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Old Mar 24, 2011, 10:13 PM
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Collective pitch vs. Fixed pitch

I have a question...

I understand the difference between collective pitch and fixed pitch helis. However, I don't understand why the blades being "fixed" causes stability, whereas a "collective/adjustable" pitched blades causes instability. What causes the difference?
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Old Mar 24, 2011, 10:15 PM
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I think you have that backwards.
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Old Mar 24, 2011, 10:30 PM
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Originally Posted by Balr14 View Post
I think you have that backwards.


A fixed pitched helicopter wants to self-stabilize (ex. SR 120 will hover just fine without cyclic input if trimmed properly). A collective pitch heli wants to roll and smash into the ground if not constantly stablized by cylic input from the pilot (ex. Let go of the right stick on a Trex 450 and....SMASH!)

Right?
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Old Mar 24, 2011, 10:45 PM
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Originally Posted by tripleup05 View Post


A fixed pitched helicopter wants to self-stabilize (ex. SR 120 will hover just fine without cyclic input if trimmed properly). A collective pitch heli wants to roll and smash into the ground if not constantly stablized by cylic input from the pilot (ex. Let go of the right stick on a Trex 450 and....SMASH!)

Right?
No. A 45 degree fp wants to self stabilize. A 90 Degree fixed pitch will roll similarly to a CP.

The reason is because on a 45 degree flybar counters the rotation of the helicopter quicker. In a 90 degree flybar it takes 90 degrees before the flybar interferes and counters the roll.

I'm not sure if I am explaining it right, but thing of it this way. A helicopter that has a 45 degree flybar (like an SR 120) will correct the instability twice as fast as a 90 degree flybar (like a Honey Bee FP). However, the 90 degree is more precise. It's faster to get to 45 degrees than 90. Now a CP can roll like a 90 degree FP, but they tend to be morestable because the head spins faster and you can reduce the pitch, which makes the blades less susceptible to the ballooning effect that happends when the wind passes over the curved FP blades.
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Old Mar 24, 2011, 10:51 PM
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No, not right. There are self-stabilizing helis that are designed with offset flybars or offset heads to provide stability. These designs are usually used on FP helis because they need it more, although there are a few CP helis with self-stabilizing heads. The Blade MSR, 120SR, Walkera CB100 and NE Solo Pro are examples of self-stabilizing FP helis.

But, if you look at conventional helis, CP helis have higher headspeed and head speed is constant while pitch changes, both of which promote higher stability. A conventional FP heli has low headspeed and you must change headspeed to change altitude, bad for stability. Most conventional FP helis use a head design that has no anti-roll geometry, most CP helis have anit-roll geometry.

This is pretty academic these days as there are very few remaining conventional FP helis. Somebody new to the hobby who sees nothing but self-stabilizing FP helis could easily make the same assumption you did.
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Old Mar 24, 2011, 11:02 PM
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Originally Posted by GetterDragun View Post

I'm not sure if I am explaining it right, but thing of it this way. A helicopter that has a 45 degree flybar (like an SR 120) will correct the instability twice as fast as a 90 degree flybar (like a Honey Bee FP). However, the 90 degree is more precise. It's faster to get to 45 degrees than 90.
No, that is not correct. A 45 offset flybar or head creates an out-of-phase condition with respect to normal gyroscopic precession. In effect, you are changing the normal order in which input signals are sent to rotating components. In the case of the 45 offset head, you are effectively overlapping movement sent to the flybar and head. In the case of the 45 offset flybar, you are sending input signals to the head before the flybar. Aside from changing normal procession, the 45 offset provides an additional point to add to the mixing arms, effectively a double average. The net effect is the 45 offset head provides very fast and smooth response but tends to swing back and forth when you want to stop. The 45 offset flybar damps head movement, but also damps the swaying.
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Old Mar 24, 2011, 11:14 PM
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Very interesting! I didn't realize it was the 45 degree flybar making the heli stable, and not the fact that he blades are "fixed".

