Fuel Flow Test - Pitch Angle

[robcaldwell]

I'm now ready for fuel flow testing. I thought I read somewhere that I need to have my deck angle at 30 degrees for the fuel flow test. Now I cannot find that information anywhere. In the Eric Newton's manual he references Russ Erb and says "According to his numbers the fuselage should be placed at about 19 degrees nose up attitude." I have the Part 23.955 guidelines for Fuel Flow, just cannot find the proper pitch angle to perform the test.

[Mark Goldberg]

The more the merrier Rob. Though it is possible to do something not very stable and have a problem. Of course that is to be avoided. I have never been very scientific as far as the deck angle when doing the fuel flow test. Just get the nose as high as you can safely and securely, and that will likely be good enough. Mark

[auburntsts]

Rob,
I don't have the answer to your question but here's the link to the EAA Webinar on aircraft fuel flow testing: https://www.eaa.org/Videos/Webinars/.../2195707082001 It might have what you're looking for.

[jaredyates]

I used 2 concrete blocks under each main wheel. I had to place them one at a time to keep the bank angle manageable.

[whee]

As I recall the guidance is the angles for the tests need to be whatever angle the max angles are that the plane can sustained during flight. I didn’t think it was very realistic (safe) to raise the nose high enough to get to maximum climb angles the BH can achieve. I stopped at 20deg and tested there. Then I made sure to not exceed that angle in my initial flight testing. Part of my later flight testing included real life flow tests, monitoring fuel flow and pressure, at the maximum sustainable AOAs.

[Battson]

We used exactly the same approach as Whee, our flight test plan which is available here online should cover this off.

[alaskabearhawk]

AC 90-89B, Amateur-Built Aircraft and Ultralight Flight Testing Handbook

Para 1-19e. 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.

(3) The formula for fuel flow rate for a gravity-feed fuel system is 0.55 times engine horsepower (HP) times 1.50. This gives a fuel flow rate in pounds of fuel per hour. Divide the pounds-per-hour number by 60 to calculate pounds per minute, and divide again by 6 to calculate gallons per minute. To get gallons per hour for Avgas divide pounds per hour by 6; or multiple gallons per minute by 60. For a pressurized system, substitute 1.25 for 1.50 to calculate the fuel flow rate.

[jaredyates]

Just for grins, I opened up a few of my flight logs on Savvy Aviation and checked what the Dynon data recorded for pitch angles. It looks like I'm seeing pretty routinely 15-16 degrees on climbout. Occasionally it gets to 17 degrees but that is never sustained and is probably maneuvering. Your sustained pitch angle will be higher than mine, having so much more horsepower, but that will at least get you in the neighborhood.

[Bcone1381]

I know we all worry about raising up off the floor an aircraft and having a tragic accident. If 150% of takeoff fuel flow happens at 17 degrees, I wonder what pitch attitude will provide "Enough" or 100%. Also I might consider how to lower the tail....dig a hole, or remove the tailwheel to raise the pitch for the test. This will be flight tested.

[Enewton57]

Here is what I did:

http://mybearhawk.com/finish/fuelflow.html

Eric Newton

[Dpearson]

That's a great writeup! How did you solve the fuel flow sensor 9 GPH flow restriction issue?

[robcaldwell]

Outstanding input and help from everyone! Thank you so much!

[Enewton57]

That's a great writeup! How did you solve the fuel flow sensor 9 GPH flow restriction issue?

Sorry there was an error in my previous response. After removing the fuel flow sensor and re-plumbing the lines, the fuel flow was fine.

I left the sensor off. Later on, I got to looking at the sensor and found a small sliver of rubber in the inlet. My theory is that it came from inside one of the hoses used at the wing to fuselage connection. I most likely pushed the hose onto the end of the tube at an angle and sliced a bit of the inside of the hose off. That sliver of hose worked it’s way through the system and became lodged in the fuel flow sender unit.

