Prop Clocking Matters

Kyle Boatright wrote:

I can tell you that 80 knots indicated is the minimum sink speed on
my RV-6 with the prop stopped, and the sink rate is 700-750 fpm at
that speed.

Huh. That seems fast, for an RV-6 (sink rate seems reasonable,
though). Van's lists the stall speed as 49 mph (43 kts). I would have
expected min. sink speed to be somewhere around 1.2 Vs, or 51 kts. 80
kts (92 mph) seems really high. My COZY has a stall speed of about 70
mph (61 kts), give or take, and the min. sink speed is around 80 mph
(70 kts), with a max L/D around 95 mph (83 kts). Sink rate at min.
sink is around 550 fpm.

Do you have sink rates for speeds from Vs up to 120 kts or so? I'd be
interested in seeing the #'s.

Thanks.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2007

"Marc J. Zeitlin" wrote in message
...
Kyle Boatright wrote:

I can tell you that 80 knots indicated is the minimum sink speed on
my RV-6 with the prop stopped, and the sink rate is 700-750 fpm at
that speed.

Huh. That seems fast, for an RV-6 (sink rate seems reasonable,
though). Van's lists the stall speed as 49 mph (43 kts). I would have
expected min. sink speed to be somewhere around 1.2 Vs, or 51 kts. 80
kts (92 mph) seems really high. My COZY has a stall speed of about 70
mph (61 kts), give or take, and the min. sink speed is around 80 mph
(70 kts), with a max L/D around 95 mph (83 kts). Sink rate at min.
sink is around 550 fpm.

Do you have sink rates for speeds from Vs up to 120 kts or so? I'd be
interested in seeing the #'s.

Thanks.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2007

A couple of things might come into play...

1) 80 knots was the indicated airspeed, and is subject to whatever errors
are present in my ASI and pitot/static system at that speed.

2) An RV-6 has a pretty low aspect ratio and therefore gets really draggy as
you approach stall speed. This may tend to increase minimum sink speed.

I didn't try to evaluate the glide outside a range of about 60 knots
indicated to 90 knots indicated.

KB

On May 4, 4:55 am, "Kyle Boatright" wrote:
"Marc J. Zeitlin" wrote in m...





Kyle Boatright wrote:

I can tell you that 80 knots indicated is the minimum sink speed on
my RV-6 with the prop stopped, and the sink rate is 700-750 fpm at
that speed.

Huh. That seems fast, for an RV-6 (sink rate seems reasonable,
though). Van's lists the stall speed as 49 mph (43 kts). I would have
expected min. sink speed to be somewhere around 1.2 Vs, or 51 kts. 80
kts (92 mph) seems really high. My COZY has a stall speed of about 70
mph (61 kts), give or take, and the min. sink speed is around 80 mph
(70 kts), with a max L/D around 95 mph (83 kts). Sink rate at min.
sink is around 550 fpm.

Do you have sink rates for speeds from Vs up to 120 kts or so? I'd be
interested in seeing the #'s.

Thanks.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2007

A couple of things might come into play...

1) 80 knots was the indicated airspeed, and is subject to whatever errors
are present in my ASI and pitot/static system at that speed.

2) An RV-6 has a pretty low aspect ratio and therefore gets really draggy as
you approach stall speed. This may tend to increase minimum sink speed.

I didn't try to evaluate the glide outside a range of about 60 knots
indicated to 90 knots indicated.

Any short-winged airplane will sink pretty good when slow. My
Jodel stalls at around 40 but it's sinking so fast by the time I get
there that the stall speed is irrelevant. If I get below 60 indicated
the bottom falls out pretty quick.
Prop indexing has something to do with the prop's orientation
during the power pulse. Most of these opposed engines have a slight
rocking moment around the vertical axis, caused by the offset of the
opposing cylinders, and if the prop is more or less horizontal during
the biggest push, it will prevent most of that rotation just because
of the location of the blade masses and felt vibration wil be
minimized. Having the prop at 10:00/4:00 has the prop passing through
the horizontal during much of the power stroke.
That's what I understand, anyway. Vibration analysis defies
intuitive thinking so I might be all wet. Something I don't understand
is the idea that old guys have told me: if the alternator belt on your
6-banger Continental with its three-bladed prop keeps flipping off,.
rotate (re-index) the prop 180 degrees and it will stop doing that. It
works, too: on a 185 we used to have, the alternator belt regularly
flew off until we rotated that prop. I didn't think a three-blade prop
would have any sweet spots, but as I said, vibration analysis defies
intuitive thinking.

Dan

"Kyle Boatright" wrote in message
. ..

"Marc J. Zeitlin" wrote in message
...
Kyle Boatright wrote:

I can tell you that 80 knots indicated is the minimum sink speed on
my RV-6 with the prop stopped, and the sink rate is 700-750 fpm at
that speed.

Huh. That seems fast, for an RV-6 (sink rate seems reasonable,
though). Van's lists the stall speed as 49 mph (43 kts). I would have
expected min. sink speed to be somewhere around 1.2 Vs, or 51 kts. 80
kts (92 mph) seems really high. My COZY has a stall speed of about 70
mph (61 kts), give or take, and the min. sink speed is around 80 mph
(70 kts), with a max L/D around 95 mph (83 kts). Sink rate at min.
sink is around 550 fpm.

Do you have sink rates for speeds from Vs up to 120 kts or so? I'd be
interested in seeing the #'s.

Thanks.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2007

A couple of things might come into play...

1) 80 knots was the indicated airspeed, and is subject to whatever errors
are present in my ASI and pitot/static system at that speed.

2) An RV-6 has a pretty low aspect ratio and therefore gets really draggy
as you approach stall speed. This may tend to increase minimum sink speed.

I didn't try to evaluate the glide outside a range of about 60 knots
indicated to 90 knots indicated.

KB

I look up the old 1993 CAFE report where they extensively flight tested an
RV-6a. That airplane's minimum sink speed was determined to be 70 knots, but
that was using their "zero thrust" testing method. With that technique, it
is as if the prop doesn't exist - it neither provides thrust or drag. Not a
terribly good measure, if you ask me, because getting rid of the prop in
flight isn't a user selectable option on most airplanes.

In addition to instrumentation differences (CAFE does a very good job
installing and calibrating the instruments in their test aircraft), my
airplane has some physical differences from the test airplane. First, mine
has the stock cowl, which may be draggier than the test airplane. My
airplane has lower drag gear leg fairings and wheel pants than the test
aircraft. In addition, the test aircraft had a nosewheel, as opposed to my
taildragger.

What does all of this tell us?

Dunno.

KB