steve111

Hi Jenny,

14000 with an 8x5 would be reasonable for a good .15, but with a 9x5 it's beyond anything I could believe. I don't think even my 19TBR would get close. Even PAW only claim .34 bhp (I think) for the .15 - the sort of performance you're talking about is close to double that. So it had to be an 8x5, I reckon, which means the good news is that your engine hasn't deteriorated nearly as much as you thought.

They're still making PAW 15s, so parts wouldn't be a problem.

Steve

Recycled Flyer

Except here we are discounting the VERY common control line use of the uniflow setup that just about ignores the changing level of the fuel and instead 'looks' at the fuel head as being a fixed distance between the fuel pickup and the inlet vent inside of the tank.

Of course (uniflow or not) the recognised head level is still effected by G forces and thank goodness it is as it can be quite the design feature!

qazimoto

My Feedback: (1)

Jenny,

Chris has pretty well bagged it.

Our friend Franchi is using a Super Tigre G20/15 Diesel mounted sidewinder style in a Flitestreak profile.

In such control line applications the needle valve is usually deliberately set richer on the ground compared with the optimum position for maximum RPM.



The amount of "richness" depends on the vertical distance between the hole in the NVA and the position of the fuel pickup pipe in the tank looking from the top of the model.

Let me illustrate this with pictures.









The first two pics show tank uniflow setups in diesel Goodyear models. The breather pipe is directly above the fuel outlet pipe. All three tanks are front feed. The third tank is in a Diesel powered Combat model . The tank in it is triangular shaped and front feed as well.





In the Racers the tank can be shaped so that the fuel pickup is directly in line with the NVA hole. This means that the ground and flight setting can be the same. Typically the tank then has to be rather long to get a decent capacity.

Often with diesels they start hot best when they're rich. Hence you deliberately set the position of the tank pickup to allow the engine to automatically leanout in the air.

qazimoto

My Feedback: (1)

The mechanism described above works exactly the same with rear feed stunt tanks.

My point is that it's the horizontal distance between the NVA hole and the pickup that controls the leaning out phenomenon in c/l models.

Yes, the Vector Resultant of the Forces of Engine suction, centripetal force and Gravity are important, as is tank shape.

If the NVA hole and the tank pickup are in line, there's no difference between optimum NVA settings in the air and on the ground.

The outboar pickup leans in the air due to the fuel mass having to "overcome" Centripetal Force. ie Effective Vector subtraction in the Resultant fuel suction.

The inboard pickup goes rich because it is "aided" by Centripetal Force ie Effective Vector addition in the Resultant fuel suction..

So moving a Flitestreak tank inboard or outboard slightly could help the engine run

If the tank pickup is inboard of the NVA, then the engine will richen in flight. I understand that this mechanism is used in c/l F2A piped speed models to allow them to have sufficient fuel to run up onto the pipe. I.E. very small adjustments in and or out to the tank position control how the engine "comes on the pipe" because of the change in fuel flow.

Fuel Tanks can be mounted on the inboard side of profile fuselages and made to work if they are made to feed a small reservoir on the outboard side.

This is how I see it anyway. :-)

Recycled Flyer

Well worded as usual Ray.

Another point with an inboard mounted tank is that (obviously) it will force feed the engine in level flight but this effect lessens the more you climb the circle as gravity starts to counteract centripetal forces.

In stunt this is a good thing as a leaner setting when flying overhead will give more power where it is needed than the standard rich burble used in level flight.

Also its worth recognising that the fuel load is always thrown in the direction of travel, so a control line model that actually wants to fly in a straight line but is constantly persuaded otherwise by its tethers will have its fuel load bulk thrown outboard and very slightly forward - hence the angled outboard wall on many tank designs that counter this effect.

Cheers.

Jennifer Curtis

Almost.

The uniflow tank does eliminate the effect of the
decreasing fuel level, but not effect the actual
"head distance."

It still isn't the distance from the pickup to the
carb that makes the difference. If you had
a very wide tank with the pickup several inches
outboard from the carb, and filled it completely,
the needle setting would be comparable to
a narrow tank with the inboard side of the tank
at the same location.

The distance from the "top" of the fuel while
under centrifugal force, would be the same
in both thanks when full. The uniflow setup
would compensate as the fuel is consumed.

Moving the pickup around would not make
any difference. Moving the whole tank would.

