During my free time i started building this KK Bantam from scratch. It is an RC assist model and weighs around 400g.,Covered with polyspan and tissue doped on for colour.

The engine is a DC Merlin diesel engine, around 0.8cc in capacity.

I hope you enjoy the pictures of the build log. The Plans are freely available on Outerzone, but some tinkering has to be done to make it RC.

Very nice Joseph, I think that you will find your Bantam is a very pleasant flying R/C sport model. My electric one has been going strong for close to ten years now. Like you I had to make some changes to adapt to R/C, the main one being to make the fuselage in two parts split along the crutch line for the first half of it's length, with the front 50% forming a detachable "pan" carrying the battery pack, ESC and Rx and the two servos mounted in the back part. The two parts are held together by dowels and magnets. Works well. I hope your model gives you s much pleasure as mine has given me.

Nice job Joseph.
I remember you asking lots of questions about diesel engines. Have you mastered their operation?

Paul

Nice to see something other than the usual high wing cabin design! Nicely done!

Originally Posted by wy8w

Nice job Joseph.
I remember you asking lots of questions about diesel engines. Have you mastered their operation?

Paul

Yes I did ask quite a bit and yes i got the hang of them now. only problem im having is that for the DC merlin to operate well the needle has to be screwed almost all the way out, almost falling off. The tank is level.

Is there something i can do to fix this problem ?

Originally Posted by Sundancer

Very nice Joseph, I think that you will find your Bantam is a very pleasant flying R/C sport model. My electric one has been going strong for close to ten years now. Like you I had to make some changes to adapt to R/C, the main one being to make the fuselage in two parts split along the crutch line for the first half of it's length, with the front 50% forming a detachable "pan" carrying the battery pack, ESC and Rx and the two servos mounted in the back part. The two parts are held together by dowels and magnets. Works well. I hope your model gives you s much pleasure as mine has given me.

I did take a look at your build log ! to acess servos and rx and battery i made a small hatch holding on with a spring pin and a magnet. My model came out close to 400g due to the linkage system. I am using 1.5 mm piano wire running in a plastic tube, aach piano wire weighed around 10g.
But if it flies, I will be happy

Check that you have the jet hole (or holes if it has two) in the spray bar correctly positioned. If there is one jet hole it needs to be facing down the venturi. If the spray bar has two jet holes at 180 degrees, they need to be set across the venturi. If that is well, check that the needle valve has not come loose in it's thimble causing the pointed part to be too long. Of course, ensure there is noblockage in the spray bar or fuel line. The centre line of the tank should be level with the spray bar and all should be well.

Originally Posted by Joseph Cassar

Yes I did ask quite a bit and yes i got the hang of them now. only problem im having is that for the DC merlin to operate well the needle has to be screwed almost all the way out, almost falling off. The tank is level.
Is there something i can do to fix this problem ?

Joseph, It sounds like you may need to "shorten" the needle.
On your DC engine, I think the thimble is alloy, so it can't be soldered to the needle. It's only a "force" fit.
This means that, with enough force, it's possible to push the needle into or out of the thimble, and this is probably what's happened with your engine at some time in the past. Also, it sometimes happens that this "force fit" becomes a bit loose.
So first, check that the needle is a tight fit in the thimble, and that it can't be just pushed in or out (you might get a surprise...).
If the needle is the correct "force fit", then you'll need to heat the thimble a little (not much), to move it (in or out).
This is best done on a gas flame and with the needle actually screwed onto the spraybar (this keeps the needle central in the thimble). You need to remove the spraybar from the engine obviously.
Before you do this, take a basic measurement, so that you know by how much you want to move the needle.
Good luck!

Another pitfall to be aware of in this case is that although DC spraybar and NV's may look ldentical-in reality they're not-the length of the tapered ground portion projecting beyond the aluminium thimble varies depending on the particular DC model....if your engine is not 'as new' then here is a chance that a previous owner may have replaced the original DC needle with another from a different DC motor-causing a mismatch-and the exact problem you describe....essentially fitting a needle valve from a Spitfire or Sabre to a Dart or Merlin can lead to exactly the problem you describe.

