why bigger rotors?

why bigger rotors?
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why bigger rotors? - 8/15/2007 8:30:26 PM

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I've got a question, I'm seeing guys put larger rotor blades on their MX's. What's the reason for this? how much bigger can you go?

seems that everyone is putting 315mm Align blades on from what I see...

thanks!

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RE: why bigger rotors? - 8/16/2007 3:50:35 AM

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Noting first that you have to get the longer belt and tail boom for the larger rotors-- well, maybe not for 315s, but there's a risk of tail interference.

Anyway, that said, the larger disc area means that the collective response is a lot snappier (I have SAB 285s and 315s both, and you have to see the difference to believe it), and the greater blade mass gives you a bit less bog on hard collective maneuvers. It should also give better autos, though I only go there when I have to

, but they are more stable as well-- the larger disc seems less responsive to cyclic, but not by a lot.

I expect that there would be a penalty in flight time from the drag, but the aerodynamics for that are complex-- the greater blade length will be tempered by higher R number giving greater lift... anyway, I haven't tested that

.

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RE: why bigger rotors? - 8/16/2007 3:01:10 PM

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I could have sworn that I read someplace that larger rotors generally lead to longer flight times..... Larger rotor disc = more lift area = less headspeed required to create the same amount of lift = lower amp draw = longer flight times????!!!!

Ever seen a heli set up for endurance flying or for filming on board video? The main rotors on these are pretty large compared to a normal heli kit. They fly alot smoother too!! Very slow and peaceful machines.

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RE: why bigger rotors? - 8/16/2007 6:59:43 PM

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I was assuming the same head speed to maintain responsiveness, so parasitic drag would dominate. You're right, though, if you slowed the head and maintained everything else you'd get longer flights if the blades were the same (general) shape.

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RE: why bigger rotors? - 8/17/2007 3:56:35 AM

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I was thinking about putting some longer blades on my MX400 because even with close to the max pitch I can set it up for the lift is still too weak. Is this something you've noticed with the stock MX or am I doing something wrong? I only have the template from the manual, but there's more than eight degrees of pitch and the washout gets pretty close to the T at full positive pitch. I've got the pitch rod in the last hole on the arm and I'm using the entire pitch range on the transmitter, plus the full EPA. I am using a 2100mAh pack, 3400KV motor and 11t pinion. I calculated the headspeed it should give me around 2500RPM and it looks fast enough to me (sorry, no tach either).

< Message edited by Orion1024 -- 8/17/2007 3:59:34 AM >

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RE: why bigger rotors? - 8/17/2007 4:15:54 AM

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What is too weak? A 3400kV motor with an 11T pinion should be turning at least 2650 RPM, and take off with at most 4 degrees of pitch. I have a 3000kV JGF on an 11T with SAB blades, and it pops my 400 around pretty well.

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RE: why bigger rotors? - 8/17/2007 6:01:42 AM

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osterizer,

I need to have the stick all the way up for even a moderate climb. I certainly wouldn't trust it to be able to pull out of a loop. It's something that's hard to describe or quantify. The rotor seems to be spinning fast enough. Maybe the pitch is being restricted somehow when it's actually flying. First thing I'm going to try is different blades. I was thinking the plastic over the hollow areas of the stock lightened blades might be caving in from the air pressure and creating turbulent airflow that's reducing lift. The motor isn't bogging either.

< Message edited by Orion1024 -- 8/17/2007 6:11:30 AM >

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RE: why bigger rotors? - 8/17/2007 6:32:41 AM

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if it isn't bogging at full pitch then something's wrong. You'd do well to find a tach, frankly, and rule that out.

I've never actually flown the original blades-- they seemed so flexy and weak they scared me off. If you don't want to spend a lot of money try some Align FG blades; if you want some you'll keep, the SAB 285 carbons are good with the short tail, 315s with the long.

