Nalon Viper 2.5cc CI Engine

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pat_pending

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Pat - You would be right but don't dismiss that info lightly. Have a look here Barton Model Flying Club :: View topic - Nylon Props - post 6 is from someone I have known for many years - a seasoned, very knowlegeable and highly experienced aeromodeller. The potential is there with these specific props - not necessarilly will but might - if it does then personal safety and that of the engine/model is of concern. I tend to work on the basis that if you don't use them the problem doesn't exist ;)

BTW If you really want to see an engine working (maybe that should be hear!) at it's best try You Tube - 'tethered model car championships'
Scary stuff with the Nylon props! I reckon I'll stay away. I like the look (and less sharpness) of wood props for bench running but the sizes are much more limited.

The tether car stuff is cool also. Perhaps i could hone my TIG-welding skills to make a tuned pipe one day 🤣
 

pat_pending

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Hi all. I took the plunge today and went for those cylinder liners. I was nervous about destroying the blanks, newly made cutter and the only 2.5mm 4-flute endmill I own so I took things really really slowly. I think the result is pretty decent and I'm sure the holes are accurate to within a metric whisker.

Still a little bit of de-burring to be done as you can see but the trick with the tight-fitting aluminium plug really worked a treat to reduce the burrs on the inside of the liner where they would be the most difficult to remove or whey they are most likely to ruin the lap.

Right I think I'm onto lapping and then cast Iron bits. Annoyingly, I have run out of aluminium bar so need to get my hands on some before I can make a lap! I have ordered a few from Noggin End so should be there in a couple of days,

Im really excited to be on the finishing straight now!

Thanks,
P

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a couple of pics from making them.

Height set from bottom of the liner seat using cigarette paper. I marked the widest tooth with red marker as a couple had been touched with the file during backing off. Overall width still good to within 0.02mm


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Cut the port using lots of cutting, a slow speed (220RPM) and enough feed so that chips were coming out and not crumbs.

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Use the needle centre-finder to precisely hit the scribed line for the start of the transfer port hole with the mill head set to 25deg.

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Use an undersized 4 flute endmill (2.5mm in this case for a #33 drill which is 2.89mm). High RPM and peck drill with a really gentle pressure particularly before the cutter was completely seated in the hole.

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Finally follow the 2.5mm endmill hole with the #33 drill. This didn't feel nice as the drill was bouncing around until fully in the hole. I reckon going all the way with a #33 sized endmill (if that exists). I was toying with the idea of using a readily available 3mm endmill bt it was too late to adjust the plans so i stuck with it.

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pat_pending

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Gday Pat
I usually use teflon for my gudgeon pin end caps.
Hey Billit. Any tips on the gudgeon pin caps? Do the caps actually fit into a drilled gudgeon pin or are they just plugs at the end of the hole in the piston using a shorter pin?

Patrick
 

pat_pending

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Got a bit of time over the past couple of days to work on the viper. I managed to get the pistons lapped and tapered, made the pistons and wrist/gudgeon pins. I think from here on in I will shift to one engine at a time as my patience is like a kid before Christmas and i have an oily/smoky workshop firmly in my sights!

Quite a collection of bits now. Engines 1 + 2 will be complete. Engine 3 I will do at some point in the future (or one of the engines explodes in which case I have spares).

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Lapping using a simple aluminium expanding lap as I did with success on the Boll Aeros. Started with 320 grit carbide grinding paste then went to 600. I will finish off with 1000 on the final piston/cylinder fitting.

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Liners all done. Really happy with the finish although the photo doesn't really show much.

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The operation for making the pistons was to turn the bottom end and bore out the insides first, cross drill (undersize 3.8mm for a 4mm final reaming for a push fit onto the gudgeon pin), then mount on the mandrel shown below to turn to final outer dimensions.

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The viper has this unusual piston 'crown' at 55 degrees. I was gong to ditch it and go for a flat piston but lost the nerve thinking how i'd need to adjust the other thins to keep the port timing right. I will try this on engine #3 when I get round to it.

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Gudgeon pins were made of silver steel and polished with 1000 grade wet-n-dry paper. i took care to make the ends as smooth as possible to avoid scoring the cylinder. They will be a tight push fit so hopefully there will be no 'floating' going on in any case.


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So now onto more lapping... I can smell the castor oil already!! I turned the pistons 0.1mm oversize which, with more confidence machining, I can drop down to something less next time. The lapping is going to take AGES! I'm on the 320 grit again and just as the piston (not the crown) starts entering the bore I will switch to 600 and 1000 for that last little bit around the exhaust port.


