Why is my throttle 3/4 closed at cruise?

Last week I flew my Grob from San Jose (RHV) to Torrance (TOA)
and back. Total time was nearly 6 hours, but burned
only about 30 gallons of fuel. The book says 9 gallons per
hour at 75% cruise. I've noticed this before, that it never
burns nearly as much fuel as I would expect. Another thing I
noticed during the flight that I had never thought much about
before is that in cruise, the throttle is about 3/4 closed, even
at 2600 RPM at 7500 feet MSL. Redline is 2700, and opening the
throttle further would push past redline. I haven't flown any
other planes in a while, so I can't recall whether this
throttle position is unusual.

All this seems to say that I am cruising around at Best Economy
(which the book says is 45%, 5.3 gal/hr). This would also explain
why I get only 105 kts IAS, rather than 115-120. I would
surely like to go faster, but I can't figure out how without
exceeding redline RPM. Is my tach in error? Do I need a different
prop? (fixed pitch) What else should I be checking? The engine
is a Lycoming O-320, same as a Warrior.


Happy landings,
Ross Oliver

It's almost certain that your tach reads higher that the real engine
RPM by about 200. Provided that you leaned properly, the best
indication of real engine power output is your fuel burn, which is low.
Your airspeed agrees with the lower fuel burn number.

The first thing you should do is to get the tach checked.

Ross Oliver wrote:
Last week I flew my Grob from San Jose (RHV) to Torrance (TOA)
and back. Total time was nearly 6 hours, but burned
only about 30 gallons of fuel. The book says 9 gallons per
hour at 75% cruise. I've noticed this before, that it never
burns nearly as much fuel as I would expect. Another thing I
noticed during the flight that I had never thought much about
before is that in cruise, the throttle is about 3/4 closed, even
at 2600 RPM at 7500 feet MSL. Redline is 2700, and opening the
throttle further would push past redline. I haven't flown any
other planes in a while, so I can't recall whether this
throttle position is unusual.

All this seems to say that I am cruising around at Best Economy
(which the book says is 45%, 5.3 gal/hr). This would also explain
why I get only 105 kts IAS, rather than 115-120. I would
surely like to go faster, but I can't figure out how without
exceeding redline RPM. Is my tach in error? Do I need a different
prop? (fixed pitch) What else should I be checking? The engine
is a Lycoming O-320, same as a Warrior.


Happy landings,
Ross Oliver

There's also a possiblity that you have a wrong prop installed on your
plane, if your tach turns out to be accurate.

Before you go too far, check your tach, as somebody else already
suggested, and -- what rpm can you really get at full throttle, level
flight, 7500 ft? You might find that the tach is off by 100 and/or the
rpm tops out at 2750 or so at 7500ft. If that is the case I'd say you
have exactly the right prop. Climb props and cruise props are
poorly named. A cruise prop that won't turn redline at 7500 ft is
definitely slower than a climb prop that will turn redline at full
throttle at 7500. Non-boosted of course. Add to that the fact that
the climb prop will get you up to 7500 quicker.

Ross Oliver wrote in message ...

Last week I flew my Grob from San Jose (RHV) to Torrance (TOA)
and back. Total time was nearly 6 hours, but burned
only about 30 gallons of fuel. The book says 9 gallons per
hour at 75% cruise. I've noticed this before, that it never
burns nearly as much fuel as I would expect. Another thing I
noticed during the flight that I had never thought much about
before is that in cruise, the throttle is about 3/4 closed, even
at 2600 RPM at 7500 feet MSL. Redline is 2700, and opening the
throttle further would push past redline. I haven't flown any
other planes in a while, so I can't recall whether this
throttle position is unusual.

All this seems to say that I am cruising around at Best Economy
(which the book says is 45%, 5.3 gal/hr). This would also explain
why I get only 105 kts IAS, rather than 115-120. I would
surely like to go faster, but I can't figure out how without
exceeding redline RPM. Is my tach in error? Do I need a different
prop? (fixed pitch) What else should I be checking? The engine
is a Lycoming O-320, same as a Warrior.


