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Contra Piston and Norvel .061 Diesel Conversion

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Contra Piston and Norvel .061 Diesel Conversion

Old 07-13-2015, 04:39 PM
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DaveyMo
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Default Contra Piston and Norvel .061 Diesel Conversion

Greetings Good People:

Running my .061 today in preparation for some weekend flying. It ran very nicely until almost the end of the tank at which time it stopped instantly (as in no winding down or sputtering; the prop came loose). I took the head off and found the contra piston lying on top of the piston! I could find no damage to the cylinder, piston or lower end as far as I could tell without a teardown. I tried to put the contra piston back into the head by hand, but it's going to take a little more force to send it home.

Wow! Any idea what gives? I thought I had good settings on the contra piston and needle valve with a slight coffee color to the exhaust when this happened. I'm burning castor 25%; ether 34%; kero 40%; and AmS 1.5%. The conversion head was provided by NV Engines.

I await the collective wisdom here, such as it is. Thanks!

Davey Mo....

PS: This Norvel makes s sweet-running conversion and I'd hate to abandon the concept.
D...
Old 07-13-2015, 09:36 PM
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Dave: a friend had something very similar happen with his converted 061 - the contra had dropped and was moving up and down with the piston. He assumed the head was wrecked. In the end, though, he replaced the copper head shim and got the contra back ,in the head, and the engine is fine. His theory is that the head shim prevents the contra dropping. I can't make a lot of sense out of that, as I'm used to contrapistons having an interference fit in the head, but in any case, we put a lot of flights on the model recently, and all is well. Hopefully yours will be ok as well.

(I like how the engine runs as a diesel - prefer it to the glow head.)

Steve
Old 07-14-2015, 05:54 AM
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Mr Cox
 
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There should be an O-ring seal on the NV heads, so check that this is not damaged. I don't think the contra will drop down to gravity, but rather from fuel seeping passed the o-ring and creating an overpressure above the contra. You could make a small vent hole above the contra to stop it from dropping down.
Old 07-14-2015, 02:23 PM
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DaveyMo
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Steve: Good to hear from you on this! You've always got my back, helping me out whenever needed - many thanks my friend! It almost appears that I could just put it back together an it might run just fine.

Mr. Cox: Now that I think back, there was some fuel leaking from the threads of the contra adjustment screw, probably indicating an overpressure situation. The NV head I have is not fitted with an o-ring so it depends only on a tight fit. My plan is to drill the vent hole as you suggested. I'll then chill and oil the contra to get it back into the head. Thank-you very much for this suggestion.

Davey Mo...
Old 07-14-2015, 05:59 PM
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Back again folks:

I took Mr. Cox's advice and drilled the head. This engine runs extremely well now! Thank-you again.

Steve: Yes, I too like this better on diesel than glow. Power output seems prodigious. If you don't mind a vocal comparison, the glow sounds like a screaming soprano while the diesel sounds like a growling alto!

Davey...
Old 07-15-2015, 02:00 AM
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Glad to hear that you've solved the problem. Do you have any favourite prop and rpm readings?

I haven't taken my NV head appart, just assumed that there would be an o-ring like there is on RJL ones etc. Need to put mine in a lathe to fix the threads, should make a hole in it too then.
Old 07-15-2015, 01:24 PM
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DaveyMo
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Mr. Cox

It turns a 6x3 Master Airscrew at 17,100. I'm happy with that.

Davey...
Old 07-16-2015, 01:22 AM
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My friend's would turn an APC 7x3 (a really good prop for it) in the mid-16000s, from memory. By comparison, my PAW 06 does about 16200, and my MP Jet 061BB manages mid-17000s, all on the same prop. So the NV is a good competitive engine, particularly since it's not ballraced.
Old 08-05-2015, 07:25 PM
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Hello Again All:

Here's a photo of the Norvel .061 diesel mounted on a 1/2A Junkers (slab fuse and slab wing). The engine runs like a charm and pulls the model around right smartly on 40-feet lines.

A question about a flight characteristic. When the plane comes around and is directly upwind, it suddenly gains a lot of elevation. Now I expect the lines to loosen a bit and can compensate, but I don't care for the elevation gain at the same time. Right now, I'm just struggling for level flight so that I can move on from there. Is there anything i can do to remedy this? By the way, this is my practice model for the Ee-Zee One and KMD combination that I've about finished building.

Here 'tis:



Davey Mo
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Old 08-09-2015, 09:23 AM
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Hi Davey, where is the CG located? (percentage of wing )

Tony
Old 08-09-2015, 11:00 AM
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DaveyMo
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Tony:

Hmm..., you might be on to something here. It is around 31% (I hadn't realized it was so high).

Dave...

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