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  1. #26
    Sport_Pilot's Avatar
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    IMO opjose has a point. To say it is absolutely critical is an overstatement. I too have run engines with tank placements that were incorrect, or at least would be incorrect if the plane was intended for aerobatics.

    If you are flying straight and level you can place the tank well below the centerline with no ill effect. Since you are not flying inverted or high G maneuvers it will work well in that instance.

    To say that the tank must be placed with the centerline to the carb is somewhat incorrect with the most common tank setup. It would only be correct if set up with a uniflow setup. With the most common setup with the vent at the top and pickup at the bottom the correct tank setup is with the tank slightly below the centerline, the actual amount is dependent on the thickness of the tank and the muffler pressure available.

    If you check out those control line planes with the adjustable tank, you will often find the tank below the centerline of the carb. And it is critical for them because they are competing in aerobatics of one kind or the other, but even if racing straight and level it can make a difference in obtaining full power through the run.
    Last edited by Sport_Pilot; 11-20-2013 at 10:31 PM. Reason: Formatting
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  2. #27

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    I personally have had much better luck with my engines when I have had my tanks placed with there center line 1/4" above the spray bar of the carb . The engines are always easy to tune ,I have no problems with flooding .
    Ken , Biker BC Cub Brother #6 Ultra Sport Brother # 100 Tiger Club # 7 Pulse Brother # 1 Sig Brother # 58 Top Flight Brother # 9

  3. #28

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    Everyone has an answer but not THE answer.

  4. #29

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    I've had three UCan46s. Wonderful flying planes. Inverted OS 70FS, no issues. Shut off the carb at the transmitter until next startup, hand start with one or two flips. Another, OS55, same protocol, no issues. I had another UCan on which I angled the engine 45 degrees to allow the exhaust to exit the bottom of the plane. Worked great, same protocol. I have several warbirds, most with OS91s. They start with one spin backflip usually. All have carbs way below the tank centerline. I'd suggest we are over-thinking this one.

    Shutting off the fuel line is the answer. Do it with a clip or do it at the transmitter. Both work. I've used these valves on my Dirty Birdy and other aircraft for decades. They really work great. That's THE answer for me.
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  5. #30
    opjose's Avatar
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    Quote Originally Posted by Jim Thomerson View Post
    I don't know about RC, but experienced folks who fly control line precision aerobatics sometimes raise or lower the tank by the thikness of a playing card. Makes me think that tank placement is important.
    That just goes to show how people buy into unsubstantiated "junk science" including some of the posters here.

    Let me give you some REAL figures.

    The thickness of a playing card, let alone 3-10 inches of tank height difference is NOT going to make a substantive or even measurable difference to the engine.

    In fact the pressure differential produced by the propeller air pressure at full throttle alone is FAR GREATER than any tank height adjustment could ever achieve.

    Don't believe me...

    Here are some HARD SCIENCE FACTS that back that up.

    If you take the thrust produced by the prop distributed by the volume of the air behind the blades up to the carburator, you will get the effective INCREASE in pressure differential when the engine is at full throttle between the carb and the fuel tank when the engine is sitting level with the fuel level at the centerline.

    Assuming a .46 size engine producing 5lbs of thrust using an 11x6 prop....

    A = π r^2

    Or 95 square inches

    Assuming that the carb intake is 1.5" behind the prop... the volume of the air behind the prop at that point is

    142 sq inches

    5lbs / 142 sq inches = .5633 oz/sq inch

    While not excessive that amount is eminently measureable.

    The tank on a typical plane sits behind the engine, approximately 8 inches behind the prop OR MORE.

    At that point the volume of the cylinder is 760 sq inches which gives us

    5lbs / 760 sq inches = .10 oz/sq inch or an effective air pressure difference of .46oz/sq inch

    That does not take into account that the pressure actually falls off far faster because the air pressure also disappates laterally the further you move behind the prop.

    So in actual practice you have GREATER than .46 oz/sq in pressure differencial moving from the carb to the tank.

    A 50 foot difference in altitude ( air pressure ) results in approximately a 1kPa difference in pressure.

    1kPa = 2.32 o/sq in.

    .46/2.32 * 50ft = 9 feet or pressure differential.


