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Jason's Firefly .46 Build

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Ian Hewson24/01/2019 18:58:46
259 forum posts
24 photos

Thanks for that Jason, just looking ahead, got the engine built up to the backplate level, going to do the crank and piston next. Lapped the cylinder with diamond paste from Arc, came up nicely.

Interesting build, makes a change from steam models.

Think I have realised what the nipple on the silencer is for, pressurising the fuel tank to feed the carb, no need for the fuel tubes to the mixing chamber in the backplate?

JasonB24/01/2019 19:07:57
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The backplate holes are really only for neater plumbing back to the tank as you can run the tubes through the middle of the bulkhead.

So from exhaust to one of the backplate pipes, from that to tank for pressure, from tank clunk to other inner backplate tube and from that to remote needle assembly and finally from there to carb.

geoff walker 124/01/2019 22:37:46
307 forum posts
131 photos

Hi Ian,

Good luck with the rest of your build.

I made two pistons for my firefly. The first one I made a poor job of lapping it into the liner. The engine ran but lacked power and when used on a model aeroplane it just had enough power to "limp" into the air.

With the second piston I made sure it had a good square leading edge and I only lapped it up to TDC. The first piston I lapped right up to the end of the liner which was a I feel the mistake I made.

If you do some calculations you will see that TDC is 0.8 mm below the underside of the cylinder head boss. I lapped up to that point and it made a big difference to the compression and the power output.

Just my thoughts Ian, hope they are of interest

Geoff

Peter Wood 524/01/2019 22:56:40
94 forum posts
11 photos

I have now almost completed my second Firefly with beam mounts, cylinder head cooling fins and the designed carburettor. My first has an ASP 46 carb.

However that has raised a question in my mind.

In a commercial RC carb the barrel rotates in a spiral groove which effectively allows more fuel to flow through the needle valve as the throttle is opened thus keeping a constant mixture.

However in the Firefl design, the amount of fuel going to the carburettor is fixed throughout the carb barrel rotation by the remote needle valve. Does that mean that the needle valve has to be adjusted (in flight) for each throttle opening?

JasonB25/01/2019 07:03:48
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Does depend on the commercial carb, I have Perry and OS ones where the barrel moves the same as the firefly rather than in a helix and no remote adjustment of needle. Also no air bleed to lean off at idle.

I suppose with modern radios you could link a micro servo for the mixture to the throttle so that the mixture is adjusted proportionally to the throttle.

Edited By JasonB on 25/01/2019 07:04:23

Ian Hewson25/01/2019 09:04:59
259 forum posts
24 photos

Hi Geoff, thanks for the info on the piston lapping, I used the method described on AdriansModelAeroEngines.com site where he advises lapping the barrel to a slight taper fit, just nipping at the top, similar to your findings.

My engine will probably never fly, but I do like to do the work to the best of my ability even so.

Appreciate the help you get on here.

Ian

Peter Wood 530/01/2019 16:31:21
94 forum posts
11 photos

Has anybody modified this design to use piston rings?

I have had two attempts to get a piston / cylinder combination with good compression. After hours spent hand lapping with the technique described by Ramon in Model Engine Maker and following Geoffs advice I achieved what I thought was a reasonable fit. Not as tight as a commercial engine but there was a plop when flicking it over. However as soon as I tried to start the motor, using a starter it quickly lost compression.

In desperation, I have just turned up a piston ring out of cast iron. It is 0.08 mm larger diam than the cylinder diameter, 2mm wide and 1.3mm thick. it was split with a diamond cutting wheel.

A groove was turned in a discarded undersize piston about 3mm from the piston head, 2.1mm wide and 1.7mm deep.

The ring was easily expanded by hand to fit in the groove and then the piston was slid into the cylinder and the ring pushed into the groove whilst pressing down.

A bit of lapping with 1200 paste was needed but now it feels good.

I need to make a new gudgeon pin before reassembling and testing tomorrow.

Any observations why this should not work?

Peter

Danny M2Z31/01/2019 04:16:53
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740 forum posts
278 photos

Nice build on the Firefly.

Just a comment about using diamond lapping compound.

It works very well, almost too well (I have tried it) but it is vital to remove all traces prior to running the engine,

I read a few years ago advice from some of the guru's who build pylon racing engines that the diamonds embed in the bore and the only hope to remove the residue is with thorough ultrasonic cleaning.

Given this info I only once tried diamond paste and had to concur so now prefer to use the traditional lapping pastes of which I have a variety ranging from 400- 4000 grit.

After all that, when building a ringed engine, a light hone of the bore resulting in a nice X-hatch pattern seems to be the way to go with a turned, split and heat treated cast iron ring (using a spacer in the gap when cooking it).

Here is a linky to what I am on about smiley **LINK**

* Danny M *

Ian Hewson31/01/2019 09:27:30
259 forum posts
24 photos

Hi Danny

Adrian’s model aero engines site also mentions very thorough cleaning after diamond lapping, but not as far as ultrasonics.

