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HelicopterMain Discussion › Hiller Vs. Bell Hiller
12-03-2007 03:19 AM  10 years agoPost 21
Fordpilot

rrNovice

Tigard Oregon USA

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Thank you dkshema and all for very informative posts on this subject.
My question is how does one control or program the amount of "control
influence" (dkshema's terms) that the hiller vs. the bell system inputs have on a bell-hiller system. As I understand it the hiller
linkage controls the flybar (kind of in theory) and the bell linkage
controls the blade pitch (kind of in theory), but since they are
mechanically linked what controls what or is that the wrong
question or the wrong way to ask the question.

To ask it another way when we are talking about control influence are
we talking about linear linkage movement or force on flybar vs.
force on blade pitch control arm. Or, in head set-up are you
simply trying to duplicate as engineered and best to follow the
right angle/perpendicular links dogma.

I had a pretty knowlegable person convince me that control input is
mechanically first to the flybar and then from the flybar back
down to the blades. From this fine discussion it appears that is
maybe not correct but input is rather to both flybar and blades
in some black magic "ratio".

Also does the swash linkage tilt degrees of lead (to go forward
the swash does not tilt forward) have a bearing on the subject.

Sorry for the diatribe questions but I have spent many delightful
hours studying my head setups and come away from each session
seeing more engineering magic than I was aware of before.

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12-03-2007 05:13 AM  10 years agoPost 22
spork

rrVeteran

Mountain View, CA

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My question is how does one control or program the amount of "control influence" (dkshema's terms) that the hiller vs. the bell system inputs have on a bell-hiller system.
The typical Bell-Hiller head has mixing arms that determine this ratio. Unfortunately, with most of the models I'm familiar with, this ratio isn't adjustable. Where these arms/levers are varies from one model to the next. But it's fairly easy to locate them. Holding the flybar level (but don't keep the paddles from pitching), you can input some cyclic. You'll see that with the flybar kept level the swashplate will still control the pitch of the main blades. But the amount of pitch on the mains will be reduced by the ratio of these mixing arms. Just wiggle your cyclic stick with the flybar held in place and watch the chain of controls to the blades. Do this with the main blades both fore-aft and side-to-side.

Next, leave the stick alone (swashplate level) and move the flybar up and down. You'll see that this also changes the pitch of the mains. On most models you'll see more pitch from this than you did when moving the sticks. This is due to the Bell-Hiller mixing ratio as well. Again, follow the control path to the main blades while holding the swashplate level and moving the flybar. You should see where this path goes through the same mixing levers. On most models it's pretty easy to approximate the Bell-Hiller mixing ratio as it's simply the ratio of the lengths of the mixing lever on either side of its pivot.
As I understand it the hiller linkage controls the flybar (kind of in theory) and the bell linkage controls the blade pitch (kind of in theory), but since they are mechanically linked what controls what
The swashplate directly controls the pitch of the flybar paddles. There are then two things that control the pitch of the main blades - the flybar position (as it teeters) and the swashplate. The Bell-Hiller ratio determines which affects the pitch of the main blades more. Let's say you have a 30/70 Bell-Hiller mixing ratio. That means 30% of the main blades pitch comes directly from the swashplate (Bell), and 70% comes from the flybar position (Hiller). The flybar position is simply the result of the air acting on the flybar paddles as the swashplate changes their pitch.
To ask it another way when we are talking about control influence are we talking about linear linkage movement or force on flybar vs. force on blade pitch control arm.
We're talking about mechanical linkages. The ratio in question is simply a matter of the lengths of the various arms and levers.
Or, in head set-up are you simply trying to duplicate as engineered and best to follow the right angle/perpendicular links dogma.
Unfortunately we can't adjust the mixing ratio on most models, so the ticket is to follow the perpendicular links dogma for the best possible setup.
I had a pretty knowlegable person convince me that control input is mechanically first to the flybar and then from the flybar back down to the blades.
In a strictly Hiller setup this would be true. With a Bell-Hiller setup there is some direct input from the swashplate to the main blade pitch in addition to the contribution that goes through the flybar.
Also does the swash linkage tilt degrees of lead (to go forward
the swash does not tilt forward) have a bearing on the subject.
That's a different topic. It's related only in that it has to do with helis, swashplates, etc. It doesn't figure into the Bell-Hiller ratio per-se.

