Go Back  RCU Forums > Glow Engines, Gas Engines, Fuel & Mfg Support Forums > Everything Diesel
Reload this Page >

MP Jet Letmo 2.5cc diesel engine replica series

Notices
Everything Diesel Discuss R/C Diesel engines here.

MP Jet Letmo 2.5cc diesel engine replica series

Old 04-04-2011, 01:18 PM
  #1  
earlwb
Thread Starter
 
earlwb's Avatar
 
Join Date: Aug 2009
Location: Grapevine, TX
Posts: 5,993
Likes: 0
Received 3 Likes on 3 Posts
Default MP Jet Letmo 2.5cc diesel engine replica series

I had bought one of the MP Jet 2.5cc Letmo replica engines a while back.
It is larger than the smaller 1.8cc Atom engine by quite a margin. I found the larger fuel tank nice as you can get some longer engine runs with it. But if you were doing Free flight I can see it being pretty easy to wind up losing a plane if the engine doesn't quit on you when you want it to.
I unscrewed the fuel tank bottom and I am surprised as to how fine the screw threads are that they used to make the fuel tank with. They are really super fine threading.
I ran a 11x6 prop on the engine, but to fit the front crankshaft stub I needed to use a 8mm reamer to ream out the prop part way to fit on the engine.
The engine has a odd three point mounting system with three mounting tabs. But the back of the engine is not flush with the mounting tabs.So you need to do some extra work to mount the engine on a plane or test stand. I fabricated a test stand adapter just for the engine, that I could use on my existing test stand. I also needed to cut in a notch and beveled it a little to clear the fuel tank and the cylinder back side protrusion as well.
The engine took a while to get running at first, but once it got a couple of tanks of fuel through it, it got much easier to start up by flipping the prop,
I was getting around 5,000 to 5,600 rpms with the Master Airscrew 11x6 prop. I used some Davis Diesel Development fuel with some extra castor oil added to it.
I like the sound the engine makes, it has a nice low speed sound to it.

I think the fuel tank needs a extra little tiny hole drilled in on the top to serve as a vent for when you fill the fuel tank. It was tricky trying to fill the tank through the small hole on top. I'll likely drill a tiny vent hole to make it easier with less waste fuel being dribbled out as you fill the fuel tank.






Here the MP Jet Letmo 2.5cc engine is compared to the smaller mp Jet 1.8cc Atom engine


Unfortunately a droplet of oil managed to get on my camera's lense, so the center is a little blurry. I'll correct that the next time I run the engine.


I had only run three tanks of fuel through it, and it was still a little stubborn.
[youtube]http://www.youtube.com/watch?v=e3HqorAmuQM[/youtube]


Old 04-04-2011, 04:50 PM
  #2  
ffkiwi
 
Join Date: Jul 2005
Location: Upper HuttWellington, NEW ZEALAND
Posts: 1,601
Likes: 0
Received 1 Like on 1 Post
Default RE: MP Jet Letmo 2.5cc diesel engine replica series

These are nice engines-I've had one for about a decade-good on big props-but as you say -a sod to mount. I made up a 'bird cage' mount-like the Cox Thermal Hopper one. One day i'll machine up and mill a solid type radial mount for it from bar stock, or possibly delrin [I favour the latter, as it works out a lot lighter in the material sizes required] I run mine-for FF- on 10 or 11" dia props-12x6 is OK for running in-but probably a bit much load for flying use. They do shake a bit (though this could have been exacerbated by the fairly light mount I used).

A couple of warnings-(1) the main bearing housing screws into the crankcase-on the bench it can-and does-sometimes unscrew partially-especially if your prop combination generates vibration-loctite or similar solves this problem
(2) the carb spigot is a tight press fit into the crankcase-it doesn't have any screw thread-so be careful if you're considering side mounting or inverting the engine and want to change the carb and tank orientation.

For FF use its safer to use an external tank and a timer, -on the integral tank you're almost guaranteed a flyaway.

FWIW, the Letmo and the full sized 60" Simplex design make a nice vintage combo................

ChrisM
'ffkiwi'
Old 04-04-2011, 05:33 PM
  #3  
earlwb
Thread Starter
 
earlwb's Avatar
 
Join Date: Aug 2009
Location: Grapevine, TX
Posts: 5,993
Likes: 0
Received 3 Likes on 3 Posts
Default RE: MP Jet Letmo 2.5cc diesel engine replica series

My mistake, I just double checked and I was really using a 11x6 prop for break in. My brain must of had a short circuit moment there.

I have attached the instructions for the letmo 2.5cc engine here too. Both inEnglish and Czech too.

Thanks for the advice. I was thinking about those screws holding the front on too.

