Fuel tank balance

"Newps" wrote in message
...
[...]
I'm not saying that what the JPI does isn't useful. Just that it doesn't
fit my definition of "direct measure".

As long as it's accurate, who cares?

It's only accurate when the engine is performing according to its design.

So I'd say any pilot using that sort of "power monitor" should care. It's
not a means for verifying actual power output of the engine. It's little
more than an electronic handbook translating the POH power charts to a
graphic real-time display.

Which is not to say it's not useful. I'd say it certainly is. But it's
VERY important to understand what it is and what it's not. Any pilot who
doesn't care is likely to wind up in the trees one day, trusting the display
too much.

Pete

On Sat, 23 Sep 2006 17:32:11 +0200, Mxsmanic
wrote:

Is there any reason to drain fuel tanks in any way other than
symmetrically in normal flight? I notice that most aircraft have
complex controls for fuel flow from the tanks, and I wonder if there
are things one is suppposed to do during normal flight, or if this is
just to provide for possible equipment failures or a need to shift the
center of gravity of the aircraft in an emergency.

Low wing aircraft generally have a set sequence or sequences of
drawing fuel from specific tanks to keep the plane balanced. If I
burn too much off one side (and it doesn't take a lot) the Deb will
get decidedly lop sided. A half hour to 45 minutes per side on the
mains is good. An hour on one side and the other wing will get heavy.
Were I to burn all the gas out of one wing tip tank while the other
was full I'd be in a heap of hurt and I'd want to get the major
portion out of the second tank before landing. The book says no more
than 5 gallons difference between the two.
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

new_CFI writes:

its a fule to air ratio. x:y... so as you climb and air density
decreases the amount of fule require to keep the ratio constant,
changes. So, the amout of fuel you send to the engine needs to be less.
Thats where the mixture controll comes in. If you don't have the Pilots
Handbook of Aeronautical Knowledge, there is a good explanation of
it....if you don't have it; you can download it from the faa website.
If you need the link Ill post it, but I have to run….

I have the handbook. I pretty much understand the explanation, I just
wonder why the pilot adjusts it. You'd think that the engine would be
able to measure the static air pressure and adjust the mixture for
itself. Cars have complex engine computers, so why not aircraft
engines? Of course, if aircraft engines had the same unreliable
software that they put in cars, they'd drop out of the sky, so it
would have to be verified a lot more, and the engine would have to be
able to run without it.

--
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Peter Duniho writes:

IMHO, the main reasons that aircraft engines require so much fiddling is
two-fold: one is that aircraft engines operate at constant settings for most
of the time they are on; another is that improvements cost big bucks in the
form of certification costs, bucks that most pilots won't pay when the
current (albeit ancient) technology suffices.

I suppose that makes sense. I know that I'd rather have older
technology that is certified than newfangled technology that isn't, at
least for aviation (and for many other things, but I guess I'm getting
tired of seeing complex, poorly designed systems fail so often).

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Marty Shapiro writes:

Not too noticeable until about 8,000'. Then it gets noticeable. Above
11,000' it gets very, very noticeable. Go to Pike's Peak and see what
happens!

Yesterday I tried flying a Piper J-3 and a Cessna 172 over the top of
(I think) Mount Rainier (the tall mountain near KSEA), and they both
seemed to struggle as we approached the altitude of the peak. I
turned around and went back when it became obvious that I wasn't going
to make it. Not sure if it was engine power that lacked, or just air
density that was too low, or what. I was flying for fun and did not
check the altimeter.

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Doug writes:

Things like this give pilots on long flights something to do....

Do you mean to imply that it's not essential? I can't imagine pouring
over paper charts with a calculator on a final approach, just to see
what engine settings to use.

If I do nothing, do I lose only efficiency, or is there a safety issue
involved if I don't play with mixture or propeller pitch?

Thrust is something a little different. Airplanes with jet engines use
thrust (whose unit is pounds) for their power settings, I believe. I'm
no expert though. I do know you don't use thrust for small gasoline
driven prop engines like in Cessnas and Pipers.

