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Minimum Fuel Test in RV-14A

J

Jumpbat74

Member
I'm getting close to doing the fuel flow test on a RV-14A. I would like to start with what minimum usable fuel in the tanks is for a Vx climb pitch attitude. My plan is to lift the nose and set it on blocks at that pitch attitude, start with a known amount of fuel, then run the fuel pump until the fuel flow quits and calculate what is left in the tanks. The next step would be to do the test with the plane in level flight attitude.

(1) Can anyone with a 14 or 14A tell me what pitch attitude you are seeing in a Vx climb?
(2) If you have done this test can you suggest how much fuel in each tank to start the test with?

Thanks!
 
I've seen threads on here where people have done all sorts of stuff to get a nose high attitude to see where min useable fuel happens in a climb. They always report back that it's useable down to just a few ounces.

The rule, at least for pt. 23 airplanes, doesn't specify a Vx climb. It just says something like; "most adverse fuel feed condition expected in intended operation."

Since the fuel pickups are in the back corner of the tank, it seems to me that fuel starvation would likely happen in a level attitude before it happened nose up where all the fuel has run to the back.

In fact, logically, the "most adverse" situation would be nose down to the point where you'd be in a steep descent which would be what, maybe 5 degrees nose down?

Am I way off base in my thinking? I've got this in my future for my RV7 so please come back with your results.
 
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Desert Rat said:
I've seen threads on here where people have done all sorts of stuff to get a nose high attitude to see where min useable fuel happens in a climb. They always report back that it's useable down to just a few ounces.

The rule, at least for pt. 23 airplanes, doesn't specify a Vx climb. It just says something like; "most adverse fuel feed condition expected in intended operation."

Since the fuel pickups are in the back corner of the tank, it seems to me that fuel starvation would likely happen in a level attitude before it happened nose up where all the fuel has run to the back.

In fact, logically, the "most adverse" situation would be nose down to the point where you'd be in a steep descent which would be what, maybe 5 degrees nose down?

Am I way off base in my thinking? I've got this in my future for my RV7 so please come back with your results.
Click to expand...

No, you are not off base. Most adverse is going to be when the potential for the fuel pickup is the greatest. In our airplanes, this is most often in a nose down attitude with very little fuel left in the tank. Nose up, fuel flows to the port and you can drawn all but a couple of ounces.
 
I’m not sure why I got focused on climb attitude. Now that I look at the fuel tanks these responses make total sense. Doing a nose down attitude is actually easier since that is where the plane wants to go as you jack it up. Incredibly helpful. Thank you!
 
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Just me

When I did my test, I did at min fuel.
The electric pump would not self prime, so I had to blow air in the tanks to provide a little head pressure to prime. then the pump ran to very low fuel level.

So if I run a tank dry, I will need more than the min amount of fuel to re prime the pump.
 
Just a thought to share here with respect to the "nose up" test condition.

If we look at the history of aircraft development we'll see that fuel systems almost all were gravity feed systems in the early days. This becomes the historical precedent which has been used to guide many regulatory and advisory developments.

In the case of fuel flow testing, the nose up flight condition is the condition where, irrespective of pick-up point location within the tank, minimum gravity-powered head pressure is present to push fuel into the carb (yes, back then they were all carb's, too!).

Even today in the context of a low-wing RV with electric and engine-driven fuel pumps the challenge of getting gas to move uphill from the wing to the engine remails a critical consideration.

With this in mind I would suggest that we not get too focused on the location of the fuel pickup but rather zoom back out to look at the bigger picture. There may be good reason to pick both a nose-high and a nose-low test configuration. In most cases the nose-high configuration will remain the worst case scenario, simply on the basis of loss of natural head pressure to help fuel move toward the engine.
 
Interesting observations from all.

OP, if you don't mind, please report back with your findings regarding nose low. I've got a taildragger and it's so light in the tail that it would be a little sporty to get the tail high enough to represent a significant descent attitude.

What does everybody think about connecting a hose downstream of the engine driven pump and extending it vertically a few feet to simulate pushing uphill against an extreme nose up attitude? I was contemplating that, but after considering Canadian Joy's comments I'm not sure it would actually be representative given the fact that the suction side wouldn't actually be pulling uphill unless I get the tail down a bit.
 
PilotjohnS said:
When I did my test, I did at min fuel.
The electric pump would not self prime, so I had to blow air in the tanks to provide a little head pressure to prime. then the pump ran to very low fuel level.

So if I run a tank dry, I will need more than the min amount of fuel to re prime the pump.
Click to expand...
I believe your engine driven pump will not require priming, plus you have air blowing in thru the vent.
 
Some additional thoughts and information.

During my building project there were two issues I wanted to resolve. One was to determine usable fuel and the other, more important was to verify the integrity and functionality of the fuel system and its ability run the motor at high power settings. AC 90-89B describes a test which covers all that. I assumed the reason for the nose-high attitude was to put the boost and engine pumps farther below the fuel servo or carb inlet and ensure the fuel system can still push fuel up-hill at the prescribed quantity and pressure.

I think the main focus is on system integrity and proper installation. We are not testing a fuel system we designed from scratch. We are making sure we put the darn thing together correctly according to the instructions. One loose fitting can cause your flow test to fail completely. Ask me how I know.

Your DAR may be impressed by your thorough testing if you do this test nose-low but I would expect them want to see it done per the AC as well. I may be mistaken on that.

My stock RV-14 fuel system works great nose-high (+15 deg) until the fuel level reaches about 4-6 ounces and then starts blowing lots of bubbles. Beyond that, I think the idea of unusable fuel becomes a little subjective as someone else pointed out. I’m confident my motor would keep running in a descent with around 1.5-2 gal in a tank but if that’s all I have onboard I’m looking for a cow pasture to land in ;)

From AC 90-89B Section 11 - Additional Engine Tests

e. Fuel Flow. A fuel flow and unusable fuel check is a field test to ensure the aircraft engine will get enough fuel to run properly, even if the aircraft is in a steep climb or stall attitude, and is accomplished by:

(1) Place the aircraft's nose at an angle 5 degrees above the highest anticipated climb angle. The easiest and safest way to do this with a conventional gear aircraft is to dig a hole and place the aircraft's tail in it. For a nose gear aircraft, build a ramp to raise the nose gear to the proper angle.

(2) Make sure the aircraft is tied-down and chocked. With minimum fuel in the tanks, disconnect the fuel line to the carburetor. The fuel flow with a gravity flow system should be 150 percent of the fuel consumption of the engine at full throttle. With a fuel system that is pressurized, the fuel flow should be at least 125 percent. When the fuel stops flowing, the remaining fuel is the “unusable fuel” quantity.

(There are more steps in the procedure. I just pasted the first two)

Anyway, hope this is useful information.
 
With minimum fuel, I would be more concern when banking steep rather than descent phase, like when we turn downwind to base or base to final.
 
Very good points about doing both nose high and low attitudes and partly why I asked what pitch attitude people were seeing in a Vx climb. Certainly there are times when one might pitch steeper than Vx but it seemed like a good starting point. When I flew the factory 14A I remember the pitch attitude being very steep but I never thought to write down the actual pitch angle for Vx. Anyone have that for your 14?
 
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Bavafa said:
With minimum fuel, I would be more concern when banking steep rather than descent phase, like when we turn downwind to base or base to final.
Click to expand...

In a coordinated turn, there shouldn’t be any difference or fuel sloshing. If you slip or skid the turn, then all bets are off..
 
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