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12-03-2007 05:30 AM  10 years agoPost 23
ZXXflyer

rrKey Veteran

stone mountain, georgia, US

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So when holding the flybar parallel to the boom, and measuring your "cyclic pitch" what is an acceptable amount to have good control response without stalling the main blades in a full pitch manuver??

I currently have 6 degrees cyclic, and 10 degrees max pitch..
Flybar paddles are set to the "3D"(rearward) hole and no weights are installed. The heli is a Hawk Pro.
It seems like the blades are stalling and bogging the motor down while doing stationary flips or rolls.

Should I lower the cyclic pitch down to 4 degrees and then just kick the EXPO up on my transmitter to get the twitchy/responsive feel I want back???

On a side note, I have several different lenght balls on hand to change the bell-hillier ratio.. I would assume I am looking for more pitch input from the swash, and less pitch change from the flybar?
This would give me a more immediate/twitchy response without over-pitchiing the blades once the flybar finally tilts to full deflection??

Believer in Weston motors!

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12-03-2007 05:57 AM  10 years agoPost 24
dkshema

rrMaster

Cedar Rapids, IA

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Also does the swash linkage tilt degrees of lead (to go forward
the swash does not tilt forward) have a bearing on the subject.
But the swashplate DOES tilt forward to make the helicopter go forward. And it tilts toward the rear to make the heli go backward, and it tilts left for the heli to go left, right for the heli to go right.

In a heli whose rotor system turns clockwise when viewed from the top, looking down, the advancing blade (would be the one on the left side of the heli) is at it's minimum pitch when it is perpendicular to the left side of the chopper. You would expect the heli to roll to the left, but the forces act 90 degrees later in the rotation. Instead of the heli rolling to the left, the heli goes forward because the nose is forced down.

Many people wonder why their heli pitches up at high speed. Most likely it's because the retreating blade (right side of the heli) is stalling. It has maximum pitch when perpendicular to the right side of the heli, and has a lower speed since it is on the retreating side. Too high an angle of attack combined with too low an airspeed is the recipe for a stall. Instead of the heli rolling to the right as you might expect, the result of the loss of lift on the right side shows up as the heli nosing UP because the effect of loss of lift on the right shows up 90 degrees later as the tail wanting to be forced down.

The "clever" design of the swashplate controls sees to it that the control input occurs 90 degrees ahead of where the control reaction is desired.

-----

And Spork did a good job of explaining the bell/hiller mix. If you were to look at the flybar carrier on a Hirobo Freya or EVO 50, you'll see that there are three different sets of holes in it to mount the levers that do the bell/hiller mix. In essence, these holes move the pivot point of the mixing levers inward or outward, allowing you to select "maximum mobility", somewhere in-between, and a somewhat stable head response based on the flybar mixing ratio.

If you take a look at some of the helis whose Bell/Hiller mixing arms are located on the main rotor pitch arm, you may find that the pivot hole for the lever is off center, or may be centered. And you may have more than one position on the end of each of those levers to mount the ball links.

Many helis don't allow you to play with the flybar mixing ratio, some of the better helis flown by the "pros" do.

Take a look at the Avant Programmable head above. On the end of each main rotor grip pitch arm are the bell/hiller mixing levers. Note that there are a couple of holes to the left of the pivot, and three holes to the right of the pivot on the mixing lever closest to you.

Also, take a close look at the flybar carrier. You'll see several tapped holes to allow you to change the flybar mixing ratio.

