Author Topic: FUEL TANK PUZZLER (Read 3216 times)

FLOYD CARTER · « **on:** July 07, 2013, 10:48:47 AM »

The needle valve on the RO-JETT 61 is 0.8" above the beam mount surface. Tank is 1" high, so the pickup and uniflo tubes are at 0.5". This means, for inverted mounting, the pickups are 0.3" higher than needle valve when plane is upright. The engine SHOULD go lean when inverted with this setup. It instead goes RICH!

I considered popping the back off the tank and raising the uniflo tube. But it is already in the direction to make the engine lean when plane is inverted.

I'm going to wait until I understand what is happening here.

Floyd

Gerald Arana · « **Reply #1 on:** July 07, 2013, 11:11:24 AM »

Floyd, is the engine upright or inverted?

Jerry

RandySmith · « **Reply #2 on:** July 07, 2013, 11:40:21 AM »

Quote from: FLOYD CARTER on July 07, 2013, 10:48:47 AM

The needle valve on the RO-JETT 61 is 0.8" above the beam mount surface. Tank is 1" high, so the pickup and uniflo tubes are at 0.5". This means, for inverted mounting, the pickups are 0.3" higher than needle valve when plane is upright. The engine SHOULD go lean when inverted with this setup. It instead goes RICH!

I considered popping the back off the tank and raising the uniflo tube. But it is already in the direction to make the engine lean when plane is inverted.

I'm going to wait until I understand what is happening here.

Floyd

Hi Floyd

Keep in mind it is not where the needle is, it is where the fuel enters the engine. on that engine, the fuel delivery system ,delivers the fuel lower than the spray bar, looking at it right side up. So you will need to measure a little lower, as the fuel is angled downward until it exits the venturie.
Now not that this is everything you consider, what matters is that you get the feed same, inverted and right side up, so either go ahead and move either the tank or the fuel tubes,
Also many use the motor sitting on top of 1/16 or 3/32 pads, so that makes the relationship between tank and fuel entry very close.

Randy

FLOYD CARTER · « **Reply #3 on:** July 07, 2013, 01:35:47 PM »

The engine is mounted inverted. OK Randy. I didn't know that needle valve position is different from where the engine thinks the fuel is entering!

I am using aluminum pads which (I think) are 0.030". That doesn't seem enough to make a difference in this case.

So. I will open the tank and move the tubing. I understand that only the uni-flo tube needs moving. Is that correct?

Floyd

Tim Wescott · « **Reply #4 on:** July 07, 2013, 03:03:58 PM »

Quote from: RandySmith on July 07, 2013, 11:40:21 AM

Keep in mind it is not where the needle is, it is where the fuel enters the engine. on that engine, the fuel delivery system ,delivers the fuel lower than the spray bar, looking at it right side up. So you will need to measure a little lower, as the fuel is angled downward until it exits the venturie.

I hope you mean that what matters is the location of the hole in the spraybar -- because that's my understanding of the point where the hydrostatic forces cease, and atmospheric pressure (well, at least the air pressure at that point) take over.

Jim Thomerson · « **Reply #5 on:** July 07, 2013, 04:28:50 PM »

Yes, just move the uniflow.

Brett Buck · « **Reply #6 on:** July 07, 2013, 04:43:59 PM »

Quote from: Tim Wescott on July 07, 2013, 03:03:58 PM

I hope you mean that what matters is the location of the hole in the spraybar -- because that's my understanding of the point where the hydrostatic forces cease, and atmospheric pressure (well, at least the air pressure at that point) take over.

Simply stated, you cannot determine the right tank position by a fixed reference on the engine as a general rule. You have to experiment and it doesn't always line up with something you can identify.

Brett

RandySmith · « **Reply #7 on:** July 07, 2013, 07:12:32 PM »

Quote from: Tim Wescott on July 07, 2013, 03:03:58 PM

I hope you mean that what matters is the location of the hole in the spraybar -- because that's my understanding of the point where the hydrostatic forces cease, and atmospheric pressure (well, at least the air pressure at that point) take over.

