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Antiques or Out of Business > GMP Cobra being revived!!

RadioFlyerMk
Senior Heliman
Location: Trenton, Ohio,

Ok, well, I finally got some time to work on the Cobra and I thought I would share my progress, post some pictures and ask some questions.
When I got the heli it was mostly complete which was a big help. The hardest thing to find was the radius arm, but I finally found a complete washout assembly and bought that. The next thing to find will be a muffler. Everything else will be a piece of cake, sort of

So far I have taken the frame apart, cleaned and checked everything and greased the bearings, and now the main frame is back together. The engine looks as if it has never been ran, it is an Enya .49. I would like to keep everything "old school" but if I cannot find a muffler and cannot make one work, then I will have to replace the engine. But I am working on the muffler issue. If I don't find one in Toledo, then I will end my search and give in to something modern.
I plan to trim the main blades and balance them and use them. The tail rotors and main blades were chewed by a dog, but I believe I can slavage the mains. The tail rotors are shot though, so I will most likely end up putting something modern on there.
The next couple of days will be spent going through the head and tail case, checking bearings, etc. Once that is done, I will get everything assembled and fabricate a rotor control rod, make a new fin set and start on setup.

I am wondering about the tail shaft, it threads on to a shaft and there is nothing to "lock" it on. I will have to read the manual when I get to that section. The canopy could really be replaced, but I like the fact that it is original, but it may get replaced??? I could repair it, but not sure I should fly it with the original. I would like to keep the scheme that it has due to it's age.

I will add info and pics as I go. Feel free to comment or post any ideas if you see something that needs attention. One thing I am wondering is, would a OS 32 work on this machine? I don't see a need for a 50 size engine and if I end up putting a different engine on it, I was thinking about a .30 size since this will only be hovered and flow in mild forward flight. I am considering a different means for mounting the tail fins as I don't care for the whole, add goop and fly it. I am also thinking about using some control rod guides for the tail control rod. Even though this is not original, I don't think it would take anything away from the model and it would be a cleaner look.

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"I'd rather hover a Heli......than fly a plane"

01-25-2008 05:28 AM

RAK402
Veteran
Location: Alhambra, CA

I never had the tail part unscrew, even after hours and hours and hours on these tail gearboxes (it was a Hirobo gearbox, and was used on all of the Falcons-Mk I through 808, at least, the GMP Cricket, Competitor, etc.).

I am not a fan of Enya's, but a .45 to .50 sized engine is appropriate. I do not think you will be happy with a smaller engine.

Good luck to you. It looks like you have a very nice machine there!

I have heard of Heloball mufflers being available on ebay, but have not looked myself. This would be appropriate for your machine.

I just picked up a .60 sized Heloball (purchased at the local flying field), with a complete and intact, but ugly, American R/C Commander for $50.00, so the stuff is out there.

01-25-2008 03:28 PM

heli_headcase
Veteran
Location: Hovering around Atlanta

Quickie Response

Forget about using any .32 or even anything under a .40. The gear ratio is set for a lower RPM, higher torque engine and you'd overheat a .32 trying to get the needed power out of it.

Is your Enya the type of engine devoid of holes on the face of the exhaust stack? If so, you have two choices:

1) Find the correct strap-mount muffler, could take a lot of searching, or...

2) Drill and tap the exhaust port to match the mounting pattern of an existing muffler. I see what look like divots on the stack but can't tell if they're drilled through and threaded.

Schluter had what was probably the largest variety of strap-attached silencers ever seen. But in many cases there was an aluminum adapter needed that matched the shape of the engine's exhaust port stack with the shape of the silencer inlet. Do you have access to a machine shop of have any shop-equipped friends?

Additional: The threads on all the Hirobo/GMP tail rotor shafts are designed to tighten under load. This goes for the tail rotor hub as well as the gears on the shafts. They will NOT come off if the heli is flown in stock configuration and rotation.

Good luck

HHC

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

01-25-2008 03:49 PM

R.J.
Senior Heliman
Location: SF bay area, CA USA

Yes, the tail rotor shaft is a left handed thread as HHC says. Keep the tailrotor on the left side as stock and it should be fine.

01-25-2008 05:09 PM

F3CWNB
Senior Heliman
Location: Napier, New Zealand

Quote
I am wondering about the tail shaft, it threads on to a shaft and there is nothing to "lock" it on. I will have to read the manual when I get to that section.

