Another ML Midge

[edholly]

Thanks Goldstar ...

A lot, maybe most, of the plans we used as youngsters or should I say when we were younger are now available on Outerzone.

http://www.outerzone.co.uk/

Simply a case of downloading the one or ones you want onto a memory stick, taking to your favourite place of printing, here in Australia I use Officeworks, and having them print it off. Cost is a lot less than we paid back then ! Some plans even have printwood, and this means you have a ready accurate template on paper to cut out the ribs etc without having to trace them.

The first model I made after a layoff of around 30 years is my DC Merlin powered Tomboy, Vic sure knew what made a good model.

The engine is based on an ML Midge, simply google that and you will see the original design.

Ed

 

[goldstar31]

Hi Ed,
Thank you for all the information- which takes me back a lot of years.
At almost 85 and with all sorts of domestic problems to manage, I doubt that I'll get time to make much.

However, my sincerest thanks and my best wishes

Norman

 

[steve-de24]

Hi Ed,
Nice work on your Midges. I'm (slowly!) making one as well. If you have the time can you tell me how you got the correct fit between the pressed in crankpin and the crankshaft web? Is there a way which avoids me having to buy an expensive reamer?
Regards,
Steve.

 

[edholly]

Steve,

Used to ream them but too inconsistent in sizing.

What I do now, is make the hole with an ordinary drill slightly smaller than the piece of drill rod I intend to use as the pin, I then machine the pin to about a thou bigger than the hole and press it in. Another advantage of this process is that you get a small shoulder for the pin to bottom onto. Haven't had a failure with this process yet. (touch wood)

Ed

 

[steve-de24]

Thanks Ed, I will store that your info away until I get to making my crankshaft.

Regards, Steve

 

[Rodrigo Castellanos]

Hi Ed!
i just finished my Midge and seems to work ok. no issues, starts easy once needle settings found, but it like to be exhaust prime for fast start... here a short video of mine...




can you share the mods (transfer ports, placement and that stuff) will be great to build one like yours.

Kind Regards
Rodrigo

 

[GreenTwin]

A friend of my Dad's gave me an airplane very similar to the one shown in the first post.
What a blast from the past; I had forgotten all about that.

It was all stick construction, and had a small glow engine on it, red body with yellow wing.
At the time, I was not sure exactly what I was looking at, but over time, I figured out that it had a 1-tube radio, and an escarpment? mechanism, where there was a big rubber band wound up, and the correct signal would allow the mechanism to make one of perhaps 4 different motions.

Most ingenious device for sure.
I flew it by control line only, and got it too high one day.
Smashed it into a million pieces.
That was a sad day for sure.
I need to find drawings for that and build a new one.

It was silk with dope, and semi-transparent, so you could see all the ribbing and such.

This was in perhaps 1968.
Those were the days for sure.
You could do a lot in the pre-transistor era.

.

 

[ruben]

hello
Good morning, I wanted to know if anyone would have the plans for the sparey 0.8cc engine or any engine forum to get them or a page to get them.

thank you

 

[Ghosty]

hello
Good morning, I wanted to know if anyone would have the plans for the sparey 0.8cc engine or any engine forum to get them or a page to get them.

thank you

This is a page for the Sparey 2cc twin, maybe you could do something with it Oz : Sparey Twin plan - free download
This is the one for the Sparey 5cc Oz : Sparey Diesel plan - free download

Cheers
Andrew

 

[ruben]

Esta es una página para el Sparey 2cc twin, tal vez puedas hacer algo con él Oz: plan Sparey Twin - descarga gratuita
Este es el del Sparey 5cc Oz: plan Sparey Diesel - descarga gratuita

Salud
Andrés

Hola Andrés, gracias por tu ayuda.

 

[jimbo7636]

Well the Midge was finished some time ago and has now had about 2 hours running.

I must say I was a little disappointed with it, as although the fits are very good, it is a trifle cantankerous not wanting to start easily on anything smaller than 8 x 4 props and it is down a full 1000 revs on a DC Merlin on the same prop at 6000 versus 5000.

