HELP - PIPER LANCE RPM FLUCTUATIONS

[John gardon]

I have a turbo LanceII that has recently had an engine rebuild. I am
currently grounded in a remote location with the following problem and
would appreciate any help from any body that may have experienced
similar problems or have some insight into its cause. Because of the
remote location it is not readily possible to change out components.

All engine run ups, static RPM, prop cycling etc are normal. The second

I release the brakes and commence rolling the RPM swings and surges
between 23-2700 RPM obviously requiring a rejected take-off Manifold
pressure remains steadythroughout. Static engine runs again produce
normal results but as soon as brakes are released the scenario
replicates.

I have changed the prop governor and thereby elimimnated it as the
cause. The prop shows no signs of problems and should not be the cause
of the problem (although I am not discounting it as the cause, but most
prop experts tell me its highly unlikely).

Any insight would be appreciated!

[Ryan R. Healy]

John gardon wrote:

> All engine run ups, static RPM, prop cycling etc are normal. The second
> I release the brakes and commence rolling the RPM swings and surges
> between 23-2700 RPM obviously requiring a rejected take-off Manifold
> pressure remains steadythroughout. Static engine runs again produce
> normal results but as soon as brakes are released the scenario
> replicates.

> I have changed the prop governor and thereby elimimnated it as the
> cause. The prop shows no signs of problems and should not be the cause
> of the problem (although I am not discounting it as the cause, but most
> prop experts tell me its highly unlikely).

Since the RPM governing system on the Lance (and all other Pipers as
well) is grossly simple, it can either be the governor or the prop.
Since you've already installed a new governor, it is reasonable to think
that the new one is working correctly, especially since a static runup
produces a consistent 2700 RPM. When you commence the takeoff roll, the
propeller will attempt to 'overspeed' due to increased airflow and the
governor will compensate by adding some oil to the hub which in turn
pushes on the piston in the propeller hub, increasing propeller pitch
and holding RPM constant. Trouble is, on your machine, the prop RPM
begins fluctuating which can either be caused by one of several things:
the governor not adjusting oil flow correctly, the prop hub not holding
the oil (oil containment ring on the hub piston worn perhaps), the
governor pump isn't supplying sufficient oil or oil of a high enough
pressure to maintain head pressure in the prop hub or finally, an oil
supply problem.

Since you stated that the governor has been replaced, the governor pump
should be functioning correctly. This leaves either a prop
(specifically prop hub) that is in need of an overhaul, or an
investigation into why the governor pump isn't being supplied with
sufficient oil.

Some questions that should be answered before you start digging into
things:

(1) Is the oil pressure well within the normal operating range?

(2) Have you done a prop governor check? By this I mean running up the
engine to say 2000 RPM and then pulling the prop back to about 1800 RPM
to put the prop on the governor and see how it maintains that selected
RPM. Try it again at full static power and then pull the prop back to
2600 RPM. If the governor will govern the RPM precisely at 2600 static,
leave the prop handle in that position and try a takeoff at 2600 RPM.
At that point, there is no reason that the prop RPM should fluctuate
radically unless you are internally leaking oil within the prop hub and
the governor just can't keep up with the props attempts at overspeeding
and thus the RPM oscillation. If it continues to fluctuate, even at a
governed 2600 RPM, I'd be looking to either an oil supply problem
(upstream of the governor pump) or a propeller problem (best diagnosed
by pulling the prop).

I had this very same problem in an Arrow a few years ago, but the
problem turned out to be the speeder spring in the governor which should
be just fine in your bird since you just replaced the governor.

--
Blue Skies,

***********************************************************
* Ryan R. Healy, C-ASMEL-I, CFI-ASMEL-I, AGI, IGI *
* E-Mail: mailto:rhea...@sprynet.com *
* Web Page: http://home.sprynet.com/sprynet/rhealy707 *
***********************************************************

[John R. Johnson]

On Tue, 16 Sep 1997, John gardon wrote:
<snip>

>
> All engine run ups, static RPM, prop cycling etc are normal. The second
>
> I release the brakes and commence rolling the RPM swings and surges
> between 23-2700 RPM obviously requiring a rejected take-off Manifold
> pressure remains steadythroughout. Static engine runs again produce
> normal results but as soon as brakes are released the scenario
> replicates.
>

These symptoms pretty positively the prop pitch system. This system is
not real complicated. In your propellor, oil pressure is used to reduce
the RPM. When the governor senses the engine speed increasing beyond
its set point it pumps some oil into the propellor hub. This increases
the pitch and reduces the RPM. Normally, the RPM should reduce smoothly,
but a hub due for an overhaul can get sticky. This causes the RPM to
surge.

Test: Run the engine up on the ground with the brakes locked.
Pull the prop control out to flush oil into the hub. Hold it until
the RPM stabilizes. Then return to takeoff position. Hold it
until the RPM stabilizes.

Repeat this several times. Watch the oil pressure during this
excercise. Does it behave normally? It may drop somewhat while
the engine is slowing down, but should come back up promptly when
the RPM stabilizes. It should not change at all when the prop
goes back to takeoff position.

If the RPM still surges after this excercise, then try an RPM about
100 RPM less that full power. This will give you a reduced power,
but will force the prop to govern before you takeoff. If the
RPM is steady at a slightly lower RPM it is probably safe to ferry.

With the engine stopped, check the blades for motion independently
of the hub. They should be tight and not wiggle or wobble in any
way.

The problem is either a bad governor, a bad prop hub, or no oil in the
engine.

John

[Gary Hillman]

Ryan R. Healy wrote:

>
> John gardon wrote:
>
> > All engine run ups, static RPM, prop cycling etc are normal. The second
> > I release the brakes and commence rolling the RPM swings and surges
> > between 23-2700 RPM obviously requiring a rejected take-off Manifold
> > pressure remains steadythroughout. Static engine runs again produce
> > normal results but as soon as brakes are released the scenario
> > replicates.
>

I have had new or fresh overhauld govoners that were not set correctly.
Make absolutely sure that the gov is correct for the engine model. The
only other thing in the system is the oil passage between the gov and
the prop. That means the front crank main journal. If the bearing has
turned slightly it may be restricting oil flow at high demand as the oil
passage may be partially covered.
Gary