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Sep 08, 2010, 07:44 PM
Registered User
Question

45 degree FB + CP ?


Is there a reason why a collective-pitch heli can't be self-stabilizing with a 45 degree flybar? There seems to be only fixed-pitch helis with 45 degree flybars.
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Sep 08, 2010, 08:44 PM
Cranky old fart
Balr14's Avatar
Quote:
Originally Posted by jazzjohn
Is there a reason why a collective-pitch heli can't be self-stabilizing with a 45 degree flybar? There seems to be only fixed-pitch helis with 45 degree flybars.
A 45 offset flybar would work very poorly on a CP heli. It's purpose is to dampen movement, so that would effect pitch. It's out of phase and trails head movement, hence the weights instead of conventional paddles. It's more common on FP helis because pitch can not be altered. The 45 offset head is a much better design with superior performance. Simple versions of 45 offset heads are used in the MSR and 120MSR, a full blown version was used in the SRB Quark. The only CP heli with a 45 offset head currently available is the Quark SG and it's quite expensive.
Sep 09, 2010, 04:58 PM
Registered User
Quote:
Originally Posted by Balr14
The 45 offset head is a much better design with superior performance.
What part of the 45 offset head is offset and what is it offset relative to? Are there illustations anywhere? I guess I don't the difference between a "45 flybar" and a "45 offset head."

Quote:
Originally Posted by Balr14
ThThe only CP heli with a 45 offset head currently available is the Quark SG and it's quite expensive.
Is it self-stabilizing?

Thanks, Balr.
Sep 09, 2010, 09:24 PM
Cranky old fart
Balr14's Avatar
It is offset 45 at the point where the bladegrips attach to the head, relative to the connection points on the swashplate. You will observe in the attached image, that the flybar is offset 45 from the blades. What you don't realize is that the flybar is really at 90 and the head is offset 45. Unfortunately, you can't can't see the linkage and attachments points on the swashplate very well. A heli with a 45 offset flybar may look a lot like this from above (with a much more simple head design), but you have to look closely at the linkage attachment points to see where the real offset is, if you aren't familiar with head design.

The 45 offset head offers self-stabilization, but it's much more dynamic than the simple damping of movement provided by an offset flybar. If anything, overall performance is enhanced.
Sep 12, 2010, 11:35 PM
Registered User
If a heli is self-stabilizing, then when the swash is level it hovers (almost) on its own. Then what difference does it make what angle the blade grip connection points are relative to the swash connection points? Wouldn't the answer be "none?" What am I missing?
Sep 13, 2010, 12:41 AM
Cranky old fart
Balr14's Avatar
Quote:
Originally Posted by jazzjohn
If a heli is self-stabilizing, then when the swash is level it hovers (almost) on its own. Then what difference does it make what angle the blade grip connection points are relative to the swash connection points? Wouldn't the answer be "none?" What am I missing?
What you are missing is an understanding of gyroscopic precession, phasing and head linkage. In simple terms, with respect to RC helis, gyroscopic precession says any action initiated at the swashplate requires 90 of rotation to be acted upon. If the object it's acting upon is less than 90 away, it will be missed. That is why in a conventional head design, the flybar is offset 90 from the swashplate and the connection points on the head are offset 90 from the flybar.

In the conventional head design used on most FP helis, the linkage is Hiller; which means the swashplate is connected to the flybar and the flybar is connected to the head. So, in simple terms, movement from the swashplate takes 90 to effect the flybar, which takes 90 to effect the head. The delay from swashplate to head results in very slow response.

With the 45 offset flybar, the flybar is less than 90 away from the swashplate, so it will be missed until the next 90 phase; it's out-of-phase. You can't use this configuration with Hiller linkage, so Bell linkage is used; which means the swashplate is attached directly to the head. Since the head is offset 90 from the swashplate, it will be acted upon before the out-of-phase flybar. In order to utilize the out-of-phase flybar, an additional linkage point is added to the Bell linkage so the flybar factors into the movement of the head. Now, remember that the flybar is out-of-phase and the head is acted upon first. Due to the linkage, at the time the head is moved by input from the swashplate (90), the flybar is still pointing where the head was, not where it's moving to (it requires 135 to received the signal). So, the flybar is used to dampen head movement. Which is why it has weights instead of paddles.

With a 45 offset head, the head is out-of-phase, because it isn't offset by 90 from the swashplate or flybar. The flybar is offset 90 from the swashplate, so it is acted upon first. The out-of-phase head is acted upon next. In order to take advantage of the out-of-phase head, Bell-Hiller linkage is used. Bell-Hiller linkage has one direct connection between swashplate and head (like Bell) and and second between swashplate, flybar and head (like Hiller). The result is movement sent to the flybar, which is offset 90, doesn't have to wait another 90 to get to the head, because it's being sent to the head by the direct link at 135. The result is movement is smoothed,and it's quicker.

