Diesels and pipes???

considering the availability of the Davis Diesel heads... Has anyone tried a piped diesel engine?

I asked Bob Davis about that, and it will work for our event, but not for RC, which is his main market. Basically, if you come off the pipe as you throttle back an engine, it is very slow to come back on. The way we use engines should be OK. But why bother? Diesels have very good characteristics for stunt with one exception.

I, at least, don't seem to be able to get consistent run times. Gobs of power, but very variable. I recently chatted with Bob, and he says "LEAVE THE NEEDLE ALONE". It seems that you should mostly tune a diesel with the compression and once you have an OK needle setting, don't touch it again....ever. He claims that then you will get runs indentical within seconds of each other, just like a glow or better.

I'll have to work on that and get back to you guys on it....

Yes, leave the needle alone! Also don't change the compression unless you really need to.

I haven't used a diesel in about 20 years. If I remember correctly, once you get a run that you like, don't touch the needle and only open up the compression by 1/4 or 1/2 turn when starting. As a matter of fact, if you feel adventurous, don't touch the compression screw either but be prepared for bloody fingers.

Most, but not all, diesels run at a lower compression than the compression which gives the quickest starts. You can also start a diesel with a starter at lower compression than you can easily by hand.

It occurs to me that fuel temperature has more effect on run time for a diesel than with glow. At a contest one time, I let my fuel sit in the sun, I could not get through the OTS pattern. I took a bulb (Sullivan black gas bulb) of fuel and put it on ice for about 10 minutes. Throught the pattern with laps to spare. In my shop, when I turn on the air or heat (eat your heart out!)in a few minutes the quart cans of diesel fuel will pop as they change shape as the fuel inside expands or contracts.

>Most, but not all, diesels run at a lower compression than
>the compression which gives the quickest starts. You can
>also start a diesel with a starter at lower compression than
>you can easily by hand.
>
>It occurs to me that fuel temperature has more effect on run
>time for a diesel than with glow. At a contest one time, I
>let my fuel sit in the sun, I could not get through the OTS
>pattern. I took a bulb (Sullivan black gas bulb) of fuel
>and put it on ice for about 10 minutes. Throught the
>pattern with laps to spare. In my shop, when I turn on the
>air or heat (eat your heart out!)in a few minutes the quart
>cans of diesel fuel will pop as they change shape as the
>fuel inside expands or contracts.
>
>Jim Thomerson

Could this be more related to ether content changes with temp?

>I asked Bob Davis about that, and it will work for our
>event, but not for RC, which is his main market. Basically,
>if you come off the pipe as you throttle back an engine, it
>is very slow to come back on. The way we use engines should
>be OK. But why bother? Diesels have very good
>characteristics for stunt with one exception.
>
>I, at least, don't seem to be able to get consistent run
>times. Gobs of power, but very variable. I recently
>chatted with Bob, and he says "LEAVE THE NEEDLE ALONE". It
>seems that you should mostly tune a diesel with the
>compression and once you have an OK needle setting, don't
>touch it again....ever. He claims that then you will get
>runs indentical within seconds of each other, just like a
>glow or better.
>
>I'll have to work on that and get back to you guys on
>it....

Yeah, I've, read or heard that on this forum (I think) before...
I've been using that technique (i.e. don't mess with the needle valve setting) on my PAW .19 and it has given consistant run times. But then, I can't say that some other technique might not work also.

Interestingly enough, even when I do mess with the needle valve, I always seem to end up back at the same setting anyway. Using a tach it does seem to be possible to consistantly lean the engine until the rpms peak then richen by 100rpm. I'm still learning and "experimenting". I suspect that the difficulty (or preceived difficulty) of getting reliable repeatable runs is one reason diesels are not more popular for PA here in the US. On the plus side it doesn't seem to be sensitive to the verticle location of the fuel tank. Centered on the needle valve works just fine. I wonder if the Davis Diesels behave more like their glow fuel counterparts?

I'm an advocate of leave the needle alone on both glow and diesel engines. An exception is when you have a big heavy airplane with an engine of modest power. Then you may have to make weather adjustments to get that last rpm that you need. I also found with the TD 09 on case pressure, on a calm day, I could richen it a couple of clicks and slow the airplane down a little. I tried this on a windy day and the engine wound up in maneuvers to where the airplane was almost unflyable.

I set up diesel fuel tanks the same as I do glow; line the uniflow outlet vertically with the spray bar as best I can, then tweak it a hair up or down if needed. So I don't know if diesels are different in sensitivity to fuel tank location. A diesel with a very small venturi probably sucks fuel pretty well.

I have an MP Jet on the Meek/Midwest P-51 replica, and I do have to adjust the tank height just a bit to get perfect upright/inverted match. I always use an electric starter with this engine and my PAW 55 on the Baby Clown. If you are careful with fueling and priming technique, there is no problem, and near instant starts.