This "programmable head" allows you to not only experiment with the flybar mixing ratio, it also allows you to experiment with the Bell/Hiller mix.

-----
Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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12-03-2007 06:10 AM  10 years agoPost 25
spork

rrVeteran

Mountain View, CA

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I currently have 6 degrees cyclic, and 10 degrees max pitch..
That sounds about right. But I'm no expert on the Hawk Pro.
Should I lower the cyclic pitch down to 4 degrees and then just kick the EXPO up on my transmitter to get the twitchy/responsive feel I want back???
I would think 4 degrees of cyclic is a bit low. I don't like Expo on anything; but I suppose that's a personal preference.
I would assume I am looking for more pitch input from the swash, and less pitch change from the flybar?
This would give me a more immediate/twitchy response without over-pitchiing the blades once the flybar finally tilts to full deflection??
You got it.
But the swashplate DOES tilt forward to make the helicopter go forward.
Dave, I could be wrong but I think he was talking about the swashplate phase offset. I avoided going too far down that road as I consider that an unrelated complication for this discussion.

By the way, nice examples of adjustable Bell-Hiller mixing. The helis I've played with don't have such nifty options.

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12-03-2007 06:36 AM  10 years agoPost 26
ZXXflyer

rrKey Veteran

stone mountain, georgia, US

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So the blade stalls I am experiencing are occuring only after the flybar reaches maximum tilt, and I am correct to try and reduce the ammount of cyclic pitch that the flybar adds?

Believer in Weston motors!

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12-03-2007 01:07 PM  10 years agoPost 27
dkshema

rrMaster

Cedar Rapids, IA

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Blade stalls are an assumption at this point.

Six degrees of cyclic (if you're measuring it correctly) on top of a maximum of 10 degrees collective doesn't sound excessive. I don't think you mentioned the motor that you're running in your Hawk Pro.

If you're using an OS32 SX-H, perhaps that ten degrees is just too much. You might need to back down to 9 degrees collective.

If you're using the 37 SZ-H, it ought to be able to do the ten degree collective thing, unless you have a muffler on your heli that's not letting the 37 do its thing.

It's easy to back off the amount of collective you have, as you can tweak your pitch curve with little effort.

-----

You may be having some linkage binding up on the head -- the reason that many people choose to use a cyclic ring, to limit control throws when your sticks are in the "corner". You can turn the head by hand, then move the aileron/collective to the corner and see if that makes it harder to turn the head. Or just give it a spin, see how many turns you get (might want to loosen the belt drive to the tail if that's what you've got), then repeat the test and move the sticks to the corner and see if things appear to get "stiffer".

-----
Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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12-03-2007 06:12 PM  10 years agoPost 28
ZXXflyer

rrKey Veteran

stone mountain, georgia, US

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OK...
there is guaranteed no binding, and I use a cyclic ring.

9 degrees pitch isn't sufficient to 3D with, and doesn't bog my 32 in FFF. I run 20% nitro with a $140 two piece Hatori tuned pipe with totally screams... head speed is locked in at 1,800 with a throttle jockey.

As for blade stalls, yes that's what I was assuming because I had read on here before that roughly 15 degrees pitch is the stalling point of a rotor blade.. is that grossly wrong??
If so, what is the total combined collective and cyclic pitch that is safe to use?

I intend to have +10 max pitch and 6 or 7 degrees cyclic directly from the swash, the only question is how much extra cyclic to get from the flybar since I can adjust the ratios with different length balls.

I'll check later today and see what my total cyclic and collective pitch is with my current setup.... might be WAY more than I think.

Believer in Weston motors!

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12-03-2007 08:51 PM  10 years agoPost 29
ZXXflyer

rrKey Veteran

stone mountain, georgia, US

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ALright..... I have +10 max pitch, and 6 degrees cyclic directly form the swash(with flybar level).

when I tilt the flybar to full deflection while applying full cyclic and full collective, I am getting a total of 29 degrees pitch on the one blade!!!!!

