Thorp Air Command - T18.net

At some point I'm going to have to do the fuel flow test to satisfy Transport Canada. What angle would the fuselage be at during climb? (I found a couple of numbers on the old Thorplist, but nothing definitive.)

-- dw

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In theory, theory and practice are the same. In practice, they are not.

Hey fellas,

I'm using 17 degrees for climb attitude with fuel flow testing. Don't recall where I got the number, but it oughta be conservative. Here's my new question: Whadya think of this? I measured 7.8 gph with gravity feed and just 6 gallons in the tank.

The FAA requirement in the US is 125% over expected worst case. I'm using only gravity feed through all 3/8" tubing and hose, a ball-valve shutoff (3/8" clear throughput), minimum bends and a gascolator. Measurements are at the float bowl drain to include effect of float needle valve. Restrictions are pretty low since before going into the carb I can get 21 gph for this condition.

Engine is O-320-B3B, and until I fly it I really don't know what fuel consumption will be. I don't know why anyone would ever want to take off with only 6 gallons, either. Any comments if 7.8 gph @ 6 is adequate? Should min. fuel requirement be higher to satisfy the 125% rule?

Thanks, Les.

With gravity feed you should be at 150% of the WOT fuel flow for your engine and it is measured at the end of the fuel line before it goes into the carb. My O290 WOT fuel flow is 12 gph so I needed 18 gph to pass the flow test. The deck angle should be at the max climb attitude which would be at Vx speed, max angle climb. Then you find what the minimum amount of fuel is required to pass the flow test, I needed 5 gal in the tank, or else you rework your fuel system to remove some restrictions.

Happy testing.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

So, would one do Vx at max gross,....max forward CG,.........does an inclinometer measure different deck angles at different flying weights and CG locations?......or is that overkill/irrelevant for fuel flow calc purposes........que Wonder, Cubes or Spanky

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Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

My thoughts

Les, you might be golden at 21 gph..... I think you only need to measure to the carb inlet, as Bruce said. You just have to make sure your line is at the same height as the fitting when you do the test. I've read some instructions that suggest you take the finger screen out and perform the test through the screen, but I don't think that's correct.

Get your fuel flow at 100% power from the lycoming operating manual. Find the chart for your engine. You'll have to convert from lb/hr.

The actual angle is not defined in the rules that I have to follow; I'm pretty sure the wording came from the FAA and/or EAA. Our requirement is for "maximum climb angle." I've been using 16.3 degrees so 17 is in the ballpark.

Maybe some of you guys with actual flying examples of the airplane can put a digital protractor on WL42 and gather some real world data for climb angle at Vx. Of course the clinb angle that can be achieved depends on HP, so we'd need to know that too.

My original system as-received was 1/4" on a 168 HP installation! The system did not actually meet 100% of demand at 16 degrees. Clearly no one had ever done a fuel flow test on the airplane. Regardless, it did survive 1100+ hrs in that condition.

So here's my data point:
I changed everything to 1/2" all the way to the gascollator outlet, and I straightened out the flow as much as I could. Now I'm well over 100% but shy of 150% of 15.5 USGPH based on the second last gallon.

I love the simplicity of gravity, and I haven't given up on it yet, but I'm starting to question if there is physically enough head between the tank outlet and the carb inlet to satisfy the 150% requirement by gravity alone on my particular airplane with the bigger engine. (The head in T18's is a small fraction of what Tony Bingelis recommends for gravity systems. Just saying.)

Bruce, I think I see what you're saying...... the lowest fuel level that is capable of supplying the flow that meets the test requirement becomes your minimum fuel that you report in your W&B, correct?

D

Don't forget that a properly designed & positioned vent system is going to increase the available flow rate when you are actually flying. In my non-expert opinion, 21 gph that you are seeing should be plenty.
Dick - 1/2" is probably overkill, but better too big than too little.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

John is correct - even before my current vent system, I launched off with a full tank for Sin City a year and a half ago, and about 10 minutes into the flight up through about 9,ooo MSL, the standpipe on my fuel level sender broke away from the flange. This resulted in fuel geysering out of the fuel tank and cascading down around my tootsies at a very alarming rate. I smelled the fuel about the same time as I felt it on my ankles - I Immediately shut down everything electrical, turned for home and reached behind the pax seat for the fire extinguisher and had it close and ready. Prolly should have landed at the only other close airport and saved a couple of minutes, but nursed it home anyway. I'd wager at 150 knots, I might have bled that tank dry in 30 minutes. If I remember correctly, there was about 10 gallons missing for a 15 minute flight back, less the 2 gallons I had prolly burned until the standpipe let go. That's 8 gallons in 10 minutes - 48 gallons an hour - I had to take out all the carpet and floor insulation and clean up the floor and under the plane. I got lucky.