I presume they just call it something different, but ultimately
powerplants push air somewhere to make the plane go.

I will try playing around with pitch and mixture and see what happens.

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Robert M. Gary writes:

Set power using throttle to the appropriate setting on your manifold
pressure. If you fly around full throttle down low you'll over stress
your engine. At cruise you usually leave the throttle full forward
(unless you have turbo charged).

How do I know when the engine is potentially being stressed based on
the instruments? Low manifold pressure?

You can't set the propeller pitch but you can adjust the propeller RPM.

On the A36 and B58 I've been trying to fly, there's a lever that says
pitch next to the throttle, and it has a feather setting (which I have
never used), so I presume it's pitch of the propeller blades.

Dumping extra fuel into the cylinders is good when climbing but not so
good during cruise. In cruise we lean the plane out for a more optimal
mixture.

Is that just for reasons of economy, or does a rich mixture damage
something in the engine?

You car does the same. For the same RPM your car will set a
higher mixture during acceleration vs. in freeway driving. Most planes
have a EGT (exhaust temp ) to measure mixture, but you can do it just
by sound in a more basic plane.

Higher EGT = rich mixture?

Planes don't do this all at once for the same reason I have to shift my
car, because they don't use an automatic system. There are such systems
out there for planes but they are very expensive since it would be very
bad if it didn't work correctly.

Understood. My main concern is whether or not I'll get into trouble
if I fail to do all the tweaking of engine parameters while flying.

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Roger (K8RI) writes:

Low wing aircraft generally have a set sequence or sequences of
drawing fuel from specific tanks to keep the plane balanced. If I
burn too much off one side (and it doesn't take a lot) the Deb will
get decidedly lop sided. A half hour to 45 minutes per side on the
mains is good. An hour on one side and the other wing will get heavy.
Were I to burn all the gas out of one wing tip tank while the other
was full I'd be in a heap of hurt and I'd want to get the major
portion out of the second tank before landing. The book says no more
than 5 gallons difference between the two.

That's what puzzles me: If you're not supposed to have a significant
difference between the two, why aren't the two tanks just connected so
that they always drain at the same rate on both sides? Why would you
_want_ one wing substantially heavier than the other?

I can see why one might want to change things for fore and aft tanks,
or between center tanks and wing tanks, but I don't see any utility to
having one wing heavier than the other, or to having wing tanks that
don't communicate with each other (provided there's a cutoff for
emergencies).

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"Newps" wrote:

As long as it's accurate, who cares?


I would not accept as accurate a calculation that leaves out essential
parameters. It's an educated guess.

No, it's either accurate or it's not. Compare what the JPI says to what
the chart in the POH says. If it's accurate then it is irrelavant how it
got the information.

So pencil-whipped data agrees with pencil-whipped data. So what? It still
doesn't really tell you the actual power output because there's no way for
it to know the load the prop is putting on the engine.

If the prop controller has to flatten the pitch to make the specified RPM
because the engine is not making rated HP, the JPI will never know the
difference.


--
Dan
C172RG at BFM

"Mxsmanic" wrote in message
...

That's what puzzles me: If you're not supposed to have a significant
difference between the two, why aren't the two tanks just connected so
that they always drain at the same rate on both sides?

On a low win airplane, you need a fuel pump to get the fuel to the engine.
If you have one fuel pump connected to both tanks, and one tank runs dry,
the fuel pump will be sucking only air, and no fuel from the other tank.

If you put in two fuel pumps, and one tank runs dry, you will probably burn
out the fuel pump running it dry.

If you put in a way for the pump to shut down when it's dry, you run the
risk of a faulire that shuts down the pump when there is still fuel in the
tank.

The goal is relable and simple. One fuel pump (actually, mine has an engine
driven pump and an electric backup pump), and a valve to select tanks.


Why would you
_want_ one wing substantially heavier than the other?

When I fly alone, the left side of the plane is heavier. If I burn fuel from
the left tank, after about an hour the plane is more balanced.