Fuel level effect on tune......"head pressure" myth?
09-16-2023, 09:06 PM
#1
Fuel level effect on tune......"head pressure" myth?
I wanted to talk about the effect of fuel level. Its said all the time everywhere that "head pressure" changes because the fuel level goes down and you have less pressure at the carb and that it leans out because of lower fuel level. Now this is specifically what I'm talking about and not a far away tank location or anything else.
From my experience the fuel level doesn't really change anything noticeable at all. I find it odd that it's also said all the time to mount the center of the tank to the center of the spray bar. And yet it seems most arf and probably even kits don't even allow you to do that. Well at least with upright mounted engine. Probably even sideways. But what I've seen is the top of the tank is about level with the spray bar. So then the tank is fully below the spray bar. That seems to be reality for a lot of glo planes. I suppose it makes sense to have it in the middle, but I believe once pressure is introduced it changes things. The carburetor is not even having to suck any fuel all on its own. It's being pushed in from muffler pressure. Like priming through the muffler line to get fuel to the carburetor. And that seems to be greater than any difference the weight of the fuel due to changing fuel levels makes. But I try to explain why I don't see the weight of the fuel makes any practical difference for how hard it is for the fuel to go to the carburetor. I have found that it is not harder for the fuel to go to the carburetor at a low tank versus full tank. The pressure is the same created by the muffler and the distance from the clunk to the carburetor is the same when in the same level position or attitude. The engine doesn't see anything or see itself from the top of the fuel level. I found no volume or flow difference with the same pressure coming out of the tank tube when blowing into it at full tank versus empty. If I'm having a hard time detecting even the smallest difference than there is no actual practical reality to leaning out in the real world due to "head" level changing. It's just as easy to pump the fuel out when it's low versus high. If the fuel line to the carburetor would suddenly go much higher than the tank that is a separate different thing. That is not fuel level related...... But that is tank position and direction to spray bar related.
All someone has to do is run their plane on the ground at the same position the whole time and tune the engine a little rich and take note of the RPMs when the tank is full. Then do the same test with less fuel in it. Obviously needs to be enough fuel so that the clunk is submerged in fuel and not getting a false lean test from sucking in air. If this head pressure was changing then you would have to move the needle a noticeable amount and change the tune to get it to the same RPMs to compensate for leaning out because of fuel level being down. That won't have to be done. Running them a little rich of peak is what to do to all two strokes anyways. But I see it's to account for attitude and not fuel level in tank.
It's said that it's harder for it to push fuel up as the tank gets lower. How? You have a clunk that's always in the same exact position at the bottom of the tank. Only a little bit of fuel is getting pushed through the clunk and fuel line any given second. The weight of the fuel going into the carburetor is always the same. The fuel line and clunk does not change as the fuel level goes down. How is it harder for the fuel to go up the same exact line with low fuel? A small amount of fuel is traveling the exact same distance and incline at all times no matter what the fuel level is. Therefore that would not cause any change in tune because of somehow becoming harder to push it to the carb. It is the same and it is not getting harder to move the same fuel the exact same distance and the exact same angles with the same pressure in the tank.
Yes the example here is going to assume level flight and normal position just for example sake to keep it consistent. What does the weight of the fuel have to do with reality here in a pressurized system? No factors have changed for the pressure in the tank to work harder suddenly.
From my experience and what I understand any lean symptoms during the flight because of fuel level going down is only due to the clunk not being in fuel at all times. Of course sucking in some air is going to give a lean symptom real quick. That's it. The engine's not running leaner all of a sudden because it hit a quarter tank or lower. It's funny because people say to use a felt clunk to pretty much eliminate all the symptoms with no other changes. That alone is saying there is no change in the tune or pressure because the fuel level is going down. How are the lean symptoms mostly eliminated with a felt clunk if it's actually getting leaner because the fuel level dropped and there is less pressure at the carb? A felt clunk wouldn't be able to compensate for the carburetor seeing less pressure due to lower fuel level.
