DIESEL FP40 FOR STUNT : UNIFLOW TANK ?

[Robin_Holden]

Greetings all from a very hot Charente.

Advice please from any Diesel users out there. I've just converted an OS40 FP to Diesel using a MECOA head.

It runs just fine on the bench and seems to like an 11x5 prop'. It's super quiet and economical to run as well.

Question : Do I need to use a UNIFLOW tank to fly with ? Might appear a daft question , but the last time I used diesels was back in the ' Middle Ages ' , circa '57. We didn't have uniflow plumbing in those days.

I only use uniflows for my glow motors so what's the right way to go please ?

Much obliged for any contributions ,

Robin.

[john e. holliday]

Where Jim Thomerson when needed?  I have seen his diesel conversion fly.  I beleive he uses Uni-Flo tank, may be mistaken.  He adjusts the engine for fly by flying.  Once it is set he leaves it unless big weather change.  He also states that by the time he reaches the handle the engine is warmed up and ready to go. 

When I was competing in F2C, we would count the clicks on the needle as we opened it up for warm up.  Did not touch compression untill after it was back in the air.  The needle was turned back in after starting and as the engine warmed up.  Of course in stunt you don't go for the absolute max on an engine and remember when you start maneuvers the engine will gain more heat.  Now to waist for Jim to correct me.  DOC Holliday

[Larry Renger]

An 11x5 sounds like a bit small or low on pitch for a dieselized .40.  I think more pitch will prove to be useful.  I run an 11x7 on my 4-stroke .40, and think the diesel would need a similar load.  My Brodak .40 turns an 11x5 perfectly well, so that is another clue that you should go bigger.

[Jim Thomerson]

I'm not here because the forum wouldn't accept my post. What Doc said.  I ran a Tower 40 with RJL head, 12x6 APC, muffler pressure to uniflow.  After some hours of running, the contrapiston came apart.  The shelf under the O ring was about 1/32 thick and the whole shelf broke off in flight.  Also broke four cranks, because of running a heavy hub, I think.  Had hundreds of flights converted back to glow with heavy hub and no problem.  So take a look at your contrapiston, and don't run a heavy hub.

I adjust for a smooth run in level fight.  If the engine sags in maneuvers, it is overcompressed.  if it misses in maneuvers, it is a hair lean.   I also look at exhaust oil color as a clue.  Clear is good. 

[Andrew Tinsley]

Hello Robin,
               I am a bit of a diesel freak, in fact I prefer diesels to glows in stunt. You get a piped engine style of run, without the hassle of setting up the pipe! You also need a lot less fuel, which helps to offset the (normally) heavier weight. There is also a lot less change in the CG of the plane during a run. O.K. I admit to more vibration with the larger diesels!
  Right, I expect a load of flak from everyone else on the forum! I don't know about the diesel conversions using the Davies and Mecoa heads. I think it depends on the individual engine, as to whether they are strong enough to take the extra stress. Contra pistons breaking up is a bit of a worry, sounds as if the design isn't good enough to me.
  As far as the type of tank is concerned, for diesels like the Oliver and PAW up to around .19 displacement, I find that there is little difference to the performance between old fashioned wedge tanks and uniflow. I fly my diesels fast or faster! anywhere between 3 and 4 seconds a lap. The uniflow does give a slightly better run (i.e. speed at start and finish is closer). Having said that, the difference isn't noticeable unless you put a stop watch on the runs.
  For bigger diesels, the difference is more noticeable. I have run a PAW .35 in a Freebird and I did settle on a uniflow tank for this set up. You could tell, without a stop watch, that the engine speeded up less during the run with the uniflow tank. Hope your engine holds together long enough for you to find out! Go with a uniflow. I hope you like it, there  are few enough diesel freaks around!


Best of luck,


Andrew.

[don Burke]

The cranks probably failed due to the increased compression load.

[Jim Thomerson]

I've run a glow plug engine on diesel fuel and had it run nicely, with no change in compression.  So I am not convinced that a properly compressed diesel is all that much higher than some glowplug engines.   It maybe a factor, but  I think it is more likely to be the shape of the combustion curve.  Diesels have a very fast burn.  It is not quite accurate to call them "detonation engines". Diesel exhaust is relatively cool, the burn is long over with; in contrast, glow exhaust comes out still burning.  So I think it is a matter of a sharp blow each revolution rather than a somewhat smoother push.  I've also had diesels throw prop blades, which I would not have expected with a glow.   

[don Burke]

"The cranks probably failed due to the increased compression load."

I guess I should have said due to the increased load on the crank from to the diesel cycle.  The first K&B 15 diesel conversions used the standard crank and had quite a few crankpin failures.  They had a pressed in hollow crank pin.  Changing to a solid pin helped the problem.  Although I think a one piece machined crank is stronger, and is used in practically all newer design engines.

[Michael Boucher]

Diesel Freaks stand tall.  Mixing diesel fuel

[RogerGreene]

Hi Robin,

Did you mount the diesel side mounted?

Roger

[Andrew Tinsley]

Robin and Michael,
 Just to say that all of my control line diesels have been side mounted. So take my earlier reply with that in mind! I have found that all the big PAW diesels from .35 upwards suffer from a diesel version of the Fox Burp. My advice is always mount your diesels as sidewinders, there may be some that will work upright or inverted, but I have yet to come across them.

Regards,

Andrew.

[Jim Thomerson]

I flew my dieselized Tower 40 sidewinder and it ran fine.  Same with ED Racer.  I've flown ST G20-15 diesels both upright and side mounted.  I'm convinced they run better upright.  I've flown a CS Oliver upright and it ran fine.  As said, all uniflow, and muffler pressure on the Tower.