So now I intend to mess with the bell-hillier mixing ratio and lower the ammount of cyclic that the flybar can add.

the question still remains what is the max pitch a blade can handle without stalling??

Believer in Weston motors!

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12-03-2007 09:10 PM  10 years agoPost 30
spork

rrVeteran

Mountain View, CA

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when I tilt the flybar to full deflection while applying full cyclic and full collective, I am getting a total of 29 degrees pitch on the one blade!!!!!
Sure, but you have 4 degrees on the other.
the question still remains what is the max pitch a blade can handle without stalling??
Unfortunately that's not a trivial question. It's true that there is a finite angle of attack at which the blade will stall, but you've only defined pitch angle, not angle of attack. To know the angle of attack you need to know the blade speed and how fast the air is moving through the disk.

But I have to admit, 29 degrees sounds like a lot when you put it that way.

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12-03-2007 09:17 PM  10 years agoPost 31
ZXXflyer

rrKey Veteran

stone mountain, georgia, US

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yeah.. I jsut did a search on blade stall, and we could discuss that for weeks, and it's going to be different for each heli, maneuver, and weather condition...

I'm just going to mess with the bell-hillier ratio to take away the flybar's authority all I can, and move my shash AFR back up till I get 7 degrees cyclic with the flybar level. Then I'll go fly and try some more stationary flips and rolls. Hopefully it will behave better because I have practiced hard on collective management.. but no matter how gentle I am on the collective, too much cyclic is occuring.

Big thanks to everyone for all the tips and advice! I now have a much better understanding of the bell-hillier relationship.

Believer in Weston motors!

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12-04-2007 02:44 AM  10 years agoPost 32
dkshema

rrMaster

Cedar Rapids, IA

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BTW -- with a 32, no matter how much it's screaming, you're going to be stuck with learning collective management. What's your Hawk weigh? Messing around with all the head stuff may simply be fruitless unless you put a 37 in it and pick up the extra horsepower, or move up to a 50 sized machine.

On the other hand, my new Caliber 4 with an OS 37 is pretty lively. Now to get hold of a better muffler than the stock one. Gotta find a good condition, used MP2-30.

-----
Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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12-04-2007 02:50 AM  10 years agoPost 33
ZXXflyer

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stone mountain, georgia, US

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I have worked on my collective management for two months both on the sim and burning gallons of fuel....

I just found my problem and posted on the Century forum....
It's not the 32's fault, it was mine for adding the two longer balls on the inner swash ring.. I now only have long balls on the pitch rods, and short ones on the washout arms..(as suggested by the hawk owners maual). I had added the longer balls to achieve faster cyclic while still using stock paddles.. This proved to be too much once switching to the 3D paddles on the rear hole.
My flybar WAS tilting to full deflection in flight due to the pitch the paddles were recieving... giving me +30 degrees pitch.
It now flies fine and behaves perfect in rolls.

The hawk weighs in at 7.5 lbs fully loaded minus fuel.

Believer in Weston motors!

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12-04-2007 11:10 AM  10 years agoPost 34
heli_headcase

rrKey Veteran

Hovering around Atlanta

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ZXXflyer,

Thanks for your comments on my Whirlybird 505 video on youtube.com. We're close (GA). You should call me some time.

MMike,

If you're still monitoring this thread, please be very accurate in describing your Axe CP's flying stability issue. I own two of these heli's and can probably explain exactly what you were feeling. And no, it's not delay in control response. There's another set of problems here and very easy to solve. Contact me direct if you want but it's better to allow everyone to read the answers to the problem.

To everyone else: Keep right on typing...


HHC

So many heli's - too little time...

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12-04-2007 03:05 PM  10 years agoPost 35
MMike

rrElite Veteran

Holland,Mi-USA

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Headcase

I'm very interested.