I still have to put in about 1/4 turn more compression to get a solid start, but don't touch the needle. One key is to put in a full tank run before your competition day starts to check if any adjustment is required. Time that run, and you should be pretty well set for the day. I do keep my fuel in an insulated box at the field (Actually one of those roll-around food chests is my flight kit, the insulation is a bonus)

30+ years ago, when I was attempting FAI TR, Super Tigre offered a G.15 Diesel with pipe timing. It ran extremely well, but I was not really able to take advantage of it, 'cause Henry and I broke up shortly afterwards.

I still have that engine, somewhere. H-m-m-m . . .

Piped operation depends somewhat on factors like compression ratio. Of course, on a diesel compression is adjustable so every time you tweak it you change the operating parameters of the pipe. Piped diesels can be done, but with that extra interlaced variable headache...

A.

By the way, the two hottest .15 size diesels I have found are the Davis converted OS 15fp and Conquest .15. The Conquest is much faster with the smaller props, by the time you get to a 10x6, they are about equal, but the OS is "happier". I am not sure I have the Conquest working right yet, as I put on a .166 dia venturi, about 1/2 the stock passage area. It might go bigger with the small props. I have heard reports of 18K with an 8x4, and I am only at 15.6K so far.

For stunt, I think I would go with the OS. Much easier handling and starting. Again, in light of testing so-far. Oh, yeah, they both turn a 10x6 at about 8500 rpm. Not bad for a teeney weeney engine!

I am comparing them to a Webra 2.5, Rivers (CS) 2.5, CS replica Silver Swallow 2.5, and MAP .15. If anyone has good numbers for other .15's I'd love to see them. I have a CS replica Oliver, but haven't been able to get that running yet, it may have a really bad cylinder/contra-piston fit. If I can ever get it running, perhaps the gap will fill with carbon? Or, if desperate, perhaps I can put the contra-piston on a tool steel plate and give it a whack with a hammer to peen the diameter out a bit. (Classic fine-tuning?)

Larry,

I don't know about the fine-tuning part, but if ever anyone refers to you as a mainstream modeler in my presence, I will use a hammer to whack 'em right between the wickets.

Much progress on your dieselized 1/2A canard?

Shhhhh! I TOLD you that was a secret!

I'm working on the pipe for it.

I thought tuning a pipe was strictly an RPM thing. How does compression enter into it?

If Diesels are outlawed only outlaws would have diesels.

Scott(THEY STINK)Riese IMHO

Scott hates diesels because he has an allergic reaction to some component of the kero/ether fuel. Sorry, Scott. Not our fault. Diesels are good. Glow engines are diesels, even the Saito .72. Stop whining.

My recollection of what pipe info I have read, is that volume of the pipe is determined by the fuel being burned. Nitro needs more volume than Alcohol. We're talking about a huge dose, here, like from zero vs. 60%+. I would guess that petroleum based fuel would need less volume than alcohol, but strictly a guess. I'd bet that Jenning's 2 Stroke Tuning would tell you. Dan has a copy. I bet Larry and Howard do, as well. Steve

>I thought tuning a pipe was strictly an RPM thing. How does
>compression enter into it?

Ron B.
F4Fguy

It does't,directly,but it does effect the exhaust gas temp,which is one of the primary design factors.The tuning is basically dependent on the speed of sound in the gasses in the pipe*.This is what determines the length of each section of the pipe,as well as the basic tuning length.This is also effected by mixture strength and by fuel.

* The speed of sound goes up directly with increasing(absolute) temperature,i.e. double the temp=double the speed of sound,and,since the resonant freq.is determined by speed of sound,the resonance changes accordingly.

The whole thing is further complicated by the fact that each section will operate at a different temp than it's neighbor,since the gas temp is changing all the time.If all the pipe elements are doing their jobs,pressures are different in each part of the pipe,ranging from well below atmospheric to well above.Therefore temperature is changing accordingly.This effect is further complicated by the fact that heat loss is occuring all along the pipe by conduction,radiation,and convection.

It's also almost impossible,with instruments available to most of us,to accurately measure local,instantaneous temp.Consequently most pipes are designed by doing a"first cut"pipe using estimated temps derived from "reverse"engineering of known pipes,then cutting,and trying until time,patience,or money run out. Incidentally,there's no such thing as "pipe"timing,any engine can benifit from a pipe if the pipe is designed properly.The pipe does allow the use of port timings which would be impractical on a non- piped engine.In a full package pipe/engine the timing can be such that the engine won't run well(if at all)without the pipe.Almost invariably such will be very powerful,but"peaky"and unsuitable for our use,great for speed,but not for us.

Sorry guys,I've gotta quit now,my typing finger is getting a cramp.