Thorp Air Command - T18.net

My aux tank is 15 inches deep, 32 inches wide and 4.5 inches tall. Using 231 cubic inches per gallon, that works out to 9.35 gallons. The tank is placarded for 8 gallons maximum. I have no drawings of the tank or its installation. My T-18 is 40 years old and the tank was in it when I bought it over 10 years ago. It is held in-place with two straps that run forward-to-aft and are bolted to the turtle deck. A vent line vents out underneath the passenger seat.

There are no issues with gravity feed. The tank is always above the carburetor of my O-360 for normal ops. With fuel in the tank, I run the engine through run-up off the aux tank. I switch to the main tank before takeoff and run off of it until level-off, then I switch to the aux. Running off the aux on the ground serves as an ops test that the tank will feed and to move the CG slightly forward for takeoff.

Pay no attention to the dirty carpets. They are a figment of your imagination.


SPANKY

Spanky : Nice simple design , just like I like it ! Save these pics as this subject seems to reappear every few years ! BING , BANG , BOOM !!

RB

Simple and straight forward. Looks good. Judging by the pictures, the only thing I would change is to add a way to check that system for contamination.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

It should be able to be checked the same way you check the standard tank... it's got a drain valve.

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

The "drain valve" pictured is at the front so, unless that tank is tapered or mounted so the existing drain is at the lowest point when the aircraft is parked, it located at the highest point on the tank floor... i.e. it can't be used for a valid sump check. My turtle deck has a 2.1 degree down slope to the rear when the aircraft is parked which would put this tank outlet about 5/8 of an inch above the lowest point. Using the numbers above, that would be close to 1.25 gallons of space sitting below the outlet. In flight, the outlet would pretty much become the lowest point but it would be difficult to do a sump check in flight.

My fix would be to move the outlet to the rear and install a gascolator on the floor to trap any contamination. That takes care of 2 problems: 1) sump checks and 2) gives a low point in the system that the hoses can drain to. Essentially, I would duplicate what I have seen on Cessna and Piper aircraft and follow the "rules" for fuel system design published by people like Tony Bingelis.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Spanky
How do you determine how much fuel is left in the aux tank as you are cruising along before you switch back to the main tank? Also, do you turn on a fuel boost pump when you switch tanks, or is it unnecessary for this setup?

My fuel totalizer is set up to show main alone or main + aux. When the totalizer shows about 29 gallons remaining plus what was used for ground ops before switching to the main for takeoff, I then switch back to the main. I cruise about 45 to 50 minutes on the aux before I start to give the totalizer the iguana stare. Of course, if the engine sputters first it's a no-brainer. No fuel boost pump required, although I do have an electric boost pump which I only use for take-off and landing.


SPANKY

You're right of course, Jeff. Can anyone confirm that the T-18 usually flies in an attitude that has the turtle deck sloping slightly nose down? In 28 years of flying mine, I don't think I ever had a reason to check or think about that. Anyway, this aux. tank's feed is going to be joining the fuel line to the engine upstream of the existing gascolator, which should trap any water, but that doesn't address your concern about getting a valid sample directly from that tank before flight.

I helped build an "under-the-turtle deck tank" like this with another builder years ago. It had a tapered shape (deeper at the rear), with a small "sump". I don't recall if it had any provision for taking a fuel sample. I do remember it held about 8 gallons, and worked well.

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

I did not intend to imply I would rip out a system that has worked fine for 40 years. Just throwing out considerations for anyone thinking about building a system. When this particular system was built it would have been a simple matter to install the plumbing or add a sump drain at the rear of the tank.

Having had to deal with leaky fuel caps and filler necks, one of the things I like about my Thorp is how the fuel cap is covered. I pulled over 4 ounces of rain water out of a PA-20 one day and Cessna has an AD concerning their flush mount caps. I think the strainer on my aircraft holds 3 ounces. The only way water can get in my Thorp is through condensation or a contaminated fuel source. I pulled about a half ounce after refueling at a public airport one day. Condensation is probably only a valid concern for anything that has sat idle for a long time.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I decided to check the angle of my turtle deck with the aircraft “level”. I get approximately an 8 degree slope going up to the back so it would not be good to plumb the fuel supply from the back of the tank since a fairly large percentage of the fuel would be unusable.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Beat me to it. Thorp's fly tail high. WL 42 is a design reference only. It has nothing to do with aerodynamics.
Cubes

Problem solved ! Plug away !

RB

Now that would be exciting!!

Tony S

This statement is wrong. The smoker tanks in the spray planes get condensation in them and I use the smoker on nearly every load.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I don't know that anyone was curious about the turtle deck angle in flight but I came up with between 8° and 9­° today. The digital protractor is sensitive and it was a little bumpy when I took the reading. I was lightly loaded (just me with 10 gallons of fuel) and about 150 mph indicated.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson