Help! - Need to find 620mm Wood Rotor Blades

[ticedoff8]

I have a GMP "King" Cobra that flys great on Aerotech 620mm Wooden Rotor Blades.
I finally broke them - and now, I can't find any replacements.

What makes the Aerotech special is two things:
a) They are thick & wide: They have a wide chord (60mm) and a think airfoil (8.2mm)
b) They are "low price" - $30 USD.

The Cobra is an antique helicopter - which mainly means:
a) It has limited collective pitch: +/-8 degrees
b) The fastest rotor RPM I can run is around 1800 (faster than that, and I think it would explode).
c) They originally had flat bottom airfoil 550mm long - high lift.

There are other wood blades, but they are all 600mm long, 51mm chord & 7.5mm thick. I think they were OEM for Raptor 50 & Swifts. At 1750 RPM the 8 lbs Cobra barely gets off the ground - into a hover. I can't fly it with these blades

Mavrikk makes the G5 620 ($69) - I have a pair of these. But they are 51mm chord.
Mavrikk also makes the G5 wide chord ($89) - I don't have a pair of these.
$69 - $89 each pair?!? - I don't need CF main rotor blades - I just need a set of simple blades for my old heli.

Century Heli was the sole distributor of the Aerotech blades - I think everyone else bought them from Century.

Does anyone have a suggestion to find a 620 (or so) wide chord (60m), low cost, blade? I could even use a 670 or 680mm blade, and cut it down.

[ticedoff8]

My Bad.
I just looked at the Century Helicopters web site, and the Aerotech 620mm blades are listed as "In Stock" - the last time I bought a set from Century, I was told that was it, never again.
I will verify tomorrow if they are actually "In Stock".

[schluterdude]

let me know if they are in stock! i have align blades on my schluter and they just don't cut it. the headspeed is so high it's scary!

nice fat wide blades. that's what the oldies like! if they have semi-symmetrical onces, that'd be great!

[ticedoff8]

My Bad.
I just looked at the Century Helicopters web site, and the Aerotech 620mm blades are listed as "In Stock" - the last time I bought a set from Century, I was told that was it, never again.
I will verify tomorrow if they are actually "In Stock".
I called Century Heli this morning.
When the web site says "In Stock", they really mean "Maybe tomorrow or the day after".
For those that are ordering online, I guess that's not a problem - they'll ship on Wednesday instead of today.
Good thing I called, I was planning to drive up there.

These are CN2342 (620mm) & CN2343 (640mm) wood blades with a fully symmetrical airfoil - not "simi-symmetrical" or flat-bottomed.
The specs say they are 58mm chord and 132 - 140g.

Until I get my hands on a pair, this is what I expect (from the previous set I bought):
I had to screw & glue the root blade-holders on, but that was an easy process (I used 5-min epoxy) and they are pre-covered.
There are "tip weights" pre-installed and neatly buried in the blades - the tip weights were still buried in the wood blade sections after "the big one", so I'm not worried about losing the weights in normal flight or major crashes.
The ones I bought before were not a white cover (like the web site picture), but an attractive yellow/black cover - but I expect the new ones to be all white. Mine were around 136g and required about 2" of 1" wide vinyl tape to match span-wise CG and weight.
The package included two 1mm(t) x 10mm(d) "shims" and 5mm-to-4mm brass insert adaptors for the bolts and some stickers to mark the "red" & the "black" blade tips.
If they are the same as the one's I bought before, they should be pretty close to the specs.

I think they lowered the price (I honestly don't remember what I originally paid for them, but these are $25 a pair)

[schluterdude]

when you get yours, do me a favor if you wouldn't mind....

hang them off a nail off the side of your workbench. then, run a piece of string with a weight on the end off the nail down the blade. mark on the blade at the tip and see if both blades line up. essentially, this is the lead/lag point of the blade (not nececeraly the helicopter!), due to cg's, and is adjusted by where the weight is positioned in the tip. it should be about 33%ish back from the leading edge on both blades. this point, should also be the same point where the blade will stay stationary (not following an arc) when you twist the blade grip. if a drawing would help, i can draw one up and post it...

[cbflys]

FYI, there's another way to find the lead/lag. If you mount the blade on the heli and pop off the link going to the blade grip, you can move the blade (lead/lag wise) until it doesn't flop. You can then extend the axis line out.

[ticedoff8]

when you get yours, do me a favor if you wouldn't mind....

hang them off a nail off the side of your workbench. then, run a piece of string with a weight on the end off the nail down the blade. mark on the blade at the tip and see if both blades line up. essentially, this is the lead/lag point of the blade (not nececeraly the helicopter!), due to cg's, and is adjusted by where the weight is positioned in the tip. it should be about 33%ish back from the leading edge on both blades. this point, should also be the same point where the blade will stay stationary (not following an arc) when you twist the blade grip. if a drawing would help, i can draw one up and post it...
I typically don't check chord-wise CG (not for 20+ years at least), but I'll check when I get them.

I called Century this afternoon: Their shipment arrived, so I'll drive up and get a couple of pairs tomorrow. After badgering him for a few minutes (Are you SURE they are there?), he went out to the loading dock and looked in the box - he says they have about 1000 sets. That should hold me for awhile.

[schluterdude]

FYI, there's another way to find the lead/lag. If you mount the blade on the heli and pop off the link going to the blade grip, you can move the blade (lead/lag wise) until it doesn't flop. You can then extend the axis line out.

i agree chuck. but, if the weights at the tip are not in the correct spot, rotational forces will try to make the blade "think" that it is located elsewhere potentially causing problems.

