Fuel flow

[Marty_d]

Hi all,

 

Trying to work out whether I can get away with 1/4" ID aluminium tube from the wing tanks down to a Y or T junction before the Savannah header tank.  From there the single hose can be 3/8" which I think is the barb size at the top of the header.  (I can also use 3/8" from the header to the engine).

 

Reason being, I have 4 panel-mount quick release fittings from Aircraft Spruce which I want to use in the wing roots, in case this becomes a trailerable aircraft.  These are 1/4".

Back of the envelope tells me the area of a 1/4" cross section would be 31.66 sq.mm, so 2 tanks = 63.33 sq.mm.  Compared to this a single 3/8" area would be 71.2 sq.mm

 

However I have had advice that it's all about ensuring sufficient fuel flow, based on a maximum of 27LPH for the 912ULS. 

 

How do you calculate this?  There are online flow calculators but one of the variables is pressure (bar).  Is there a set variable for this based on the mechanical fuel pump at the front of the engine?

 

Thanks, Marty

[APenNameAndThatA]

Net says: “According to Poiseuille's law, the flow rate through a length of pipe varies with the fourth power of the radius of the pipe.”

 

So, if your new pipe’s radius is ⅔ of the old pipe’s radius, the new max flow rate is 1/5 of what it was. You have two pipes, so the max flow is 2/5 what it was. (assuming the same length pipe). Based on that (not other things) the answer is no.  

 

Disclaimer: have never built an aircraft. Have built a tree house.

Edited September 17, 2021 by APenNameAndThatA

[facthunter]

Check your fuel flow by timing what goes into a known volume of a container. (calibrated and derive rate by timing it) You need more than the maximum engine requirement. I'd want 50% more . Also you may be on one tank so it applies to single tank as well. Length of a pipe affects a flow rate. I wouldn't go for the smaller dia pipes. nev

[rgmwa]

For the RV-12 the required flow rate for the 912ULS is 1 US gal from the gascolator outlet in 180 seconds or better.  Not sure if this applies to the CH-701, but imagine it would be similar as the engine is the same. I'd be wary of using 1/4" tubing as the flow rate will be a lot less than 3/8".  Feed lines in the RV-12 are 3/8" although the return line is 1/4".

[IBob]

Marty, it's pretty straightforward:

You know the height difference between your tanks and the receiver tank behind the seat.

You've got your fittings.

Get the hose you're looking at using, and the fittings and set it up at those heights on the bench, with some sort of vessel at the top, and another at the bottom. Put in a known amount of fuel and time how long it takes to arrive.

 

Online calculators are more normally used for longer runs of larger pipe at higher pressures, all sitting on the ground.
They're good for sizing that sort of thing up. But in an aircraft, you're looking for the reasonable minimum (weight) that will do the job in practise.
So, you're not looking for opinions, or for theoretical answers.

You're looking for a practical answer as to whether your aircraft fuel system will deliver through 1/4" pipe fittings.

And you check that by trying it.

Edited September 18, 2021 by IBob

[Thruster88]

The other thing to consider is what might happen if air enters the system due to tank un porting. Air in gravity feed lines can cause vapour locking, will it be worse in the small line? I would use the size called for in the plans. 

[facthunter]

Fitting of individual tank pumps can assist with that but there may be  the possibility of inadvertent fuel transfer and venting overboard. You won't get transfer if non return valves are fitted to each tank. With THAT set up you could just turn on both pumps and you will exhaust all fuel with no air getting into the fuel line till then. You should be able to work out why that is the case. Nev

[IBob]

Fortunately, the 701 has an admirably simple and reliable fuel system layout: high wing tanks feed into a receiver(or header) tank behind the pax seat.

The receiver tank holds 20mins of fuel and has a switch at the top wired to a panel light, which comes on after about 2.5mins in cruise if no more fuel is arriving.

On newer builds, the receiver tank also has a vent line running back up to a high point in one of the wing tanks.

Provided the builder does a reasonable job of running the pipes from tank to wing root level or downhill, and without undulations, it takes care of itself without any need for pumps or valves.

Crossporting the tank vents would put the icing on the cake...

