I converted the LA .46 with a Davis Diesel head today and the results are great,
Davis, Oily Mix/ Plane fuel
MasterAirscrew 12x6 three blade 8,800 rock steady, does not wander, still working on the idle 2,500 so far. I'm flying it tomorrow. Throttle up is clean and crisp.

These are great motor's, I run a 12-8 apc on mine and it's great!!

Hey Cap, Francis Person and I flew that .46 on an RCM 40 trainer, we came to the conclusion that there is a fuel tank issue to deal with but while the tank was full it flew and ran quite well. I think a smaller tank will be the fix. We also flew my Sig Rascal 40 with an inverted PAW 40 turning a 12x6 three blade a 8,200, what a pleasant surprise, it leapt of the runway in about 4 feet and flies like it is on rails, the PAW ran flawlessly, my first inverted Diesel. We also flew his Goldberg Tiger 60 with a Fox Eagle 60 and Davis Diesel head. It would make very long verticle runs, all in all we had a great Diesel day. I greatly appreciated his invitation and hospitality. These were also my first ever flights off of asphalt and DCRC.

The tank issue turned out be that the clunk had flopped around to the front of the tank.

Good job, finding the problem=it's always the stupid stuff. I've never been to DCRC but I hear it's a great field. I fly at CBRC in Crownsville, sadly my work hour's have doubled, but FEEL FREE to come by and be the second person to blow kerosene out the pipe! Happy Trail's of diesel smoke that is!!!!

BARKAI, I have some new numbers for the LA .46, I got a Pro-Zinger 12x7 which it turns at 9,000 rpm.

Dave,


I know we usually don't want to lug down our glow engines.
It would, however, be interesting to see what a glow OS.46LA would do with a 12x6 3B.

This to see how much low RPM torque is added as a result of converting it to Diesel.

The RPM you got on the 12x7 Pro Zinger represents a drop from 0.69 HP @ 8,800 RPM, to 0.63 HP @ 9,000 RPM.
It could be just a dip in the torque curve, with a rise to follow at higher RPM, or it could be the engine is peaked out below 9,000 RPM.

I don't think this is very likely...

hobbsy

Your buddies Tiger with the Fox 60, What size prop was he turning? I am in the process of building a tiger for a Diesel conversion. and was curious.

Funny, Dar- here recently I've been running a 13-7 2 blade apc and it run's great I'll have to tach it when the new batteries get here! Brian

Brian,


A 13x7 on an OS.46LA???

I would recommend this to no one. Not in a Diesel conversion and certainly not in a standard glow.

This would very likely cause overheating and even 'egged out' rod bearings.

It is sort of like running your standard shift car in top gear, uphill, at 30 mph... at full throttle.


This cannot sound very good, can it?


Model engines run best when peaked to achieve their maximum real HP RPM, in high speed flight.

Most .46 sport engines with the standard silencer they come with, usually top out around 14,000 RPM.
This means they should be propped to about 12,500, on the ground, for best all-around performance.

Not to 8,500 RPM...

PAW 40TBR R/C are designed to run with maximum 13x8 ..

Jens Eirik

Hobbsy / DieselDan
I am using 12x8 or 13x6 at both prop I never get over 8100 rpm. Most the time only 7500rpm
I blend the fuel as Davis wrote me (I try to made some minor changes) I use 1.5% MEKP
I adjust the main needle for max rpm and then the the press. screw to get max rpm
and go again to the main needle I repeat this until I get the max rpm in both.
I have the "Dr Diesel" booklet.

Do you have any idea to get better?

Thanks
Barkai

Jens,


This is correct, but PAW engines are all Diesels, timed like Diesels and intended to run at lower RPM levels from the start.

An LA is designed as a glow engine, with all the typical glow numbers. The conversion to Diesel, with a contra-piston head, does not change these numbers, so this is definitely not a fair comparison.

Funny that after 2.5 gallons of fuel it still run's great. That is alot of time for a diesel to run while sipping fuel and have no problem's. I just pulled the cover and it look's great no wear and the compression is great. I tried numerious prop's and this fit the package best !!!

Brian,


2.5 gallons represent only about 13 hours at full throttle...

What wear did you expect after such a short period? Especially since in post #2 you wrote you are using a lighter 12x8 APC prop.

As a glow, it would have already consumed itself by now.

Dar< Davis has been making conversion heads for at last 30 years, He recomends the larger prop loads and the engines thus produce
more thrust at a lower rpm. Also they run cooler than glow, if running hot they are overcompressed (pre-ignition) which will take its toll
on any engine glow or diesel. The fuel formula with the kero is also has more of a lubricant action, As far as made as diesels I believe
on the MVVS 12,15,and 61 the only variance is the diesel head and a smaller bore carb. The Irvine Q40 and Irvine diesel are the same engine except for the head. In enya the CX11, CX15 and 25 are the same except for the head (the 25 does have a restrictor in the carb
barrel).It also follows that an engine will last longer at 9 or 10K rather than 14K. There are a few engines out there that may have weak
cranks or rods that might not make a good conversion but also how long do they last as glow?. My case in point at least as far as MVVS
and Enya (and only the one irvine as far as I know) they are factory issues the only difference being the head martin
One more thing the MVVS 40 and 49 make super conversions massive rods, I have the 40 with the davis head, my only regret was not getting the 49 instead, and back to the orginal issue the LA 40 and 46 are good inexpensive glow engine but really turn on as diesels

Martin, Brian, Jens,


The question is 'how big is too big?'.

I know Diesels, for all the various reasons, even when they are converted glow engines, peak significantly lower in the RPM range than equivalent glow engines.

