Author Topic: Building the little dragon engine.  (Read 4706 times)

Stoveitin

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Building the little dragon engine.
« on: October 16, 2014, 06:52:10 PM »
Hello,
Recently decided to try make the little dragon engine with no experience as of yet .

How ever its made and having a few issues.

First of all it does not fire atall im guess its lack of compression due to did have it firing occasionally with lots oil bringing the compression up.
is there any easy way of checking the compression ?

Also the inlet sucks but at one point in the revolution the inlet has pressure and blows the fuel out . Is this normal on this engine if anyone has made It before ?
I have noticed it has positive pressure as the piston is on its down stroke soon as the transfer port/inlet port on liner opens .. which im guessing as the pressure drops in the inlet chamber it doesn't hold the cam valve seal tight against its seat but im wondering if my inlet is not as free flowing as it should be.

Ill get pictures up when can.

Cheers for any help if anyone who has made this engine can shed some light.

Tom.

Offline Jasonb

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Re: Building the little dragon engine.
« Reply #1 on: October 16, 2014, 06:58:57 PM »
With the glow plug in you should be able to feel the compression as the piston nears the top of its stroke - the prop should bounce as you ease off the pressure when turning it slowly with one finger and then kick down once past top dead centre, compare that to when the plug is removed, if there is not much resistance without a plug then you are likely to be lacking compression.

How did you fit teh piston to the liner? was it just a turned finish or did you lap the parts and if so with what.

J

Stoveitin

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Re: Building the little dragon engine.
« Reply #2 on: October 16, 2014, 09:17:01 PM »
With the glow plug in I do get Extra resistance with the glow plug but not much kickdown at all after top dead centre.
I just done a fine turned finish best fit I could get at time using bronze for the liner and piston .

Cheers for the response I guess is lack of compression due to no much kick back if any

Offline GailinNM

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Re: Building the little dragon engine.
« Reply #3 on: October 16, 2014, 09:31:48 PM »
Tom,
First off welcome to the MEM forum.  If you have a chance, tell us a little bit about yourself in the welcome section.

I built a Little Dragon about 8 years ago and had a few problems with it at first. Most were compression related.  It piston cylinder fit needs to be very good.  I ended up with lapped cast iron piston and cylinder. Made cylinder first and lapped it with a brass lap.  Then made the piston and lapped it independently to fit. As I remember it runs at about 9500 RPM with a 7-2 propeller and this is probably about all you will get out of a long stroke engine like this.

With just a light coating of oil wiped on it will hold compression about 2 seconds at top dead center when turned over slowly by hand. Turn it to TDC and it needs to hold compression for at least one second and when the prop is moved off TDC it should rotate 30 degrees or so from the compression.  Again, only with a light oil coating.

I still have some notes on it here (somewhere) and will try to answer any questions you might have. 

You probably have read the build log that Ron did on Model Engine News, but if not here is a direct link to the first page.
http://modelenginenews.org/lildragon/index.html

And a link to the blurb he did on mine in MEM.
http://modelenginenews.org/ed.2006.05.html#t8

Gail in NM



I would like to be the kind of person my dog thinks I am.

Offline steamer

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Re: Building the little dragon engine.
« Reply #4 on: October 16, 2014, 11:22:04 PM »
Glad to see you post Gail!

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline b.lindsey

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Re: Building the little dragon engine.
« Reply #5 on: October 17, 2014, 01:27:10 AM »
Gail, can you post a picture of yours here. There is a picture on one of the links you posted but others might want to see a picture here. Nice looking little engine by the way :)

Bill

Offline GailinNM

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Re: Building the little dragon engine.
« Reply #6 on: October 17, 2014, 02:03:01 AM »
Glad to Bill.
A few notes on the Little Dragon. 
The plans for the Little Dragon were first published in Model Airplane News Magazine in 1950 by Roy Clough Jr.  Roy was a well known model airplane designer but a not a brilliant engine designer.  The Little Dragon was one of the first designs published using the newly developed Glow Plug by Ray Arden that was introduced in 1947.  I built it because of it's historical value as many later designs were influenced by it.
Gail in NM
I would like to be the kind of person my dog thinks I am.

Offline Jasonb

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Re: Building the little dragon engine.
« Reply #7 on: October 17, 2014, 08:20:10 PM »
It does sound like the turned finish is causing a loss of compression. Although the materials are not ideal you could try lapping your existing cylinder liner and then making a new piston from Cast Iron and lap that into the liner

Stoveitin

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Re: Building the little dragon engine.
« Reply #8 on: October 17, 2014, 09:45:33 PM »
Cheers for the information GailinNM your build looks pretty amazing compared to mine !
I have already seen the links you have posted but thank you for your time.

I have looked into it more and yes it seems my linear is the real major problem! its not very even has high and low spots.

I used a drill bit to get 0.5 mm within the size and a mill bit to take it the rest of the way ..( I have been working in metric by the way converting most the figures).. The reason I done the cut like that on the linear is that it was the best finished and straightest cut I could get with the tools I have maybe I should invest in a reamer !

I choose this material Jasonb due to seemed to give me the smoothest finish and was reasonably hard I had it laying about and thought would give it ago but if you think Cast iron is the way to go ill give that ago.

Ill have to have a read into lapping techniques and remake the linear and cylinder when I get a spare moment .

Cheers for the help so far great stuff !

 

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