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RE: why bigger rotors? - 8/17/2007 7:23:15 AM

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Yeah, I just mean bogging excessively like the motor or battery is the cause. It may bog a little. One thing I noticed right away is that the airflow over the stock blades don't sound smooth...and they are tracking perfectly. I've got some other (solid) 290mm woodies I use on the 36 and you can tell by the sound that the airflow is nice and smooth. I'll make some spacers and try them tomorrow. I might break down and get some of the $24 CF blades from Helidirect when they come back into stock. Yes, I would like to get a tach but I want an optical one that lets me tach it at any distance. We have a local surplus store here that might have some LCD panels I could use to build one, but the projects are many and time is limited. Maybe one day.

< Message edited by Orion1024 -- 8/17/2007 7:51:40 AM >

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RE: why bigger rotors? - 8/17/2007 2:04:42 PM

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quote:

ORIGINAL: Orion1024

osterizer,

I need to have the stick all the way up for even a moderate climb. I certainly wouldn't trust it to be able to pull out of a loop. It's something that's hard to describe or quantify. The rotor seems to be spinning fast enough. Maybe the pitch is being restricted somehow when it's actually flying. First thing I'm going to try is different blades. I was thinking the plastic over the hollow areas of the stock lightened blades might be caving in from the air pressure and creating turbulent airflow that's reducing lift. The motor isn't bogging either.

woah! that don't seem right, I do decent climbs at half stick...

seeing how you are close, if you want my limited knowledge, or to borrow my tach, I'll gladly give you what I have... I can turn you on to my heli-lama dude if all else fails...

(look at your heli when you get a chance, the servo up front that's on the arm was hitting the servo up front on the right side of the heli, it was actually keeping me from lifting, I took an x-acto knife to it, and all is good, maybe this is what's going on? It was contact between the servo and the servo mount on the frame)

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RE: why bigger rotors? - 8/17/2007 2:56:10 PM

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I would check my head setup before muckin about with tachs and stuff.... 8 degrees max pitch?! Doesn't that sound like far too little?

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RE: why bigger rotors? - 8/17/2007 4:32:08 PM

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8 degrees should be OK for most flying-- over 10 is usually just making noise on a 400, particularly on hard reverses when there isn't any induced flow or what there is is going the wrong direction.

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RE: why bigger rotors? - 8/17/2007 6:49:19 PM

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thestratcat,

Wow, thanks for the offer! That's quite generous to extend to someone you don't know. Hopefully I can figure out what the problem is this weekend. I hate these mystery problems! (intermittent ones as well). It's definitely over eight degrees on the bench...maybe nine. I'll have to look at the servos and linkages as you suggested. I know the rotor has some resistance to it (not from the belt, though), but I don't think that's enough to be causing the problem. After that, I guess I need to clamp the heli down to my test platform and observe what's going on with the pitch. My battery and motor are running cool (bout 120F on the batt and 110F on the motor), so I wouldn't think it's a mechanical resistance problem, but you never know.

< Message edited by Orion1024 -- 8/17/2007 6:50:43 PM >

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RE: why bigger rotors? - 8/17/2007 6:53:25 PM

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quote:

ORIGINAL: Orion1024

thestratcat,

Wow, thanks for the offer! That's quite generous to extend to someone you don't know. Hopefully I can figure out what the problem is this weekend. I hate these mystery problems! (intermittent ones as well). It's definitely over eight degrees on the bench...maybe nine. I'll have to look at the servos and linkages as you suggested. I know the rotor has some resistance to it (not from the belt, though), but I don't think that's enough to be causing the problem. After that, I guess I need to clamp the heli down to my test platform and observe what's going on with the pitch. My battery and motor are running cool (bout 120F on the batt and 110F on the motor), so I wouldn't think it's a mechanical resistance problem, but you never know.

I'm telling you man, check that servo up front on the whatever the hell.. I went out last night, tried mine, it would spin but no fly! I scratched my head (and probably scratched some other things) while thinking then it hit me that the servo's were just barely catching each other, not giving me full throw.. if you want, I can PM you my number and you can give me a call...

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