It’s been a while, so I had to go back and look at my old notes to jog my memory.

[Battson]

Sorry there was an error in my previous response. After removing the fuel flow sensor and re-plumbing the lines, the fuel flow was fine.

I left the sensor off. Later on, I got to looking at the sensor and found a small sliver of rubber in the inlet. My theory is that it came from inside one of the hoses used at the wing to fuselage connection. I most likely pushed the hose onto the end of the tube at an angle and sliced a bit of the inside of the hose off. That sliver of hose worked it’s way through the system and became lodged in the fuel flow sender unit.

It’s been a while, so I had to go back and look at my old notes to jog my memory.

I also found two little rings of rubber stuck in a hose near the engine. We never found them during the testing, because they were in a house downstream of the required test point... they were in the line from the fuel servo to the fuel distribution spider. Once the engine starting running, they were blocking fuel flow.

The engine would not develop full power, then I went looking and found them by poking a spring-wire offcut from a control cable through each hose. I was not expecting to find anything, so I was shocked to find these little guys.

20131025_092505_zps9350f9f7.jpg

I hypothesized that they were created when I pushed the mandrel in to assemble the hose connectors. They could not have come from anywhere upstream in the system, because the next part upstream (servo) has the finest filter in the whole system.

P.S. never use photobucket.

[JimParker256]

+1 on Photobucket. Used to be a great utility, but nowadays, it's just annoying as can be. Their dang logo is ALWAYS right on top of whatever it is I'm trying to see, and I really hate having to open the link, sign into their site, just to see an image that was 99% visible before going to all that trouble. As a result, I'd never use them, just on principle... What were they thinking?

[Aero_tango]

I initially raised the aircraft off the floor, but it wasn’t enough to get the “extreme” condition.
Instead of pushing my luck with an unstable condition, I decided to disconnect the fuel line from the carb, and connect the line to a temporary aluminum tube. I raised the tube up to a 5 gal container and run my test. The idea is to mimics the “head pressure” from the fuel tank to the carb, for a given deck angle. I used the side view drawing (page 1 of your plan) to calculate the height difference (head pressure) between the lowest (aft) fuel pickup point of the tank and the carb fuel inlet, for the given deck angle you want).
Let say that this height difference is 10”, then have your aluminum tube end point 10” lower than the fuel pickup point in the tank, and run your test. Mind you that the aluminum tube adds some more restriction to the flow, but if you can get your targeted fuel flow, you have more than enough flow...it works for me.
See attached photos.
Mike.

DF49C9F3-79FB-4B2F-B7E9-97089D53E660.png35668E28-CAEF-4541-AFCA-4EAF58875142.png

[marcusofcotton]

Mike, might it be possible that with your method you could be getting flow from front and rear ports but if you had the plane at the steeper angle it might be only coming from the rear ports - thus possibly resulting in a lower flow rate?

[Aero_tango]

I ran my test with only 3 to 5 gal in the tank. The plane being in a 3-point attitude, the fuel was from the rear port only. Actually, that’s a good point, the test should be run with minimum fuel, which gives the least head pressure.

[Battson]

Mike, that is a clever idea and I like that you are thinking outside the box.

The other potential issue with that test, the flow restrictions are much lower than they would be in a real situation, which will change the pressure they experience and the way fuel passes over them. Fuel flowing at high pressure will behave differently to fuel flowing at lower pressure. I can't say whether how much of a difference it would make, however mathematically it would be a little different.

An aside,
In case it hasn't been mentioned yet, you can also remove the tailwheel to increase the angle.

[Aero_tango]

In my case, my O-540 has a mechanical fuel pump. I also have an electrical boost pump. I ran my test with the boost pump on. With gravity only, I wasn’t able to get my targeted fuel flow at high deck angle and minimum fuel...to much restriction in my system, I guess.
With the boost pump on, I pass with flying colours. Mike.