ML Midge Build

Help Support HMEM:

jack620

Well-Known Member
Joined
Jan 5, 2013
Messages
232
Reaction score
67
Location
Melbourne, Australia
I popped over to Paul's house today and he gave me a piece of bronze. Thanks Paul. :)

I will make 2 more conrods out of this bronze. I see no point in persisting with the aluminium ones. Paul showed me his almost finished inline twin diesel. It looks very nice. I hope to see it running soon.

Chris
 

canadianhorsepower

Well-Known Member
HMEM Supporter
Joined
Oct 22, 2011
Messages
1,671
Reaction score
320
On closer inspection there doesn't seem to be much point in 'growing' the CP. It's cracked.

Damm is Murphy's law following you :hDe:

I guess it's better to see it now then kicking a rod out
does look very good Thm:
 

jack620

Well-Known Member
Joined
Jan 5, 2013
Messages
232
Reaction score
67
Location
Melbourne, Australia
I got my other Midge running today.

[ame="http://youtu.be/v4mPGnQNQA4"]http://youtu.be/v4mPGnQNQA4[/ame]

Needless to say I was thrilled. It ran very smoothly for two tanks of fuel, then it wouldn't start again. I pulled the head and liner off and discovered the wrist pin had broken in half (sorry about the blurry pic). The wrist pin is W1 steel. Any ideas what might cause it to break?

When starting the second engine today I was surprised at how little compression was needed to get it to run. This has got me thinking that I had the other rough-running engine over-compressed as Ramon suggested. The piston and rod in the picture are from that engine. Check out the bend in the conrod big-end. I'm obviously doing something wrong. Could over-compressing the engine cause the conrod to bend?

Chris

broken wrist pin engine #1.jpg


bent conrod engine #2.jpg
 

canadianhorsepower

Well-Known Member
HMEM Supporter
Joined
Oct 22, 2011
Messages
1,671
Reaction score
320
Check out the bend in the conrod big-end. I'm obviously doing something wrong. Could over-compressing the engine cause the conrod to bend?
Hi Jack, to answer your question over compressing would not do this to your rod. I wonder if your cylinder is not slightly out of alignment with your crank. This would cause a ton of stress on the pivot points and that's what you're getting
my 2 cents
 

Ramon

Project of the Month Winner!!!
Project of the Month Winner
Joined
Jan 30, 2011
Messages
272
Reaction score
28
Hi Chris congratulations on another runner and this one far better than the first ;) It certainly sounds how it should and appears to be running well. Thm:

If the wrist pin was hardened it looks like it was not tempered through enough -W1 steel ? is that equivalent to silver steel? If so, I know its small diameter, but try drilling a small hole - say 0.8mm through its centre and temper it in hot sand to a deep blue/brown colour. Sand allows the heat to build up slowly.

Whilst I agree with Luc that overcompressing per se would probably not cause the rod to bend in a usual 'over compressed' state the fact that you appeared to be oscillating back and forth on that first video possibly between a well overcompressed contra piston certainly could. Something led to that cracked the CP and probably that score across it. To bend a rod normally requires the piston to come up hard against something and I would guess that was the culprit.

Anyway - you've got your first runners - great work Thm:Thm:

For myself I'm taking a break from engine building for a while - off on a new 'tack' - I'm going to have a go at building a model yacht :eek:

Good luck with your future builds.

Regards - Ramon
 

jack620

Well-Known Member
Joined
Jan 5, 2013
Messages
232
Reaction score
67
Location
Melbourne, Australia
Luc,
Thanks I'll check the alignment, but I'm confident it is OK. I think Ramon is right- I probably drove the CP into the piston. Hence the cracked CP and bent rod. I wish I had asked an expert to show me how to start it. I would probably have avoided all this damage. :wall:

Hi Ramon,
The W1 is water-quenching silver steel. I didn't temper it at all. :Doh: Being such a small engine I didn't think it would need it. I will make 2 more using the method you suggest.

Thanks for all your help with this build. I couldn't have done it without you. Good luck with the model yacht. I wish I could return the favour and offer you some advice, but I know nothing about them!

Chris
 

wakeup

Member
Joined
Jun 19, 2013
Messages
7
Reaction score
1
6060 is pretty much only good for window frames, sad to say. Recently I was trying to buy some 2024 T3 or better material for con rods. It was very expensive, as in not practical. Legend goes that 2024 is the preferred material because of its high temperature strength. So I did some digging, what I found was that, certainly 2024 does have better high temp properties, but not to the exclusion of any others, especially 7075 T6, which is available at a sensible price.

If you want a small piece of 2024 T3 PM me your details and I'll stick a piece in the post.

Ramon was spot on with what he said.
cheers
Bill
 

Ramon

Project of the Month Winner!!!
Project of the Month Winner
Joined
Jan 30, 2011
Messages
272
Reaction score
28
You're welcome Chris - it was a pleasure to assist and especially so to see them run at the end despite the small set backs.