How do you solve the "Axe Issue"

MMike

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12-04-2007 07:27 PM  10 years agoPost 36
heli_headcase

rrKey Veteran

Hovering around Atlanta

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You first...

How do you solve the "Axe Issue"
I want you to tell us what you think the Axe was doing that was giving you problems. I'm not going to put words in your mouth and give up the answer that easy. Tell us, how did the flight quality problem manifest itself? (I'm trying not to sound like a heli 'shrink', but hey...)


HHC

So many heli's - too little time...

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12-04-2007 07:47 PM  10 years agoPost 37
spork

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Mountain View, CA

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Ahhh yes - the heli whisperer!

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12-04-2007 07:57 PM  10 years agoPost 38
heli_headcase

rrKey Veteran

Hovering around Atlanta

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That's me...

Ahhh yes - the heli whisperer!
As I have been known...


HHC

So many heli's - too little time...

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12-04-2007 09:54 PM  10 years agoPost 39
MMike

rrElite Veteran

Holland,Mi-USA

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Headcase

I actually own two.

I bought the second one from my son to help him raise $$ for a T-REX.

After and before installing a heading hold gyro and brushless motor:

I would say the flight problem manifested in:

1. Tail "bobbing" around.
2. Hover needs to be chased, but slow to extreme input.
3. Always "sliding around" feeling.

This is a little more detail to what I meant by "unstable".

BTW, On G4 sim, They have it pretty close to what I experienced.

I will look forward to your response.

Thanks

MMike

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12-05-2007 12:20 AM  10 years agoPost 40
heli_headcase

rrKey Veteran

Hovering around Atlanta

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OK, here we go...

I forgot to subscribe to this but I'm here now.
1. Tail "bobbing" around.
2. Hover needs to be chased, but slow to extreme input.
3. Always "sliding around" feeling.
Tail bobbing is a sign of either the blades are out of balance, flybar not balanced, bent rotorhead component or bent main shaft. Fixing these will provide a better platform for the gyro but won't really affect rotorhead dynamics. If the head speed is too slow you can have all sorts of vibration issues too. You need to be over 2000RPM for solid control. Do you know your head RPM?

Sounds like #'s 2 and 3 are related. This can be caused by having an early AXE CP with less than the best servos. There was a production change that added more precise servos in the later production run. I have both early(ish) and later AXE CP models. The purple AXE in my gallery had the earlier servos and I replaced them with Hitec HS55. Helped quite a bit. The yellow AXE isn't perfect but didn't need the servos swapped out like the purple one did.

Add flybar weight. The mixing ratio of the seesaw tilt to main blade pitch change (blade feathering) is wrong as far as I'm concerned. The blades change pitch at a greater rate than the flybar, giving a 1.1-1 Bell ratio, maybe more like 1.3-1. This has a good and bad effect. The good is the "gain" of the flybar assembly, which remember, is a gyro, helps in making the rotorhead resist outside disturbances. BUT, the downside is the main blades have a bunch of leverage available to influence the flybar seesaw angle depending on the lead-lag of the blades and related to the center of mass of the blades in the for/aft (chordwise) direction.

This last problem can make the heli appear to slid off in a direction all by itself without any input from you. Since changing the Bell mixing ratio isn't an easy option (I've wanted to try this but can't think of a clean way to make the mod), you can try different sets of main blades and hope to find a set with a more fore or aft center of gravity, increase the head speed and/or add wheel collars as weights to the flybar to let the flybar have more resistance to influence from the main blades.

You saw the mod I made to my AXE blade grips - the screw inserted into the grip arm - but that had only one purpose, to help stiffen the arm and harden it against breakage in crashes. My first AXE crashed on every flight and the pitch arm was the only thing to fail in these contacts with the ground. Did it help? Yes but then the arm broke in a location closer to the grip body. Oh well...

There's my diagnosis. The doctor has spoken

Hope this helps.


HHC

So many heli's - too little time...

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HelicopterMain Discussion › Hiller Vs. Bell Hiller
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