I don't believe it's happening. From testing this is my experience and what I see with nitro cars also.
I have found the shape of the tank to be incredibly important. If it's not a mostly symmetrical tank the tune will absolutely change as the fuel level goes down. Take the HPI Savage for example. Still making the same tank over two decades later 😑 and they know it's a problem and don't care (as usual for everyone)
It goes lean right at the halfway mark because that's where the tank steps down and becomes a smaller area. The muffler pressure didn't change. Nothing else changed except the shape of the tank and the surface area of the fuel now. So then the exact same pressure in the tank is pushing on a smaller surface area and therefore is pushing a little bit less fuel in the carburetor. That little difference makes a difference and leans it out due to the tank not being symmetrical. I've never seen crazy tank shapes in glo airplanes. For good reason. Although the dubro has that hump and I'm not sure why and wouldn't mind knowing why they do that?
Now the Tmaxx tank for example starts getting wider and wider as the fuel level gets lower. The tune absolutely changes from my experience and becomes richer instead of leaner like the savage. Although because it's somewhat gradual it's more deceitful and creeps up on you. Now the exact same pressure in the tank at the same throttle position has a much bigger surface area to push and work leverage, and therefore push a little more fuel in and richen the tune. I know this is happening without a doubt. And I know the savage tank goes leans suddenly without a doubt. And it's 100% due to the shape of the tank. Nothing else. A symmetrical tank will make it consistent from full to empty. A losi tank with a clunk fixed my T-Maxx problem and the thing tunes great full to empty over and over again.
I played with the needle and done testing when the tank is mostly full versus empty and haven't seen any noticeable difference of leaning due to the fuel level dropping. I richened it up with plenty of fuel to the point it slows down noticeably and take note of the needle. When doing the exact same test with low fuel level it took the same amount of richening on the high-speed needle to get it to perform slow again. I didn't notice that I had to compensate by tuning it richer because the fuel level was lower.
If you take a soda bottle and take the cap off and try to blow in it...... You'll find that it pressurizes pretty much instantly. The only difference in RC is there's a small restriction and a fuel line and a little bit of a leak for fuel going to the carburetor. That tells me that the tank pressurizes basically instantly as soon as you're on the throttle. Whatever the PSI is created by the muffler, it gets to that point pretty quickly. I don't see how that pressure can change as long as the throttle position and the pressure coming out of the muffler is the same. Which it always should be if everything is working right and it's the same throttle position. When the tank is full at full throttle, let's say it puts 1 PSI in the tank. Now when the fuel level is low at full throttle it should put 1 PSI in the tank just the same. How fast? As fast as it takes to pressurize the soda bottle with your mouth. You can change the amount of liquid in the bottle and try it with a full bottle or with very little liquid and it will make no difference. It pressurizes pretty much instantly the exact same if you're blowing the same amount from your mouth.
Therefore I don't believe there is any noticeable change in pressure the carburetor sees *due to the fuel level going down* Any lean symptoms is due to the clunk not being in the fuel at all times. Or some other problem besides the fuel level in the tank. Like a pressure problem or tank being too far away.
Of course when there's just very little fuel left it's normal to be sucking in a little bit of air at that point and have some lean symptoms. That's not due to losing pressure from the fuel level but only because it can't suck in fuel 100% of the time. You could blow through the muffler line going into the tank and see how long it takes to pressurize with your mouth. It'll be just like that bottle test and it will make no difference whether there's fuel in it or not. The size of the tank is always the same. And if you have liquid in it it will push the liquid just as fast as soon as you put pressure to it whether it's full or almost empty.
Now mounting the tank really far away and starting to have problems is a different separate subject. Since that would require a little more pressure to go through longer lines that would be a separate issue.
Why do people constantly say that the fuel level is changing the pressure the carburetor sees?