A second-though unrelated issue with the Merlin is that they are a bit generous in the intake department-leading to poor suction and (sometimes) an inability to keep the fuel line full.....this can be solved quite easily by making a wooden plug -out of balsa or dowel to block half the intake-i find the rear half best-simply by cutting it to shape and wedging it in the venturi. This improves the fuel draw considerably-with no real effects on the power output. Not all merlins suffer from the problem-but if yours does-it is a simple fix-and easily reversible. [a few 'classic' engines suffer from this issue-the Frog 500 being another one which often benefits from having the intake sleeved down somewhat. Of course your exact tank setup and geometry also affects things....]

ChrisM
'ffkiwi'

A lot of experience distilled in the last few posts.

One more possibility. Sometimes original NVAs have been replaced with more readily available PAW ones. They have two jet holes which are off-set. These spray-bars should be set with the the jet holes "down wind" - when you look into the intake you shouldn't be able to see the jet holes.

A leaking backplate gasket is another conceivable reason to poor suction.
Also, the cylinder base gasket may be worth checking up, for the same reason...

----------------------------

More unusual, but worth to mention:
Some years ago, I purchased an M.E. Heron, apparently in good condition.
When bench running, the engine was easy to start and behaved faultless, except the fact that the needle had to be a bit more open than "standard", This didn't affect running and handling though, as long as the engine was fitted to the test stand, i.e. when the engine operated "horisontal"....
However, when I later on tilted the stand to emulate a "raised nose" (or lower tank position), the engine stopped promptly....each time....
Gaskets were checked, as well as all various NVA-related issues, but the problem remained.
This mystery remained unsolved, UNTIL I later on came by another M.E. Heron, and compared the intakes:
The intake was drilled out.....!...as a meaningless "go faster" measure, which of course was the reason to the poor suction...

Thanks for all the information !

I managed to solve the needlevalve problem. I just put the wire bit in a vice and slowly used pliers to push the alloy part further inwards and now it works well.

The only trouble that remains is that on my model when i simulate a nose up or down the engine does suffer a bit and runs a bit unsteady irrespective of all the tuning effort that i do. The tank is level with the engine (sitting on engine bearers) and isnt very high (only 2cm in height). Is there something that im doing wrong ?

Also for those people out there who want a bantam I know you must be thinking "How in the world will I adjust the compression screw with that cowl in the way without getting my fingers chopped off", well i found an easy solution for that;

Took a piece of stainless steel tube with the inner diameter matching that of the comp screw and sawed a slit. Then i just put this tube in a screw driver handle and there you have it.

I hope someone can come forward with a way to fix this engine running issue when i tilt the model up or down. Also by any chance does somebody have any spares for the merlin

I also want to say that the cylinder gasket is ok but the backplate gasket is torn in 1 part... i just aligned that it matches and screwed the backplate back on (hope this isnt the cause of my trouble) I checked for airleaks at the backplate and none seem to be present.

If it's sagging and going rich easily from tilting the model either the fuel tank is too far behind the engine or you don't have quite enough compression.

It's been a lot of years since my diesel days but I do recall that over the range of compression to mixture that seems to work that on the side of the peak power that has a trifle more compression and a slightly leaner needle valve mix that the engines are a little more tolerant of fuel feed pressure changes. It's a fine line though. Somewhere between peak RPM of compression and needle setting and the point where it sounds a trifle labored and the exhaust starts to look black is the point where it is more tolerant of the fuel tank height. Or in the case of the CL combat models I flew of being shaken up and down obscenely hard with little to no change in RPM prior to being launched.

And when I say a small ways from what you think is the peak and where it starts being so labored that it pukes black oil it's a lot closer to the peak than the black exhaust oil setting. A whisper of more compression and a very slight leaning will find the setting where it sounds like it is peaked and running unlabored. If it sags at all or starts to sound like someone has their hands wrapped hard around the cylinder to choke the engine then it's too much to find this sweet spot where it pretty well ignores changes in the tank feed pressure.

Joseph, did you read ffkiwi's post? I have used Merlins a lot and always restricted the intake to increase gas velocity past the jets. Also, can we assume you are using the 7 X 4 prop in the photo? That is fine but if you are using a bigger prop to get a gentler performance the intake restriction becomes essential. You should replace that torn gasket too.

DC Merlin’s do fine on 8 x 4 props, but are designed to rev. The reviews show bhp peaking at over 12,000 rpm and torque staying well up at over 10,000 rpm.

The Merlin comes alive and delivers on 7 x 3, 7 x 4, 6 x 4 or 6 x 5 props. These prop sizes suit the large-ish intake.

Early examples had forged connecting rods. Those rods are more durable than the later turned rods.