Feathering an engine

In article o_r_fairbairn-B758D1.12591306052009@70-3-168-
216.pools.spcsdns.net, says...
In article ,
"Tim" wrote:

"Orval Fairbairn" wrote in message
news
In article
,
wrote:

On May 5, 1:06 pm, Scott Skylane wrote:
As an aside, the drag created by a windmilling propeller, i.e. one not
feathered and attached to a dead engine, creates as much drag as a flat
plate the same size as the area of the prop arc.

Nope. The blades cannot be everywhere at once, and so the area
affected is no larger than the blade area.

Dan

YES! In aeronautical engineering analysis, a windmilling prop is
considered to be a flat disk, with drag numbers to match. Feathering the
prop greatly reduces drag.


So you are saying if I loose power at high altitude in a fixed pitch prop
aircraft, like a Skyhawk, I will have less drag if I stop the prop, as
opposed to letting it windmill?

That is correct!

Please provide some evidence.

--
Duncan

"Flanagan" wrote

It is so kind of you to reply, and your explanation is so interesting.
Thank you!

In addition, I would add that though it seems counter intuitive, a rotating
unfeathered prop has more drag (much more) than a unfeathered prop that is
not rotating. The feathering has two bonuses, in sorts then.

An unfeathered prop has lower drag stopped, so feathering the prop stops the
prop for the first reduction in drag, and feathering it after it is (or
during stopping it) reduces drag even again.

The reasons given about lessening damage, although true, are a very very
distant reason on why to feather. The MOST important reason is to reduce
drag, increase time in the air and gliding distance, because both of those
reasons mean LIFE. Never seen a better reason than that one, yet. g
--
Jim in NC

"Flanagan" wrote

It is so kind of you to reply, and your explanation is so interesting.
Thank you!

In addition, I would add that though it seems counter intuitive, a rotating
unfeathered prop has more drag (much more) than a unfeathered prop that is
not rotating. The feathering has two bonuses, in sorts then.

An unfeathered prop has lower drag stopped, so feathering the prop stops the
prop for the first reduction in drag, and feathering it after it is (or
during stopping it) reduces drag even again.

The reasons given about lessening damage, although true, are a very very
distant reason on why to feather. The MOST important reason is to reduce
drag, increase time in the air and gliding distance, because both of those
reasons mean LIFE. Never seen a better reason than that one, yet. g
--
Jim in NC

On May 6, 6:44*pm, Dave Doe wrote:
In article o_r_fairbairn-B758D1.12591306052009@70-3-168-
216.pools.spcsdns.net, says...





In article ,
*"Tim" wrote:

"Orval Fairbairn" wrote in message
news In article
,
wrote:

On May 5, 1:06 pm, Scott Skylane wrote:
As an aside, the drag created by a windmilling propeller, i.e. one not
feathered and attached to a dead engine, creates as much drag as a flat
plate the same size as the area of the prop arc.

Nope. The blades cannot be everywhere at once, and so the area
affected is no larger than the blade area.

Dan

YES! In aeronautical engineering analysis, a windmilling prop is
considered to be a flat disk, with drag numbers to match. Feathering the
prop greatly reduces drag.

So you are saying if I loose power at high altitude in a fixed pitch prop
aircraft, like a Skyhawk, I will have less drag if I stop the prop, as
opposed to letting it windmill?

That is correct!

Please provide some evidence.

--
Duncan- Hide quoted text -

- Show quoted text -

Here is a URL to a thesis that addresses the question. The answer,
based on his evidence, is, it depends.

http://www.goshen.edu/physics/PropellerDrag/thesis.htm

Dave Doe wrote:
Q2. I have a prop and I drag it through grease - as I do so it turns
to "allow" it it's passage through the grease. Now if I was to hold
the shaft so the prop does *not* rotate - surely that would be harder
to pull through the grease now. ?

Sounds like a slick idea, but I think you'd have to deep fry an awful lot
of french fries before you'd have enough to perform the experiment over a
reasonable distance. That assumes you don't die from a coronary first, what
with having to dispose of all that greasy food first....

In article ,
says...
Dave Doe wrote:
Q2. I have a prop and I drag it through grease - as I do so it turns
to "allow" it it's passage through the grease. Now if I was to hold
the shaft so the prop does *not* rotate - surely that would be harder
to pull through the grease now. ?