Hi Tim

No that is not what I mean, in some engines it matters NOT where the hole in the spraybar is located, Take for instance a true venturie with 1 hole feeding the engine, the hole in the spraybar is not where the hydrostatic forces cease, the small hole in the venturie. or in the case of others the small hole in the Fuel post is up or down farther in the engine, this is just a continuation of the venturie feeding the engine..what matters is the point it enters that atmosphere ...so to say... With an angled forward venturie or fuel post it is lower than the point at which the spraybar has a hole in it.
I have proved this beyond doubt with my adjustable venturies, They have a fixed spraybar...but.. the venturie is multi pieces and have a separate center piece that seals against the inner wall of the outside venturie part , the "hole" in the venturie center can be moved up or down without taking the venturie out or touching the spray bar.
This is the way I adjust some of my engines without touching the fuel take...moving the inside piece up or down moves the engine..or tank..effectively if that is how you want to think of it.
I have made these for many people over the years, Example I made a set that Kaz Minatp used in his BM comp ST60 powered, and Eric V is using a copy of the ones I made in his 75 right now.

Randy

RandySmith · « **Reply #8 on:** July 07, 2013, 07:17:35 PM »

Quote from: Brett Buck on July 07, 2013, 04:43:59 PM

Simply stated, you cannot determine the right tank position by a fixed reference on the engine as a general rule. You have to experiment and it doesn't always line up with something you can identify.

Brett

Brett has it right, and many times you cannot see exactly where the uniflow and pickup tube is also.
Many engines follow this pretty much dead on, other need to be slightly up or down, take for example the point I made above about where fuel enters, that is something many people don't see and they are measuring from the center of the spraybar.
Then take the FOX 35 for example, it normally runs very close upright or inverted, but side mounted many need to be 3/16 to 1/4 inch lower ??
I have seen some ST 60s and 46s that run slightly off from dead center of the point where the spraybar is..and.. where it enter the open air.

Randy

Tim Wescott · « **Reply #9 on:** July 07, 2013, 07:22:37 PM »

There are more things in this world Horatio, than I have imagin'd in my philosophies.

I'd argue about calling it a "spraybar" if it's not where the spray comes out, but the word gets the point across, so I guess I'll keep mum on that. "Point at which it enters the atmosphere" is what I was going to say before I decided it was too complicated, and that's what it looks like you're saying.

Brett, I know you can't line things up 100%, but I would assume that the point where the fuel enters the atmosphere (regardless of how the clever engine designer has arranged for that to happen!) would be the starting point for adjustment.

RandySmith · « **Reply #10 on:** July 07, 2013, 07:33:16 PM »

Quote from: Tim Wescott on July 07, 2013, 07:22:37 PM

There are more things in this world Horatio, than I have imagin'd in my philosophies.

I'd argue about calling it a "spraybar" if it's not where the spray comes out, but the word gets the point across, so I guess I'll keep mum on that. "Point at which it enters the atmosphere" is what I was going to say before I decided it was too complicated, and that's what it looks like you're saying.

Brett, I know you can't line things up 100%, but I would assume that the point where the fuel enters the atmosphere (regardless of how the clever engine designer has arranged for that to happen!) would be the starting point for adjustment.

Hi Tim

The spraybar has been called that for many decades, I guess people will still always refer to it as that, and in many engines it does enter the air directly from the hole in the spraybar.. such as FOXes OS, McCoy, Some K&B ...others it does not.... such as ST, PAs, Jetts, many Thunder Tigers, Aero Tigers, OPS etc

You are right about it being complicated to explain, but the telescoping venturie shows you that it is that point that counts, where it enters not where the bar is.

Your also right about the point you should measure from, and things will most always be pretty honest as far as tuning it goes, and you do have to pick a point. What really matters is that you move it the right way and the right amount. :-)

Randy

Tim Wescott · « **Reply #11 on:** July 07, 2013, 08:14:44 PM »

Quote from: RandySmith on July 07, 2013, 07:33:16 PM

What really matters is that you move it the right way and the right amount. :-)

Yup. Just last Saturday I was flying along inverted, faster than I had just flown level, saying to myself "now, does that mean I need to move the tank up or down?" Fortunately I figured it out, and without engaging so many brain cells in the problem that there weren't enough left over to keep me from crashing.

