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Old 12-03-2021, 05:59 PM
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Fuel Tank Plumbing - Internal

Ok - A bit of background - I have been a RC Heli flyer for 30+ years and I recently have been interested in fixed wing with the intention of moving over to Jets when my flying skills are up to it.. So I am a bit of a noob with certain tips/tricks when it comes to fixed wing craft.



I am currently modifying a Spacewalker airframe by installing a Saito FA82 and a larger fuel tank.


I have a question regarding the typical installation of the clunk line in aircraft fuel tanks.


With helis it is imperative that the clunk line is flexible enough that the clunk reaches all areas of the tank if the tank is rotated in all orientations. This is obviously to maintain reliable fuel pickup in any orientation (esp. when performing any 3D flying). Tanks also tend to be more of a cube looking shape rather than the typical rectangular shape you tend to see in aircraft.


What I have noted with many aircraft tanks is that the clunk line is not all that flexible and the clunk tends to sit at the end of the tank with limited up/down and sideways movement available. I gather this does not cause too many problems for most pilots..???


I realise that many fixed wing planes generally tend to always fly forwards Vs helis which can change direction in any attitude so should I not worry about the clunk line flexibility as much as I would for a heli.. or is it good practice to still utilise the same (heli) method of installing a clunk into my fixed wing aircraft - Am I 'overthinking' it for fixed wing aircraft?



Here is my Dubro tank undergoing pressure testing before being fitted to the craft. Fitted with Hayes flexible clunk line and an OS bubbless clunk as per my heli builds.


Thanks


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Old 12-03-2021, 06:32 PM
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You want your clunk like to be flexible… and about 3/4” off of the bottom of the tank if you were to hold it straight up and down. What you dont want to happen is the line get folded forward during your flight and pinch off your fuel supply… when you get in to gas and tygon lines we sometimes will put a piece of hard line in between our clunk line so it can only pick up fuel from the bottom of the tank.. if you spin the tank it should be able to hit all the sides and corners.
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Old 12-03-2021, 07:35 PM
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Originally Posted by 3DJP View Post
so it can only pick up fuel from the bottom of the tank..

I have seen this on many builds (foam style clunk also helps) - What confuses me (as a heli flyer) is what happens in a sharp nose down attitude - ie the rear/bottom of the tank now becomes the 'top' where the clunk is.


In this case how does the fuel supply remain consistent if the clunk cannot bend and reach the front of the tank if there is not all that much fuel in your tank.
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Old 12-03-2021, 07:55 PM
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it's really a non issue, but has morphed into one of the better old wives tales of RC. there's plenty of gas in the fuel line to last the few seconds in vertical nose down at idle attitude. the foam clunk thing is personal preference. I don't use them.
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Old 12-03-2021, 08:36 PM
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Quote:
Originally Posted by Costas View Post
I have seen this on many builds (foam style clunk also helps) - What confuses me (as a heli flyer) is what happens in a sharp nose down attitude - ie the rear/bottom of the tank now becomes the 'top' where the clunk is.


In this case how does the fuel supply remain consistent if the clunk cannot bend and reach the front of the tank if there is not all that much fuel in your tank.
As Newbie stated… you should never be nose down long enough to draw the fuel you have in the pickup dry… on top of that… your not full throttle pointed nose down 99% of the time! Normally im doing a maneuver at just above idle.. or if im flying a warbird ill drop in at full throttle but thats not straight down either.. lol
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Old 12-03-2021, 08:54 PM
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Thanks to all - Makes sense regarding the relatively small amount of time spent in a nose down attitude.


With helis it is different and common to be at, say 75% throttle in nose down attitudes for extended periods.


One of my favourite maneouvres is to perform a vertical climb with an aileron roll to a peak altitude and then perform a 540 degree pirouette at the top stall point. I then head straight down nose first and perform another roll or two on the vertical descent.


The heli maintains its nominal head RPM throughout which is usually around 75-100% throttle depending on head loading.
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Old 12-03-2021, 09:49 PM
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Originally Posted by Costas View Post
Thanks to all - Makes sense regarding the relatively small amount of time spent in a nose down attitude.


With helis it is different and common to be at, say 75% throttle in nose down attitudes for extended periods.


One of my favourite maneouvres is to perform a vertical climb with an aileron roll to a peak altitude and then perform a 540 degree pirouette at the top stall point. I then head straight down nose first and perform another roll or two on the vertical descent.


The heli maintains its nominal head RPM throughout which is usually around 75-100% throttle depending on head loading.
Yeah i have a 600 nitro i used to fly.. but im much more comfortable behind a plank!! Lol a few buddies of mine fly helis that are really good!! I was always amazed how these guys can keep track of whats going on… its a whole different ball game flying a fixed wing.. hope this helped and good luck
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Old 12-04-2021, 12:45 AM
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hope this helped and good luck

Yes it cleared up my initial query... and thanks for your help....
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Old 12-04-2021, 08:48 AM
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They're get'n lower mate.....
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There was a video of an RC tank mounted to the top of a wing with a camera filming as the guy went thru some aerobatics showing what happens to the fuel -
Conclusion is ...... dont over think the obvious
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Old 12-04-2021, 10:10 AM
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Quote:
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There was a video of an RC tank mounted to the top of a wing with a camera filming as the guy went thru some aerobatics showing what happens to the fuel -
Conclusion is ...... dont over think the obvious
+1 .... video made it pretty obvious that concerns over 'running out of fuel' during routine, especially when the plane's attitude was nose down, was not gonna happen. Now if I could only find it
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Old 12-04-2021, 11:23 AM
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https://www.flyinggiants.com/forums/...eo#post2354372

This might be the video thread. Looks like the link goes to post 7. Need to scroll up to first post.

Jeff
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Old 12-04-2021, 12:13 PM
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I fly gas aircraft only.

I use a substantial metal clunk, the large Dubro or similar, and nothing but Viton 1/4 OD, 1/8 ID inside the tank. Extremely flexible and very long lasting. Tygon inside the tank will eventually stiffen up and create some issues for you that you will likely discover in extended inverted flight when the carb gets starved. I still use tygon outside the tank but eventually have to replace at least the fill line when it gets too stiff. All my tanks are 50 to 64 oz and I never have any tank related issues.

Works for me....
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Old 12-05-2021, 04:57 AM
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Thanks for that video guys - Like you stated, I shouldn't be holding a vertical dive for all that long and if I did ... well that's probably the end of that model...
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Old 12-05-2021, 10:09 AM
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https://www.flyinggiants.com/forums/...d.php?t=285291
Bottom line: Viton is the only thing tested so far that does not change. All the blue, yellow and red fuel line changes dimensionally and degrades over time.
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Old 12-05-2021, 02:50 PM
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Airplanes and helicopters are two different things as to how they behave inflight with g loading. And as the infamous video shows the fuel in a plane usually stays towards the back.. that’s why a semi rigid semi rigid clunk line works best in planes. And a heli needs a floppy one that can get into every nook and cranny. Don’t try to reinvent the wheel...
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