Fuel tank balance

new_CFI wrote:


as far as left/right/both/off, normaly you set it on both and forget it.

Even Cessna recommends using Left or Right at high altitude cruise to
alleviate vapor locking problems. Of course, which tank level is
going down on a 172 is only marginally related to the selector
position.

Mxsmanic wrote:
Robert M. Gary writes:
working by itself.

Okay, but that's an exceptional situation. For a normal flight, do
you have to change the fuel settings? I know Lindbergh did, but that
was an unusual aircraft.

If you ran all your gas out of one tank you would only have 1/2 the
capacity. I switch tanks every hour.
-Robert

Yup pretty darn complex. In Our Mooney you have to go from right to left
(or was that left to right?) about ever 30 miinutes or so... I forget...
I think the procedure takes up 10 or so pages in the POH though....

Jon Kraus
'79 Mooney 201
4443H @ UMP

Mxsmanic wrote:
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.

Jon Kraus writes:

Yup pretty darn complex. In Our Mooney you have to go from right to left
(or was that left to right?) about ever 30 miinutes or so... I forget...
I think the procedure takes up 10 or so pages in the POH though....

Perhaps this will sound stupid, but why wasn't the aircraft designed
to pull fuel simultaneously from both, or at least to connect the two
together?

--
Transpose mxsmanic and gmail to reach me by e-mail.

Typically low wing aircraft "can't" have a "both" setting. Now there
are exceptions (if it has a header tank). So on low wing aircraft
(which HAVE to have fuel pump(s)), you typically have just left and
right. Now I am talking about small GA aircraft, not military or
transport aircraft.

If you have to have a fuel pump, then usually you have two, so you have
a backup if one quits.

As for the mixture, that is very important and often used. Most systems
now have an EGT (exhaust gas temp) guage and you use that temperature
to set the mixture. There is also a procedure for setting the mixture
based on rpm. At altitudes above 3000' (some use 5000), the mixture is
set leaner than full rich on the ground before takeoff at runup.

Doug writes:

If you have to have a fuel pump, then usually you have two, so you have
a backup if one quits.

As for the mixture, that is very important and often used. Most systems
now have an EGT (exhaust gas temp) guage and you use that temperature
to set the mixture. There is also a procedure for setting the mixture
based on rpm. At altitudes above 3000' (some use 5000), the mixture is
set leaner than full rich on the ground before takeoff at runup.

How do I determine how much actual propulsive thrust I'm generating?
I see a throttle setting, manifold pressure, RPM, and pitch, but I'm
not sure how to set all this in order to increase or decrease total
thrust. I've been reading the FAA's handbook, but I'm still not very
clear on how it works. My _impression_ is that I advance throttles to
provide more power, and then set propeller pitch to the green range in
order to translate engine power into optimum thrust. Is that right?
But apparently manifold pressure is supposed to be telling me
something, too.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Piston engines use percent power. So if you want to cruise at 65% power
(and burn fuel at the rate stated for that power setting), you look in
the aircraft manual for the rpm setting and manifold setting that gives
65% power (there may be more than one). For any given percent power,
you will get better fuel economy if you use the lower rpm and higher
manifold pressure setting. (This is akin to going up a hill in a car in
a high gear and full throttle. Such procedure uses less fuel than
downshifting and using say, 3/4 throttle).

There is no percent power guage that gives a direct measure of the
engines output.

You can also derive percent power from fuel burn if you have an
accurate fuel flow meter. The rule of thumb is 12 horsepower per gallon
per hour. So if you are burning 12 gallons an hour that is 120
horsepower. If the engine's max horsepower is 180 then you are at
120/180 percent power.

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

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.

Mxsmanic wrote:
Doug writes:

How do I determine how much actual propulsive thrust I'm generating?
I see a throttle setting, manifold pressure, RPM, and pitch, but I'm
not sure how to set all this in order to increase or decrease total
thrust.

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).
You can't set the propeller pitch but you can adjust the propeller RPM.
Higher RPMs are good for higher power but for cruise you want something
less. Your POH will show you a manifold pressure(throttle)/RPM (prop
control) combo for the power setting you are looking for. We usually
refer to it as say 23/25 (meaning 23" manifold pressure, 2500 RPM).

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. 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.
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.
-robert

with 4 main body tanks fore to aft
2 wing tanks (1 each wing)
2 Main tanks 1 each side over each nacelle of 2 engines each (4 engines)
and 1 weapons bay tank added, #20,000# apprx
other two weapons bays carry 54, 500# gravity weapons
Total full up fuel load, apx 210,000#, aircraft weight, apx 200,000#,
weapons, 27,000# plus racks (4,000#) Take off weight, apx 441,000#, Max GW
for take off 477,000#

-Starting with full fuel all tanks, all tanks feed to the Mains.. keep the
mains above 10,000 #
-Burn from tanks 1 and 4 first (apx 18,000pph at this weight)
-Trap 35,000# fuel between the body tanks number 1 and 4 tanks for CG
control later in the flight
-Move 35,000# trapped fuel between tanks 1 and 4 to maintain CG when wings
sweep
-Burn off body tanks 2 and 3
-Burn off the weapons bay tank
-Burn off the wing (if fuel imbalance in wings we can cross feed, more than
10,000# fuel imbalance will cause roll control problems) may have to cross
feed if one or more engines on the same side are shut down
-Now down to 10,000# in each main and 35,000# in tanks 1 and 4 (total
55,000#)
-You best be at the IAF for the primary airport, 55,000 is enough to weather
divert to the approved alternate (apx 13,000pph fuel flow) and land with apx
20,000-25,000# in the tanks..

-Fuel slosh.. 20,000# is just 10,000# in each main tank, direct feed to the
engines
-Not enough weight forward to keep nose wheel steering squat switch engaged
for ground taxi,
you need to be above 7,000# each tank to start the approach

BT

"Mxsmanic" wrote in message
...
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.

--
Transpose mxsmanic and gmail to reach me by e-mail.

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