I was thinking of a 72 inch wingspan Mysteryman myself, but maybe it is a little too small for it. That Simplex looks like a good choice though.

Attached Thumbnails Click image for larger version

Name:	Pn35217.jpg
Views:	132
Size:	271.3 KB
ID:	1589134   Click image for larger version

Name:	Ql32756.jpg
Views:	128
Size:	290.3 KB
ID:	1589135  
Old 04-04-2011, 06:11 PM
  #4  
ffkiwi
 
Join Date: Jul 2005
Location: Upper HuttWellington, NEW ZEALAND
Posts: 1,601
Likes: 0
Received 1 Like on 1 Post
Default RE: MP Jet Letmo 2.5cc diesel engine replica series

I lost my small (40"-the Aeromodeller plan) Simplex a few weeks ago, so now I have the motivation to build the full size one and put my Letmo to work. Gary Edson who frequently posts here is a fan of the Letmos as well. I may well build mine set up for FF but fitted for (but not with) RC for a later date. I know they fly well-they were quite popular for Class A Texaco at my previous club (Christchurch MAC, NZ) with a PAW 249 or the like up front-throttled right back and running forever on a 14cc Humbrol tin tank-probably about the same size tank as the Letmo comes with now that I think of it...........

ChrisM
'ffkiwi'
Old 04-05-2011, 04:03 PM
  #5  
gmedson
Senior Member
 
Join Date: Mar 2005
Location: Sanford, NC
Posts: 127
Likes: 0
Received 0 Likes on 0 Posts
Default RE: MP Jet Letmo 2.5cc diesel engine replica series

Hi Chris! Hi Earl! I have a LOT of fun with my wee MP Jet repros, including the Mighty Letmo 2.5 (...mighty compared to its brothers).

Do either of you notice objectionable vibration from your Letmo? I did. Bought mine over ten years ago, and it shook more than I would want my radio stuff to endure. However, a lot of the prob had to do with my "birdcage" motor mount, which I should strengthen with diagonal bracing. I cut away part of the full-circle crank web to achieve better balance. And Dave Owen suggested that I also lighten the piston. With those two mods, plus using an unbalanced propeller, the engine smoothed out nicely.

I promise to follow up with more info on this. But I am also procrastinating on doing my Income Tax return. And I guess I really should do that first?

Regards. -Gary
Old 04-05-2011, 06:29 PM
  #6  
earlwb
Thread Starter
 
earlwb's Avatar
 
Join Date: Aug 2009
Location: Grapevine, TX
Posts: 5,993
Likes: 0
Received 3 Likes on 3 Posts
Default RE: MP Jet Letmo 2.5cc diesel engine replica series

My Letmo didn't seem to vibrate more than I was expecting. Maybe when they did the cutouts on the crankweb, they accidentally balanced it a little better. Anyway the engines that used iron pistons tend to have more reciprocating mass that makes it harder to balance a engine resulting in more vibration. Then maybe the heavy Master Airscrew 11x6 prop might have dampened some vibration too. I think my Marz 2.5 engine seemed to vibrate more. But now that I think about it, the Atom 1.8cc was harder to adjust the compression when the engine was running. As it was hard to hang onto the lever and turn it.


Old 04-05-2011, 10:57 PM
  #7  
ffkiwi
 
Join Date: Jul 2005
Location: Upper HuttWellington, NEW ZEALAND
Posts: 1,601
Likes: 0
Received 1 Like on 1 Post
Default RE: MP Jet Letmo 2.5cc diesel engine replica series

Gary-as I recall, [and my Letmo is packed away with my house pack whilst I'm living in barracks, so not accessible] the Letmo 2.5 has a very thin crankweb with two sizeable holes flanking the crankweb to give some degree of balance. From what you've said I take it you took a Dremel to the shaft and cutway more of the web? as well as lightening the piston-by drilling holes internally or milling out op[posite the gudgeon pin bosses? Its been quite a few years since I ran mine-but I recall it being a bit of a shaker! I don't think I experimented much with unbalanced props at the time-and I certainly didn't shift the prop around on the shaft to try out different orientations and see if that affected the vibration harmonics. The classical wisdom has it that you put the heavy blade opposite the piston at TDC-but this doesn't necessarily place the prop in the ideal position for flick starting (personally, I prefer a '20 to 2' or a '10 past 8' position for flick starting a diesel)

ChrisM
'ffkiwi'
Old 04-06-2011, 11:32 AM
  #8  
gmedson
Senior Member
 
Join Date: Mar 2005
Location: Sanford, NC
Posts: 127
Likes: 0
Received 0 Likes on 0 Posts
Default RE: MP Jet Letmo 2.5cc diesel engine replica series

OK, I have calculated my Federal income taxes and now will let the mess sit for a day and then go back over it before sending it in. So now's a good time to give more info on LETMO 2.5.