    So if you take the air pressure of the prop alone, that is equivalent to moving the tank 9 feet above or below the centerline!!!

    If someone thinks a few inches matters, then they need to go back to simple high school physics and math....
    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  6. #31
    opjose's Avatar
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    Quote Originally Posted by blhollo2 View Post
    Everyone has an answer but not THE answer.

    The answer I gave is substantiated by science not speculation.
    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  7. #32

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    The thing with numbers and science reminds me of one of my favorite quotes (have no idea who said it though): "In theory the practice is the same as the theory. In practice it's not."

    Now I agree that all the doom and gloom probably isn't warranted. But the thing is a lot of fuel feed problems in the history of flying model aircraft have been resolved by moving the tank.

    Saying that position doesn't matter is saying that when I point the nose of my plane up that the engine isn't leaning at all when it obviously is.

    In every club I've ever belonged to there were several guys who had tons of planes including 3D, classic pattern, turn-around pattern, and various sport models (stiks, etc.). But these guys didn't ever fly any aerobatics. They had the planes but not the guts or talent. A roll every once in a while, some loops and maybe inverted flight for five seconds and that was it.

    We had one guy who was the resident "engine expert". At least all the noobs thought so because he was the guy who was ALWAYS screwing with his engines. And any time anyone else is having any kind of engine problem he invites himself in and starts screwing with it.

    In fact, that's how I met him. Admittedly what he saw me doing probably looked to him like I had no clue. What I had done is disassembled the entire engine including the carb and cleaned it out well and oiled it because it was going to be stored for a while and it was an awesome engine that I wanted no rust in.

    So when I replaced the needles I left them very rich. So rich that the engine wouldn't run at full throttle. So when he happened along he saw me adjusting the high speed needle at less than half throttle. Once I explained to him that why I was doing what I was doing and that I knew what I was doing he sort of hrumphed and moved along.

    But every time he ever messed with someone's engine they had engine problems the rest of the day. What that has to do with anything...

    I never really thought about it much until I met that guy and it dawned on me that the guy to ask for help with engines is the guy who is flying and not the guy who is always screwing around with his engine. If he knew what he was doing he'd be flying.

    There is also a huge difference between us mere mortals and those guys who can compete on a national and international level. Another guy in our club used to buy gas engines and start drilling holes the carb before he ever ran the engine because he knew better than the manufacturer how the engine should be. He could never get an engine to run long enough to take off which is a good thing because every time he did take off he crashed immediately after including a few times in the pits.

    But if a control line guy who is competing and knows what he's doing is moving his tank a smidge he can probably tell the difference. It might prevent loss of a couple hundred RPM toward the end of the flight which that loss would change the size and rate of his maneuvers. I don't know what the issue was but I'm not going to tell someone he doesn't know what he's doing when he knows more about it than I do.

    Anyway... bottom line for me is that even though tank position isn't critical in a trainer, all trainers that I've ever seen allowed the tank to be in the right place. A trainer is supposed to train, right? Not just flying, but building, set up, radio installation, engine installation, maintenance, post-flight, pre-flight... all that stuff.

    Just because a trainer will let you get away with a lot of wrong things doesn't mean you should attempt to learn to do things the right way. It will make a difference when you advance.
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  8. #33
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    The key element is the "criticality" of all of this.

    Some here would have you believe that 1"-2" height difference is going to make your planes crash. Nothing is further from the truth.

    Moving the tank a few credit cards thickness has less of an effect than rubbing a rabbits foot charm.

    It makes no difference to the engine. The numbers don't lie nor does gravity. There is NO "THEORY" behind any of this except on the part of those who speculate instead of using applied science, which unfortunately include almost all of those who compete nationally in RC. In full size high end car racing the science is king now-a-days.

    There is the issue of siphoning, but when you get right down to it, if you want to cure siphoning you would need to have your tank height set so that the TOP of the fuel when the fuel tank is full, is below the spray bar. That gives you a 1" or more off set from the start.

    Is the tank height difference critical? Nope not by a long LONG shot.

    Desirable? Sure to lessen the chance of SIPHONING it is... but don't go chopping up your plane to to achieve it nor think you MUST "lower the tank" as often espoused here.