A friend of mine had a Kalt heli that had the tail shaft threaded to the tail yoke. Although the thread tightened in operation, he said that one day it actually came loose!!! On the other hand, when I once had a Kalt, I had to fully screwed the yoke on after a rebuild, well it tightened it's self in flight, changed the tail pitch, caused the heli to pirouette, and necessitated another rebuild (I was a beginner at the time).

Proceed with caution and use plenty of Loctite.

'Life' is Looking Up!!!

01-25-2008 06:20 PM

heli_headcase
Veteran
Location: Hovering around Atlanta

Why?

Quote
Keep the tailrotor on the left side as stock and it should be fine.

I'm going to play 'Devil's Advocate' here and ask why having the tail rotor on the left has anything to do with the threads?

Thought I'd use this as an opportunity to dispel any myths...

HHC

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

01-25-2008 09:02 PM

RAK402
Veteran
Location: Alhambra, CA

Tail rotor was on the right side, if memory serves, on the Cricket.

I will have to examine it, but perhaps, if the tail is on the right side, and the ascending blade is coming up through the rotor wash, turning that direction will tend to unscrew the threads?

01-25-2008 09:19 PM

GMPheli
Veteran
Location: W. Bridgewater, MA USA

I crashed my Competitor because I did not use loctite on the tail hub. I reasoned that it would tighten under load and be fine. Only thing, at the end of every flight when the main gear stopped turning the mass of the spinning tail hub would cause it to unscrew itself. I just threaded it back on and continued to fly. Turns out when trying to tighten it, I pulled to much and loosened the tailbox to the boom (the worm clamp). Next flight the tailbox fell off. I was using a torque tube so there was nothing to hold it on. I hit hold but it piroed so hard it disconnected the battery. It was not pretty. I would loctite and make sure the worm clamp is tight.

01-25-2008 09:40 PM

RadioFlyerMk
Senior Heliman
Location: Trenton, Ohio,

Thanks for the input guys. After posting last night I dug out the manual and started reading. I tightened the tail up using lock tite and all is well. I picked up some more parts today at the LHS and will doing some more work on it here soon. I am going to use a Raptor tail control rod setup, as I like it, and the length is almost perfect. It may little a small amount of trimming. But considering how the raptor rod goes together, that is a super simple fix. I am leaning towards ordering a new canopy, but not sure yet. I also picked up links so I can make new double links.

As for the muffler, I have one here I am going to try. There are holes drilled and tapped in the exhaust side of the engine. They look like 3mm holes? Anywho...I have a muffler, that i do not even know where it came from, but I think I can drill new holes in the muffler and using a ball link allen, I think I can fake out the screws and get 'em in there. I'll see how it goes, and post pics when it is done. It is not going to be a ball muffler, but at this point, if it works, it will be fine. I can always pick up a ball muffler if and when I find one. Besides, it is always easier to find stuff when you don't have to have it.

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"I'd rather hover a Heli......than fly a plane"

01-26-2008 03:17 AM

heli_headcase
Veteran
Location: Hovering around Atlanta

Quote
Besides, it is always easier to find stuff when you don't have to have it.

Ain't that the truth!!!!!

If you're unsure of the thread size, try a screw the same as one of the screws that pass through the Cobra's side frames. These are M3 or 3mm size. I'm guessing the threads are bigger, more like M3.5. I'm not remembering very well but the screws used to hold the engine to the mount may be 3.5mm, maybe someone else can verify this.

*Note* Engine holding screws are M4, not M3.5 as guessed. Try those but I'll bet M4 is too big. If you own any OS .61S series engine (61SF, 61SF-N, 61SX), you can pull a cylinder head screw and you'll have an M3.5 to play with.

Enya has had a history of using JIS (coarse) thread screws in their engines vs the finer ISO threads. If the M3 screws give any resistance or bad wobble when inserted, DO NOT USE them. We'll have to try something different.

HHC

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

01-26-2008 07:52 AM

bigdad390
Veteran
Location: e. liverpool, ohio

Looks real nice. My Cobra had an OS 50 with heliball back in the mid to late 80's but I used to have a strap on heliball that was used in a Kalt Cyclone with an Enya 50 engine. I will look around and see if I can find it.
With the gearing and overall weight, I dont think a 30 size engine would have enough power to hover and do any FF.

