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Cox Engine of The Month
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Mudhen's

"Prototype T.D. .010 c.1960"



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Russian VA .020 engines

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Re: Russian VA .020 engines

Post  Surfer_kris on Sat Aug 25, 2018 2:47 pm

How are people getting on with their little VA engines?

Here is a little update on mine. I've finally finished the carb that I started making for it. I've used the design idea of the Norvel engines, and below is the result (with a Cox .05 RC in the last image for size reference).

Lots of really tiny parts to make, the needle is made from brass with an M1.2mm thread (about 0.047 diameter) for instance. The idle is not super low, but it works. Smile
Might have too increase the throttle bore a little though, I think a lost a little too much at full throttle while the fuel draw seems good.





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Re: Russian VA .020 engines

Post  1/2A Nut on Sat Aug 25, 2018 3:12 pm

Most excellent Kris, I saved a few pics of your work. Thumbs Up
With the header I am looking forward to what sort of rpm's it will make with the APC 4x2!
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Re: Russian VA .020 engines

Post  Surfer_kris on Sun Aug 26, 2018 1:13 am

Thanks for the kind words. I think for all out rpm the larger of the two stock venturis and a bladder tank would be the way to go.

I could make a few different throttle barrels though with different bore diameters and do some testing I guess. It would be fairly easy to swap barrels at the field, as they are simply threaded onto the opposing fuel inlet part.
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Re: Russian VA .020 engines

Post  1/2A Nut on Sun Aug 26, 2018 2:48 am

I think the time spent was worth it to have control over engine rpm. I really prefer that
over tossing a screaming engine with no way to shut it down if the radio acts up etc.

The barrel size matches the bore in the carb currently by the looks of it.



Might be less restriction / improvement if you bring the carb body down closer to the crank case.

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Re: Russian VA .020 engines

Post  Surfer_kris on Sun Aug 26, 2018 6:04 am

It doesn't show up well in that picture but the bore in the barrel is 2.0mm, while the hole in the carb is 2.4mm. The smaller stock ventury is 2.0 in diameter, and it is unrectricted, so therefore I think I would be better off with around 2.5mm both in the carb and int the barrel. With the Norvel style there is quite an obstruction in the center, as both the needle and needle seat are exposed there.

The carb could be lowered a little bit but I don't think that it would change much, it is the hole in the barrel than limits the flow, as this bart is obstructed by the needle and its seat.
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Re: Russian VA .020 engines

Post  getback on Sun Aug 26, 2018 7:00 am

Good looking Carb. Kris , the fact it works is a + with a little tricking you can get it the way you want it to run . Very Happy
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Re: Russian VA .020 engines

Post  1/2A Nut on Sun Aug 26, 2018 12:07 pm

2.5mm would be a prudent way to compensate for the obstruction, what is the dia. of
the larger stock venturi bore insert?

Given the shorter length stock inserts the difference of the entire intake of the RC carb
in it's current length has the mouth of the intake much further away from the crank window.

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Re: Russian VA .020 engines

Post  GallopingGhostler on Sun Aug 26, 2018 2:34 pm

Actually, in some cases the extra length can help the airflow straighten itself from the turbulent conditions at the mouth of the venturi. I think it was Joe Wagner, engine guru who one time mentioned this as to why some of the manufacturers had a lengthened venturi. The static pressure loss in such a short distance is negligent. Of course this is in reference to inlet, but the short distance after the NVA would also in a similar vein make little difference, IMO. If you notice in some of the 4 strokes, they have a lengthened piece of pipe after the carburetor to carry the air-fuel mix to inlet at top of the head.
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Re: Russian VA .020 engines

Post  ticomareado on Sun Aug 26, 2018 8:06 pm

Be sure to put a drop of hot candle wax on glow element before starting lathe work.
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Re: Russian VA .020 engines

Post  Surfer_kris on Mon Aug 27, 2018 8:16 am

1/2A Nut wrote:what is the dia. of
the larger stock venturi bore insert?

Larger one is about 3.0mm in diameter (!), a big jump in area compared to the smaller one and it will not really run without pressure. Something around 2.5mm might still work on muffler pressure though.

So, 2.5 for the carb which has an obstruction should be fine, could possibly try go a little larger even.


Last edited by Surfer_kris on Mon Aug 27, 2018 8:34 am; edited 1 time in total
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Re: Russian VA .020 engines

Post  Surfer_kris on Mon Aug 27, 2018 8:17 am

ticomareado wrote:Be sure to put a drop of hot candle wax on glow element before starting lathe work.

Why?

I don't see how/why the glowplug filament would care if the carb is in a lathe?
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Re: Russian VA .020 engines

Post  ticomareado on Mon Aug 27, 2018 8:49 am

My comment was in reference modifying Merlin .049 inserts to fit the .020.
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Re: Russian VA .020 engines

Post  Surfer_kris on Mon Aug 27, 2018 10:21 am

Ahhh, okey.

I still don't know if there is enough meat on those, or the Norvel ones.
I think the Turbo plug is a much better option anyway, as you then get a choice of heat ranges to play around with too.
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Re: Russian VA .020 engines

Post  1/2A Nut on Mon Aug 27, 2018 3:02 pm

Yes go for the ceiling at 3mm the obstruction takes it down some as is.
The throat length helps to increase draw so you should do just fine with exhaust pressure.

The next question is what plane are you going to build for it Eyebrows
Should be a strong engine for something like a GLH .020 size.
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Re: Russian VA .020 engines

Post  Surfer_kris on Tue Oct 02, 2018 12:44 pm

Here is a little update. I have now increase the throttle barel bore to 2.4mm in diameter. Seems to work pretty well, perhaps not quite as crisp throttle respons as before but the top end has improved. It will now do about 7000-19700rpm on the Cox 4.5x2 prop and 10% nitro. Not too shabby and it is good enough to fly something I think.  Smile

It was rainy this evening so I only gave it a short test-run in the workshop with a long hose to the outside for the exhaust....
Not recommendable, I know, but I also let the door a little open to the outside. Wink
Here is a short film:

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Re: Russian VA .020 engines

Post  balogh on Tue Oct 02, 2018 1:23 pm

Given the backpressure of the long exhaust duct I still find it running very impressively. Does it have SPI?
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Re: Russian VA .020 engines

Post  Surfer_kris on Tue Oct 02, 2018 1:32 pm

Yes, it does have SPI and that will make it more sensitive to the exhaust backpressure.

I had intended to change the piston height (it should be adjustable like the on their .049 engine I think), in order to reduce the SPI, but I haven't managed to take it appart. I simply cannot get the conrod off from the crankpin and I don't want to force it too much...

I could always try shimming the cylinder instead.
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Re: Russian VA .020 engines

Post  1/2A Nut on Tue Oct 02, 2018 7:25 pm

Carb is doing great Kris Thumbs Up , it was worth opening up the barrel and good for another 1.2k + rpm with the open header.
Have you thought about what plane you want to fly it with?

RC Plane
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Re: Russian VA .020 engines

Post  Surfer_kris on Wed Oct 03, 2018 6:54 am

I'm not sure what to put it in yet. I have been looking for a small RC engine for a bit of relaxed sport flying, but this one deserves something faster I think. Perhaps another "stick" these do have a nice compromise between high and low speed capabilities.

My lates .10 stick is performing really well now that I have it trimmed in properly. Added some engine downthrust and less wing incidence and it is now a real smooth flyer. With an OS .10 and a 7x5 prop in a 20oz plane, one can cruise around at half throttle or speed it up and do unlimited verticals etc. Smile

So something around (.020/.10)^1/3 ≈58% could be a fun addition to my "stick-fleet".


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