Thorp T-18 Mutual Aid Society
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 Fuel Systems
 Gravity fuel flow
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Richard H Woodcock

151 Posts

Posted - 11/23/2009 :  10:53:12  Show Profile
I posted this on the "old" forum in response to a discussion about gravity feed systems, and someone suggested that I post it here as well, so . . .

I've just finished fuel flow testing my S-18, so I thought I'd offer
some comments. :

The FAR's require 150% of maximum flow for a gravity-fed system. I'll
assume that provides an ample safety margin, but not an overly excessive one.

With all due respect to some of the comments before, I think it's
important to KNOW what the plane will do with low fuel. I'd hate to
attempt a go-around after searching for my destination with low fuel and have the engine die during the go-around. For simlilar reasons, I
wouldn't add ONE fuel pump to be sure, without a second one for backup, and a fuel pressure gauge if one pump was engine driven.

My flow tests were conducted as follows:
1.) Tank dry.

2.) Fuselage set at 17 degree climb attitude.

3.) Fuel added and flow measured measured with the following results:
1 gal. - less than 9 gal/hr.
2 gal. - 11.25 gal./hr
4 gal. - 14 gal./hr.
6 gal. - 18 gal/hr.

5.) I tested the flow at landing attitude (with the "long" gear)
and 6 gallons, and got about 125% of the flow obtained at 17
degree deck angle.

Things not considered:
1.) Ram air pressure from vent may be substantial.
2.) The plane may be capable of extended climb at a higher deck angle
than 17%

I have a 150HP 0320 which will burn (I estimate) 11-12 gal/hr. on
takeoff, and 8.25 at 75% power, in cruise. Given the FAA 150%, this
means that minimum flow for takeoff would require 18 gal/hr (6 gallons), and cruise climb at 75% power and 17 deg. deck angle would require just under 13.5 gallons or about about 3 gallons in the tank.

Tentative Conclusion:
I'll write in the operator's manual that "takeoff with less than 6
gallons fuel is prohibited" and that "due to possible fuel flow
limitations, power settings above 75% power with less than 3 gallons fuel is not recommended."

Any thoughts or comments?

Rich Woodcock

Rich Woodcock
N114RW - T18CW

Richard H Woodcock

151 Posts

Posted - 11/25/2009 :  20:40:26  Show Profile
I had some brief comments via Email (off-forum), and thought I should add something to the above.

RAM air pressure into the tank vent, assuming there is no leakage through the filler cap or elsewhere, would amount to 15" water at 150 kts indicated. To put it differently, ram air at that speed would pressurize the fuel line to approximately the equivalent of full-tank pressure. At 40 kts, it's slightly less than 1" water. Presumably it would take a substantial amount of time to pressurize or de-pressurize a near-empty tank, so I doubt there would be much help from ram air pressure on takeoff.

A go-around might be a different matter. 70 kts approach speed after a cruise-speed flight would leave the tank pressurized to about 4" higher than the actual fuel level, which would help.

This might explain the opinion of some that a T18 must have a fuel pump to operate safely, and the experience of others who have successfully operated for years without one.

In any case I'll be putting a screen over my vent.

Hope this helps.

Does anyone know the deck angle at Vx?

Rich Woodcock
N114RW - T18CW
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50 Posts

Posted - 11/26/2009 :  15:05:59  Show Profile
Would you be willing to list your fuel system componets with sizes so we could compare our fuel system (or planned fuel system)?

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Richard H Woodcock

151 Posts

Posted - 11/26/2009 :  20:39:02  Show Profile

Not at all, but I'm not sure how specific information you are looking for, or how much it matters. The short answer is 3/8" fuel lines, with only a gascolator and shutoff in the lines.

I have a very course finger strainer in the tank, which fits onto a 3/8" outlet valve, connected directly to the tank. (Yes, I know I should have mounted it separately, but did not.) An aluminum tube was used to extend the shutoff handle rearward to make it accessable during flight without removing seatbelts or shoulder harnesses. A 3/8" standard elbow connects to the fuel shutoff with a 3/8" hose bulkhead fitting, with a 90 deg. bend on the firewall side. Gascolater is located on the port side at the bottom of the firewall, with an internal fine mesh screen and a 3/8" aeroquip hose connecting to the MA4 SPA Carb.

There is no flow transducer or other obstructions in the fuel lines, and the lines are run as direct as managable. The only substantial curves are the ones mentioned and the connection from the firewall bulkhead fitting to the gascolator, to clear the landing gear/motor mount connections. Curves in flexible tubing are large enough to allow for minimal flexing, but small enough to prevent "low spots" which could collect water.

Fuel flow tests were conducted with the gas cap and the carburator connection removed. The carb connection was wired to the same height as the carb inlet. Were I to do it over, I'd consider doing it by removing the float bowl, to include any obstructions in the carburator, but I did not do so.

The fuel vent connects to the top of the tank and faces the wind on the lower center of the fuselage. The fuel cap is the "thermos bottle" type per the Thorp plans, and I have no "scupper" or the like.

I haven't yet run the engine, and am working on the canopy, which I think will end up temporary, as I am not satisfied with it. But, I wanted to test the fuel flow before running the engine and particularly before flight.

As I wrote before, my plan is to run without a pump, as I believe most Thorps do, after establishing appropriate operating limits. I would however, add fuel pumps if I were to add a fuel flow transducer. The Flowscan 201A-6, for example, requires 0.6 psi at 15 gph, which amounts to 22 inches of fuel level (!)

Please let me know if I've missed anything.

Rich Woodcock
N114RW - T18CW

Edited by - Richard H Woodcock on 11/27/2009 09:55:18
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50 Posts

Posted - 11/27/2009 :  22:16:21  Show Profile
thanks Rich

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