Both offset methods provide stability by smoothing movement. But, the offset flybar is reactive, the offset head is proactive. This discussion is an oversimplication, since the swashplate, flybar and head are all linked together and you can't move one without the others. The effects I stated actually describe resistance to movement and how they are influenced. There are other factors to consider as well, such as linkage leverage, length and timing. Remember, this is all occurring in milliseconds, where slight differences have big impact.
Last edited by Balr14; Sep 13, 2010 at 12:49 AM.
Sep 15, 2010, 06:35 PM
Registered User
Balr,

Thank you for your experienced and knowledgeable overview of rotor head theory.

What I am interested in (and still confused about) is self-stabilization. My concept of self-stabilization is that without cyclic input, the heli will return to the horizontal and almost hover itself (like the mSR does). No cyclic input means the swash is level. That tells me that the self-stabilizing action involves the flybar and the blades only -and not the swash. Or are the swash connection points involved when an outside force (like wind) acts on the body by tipping it (and the swash) relative to the flybar and disk?

Thanks for your patience.

John
Sep 15, 2010, 06:46 PM
My other addiction!
norcalheli's Avatar
Quote:
Originally Posted by jazzjohn
Balr,

Thank you for your experienced and knowledgeable overview of rotor head theory.

What I am interested in (and still confused about) is self-stabilization. My concept of self-stabilization is that without cyclic input, the heli will return to the horizontal and almost hover itself (like the mSR does). No cyclic input means the swash is level. That tells me that the self-stabilizing action involves the flybar and the blades only -and not the swash. Or are the swash connection points involved when an outside force (like wind) acts on the body by tipping it (and the swash) relative to the flybar and disk?

Thanks for your patience.

John
On a regular, 90* fly-barred heli, no cyclic input means the swash is level relative to the heli frame, not the ground, and at 90* to the main shaft. That is why when you give a CP heli a command to go in a particular direction, it will continue in that direction until you give it a command that counteracts the first command. With the offset heads, and/or flybar, the same command is counteracted by the flybar itself dampening the command, allowing the heli to self-stabilize. There is no such self-stabilizing effect in a 90* flybar heli.
Sep 15, 2010, 07:10 PM
Cranky old fart
Balr14's Avatar
Self-stabilizing only helps maintain current position, by smoothing response, regardless of it's current position. The MSR tends to return to horizontal because of a long center of gravity; nothing to do with flybar or blades. The extra linkage and head components required to provide the offset head or flybar makes the center shaft longer on most self-stabilizing helis. Hence the long CoG and the pendulum effect, you probably noticed. Combine that with the response smoothing and you have what you view as self-stabilizing.

Helis like the Hirobo SRB Quark self-stabilize but do not return to level, as do many of the new 3 axis gyro helis by Walkera.
Sep 15, 2010, 09:33 PM
Registered User
so if you put a longer center shaft on any fp it would be more self levelingcorrect
Sep 16, 2010, 01:43 AM
Cranky old fart
Balr14's Avatar
Quote:
Originally Posted by crashj
so if you put a longer center shaft on any fp it would be more self levelingcorrect
Of course. Think about it for awhile. What else other than gravity can make a heli return to a level position? Why do you think coaxes remain level? Think about how a plumbob on a string works. Now imagine the rotor head is the end of the string and the bottom of the heli is the plumbob. The longer the string and the heavier the weight at the other end, the greater the inertial moment. C'mon, this isn't difficult. It's all elementary physics. It's been tried many times, with predictable results. Watch the video and see how the heli swings back and forth: https://www.rcgroups.com/forums/show...&postcount=422. Then imagine that same action on a much larger heli. I'd be standing behind a tree.
Last edited by Balr14; Sep 16, 2010 at 09:23 AM.
Sep 16, 2010, 07:38 PM
Registered User
APJudson's Avatar
Would replacing a paddled flybar with a weighted flybar accomplish something similar?
Sep 16, 2010, 09:17 PM
Cranky old fart
Balr14's Avatar
Quote:
Originally Posted by APJudson
Would replacing a paddled flybar with a weighted flybar accomplish something similar?
You are misjudging the purpose of a flybar. Paddles on flybars are used to influence head movement. Weights are used when an out-of-phase condition exists and all the flybar can do is dampen head movement. A flybar has no bearing on returning a heli to level. It's motion is influenced by centrifugal force, so it will resist all changes in the plane of it's rotation. Tie a brick to the end of a 20' rope and swing it around, say at an angle about 20 from level. Get it going real good, then try to change the angle to level. I'll bet you'll find it's not easy. Now, get that brick spinning at about 2000 rpms and try it! The point is, you can force it to change planes, but it certainly isn't going to do it on it's own. So, it can't help level a heli. The same holds true for the blades; centifugal force. Again, simple physics at work.


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