ETLB Squawk Forums — Archive

I brought this over to a new thread because it is more about CG than about the fuel system itself. Also didn't want to mess up the other thread.

Quoted from Keith103 Many ultralights have no Facet pump.
The pulse pump does the fuel feed.

I have a different issue altogether.
My CG is at 12 inches back from Leading edge.
I had to add almost 6.25 lbs of weight inside the
rear fuselage, under the stabilizer.
This moved the CG back by just 1 inch to 13 inches.
Can't afford to add any more weight.

I would still like the CG to move back by one more inch to at least 14 ".
I have a 5 gal poly tank inside the forward fuselage.
Not sure if this is what is called a header tank.

I am thinking of adding a small 1.5 gal tank behind my seat.
Unfortunately it can only be fitted inside the turtle deck
at my neck / head level.  (Already ordered a small tank.)

The plan is to fill forward main tank with no more than 2 or 2.5 gal.
1.5 gal fuel is available in auxiliary tank.

I was thinking of adding a facet pump.
But on second thought, I felt it may not really be
necessary. The pulse pump should do its job.

I am doing all this by trial and error.
Has any other builder tried to solve CG issue by
dispersing fuel to the back ?

https://www.wish.com/product/5.....l=true&share=web

Quoted from PUFF why not add a battery in the back, instead of doing all this? I had the opposite issue, so added a battery up front.

Quoted from radfordc

Your plan is certainly feasible but it does introduce a potential problem.  You have to be very aware of the fuel level in the front tank so that when you are running it "low" you don't run it "empty".  You say that you would never do such a thing....well, if you do you won't be the first pilot to accidentally run a tank dry.  Managing the switching of tank valves has been a factor in many accidents.

BTW, how far will the CG move aft with 9 lbs of fuel behind you?

Quoted from Stilson Replace the weight you installed in the tail with an atv battery (the same weight pretty much), and install a facet pump from the front tank to the back tank with the back tank overflow routed to the front tank. The front tank will run empty first and you can use all of it and not have to worry about switching valves.  Have two little posts to hook up a battery tender to charge up while it's parked.  An atv battery should be able to empty a few tank fulls.🤷🏻‍♂️

Quoted from beragoobruce You do need to be a bit wary of using fuel as ballast, because it means your c.g. changes during flight. This is not ideal, and could catch you out.

A better idea is a big battery in the tail, as has been suggested. At least the weight is doing something useful!

Quoted from bob.hood Just a suggestion,

but perhaps you could fit your 'header' tank low and somewhere that's CG neutral or aft of CG, and then drain your main tank into it. You wouldn't need to worry about having to switch tanks as the 'header' tank will be the one the engine gets its fuel from. That's the setup I have on my Max, where I have the standard tank, and an 11 ltr tank bolted under the fuselage just under the seat. I've attached a diagram showing the approximate location of the tankage, and a couple of photos of the tank in place.

Posted by Bob.Hood

Thanks for all the replies and suggestions. Much appreciated.

Yes I will certainly remove the 6.25 lb of dead weight and replace it  
with something more useful. This is merely a temporary set up till
I settle on a more enduring solution.

The 12V  Lithium battery in my cockpit weighs only 2.4 lbs. Moving that
to the rear fuselage was the most appealing option. But then I realized
that would move the CG by a mere 0.3 inch, which means I would
still need to add more weight at the rear.

Even if I keep only one tank, CG will continue to change throughout
the flight as fuel is being consumed, unless the only tank is located
on or very close to the CG. In my plane it is not. I agree, while using 2 tanks, I
need to make sure that for all possible combinations of fuel levels
between the 2 tanks, CG remains within the desired range.
Also goes without saying,  the possibility of mismanaging tank
selection is always a very real possibility.

RadfordC - regarding your question of how much will the CG move
in the suggested layout: I guess once the gross weight is known,
which in my Max is 470 lbs including the 6 lbs of added weight,
displacement of CG due to re-distribution of weight along
the longitudinal axis can be calculated even without a spreadsheet.

In this case to move the CG to the aft by 1 inch, I need to change
the moment arm by 470 x 1 inch = 470. For moving it 2 inches, the
change needed would be 470 x 2 = 940.

To move the CG aft by 1 inch, I need to move the 1.5 gals (9.5 lbs)
of fuel from main tank to the rear tank by a distance of 470 / 9.5 =
49.5 inches.

I need to measure the distance between the center of existing tank,
and the center of the new tank, and if it is going to be close to 4 feet,
then the CG would have moved 1 inch to the back. A fresh W& B may
not even be necessary, since no new weight is being added.

I hope the above calculation is right ( ?? ).  

Hi Keith

I recently cam across this article on fuel systems in general. you might find it interesting.

https://heavenboundaviation.co.....rticle-with-logo.pdf

Bruce

Thanks, Bruce. It is a very nice write-up on fuel delivery system.
I recall ULBuilder had also posted this link sometime back on another thread.