Diesels and aerobatics

[Plummet]

I have been thinking about building a model for a R/C PAW diesel engine I have. Is it practicable? I know that it is no problem for vintage designs that are more or less horizontal for their entire flights, but what about more agile models?

My main concern is the fuel tank. In a glow model we use exhaust back pressure to encourage the fuel to reach the carb whatever attitude the model is in. I understand that the idea of feeding nice hot exhaust into a fuel tank containing an ether mix might be somewhat short lived. (Am I correct in this?)

I also understand that diesel engines are less thirsty than glow. I assume that this means that they "suck" fuel less powerfully.

The fuel tank in a glow model is often a little way behind the engine.

Will the diesel still be able to get its fuel in, say,a vertical climb?

Plummet

[Bob Cotsford]

My thoughts are that glows use exhaust pressure to allow a larger carb venturi giving more power by sucking in a larger volume of air and fuel. Olde-worlde glows had small carbs and coped with aerobatics just fine - look at control line stunters if you want confirmation. Diesels usually have smaller carbs and will happily suck the tank dry as the small venturi gives a greater pressure drop - ie more suction.

Where you run into problems with a diesel is in the clunk line, you cant use silicon and other tubes are a bit stiff for the smaller tanks needed.

I've made clunk tanks for diesels using black rubber tube but the tube needs replacing often as it will go stiff with constant immersion in diesel fuel. Maybe a felt clunk would help as used in petrol motors?

As for the hot gases, the rate of flow from the exhaust into the tank is the same as the flow rate of the fuel into the engine, very low, so the gases wouldn't be very hot by the time they get to the tank.

I've got an Evo 10GX petrol motor and that uses exhaust pressure without igniting the petrol in the tank.

[Solly]

Plummet, you should not have a problem using a diesel, after all, diesels were used for many years for control-line stunt and combat. Also from personal experience (I flew a small PAW 09 R/C powered model for a time) I never found aerobatics to be a problem for the engine, just for me!

[Frank Skilbeck]

If anything a diesel engine with it's smaller carb has a greater venturi effect so sucks harder, they just don't use as much fuel as a normal glow. As others have noted they were often used in stunt control line planes where they rarely spent anytime straight and level.

Not sure if you are using a throttled glow or not, I found with the throttled 35 PAW I ran many years ago that it wasn't as responsive to the throttle as a glow engine, it didn't stop but it didn't pick up as quick. But that may be because i didn't posess the "diesel" knack.

BTW the exhaust gases from any engine are oxygen deficient and can be consider an inert gas as they don't have enough 02 left for ignition to take place. On old oil platforms the exhaust gas from the engines was often used as an inert gas to purge out hydrocarbon lines.

[Greybeard]

I used a diesel in a model that I would describe as aerobatic some years ago. I think it was an Irvine 40 with an r/c carb which gave me no trouble whatsoever other than the cost of the fuel that it used.

[Erfolg]

The very idea that diesels are not suitable for aerobatics, tut, tut!

Used extensively for stunters, especially smaller ones.

I guess the main difference is that most Diesels had no silencers, so a pressure tapping to the tank could not be taken from the manifold/silencer. What was done that the tank was arranged with two forward facing vents, in those days top and bottom. The inlet to these was chamfered, inlet facing forward. The tank was pressurised by forward flight and propeller draft.

Using some variation on this idea, almost certainly would work. The difference would be that a cylindrical tank would be needed with clunk. In the past it was a wedge tank, relying of centrifugal forces moving the fuel to the pick up. On that basis, prolonged knife edge could cause an issue.

[Myron Beaumont]

Just a thought but from memory control line stunt glows were set to what we then called "4-stroking" on the ground and for level flight. As soon as a loop or similar the engine would lean out and run smoothly (2-stroke).This might of course be partially due to lack of centrifugal (centripital?) force and the height of the A/C .Certainly nothing to do with forward facing fuel pipes which simply stopped fuel being sucked out the tank as would have happened with our tanks used in RC .

This is all IMHO of course

Edit --Sorry ,just reread the title -it's about diesels -whoops!   My last one was a Frog 249 many moons ago -Wish I had it now .