I also know that by reducing the compression, pre-ignition and thus overheating can be easily prevented.

But are you getting what you bargained for?
Even a Diesel has a horsepower curve, which peaks at a certain RPM level. To get the best performance out of your Diesel, you would want it to just reach its HP peak, when unloaded in high speed flight.

If a glow engine peaked (actually - not according to its manufacturer...) at 14,000 RPM and its Diesel conversion, with the smaller carburettor and all, peaks at 12,000 RPM. And you prop it for 8,000 RPM, it will unload in flight to 9,200 RPM...

This is still a good distance from making its best performance. It may even be short of the peak of its torque curve...


So reason is what is needed here. You can use a good HP calculator, to plot the HP curve of your converted Diesel.
You do it by taking tachometer readings on various props (many sizes) and entering them into the HP calc, to calculate the HP@RPM.

Using a typical glow .91 size 13x8 prop, on a .40 Diesel? It seems like overdoing it to me.


I can send the Prop Power from Pé Reivers, if you have an electronic spread-sheet program and send me your E-mail.


I have not made my .49 MVVS conversion run properly yet... It may be that my ether.... isn't...

Dar my Blue Max sort of a WW! looking mid wing has a 13x6 zinger and flys quite nicely at "scale speed" a 13 x8 would be a little too
much, my vmar hot rod with the irvine 53 uses a 12x6-10 zinger which unloads if that is the right term and goes like a stripped ape
with that prop it would be nice to see the real speed and rpm with an onboard RCATS unit for rpm/speed meaurements but I do not have the bucks for that unit at this time martin
Also the smaller PAWs have been used in high speed C/L for years and they do state in their info sheets to check the rod after 3 to 5 hrs of run time if run at 15K high rpms in any engine requires checking after a few hrs
try and get some new ether too for your 49

Dar, I'm not trying to start a conflict but Bob Davis said you could use a 12-8 13-7 with no problem and I have. I would never try such a prop on glow but they fit my deal prefectly and it worked great! The la 46 is not only cheap but pretty cocky when diesel! Brian

I seldom prop a Diesel for above 9,500 rpm, when you do it negates your fuel ecomomy, requires higher compression and makes the engines run hotter. Also a restrictive exhaust that would not normally be a problem becomes a problem.

I prop mine the same as Hobbsy for about 9500 max, also as the prop load increases you lower the compression this certainly makes life easier for the engine, by running small props at high rpms and higher compression the advantage of the diesel is diminished. The
exception to this set up of course is the speed control line class martin

When talking about any Diesel, like the LA in question, I would say propping it about 1,500 RPM shy of the calculated HP peak, would be the best way to do it.

Propping any engine to spin significantly below its torque peak, is bad for any engine, Diesels and converted Diesels included.
I believe, Dave, you would agree to this.

The question is how much the conversion lowered the HP peak and the torque peak, in RPM terms.


So, before trying several props, to plot a HP curve, none of us would really know the best power RPM to prop the engine (first paragraph).

The torque peak on a two-stroke Diesel is usually about 3/4 of the way to HP peak (my estimate), so if this converted engine has its peak HP at 12,000 RPM, I believe propping it for less than 9,000, even from Martin's and Brian's POV, is useless.


I believe 9,500-10,500 is OK for the .46LA. Lower RPM would probably be usable for larger engines.


As I wrote, I could not yet run my .49.
Barkai will be coming for a visit soon, with fuel that can work and we will work on this...

Besides!

While reduced RPM and lower power extraction have a certain contribution, to the lower fuel consumption, of a glow engine that has been converted to Diesel, the main factor to this is the higher potency of the fuel.

An ounce of model Diesel fuel has nearly twice the BTUs of an ounce of low nitro model glow fuel.

It is this that allows a diesel to make a comparable amount of HP, at similar RPM levels, while using much less fuel to do it.

This does not necessarily mean that we must all run our Diesels at the same RPM, they ran in their previous 'glow' life, but a reduction in fuel consumption should not promote us to running them at very low RPM, as this contribution is only partial.


And as to what Martin wrote; using lower compression to lower the load on the engine's working parts... Running any engine at a lesser load AND at lower RPM (but at proper working temperature - i.e. not too cold), will contribute to its longevity.
But don't some well made glow engines (Jett, for instance) run for over 100 hours at 17,000 RPM and at very high load?


I have no doubt the muffler's back pressure comes deeper into play, as RPM rises further.

Several messages have touched on this point. One of the most important factors in prop selection is that it match the needs of the aircraft and your style of flying. To see how large a prop your Diesel will turn is like the guy with the glow engine that wants to see how fast the engine will turn on a small prop, even tho that prop would be impractical on anything but a racer - maybe. A large diameter slow-turning prop might fit a slow flying aircraft just fine - or maybe not.

For example, a fellow at our field has an Extra 300 with an OS90 on glow that flies well. He converted the engine to glow and tried flying on larger props to get better fuel efficiency. He discovered that the aircraft could not move fast enough at the lower prop speeds to do much more than circle the field. The best he could do was to use the same prop he was using on glow at roughly the same RPM and the about the same fuel consumption. No gain there. The power requirements of the aircraft were not compatible with the benefits of Diesel operation.

I am most interested in a Diesel engine where I want to swing a somewhat larger diameter prop than a glow engine can handle because it better fits the aircraft design. Generating a power or torque curve for the engine would be very helpful here.

derf

That doesn't make sense, if he changed from a 14x6 as a glow to a 14x7 three blade or a 14x8 two blade as a Diesel he would have had more than enough speed, possibly more than he had before. No way would he have less.