Steve Helmick · « **Reply #12 on:** July 07, 2013, 09:43:51 PM »

Randy...this is curious! Both Ted Fancher and Al Rabe, independently, made some sort of telescopic venturii and found that the spraybar location had no effect. It would be very interesting if the three of of you would compare venturi designs and figure out why the results varied. Well, interesting to me, anyway.

Steve

Lauri Malila · « **Reply #13 on:** July 08, 2013, 04:15:42 AM »

Wouldn't it be easiest to test the effect of needle valve position (the very point where the flow regulation happens) vs fuel post location, with a remote needle valve? With a "normal" setup you are still talking about a height difference of only a few millimeters. A remote needle unit can easily be moved several cm up or down, that way it would be easier to spot the differences.
I think, that fuel behaves in differend ways before and after the point of where flow regulation takes place. Before that point it's the hydrostatic pressure from tank that dominates, after the needle valve it's the suction of venturi that counts. I see that the difference is immeasurable in a static situation, but with bigger G's it may cause some issues.
Another funny thing I noticed with a remote needle is that just before stopping, the engine goes rich instead of leaning out. But that's not really related to the subject here. L

Andrew Tinsley · « **Reply #14 on:** July 08, 2013, 04:53:23 AM »

Hello All,
Some very interesting results here, I am very grateful that this topic has come up and that many very able people have contributed to helping me understand what is going on.
I have a connected query, most of the engines that I have used for stunt, fit into the general pattern described above. for example the old cross flow Mercos and the LA46 are text book examples of engines that are sensitive to tank height. With these type of engines, positioning the tank so that the spray bar is mid tank height is a good start position, to begin the final trimming. Other engines are not quite correct at spray bar being at half tank position, but not too far out. I also fly the TTGP42 and with a custom made venture, I find the tank position to be very insensitive to spray bar location. You really have to make a very large alteration in tank height before you see much change in engine revs! I am not complaining as this removes one of the trimming tasks. However I am at a total loss as to understanding what is going on here. The TTGP42 seems to be a very conventional engine, so why the odd behaviour?

Regards,

Andrew.

P.S. Sorry if I am hijacking the thread, but the odd behaviour is bugging me. I have several GP42s and they all show the same behaviour and others have noticed it too in their Gp42s.

Tim Wescott · « **Reply #15 on:** July 08, 2013, 09:23:42 AM »

Quote from: Lauri Malila on July 08, 2013, 04:15:42 AM

Wouldn't it be easiest to test the effect of needle valve position (the very point where the flow regulation happens) vs fuel post location, with a remote needle valve? With a "normal" setup you are still talking about a height difference of only a few millimeters. A remote needle unit can easily be moved several cm up or down, that way it would be easier to spot the differences.
I think, that fuel behaves in differend ways before and after the point of where flow regulation takes place. Before that point it's the hydrostatic pressure from tank that dominates, after the needle valve it's the suction of venturi that counts. I see that the difference is immeasurable in a static situation, but with bigger G's it may cause some issues.
Another funny thing I noticed with a remote needle is that just before stopping, the engine goes rich instead of leaning out. But that's not really related to the subject here. L

Hey Lauri:

If we were in the same room, I'd put $20 down on the position of the needle not affecting the run much at all. There's a solid column of fuel from the tank to the fuel post (or wherever). That column is restricted by the needle, but it is not broken. So the density and flow of the fluid will remain constant from end to end. This means (if I'm not overlooking anything, which is why I won't bet $50) that the hydrostatic and hydrodynamic pressure drops should be the same no matter where in the column the needle goes.

The engine going rich when the tank empties is because the needle is suddenly constricting the flow of a much less viscous fluid, so it allows much more flow and the engine richens.