Edited By Myron Beaumont on 28/03/2013 15:41:09

[Bob Cotsford]

Wasn't the 4 stroking setup in order to get a more consistant speed through manouvers by giving a bit more oomph on the up lines?

[Myron Beaumont]

Bob

That's the way I see it .I just wonder the exact reason the engine leaned out ? Maybe a combination of factors .With my diesel powered control line stunters & combat wings ,they weren't affected by orientation . Set to max power and go fly .( THINKS! Which might help answering the original question actually ) Team racers as well come to think of it .Now what did I do with my Olly Tiger and my Enya 15D ??

Edit .I'd almost forgotten what nostalgia is about -now I'm beginning to remember

Edited By Myron Beaumont on 28/03/2013 16:01:32

[PatMc]

In the past I've made clunk tanks for diesels using the insulating sleeve tube of Pyrotenax cable for the clunk pick up. This tubing doesn't rot or harden with diesel fuel. However for most sports aerobatic models a non-clunk tank with a slighty longer than usual tube between tank & carb is OK so long as you don't indulge in prolonged inverted. The fuel in the longer pipe keeps the engine going when the feed is drawing air & quickly fills up when the feed is back drawing fuel.
That said the biggest drawback is that if the engine is throttled back for long it can cool sufficiently to affect the compression setting so that when the throttle is opened the engine mis-fires & takes a relatively long time before it comes back on song.
Another drawback I found is that diesels larger than about 09 - 15 generally cause more vibration than glow engines.

IMO the carbs on PAWs are carp, with the non adjustable air bleed & rotating fuel pick up. I changed the one on my PAW 19 for a carb from an old OS15 & that was much better.

Erfolg, there are & were plenty of diesels with silencers but IIRC the instructions with Irvine engines diesels advised against using the pressure nipple, I can't remember the reason but as Frank mentioned diesels generally have a smaller venturi than a similar capacity glow engine so have greater suction anyway.
A wedge tank is only useful with a C/L model where there's a constant centrifugal (centripetal ?) force so no good for RC.

[Erfolg]

I do agree Myron that the pressurisation effect from forward facing vents, would be extremely slight and yes the primary reason was to prevent syphoning.

Bob has reminded me that a degree of throttling was obtained by the 4 stroking. I cannot make my mind up if the primary effect is due the relative position of the needle valve, or the change of loading when climbing or diving.

One thing I am confident about is that a Diesel would do the job.

We have been through the throttling issue before. Many say they work well. My experience with a retro fit ED Racer plastic rear throttled disc arrangement was that my own did not. The slow running was erratic and pick up was poor. That is compared to a glo.

Yet I have been told that the PAW throttled engines as built, work well.

Like many I have a soft spot for Diesels, yet the fuel situation is the killer for me, difficult to get and at a national debt cost.

Other than the nostalgia thing, or a SAM type model, or a personal challenge, I would go with a modern glo, particularly if the model is small. Particularly if reliability of operation is a priority.

The one thing I am certain of, a Diesel would do aerobatics.

[PatMc]

Posted by Myron Beaumont on 28/03/2013 15:59:38:

I just wonder the exact reason the engine leaned out ?

Gravity.

But it was only evident with glow engines - some more than others.

[Bearair]

Diesels in aerobatics! Bill Kits FunFly 15, RC P.A.W.,TX and a small can of diesel was my minimalist flying kit for strapping to the pillion on my moped many moons ago. Very happy days.

[PatMc]

Older diesel engines, like the original ED Racers, don't throttle well because the have sub piston induction. They lose a lot of power when fitted with silencers for the same reason.

[Erfolg]

PatMac

The outer sheaf used on Pyrotechnic Cable was PVC, with flame retardants added, that is over the metal outer sheaf. If it is the sleeve that went over the metal conductors, when potting I have not a clue. We used to make it, if of interest, by filling a short tube containing two conductors with insulating powder and than pulling the whole assembly into a much longer assembly, that was then fed through a extruder to put on the outer plastic sheaf. I guess the company now belongs to Gerhard.

