Fuel Flow Tests

[BillL]

Originally Posted by rmartingt

Why restrict the flow? Bret has an electronic injection system so you expect to see a substantial return flow to the tanks even at full power.


I'm asking because I'm basically going to be using the same thing in a few years...

I did not know that, never mind. I just saw the 45 GPH and it is typical of the free flowing standard pump. Some thinking would have to occur to determine how to test this in a representative manner. He still might need to just run the system to charge the loop, then bleed off the 125% number from there. That might take a ball valve.

The point is to have pressures and flows representative of max fuel flow x 125% while the tank is being emptied. This way an accurate remaining (unusable) fuel amount can be measured. It might take a different technique to assess the shut off point, as it can suck air and still have some loop pressure. The basic test principle and objective is the same, but not the exact process.

[N427EF]

The unusable fuel test is best done in real flight testing.
There is no good way of simulating aircraft attitude for this test unless you have a hoist to elevate the entire aircraft and position it in different attitudes and don't forget you'll be fooling with fuel flowing at all times and collecting measurements, so save this one for flight testing.

How I did it:
Running the left tank down to 2 Gallons and start testing.
Establish Vx until engine quits.
Land airplane on full tank and refuel.
Note quantity and calculate unusable fuel.
Repeat for right tank.


For the RV-10 and the RV-8 it was less than a gallon and for good measure I
use 2 gallons as the absolute minimum and have my fuel warning level set a 5 Gallons. In 400 hours of flying the 10 I have yet to get anywhere near that 2 gallons of unusable fuel.

[DanH]

Ya'll are talking about two different things, (a) fuel flow and (b) unusable fuel.

In terms of fuel flow, Bret is good to go. He has 45 GPH after the regulator, while maintaining 35 PSI in the fuel rail. Tee a valve into the fuel rail and bleed off the equivalent of fuel burn at WOT full rich plus some for the FAA (say 17 x 125%, or 21.25 GPH), and the tank return will flow at 23.75 because rail pressure is regulated. The pump supplied the rail with 45 GPH either way.

Unusable fuel is not found at Vx, i.e. highest expected angle, nose up.

Here's what the FAR dictates for a certified airplane:

23.959(a) The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank.

Most RV models place the pickup in the bottom rear of the tank. This SB provides an illustration:

http://vansaircraft.com/pdf/sb06-2-23.pdf

So, the most adverse fuel feed condition of all possible operations would be nose down. How far down? Ahhh, let the debate begin! One camp will say pitch down so as to result in best glide, another as appropriate for a typical power off descent, and a third as appropriate for nose down at Vne, throttle against the idle stop. In all of these, the fuel pool in the front of the tank is unusable, the only difference being how much.

Of course, we could argue the most adverse condition would be hard slip toward the tank in use. That would make about half the tank volume unusable

[bret]

Good points from everyone, thanks for the input, so with those that are flying, what is (your) unusable amount for nose down, best glide angle, I think that will be the most important figure as far as unusable, like Dan noted, the pickup is in the rear of the tanks and while pointed down, we will have (gallons) I think? unusable?

[Robert Anglin]

Originally Posted by bret

Good points from everyone, thanks for the input, so with those that are flying, what is (your) unusable amount for nose down, best glide angle, I think that will be the most important figure as far as unusable, like Dan noted, the pickup is in the rear of the tanks and while pointed down, we will have (gallons) I think? unusable?

About 1.5 in the left tank an about 3 in the right tank. We have a full mil.spec type inverted pickup in the right tank. I set my red alarm at 5 on each side, with yellow set at 8. That's just me, I know I want hurt anything running a tank dry, but I just don't like to do it if I don't have to. So far, so good.
I would still check with your DAR or FSDO and look the system over very well. Van's fuel tanks and stock plumbing if installed per plans is good as is for the engine rated for that air frame. Just me talking, Yours, R.E.A. III #80888

[Jesse]

All regulations aside (I think that has been covered), when I am planning a flight considering max range I burn both tanks down to half or third tank then I run one tank dry, slipping the plane so the fuel in that tank comes inboard. I always do this in cruise, so level flight attitude, and run it until the fuel flow starts to drop in a carb or until fuel pressure starts to drop when fuel injected,mother I switch to the other tank. In real life, that leaves probably a quart of fuel or less in the tank run dry. Then I know where all of my fuel is and have less chance of unporting that tank. Again, this may not be able to be used in an official unusable fuel calculation, but in real life, it works this way if handled right.

[N427EF]

Unusable fuel is not found at Vx, i.e. highest expected angle, nose up.

Here's what the FAR dictates for a certified airplane:

23.959(a) The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank.

Most RV models place the pickup in the bottom rear of the tank. This SB provides an illustration:

I knew this was coming and I can't argue with what the regs say.
In practice however, you don't need fuel in a nose down attitude glide.
If you need altitude you climb and the fuel will be right where you want it,
all the way back in the tank where the pick up tube is located.
I know, it is a simplistic approach but very reliable no matter what the regs
would have you do.

[az_gila]

Originally Posted by N427EF

I knew this was coming and I can't argue with what the regs say.
In practice however, you don't need fuel in a nose down attitude glide.
If you need altitude you climb and the fuel will be right where you want it,
all the way back in the tank where the pick up tube is located.
I know, it is a simplistic approach but very reliable no matter what the regs
would have you do.

Since we don't have to meet the certified aircraft requirements, but can follow the suggestions in AC 90-89a, I say just to follow the Advisory Circular and do the test nose high. This is the first test flight condition the FAA/EAA is addressing.

page 23 here (PDF page 30) -

http://www.faa.gov/documentlibrary/m...c%2090-89a.pdf

[DanH]

Geeezzz Louise....forget the regs.

Given a standard Vans tank for 3-4-6-7-8, what normal flight condition results in the most fuel unreachable?

What flight flight condition results in the most fuel available?

Which one were we trying to determine?

[ILikePike]

I am about to do the nose high test in my RV-8. The form that came in the EAA registration set of documents refers to " most critical attitude (such as initial take-off attitude". What angle is this? Three point attitude, Vx or critical AOA (stall)? From a practical perspective should I test with the tail wheel on level ground or put the tail wheel in a ditch to achieve some greater angle of the canopy rail?

What does Lycoming say about the fuel consumption at max take-off power for an XIO-360-M1B?