At speed there is some decent pressure in the tank. Having said that, you have to measure it on the ground. But know whatever flow rate you get on the ground will be increased in the air.

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Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

Man, that's a pretty graphic demonstration of the pressure created Frase! Glad you survived!
I'd sure like to hear if anyone has measured the pressure created in the tank by the vent. All it would take is a tee off the vent line with a tube going to a gauge. Maybe the pressure is actually equal to or greater than the pressure created by a common low-pressure (for carburetor use) fuel pump. I'm still a big fan of all gravity feed in a T-18 - not that I want to start that whole discussion again!

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

WHOA HEY!!!!!!! Let me be clear, guys, I was in no way trying to ignite the pump-no pump debate which has been discussed ad-nauseum already since the dawn of the newsletter. Let's not do it again.

I absolutely acknowledge and agree that the contribution of a PROPER vent installation will provide all the additional pressure (and more) required to meet engine demand for the biggest of engines in the T18 fleet. I can put my hand on my heart and say that my redesigned gravity system is perfectly safe and I would take my 87 year-old mother flying today if I could. (And if she'd agree to it. )

I was talking strictly about meeting the letter of the law with respect to the fuel flow test. That's different. (Because the law is an ass.) Unfortunately, as far as my government is concerned, the dynamic pressure contribution of the vent system is not credited. It clearly should be because it is an integral part of the T18 fuel system, but the Feds don't recognize it.

So, doing the math, Les, I looked up your FF at 100% power and the chart says 14 gph or maybe just under. So 21 gph at the carb inlet fitting is right on the money. If I understand correctly from what Bruce F said, if you needed 6 gallons in the tank to get 21 gph flow at the carb, then 6 gallons should be stated (placarded?) as your minimum fuel required for takeoff. This is a little subtlety of the fuel flow test that I did not understand until now, but it makes sense. I'm very thankful that Bruce mentioned it in this thread.

..... I'm also a big fan of gravity; it keeps me grounded.

D

Its not just takeoff but also go around on landing. So, for my case, 5 gallon is my absolute minimum fuel, actually I want to be on the ground with no less than 7 gallon, 1 hour reserve

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

That's how I plan also, Bruce.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

You guys cut it close! I believe bout 9 gals is the lest I have landed with, go ahead and say it I know I'm chicken, but I am alive after flying behinds an un tested fuel delivery system. I too did the 1/2 in tube to the gascalator, 3/8 to the carb. I have 3 - 90 deg turns in the fuel line, the Gasgalator itself accounts for one of em. I fabricated the bulkhead fitting where the line goes through the fire wall, it is a sweeping turn. I can say this, with the amount of fuel that runs outta that fuel line with 5gals in the tank, if it takes more than that, at full throttle I will be empty in less than 5 min after topping the tank off.....I did the fab work on the gaskalator also, it is effective and feather weight. Photos on the old website........Dan

I'm with you Dan . When my fuel gauge approaches 1/4 ( 7 gallons) of a tank my Farfannewgen starts to get a rash !

I feel about the same: I won't take off with only 8 gallons, I prefer 16.
On the other hand, I'm not too sure about filling the tank to the top?
I have both mechanical & electric fuel pumps so I might not be effected, but I've heard the tank vent can get fuel in it and cause fuel starvation due to the weight of the fuel being pulled up the vent could create a suction in the tank?

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Scott Emery
EAA Chapter 326
T-18 N18TE

You guys are absolutely right ....... from the practical point of view we always land with a good amount of fuel left in the tank. That happens very naturally for me because I have TB. (Tiny Bladder.)

So, has anyone gone out and measured their climb angle at Vx yet?

D