[Yenn]

There is a specified method of checking for correct fuel flow on a new aircraft. Briefly is needs to have the aircraft in the steepest position it is likely to ever reach and measuring the fuel flow at near empty tanks. You should get at least 150% of the fuel flow needed at Takeoff. I use a figure of 0.5lb of fuel per hour per horsepower. ie 80 hp engine needs 40lbs of fuel or about 39l per hour or 0.64 l per minute.

[Marty_d]

So it seems like I should stick with 3/8" from the tanks onward.

 

On the R/H tank I have a 1/4" fitting at the top for fuel return.  According to the Rotax Heavy Maintenance manual, this comes from a 5-way fitting bolted to the compensation tube running between the carbs. 

1) Fuel in from the mechanical fuel pump

2) and 3) Fuel out to the carbs

4) smaller fuel out to the fuel pressure gauge

5) smaller fuel out (return) to the R/H tank.

 

That's fine.  But the other thing I have on the Savannah header tank is a breather tube that's meant to go to the top of a tank (usually the L/H one I believe).

 

My tanks are hard to get to - unlike the Savannah, the 701 doesn't have an access panel below the tanks.  So if I want to get a tank out to modify it, I either have to drill out a fairly large area of wing skin rivets or cut the aluminium away under the tank and replace with a panel (either riveted or screwed) like the Savannah.  So I'm trying hard NOT to modify my tanks in any way!

 

Question 1:  Could I put a T fitting on the 1/4" hose that comes from the TOP of the R/H tank (this is the fuel return) and run the header tank breather to that?  If the T was very close to the tank would that work?

 

Question 2:  What diameter hose/pipe do people use for the line to the fuel pressure gauge, and also the fuel return?  I'm thinking 1/4" for the return because that's what I have at the tank end - is that standard?

 

Below is a photo of the R/H wing root with top root skin removed, showing the return line at the top left side and the fuel main line at the bottom right side.  So question 1 refers to plumbing a T into that top left return line, right before that rib beside the tank, then running that down to the header tank as a breather line.

 

Thanks!  

Cheers, Marty

 

 

 

[IBob]

1. In my view, T- ing in as you suggest will likely work very well for the header vent, and I can see no possible drawbacks.

2. The Sav uses 1/4" for the vent. I can't remember for the Fuel return and pressure gauge. It has to be small enough to match the fitting on the gauge. The flow is minimal so as small as you like there, but 1/4" is fine if it fits.

[facthunter]

IF you T into the vent line you would have to make sure there is no chance of that return fuel venting. I don't see how you could meet that requirement. The size of pressure gauge pipe is not critical at all.

   The only pipes affecting fuel flow are from each tank to the engine.  Nev

[IBob]

Facthunter I don't think he's talking about T into the wing tank vent line: he's looking at T ing the vent from his header (receiver) tank into the fuel return line that goes into the top of the RH tank.

[Marty_d]

1 hour ago, facthunter said:

IF you T into the vent line you would have to make sure there is no chance of that return fuel venting. I don't see how you could meet that requirement. The size of pressure gauge pipe is not critical at all.

   The only pipes affecting fuel flow are from each tank to the engine.  Nev

Hi Nev,

As Bob says the T would be on the fuel return line.  Theoretically if the fuel return line were full of fuel (not sure how likely that is with 1/4" line) then it could spill down the T'd line into the header tank - so that doesn't matter either.  I guess the main thing is that if any air does get into the header tank, it finds the highest point and goes back into the top of the tank.  This is just to keep the low fuel sensor off so I don't have unnecessary involuntary clenching.

 

A diagram of what I'm thinking is:

 

Edited September 19, 2021 by Marty_d

[facthunter]

Your diagram doesn't show tank vents which I thought you were referring to. The minor thing wrong with your set up is the fuel temp in 4 might elevate a bit.. I'd insulate all fuel line s forward of the firewall anyhow. Nev

[Marty_d]

17 minutes ago, facthunter said:

Your diagram doesn't show tank vents which I thought you were referring to. The minor thing wrong with your set up is the fuel temp in 4 might elevate a bit.. I'd insulate all fuel line s forward of the firewall anyhow. Nev

Sorry, should have mentioned - tank vents are in the fuel caps, furthest point outboard on the top of the tanks.

