Rebuilding N9234N 1993 Maule M-7 235

Island Flyer · Post by **Island Flyer** » Fri Jan 28, 2011 10:35 pm

After thinking about the fuel vent line over pressurization potential with the two check valves, we have decided to go back to the original Maule design, even though it does leave a possibility of siphoning from the main tank to the aux tank on a slope. But we figured, at the max, you could probably only get 7-8 gallons into the outboard tank which is still about 48 lbs. Just have to be careful to check in pre-flight.

New entry on lights, heater and frame mods.

http://web.me.com/rhysspoor/Site_2/Blog ... _Ends.html

Post by **andy** » Sun Jan 30, 2011 7:47 am

Looking good! Are you modifying your fuel plumbing to feed the aux tanks into the selector switch instead of through the aux fuel pumps? The only way I can get fuel from my aux tanks to the mains in my MX-7-180 is by switching on the aux fuel pumps. I put the fuel selector switch on left or right on a slope to avoid the uphill main feeding into the downhill main through the fuel selector. I don't have the check valves and have always wondered if they are worth the money. To avoid fuel siphoning out the vent tubes, I fill the tanks to about 2 inches below the filler necks.

SkyMaule · Post by **SkyMaule** » Sun Jan 30, 2011 8:43 am

very interesting. Thanks for posting your process and progress.

Mark

BudG · Post by **BudG** » Sun Jan 30, 2011 9:53 am

Hey Rhys,

Excuse my lack of knowledge in the subject. I don't have a great deal of experience in off airport or unimproved runway parkings; but the times that I did go to airports where there were inclines my aux. tanks were either empty or were well below the vent lines. I haven't had a problem with overflow. I usually don't fill the aux tanks unless it's winter and the bird is nested for more than a few days, or going cross country, usually just a few gallons for slosh. Normally I turn the fuel selector to off to avoid siphoning.

Do the fuel pumps act as a check valve? I assumed that without the pump being on fuel wouldn't bypass the pump.

My airframe should be getting powdercoated as I write. IhopeIhopeIhope.

CAVU2U

BUD

maules.com · Post by **maules.com** » Sun Jan 30, 2011 10:43 am

BudG the fuel will crossfeed when the selector is on BOTH or OFF.
It will only NOT crossfeed if LEFT or RIGHT are selected.

Island Flyer · Post by **Island Flyer** » Sun Jan 30, 2011 9:45 pm

One of the best things I have done as a pilot is to learn the ins and outs of the workings of the plane. At the very least if you haven't been with your mechanic at the annual, I highly recommend it. Better yet, take the whole thing apart and reassemble it

A little basic anatomy about the fuel system. Your two main tanks feed to a "T" valve "fuel selector switch" that has 4 positions, Off, Right, Both and Left. It works like a "T" but it isn't exactly shaped like it. Each leg off the top of the "T" is fed by two lines that merge into one. These come from the front and back of each main so when you are in a prolonged climb or descent you don't stop drawing fuel. The fuel is flowing into a space in the fuel line by gravity but that space in the line is created by the vacuum at the far end of the line from the fuel pump. In the Off position, the base of the "T" is closed so the fuel supply to the engine is shut off. This stops fuel to the engine which I would use every time I would change the oil because to get to the oil screen on the Lycoming IO-540 you had to disconnect the fuel line. In reassembling the engine we put in a extra elbow (see in blog) so the fuel line won't need to be disconnected any longer. However, as Jeremy pointed out, when the fuel selector is "Off" it does not close off the connection across the top of the "T". So if you are on a slope with one wing lower than the other, then it will flow downhill from the high tank to the low. If you have the difference big enough you can drain the whole tank. If you don't have a check valve on your vent line, then the excess, once the low main tank fills goes out the vent tube on the wing. (Another thingcan happen is if you have full mains and aren't "on the ball" and skid the turn, you will be throwing fuel out the vent). The only way you can stop the transfer of fuel is by placing the selector on Right or Left, either will work. I tend to park with mine on the Right because the length of the fuel lines from the mains to the fuel selector switch is shorter on the left and consequently the left side drains faster than the right in flight if you are on Both because of less resistance in the line. If I have gone up to Stuart Island which is about a 40 minute flight and park unlevel (which always happens) then having the selector on Right stops the siphoning and sets up the fuel supply for the return flight. So the fuel selector primarily controls the supply lines but in an unlevel parking situation can indirectly affect the vent line. The flip side of all this is that if you have flown and have uneven fuel levels in the mains and want to let physics equalize them with out having to add fuel, just leave your selector on Off or Both and if you are level, the level of fluid in the mains will balance given a little time.

Now for the Maule designed check valve in the vent line another "T" is placed in the vent lines from the main and the aux tank. At the base of the "T" is the check valve which allows the air to come in through the vent line but blocks fuel from escaping out. However, if you take the above scenario and leave your fuel selector in Off you could now potentially fill up your low side aux tank instead of having the fuel dump overboard. Potentially putting 15 gallons of fuel (90lbs) way out on the end of the wing without a balancing factor on the other side. I haven't never tried it, but I suspect it could lead to a significant lateral W/B problems on lift off. Obviously, the wise thing would be to check all of the tanks for fuel levels during the preflight.

