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Old 09-17-2007, 03:59 PM   #1 (permalink)
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Default Another question about 4 and 5 blade rotors

I think I understand what people mean when they say the helicopter will be more 'twitchey' with a 4 or 5 blade head. I'm guessing this is because the flybar is removed. I have watched how flybar-less helicopters fly and it seems they don't return to a position once moved if that makes sence.
So here is the question.
If I put a 5 blade head on, basically flybar-less, how can I stabilize the helicopter so it acts like a heli With flybar?

Could I use 2 more gyros that control pitch and roll?
I tried to do this with the real flight simulator and don't think I got it right or it doesn't work or I don't understand the concept.

Can someone help?

Thanks!
Mike
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Old 09-18-2007, 01:03 AM   #2 (permalink)
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More weight in the blades. Shoot an E-mail to JMD Models.. These guys run all kinds of flybarless heads with NO mixers or gyros to help out. They add weight to the blades. I had a great conversation with those guys this past weekend about a 5-bladed setup for a Trex450 and checked out two 450 sized 5-bladed prototype heads, including one they were flying....
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Old 09-18-2007, 09:05 AM   #3 (permalink)
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On the larger scalers (nitro, gas and turbine) gyros are commonly used on the pitch axis, and sometimes roll axis as well. There are even electronic mixers available that will make them fly like a flybarred head. Then again, as Skiddz noted, there are many who fly them with no gyros.

I'm currently test flying a 3-blade head on one of my TRex 450s in preparation for a scale fuselage. It's twitchy, but flyable. Adding weight to the blades will help, and slowing the headspeed down helps as well.

I'm still researching the possibility of putting a 5-blade head on my Raptor/MD500, but the cost is prohibitive...
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Old 09-18-2007, 09:22 AM   #4 (permalink)
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thanks guys,

Do any of you know how gyros are added to a 3 post swashplate? I forgot the name of this type of swashplate, but it only has 3 points of contact and requires servo mixing. It's what comes on the preditor.

got any links to how this is done?
I'm not in a rush to do this right away or anything, but think its required knowledge and I love 5 blade rotor heads. trying to get into the technology.
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Old 09-18-2007, 09:39 AM   #5 (permalink)
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It's called a 120 degree swash, and I think you will need a mixer, as you can't plug in the gyros on a single servo like you can with the tail.

Here's the one most of the scale guys are using: http://www.helitronix.com/index.html
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Old 09-18-2007, 07:26 PM   #6 (permalink)
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Thanks Rodan, That is just the info I was looking for.
I wish I knew more about it cause i'm really not understanding how it works with the 120 swashplate.
I guess this is going to take time.
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Old 09-19-2007, 01:01 AM   #7 (permalink)
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Right, so, with a 120 ccpm swash, you will need to get what's called a ccpm mixer and also two gyros, (gy240's will work fine,) each set perpendicular to the helicopter's pitch and roll axes. You will also need to set up your radio for a single servo 90degree mccpm swash. Plug the gyros in line with the aileron and elevator channels to the mixer, as well as the pitch channel. This way, the aileron and elevator channels are dampened by the two gyros before they get to the mixer. The mixer will then do it's job of converting your 90degree mccpm commands, elevator and aileron dampened by your two cyclic gyros, to commands for the servos on your 120degree swash.
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Old 10-11-2007, 01:33 PM   #8 (permalink)
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Quote:
Originally Posted by Mike SVOR View Post
Thanks Rodan, That is just the info I was looking for.
I wish I knew more about it cause i'm really not understanding how it works with the 120 swashplate.
I guess this is going to take time.
Check out this website. I'm going to build one of these; already ordered the head. Excellent description.

http://www.deeteeenterprises.com/NS....ughes.500E.php
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Old 10-11-2007, 06:39 PM   #9 (permalink)
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That thing is sick!

Got a question though:

On all the helicopters I've seen, it seems like the more blades it has, the less width is put on the blades. I don't know the technical term of that width. Not length (rotor diameter), not thickness, but width (from leading edge to trailing edge).

I would think that loosing that width on a 5 blade head would make for a better performing machine?
Does anyone even make a blade that i'm describing?
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Old 10-11-2007, 06:44 PM   #10 (permalink)
 
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It is called chord wise from leading edge to trailing edge. When you see 60 mm chord, it is referring to the width of the blade.

Yes multi blade helis ( 3 blades and up ) use blades that have a narrower chord than most 2 bladed helis. The blades are more efficient with a narrow chord and there is more blades on a multi blade heli to provide more lift.

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Old 10-11-2007, 06:48 PM   #11 (permalink)
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Quote:
Originally Posted by DavidH View Post
It is called chord wise from leading edge to trailing edge. When you see 60 mm chord, it is referring to the width of the blade.

Yes multi blade helis ( 3 blades and up ) use blades that have a narrower chord than most 2 bladed helis. The blades are more efficient with a narrow chord and there is more blades on a multi blade heli to provide more lift.