Even on RC cars I noticed that typically the top of the tank is about even with the spray bar which means the whole tank is below the carburetor. Or most of it is. They seem to run great that way. Same with the airplanes. Of course with cars you aren't driving inverted so that's not as important.
When inverted on an airplane though the tank supposedly becomes a uni flow system. So in theory it shouldn't matter now that the tank is completely above the spray bar when inverted. Since supposedly with uniflow the weight of the fuel makes no difference because fuel wouldn't come out no matter if you have little fuel or a full tank because its equalized from the pressure line being at the same level with the fuel pickup when inverted. In theory it shouldn't make a difference that the tank is fully above the spray bar. But I don't don't believe that it's even working like this like people say. Once the pressure and flow starts going it will still be easier to flow fuel to the carburetor with the tank fully above the carb. It'll work just like a standard two line setup from what I see. But also..... Is the end of the muffler line the same level as the end of the fuel line to the carburetor on the RC airplane in reality? So then it would gravity feed if one line is lower than the other. I'm pretty sure that ruins "uni flow" now because it doesn't matter that the clunk and the pressure line in the tank is the same level. The lines they are connected to are not at the engine and are at different levels. Now you will get flow out of one of the lines that is lower than the other which somewhat cancels out uni flow. Doesn't it? It seems the only thing left is to get the muffler line filled up with fuel or your carburetor filled up with fuel and choked out because the pressure has nowhere to go and equalize except the carb and muffler. If it's by chance not going all the way to the carburetor then it's spitting out the muffler and wasting expensive fuel and run time on a good day at best.
If I'm not mistaken this is really not working like people say and it's stupid because it just blows out fuel out of the muffler on a good day and doesn't have any benefits. I've seen people say themselves that use uni flow.....that it makes no noticeable difference in reality to the 2 line setup for how it flys. I guess they just like wasting a few ounces of fuel per flight out of the muffler? I don't know. And that it absolutely has to do and will do because pressure has to go somewhere. I'm sure there's some that purposely don't pay attention and have a dark colored muffler fuel line with the engine mounted sideways and the muffler at the bottom....... Purposely and willfully being ignorant to it and not looking or testing/checking and then saying......."I don't see any fuel coming out of the muffler or any problems with uni flow"
Somehow their uniflow setup doesn't do what everyone else's does and is better....... Somehow the tank is being pressurized but also equalizing just like a two line setup with no fuel going up the carb or muffler.........hmmmmmm
As they say.....the devil is in the details. Partial truths are always 100% lies and deceptions. No such thing as partial truth in reality.
But wherever the exact tank position is the final tune accounts for any differences. And then it's going to be the same from that point because the pickup is always at the same spot and the incline or decline is always the same going to the carburetor. (Just talking about fuel level in tank and not attitude of plane) If the tank is fully below the carburetor...... Well then that little bit of up travel through the clunk to the carburetor s going to be same for the whole entire tank. All of the fuel in the tank enters the clunk and travels the same distance no matter what the fuel level is. A little bit of fuel at a time. The same pressure from the throttle position is going to push that fuel the same way all the way to the end. If it's 1 PSI at full throttle it's 1 PSI at all times at full throttle till it empties. Blowing into an empty bottle or a bottle full of water has the same characteristics and the liquid doesn't seem to make any difference for pressure.
But a tank with a crazy shape is a whole different story. And the only thing that makes sense is the surface area that acts like more or less leverage from the same pressure in the tank that changes the fuel flow as the shape of the tank changes due to fuel level going down.
And if I'm not off on this.....well there is too many myths and lies everywhere.....that will never change. Lies will always be fully in stock. And whatever truth is will be out of stock and discontinued on purpose.
Airplane guys taking ABC engine straight to full throttle like the manual says and not tune it too rich. All car guys run their new engines at idle and tune it filthy rich and only use part throttle and run slow cold and rich to run them in. And many will take a lot of time to warm up their engine every start. Whereas the airplane engine you just go straight to full throttle and it's warmed up in 3 seconds. Same identical ABC engine design with two opposing and perfectly opposite ways of running these engines between the airplane world and the car world. Pretty astonishing.