Jenny

Jennifer Curtis

I tried to get parts for a PAW 19 of nearly the same vintage,
but they would not fit. They changed the design over the
years. I wouldn't be surprised if they changed the 15 as well.

Jenny

steve111

Yes, there were some significant changes when they changed over to the angled-venturi models - not just the obvious external stuff, but things like crankpin diameter (& hence conrods) as well. Pretty sure that was in the early 80s though, which would make your 23-y.o. .15 one of the current ones. Anyone?

Out of interest, was your .19 a straight or angled venturi one?

Jennifer Curtis

Angled on both 15 and 19.

Jenny

Recycled Flyer

Hi Jenny,
I thought that I covered that off with "Of course (uniflow or not) the recognised head level is still effected by G forces ............." so that the actual effect is still valid.

Anyhoo, this thread does perhaps contain too much ether as its getting waaaaay off topic.

Happy to continue with a separate and dedicated thread as tank design can get very theory intensive.

Thanks.

qazimoto

My Feedback: (1)

Jenny, I don't understand your point here. To me this statement is contradictory.

Jennifer Curtis

This drawing shows what I mean

Jenny

DaveyMo

Qazimoto: Each of your first two photo's shows a fuel tank with a large fixture installed (hex head with a central hole). Inquiring mind(s) would like to know what the purpose of that fixture is. Perhaps used for fueling? Certainly nothing I've seen 'round these parts.

Jenny: Your "watering can" example was most helpful. Thanks for it and the drawings above.

Davey Mo...

Recycled Flyer

Nice drawings and I appreciate the trouble that you have gone to here.

My 'interpretation' of these are -

Pic #1 is that the fuel level is in fact even with the inlet vent not the physical meniscus of the fluid so the recognised level is outboard.
(That's the whole point of uniflow, it ignores the head distance above the vent. See Marriott bottle principle for an explanation.)

Pic #2 Same but grossly outboard and I agree that it needs a richer setting because the recognised fuel level (again determined by the vent pipe not the container or the amount present in it ) is further outboard.

Pic #3 Same as pic 2, it just a a smaller tank volume with the same uniflow mechanism in the same place. The difference is the head level and that is ignored with this system.

So all of the pics show an outboard fuel head level, again that is the point of uniflow as it ignores head pressure above the vent but I agree that is the distance between the fuel level and the jet.

The point of variance seems to be exactly where the level is determined.

And of course all of this ignores varying G forces.

Cheers.

qazimoto

My Feedback: (1)

Jenny,

now I'm really confused as to how these examples advance the discussion. The fuel level "wall" will only be inline with the NVA for a short time in each run through a tank. So won't the optimal (sic) needle valve setting change through the c/l flight?

qazimoto

My Feedback: (1)

The Hex shaped device is a home made fuel "quick-fill". It's mostly made from a Brass RCA SCAT electrical connector. I'll post an assembly picture later..

qazimoto

My Feedback: (1)

These are quick refueling valves made out of RCA SCAT Electrical connectors. They have a fine 3/8" x 32 tpi UNEF thread. Taps are cheap for this common screw size which just happens to be the same as the venturi thread on a Cox TD .15.

The pics tell the story of how the valve is fabricated and assembled. The additional materials are Brass and Aluminium.

Jennifer Curtis

With the uniflow configuration, the fuel in the tank wants to
seek its level in the vent tube. The suction from the carb
pulls it down as fuel leaves the tank. When the tank is
full, the carb has to suck harder because there is a higher
level of fuel in the vent tube to pull down. But since
the fuel level is higher, less suction goes to lifting the fuel
to the spraybar.

As the fuel level decreases from being burned, the carb
doesn't have to suck so hard to keep fuel from trying to
go up the vent tube, but it does have to suck harder
because the fuel level is lower. The two amounts of
suction add up to the same total for all different
level of fuel in the tank. The result is that the mixture
doesn't lean out during flight.

All three drawings show this configuration. The first two
have the fuel level near the fuselage (which is the "top"
of the tank considering the centrifugal force in a
control line plane in flight)

The third drawing shows the fuel level far to the right,
which means that the carb needs to suck the fuel a
longer ways "uphill" from the start of the flight, thus
needing the needle screwed out more. The uniflow
configuration will still keep the mixture from leaning
out during flight. The needle setting will be at the
more open setting.

