Another ML Midge

[edholly]

Spent about 7 hours so far on a ML Midge build - crankcase essentially done except for adding a 6x3x12mm ball bearing in front of the crank-throw and a bronze bush in the nose. Haven't made an engine for some months - built a Tomboy in that time and flying it with 2 channel radio and a DC Merlin. Can't believe how little fuel it uses and the power match is terrific about 150 foot climb over 3 minutes !

Plan is to build a Cardinal (Veron design) for the Midge when it is completed.

Will post a few photos as the build progresses. Note the crankcase is as machined - still to be fine finished.

Photo of crankcase and the Tomboy. Which incidentally flew beautifully - balsa and tissue just like it used to be !

 

[edholly]

Venturi done, backplate done, muff done, cylinder done and lapped, crankcase done except needs boring for 6x10x3mm ball bearing and nose bronze bush.

Almost there really - I wish ...

Seems pretty small compared to the BollAero 18 - the first engine made, the Midge is No.6

Inside the crankcase I machined a 15 thou groove up to the point where the transfer port ends in the steel cylinder to increase the flow of fuel air mixture into that area, note this does not go all the way as it would spoil the seal of the cylinder to the crankcase, but far enough to blend into the transfer port holes, although having said that there is a witness of a few thou where the burr cma e a tad further than planned, difficult as the burr was a tear-drop shape, but happy with the result and it still seals with plenty of meat.

Also - would appreciate some thoughts, the crank-throw / conrod etc rotates clockwise viewed from the rear - the question is does the transfer port on the upstroke carry the bulk of the charge to the cylinder due to windage in the crankcase ?

Photos attached of the BollAero alongside the part built Midge and the internal transfer machining.

Ed

 

[oneKone]

This build is going to be awesome!! I love rc planes and love building ic engines, I am genuinely excited about this build.

 

[edholly]

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?

 

[edholly]

Well I did make the longer conrod and it changed the character of the engine completely. Docile as a kitten starts easily sometimes first flick, and a bit more power.

Put it in the Cardinal which weighs 12.5 oz and it climbs about 70 feet a minute. Had 5 flights now and it is a suburb little engine.

.

 

[GailInNM]

Very nice Ed.
I like working engines that earn their keep. Looks like your conrod solution was the right decision.
Gail in NM

 

[jack620]

Nice build Ed. I'm having trouble with one of the two ML Midges I built. It runs very rough and only with the comp screw wound in pretty far.

So this morning I took the inlet venturi off and with a stong torch checked what the piston was doing at tdc - ...

Did you mean bdc? I'll go and have a look in the inlet port of mine. I might have the same problem.





Sent from my iPad using Model Engines

 

[edholly]

Hi Jack,

tdc is correct - you want to see what the underneath of the piston is doing in relation to the inlet hole.

In my case it was literally a hair's width uncovering the port, you could JUST make it out as the piston dwelled there, BUT, what it was doing was allowing the crankcase pressure to be lost to a small degree and pushing air out the inlet venturi the exact opposite to what a venturi needs to do.

A slightly longer rod made my engine run beautifully, but if I was building another, I would leave all as on the plan and move the venturi/inlet hole up by about 15 thou. Actually just a new piston and cylinder is all that's needed to achieve this.

They sure are a cute engine...

Ed

 

[jack620]

tdc is correct - you want to see what the underneath of the piston is doing in relation to the inlet hole.
Ed

Hi Ed,
Now I'm really confused! At tdc (top dead centre) the piston skirt should uncover the inlet port to draw fuel/air into the crankcase. At bdc (bottom dead centre) the piston should uncover the exhaust and transfer ports but keep the inlet port covered.

http://www.green-planet-solar-energy.com/2-stroke-exhaust.html

I could see a problem if the TOP of the piston was uncovering the inlet port at BDC. In that case exhaust gas would try to escape out the inlet port and mess up the venturi effect. A longer conrod would fix this.

 

[edholly]

Hi Jack,

Must have been looking at the engine upside down - you are correct it is at BDC that the top of the piston uncovers the inlet from the venturi and the residual exhaust gases mess up the incoming air. Apologies ..

I did make a longer conrod and it changed the behaviour to a sweet little engine - or if I was making another, I would just extend the skirt at the bottom of the piston by 10 thou and think about relieving it 10 thou adjacent to the transfers.

Ed



This is from text above - when I was standing right way up !!



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.

 

[jack620]

No worries Ed. I'm glad we're on the same page! I think this might be the problem with my recalcitrant Midge. I'll have a good look tomorrow.

 

[jack620]

BTW Ed- How did you make the finger grip for the needle valve assembly?

 

[edholly]

How did you make the finger grip for the needle valve assembly?

Simple - paid David Owen about $14 for an MP Jet needle valve assy ...