First picture shows LETMO's latter-day incarnation, with RC carburetor and birdcage engine mount. Second pic, its guts, including original induction pipe, NVA, and tank, as well as RC carb and wrench for removing the crankshaft housing.You have a good memory, Chris. The LETMO crank web has holes to improve balance. Mine, bought ten years ago, had 4 holes. Back then I cut away part of the full-circle web, following the inside radius of the holes flanking the crank pin.. See pic 3 below. Also the bottom of the unmodified piston.

Following David Owen's suggestion from years ago, I Milled out the inside of the piston, thinning the walls to match the bottom of the skirt but leaving full thickness on the sides to bear wrist-pin stress. I also thinned the crown a bit. Altogether, reduced piston weight by 20 % , from 10 to 8 grams. Fourth pic shows milling operation. I did this final modification just a few weeks ago because the milling machine is a recent acquisition.

Propeller is a wood Master Airscrew 11x6, which I balanced and then ran on the engine. Vibration not bad at all. Then I sprayed some rattle-can polyurethane on one blade and mounted the now heavier (can't say how much heavier...) blade opposite the crank pin. Seemed to help.

LETMO is awkward to mount because of the induction pipe in back but even more because of the shape of the back of the engine, relative to the three mounting tabs. Birdcage motor mount is fabricated from brass. Works well but is not rigid enough. I need to add diagonal bracing. Mount contributed to initial vibration problem.

Original induction pipe is friction fit into a boss in the back of the engine. I made an aluminum adapter for RC carburetors and made it a friction fit too. This was a mistake because I can't get it back out to reinstall the original pipe and NVA (which now themselves plug into the adapter...with an adapter inside the adapter...Jeees...).

Anyway, I used Thunder Tiger .10 and .15-sized air-bleed RC carbs, and they both work real well. Just a bit higher rpm for idle and full-bore with the bigger carbie. 2,000 and 5,400 rpm. In the same range as your results, Earl. For some reason, TT carbs work well for me on a variety of engine conversions.

One reason long-stroke side-port engines run best at low rpm is that they vibrate too much at higher speed. As Chris warns, the LETMO will vibrate its crankshaft housing right off the front of the crankcase! If it's not screwed on real tight.

In general, the smaller the engine, the less it will vibrate and the faster it will run. E.g. .75 Mills which does 10,000 rpm without complaining too much. I have not run my David Owen GB 5 (twice the displacement of LETMO). So how fast will it go, and how much will it dance?

Good luck with your relocation, Chris. Thanks for starting this thread, Earl.

Regards. -Gary
Attached Thumbnails Click image for larger version

Name:	Cz79806.jpg
Views:	184
Size:	183.7 KB
ID:	1590216   Click image for larger version

Name:	Gb90145.jpg
Views:	273
Size:	197.1 KB
ID:	1590217   Click image for larger version

Name:	Mr40056.jpg
Views:	125
Size:	77.6 KB
ID:	1590218   Click image for larger version

Name:	Ic83417.jpg
Views:	160
Size:	87.1 KB
ID:	1590219   Click image for larger version

Name:	Pa33123.jpg
Views:	133
Size:	71.5 KB
ID:	1590220  
Old 04-06-2011, 12:07 PM
  #9  
earlwb
Thread Starter
 
earlwb's Avatar
 
Join Date: Aug 2009
Location: Grapevine, TX
Posts: 5,993
Likes: 0
Received 3 Likes on 3 Posts
Default RE: MP Jet Letmo 2.5cc diesel engine replica series

Cool pics, thanks for the info.
yeah I got a couple of Thunder Tiger GP10 carbs myself, I could use on a small engine.

I can see now where there can be some vibration, that long heavy piston would have a lot of reciprocating mass there.
I can see how your motor mount would flex a little too much with those long standoffs.

I made a test mount out of plywood epoxied together and with a cut out and some small plywood spacers for the mounting tab. I didn't notice that the back of the engine protruded back a little more than the mounting tabs on the engine did, until I was fitting the engine on the test mount.
I'll take some pics of it later. I forgot to do that after cleaning off the camera lense.
But my test mount was designed to be clamped down into the regular engine test stand that I use.

I suppose if we got more fanatical about it, we would weigh the piston and rod assembly, and then fabricate a brass or lead weight to fit onto the crankpin and use a couple of knife blades to balance the crankshaft better to match.
ref: modelenginenews.org/etw/etw_bal/p2.html
and modelenginenews.org/cornell/p6.html




Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service -

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.