[facthunter]

Well you know what happens if you park on a slope sideways or fly unbalanced. With fuel return you have to consider where it goes and be using fuel from the tank where it returns to. Your set up covers that. Nev

[Marty_d]

New diagram:

 

 

[facthunter]

Plenty of others are similar. At least you don't have bladder tanks.. It is possible to design a cross ship vent system where the balance issue is covered.  Someone may be able to post one. It's only a problem when you're really filling right to the brim, but a lot of us like to do that as you know exactly what fuel you have on board. Hope you enjoy  this thing when it's all done. Nev

[Marty_d]

1 minute ago, facthunter said:

Plenty of others are similar. At least you don't have bladder tanks.. It is possible to design a cross ship vent system where the balance issue is covered.  Someone may be able to post one. It's only a problem when you're really filling right to the brim, but a lot of us like to do that as you know exactly what fuel you have on board. Hope you enjoy  this thing when it's all done. Nev

I was discussing a cross-porting system with Bob, but the trouble is, I don't have a barb high on the L/H tank and there's no easy way to get to it.  The other thing is the wing roots on the 701 come downward which means any cross port could not be straight across, it would dip to a low point in the middle. 

Thanks Nev - I hope I enjoy it too!

[facthunter]

Not sure if a dip matters. If it goes UP you break the siphon. Nev

[skippydiesel]

One , admittedly unlikely, scenario is that your(13)  Header tank vent will allow fuel to move from a full (1) RH Tank or from the (12) Return line into the (4) header tank bypassing the (3) Individual tank shut of valves.

[Marty_d]

1 hour ago, skippydiesel said:

One , admittedly unlikely, scenario is that your(13)  Header tank vent will allow fuel to move from a full (1) RH Tank or from the (12) Return line into the (4) header tank bypassing the (3) Individual tank shut of valves.

That's true, but I don't think it's a big issue.

 

I can't think of a reason that I'd want to shut off a full tank individually - and if I need to shut off both tanks there's the master valve.   Fire in the rear fuselage is the only reason I can think of, and with 7 litres of fuel sitting in the header, I'm pretty well screwed anyway if that happens.

[Marty_d]

Hi all,

 

More questions...

 

Firstly, NPT fittings (these are the tapered threads).  What sealant do you use?  Standard automotive stuff like this - https://www.repco.com.au/en/car-care-panel/adhesives-sealants/fastener-locking/permatex-high-performance-thread-sealant-50ml-px56521/p/A9693218?rgfeed=true&cid=google-shopping&utm_source=google&utm_medium=cpc&region_id=100481&gclid=CjwKCAiAp8iMBhAqEiwAJb94z2XFlj83Zns6qRELRiVW_YIC805DgUNqZA0JUHW_fF0o6sYq8K95cxoC6o4QAvD_BwE ?

 

Secondly - hose clamps.  What sort of clamps come standard with Rotax fuel fittings?  Guessing whatever they use must be ok.   Worm drive, 1 or 2 ear compression ones - what's the usual?

 

Thirdly - fuel taps, for the tank lines down to the header.  What sort of taps do you use, and where did you get them?  There's a few cast or aluminium cheap ones for motorbikes and ATV's on Ebay, is that the sort of thing or are there better brass ones?

 

Finally - is there a standard 5-way fuel fitting for the bit after the mechanical pump where it goes to the carbs, fuel pressure gauge and return line? (number 9 on the diagram about 5 posts ago).

 

 

Thanks people!

Cheers, Marty

[skippydiesel]

Hi Marty_d, 

 

My Rotax installation uses T pieces/connectors to achieve the 5 way split you are referring to.

 

Hose clamps: I am a strong believer in using fuel injectors style clamps. There are 2-3 designs but all have common features;  360 degree pressure (no pinching) ; smooth edges (sharp edges can cause damage) ; can be reused indefinitely (if not overtightened. ; redilly available from the better automotive stores (eg Repco - purchase by the box, way cheaper).

 

No experience with fuel taps in aircraft, however there is a good selection available for mower/stationary engine applications in "plastic" & metal.

 

Hope I have been of assistance.