David's details can be found at

http://oea.modelenginenews.org/pricelist_2013-05.pdf

I must admit I've never made a NVA - I cheat and always buy one from David that suits !

Good luck with the longer conrod, sure made a huge difference to my engine.

Ed

 

[jack620]

Thanks Ed.

 

[edholly]

When I made the 60 degree Vee Twin based on the Midge, I used the best piston / cylinder combos in it and the worst one of the 3 - stayed in my Midge.

A couple of days ago, I decided to "fix" the Midge and get it running nicely. It had bags of compression but spat the fuel/air mixture back out the venturi. It would run on prime but it would not run as it wasn't getting any fuel inducted. I pulled it apart, back together inspected it so closely, and could see that the piston again was uncovering the inlet ever so slightly.

As I classed this piston/cylinder combo as unserviceable, I decide to try some mods. Firstly 12 thou off the bottom of the land lowering the cylinder by that amount, so the piston didn't vent the exhaust into the venturi. Still the same. So another 10 thou off just to be sure - still the same.

This really has me stonkered - I can see no reason why this should happen, but it does the incoming charge has no chance of going down the venturi - it gets spat out, and when you cover the venturi hole to suck fuel up the line - it actually gets pushed back down the line.

So .....

Knowing the 60 degree Vee twin had the good components - and knowing I won't run it again as it really lacks any power, I pinched what was the best piston/cylinder combo from it and replaced it with the recalcitrant one.

The Midge then started behaving quite nicely, but it would run then stop after about 20 seconds - seems like something to do with getting warm.

Traced it to not having the muff head capbolts done up dead tight, seems the seal around the bottom of the cylinder wasn't effective enough without bolts being real tight.

After many hours of messing around, now very happy with the Midge again, runs to over 6,000 with a 7 x 5 wood Turnigy prop.

Anyone got any thoughts on why the first cylinder/piston combo does this when the compression is very good - or has anyone come across this before?

Ed

 

[edholly]

Just ran it again now - 2 days later - no change to settings from then - 3rd flick - consistent 5600 on the 7 x 5.

What a difference that cylinder/piston combo makes ... from a non-runner to a beauty !

.

 

[edholly]

Here's engine build No.12 - this time a modified Midge.

Mods are

LH thread screw on nose
ball bearing inside
two wider exhausts around 300% orig size
enhanced transfer ports both in passages placement and size
front rotary induction with conservative timing
steel very thin wall contra piston.

Initial runs - couldn't get to start. Had me really baffled, seemed as though something out of alignment, would turn through non-induction side about 90deg free then tight, yet piston to bore was not tight, in fact almost perfect.

Preservered for ages till finally resorting to the dreaded electric starter, within a half minute, it seized and the problem came to life. It seems the cheese headed screw I chose for the conrod, and which was definitely some exotic material, was galling on the crankpin, and the electric starter brought it to an end. No damage that a new press in crankpin and a bronze bush in the little end didn't fix, and presto, a beautiful feel to it and it ran happily for 5 minutes within a few seconds of trying.

When run in will do a back to back test on my trusty Merlin .75 and see what it's power is like but even its first run gave 6200 on a wood 7 x 5

Ed

 

[edholly]

OK now had about 25 minutes running - so tried a max power run with the 7 x 5 wood prop - got rock steady 6900.

Put the prop on the trusty Merlin in the Tomboy where it has had over 500 flights with an engine run of around 1.5 minutes, and it was well used before that, and it ran to 7,500.

So for a sport engine to power something like a Tomboy or a Cardinal, plenty of power. Very easy to start too hot or cold. But a bit of a finger biter if you aren't careful. So very happy with it - compression hot or cold is extremely good.

Photos show the nose unscrewed and the pressed in crankpin.

 

[edholly]

Done about 10 flights now with the modified Midge in my Veron Cardinal.

This model started out with a standard Midge and it was very marginal on power, now it is a perfect match. One small thing haven't got on top of is that it wants to "grab" a bit on one side of BDC. I have had it apart spent about 2 hours trying to work it out, redone the backplate clearance and still the problem exists. Almost like the conrod is grabbing slightly or cocking over, but cannot pinpoint it. Maybe it is my imagination as the piston/bore fit is really good and tight. It is happy enough to run beautifully though and usually starts first or second flick.

It is a super little motor, and an easy mod to the original Midge.

 

[goldstar31]

Good gracious! Is Veron still in existence? Again, I noted that Vic Smeed who designed Tomboy along with other very nice stuff like Debutante had died. I'm a bit baffled with 'Midge' and politely ask is this a Mills .75 or whatever.

After all, I was flying these things in 1951. 'Charlie' Lutman of the Model Shop, in Newcastle upon Tyne and I were National Service before then. Charlie became a pilot and I refused my commision in the field. I was too poor to afford the Officer's Mess Best thing that I ever did!

Cheers

N