AOA vs fuel level in main tank

[PA1195]

Re: AOA vs fuel level in main tank

In the UK, the old BCAR* requirements for fuel flow, required 50% more than the normal flow at maximum angle of climb at stated minimal fuel capacity. These regs are not dissimilar (in principle) to the FAR regs.

In essence, these regs mean that the carb should see a head of pressure of 150% of the carb's normal intake.

A few years ago we did such a test on a Taylorcraft fitted with a C85, in the hangar. Keeping the tailwheel on the ground, we raised the main gear 12" to simulate an excessive angle of climb (first picture below).

Then we opened the fuel pipe at the carb and verified that we had the 150% of fuel flow (second picture below).

It is in this type of way that aircraft manufacturers determine the minimal fuel requirement in the header tank, and to approve fuel flow modifications. That is why there is a minimum fuel level in the likes of the F19. You may notice similar restrictions in all sorts of aircraft.

Rob

*BCAR = British Civil Aviation Requirements (now superseded, in the same way that US CARs have been superseded by FARs)

Very good information Robert. This is what I wondered about at first post, as I'm sure others who unfortunately have had a stoppage.

The Elephant in the room as I see it...I'll say it and take the heat as I'm too old to care about criticism...is what happens when an STC or combination of STC's is approved, and except for fuel line diameter and other system requirements like venting, there's no mention of limits to useable fuel?

For example, the C-85 installation STC allows for two motor mounts, the longer raises the carb inlet I assume to some degree. If the power-driven demand for fuel approaches that of the F-19 via the Stroker STC, then shouldn't minimum fuel be considered in aircraft that employ non-wing fuel tanks? If all or a combination of three STC's are employed...the C-85 installation, the C-85-Stroker, and the Airframes large tires...then certainly that combination should be tested for compatibility and thrust reliability.

Just some suggestions for discussion and not trying to shoot the elephant. I realize there's shared responsibility when it comes to combining multiple STC's.

Typically it goes like this: "Compatibility of this design change with previously approved modifications must be determined by the installer"

Gary

[Hank Jarrett]

Re: AOA vs fuel level in main tank

This is similar to one of the flight tests I planned to do in my plane (engineers can't resist doing flight testing). One thing that isn't properly being considered is that ground angle is NOT the measurement you need. The EFFECTIVE angle is the sum of the vector shift due to acceleration PLUS the angle of climb. To find that you need to know the maximum sustained climb angle possible for your plane plus the maximum sustained acceleration possible. Along with that you need to know how long your plane can sustain that acceleration and angle since un-porting the pickup is not necessarily going to stop the engine (remember you are drawing fuel from the float bowl and gascolator and they will delay fuel starvation, but will NOT help with pressure). Luckily it turns out MEASURING these things is a LOT easier than calculating them.

What I planned was to do some rapid accelerations and climbs with a recording protractor and "G" recorder mounted in the cabin. After the flight the maximum aft angle of the indicator will have been recorded and THAT is the angle we want for ground testing to insure we will not un-port the pickup or loose too much pressure (calculated with the "G" measurement). This will be a worst case condition since I know I can't sustain either the angle or acceleration for long with a little 65HP engine.

I am very disappointed that the FAA never funded our follow-on contract for AoA indicators since I was really looking forward to using the AoA probe to do some of my own testing (we proposed that we get to keep the test instrumentation to reduce the cost of the contract to the FAA), after all, what were THEY going to use them for? My partner really wanted the PC we proposed and I wanted the probe and we were going to buy them after the contract through an invoice reduction in lieu of payment.

Oh well, I guess we will have to build the probe anyway and our PCs are getting a little old. ;-)

Hank

[PA1195]

Re: AOA vs fuel level in main tank

Thanks Hank for an engineer's perspective. I see what your noting...a combination of acceleration "G force" plus climb angle. I'd like to see a drawing or link to such equipment - recording protractor for example. That NACA Report where they tuft-tested the T with various washout and LE slots had some instrumentation but likely not what you're suggesting.

That report with test instrumentation described. Note that the main fuel tank was disabled (to allow for later CG adjustment) and the engine fed from the wing tanks: http://ntrs.nasa.gov/archive/nasa/ca...9930083935.pdf

It all begins on T/O roll and extends through climb. My friend and others that have experienced the sudden quiet should not have to repeat that event. Maybe at altitude during a test but not close to Earth.

This brings another question: What effect if any would the G and climb angle have on float's behavior in the carb? It would seem it might initially raise the fuel level and/or raise the float, which might respond by restricting the fuel inlet. Maybe that's not an issue.

Gary

[M Towsley]

Re: AOA vs fuel level in main tank

Hank,

I just read that NASA did some testing and realized that AOA's really did not help folks from exceeding AOA, etc. I will see if I can find that article.

Here it is, AOPA magazine: https://www.aopa.org/News-and-Video/...pkmKEVw9F_xtfA

As to whether the gas tank indicator wire reads on the ground or in-flight I have mine, (obviously), set for in-flight. I have a dipstick that was calibrated for each gallon from an empty tank. Never any question as to how much fuel is on-board prior to flight.

[3Dreaming]

Re: AOA vs fuel level in main tank

On the different aircraft I flew with a wire type fuel gauge I could tell how much was in the tank in either 3 point or in flight attitude.

[astjp2]

Re: AOA vs fuel level in main tank

I have two modes of fuel calibration...I can see fuel and the tank is full, or I just consider it empty and get it filled.....never a question then. Tim

[Hank Jarrett]

Re: AOA vs fuel level in main tank

We found LOTS of problems with the "recommended" and commonly available AoA indicators. There were even worse problems in how they were being used and the training the FAA was requiring to use them.

PROPERLY designed indicators for AoA were very effective but only if the pilot was trained to understand what he was being told by it and how to use the indicator to stay out of trouble, and if he was already IN trouble, how to get back out.

Unfortunately the "Approach Indexer" is the most common AoA indicator, mostly because it is the cheapest to make. It works GREAT.....if you are trying to land on an aircraft carrier. Otherwise it is only marginally useful, and then only if you fully understand what it is telling you. Most pilots have no idea how to use one properly.

A well integrated AoA with proper human factors built in can be VERY useful in preventing stalls and making the most efficient recoveries possible. Until the people making the decisions on equipage actually understand the problems and the proposed solutions, they are pretty much a waste of money......a lot like current ELTs.

Hank

I hope to put a USEFUL AoA on my plane one day and see just how efficient it can make my slow flight and stall recoveries. Based on the simulations, it can be a very useful instrument.