I think how long the airplane engines last vs the car guys engines says it all right there for which way is right and which way is wrong and bad for them.
Maybe I'll go run a glo engine the adam drake method so that I can wear out all my pinch in 5 minutes and get rid of that disease and resistance that is called mechanical pinch out of the way completely. Then it will be race ready within half a tank and make full revs and power in half a tank. That nasty old pinch. Get rid of it!
From my experience the fuel level doesn't really change anything noticeable at all. I find it odd that it's also said all the time to mount the center of the tank to the center of the spray bar. And yet it seems most arf and probably even kits don't even allow you to do that. Well at least with upright mounted engine. Probably even sideways. But what I've seen is the top of the tank is about level with the spray bar. So then the tank is fully below the spray bar. That seems to be reality for a lot of glo planes. I suppose it makes sense to have it in the middle, but I believe once pressure is introduced it changes things. The carburetor is not even having to suck any fuel all on its own. It's being pushed in from muffler pressure. Like priming through the muffler line to get fuel to the carburetor. And that seems to be greater than any difference the weight of the fuel due to changing fuel levels makes. But I try to explain why I don't see the weight of the fuel makes any practical difference for how hard it is for the fuel to go to the carburetor. I have found that it is not harder for the fuel to go to the carburetor at a low tank versus full tank. The pressure is the same created by the muffler and the distance from the clunk to the carburetor is the same when in the same level position or attitude. The engine doesn't see anything or see itself from the top of the fuel level. I found no volume or flow difference with the same pressure coming out of the tank tube when blowing into it at full tank versus empty. If I'm having a hard time detecting even the smallest difference than there is no actual practical reality to leaning out in the real world due to "head" level changing. It's just as easy to pump the fuel out when it's low versus high. If the fuel line to the carburetor would suddenly go much higher than the tank that is a separate different thing. That is not fuel level related...... But that is tank position and direction to spray bar related.
All someone has to do is run their plane on the ground at the same position the whole time and tune the engine a little rich and take note of the RPMs when the tank is full. Then do the same test with less fuel in it. Obviously needs to be enough fuel so that the clunk is submerged in fuel and not getting a false lean test from sucking in air. If this head pressure was changing then you would have to move the needle a noticeable amount and change the tune to get it to the same RPMs to compensate for leaning out because of fuel level being down. That won't have to be done. Running them a little rich of peak is what to do to all two strokes anyways. But I see it's to account for attitude and not fuel level in tank.
It's said that it's harder for it to push fuel up as the tank gets lower. How? You have a clunk that's always in the same exact position at the bottom of the tank. Only a little bit of fuel is getting pushed through the clunk and fuel line any given second. The weight of the fuel going into the carburetor is always the same. The fuel line and clunk does not change as the fuel level goes down. How is it harder for the fuel to go up the same exact line with low fuel? A small amount of fuel is traveling the exact same distance and incline at all times no matter what the fuel level is. Therefore that would not cause any change in tune because of somehow becoming harder to push it to the carb. It is the same and it is not getting harder to move the same fuel the exact same distance and the exact same angles with the same pressure in the tank.
Yes the example here is going to assume level flight and normal position just for example sake to keep it consistent. What does the weight of the fuel have to do with reality here in a pressurized system? No factors have changed for the pressure in the tank to work harder suddenly.
From my experience and what I understand any lean symptoms during the flight because of fuel level going down is only due to the clunk not being in fuel at all times. Of course sucking in some air is going to give a lean symptom real quick. That's it. The engine's not running leaner all of a sudden because it hit a quarter tank or lower. It's funny because people say to use a felt clunk to pretty much eliminate all the symptoms with no other changes. That alone is saying there is no change in the tune or pressure because the fuel level is going down. How are the lean symptoms mostly eliminated with a felt clunk if it's actually getting leaner because the fuel level dropped and there is less pressure at the carb? A felt clunk wouldn't be able to compensate for the carburetor seeing less pressure due to lower fuel level.