    As far as a running engine is concerned even a 3" tank height difference simply doesn't exist.

    If anyone wants to argue the nose-up/down situation I have theoretic and demonstrable numbers for that too.
    Last edited by opjose; 11-21-2013 at 03:46 PM.
    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  9. #34

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    Couple of things.
    1) Clarence Lee is the pre-eminent authority in the world on model aircraft engine operation. I consider it an honor to have done business with him and have 2 engines that he modified for me.

    2) In what part of your calculation do you account for the difference between the volume of the air behind the full prop arc and the volume of air available at the carb inlet?
    3) In what part of your calculation do you account for air flow turbulence in the area between the prop arc and the carb? hint- there is a pressure differential that needs to be accounted for.
    4) In what part of your calculation do you account for the atmospheric pressure differential between the carb, and the "high" pressure air from the prop.
    5) Air pressure exponentially drops and returns to ambient air pressure as you move away from the propellor arc. Where is this in your calculation?

    Any way I calculate it your air pressure "calculation" is about 99.3% too high... But frankly this calculation really does not explain the fluid dynamics behind the prop, carb, and fuel pressure. Sorry, it is not simple "physics and math" it is actually fluid dynamics.

    edit: rechecked my calculator -it was 99.3% and not 99.7% Based on a carb air intake volume of 1 sq in. Which if you fly a .46 engine you will see is very generous.
    Last edited by kdunlap; 11-21-2013 at 05:07 PM. Reason: math error

  10. #35

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    Opjosem, I take it you do not fly control line precision aerobatics at a competitive level. I no longer fly, but when I did, I got an occasional third place in Advanced at a local contest. The playing card thickness came from a person who is competitive at the national level. Thinnest shim I have used is 1/32 ply. The experts can tell differences which I did not notice.

    Generally speaking, in CLPA we want the airplane to fly at the same speed throughout the entire flight, maneuvers, upright, inverted ,etc. so we do not see the variation you calculated, which seems valid for a throttled engine.

  11. #36
    Wagon1's Avatar
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    I just started a Top Flite Corsair and was taken back a bit by the fuel tank placement on the plans. The centerline of the tank is more than 3" above the spraybar. I intended on using the same engine (OS 120 4 stroke inverted as on the plan). The tank placement spooked me a bit so I bought a pumped engine. I'm still learning. I guess I was also taught to keep the spraybar in the middle of the tank.....

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  12. #37

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    @opjose,

    By your math example a spray painting gun won't work, but moving air that's higher than ambent pressures air can and does create areas of below ambient pressure. Assuming that the pressure gradient from the carb to the tank is easily calculated and static is simplistic, although you might be presenting it this way to make a point.

    I agree some respected authorites in the past probably should have had some of their assumptions challenged, but it's a hobby for most people.

    I think the real issue which is for every installation/motor type is just how much diffential pressure is generated across the needle valve via the pressure in the tank and the vacuume at the spraybar. The larger this value is, the less influence small changes in fuel pressure due to tank height will have on the total fuel flow.

    Control line stunt guys tend to have a very specific engine run requirement, tank position (up and down, further in or further out) as well as positioning of the vent to take advantage of the changes in air pressure due to changes in airspeed or RPM, are all critical for them.

    To the other poster complaing that there's plenty of answers but no one given THE answer, the reality is tank position is only critical if having it in the wrong spot is the cause of engine problems. By placing the tank where the engine manufacturer recomends you are reducing the likelihood you'll have a tank related problem.

  13. #38

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    Not to change the subject too much, but fuel foaming because of rigid tank mounting is a lot more problematic than tank position with nitro power. A fuel filter near the carb helps.

    Slightly high or low tank position will make a difference in some maneuvers, but if you back the high speed needle off about 400 rpms from max on the ground, generally speaking, you should be fine. The correct prop helps, and 5 percent extra nitro helps too.

  14. #39

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    Quote Originally Posted by opjose View Post
    The key element is the "criticality" of all of this.

    Some here would have you believe that 1"-2" height difference is going to make your planes crash. Nothing is further from the truth.
    I didn't see anyone even suggest that. I certainly didn't and the responses I read don't say that either.