It must be the way I have written the sentence with reference to wedge tanks. I was not suggesting there use, referencing how the then stunt tank worked. I do know that there were more sophisticated tanks in use, primarily to obtain a more consistent run, such as "Chicken Hopper" tanks and bladder tanks.

I am sure you are correct that modern Diesels do or can have pressure tappings from the exhaust. It is just that no one had them, when I used Diesels, right up to my power egged thermal glider. Even in the 70's, getting fuel was an issue, and even then the cost was eye watering.

I do not think that the ED Racer had sub piston induction. I have just had a look at both of my own, but they are gummed up and will not turn over.

Edited By Erfolg on 28/03/2013 16:34:26

[Myron Beaumont]

Bearair

Now you've got me going ! Yes I remember strapping my controliner to my school haversack every thursday with no more than a half pint of diesel fuel in my pocket (never used it all!) to fly on the school playing field before having to "play" rugby .ie Stand well away from the maniacs and sometimes freeze to death .The guys who played rugby got all the accolade .ME -Well I had the backing of the metalwork and woodwork teacher and started the school flying club .Not bragging but I managed the only distinction in woodwork O'Level that time .I wonder why -'cos I just loved it .

Apologies for going off thread like the others

Edited By Myron Beaumont on 28/03/2013 16:49:50

[Erfolg]

Patmac

I have just put some 4 in 1 oil into one ED Racer, it does not have sub piston induction.

Once I found it no longer turned I had to free it up, it was a matter of it should, shouldn't it.

Whatever the poor throttling it was not down to that particular issue.

Unlike others, I have found that PAW are good, I principally used my own for the glider application.

Unlike many others, I found that not all my old motors were that easy to start. In particular, a 1.49 Elfin i had, rear mounting, and front induction, situated underneath, I understand supposably to make inverted fitting easier. Not convinced myself.

[Myron Beaumont]

Erfolg

Reminds me of a DC Super Merlin (red spinner ) I always had problems with after a previous life of Mills . 75s & 1.3s that started first flick .

Maybe a "nostalgia" thread should be considered to carry on our conversations considering todays response??

[IanN]

Plummet, it won't be a problem

[Plummet]

Is it safe to use exhaust tank pressure?

Or is it just not needed?

Plummet

[Myron Beaumont]

Plummet

From the replies so far .it seems to be safe ,Whether it is needed or not nobody seems to know .Ok smaller venturi -But -less fuelflow . In other words you are entering the world of the unknown .Let us know how you get on .I'm seriously thinking of going all diesel again .I just love 'em (and the smell)Pint for pint and hours of enjoyment I don't think the cost of fuel comes anywhere near the cost of running your car for the same amount of time for fun

[PatMc]

Posted by Plummet on 28/03/2013 17:43:55:

Is it safe to use exhaust tank pressure?

Or is it just not needed?

Plummet

Not needed, it's just one more thing that could go wrong.

[PatMc]

Erfolg, it's the Pyro conductors' insulating sleeving that I used, the outer would have been too big for my car.

Re the ED racer SPI - I've no easy way of checking as mine's in a box in the loft. I was under the impression that at least the original versions of the Racer had SPI. This could very well be wrong but IIRC that ED copied Mills by modifying their existing 2cc engine to have SPI & re-naming it as the Competition Special. I pretty sure it was also a feature of the Mark III which was the predecessor to the much modernised Racer.

PS The ED Competition Special & DC wildcat must be the most misleadingly titled model engines ever.

[Myron Beaumont]

I suppose that if you keep the fuel tank close as you can to the carb then there should be no unknown problems based on everyones past experience .As PatMc says -- The elimination of one more potential unknown that could go wrong .

[Lee Damms]

I did't think exhaust pressure was used on diesels because the hot exhaust onto cold fuel could cause condensation in the fuel tank. Water and diesel fuel does not mix sinks to the bottom and your clunk sucks it up and one dead engine. Glow fuel on the otherhand will readily mix with water and does not normally cause a problem. I used to fly stunters with pressure pipes facing forward or the more normal stunt tank.

Regards Lee