I don't believe it's happening. From testing this is my experience and what I see with nitro cars also.
I have found the shape of the tank to be incredibly important. If it's not a mostly symmetrical tank the tune will absolutely change as the fuel level goes down. Take the HPI Savage for example. Still making the same tank over two decades later 😑 and they know it's a problem and don't care (as usual for everyone)
It goes lean right at the halfway mark because that's where the tank steps down and becomes a smaller area. The muffler pressure didn't change. Nothing else changed except the shape of the tank and the surface area of the fuel now. So then the exact same pressure in the tank is pushing on a smaller surface area and therefore is pushing a little bit less fuel in the carburetor. That little difference makes a difference and leans it out due to the tank not being symmetrical. I've never seen crazy tank shapes in glo airplanes. For good reason. Although the dubro has that hump and I'm not sure why and wouldn't mind knowing why they do that?
Now the Tmaxx tank for example starts getting wider and wider as the fuel level gets lower. The tune absolutely changes from my experience and becomes richer instead of leaner like the savage. Although because it's somewhat gradual it's more deceitful and creeps up on you. Now the exact same pressure in the tank at the same throttle position has a much bigger surface area to push and work leverage, and therefore push a little more fuel in and richen the tune. I know this is happening without a doubt. And I know the savage tank goes leans suddenly without a doubt. And it's 100% due to the shape of the tank. Nothing else. A symmetrical tank will make it consistent from full to empty. A losi tank with a clunk fixed my T-Maxx problem and the thing tunes great full to empty over and over again.
I played with the needle and done testing when the tank is mostly full versus empty and haven't seen any noticeable difference of leaning due to the fuel level dropping. I richened it up with plenty of fuel to the point it slows down noticeably and take note of the needle. When doing the exact same test with low fuel level it took the same amount of richening on the high-speed needle to get it to perform slow again. I didn't notice that I had to compensate by tuning it richer because the fuel level was lower.
If you take a soda bottle and take the cap off and try to blow in it...... You'll find that it pressurizes pretty much instantly. The only difference in RC is there's a small restriction and a fuel line and a little bit of a leak for fuel going to the carburetor. That tells me that the tank pressurizes basically instantly as soon as you're on the throttle. Whatever the PSI is created by the muffler, it gets to that point pretty quickly. I don't see how that pressure can change as long as the throttle position and the pressure coming out of the muffler is the same. Which it always should be if everything is working right and it's the same throttle position. When the tank is full at full throttle, let's say it puts 1 PSI in the tank. Now when the fuel level is low at full throttle it should put 1 PSI in the tank just the same. How fast? As fast as it takes to pressurize the soda bottle with your mouth. You can change the amount of liquid in the bottle and try it with a full bottle or with very little liquid and it will make no difference. It pressurizes pretty much instantly the exact same if you're blowing the same amount from your mouth.
Therefore I don't believe there is any noticeable change in pressure the carburetor sees *due to the fuel level going down* Any lean symptoms is due to the clunk not being in the fuel at all times. Or some other problem besides the fuel level in the tank. Like a pressure problem or tank being too far away.
Of course when there's just very little fuel left it's normal to be sucking in a little bit of air at that point and have some lean symptoms. That's not due to losing pressure from the fuel level but only because it can't suck in fuel 100% of the time. You could blow through the muffler line going into the tank and see how long it takes to pressurize with your mouth. It'll be just like that bottle test and it will make no difference whether there's fuel in it or not. The size of the tank is always the same. And if you have liquid in it it will push the liquid just as fast as soon as you put pressure to it whether it's full or almost empty.
Now mounting the tank really far away and starting to have problems is a different separate subject. Since that would require a little more pressure to go through longer lines that would be a separate issue.