Thanks for the info!
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Old Mar 24, 2011, 11:38 PM
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Another point about FP versus CP helis, is for all intents and purposes, the rotating blades are seen as a solid disk. Air will not penetrate it. As far as wind is concerned the rotor disk is just like a kite. If wind gets under it, it will lift. With an FP heli your only choice is to cut throttle. Of course when you do that, you have no control. A CP heli has negative pitch. It can drive downward to counteract wind without reducing throttle; in fact you can often increase throttle. People who can fly FP helis in wind are essentially "wind surfing" while attempting to angle the head downward enough where the wind is pushing at an angle; kind of like tacking into the wind on a sail boat.
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Old Mar 25, 2011, 12:19 AM
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Lotta variation.

Hiller-only heads like HBFP don't stabilize at all. You REALLY have to stay ahead of them.

Bell-Hiller + 45* heads like 120SR will reach a null with no control inputs, given enough time and space. But the price for that is staying ahead of the head's own corrections. IOW, when you tell it "stop" it kinda resents that and backlashes and you have to anticipate that too.

Given a slight breeze, all 'stability' bets are off as "breeze" can come from ANY direction at any time. That's what makes it fun, eh?
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Old Mar 25, 2011, 07:42 AM
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Lotta variation.

Bell-Hiller + 45* heads like 120SR will reach a null with no control inputs, given enough time and space. But the price for that is staying ahead of the head's own corrections. IOW, when you tell it "stop" it kinda resents that and backlashes and you have to anticipate that too.
Yea, the pendulum effect is a real pain in the butt in my opinion. I plan to buy a RC simulator and pick up a CP heli soon. The SR 120 is fun, but as Balr14 was just saying in his last post, it definately has its drawbacks.
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Old Mar 25, 2011, 08:28 AM
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Yea, the pendulum effect is a real pain in the butt in my opinion. I plan to buy a RC simulator and pick up a CP heli soon. The SR 120 is fun, but as Balr14 was just saying in his last post, it definately has its drawbacks.
to be honest that is the best thing you can do in my opinion. them FP helis are usually so unsimilar that its not worth it. get some sim time a 450 and get ready to start fixing stuff. lol
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Old Mar 25, 2011, 08:49 AM
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FP helis are no good in the wind and for that matter unless they have a 45 degree flybar they are no good for indoors. i would suggest the MCPX for a first CP heli. it is very similar to a 120sr IMO but you just have to fly it a little more. Sim time is a good thing as i think the sim is harder than actually flying, however crashing is a lot less expensive on a sim! if you plan on, staying in the hobby, get a good radio first, learn to program it and get comfortable with it on the sim.

CG

I am beginning to think Balr is not human, i picture him as a super computer with reel to reel tapes ect... with everything about helicopters stored in the memory banks.
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Old Mar 25, 2011, 09:04 AM
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CG

I am beginning to think Balr is not human, i picture him as a super computer with reel to reel tapes ect... with everything about helicopters stored in the memory banks.
haha or maybe just an engineer cuz he is speaking my language. lol
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Old Mar 25, 2011, 09:32 PM
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I picked up Phoenix v3 today at the LHS. I already have a DX6i, so I got the cheaper one without the transmitter. To be honest, I am a little disapointed. It doesn't come with any instructions to speak of, and calibrating the transmiter to the software is a little confusing. For example, Phoenix says to do calibration in Heli mode, not fixed wing. Ok simple enough...but then it comes time to calibrate the "gear" and "flaps" switches. How in the heck do I do that in heli mode? Those aren't even options in heli mode. It just doesn't make sense. Anyways, rant over. I will carry this over to my other thread in the RC Sim forum

Anyways, another question regarding CP helis...and a very noob one at that. Is collective controlled independently, or is it automatically mixed with throttle? I was thinking it was an automatic thing, but Phoenix asked me which channel I wanted to use for collective.
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Old Mar 25, 2011, 09:56 PM
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throttle and pitch are on the left vertical axis (mode 2) Its a mix in the radio....they have seperate channels but all work together when the stick is raised and lowered...Hope that is clear as mud...Look in your dx6i manual to be sure but I believe it like channel 2??? Cant member at the moment...I know throttle is 3...

-Jeff
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