01-26-2008 03:14 PM

RadioFlyerMk
Senior Heliman
Location: Trenton, Ohio,

Thanks guys. I have given up on the 30 size idea. I was only thinking about the .32 to keep costs down on a bird that will only be flown a couple times a years, ok maybe a few times. I am considering the use of an old mechaincal gyro. I have one here somewhere.
Hopefully I will get to work on this thing sometime today...I hope to get the tail control system installed and finished, then on to making double links and going through the head. The tail has been lubed and tightened up and is ready. Still needing tail blades, no decision on what to use yet. I read that some guys are using trex 600 tail blades, that may be an option if I dont go with original.

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"I'd rather hover a Heli......than fly a plane"

01-26-2008 08:46 PM

RAK402
Veteran
Location: Alhambra, CA

The originals were semi-symmetrical wooden blades.

They worked pretty well.

I do remember people substituting Kavan Alouette tail rotor blades (probably the only useful part a of Kavan Alouette) on some things, but not specifically on your machine.

01-26-2008 09:32 PM

R.J.
Senior Heliman
Location: SF bay area, CA USA

Quote

Why?

Quote
Keep the tailrotor on the left side as stock and it should be fine.

I'm going to play 'Devil's Advocate' here and ask why having the tail rotor on the left has anything to do with the threads?
Thought I'd use this as an opportunity to dispel any myths...

HHC,
Hey, I thought I was agreeing with you

You're right, if you stick with the stock gearing configuration and rotation, it doesn't matter if the tailrotor is on the right or left as far as the hub tightening/ left handed thread is concerned.

01-27-2008 03:10 AM

RadioFlyerMk
Senior Heliman
Location: Trenton, Ohio,

HHC, your right, those screws are a different size from anything I have. I need some set screws for the tail yoke, so I will look for some at the local hardware store. I am pretty sure they will only have 3, 4 & 5mm though. They may prove difficult to find. Who would've thought. Not to mention the headache with the muffler.

I have some K&B blades on the tail as of now, but I don't think they are staying. I may opt for some wood blades... just in case of a tail strike. I don't want to have to be looking for more parts, and I could make the tail blades if need be. I believe MP Heli's has the wooden blades though.
My thoughts are that the wood blades would break in case of a tail strike vs. a difficult to find part. Of course my main goal will be, "Be careful". I have sorted out most of the linkage, installed some ball ends and I still need to take that head apart, guess I have been putting that off. Once that is done, I will be well under way.
I am thinking I will order a canopy and tail blades from MPHelis.com. I might as well order a fin set since I will be ordering. That would save me the trouble of cutting my own for $3.50. Good progress so far, but I am creeping towards a standstill point. Between the muffler issue and bolts. Hopefully something will give soon, I need to get it done and tested well before IRCHA. There are several that are wanting to make a show of vintage heli's.

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"I'd rather hover a Heli......than fly a plane"

01-27-2008 07:59 AM

heli_headcase
Veteran
Location: Hovering around Atlanta

Quick Test...

RadioFlyerMk,

Try this - Remove one of the Enya's cylinder head screws and see if it will thread into the muffler mounting hole. If it works, you'll have something you can use as an easy reference but not necessarily easy to find. The screws may be Philips head and if so, please use an unworn #2 Philips driver to remove it. You don't want to go tearing up this thing.

HHC

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

01-27-2008 04:03 PM

GMPheli
Veteran
Location: W. Bridgewater, MA USA

Anyone seen these tailblades before? They are flat bottomed and 91mm. I am going to use them on my Legend. And speaking of Enya screws, does anyone know the size that holds the carb on to an Enya 60X? They are not 4x.70 or American. I think they must be 4x.75

Alan

Fastphoto is not working right now. I will try later

01-27-2008 05:05 PM

GMPheli
Veteran
Location: W. Bridgewater, MA USA

Radioflyer, what is the distance between the mounting bolts on your Enya? I found a heliball that is in very good shape that has mounting holes 30mm apart

01-27-2008 06:18 PM

RadioFlyerMk
Senior Heliman
Location: Trenton, Ohio,

30mm....Sounds like a fit. The take off is 9.7mm thick and the opening is about 22 X 6.6mm. If it'll fit and you wanna sell it let me know. I'll have to wait til pay day though. But this sounds good so far. Thanks a bunch for checkin'.

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"I'd rather hover a Heli......than fly a plane"

01-27-2008 08:10 PM

GMPheli
Veteran
Location: W. Bridgewater, MA USA

Sell it, hell I'll give it to you. PM me with your address.

Fastphoto is still not working for me, anyone else having trouble? Says:
FastPhoto processing underway. Click Preview below to refresh status

And has said it all day

01-27-2008 10:57 PM

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