Why do people constantly say that the fuel level is changing the pressure the carburetor sees?
Even on RC cars I noticed that typically the top of the tank is about even with the spray bar which means the whole tank is below the carburetor. Or most of it is. They seem to run great that way. Same with the airplanes. Of course with cars you aren't driving inverted so that's not as important.
When inverted on an airplane though the tank supposedly becomes a uni flow system. So in theory it shouldn't matter now that the tank is completely above the spray bar when inverted. Since supposedly with uniflow the weight of the fuel makes no difference because fuel wouldn't come out no matter if you have little fuel or a full tank because its equalized from the pressure line being at the same level with the fuel pickup when inverted. In theory it shouldn't make a difference that the tank is fully above the spray bar. But I don't don't believe that it's even working like this like people say. Once the pressure and flow starts going it will still be easier to flow fuel to the carburetor with the tank fully above the carb. It'll work just like a standard two line setup from what I see. But also..... Is the end of the muffler line the same level as the end of the fuel line to the carburetor on the RC airplane in reality? So then it would gravity feed if one line is lower than the other. I'm pretty sure that ruins "uni flow" now because it doesn't matter that the clunk and the pressure line in the tank is the same level. The lines they are connected to are not at the engine and are at different levels. Now you will get flow out of one of the lines that is lower than the other which somewhat cancels out uni flow. Doesn't it? It seems the only thing left is to get the muffler line filled up with fuel or your carburetor filled up with fuel and choked out because the pressure has nowhere to go and equalize except the carb and muffler. If it's by chance not going all the way to the carburetor then it's spitting out the muffler and wasting expensive fuel and run time on a good day at best.
If I'm not mistaken this is really not working like people say and it's stupid because it just blows out fuel out of the muffler on a good day and doesn't have any benefits. I've seen people say themselves that use uni flow.....that it makes no noticeable difference in reality to the 2 line setup for how it flys. I guess they just like wasting a few ounces of fuel per flight out of the muffler? I don't know. And that it absolutely has to do and will do because pressure has to go somewhere. I'm sure there's some that purposely don't pay attention and have a dark colored muffler fuel line with the engine mounted sideways and the muffler at the bottom....... Purposely and willfully being ignorant to it and not looking or testing/checking and then saying......."I don't see any fuel coming out of the muffler or any problems with uni flow"
Somehow their uniflow setup doesn't do what everyone else's does and is better....... Somehow the tank is being pressurized but also equalizing just like a two line setup with no fuel going up the carb or muffler.........hmmmmmm
As they say.....the devil is in the details. Partial truths are always 100% lies and deceptions. No such thing as partial truth in reality.
But wherever the exact tank position is the final tune accounts for any differences. And then it's going to be the same from that point because the pickup is always at the same spot and the incline or decline is always the same going to the carburetor. (Just talking about fuel level in tank and not attitude of plane) If the tank is fully below the carburetor...... Well then that little bit of up travel through the clunk to the carburetor s going to be same for the whole entire tank. All of the fuel in the tank enters the clunk and travels the same distance no matter what the fuel level is. A little bit of fuel at a time. The same pressure from the throttle position is going to push that fuel the same way all the way to the end. If it's 1 PSI at full throttle it's 1 PSI at all times at full throttle till it empties. Blowing into an empty bottle or a bottle full of water has the same characteristics and the liquid doesn't seem to make any difference for pressure.
But a tank with a crazy shape is a whole different story. And the only thing that makes sense is the surface area that acts like more or less leverage from the same pressure in the tank that changes the fuel flow as the shape of the tank changes due to fuel level going down.
And if I'm not off on this.....well there is too many myths and lies everywhere.....that will never change. Lies will always be fully in stock. And whatever truth is will be out of stock and discontinued on purpose.