RandySmith · « **Reply #16 on:** July 08, 2013, 09:38:17 AM »

Quote from: Steve Helmick on July 07, 2013, 09:43:51 PM

Randy...this is curious! Both Ted Fancher and Al Rabe, independently, made some sort of telescopic venturii and found that the spraybar location had no effect. It would be very interesting if the three of of you would compare venturi designs and figure out why the results varied. Well, interesting to me, anyway. Steve

Steve
In my venturies the Spray bar never moves, I do not know exactly what you are saying?? the spraybar had no effect on what???
My venturi has a center tube that will slide up and down inside the venturi, the center sliding,or telescoping venturi has the exact same effect as if you had moved the tank up or down, or the engine, because the point where the fuel enters the air moves up and down with the center part that telescopes. It does work that way

Randy

Lauri Malila · « **Reply #17 on:** July 08, 2013, 10:25:17 AM »

Yep Tim, you're correct with that. I have gone through it with our engine when I wanted to eliminate all possible uncertain variables from the system.
Now I have everything that I thought that may cause something weird as much in line (symmetrical) as possible but the fundamental problem is still present. I mean the non-symmetrical engine behaviour in outside and inside loops.
However, I have managed, mostly by manipulating scavenging and combustion process, to make the symmetry problem so small that I can compensate it by moving the fuel tank. Now the problem is absolutely tolerable.
The funny thing is that when I reverse the running direction, the symmetry issue also reverses. I started with "normal" tractor props and with those the outside loops were slower. Now I prefer pusher props because I prefer to have slightly faster outside loops, that actually helps a bunch in fig. 8 and hourglass, plus of course the gyroscopic @#$% that makes pushers preferrable.
So it seems that it's something inside the engine, in gas dynamics, that cause the symmetry problem. Propably crankshaft counterweight or conrod movement causes pressure differences or just guides the gas flow more to the other side of cylinder.
The last experiment was to connect the channels on both ide of cylinder with a horizontal channel, the idea is to even out pressure differences. It clearly heps a little, but with cost of efficient scavenging and minimal bottom end volume.
So far the most efficient cure has been the work with combustion process, I made a head with a good squish band and minimal squish clearance (0,25mm), and a good double-bubble combustion chamber.

So, what I try to say, is that when fuel tank height ends up far from fuel post height, then you are compensating something else than hydrostatics. Something strange inside the engine. (Especially with cylinder pointing down it's easy to imagine). The East-European way is to find a position where the engine runs symmetrically (not cylinder down nor horizontally but something inbetween). L

RandySmith · « **Reply #18 on:** July 08, 2013, 11:18:33 AM »

Hi Lauri
That symmetry problem you speak of has also been made much better, or worse with Glow Plug depth, It also has been completely banished in some motors with a deep hot big bore glo plus, other that has only helped a lot, still others had no affect on. Without going into a long winded post there are many many things that affect symmetry. Even changing heads will do that many times, as will prop pitch, mixture,and needle settings
Seems as if each engine type may act much differently than others to to this.
I have seen some pilots run engines at the 7 O,Clock, or 4 O Clock position to combat the problem, while other have ran sidewinder with the head pointing to the inside of the circle...

Randy

Lauri Malila · « **Reply #19 on:** July 08, 2013, 12:38:08 PM »

Hi Randy.

Yes, I have noticed how big difference the plug can make. I have tried about a million differend plugs and head shapes/plug depths and so far the best is Thunderbolt RC long with idle bar removed. Thread lenght in head is 4,70mm. Also what helps alot is the racing-style double-bubble combustion Chamber. It protects the coil from incoming charge and once it explodes, makes some nice turbulence.
But whatever I do, the symmetry thing is still there. It just gets smaller and smaller but never seems to disappear. Besides, the plug desn't really explain why the asymmetry is opposite with opposite running direction.
The main problem with our engine is that it's too light, too powerfull and it tends to run too cool. I want to make the scavenging and burning as efficient as I can, before reducing cooling. I've been pondering a ceramic combustion chamber too, but there are some technical problems to solve first. L

RandySmith · « **Reply #20 on:** July 08, 2013, 01:17:10 PM »

Quote from: Lauri Malila on July 08, 2013, 12:38:08 PM

Hi Randy.

