Ground speed control with leaning

I like to keep RPM in ground operations just over 1000 to minimize fouling.
This is always a compromise with brake usage however. Controlling speed by
riding the brakes is a clear no, no. One suggestion a CFI made to me
recently is to just leave the throttle alone, apply brakes briefly, let the
plane pick up speed again, repeat as necessary.

I had an idea today which worked well and I wonder if anyone sees a
downside. I always operate our 172 N on the ground leaned to the max.
Instead of using the brakes this morning, I just leaned some more until the
engine sagged and controlled my speed that way. Turning the vernier knob
didn't seem much more work than pushing the throttle in and out. Running
super lean like that must be good for the plugs.

I understand that early aircraft engines didn't have throttles, climbed and
flew flat out, and then landed this way. That always seem clumsy to me but I
can see now that it would have worked fairly well.

--
Roger Long

"Roger Long" wrote in message
...
[...]
I had an idea today which worked well and I wonder if anyone sees a
downside. I always operate our 172 N on the ground leaned to the max.
Instead of using the brakes this morning, I just leaned some more until
the
engine sagged and controlled my speed that way.

I'm surprised that taxiing with the mixture leaned you have any trouble with
plug fouling at all. Have you brought the issue up with the mechanic(s) who
maintains the plane?

In my plane, with a Lycoming 540 engine, I get plug fouling if I don't lean
during taxi. But by keeping the engine leaned out during taxi, I have no
trouble at all with fouling, and I can still use the throttle to adjust
power. There's no need to keep the RPM above 1000 in order to prevent
fouling.

Since you're already leaning, I don't see why you'd need to also keep the
RPM above 1000. The leaning alone ought to take care of the issue. If it
doesn't, it seems as though there might be some adjustment needed. The 172N
is carbureted, right? Maybe the idle jet is set too rich?

In any case, if the mechanics say everything's set perfectly, sure sounds
like a winner solution. At low power settings, there's nothing you're going
to harm by leaning, and as you've found, it's a fine way to limit engine
power. Just don't accidently shut off the engine; that sure can be
embarassing.

Pete

Isn't it fun learning all the things you should have been taught in
primary flight training.
Oh well, I guess that's why they say the PPL is a license to learn.

We're not actually having any problems with fouling. I'm just looking for
new techniques to keep it that way.

I've heard that the desirability of keeping ground RPM and temperature up
has more to do with lead build up on the exhaust valve stems than plug
fouling but I don't know how good that info is. I would think that a very
lean mixture would benefit the valve stems as well.

Probably no good or compelling reason to run the engine this way but it's
interesting to find out that it can be done.

This engine has very good mixture distribution and I've been experimenting
with running it slightly lean of peak in cruise. Since we only have a
single cylinder EGT and CHT set up, I restrict that operation to 60% power
as I don't know for sure where the other cylinders are operating. It's a
little rough if you listen for it but CHT goes down a bit. This worked very
well just after the engine was overhauled and it then got rougher. Now,
it's getting better. The variable is probably very mild plug fouling.
Staying lean and clean, even below what is required for smooth run ups, may
pay off in this regard.

--
Roger Long

On Fri, 04 Jun 2004 17:33:06 GMT, "Roger Long"
wrote:

I like to keep RPM in ground operations just over 1000 to minimize fouling.
This is always a compromise with brake usage however. Controlling speed by
riding the brakes is a clear no, no. One suggestion a CFI made to me
recently is to just leave the throttle alone, apply brakes briefly, let the
plane pick up speed again, repeat as necessary.

I had an idea today which worked well and I wonder if anyone sees a
downside. I always operate our 172 N on the ground leaned to the max.
Instead of using the brakes this morning, I just leaned some more until the
engine sagged and controlled my speed that way. Turning the vernier knob
didn't seem much more work than pushing the throttle in and out. Running
super lean like that must be good for the plugs.

I understand that early aircraft engines didn't have throttles, climbed and
flew flat out, and then landed this way. That always seem clumsy to me but I
can see now that it would have worked fairly well.


If you're already lean, you don't need to maintain 1000 RPM, I'd taxi
at idle and save the brakes. If you do get fouling when lean and
below 1000, you may want to check out the engine set up cause
something may not be correct.

z

If it is severely leaned out, you will be hard pressed to make more than
1000 rpm. I have to enrichen the mixture to get more than 1000 rpm when I
taxi. I lean until I get a slight RPM rise and have never ground fouled a
plug.

jerry

Questions...

What is the recommended idle speed listed in the POH?

If the recommended idle speed results in aircraft movement when idling with
brakes off, does the POH address the issue?

This just sounded like something the manufacturer might be aware of and have
some recommendations for...




"Roger Long" wrote in message
...
I like to keep RPM in ground operations just over 1000 to minimize
fouling.
This is always a compromise with brake usage however. Controlling speed by
riding the brakes is a clear no, no. One suggestion a CFI made to me
recently is to just leave the throttle alone, apply brakes briefly, let
the
plane pick up speed again, repeat as necessary.

I had an idea today which worked well and I wonder if anyone sees a
downside. I always operate our 172 N on the ground leaned to the max.
Instead of using the brakes this morning, I just leaned some more until
the
engine sagged and controlled my speed that way. Turning the vernier knob
didn't seem much more work than pushing the throttle in and out. Running
super lean like that must be good for the plugs.

I understand that early aircraft engines didn't have throttles, climbed
and
flew flat out, and then landed this way. That always seem clumsy to me but
I
can see now that it would have worked fairly well.

--
Roger Long

The engine was designed for a different fuel and 100LL was just coming into
use when the POH was written. Plug fouling wasn't on the radar screen.
--
Roger Long

What is the recommended idle speed listed in the POH?

If the recommended idle speed results in aircraft movement when idling
with
brakes off, does the POH address the issue?

This just sounded like something the manufacturer might be aware of and
have
some recommendations for...

"Roger Long" wrote
I had an idea today which worked well and I wonder if anyone sees a
downside. I always operate our 172 N on the ground leaned to the max.
Instead of using the brakes this morning, I just leaned some more until the
engine sagged and controlled my speed that way. Turning the vernier knob
didn't seem much more work than pushing the throttle in and out. Running
super lean like that must be good for the plugs.

There is no downside whatsoever to your idea. You can't possibly hurt
your engine by overleaning at these power settings.

However, if you're running so lean that you're reducing power by
leaning, you have no need to maintain 1000 RPM. When you're running
that lean, you can run the RPM as low as you like and you won't foul
plugs.

Michael

In article , Roger Long wrote:
This engine has very good mixture distribution and I've been experimenting
with running it slightly lean of peak in cruise.

It's funny, for all the ill reputation the O320-H2AD engine got as
fitted to a C172N, all the ones I've flown have had very good mixture
distribution and don't go rough as you lean the mixture out - the RPM
just starts dropping as you get too lean.

--
Dylan Smith, Castletown, Isle of Man
Flying: http://www.dylansmith.net
Frontier Elite Universe: http://www.alioth.net
"Maintain thine airspeed, lest the ground come up and smite thee"