    What we're saying is that the farther the tank is from the ideal location the more your engine will lean and richen when forces change. Even if it kills the engine the plane shouldn't crash unless it's one of those that is incapable of gliding when dead stick and I've never seen a plane like that.
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  15. #40

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    I always tell anyone I'm helping out that the tank should be completely wrapped in latex foam rubber. If they have to use a smaller tank to accomplish that then it's better than having inconsistent engine runs for longer flights.
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  16. #41
    opjose's Avatar
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    Quote Originally Posted by kdunlap View Post
    Couple of things.
    1) Clarence Lee is the pre-eminent authority in the world on model aircraft engine operation. I consider it an honor to have done business with him and have 2 engines that he modified for me.
    .
    Yup, he writes great columns too. But the example I gave is indicative of how people buy into often repeated ideas that have no basis in fact.

    He bought into the "fuel stays at the back of the tank during downlines" nonsense which made absolutely no sense from a physics standpoint.
    I challanged that here and a forum poster here provided a great video showing definitive proof that it does NOT.

    Quote Originally Posted by kdunlap View Post

    2) In what part of your calculation do you account for the difference between the volume of the air behind the full prop arc and the volume of air available at the carb inlet?
    3) In what part of your calculation do you account for air flow turbulence in the area between the prop arc and the carb? hint- there is a pressure differential that needs to be accounted for.
    4) In what part of your calculation do you account for the atmospheric pressure differential between the carb, and the "high" pressure air from the prop.
    5) Air pressure exponentially drops and returns to ambient air pressure as you move away from the propellor arc. Where is this in your calculation?

    Any way I calculate it your air pressure "calculation" is about 99.3% too high... But frankly this calculation really does not explain the fluid dynamics behind the prop, carb, and fuel pressure. Sorry, it is not simple "physics and math" it is actually fluid dynamics.

    .
    Right! And going into all of that is WAAAY beyond those who are posting a lot of this nonsense.

    To answer your questions, my attempt was to demonstrate the air pressure differential between the two points on the plane.

    Air pressure actually drops off far faster and there is turbulance to account for as you've said.

    Yes it's all fluid dynamics and a proper model is beyond the scope of a hobbiest discussion.

    The simple stuff I posted assumes a simple volumentric area behind the prop.

    In actual practice the air pressure drops off FAR FASTER than what I put forward. Forgetting third order effects at the moment, the area behind the prop is NOT a cylinder in terms of air pressure effects, It is more rightly shaped like an egg if you were to perform actual measurements or modeling

    Immediately behind the prop ( close to the carb intake ) you have the greatest amount of pressure, and the dropoff is extremely quick as you move back, far quicker that what I showed above.

    That in itself means that my rudimentary calculations are indeed off... off by a big facter... meaning that the air pressure differential from the carb to the tank is potentially MUCH MUCH higher, equivalent to a higher atmospheric pressure altitude difference.

    The bottom line is that as you move from the cab to the tank with your small engine at full throttle the pressure variance alone is FAR GREATER than that seen moving the fuel tank up or down an inch.

    I'm indeed off by being TOO conservative in stating the difference in such a rudimentary way.

    Bottom line:

    The engine simply doesn't care about that shim, or even 1-2" of tank height difference once it is running.

    Of course with some people insist the earth is 4000 years old too, no matter the facts.
    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  17. #42

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    Hi!
    Don't know what to say when I read what you have written...Sorry!
    All experienced fliers know that tank height matters, not from any teoretical aspect but from practical use.
    Ofcourse tank height doesn't matter if you run the engine at constant speed, it what happens when you thottle it that counts.
    Try throtteling a glow engine with the tank 2- 6centimeters above the engine and see what happens.
    Jan Karlsson - Supplier MVVS Products

  18. #43

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    Geez guys, this isn't rocket science and we're not talking about national level competition planes or pilots.
    One of my models has twin 8 ounce tanks side by side over a foot from and centered fully below the carb. They are fed pipe pressure to force the fuel uphill to the carb and, to this day, I haven't had a problem. I guess it's all in your application and how willing you are to think outside the box

  19. #44
    opjose's Avatar
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    Quote Originally Posted by jaka View Post
    Hi!
    Don't know what to say when I read what you have written...Sorry!
    All experienced fliers know that tank height matters, not from any teoretical aspect but from practical use.
    None of this is theory. Our planes would not fly otherwise.