Now I know a sideport won't give the same power as an advanced timed rotary port - but I thought it should do better than this - and as I am 80% finished a Veron Cardinal just for this motor - I feel that it will be very marginal on power.

Another intriqing thing it has is that it wants to bite badly on tying to start on small props and I use a chicken stick as a result. It also has little suction and will actually spit out any prime from the venturi instead of sucking it in ! Clearly not good for performance when its trying to trying to throw the incoming fuel/air mixture back out the venturi.

So this morning I took the inlet venturi off and with a stong torch checked what the piston was doing at tdc - well maybe I found the cause of all these little problems. The piston was uncovering the inlet by about 10 thou as best I can judge. Obviously any uncovering of the inlet will allow residual exhaust pressure to escape down the venturi reducing inlet flow and thereby reducing power. It also has this very annoying habit of oscillating back and forth, rather than rotate when trying to start.

So I went back to the drawing to see what the specs were, and I think that maybe the problem is in the original design - unless it was meant to be this way.

I would welcome any comments pro or con ...

The way I see it is this - at BDC

Centreline Crankshaft to datum land on cylinder (all in inches) 0.906
Conrod length 0.787 plus piston above 0.197 = 0.984 less 1/2 crank rotation 0.200
therefor top of piston is 0.784 above CS centreline

The inlet hole is 0.157 below datum and 0.078 diameter so top of hole is 0.118 below datum or
0.788 above CS centreline

So therefore the piston uncovers the inlet by 0.004 according to the drawings as far as I can see - and if a small error occurs in maching to these tolerances then it can "fix" the problem or exacerbate it.

Over to the boffins now - do you think that this would rob the engine of some power?

Also ... if I made a new conrod say 10 thou longer - would this help ? I know it will throw out the timing but what I lose in timing might that be made up for by better inlet tract flow?

View attachment 67882

View attachment 67888

yes there is something not quite right about the 0.004 inches. I think the best thing to do is to lower the position of the inlet in the cylinder so there is not a position where the inlet opening is above the top of the piston. Also, I like your idea of lengthening the conrod a bit. I did put my hands on the original AMI articles and ML does mention tweaking the conrod length if you are not satisfied with the as-built engine.

 

[stanstocker]

yes there is something not quite right about the 0.004 inches. I think the best thing to do is to lower the position of the inlet in the cylinder so there is not a position where the inlet opening is above the top of the piston. Also, I like your idea of lengthening the conrod a bit. I did put my hands on the original AMI articles and ML does mention tweaking the conrod length if you are not satisfied with the as-built engine.

It may just be coincidence, but having played with metric and imperial simultaneously for years whenever I see 0.004 inches I think 0.1 mm and the other way round. Any chance a measuring system conversion introduced this difference? Not knowing the engine, would a piston with a lowered pin location or taller top surface be quicker than making a new con rod? Hopefully it's a rod or piston issue and not a port location error.

Perhaps to verify this is in fact the source of the problems a disc of 0.1 mm / 0.005 inch shim stock bonded to the top of the piston might let you test things out before investing effort in the wrong place. Could be talking from something other than my mouth here, I'm not an aircraft engine guy. Any time a port hangs open though it's usually a bad thing

 

[jimbo7636]

It may just be coincidence, but having played with metric and imperial simultaneously for years whenever I see 0.004 inches I think 0.1 mm and the other way round. Any chance a measuring system conversion introduced this difference? Not knowing the engine, would a piston with a lowered pin location or taller top surface be quicker than making a new con rod? Hopefully it's a rod or piston issue and not a port location error.

Perhaps to verify this is in fact the source of the problems a disc of 0.1 mm / 0.005 inch shim stock bonded to the top of the piston might let you test things out before investing effort in the wrong place. Could be talking from something other than my mouth here, I'm not an aircraft engine guy. Any time a port hangs open though it's usually a bad thing

I think you are correct. in the original work written up in AMI it was all metric. the exhaust was 3 mm diameter. So that is 0.118, not 0.120 (drill #31). Also, the top of the inlet hole was at 3 mm, so that would be a theoretically perfect line up of the bottom of the exhaust and the top of the inlet. probably need to separate those a bit...