[ticedoff8]

Picked up three sets of blades this morning.
Two CN2342 (620mm) & one CN2343 (640mm) - $83 total USD out the door.

I checked the specs on the CN2342:
Out of the package, you get two rotor blades finished in a thin, white, shrink wrap covering (simi-translecent), a "decal sheet" with Red & Black tape and a separate baggy. The baggy contains 4 plastic blade grip halves (2 complete), 2 brass inserts, 4 1mm-shims, and about 6 tiny screws.
The blades are protected in the package by a thin cardboard "taco".
The plastic shrink wrap is perfectly applied and sealed at the ends.

You have to screw & glue the blade grips on yourself. The plastic covering is already cut out where the grips glue to the wood.

Both blades are exactly the same length: 619.1mm measured along the "plumb-line", bolt-hole to tip.

They have a fully symmetrical airfoil that is 8.5mm thick & a constant chord of 58mm.

Without the grips installed, the blades weigh 140.5g (+/- < .1g) each (I use an Ohaus 2610g Triple-beam balance).

The span-wise CG is off by 5mm from blade to blade (331mm "Red" blade vs. 336mm "Black" blade). This is not good.

The blade bolt hole is centered 13.5mm from the leading edge. I hung the blades from a nail and hung a plumb-bob from the nail. I measured the chord-wise CG at 10mm from the tip (because of the curved shape of the tip).

You can easily see the blade weight grooves under the covering. The grooves are exactly the same between the blades - length, width and distance from the leading edge (5mm) - and perfectly finished (you can not feel a transition under the covering).

The plumb-bob line was 20mm from the leading edge on the Black blade and 19mm on the Red blade. This would be a 5.5mm (Black) & 6.5mm (Red) "lag".
Makes sense: The Black blade needed more weight to match the Red blade, and the extra weight moved the chord-wise CG forward.
They should move the weight groove back from the leading edge, and additional weight won't have as much effect on chord-wise CG. But it isn't enough for me to worry about.

Other than the covering, these look just like the older AeroTech blades I got from Century.

I'll have to add weight to both blades to match the span-wise CG - but this will give me a chance to also match the chord-wise CG. Besides a final balance and matching span-wise CG, these are pretty close to ready to fly.

Now, I'll fly them, but I expect they'll do what I need.

I'm glad Century Heli made the investment to bring them back. I think they fill a gap.

[cbflys]

Ray Hostetler has a very good DVD on getting your blades perfect. I highly recommend it, especially if you're using wood blades.

https://www.rayshelitech.com/store.html?product=37

[ticedoff8]

For anyone that cares: A few final notes about these blades.

First: All three packages had the wrong blade grips. The kits include brass inserts for the blade bolts. These inserts are 4mm ID and 5mm OD. The grips are drilled out for 4mm OD inserts.
Opps.
I didn't want to chance screwing them up by drilling them out myself, so I ended up driving back to Helicopter World and exchanging the blade grips for the "right" ones.
Except, they just drilled them out themselves. I assume they have a "special jig" they used to get it right. Anyway, if you buy a set of AeroTech blades form Helicopter World - first thing is to check you got the right blade grips. If you didn't get the right grips, let the world know they screwed up (again).

Second: The blades are matched perfectly for weight - each blade was less than .1g different from each other (from the same pack). But the span wise CG did not come close.
I bought two sets of 620mm blades - and the blade to blade CG was off by 4 to 5mm to its mate from the same pack. To get the CG to match, I would have had to add about 3g of weight to the tip of one of the blades - and a matching 3g at the CG of the now "light" blade.
Lucky for me, when I compared the blades between the two packs, I found I could mix-n-match blades to get 2 pair with good span-wise CG - close enough that I could get them perfect with a little extra tape.
I understand how this can happen - one blade needs more weight than another to get to a desired final blade weight - and the weight is added close to the tip, so that screws up span-wise CG. But I would think Helicopter World would do a better job overall matching blades. - not just weight, but also CGs.

I guess maybe I expect too much from a $25 pair of blades.
But, I think I got "lucky" that I found a matching pair of blades out of 2 packages.

Now that they are mounted and I've flown them - they seem to be okay. They track well and there is no vibration that I can detect. They didn't flutter at 1800 RPM and they seem solid.

My vintage "King" Cobra really flies a LOT better with the extra lift from the wide-chord blades. Maybe I'll buy a set of the Mavrikk G5 Wide-Chord blades one day, but until then, I guess I'll stick with these Helicopter World blades.

[vermonster]

I wouldn't buy an effin thing from Helicopter World. Biggest rip off going. I bought a manual for a Century Jet Ranger fuselage for $19.50 plus shipping and received 1 piece of paper that my black labs could of wrote.

[blax1]

http://www.centuryheli.com/products/productdetail.htm?prtnm=CN2342&currentid=613

We care..... ;)

[ticedoff8]

I wouldn't buy an effin thing from Helicopter World. Biggest rip off going. I bought a manual for a Century Jet Ranger fuselage for $19.50 plus shipping and received 1 piece of paper that my black labs could of wrote.
I don't want to throw them under a bus, but I haven't had a lot of good luck with them either.

I bought some machined aluminum pieces from them, and they were supposed to be tapped for a 3mm screw - the hole was so oversized the screw wouldn't "bite". I ended up doing a lot of extra work I shouldn't of had to do. I told them about it, and they didn't seem too concerned.

There's been a couple of other things that weren't right either.

I can only image what it would be like if I was buying from them mail-order - and kept getting screwed up parts.

I like the looks of their new Radikal gasser - but I would be worried some critical part would have to be returned because of QA problems.