it. Thanks for your comments.

Russ
Not your fault, sometimes I'm a lot better at thinking things than saying them. Heli physics are a great deal more complicated than most people realize. It's quite common for people to do something that fixes a problem, but don't really know why it worked.
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Old Jun 26, 2011, 10:58 AM
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We value your opinions Balr.

As soon as you mentioned a long CoG, I "got" it, concerning banking causing the death dive. I guess once you can visualize something, it helps. Now I can vizualize the wobble caused by the fighting forces.

Are we essentially talking about something like centrifugal force versus traction when a car turns fast? Obviously the car spins out, because of that pull away from gravity, like banking throws the heli out.

Well, that's how I see it, probably talking baba-nonsense.
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Old Jun 26, 2011, 11:04 AM
"whirlydude"
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Originally Posted by rampa202 View Post
@ Whirlydude - Wow, nice heli - is that BL motors I see? You have the complete Skytec set (except maybe the swashplate?) Looks like the stock inner shaft length?

Thanks for the advice about the links - Ill order some more. I think I'm going to try out the Xtreme head for a while before I go all the way (to skytec that is.) I have some on my CX3 and I love it.

Russ
Yes, I do have a boardless brushless setup on my 400D, and the inner shaft is from the Xtreme extended upper head. I lowered the Skytec lower rotor head to create the same distance between rotor blades as one will get from the Xtreme extended head. (+15mm)
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Old Jun 26, 2011, 02:07 PM
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Originally Posted by whirlydude View Post
Yes, I do have a boardless brushless setup on my 400D, and the inner shaft is from the Xtreme extended upper head. I lowered the Skytec lower rotor head to create the same distance between rotor blades as one will get from the Xtreme extended head. (+15mm)
Oh 400D - I didn't notice. I wanted to ask your opinion about this, but maybe you don't have one:

http://www.rtf-heli.com/ESKYBIGLAMAbrushless.html

I'll have to look-up what "boardless" means in this situation.

Thanks again,

Russ
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Old Jun 26, 2011, 02:23 PM
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boardless means you dont have a 4in1 (board) but all "important parts" (ie, the 2 speed controllers, the mixer and the gyro) are individual components ...
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Old Jun 26, 2011, 02:25 PM
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Thanks for that E2
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Old Jun 26, 2011, 05:22 PM
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Originally Posted by Microbaba View Post
We value your opinions Balr.

As soon as you mentioned a long CoG, I "got" it, concerning banking causing the death dive. I guess once you can visualize something, it helps. Now I can vizualize the wobble caused by the fighting forces.

Are we essentially talking about something like centrifugal force versus traction when a car turns fast? Obviously the car spins out, because of that pull away from gravity, like banking throws the heli out.

Well, that's how I see it, probably talking baba-nonsense.
That's a pretty good analogy. You would be surprised how many people think the flybar is what makes the heli want to remain upright. It's just good old gravity.
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Old Jun 27, 2011, 05:54 AM
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Quote:
Originally Posted by rampa202 View Post

It seems the consensus on the death dive (stall) issue was the flybar reaching the limit of its travel. I've heard that shortening the flybar eliminates the problem, but I'm wondering if fixing the flybar travel asymmetry doesn't also have some benefit.

Russ
Russ, the asymmetry of the flybar movement certainly can't do the BOL any good as far as death dives, but having said that, the free movement that the Xtreme flybar holder allows seems to be enough to almost eliminate the problem. Combined with a shorter flybar, I've never had it happen again. On a related note, today I added a longer servo arm to both servos and then did some test flights to see if the substantially increased banking ability could cause a death dive. It did not, and I purposely banked and turned steeply at low speeds trying to cause the death dive. Yes, I'm mean to my BOL.

This reinforces my belief that the death dives are not caused by the lower blades stalling, but by a mechanical force such as the flybar bouncing off of its limits.

Owner1
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