Yes, I have noticed how big difference the plug can make. I have tried about a million differend plugs and head shapes/plug depths and so far the best is Thunderbolt RC long with idle bar removed. Thread lenght in head is 4,70mm. Also what helps alot is the racing-style double-bubble combustion Chamber. It protects the coil from incoming charge and once it explodes, makes some nice turbulence.
But whatever I do, the symmetry thing is still there. It just gets smaller and smaller but never seems to disappear. Besides, the plug desn't really explain why the asymmetry is opposite with opposite running direction.
The main problem with our engine is that it's too light, too powerfull and it tends to run too cool. I want to make the scavenging and burning as efficient as I can, before reducing cooling. I've been pondering a ceramic combustion chamber too, but there are some technical problems to solve first. L

Hi Lauri
Best I have used is the T Bolt Big Bore, it has a 30% rhodium element instead of the normal 20% and it also has a big hole which helps it stay hot, it is a non idle bar
The Enya #3 is good but the head needs to be machined with more plug depth.
on the other subject, I did not mean that it "explain" why, what I meant was these things all affect the run.
My 40 and 51 some 61s used the HC double bubble, later motors the shallow High Compression squish Hemi worked best.

WE made and used ceramic heads, not something you will want, they keep the chamber way too hot, and in test the motor could not be gotten out of a 2 cycle. They are not much problem to machine but..the inserts tend to crank after multiple runs.

maybe better to try a layered head of multiple parts , so that you can make little dimples in the button insert to deter contact with the outer finned part of the head, this way you can control the heat build up your self by using more or less contact/cooling are.

Randy

Chris Wilson · « **Reply #21 on:** July 08, 2013, 04:13:05 PM »

I have a friend of mine who has a Rustler Belko long shaft 75 that has the intake rotated 180º from the normal position (in other words an invert mount would have the down draught intake coming through the top block and the spray bar about 1" above the thrust line.)

Now according to the theory of aligning the fuel jet with the tank centre line this would place the tank almost entirely outside of the model, and yet I don't think that that's the intended position for the tank.

(Must ask him where the tank actually is next time we talk.)

Steve Helmick · « **Reply #22 on:** July 08, 2013, 08:09:44 PM »

Quote from: RandySmith on July 08, 2013, 09:38:17 AM

Steve
In my venturies the Spray bar never moves, I do not know exactly what you are saying?? the spraybar had no effect on what???
My venturi has a center tube that will slide up and down inside the venturi, the center sliding,or telescoping venturi has the exact same effect as if you had moved the tank up or down, or the engine, because the point where the fuel enters the air moves up and down with the center part that telescopes. It does work that way

Randy

Randy, it was quite a few decades ago that both Ted and Al wrote about their experiments, so I don't even remember if there were any drawings, just the results. I believe that both moved the spraybar up and down. I don't have a drawing of either of their experimental parts, and I don't have a drawing of yours, or understand how you move the venturi while leaving the spraybar location alone. I was just saying that it would be interesting to compare the designs and see why your results are different than theirs. I'd like to know "what's the haps".

Steve

RandySmith · « **Reply #23 on:** July 08, 2013, 08:18:27 PM »

Quote from: Steve Helmick on July 08, 2013, 08:09:44 PM

Randy, it was quite a few decades ago that both Ted and Al wrote about their experiments, so I don't even remember if there were any drawings, just the results. I believe that both moved the spraybar up and down. I don't have a drawing of either of their experimental parts, and I don't have a drawing of yours, or understand how you move the venturi while leaving the spraybar location alone. I was just saying that it would be interesting to compare the designs and see why your results are different than theirs. I'd like to know "what's the haps". Steve

Hi Steve
Moving the spray bar up and down with keeping the same point that the fuel comes out doesn't really do anything.
My system has a venturie inside a venturie, that telescopes up and down, it is simple when the inside moves up and down it changes the point where the fuel comes out of the venturie, and enters the atmosphere.
But the spraybar stay put and never moves.
Randy