David
thank you,
Can you show me an example of a narrow chord blade for an rc heli?
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Old 10-11-2007, 07:40 PM   #12 (permalink)
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Quote:
Originally Posted by Mike SVOR View Post
thank you,
Can you show me an example of a narrow chord blade for an rc heli?
Link here: http://www.centuryheli.com/products/...#MainBladeSets
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Old 10-11-2007, 07:44 PM   #13 (permalink)
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Quote:
Originally Posted by DavidH View Post
The blades are more efficient with a narrow chord and there is more blades on a multi blade heli to provide more lift.
That's correct, but it not proportional, i.e. 4 blades generate more lift but not twice as much as 2 blades because of inefficiencies introduced by each blade flying in the wake of the previous blade. Less of an issue for full scale heli's but the airflow doesn't exactly scale (Reynolds numbers, etc.).
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Old 10-11-2007, 07:46 PM   #14 (permalink)
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I really don't see any difference in those chord widths and a regular 2 blade design.
I see this helicopter and look at its chord width and say, there should be some blades that mimic that, right?
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Old 10-11-2007, 08:18 PM   #15 (permalink)
 
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Quote:
Originally Posted by cbdane View Post
That's correct, but it not proportional, i.e. 4 blades generate more lift but not twice as much as 2 blades because of inefficiencies introduced by each blade flying in the wake of the previous blade. Less of an issue for full scale heli's but the airflow doesn't exactly scale (Reynolds numbers, etc.).
Never said it provided twice as much lift. Just said it provides more lift with multi blades over 2 blades. Yes I know about Reynolds numbers and such. I was just trying to provide a simple answer to the original question.

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Old 10-11-2007, 11:55 PM   #16 (permalink)
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In 6sic6s (sic) explanation. Some more clarification with what is happening with the pitch inputs and outputs? I asume the seperate mixer is taking care of the pitch servo. But alittle confussed with the Rx pitch out send (usually aux1). Or is the radio just sending elev and aileron. Not really familiar to 90deg settups.
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Old 10-12-2007, 01:04 AM   #17 (permalink)
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Quote:
Originally Posted by DavidH View Post
Never said it provided twice as much lift. Just said it provides more lift with multi blades over 2 blades. Yes I know about Reynolds numbers and such. I was just trying to provide a simple answer to the original question.

David
You must have read something into my reply that wasn't intended. Just adding to what you said. Sorry to offend.
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Old 10-12-2007, 04:00 AM   #18 (permalink)
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While we're already talking about multi-blade...

I've never been able (yet) to have one of those heads in my hands...

What I always wondered, whether these feature damping or not.

At least from what I think I can see on the pictures from vario, those heads seem to have the feather-shaft fix imbedded into the center-piece...
I guess there isn't much of damping needed since the rotor has greater mass and thus is more stable in itself...

Anyone please enlighten me?
Is there any damping or not?

Cheers already in advance!

Daniel
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Old 10-12-2007, 12:48 PM   #19 (permalink)
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Quote:
Originally Posted by Danyboy View Post
While we're already talking about multi-blade...

I've never been able (yet) to have one of those heads in my hands...

What I always wondered, whether these feature damping or not.

At least from what I think I can see on the pictures from vario, those heads seem to have the feather-shaft fix imbedded into the center-piece...
I guess there isn't much of damping needed since the rotor has greater mass and thus is more stable in itself...

Anyone please enlighten me?
Is there any damping or not?

Cheers already in advance!

Daniel

I believe (he says cautiously, nervously glancing about for lurking curmudgeons) that Vario relies on the mechanical compliance of the plastic grips to provide any necessary damping. I think that I remember reading this in their 20lb () catalog that I ordered from EastCoastVario. Interestingly enough, though, the Century blurb on their website for their all metal head says:

Quote:
Originally Posted by Century Heli
These high performance scale rotor heads consist of an all metal hub assembly, dual ball bearing blade grips with thrust bearings plus thrust bearings which feature a solid damper-less system to eliminate turbulence vibration which is crucial for super smooth flying with multiple rotor blades. The center hub is a machined single piece unit which provides the rigidity needed to perform under any conditions.
Also, Vario makes a big deal about the fact that their multi-blades are held with two bolts so they can't rotate in the grips. The Century head uses a conventional single bolt in the grip.

The DeeTee MD500E (see link in most post earlier in this thread) flies fantastically with the Century all metal head and I fell in love watching the videos. However, if you look at their website starting just yesterday evening, it says that it crashed! They explain that the link popped off one of the blades and caused a major boom strike; I assume not related to damping. I'm trying to copy their setup.
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Old 10-12-2007, 01:38 PM   #20 (permalink)
 
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Quote:
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You must have read something into my reply that wasn't intended. Just adding to what you said. Sorry to offend.

I am not offended. Just stating I did not say it. Perfectly fine to add to what I said. LOL


As for the plastic grip verses metal grips. I am a plastic grip fan. I also believe it helps with the dampening. For example I have an MA rotor head with plastic grips and one with metal grips. The both use the same CNC metal hub in the center. The dampening set up on the metal grips has to be softer than the dampening on the plastic grips. If the dampening on the metal grips is set the same as the plastic grips. The rotor head will oscilate, due to the dampening being too stiff.
There is a couple of manufactuers that use solid rotor heads. They are 2 bladed setups also and not multi blade. Century was not the first to introduce this concept of the solid head. Don Chapman worked on the concept back in the early to mid 90's.

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