    Quote Originally Posted by jaka View Post

    Try throtteling a glow engine with the tank 2- 6centimeters above the engine and see what happens.
    Not much.

    I've demonstrated this time and again at our field with the engine in a stand.

    Heck I've put the tank 2 feet over the carb line to show how minor the effect actually is.

    Again remember that as the plane is flying the tank WILL sit a full 6-8" above or below the engine during downlines and uplines.


    Simply put, tank height is NOT critical to engine operation especially over the rather trivial distances being discussed here.

    Tank height affects siphoning, but even when that occurs it can be ameliorated by proper startup technique. It is not as critical as it is being made out to be.
    Last edited by opjose; 11-22-2013 at 12:39 PM.
    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  20. #45

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    Inverted asp fs91 tank height?

    Hi, looking for advice here especially from someone who has experience with a similar setup... i am installing a ASP FS91....(OS copy)into a P51 Mustang and the engine has to be inverted..... this puts the tank centreline at 10mm above the spray bar... the Magnum XL .91RFS manufacturers instructions (the closest instructions i could find to the ASP) says this:........... "Ideally , the stopper in the fuel tank should be even with the high speed needle valve or just slightly below it. Some models will only allow the fuel tank to be mounted higher than the ideal location. A fuel tank that is positioned higher than the ideal location usually doesn't pose any problem except when it is mounted excessively higher and/or is used in conjunction with an inverted mounted engine or during extreme aerobatic flight. If you mount the engine inverted ,we strongly suggest lowering the fuel tank so the stopper assembly is slightly below the high speed needle valve. Doing this will prevent fuel from siphoning into the engine into the engine and flooding when the tank is full .If you cannot lower the fuel tank far enough we suggest lowering it as far as can be allowed in your particular application. The size of the fuel tank used should be 12oz - 14oz depending on the model and the lenght of the flights desired . Use of a fuel tank any larger than 14oz can lead to excessive leaning of the engine during flight and is not recommended" .......not sure which way to go here so any help would be appreciated .....Ken....

  21. #46
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    I have to say I have been getting a great deal of entertainment from this thread so far. Lots of mis information. I figure what the heck I will throw in my experiences as someone who has been flying models for 40 years most of which has been competitive flying in pylon, pattern, IMAC and helicopters. Does tank location make a difference? You bet your rear it does. Pylon guys tend to run the tank a bit above the needle center because during high G turns it helps keep the engine from going lean. The engines usually only have a Venturi so throttle response is not an issue. On my IMAC airplanes I always mount the tank at CG and as high as I can get it. The height of the tank helps get the vertical CG of the airplane as close to thrust line as possible. Obviously this is a trade off as even with a walbro pumper carb the mixture does change from upright to inverted. If your low mixture is set a bit lean you are going to know it the first time you do a negative spin. Up and down lines are not affected because there is no head pressure differential happening from one up line to the next. Look at just about every glow powered helicopter. Notice how the tanks are almost always below the needle CL? This is not by chance. Back in the early days of helicopter aerobatics we found the the rotors were more efficient when the helicopter was inverted. This is because the airflow stream is not obstructed by the mechanics. The result of higher efficiency was a jump in RPM, the engine would unload and go lean. By offsetting the tank location the engine would richen up slightly when inverted and help maintain constant RPM. CL stunt guys have the advantage of being physically connected to their airplane. Someone with enough experience can feel an RPM change in the handle. Their goal is to fly consistent 5 second laps whether inverted or right side up so a couple hundred RPM shift is something that needs to be tuned out. Does any of this apply to the OP? Not much. He just isn't at the skill level yet for this to really be an issue. Neither is Jose. The OP however does seem to have the advantage of asking a justifiable question and then keeping an open mind to how the answers may affect his particular airplane. IMO this means that he will eventually become a better pilot then the guy who dosent know what he dosent know and then wants to argue the point. As long as the fuel is not being siphoned and creating a flooding situation I would say it's going to be just fine on a trainer and most sport airplanes. Just remember that a flooded inverted engine can and will bend a rod quite easily when forced with a starter.