We put the extra check valve in the horizontal vent tube coming off of the main to eliminate that possibility. But that leaves the main tank isolated for expansion, if it were full to the top, it got hot and you had the fuel selector on the opposite side. What Tim tested and found out to the positive, is that if the tank starts to pressurize, the fuel cap will vent outward at a very low level of pressure (he could produce enough pressure by blowing into the tank , Hmmm... maybe that means he is full of hot air

The good is we have a relief system already in place for not only expansion by heat but also by overflow from the aux pump if the main tank were overfilling. I usually wait until my mains are below 1/2 to start a fuel transfer from the aux. If I happened to start too soon before it would just dump overboard through the vent. With the Maule design, it would dump back into the aux tank in a circle. And with our design it will vent through the fuel cap. If you are watching the fuel carefully, overflowing shouldn't be an issue, but the W/B issue to me seems worth trying to avoid.

So Andy, the selector valve pathways are still the same and to transfer fuel from the aux to the mains still requires the aux pump. And Bud, the pump does stop the flow of fuel from the aux tank to the main when off.

BudG · Post by **BudG** » Mon Jan 31, 2011 7:21 am

Thanks Rhys,

Man, sounds like you are gona be the go to guy on fuel systems. Thanks for taking the time to give up that info. and thanks to Tim for taking the time to finger it out.

Now I'm gona go get another cup of coffee and read that again.....was it the rib bone connected to the elbow bone...hmmmmm ahahahaha.

CAVU2U

BUD

maules.com · Post by **maules.com** » Mon Jan 31, 2011 9:19 am

The problem that becomes apparent if venting excess maintank fuel through the cap is that if the airplane is stationary, the fuel will flow rearward but also inward because of dihedral.
This fuel flows under the wingroot fairing and eventually compromises the sealing tape, leaking through and down to stain the headliner.
The T vent valve is designed to open at low pressure, about 1/2psi to let crossflow and expanded fuel out away from the cabin.
As fuel is used from either main or aux, air passes through the T vent to whichever tank requires it.
The Aux tank being higher than the main (dihedral) allows overfill or expansion fuel to escape out the communal vent before entering the Main and pushing out the main tank cap.
For the Maules with the individual vents this does not apply.

Island Flyer · Post by **Island Flyer** » Mon Jan 31, 2011 4:34 pm

The "T" valve in the vent is an Andair CK250 and was an addendum to the original vent valve and approved for use by Maule on 01/29/02. I don't know what the prior valve specs were but assume similar to the Andair. The Maule drawing shows the valve with the arrow up which allows air to be pulled into the valve from under the wing as fuel is taken out of either the main or aux tank. The presure release on the valve in that direction (the direction of the arrow) is < 1 psi. However, the pressure to release liquid back out the valve according to the manufacturer's spec sheet is 50 psi. That is why, for this valve design, you can't vent back out through it. If you turned the valve over it could work to vent excess pressure, but then you could not draw air back in unless you exceeded 50 psi which I assume would not work. But it would draw through the fuel cap. The problem is that it's not the approved way from the drawing from Maule. I agree with Jeremy that I don't want fuel venting out the top of the fuel caps especially with the dihedral of the wings. So our solution will be to not overfill the tanks. However I have had a few times where I've left the palne at an FBO and the tanks got overfilled. The whole system isn't perfect no matter how you slice or dice it. What would really work great is to have another tube out of the top of the tank with a check valve going out (<1 psi) and vent that through the bottom of the wing in case it overflowed. I suspect that would take more than a field approval. All of this is a whole lot easier if you just keep the original open vent design and dump a little fuel out once and a while.

Island Flyer · Post by **Island Flyer** » Sun Feb 13, 2011 11:16 pm

Another chapter added. Polishing the spinner was pretty satisfying. When Tim put on the top skins he built in the proper wash out so the wing will stall first from the root and we won't end up with creases in the top skin when the wing struts are adjusted. http://web.me.com/rhysspoor/Site_2/Blog ... hings.html

Island Flyer · Post by **Island Flyer** » Mon Feb 21, 2011 10:41 am

The first fabric went on yesterday. It's starting to look like an airplane again.
http://web.me.com/rhysspoor/Site_2/Blog ... _Deep.html

Skeletool · Post by **Skeletool** » Mon Feb 21, 2011 12:47 pm

Thanks for posting the progress of your Maule. I don't own one yet, so it''s great to be able to see the parts that are normally hidden, and to read about improvements.

Island Flyer · Post by **Island Flyer** » Sun Feb 27, 2011 10:10 pm

The last of the fabric has been put on and we replaced the rudder and elevator cables.
http://web.me.com/rhysspoor/Site_2/Blog ... ables.html

Island Flyer · Post by **Island Flyer** » Fri Mar 04, 2011 11:57 pm

Blog update: New panel installed, the new ailerons (6" longer) and flaps (6" shorter and 7 rivets over the inside ribs) came all boxed up from the Maule factory and the landing light lenses get buffed up.

http://web.me.com/rhysspoor/Site_2/Blog ... enses.html

maules.com · Post by **maules.com** » Sat Mar 05, 2011 12:09 am

Nice work you two.
I think you'll find the aileron/flap change is 7" though.
It's also advisable to add two rivets in the first trailing edge rib outbound from wing roots.

Rebuilding N9234N 1993 Maule M-7 235

fuel check valve

fuel check valves

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