Fuel Selectors...?

[Daifly]

I posted this in Private Flying, but haven't had a response. Perhaps it's too techie?!

"Here's one for the technically minded out there.. .Why do training aircraft like PA38's have tank fuel selectors when others like the C152 have a basic on/off one. I don't need to know all about gravity feeds and fuel pumps though. This isn't a question about the good/bad points of the C152/PA38 either, merely why don't all aircraft just have on/off valves?

I can appreciate in larger multi-tank a/c that there have to be selectors from the weight and balance and fuel planning points of view but in training aircraft is seems like another potential problem.

If this is just for the "added realism" factor, then do you think/know whether it increases fuel burn or not.

The fuel burn question is the one that's been bugging me all day...!

Ah, 10 years since Trevor Thom means it's all gone hazy!

Thanks, Dai."

Perhaps it's a bit waffley, but I am intrigued whether fuel burn increases on a/c with an on/off selector compared to a left/right one?

[Kermit 180]

Experiment to try at home:

Take two glasses. Fill one with air, the other with water. Put a straw in each glass. Put both straws in your mouth. Suck. What do you get?

Answer: More air than water, as air is easier to suck than water.

With a gravity feed system both tank contents are drawn into the fuel system by gravity and by differential air pressure. Simple. A low wing system relies on a pump (which you know about already). Run one tank dry on a low wing system with a tank selector on BOTH and.... you get air.

Hope this suffices to answer your query.

Kermie

[juggernaut]

Years ago I used to be a tuggie and flew a Piper Cub and a Citabria, the Cub had a left tank and right tank selector the Citabria an on/off selector, they were both gravity fed systems, the interesting thing is that the Piper Cub ran out of fuel 7 times and the Citabria not even once. The point is regardless of fuel burn the average pilot is an optimist and will happily run on low fuel if he/she thinks that there is another tank to switch to if the engine stops. In the Citabria if you ran out of fuel it was forced landing time. Does this mean the Cub has a lesser fuel burn? the pilots thought so!

[Tinstaafl]

Just one of the many decisions the designer makes.

Not really relevent to 'trainer aircraft' per se.

On/Off: simple but controlling a fuel imbalance is awkward.

Tank selector: Have to change tanks but then a fuel leak in one is isolated from the other.

Also the general system or some parts might be used across, or have evolved from, a variety of models, with various combinations of tanks. Easier to adapt if some semblance of multiple selection was already built in, or not worth the bother if it was there already.

[Icarus Wings]

If you select the tank with least contents and run it dry you now know you have at least that length of time again before you run out completely