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Thanks,
Patrick
 

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Tim Wescott

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That 55-degree "crown" looks like could be there to help scavenging, by guiding the puff of fuel/air from the transfer ports to the top of the cylinder. Flat-topping the piston, even after adjustments for proper timing, may not give good results.
 

pat_pending

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That 55-degree "crown" looks like could be there to help scavenging, by guiding the puff of fuel/air from the transfer ports to the top of the cylinder. Flat-topping the piston, even after adjustments for proper timing, may not give good results.
Hi Tim. Yeah that's what I figured in the end. I was a bit worried about the lapping with a chamfered bit on the top of the piston but it turned out to be exactly the same as for a flat piston.
 

pat_pending

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Hi All,

some good progress with the lapping on engine #1 today and the liner + piston are a great fit. I need to make the contra-piston now and I'm ready to move on to the FFFA phase (Final Fettling Filing and Assembly). I took care on all fits so this should not be too much of a job. Hopefully by tomorrow we'll get some puffs of smoke at least.

I have 2 quick questions for the seasoned engine builders out there if I may.

1) For a 'tight push fit' on the gudgeon pin in the piston i have been relying on the taper of a hand reamer and coming in very gradually from one side until the pin wedges in the other (so far so good). I was wondering how tight a push fit this should be? (a) thumb tight (b) tap with a mini hammer tight (c) gentle arbor press fit. Apologies, I'm not an engineeer and these might have more specific names.

2) Im thinking of drilling some 1mm oil holes in both ends of the conrods to help with lubrication. Would these be preferable on one side for the direction of travel of the crank / stroke or is at the bottom OK?

Thanks in advance.

Patrick
 
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pat_pending

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Hi all, good progress on the lapping then a bit of a setback. I had the engine assembled and victory was firmly in sight.

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It was firing nicely on primer but would never catch and run. The mystery here was that, as opposed to the last engine, this one was really airtight. I put soapy water all around the crank case and not a single bubble blown anywhere. Furthermore, fuel would be sucked up the tube when covering up the carb choke.

After I could hardly move my arm after all the propeller flicking (no exaggeration) and I got hit in the finger half-dozen times by the prop, I went back to the plans and did a lot of head scratching (and being generally in a sh**ty mood).

Turns out I missed a detail in the plans which means the transfer ports CAN'T WORK. So obvious in hindsight and bloody annoying!!

I missed the recess in the top of the crankcase.

My engine below. No passage from the crank to the transfer ports.
Screenshot 2021-05-26 at 18.54.30.png



The plans have this rather delicate cutout so the transfer passage runs behind the liner.

Screenshot 2021-05-26 at 18.55.37.png


I have two options now to save the engine.

1) Mount up the crankcase on the 4-jaw indicated on the bore and turn a recess using some sort of boring/internal grooving tool.
2) Go 'off-piste' and modify the cylinder liner to have a channel that connects to the crank case.

Both are equally risky in terms of wasted time to re-make the part if it doesn't work out as the lapped liner/piston and contra combo took 2 days at least. Out of the two operations, mounting the liner in the rotary table and extending the transfer passage is probably easier.

I will sleep on it and it will be decided by morning. Lesson learned here RE looking at plans properly. I had this niggling feeling that something wasn't right as i couldn't understand how the design was supposed to work and should have stuck to my gut instinct.

Not too much of a detour and the engine felt and sounded lovely on those few seconds running on primer so very excited to bring them to life soon.

Thanks,
P
 

Peter Twissell

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If it were me, I would definitely go for the crankcase modification.
The space you need to create in the crankcase is far less critical than the risk of distorting the cylinder.
 

Jasonb

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Should be easy enough with a standard boring bar, set the topslide/compound over 25deg and use that to feed in for the tapered part and then the carriage to carry the cut on down to meet the crank bore
 

Ramon

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Pat
Boring out the crankcase to give a transfer passage as the original design is probably 'overkill' for what is actually needed. It will certainly lead to an excess increase in crankcase volume. As an alternative passages can be cut into the side of the case bore using an homemade cutter that match the position of the passages in the liner...

This shows it being done on the Oliver Tigers - the cutter having been used on previous engine builds


What ever way you do it stop the taper below the top face and not bring it to a knife edge as shown - you can see what I mean better here.


If you bore it to a knife edge and you overshoot slightly you stand to lose the liner locating diameter as well as potentially affecting the seal.

As I see it, whilst passages need to be big enough there is a point where transfer passage volume becomes too much and crankcase volume (and pressure) suffers as a result.

Tug

PS Another alternative would be to modify your liner as you suggest - these are the Eta Liners but the volume on their own is marginal - the cases still have passages in them as above
 
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pat_pending

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Pat
Boring out the crankcase to give a transfer passage as the original design is probably 'overkill' for what is actually needed. It will certainly lead to an excess increase in crankcase volume. As an alternative passages can be cut into the side of the case bore using an homemade cutter that match the position of the passages in the liner...

This shows it being done on the Oliver Tigers - the cutter having been used on previous engine builds


What ever way you do it stop the taper below the top face and not bring it to a knife edge as shown - you can see what I mean better here.