Airplane guys taking ABC engine straight to full throttle like the manual says and not tune it too rich. All car guys run their new engines at idle and tune it filthy rich and only use part throttle and run slow cold and rich to run them in. And many will take a lot of time to warm up their engine every start. Whereas the airplane engine you just go straight to full throttle and it's warmed up in 3 seconds. Same identical ABC engine design with two opposing and perfectly opposite ways of running these engines between the airplane world and the car world. Pretty astonishing.
I think how long the airplane engines last vs the car guys engines says it all right there for which way is right and which way is wrong and bad for them.
Maybe I'll go run a glo engine the adam drake method so that I can wear out all my pinch in 5 minutes and get rid of that disease and resistance that is called mechanical pinch out of the way completely. Then it will be race ready within half a tank and make full revs and power in half a tank. That nasty old pinch. Get rid of it!
09-17-2023, 01:46 AM
#2
Without reading the full story, I can say this from experiments I have done:
At full throttle, muffler pressure is in the order of mangitude of about 20"of fuel column (very easy to test by simply using a 3-line set-up and attaching 3 or 4 ft of fuel tubing to the filling connection, leading it straight up and see how high the fuel will be pushed up at full throttle). Not saying it IS 20", I have seen 25, I have seen 15, but in general, "roughly 20"
A typical glow engine also has a suction head of at least that amount or more, which can simply be tested by placing an engine on a bench, using 3 ft of fuel tubing and a loose fuel tank. No muffler pressure. Run the engine at WOT, gradually lower the tank while simultaneously readjusting (opening) the HS needle. At some point the engine will not draw fuel anymore regardless of how far you open the needle. This height that the engine manages to pull up fuel is the suction head.
All in all, that means that there is roughly about 40" of fuel pressure difference present at the NVA.
So yeah, that 1 or 2 inches of fuel level change in the tank from full to empty really is not going to do very much, and even holding the plane vertical (maybe 4" of height difference between fuel level and spraybar?) is not going to really affect things much. A bit, because the leaning out in this position IS noticable after all (engine will gain a few hundreds of RPM).
Things change though, when doing sharp manouvres. Sportier RC planes have no issues pulling 10G or more. and suddenly a 2" height difference becomes the equivalent of 20"...
Nothing to worry about for a sedate floater, but for an F3A plane proper tank location DOES make a difference.
At full throttle, muffler pressure is in the order of mangitude of about 20"of fuel column (very easy to test by simply using a 3-line set-up and attaching 3 or 4 ft of fuel tubing to the filling connection, leading it straight up and see how high the fuel will be pushed up at full throttle). Not saying it IS 20", I have seen 25, I have seen 15, but in general, "roughly 20"
A typical glow engine also has a suction head of at least that amount or more, which can simply be tested by placing an engine on a bench, using 3 ft of fuel tubing and a loose fuel tank. No muffler pressure. Run the engine at WOT, gradually lower the tank while simultaneously readjusting (opening) the HS needle. At some point the engine will not draw fuel anymore regardless of how far you open the needle. This height that the engine manages to pull up fuel is the suction head.
All in all, that means that there is roughly about 40" of fuel pressure difference present at the NVA.
So yeah, that 1 or 2 inches of fuel level change in the tank from full to empty really is not going to do very much, and even holding the plane vertical (maybe 4" of height difference between fuel level and spraybar?) is not going to really affect things much. A bit, because the leaning out in this position IS noticable after all (engine will gain a few hundreds of RPM).
Things change though, when doing sharp manouvres. Sportier RC planes have no issues pulling 10G or more. and suddenly a 2" height difference becomes the equivalent of 20"...
Nothing to worry about for a sedate floater, but for an F3A plane proper tank location DOES make a difference.
Last edited by 1967brutus; 09-17-2023 at 01:49 AM.
09-17-2023, 05:42 AM
#3
Tank location does make a difference if you fly a F3A type airplane or Pylon racer or a sport plane capable of doing aerobatic maneuvers (this includes 95% of all R/C planes).
That is my experience from flying R/C for 47 years flying all kinds of R/C planes powered by glow engines.