The point of the drawing is to show that it isn't the
distance from the carb to the pickup line that requires
a more open needle. It is the distance from the fuel
level to the carb. The second and third drawings
have the same distance from the pickup to the carb
but the third needs a more open setting even though
it is a uniflow system.

The first and second drawings have different distances
from the carb to the pickup, but since the distance
from the fuel level to the carb is the same they will
use the same needle setting.

Jenny

Lou Crane

Jenny,

I'm delighted with your clear, well phrased comments in here. Thank you!

This probably belongs more in the Control Line forum, and I may offer it there as well.

I'm a dedicated CL flier, more in CLPA than any other form. I've flown the George Aldrich pattern since it emerged in about 1957, before it became the AMA Pattern, later adopted as the FAI Pattern, both of which have experienced numerous "advances" - in wording, scoring and details - ever since.

The matter of forces in flight - for most stunt and sport on a CL models, centripetal load can be crudely, but fairly well, considered as about 3g, horizontally out from the center (or central vertical axis) of the flight circle. Gravity always applies 1g vertically downward, of course. Maneuvers apply their own temporary g to the picture.

Vectors are forces with a quantity, a direction of action and an identifiable point of origin. The 'quantity' of each force can be represented graphically by units of measure along their length. Each vector can be moved around in certain ways, but the direction and length (quantity) of the force must be preserved.

So, visualize - or diagram - a line 3 units long, horizontally away from the center (or central vertical axis, if above level flight at shoulder height ) of the flight circle. Add a gravity vector line one unit long, vertically downward. You could place the both points of origin at the center of mass of the fuel in the tank, for a simpler view of things..

"Resolve" the vectors: I.e., find one single force that represents their combined effect. Visualize the gravity vector moved to the outer end of the centri******al vector, with a line joining the tip of the grav vector back to the beginning of the horizontal vector. If my description works, you see a right triangle with the hypotenuse slanting outward 3 units and down 1 unit.

That's the effective "local gravity" direction and 'length'. The surface of the fuel in the tank is perpendicular to that "local gravity," For a 3:1 force condition, the "resultant," i..e., the "local gravity" (hypotenuse in our sketch) measures 3.162 units, so it "is" 3.162 g in that direction.

The uniflow vented tank is affected by the quantity of the resultant force and its angle of application, too.

Jenny, your description of engine fuel draw as pulling the fuel down against reaching the same level inside and outside the vent tubes' standpipe height is very clear, and we only need to add that the 'effective fuel head' inside the tanks is not to the actual fuel surface there, but the height at which the vent admits air into the tank shell. With uniflow vent tubes barely 'inboard' of the fuel pickup tube, that is practically a negligible distance, and it remains constant as the fuel is exhausted.

Yes! Spanwise and 'vertical' location of that vent end affect things. For stunt models, tank "height" is a crucial factor for establishing an equal run upright and inverted, and in positive and negative transient maneuvering g's. Spanwise location is less crucial, as the engine is likely to find a compatible needle setting in a moderately fast model - as most stunters are. For racing, maneuvering g's are less of a factor, provided the needle suits takeoff acceleration; for combat, of course, all goes wild.

Recycled Flyer

I think that here is the conflict, both points are literally ON the same point.

Any decent uniflow tank will theoretically have the pickup line and the vent ON the same point.
Practically this is not possible as air bubbles will be drawn into the pickup line so a slight separation is highly recommend.

When the fuel level reaches this separation distance and the vent becomes uncovered by fuel you will get a slight enriching out effect as the incoming air no longer has to push past the fuel.

The only other time a uniflow does not give you a constant head pressure is upon initial equalisation but that is indeed very brief.

Outside of the tank you will have a negative or positive siphoning effect dependant upon where you mount the tank but by virtue of the uniflow flow effect this is almost a constant ie once set is does not vary "if' the G forces similarly remain constant. This siphoning is a separate issue and should not be confused as part of the so called 'uniflow' effect.

Ray's front feed tanks take less notice of G forces generated by the models manoeuvres but are effected more by forward speed changes and thus suit violent turns found in combat.
Rear feed relies upon the opposite and suit the 4/2 break or leaning out effect so sort after in stunt models.

Thanks.

Hobbsy

My Feedback: (102)

A lean mixture burns hotter and faster than a just right or a rich mixture giving the appearance of being over compressed when in fact the compression did not change. When an engine unloads in the air it intakes more air but cannot intake more fuel due to the needle restricting it. To get the correct fuel mixture correct in flight you need to be a little rich at take off. A Cline regulator is a wonderful fix for this. But hardly necessary in this case.