If you bore it to a knife edge and you overshoot slightly you stand to lose the liner locating diameter as well as potentially affecting the seal.

As I see it, whilst passages need to be big enough there is a point where transfer passage volume becomes too much and crankcase volume (and pressure) suffers as a result.

Tug

PS Another alternative would be to modify your liner as you suggest - these are the Eta Liners but the volume on their own is marginal - the cases still have passages in them as above
Thanks for the advice Tug. That makes a lot of sense. I still have a bit of trouble getting my head around the cutter profiles. You don't happen to have any pics of making them do you? I don't recall seeing it on the Super Tigre thread.
 

pat_pending

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SUCCESS!! :cool::):):):):):):):):):):):):):):):):):):):):):):):)🤟🤟🤟

Thanks to all the great advice from you guys, my engine #1 runs. Lots of room for improvement (especially as this was the one made from all the blemished/less than perfect bits).

Chuffed to bits! (but a bit lightheaded from breathing in those fumes. I should really run these outdoors.

I am following this guide on running in from the Adrian's model aero site.


I have the first 4 minute ultra-rich run with the 9x5 on low compression (missfiring) under the belt and will switch to the 8x4 for the short hot runs.

again, thanks everyone!

perma-grin Patrick

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Ramon

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Congratulations Pat - well done on acheiving a good runner 👍 Been pleased to help and answer all those questions. You've got what it takes now (or should have :)) so hope your future builds meet with similar success.

Re cutter making heres the above cutter being made for the Eta engines








The cutter was plunged in sideways to depth then the passage cut 'upwards'

Hope that helps - Tug
 

pat_pending

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Congratulations Pat - well done on acheiving a good runner 👍 Been pleased to help and answer all those questions. You've got what it takes now (or should have :)) so hope your future builds meet with similar success.

Re cutter making heres the above cutter being made for the Eta engines








The cutter was plunged in sideways to depth then the passage cut 'upwards'

Hope that helps - Tug
Thanks Tug. I’m dying to try one of these. Will report back. Also do a video of running engine once I’m through the excitement of playing with it.

P
 

pat_pending

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Hi, I haven't reported back as I've been having too much time playing with the engine :cool:.

Here she is running an 8x4 prop after a few running in-runs. RPM there was about 11K but that wasn't fully 'leaned-out'.


I will turn my attention to the other Viper and seeing if I can't improve on how she 'breathes' by using the transfer passages Tug suggests.

I couldn't be happier. Thanks again all for the help.

Patrick

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Mechanicboy

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Patrick. Well made to run the model engine, but I spotted the propeller is in wrong side. Is it right the propeller is in wrong side?
 

pat_pending

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Patrick. Well made to run the model engine, but I spotted the propeller is in wrong side. Is it right the propeller is in wrong side?
hi mate, I’m pretty sure it was pulling rather than pushing. I’ll double-check next run. Don’t want anything overheating! I’m just about to switch over to some larger props and cooler runs from now on in. P
 

Ramon

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Hi Pat

Nice run but I too had to look at the prop hard to see if it was back to front. I found it difficult to determine as it does look as if you have it that way. Turning the prop such does not turn it into a 'pusher' BTW :) Turning the prop around was a recommended way to reduce the thrust on free flight models in the early stages of trimming.

Easiest way to tell is the leading edge is more rounded and thicker than the trailing edge - what make of prop is it?

Surprising how much you have to make to make an engine eh?

Don't over load the engine with excess diameter (or pitch) - a 9 x 4 should be ideal for further running

It will be interesting to compare the different results with differing transfer passage volumes

Tug
 

pat_pending

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Hi Pat

Nice run but I too had to look at the prop hard to see if it was back to front. I found it difficult to determine as it does look as if you have it that way. Turning the prop such does not turn it into a 'pusher' BTW :) Turning the prop around was a recommended way to reduce the thrust on free flight models in the early stages of trimming.

Easiest way to tell is the leading edge is more rounded and thicker than the trailing edge - what make of prop is it?

Surprising how much you have to make to make an engine eh?

Don't over load the engine with excess diameter (or pitch) - a 9 x 4 should be ideal for further running

It will be interesting to compare the different results with differing transfer passage volumes

Tug
You are both right (of course) :oops: I thought the ‘sharp end’ would be better at cutting through the air so made that the leading edge... and of course the more rounded end better for not cutting my fingers on the ‘flick’ especially in the event of flooding the engine. every day is a learning opportunity.

The make of prop is APC and they seem to have most sizes available. 9x4 on order.

RE the bits you have to make to make an engine: I think that they are roughly equal! I reckon I’ll get a load of reuse though especially if I continue on the 2.5cc path. The scrap bucket should go down too.... who knows 😀
 
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