I've messed around with these little power plants since the age of thirteen but the buzz I got from that Nova when it burst into song was just like that first engine so I know just how you feel ;)

Regards running it's always best to start with the contra piston well backed off to begin with, gradually turning the compression in a third of a turn or so (depends on how coarse a thread on the comp screw) flicking all the time until you begin to feel the compression building. Looks like you had that first one well screwed in to begin with. Never mind it's all part of the learning curve

My wife Sue and I took up R/C sailing last year. Sue's as keen as I am so it's very much a joint interest. The need is there to build a yacht (or two!) and to some extent that will take over most of my attention for a while but I'll be back on another engine in due course Thm:

Well done Chris - keep on going

Regards - Ramon
 

jack620

Well-Known Member
Joined
Jan 5, 2013
Messages
232
Reaction score
67
Location
Melbourne, Australia
Here's a belated follow-up on my Midges. I finally remade the 2 conrods using the 2024-T351 that Bill (wakeup) kindly sent me.

Both engines have been reassembled and one of them has been run about 6 times. I actually re-used the cracked CP (see post 123) because the crack doesn't go all the way through and it runs very nicely! I'm very glad, because I really didn't feel like making a new CP.

The other engine is fairly stiff and needs to be "run in" with the lathe before attempting a start. I'll get around to that on the weekend.

A big thank-you to Bill for giving me a piece of hard-to-source 2024.

Chris
 

modelmotor

Well-Known Member
HMEM Lifetime Supporter
Joined
Feb 8, 2013
Messages
273
Reaction score
185
Hallo Chris,
Your ML Midge model engine looks great.You are a well skilled person and you might be proud with the results now your engine is ready.
I like to make small engines just like you ,in my case the Boll Aero,Boll Major,B.J.Cicada and now i will start with probably the Little Dragon 1 cc glow engine.
I could n't obtain a plan from the EZE model engine serie,neither a plan from the GNEZ-1.They all are designed a bit equal to each other.
I will start building in the Autumn time so that will be in about 3-4 months.

many greetings from modelmotor-Gerard-The Netherlands-Europe.
 

jack620

Well-Known Member
Joined
Jan 5, 2013
Messages
232
Reaction score
67
Location
Melbourne, Australia
Hi Gerard,
Thanks for the compliments. Just today I tried to get the other Midge running. It's been sitting on the bench in my workshop waiting for me to find the motivation to try and get it running again. It needs to have the compression screw wound in a long way for it to fire. It will then start and run very roughly. If I back the screw out it stops running.

I also think there may be something wrong with my carb. The engine will run (roughly) with the needle jet removed and my thumb placed over the hole that the jet screws into. I have no idea what that means.

Here's a video of it running. I hope that somebody will be able to diagnose the problem for me.

http://youtu.be/_ozAWq1RZCA
 

Chris Murphy

Member
Joined
Aug 20, 2018
Messages
19
Reaction score
2
Location
Wellington, New Zealand
Hi Gerard,
Thanks for the compliments. Just today I tried to get the other Midge running. It's been sitting on the bench in my workshop waiting for me to find the motivation to try and get it running again. It needs to have the compression screw wound in a long way for it to fire. It will then start and run very roughly. If I back the screw out it stops running.

I also think there may be something wrong with my carb. The engine will run (roughly) with the needle jet removed and my thumb placed over the hole that the jet screws into. I have no idea what that means.

Here's a video of it running. I hope that somebody will be able to diagnose the problem for me.

Your Midge is running under-compressed and quite rich-you need to wind the compression screw in further still yet. All these model diesels have a sweet spot of about 1/8" turn (it depends a bit on how fine or coarse the comp screw thread is) for any particular prop load-at which the engine will run smooth without misfiring-less than this optimum and it will run 'rough' as you call it-more than the optimum-and it will start to run 'hard' and laboured. Once you have the comp setting right, then you can play with the needle valve to get it running most economically (ie the setting that it will run longest on)-again-go too far and it will start to misfire and labour to a halt.

The Midge was designed to be easy to make-there was no effort whatsoever put into optimising the design in terms of venturi and porting-for someone prepared to experiment with port sizes, timing, transfer passages, intake geometry etc you could get a bit more performance out of it....but that was never the point of the design-it works, and produces sufficient power to fly a model-so meets the designer's intent admirably.

ChrisM
'ffkiwi'
 

jack620

Well-Known Member
Joined
Jan 5, 2013
Messages
232
Reaction score
67
Location
Melbourne, Australia
Your Midge is running under-compressed and quite rich-you need to wind the compression screw in further still yet.
Hi Chris,
I don't know if you are aware, but my post to you was six years ago! Thanks for the response though. Since then I managed to get the Midge running by placing a seperate fuel tank above the carb so gravity would assist the flow of fuel. I was able to get it to run continuously but it was still a little rough. I was happy with that. I built a pair of Midges and the other one runs beautifully. They were never intended to power a model and both now reside on display stands.

Cheers
 

Latest posts

Top