Link is useful for piston rings, but Firefly is not designed for use with one.

Ian

Peter Wood 531/01/2019 10:55:29
94 forum posts
11 photos

Hi Danny

The observations on using diamond lapping paste may be the explanation for the problems I have been experiencing. When I was trying to start the engine I noticed that the unburnt fuel that was coming out of the exhaust was black as were the residues inside the engine when dismantled.

I have read the article on making rings with interest but wonder whether I really need to go to all the trouble of annealing the ring for a simple engine like the Firefly? Why wont my approach of making the ring marginally oversize work in a simple engine like this. It just needs enough power to lift a Flair Cub into the air.

Anyway I have run out of Meehanite for the moment so will have to take stock whilst I decide whether to have another go at lapping.

Thanks for the comments.

Peter

Martin Harris 931/01/2019 11:22:40
13 forum posts
Posted by Peter Wood 5 on 30/01/2019 16:31:21:

Has anybody modified this design to use piston rings?

Any observations why this should not work?

Peter

If you decide to give it a try, don't forget that normal 2 stroke practice is to peg the ring to stop it turning and catching the ends in any of the transfer ports.

Edited By Martin Harris 9 on 31/01/2019 11:23:50

geoff walker 107/02/2019 19:43:49
307 forum posts
131 photos

Hi All,

Dismantled the firefly yesterday for a winter service.

My "pilot", John, a friend and experienced aeromodeller felt that the engine was starting to labour over the few months of flying. I wasn't really surprised because it's been worked hard over the last year.

I was surprised when I opened it up and discovered the wear in the con rod. The big end hole was elongated by a good 0.010" so much so that the underside of rod had been rubbing against the crankcase. The small end was also a sloppy fit. I'm curious as to why it has worn so much. I used H15 alloy as specified and the crank and gudgeon pin were polished before assembly two years ago. Both ends had lubrication holes to feed the bearing surface. Is this sort of wear to be expected from a plain con rod with no additional bearing material?

Anyway using my new super dooper Seig mill it was a simple task to open the holes up and insert slimline phosphor bronze bearings. You can see the renovated rod in the photo. Let's hope they stand up better than the plain alloy bearing, time will tell.

20190207_190409.jpg

Geoff

Jeff Dayman08/02/2019 06:38:19
1563 forum posts
37 photos

How does oil get to the rod ends? If it doesn't have a path to get there it may not, and if there's not enough lube you will get wear. Maybe you could pre-oil by hand before flights. Just a thought.

JasonB08/02/2019 06:57:06
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"Both ends had lubrication holes to feed the bearing surface"

Peter Wood 511/02/2019 14:52:54
94 forum posts
11 photos

img_6826a.jpgA photo of the two Firefly's I have built. I have had no success in getting the top one to run. I have not tried the front version as it currently contains an early attempt at the cyl /piston assembly.

Basically I don't seem to be able to get a fit of the piston in the liner to give good compression. My attempt at using a piston ring did not work (probably because I did not temper the ring).

I suspect that the venturi on the commercial carb is too large as well and the poor crankcase vacuum is not sufficient to draw fuel.

So the next (and probably final) step will be to make a new cylinder/ liner for the front version with the design carb adopting all the excellent advice I have been offered on this thread

geoff walker 111/02/2019 18:26:38
307 forum posts
131 photos

Hi Peter,

If you are making a new liner and presumably a new piston then it might be of interest to you to have a look at the website referred to in an earlier post by Ian Hewson

The site is Adrians model aero engines. In the technical topics section there is an excellent article by a guy called Ken Croft on cylinder liners and pistons including lapping and fitting.

I found it really interesting, maybe you will too.

Geoff

Brian Oldford11/02/2019 19:45:33
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533 forum posts
4 photos
Posted by JasonB on 25/01/2019 07:03:48:

Does depend on the commercial carb, I have Perry and OS ones where the barrel moves the same as the firefly rather than in a helix and no remote adjustment of needle. Also no air bleed to lean off at idle.

I suppose with modern radios you could link a micro servo for the mixture to the throttle so that the mixture is adjusted proportionally to the throttle.

Edited By JasonB on 25/01/2019 07:04:23

A fairly common-place practice by the RC racing power boat fraternity.

Peter Wood 512/02/2019 17:35:58
94 forum posts
11 photos

Geoff.

Thanks for the link to Adrians Model Aero Engines. An interesting site and the article by Ken Croft was really helpful. Full of useful tips such as using Turps as a lubricant in the lapping process.

I made the cylinder lap he described but found that it tended to expand at the end as the grub screw was tightened which is exactly what you don't need so I have reverted to Adrians design which gives a parallel expansion along the laps length.

Now for several hours of patient work on the liner.

Peter

Tajinder Khambay 117/06/2019 10:17:34
1 forum posts

Hello All

I am new to engineering and would like to build a IC engine. I am interested in the Firefly .46 but can not find any plans.

Please could I ask anyone if there is a link to where I can find these plans.

Many thanks in advance.

Taj

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