That is my experience from flying R/C for 47 years flying all kinds of R/C planes powered by glow engines.
09-17-2023, 08:27 AM
#4
Tank location does make a difference if you fly a F3A type airplane or Pylon racer or a sport plane capable of doing aerobatic maneuvers (this includes 95% of all R/C planes).
That is my experience from flying R/C for 47 years flying all kinds of R/C planes powered by glow engines.
That is my experience from flying R/C for 47 years flying all kinds of R/C planes powered by glow engines.
My 40 years of model flying says that in 90% of the planes I have owned, tank location 1 or 2" up or down did not make one iota of difference.
09-17-2023, 12:21 PM
#5
My 47 years flying sport, scale, twin engined planes , pylon racers, aerobatic F3A and CL aerobatic says that tank placement is vital.
If you do not place the tank right (Have the tank as close to the engine as possible, have a tank not too big and have the center of the tank in line with carb orifice) you do not get a reliable engine run. Reliable in my case means I do not notice any different engine behavior from the tank is full to that it is nearly empty and that my engines can cope with any behavior the plane is doing in the air, be flying right side up , inverted flight, knife edge or doing fast rolls,snaps or any maneuver.
Take pylon racing for instance, in F3D (International pylonracing) and in any other pylon racing class it has been mandatory since the nineties to use the Tettra Bubbleless tank to have a more reliable engine run compared to the previously used two clunk type tanks that every sport flier uses. Why? Because they give a more reliable engine run.
In my sport planes I use ordinary tanks like DuBro or Sullivan but I use tree lines, one for pressure that goes down at the bottom of the tank, one straight up which is for overfill and one line that goes to the carb. Why do I rout a third line with clunk for pressure down at the bottom of the tank you may ask..The answer is that because that makes the fuel pressure to the carb more uniform compared too that of a two line clunk tank where pressure from the silencer is let in the tank at the top.
Curare
If you do not place the tank right (Have the tank as close to the engine as possible, have a tank not too big and have the center of the tank in line with carb orifice) you do not get a reliable engine run. Reliable in my case means I do not notice any different engine behavior from the tank is full to that it is nearly empty and that my engines can cope with any behavior the plane is doing in the air, be flying right side up , inverted flight, knife edge or doing fast rolls,snaps or any maneuver.
Take pylon racing for instance, in F3D (International pylonracing) and in any other pylon racing class it has been mandatory since the nineties to use the Tettra Bubbleless tank to have a more reliable engine run compared to the previously used two clunk type tanks that every sport flier uses. Why? Because they give a more reliable engine run.
In my sport planes I use ordinary tanks like DuBro or Sullivan but I use tree lines, one for pressure that goes down at the bottom of the tank, one straight up which is for overfill and one line that goes to the carb. Why do I rout a third line with clunk for pressure down at the bottom of the tank you may ask..The answer is that because that makes the fuel pressure to the carb more uniform compared too that of a two line clunk tank where pressure from the silencer is let in the tank at the top.
Curare
09-17-2023, 04:03 PM
#7
Perry pumps are a joke unless you have a Perry pump carb. Most engines will not benefit from a pump. It will actually cause more chaos than it’s worth.
Frankly, Perry carbs kinda suck IMO. Too sensitive to minuscule specs of debris cause all sorts of hell.
Frankly, Perry carbs kinda suck IMO. Too sensitive to minuscule specs of debris cause all sorts of hell.
09-18-2023, 10:30 AM
#9
09-18-2023, 06:25 PM
#11
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Sorry fellas. YMMV truly applies here. I have not had the same great success as others. I have a different expectation for a model engine carburetor. I have used some stellar carburetors, and used many poor ones as well. While I don’t consider a Perry carb a poor choice, I would say that I would exhaust a few other options before going down the rabbit hole that is using a Perry carburetor.
This suggestion question was not about Perry Carbs???
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TheEdge (09-19-2023)
