tweedy

My Feedback: (1)

Anyone know the correct Orientation of the spray bar, in relation to the barrel, on PAW .033 R/C.

Thanks

Richard Browning

AMB

Tweedy I would guess about 4 or 5 o clock it would bounce off the lower part of the venturi and allow a good mix before entering the crank
rather than a squirt strait down at the crank martin

my reasoning the hole drilled in the crank not that big and not that much mixing fuel with air

AMB

[quote]ORIGINAL: AMB

Tweedy I would guess about 4 or 5 o clock it would bounce off the lower part of the venturi and allow a good mix before entering the crank
rather than a squirt strait down at the crank martin

my reasoning the hole drilled in the crank not that big and not that much mixing fuel with

Mr Cox

There are two holes and they should both point downstream, i.e. you shouldn't be able to see either of them when looking down through the barrel at full throttle.

AMB

Mr Cox not aware 2 holes makes sense ,for a good mix air and fuel martin

tweedy

My Feedback: (1)

Thanks, If I get it right the holes in the spray bar are at 90 degrees to the throttle barrel hole.

Richard

ffkiwi

Just be aware-that depending on the age, history and usage, the engine might have a single hole spray bar or a two hole one-and if it is a 2-hole one then there are TWO variants out there-one has the holes drilled diametrically-ie right through from one side to the other, the other (and it was known as the 'RC' spraybar) also has two holes and these are at 90 degrees to each other. The correct orientation in the carb is different for each:
(i)single hole-generally traditionally pointed straight down (but there is some recent debate about whether this is the 'best' position for a single hole jet)
(ii) two holes diametrically opposite-positioned fore and aft with the holes horizontal (and not visible from above) as stated by Mr Cox
(iii) two holes at 90 degrees apart,-positioned so that each hole is 45 degrees to the carb axis, pointing downwards-so not visible from above either

What spraybar is fitted to your engine will depend on when it was made, and also if any previous owner has had to replace the original spraybar (PAW spraybars bend easily, and almost always break when you attempt to straighten them)

You need to check (by loosening, or even removing completely) the spraybar and confirm what type it is. I find it useful to mark the hex in some way-a small file notch, or a touch of a drill point to make a clear 'dimple' to indicate the jet hole location, so that you can maintain the correct orientation when tightening up and also have a reference point against any rotation or creep in use.

ChrisM
'ffkiwi'

tweedy

My Feedback: (1)

Thanks so much it was the latter, holes 90 degrees from each other, I would never have caught that. any other tips on this engine? I'm a bit of a nube when it comes to diesels. I am replacing the Tee Dee .020 in the model pictured with the PAW .033, any sugestions as to what prop to start with?

ffkiwi

If you haven't much experience with diesels then I suggest you bench mount it first to get a bit of experience. The 55 produces about 40% more power than an 020-at about half the revs! [0.07BHP at 12500 vs 0.05BHP at 23,000] Obviously the 55 turns much bigger props! Maximum power is probably obtained on a 6x4-but you probably don't need this at this stage-diesels can get a bit 'snappy' on small props-ie they can 'bite'-which believe me hurts-even from small ones! A bigger prop improves starting-at the expense of some power-but you gain in prop efficiency, so you don't actually lose as much as you might think simply from looking at the power curve profile. After all THRUST not power or revs-is what flies aeroplanes-even though there is a direct relationship!
I'd suggest-if you're inexperienced with diesels-and the small ones are fussy (compared to the bigger ones of 1.5cc upwards) that you could start with a 7x4-starting will be easy on this size, though the engine will not produce the max power it is capable of. I'd recommend that you hunt around and try and find a flexible nylon one-a Topflite, or Kavan or similar. APCs are damn sharp, and Master not much better (nothing wrong with the props themselves-it's your fingers I'm thinking of here!)-then later perhaps you can look at a Master 7x3 or a 6.5x3 or any of the 1/2A 6x3-6x4 options-which essentially boils down to Cox, APC and Master again.
I doubt that you would ever find any need to go below 6x3 on the PAW55-unless you felt a strange need to fly control line speed or proto with it..........(in which case a 4.5x5 or something might be interesting....)

And hopefully-and before you attempt to-DON'T use an electric starter on it. While you can use starters on larger diesels, extreme caution is required-and plenty of diesel experience, and the smaller the engine the greater risk of damaging it.

ChrisM
'ffkiwi'

PS FWIW, my Vic Smeed 'Poppet' is powered by an 0.46cc ED Baby older than I am-and flies quite happily on a 7x4 Topflite at about 7,000 revs flat out-and I don't usually run it flat out. That's a 32" span model weighing almost 10oz (yes-very heavy-from 'oak grade' balsa-and with 2-part paint on the fuselage-which explains why its still flyable after 30+ years!)

Recycled Flyer

Heya Chris,
its been lab tested by Bill Lee that a single hole should be just below the horizon or about 5º below level for maximum fuel draw.

But here is the problem with that, is it can not really be verified in the air and it seems to be a critical position as a few degrees off that sweet spot will render the efficiency to be jus the same as any other workable position. In my humble opinion though, the 'just below the horizon' is something to aim for as you have nothing to lose to you are off a tad and everything to gain if you happen to get it right.

What Ihave against the straight down position is possible flow reversion - just stand on a bridge, look down and view from above the water flowing around a piling. On the downstream side the flow wants to revert back to the direction it came from (or at least tries to) and in the process causes swirling.
Now what Idon't know is if this matters in a pulsed system like that found in an ICengine as flow is built up and then stripped away many times a second.

With true venturis (no spraybar) the most consisent place for a jet is indeed 'just below the horizon' on its side wall so I would run with that.

Thanks.

ffkiwi

I did say that there had been recent debate on the subject. FWIW the wick jet was proved superior to all others back in the 80's-but not a lot of people use it due to the hassle of getting both the right size tubing and the location dead right. I think it depends on usage-and on relative size of venturi, jet hole etc-there are too many variables to hang your hat on one single solution............and in some cases other factors may be dominant to the point where the exact jet hole location becomes insensitive............

ChrisM
'ffkiwi'