Happy landings,
Ross Oliver

I assume you are flying something like a 115c or other aerobatic trainer?
Like another poster mentioned about props, this machine was probably built
with emphasis on climbing, not cruising.
How about takeoff? Do you only use partial throttle? What does the tach read
when you throttle up?

On Fri, 21 Jan 2005 22:01:14 -0500, Mike W. wrote:
I assume you are flying something like a 115c or other aerobatic trainer?
Like another poster mentioned about props, this machine was probably built
with emphasis on climbing, not cruising.
How about takeoff? Do you only use partial throttle? What does the tach read
when you throttle up?


Yes, it is a 115C. I do use full throttle at takeoff and climb. Usually
around 2300 at the start of the roll. After takeoff, I have to be careful
to keep the pitch to about 80 kts to stay under redline. At level-off,
power reduction always comes before pitch down.

Ross Oliver wrote:
: hour at 75% cruise. I've noticed this before, that it never
: burns nearly as much fuel as I would expect. Another thing I
: noticed during the flight that I had never thought much about
: before is that in cruise, the throttle is about 3/4 closed, even
: at 2600 RPM at 7500 feet MSL. Redline is 2700, and opening the
: throttle further would push past redline. I haven't flown any
: other planes in a while, so I can't recall whether this
: throttle position is unusual.

It really sounds like the tach is off. An easy way to check (at least at one
RPM) is to fire it up under some orange sodium lights at night. Run it up to where
the prop "stops" by strobe effect of the lights. Assuming a 60Hz country, a 2-bladed
prop will stop at 1800 RPM, a 3-bladed at 1200.

That's only a single datapoint, but it's quick and easy. I've also used a
laptop with microphone to record the noise the engine makes. A quick frequency
analysis will show the fundamental frequency.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

wrote:
It really sounds like the tach is off. An easy way to check (at least at
one
RPM) is to fire it up under some orange sodium lights at night. Run it up to
where
the prop "stops" by strobe effect of the lights. Assuming a 60Hz country, a
2-bladed
prop will stop at 1800 RPM, a 3-bladed at 1200.

Does this work at a big field with lots of lights? The power feed will
certainly be 3-phase, with various lights randomly fed from different
phases. Do you really get enough of a strobe effect at 60 Hz to make
this work?

On the other hand, if you've got a friend who's a musician (or a piano
tuner), they might have a electronic pitch meter you can borrow.

Several people have mentioned checking your tach. Try this program -

http://www.tunelab-world.com/rpmsound.html


Last week I flew my Grob from San Jose (RHV) to Torrance (TOA)
and back. Total time was nearly 6 hours, but burned
only about 30 gallons of fuel. The book says 9 gallons per
hour at 75% cruise. I've noticed this before, that it never
burns nearly as much fuel as I would expect. Another thing I
noticed during the flight that I had never thought much about
before is that in cruise, the throttle is about 3/4 closed, even
at 2600 RPM at 7500 feet MSL. Redline is 2700, and opening the
throttle further would push past redline. I haven't flown any
other planes in a while, so I can't recall whether this
throttle position is unusual.

All this seems to say that I am cruising around at Best Economy
(which the book says is 45%, 5.3 gal/hr). This would also explain
why I get only 105 kts IAS, rather than 115-120. I would
surely like to go faster, but I can't figure out how without
exceeding redline RPM. Is my tach in error? Do I need a different
prop? (fixed pitch) What else should I be checking? The engine
is a Lycoming O-320, same as a Warrior.


Happy landings,
Ross Oliver

All this is why I advocate a manifold pressure gage for even fixed
pitch aircraft. It is ridiculous how little full scale error in a
tachomenter can influence such important things like fuel consumption
and engine evaluation. I've never tried the flourescent light trick at
1800 RPM. Does it work in practice?

Any audio measurements using a computer as a timebase would have to be
made in the aircraft or with the aircraft sitting still on the ground,
to eliminate sonic velocity errors. Even a musician who allows a
quarter step error to creep in would have a 2 1/2 percent error. Maybe
if he carried the tuning fork (or whatever) in the aircraft?