Lou Crane

Jenny, Recycle and Hobbsy

You have a great discussion going. I hope my thoughts add a bit to it.

Thoughts on the above 2 posts: (References to up and down, front and back, in here are as the model is seen at rest, upright on wheels or whatever.)

Recycled - Having the vent tube end and the fuel pickup tube end at the same "POINT?" No, not if the tank has any fore and aft length. Aside from those base-forward triangular forms, most tanks do... Even in those, the height of the vent opening is the critical item to balance inside and outside g effects. In most tanks for other uses, the outer wall or wedge has a length. Placing the fuel pickup near the rear wall, in the vertex of the wedge is common.

The pickup tube opening need not be at the same height as the uniflow vent opening. ...within reason. Tommy Lay, a recently departed expert stunt engine reworker, made some tanks for ST.46 Stunt engines with the vent end and the pickup end at different heights. One was soldered either above or below the other, and the vent tube was at least 1/2" (12mm ?) shorter to allow bubbles to rise in the "local gravity" direction without getting near the pickup tube inlet.

For esthetics, if nothing else, I like to center the tank's physical dimensions on the critical uniflow vent end position. That tube is soldered to the inboard side of the pickup tube, which in turn is centered in the wedge. The vent end goes that same 1/2" or so forward of the fuel pickup end. Soldering them together to the tank shell is a way to strengthen them against shifting, or worst case, flailing unsupported inside the tank shell. Our models vibrate; vibration "flailing" can fatigue brass, leading to lengthwise splitting and undesirable churning/frothing of the fuel inside.

Jenny - the initial drop in interior pressure to set the standpipe draw-down condition is very slight, and in effect remains almost identical throughout the flight. Remember that the fuel outside the "standpipe" is under the same "fuel head" pressure conditions as that inside the standpipe. The actual head pressure may diminish, but equally so for both. They effectively cancel out. When the uniflow vent end finally uncovers, that slightly lower pressure returns to ambient atmospheric. For stunt flight, it is nice to have a lap or so warning that the fuel is finished. With the uniflow tube soldered inboard of the pickup, the sign is a definite. brief richening before the final bubbles lean it out at the very last.

Hobbsy - I'm not disputing frequently observed experience, but the I think the rate at which the fuel is admitted in the intake air should depend on the air velocity through the venturii's smallest cross-section. At higher RPM, I expect that that airflow velocity increases, unless we're getting into compressibility velocities. There should be a tendency to draw more fuel accordingly. Whether that meets the needs of the increased air volume entering the engine depends on many things. One may well be that the needle and jet restriction to fuel flow is NOT usefully proportional to velocity over the critical minimum section.

Finally, yes, the spanwise location enters the picture, particularly with non-uniflow tanking. Remember that the air inlet system is already pulling fuel "up" from the fuel pickup's. or uniflow end's location. Uniflow allows the physical 'fuel head' to go from full tank to empty without effect on needle setting. Physical 'fuel head' relates only inside the tank. From there forward we have a small height to climb to the spraybar. That is a smaller distance than from full to empty in the original over-and-under vented tanks, AND half of that was done with a positive head (fuel draining "downhill" in "local gravity" direction.) Engines worked then, tolerant of that fairly large range of 'head' change; the features of our engines, related to that, haven't changed significantly

Recycled Flyer

Yep, theoretically this would be the best in a dynamic and moving situation but sadly all references to Mariotte principles only show static and unmoving examples and in the real world air entering would contaminate the fuel feed so we employ a reasonable separation between the two.
(Placing two two pipes side by side via a simple partial V cut and bend back to back has been tried before and does work apart from the above issue.

In control line that fly on a radius we choose levels in which to place pressure points.

Anyway the point is that the internal uniflow principle is separate from the external siphoning.

Thanks Lou.

PWF63

The correct fuel for a G20 diesel is 20% Castor

30% ether

50% kero or A1 Jet

and only 1% Amsoil.
I use this fuel on Super Tigre's, ETA's, Olivers, PAW's, and Rossi's. Never had a problem. Even my Rothwell R250 running only 18% oil I still only use 1% Amsoil. If your using IPN then run 1.5%.
At least Try It and See! Extremely cold weather you could go the 35% ether and 45 Kero. . Cheers
Fredo