  22. #47
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    Hi Ken and welcome to the forum. It sounds as though you already have found some sage advice in those included engine instructions and my advice is to simply follow them. Do attempt to lower the lank as low as possible.

    Now if the tank attempts to empty itself after fuelling or if the tank starts emptying itself only after sitting out in the hot sun then the only solution is if you care is to lower the tank.

    If you forget to put the Hemostats on the carburetor fuel line before and after every flight to avoid syphon and hydrolock possibilitys then you might just want to lower tank.

    If someone goes to pick up your airplane for you (something often gratefully done for us wheelchair folks) after a flight and you or he forgets to carry the ship nose up then you just might want to lower the tank.

    If you forget to cut a pair of holes in the cowl to run the carb line out and back in agine to have a place to even put the hemostats then you may want to lower the tank.

    if you fail to check the engine for hydrolock everysingle time before applying a starter (this is important for any inverted situration even with a tank at recommended tank/spraybar relationship) then you may want to lower the tank.

    If you forget to clear a hydrolock everytime by removing the plug and spinning the engine with the fuel line hemostated then you may want to lower the tank.

    If you have to start you airplane upside down UGH! then you may want to lower the tank and no I won,t go into the safety thing on this practice.

    So while the engine may work OK when its running however the required extra procedures and handling methods are just not acceptable to most sport flyers and certainly not the average trainer with someone who may be just learning. Sure I have some airplanes that are 'special purpose' airplane that are not even remotely within the normal height relationship especially my two cross country airplanes. Heck one of them even carries almost its entire fuel load on top of the fuselage oh maybe five inchs above the recomemended height and it works in flight but it took lots of effort to get the tank system to where it would work acceptably and of course very careful attention to the use of the hemostats must be observed. Forget the hemostats just once and the days attempt may be over for that day. I will attach a photo in a little while of that ship.

    So it all boils down to what you are prepared to deal with. I recommend for any trainer or sport plane including first warbirds to lower the tank' always have and always will. I have in this same thread offered ways to lower the tank for your consideration if choose to do that and avoid some or all of the procedures I have outlined above.


    Oh yea almost forgot one that really sucks and this is for my fellow wheelchair dudes. You see for us the normal tendency when you are in a wheelchair is to pickup and hold the airplane with the engine in your lap and the tail stickijng out forward and up. Man it really stinks when you get a whole tankfull or what feels lite a whole tankfull of glow fuel right in your crotch and belly. No tank height just might not help this situation but sure sucks when I forget

    John
    Last edited by JohnBuckner; 11-23-2013 at 08:00 AM.
    \"Keep your controllines tight\"

  23. #48
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    Click image for larger version. 

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    Here is the airplane I mentioned and almost all the fuel is above the fuselage however also note in the first photo the hemostats that are mandatory right up to the point of hitting the starter and immediately after a flight before picking up the airplane. So its all up to what you prepared to deal with?

    John
    Last edited by JohnBuckner; 11-23-2013 at 08:03 AM.
    \"Keep your controllines tight\"

  24. #49

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    OK, if your engine is on a stand or something like John just posted, then yeah, you can put your tank anywhere and set the engine to work with it. As long as it can draw fuel you're fine.

    The problems start when your model is flinging around and flying upside down for extended periods. In that case your tank in relation to the needle is the exact opposite of what it was before. If it were 2" low then now it's 2" high. If you roll then it will probably be fine. If you try to fly circuits inverted then your engine will do something different.

    Let's say your tank is way high. If you lean your engine that way and you try to fly inverted for long it's probably going to quit.

    If your engine is running rich that way then it will probably run the way it's supposed only when the plane is inverted.

    I didn't read all the math by opjose, but it looked like he's claiming that an engine can draw fuel 96" straight up. If that's what his math is saying then I'm saying the math is wrong. If that's not what it's saying then ok.
    Work is what I do for the love of it. A job is how I pay for it.
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    I am in agreement with John . He knows what he is talking about ..simple as that.
    Ken , Biker BC Cub Brother #6 Ultra Sport Brother # 100 Tiger Club # 7 Pulse Brother # 1 Sig Brother # 58 Top Flight Brother # 9


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