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  #1  
Old 09-18-2013, 08:26 PM
ifixf15's Avatar
ifixf15 ifixf15 is offline
 
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Default Left, Right or Both?

One of the things I love about VAF is that it seems that I learn something new everyday. I probably enjoy the Point A vs. Point B topics the most, since I seem to learn the most from them. For some reason lately my attention has been diverted to Fuel Selector Valves and I have learned a few things.

1) Normally aspirated engines generally do not require a return line.

2) Fuel Injected engines generally do.

3) Andair vs. all others

I have noticed that in every conversation I have observed to this point all the possible tank selections are Left or Right. I get the distinct impression that BOTH = Bad, or at least, not discussed and I am wondering what the Pros and cons are. I am currently training in a 172 and it has BOTH. It seems to work well. I worked on F-15's and F-22's for 30 years and they don't get a choice (its BOTH by the way). Having no data it is hard to form an opinion myself, so I am soliciting input.

Hopefully I haven't waded into a "Nose-wheel vs. Tail-wheel" or a "To Prime or Not To Prime" topic. If it is one of the 'Stern Verboten' topics, I apologize in advance.
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  #2  
Old 09-18-2013, 08:33 PM
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I personally don't like "both". With "both" you can run both tanks dry simultaneously!

Ask me how I know!
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  #3  
Old 09-18-2013, 08:33 PM
PaigeHoffart PaigeHoffart is offline
 
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If you had a boost pump in each tank, you could have both. But, with just one boost pump sucking fuel through the selector valve, you would suck air as soon as one of the two pickups became uncovered.
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  #4  
Old 09-18-2013, 08:35 PM
F1R F1R is offline
 
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With a low wing RV both is not an option. If you poke a hole in a straw above the cup you can not drink a milkshake. Likewise if you were on both but one tank was low to the point that you uncovered the intake end of either side, your fuel pump/pumps would not be able to draw fuel.

It does however work on a high wing with the tanks in the wings like a c-172.

Also, some low and mid wing AC have a central collector tank that both wings drain into by gravity, but not so with RVs to date.
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  #5  
Old 09-18-2013, 08:38 PM
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ifixf15 ifixf15 is offline
 
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Default Thanks

Wow, that was fast. I love VAF and physics. Makes all the sense in the world once it is explained. Just like the 172, the fighters mentioned are High wing, at least in relation to the engines, and they have boost pumps in every tank, so I guess all the bases are covered.

Thanks
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Last edited by ifixf15 : 09-18-2013 at 09:08 PM.
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  #6  
Old 09-18-2013, 08:39 PM
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Mel Mel is offline
 
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In a low wing aircraft, the fuel pump is "sucking" fuel and pumping up to the engine.
If you have a "both" selector and one pick-up is somehow unported, you will be sucking air. The pump does not suck air very well and will cavitate.

Check Standard certificated low wing aircraft. You are not likely to find one with a "both" selection.
The Cessna 172 has gravity to help.


edit:
Looks like my typing is slow tonight. Just finished paperwork for 4 upcoming aircraft inspections. Must be getting close to bedtime.
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Last edited by Mel : 09-18-2013 at 08:42 PM.
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  #7  
Old 09-18-2013, 09:00 PM
BobTurner BobTurner is online now
 
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"normally aspirated" means not turbocharged. Could be injected or a carb.
Fuel injection systems found on TCM engines usually have return lines. Those on Lycoming usually do not.
The fuel boost pump sold by Vans uses a return line as part of its pressure relief system.
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  #8  
Old 09-18-2013, 09:07 PM
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Toobuilder Toobuilder is offline
 
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Couple points of correction:

"Naturally aspirated" engines are both carburated and injected... The term refers to a form of induction free from forced induction such as a turbo or supercharger.

Certain types of injection require a return line, but most Lycoming systems don't.

A "both" position is possible and workable in certain conditions such as full tanks and level flight, but highly unforgiving otherwise (see comments above). Yes , it works (in some cases) but ultimately, the "Both" position is more of a fuel management headache than its worth. Go into it with your eyes wide open.
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Last edited by Toobuilder : 09-18-2013 at 09:12 PM.
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  #9  
Old 09-18-2013, 09:24 PM
aerhed aerhed is offline
 
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Just to keep it we'll muddled, sometimes turbocharged engines have a carburetor, but turbosupercharged engines don't.
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