Sounds like a slick idea, but I think you'd have to deep fry an awful lot
of french fries before you'd have enough to perform the experiment over a
reasonable distance. That assumes you don't die from a coronary first, what
with having to dispose of all that greasy food first....

Well Jim, when yer prepared to give me a scientific evidence based
answer....

Hey ok, I admit that, the Reynolds numbers are quite different due to
the grease vs air (grease is almost 100% friction based drag, air is
largely pressure based drag). And perhaps therein lies the answer -
however I wanna see the maths.

Reading up on 'a's' link now...
http://www.goshen.edu/physics/PropellerDrag/thesis.htm

--
Duncan

In article c8d0834f-8e51-42ad-add6-
, says...
On May 6, 6:44*pm, Dave Doe wrote:
In article o_r_fairbairn-B758D1.12591306052009@70-3-168-
216.pools.spcsdns.net, says...





In article ,
*"Tim" wrote:

"Orval Fairbairn" wrote in message
news In article
,
wrote:

On May 5, 1:06 pm, Scott Skylane wrote:
As an aside, the drag created by a windmilling propeller, i.e. one not
feathered and attached to a dead engine, creates as much drag as a flat
plate the same size as the area of the prop arc.

Nope. The blades cannot be everywhere at once, and so the area
affected is no larger than the blade area.

Dan

YES! In aeronautical engineering analysis, a windmilling prop is
considered to be a flat disk, with drag numbers to match. Feathering the
prop greatly reduces drag.

So you are saying if I loose power at high altitude in a fixed pitch prop
aircraft, like a Skyhawk, I will have less drag if I stop the prop, as
opposed to letting it windmill?

That is correct!

Please provide some evidence.

--
Duncan- Hide quoted text -

- Show quoted text -

Here is a URL to a thesis that addresses the question. The answer,
based on his evidence, is, it depends.

http://www.goshen.edu/physics/PropellerDrag/thesis.htm

Thanks, so it really depends on the pitch of the propellor.

qv... you have a prop on the end of a shaft that has no engine, just a
braking mechanism (this is where I find it hard to get my head around
the maths! ...

You're say gliding through the air, the prop is freely spinning. Now,
we apply some braking to the shaft and slow down the prop. Basically
(and according to the maths you've shown), the drag will (dependent on
pitch, but for most fixed pitch props), increase. And "at the other
end" the brake will produce heat. The prop will slow and I would expect
the drag to *increase* and the aircraft attitude will need to be lowered
to maintain the same airspeed. But... according to the maths you've
shown, this is all dependent on the pitch of the prop. And, I *assume*
that fixed pitch props are too fine in pitch to be good windmills.

According to the maths, I assume that wind turbines are more efficient
if built really large, and spin slowly, rather than fast (which kinda
makes sense - certainly in known (expected) wind strengths.

--
Duncan

In article c8d0834f-8e51-42ad-add6-
, says...
On May 6, 6:44*pm, Dave Doe wrote:
In article o_r_fairbairn-B758D1.12591306052009@70-3-168-
216.pools.spcsdns.net, says...





In article ,
*"Tim" wrote:

"Orval Fairbairn" wrote in message
news In article
,
wrote:

On May 5, 1:06 pm, Scott Skylane wrote:
As an aside, the drag created by a windmilling propeller, i.e. one not
feathered and attached to a dead engine, creates as much drag as a flat
plate the same size as the area of the prop arc.

Nope. The blades cannot be everywhere at once, and so the area
affected is no larger than the blade area.

Dan

YES! In aeronautical engineering analysis, a windmilling prop is
considered to be a flat disk, with drag numbers to match. Feathering the
prop greatly reduces drag.

So you are saying if I loose power at high altitude in a fixed pitch prop
aircraft, like a Skyhawk, I will have less drag if I stop the prop, as
opposed to letting it windmill?

That is correct!

Please provide some evidence.

--
Duncan- Hide quoted text -

- Show quoted text -

Here is a URL to a thesis that addresses the question. The answer,
based on his evidence, is, it depends.

http://www.goshen.edu/physics/PropellerDrag/thesis.htm

Oh, BTW, it is not correct to for Orval (according to your link) to
*simply* say "that is correct" - as it is not.

Just looking at the conclusion...
.. Conclusions:

From the experimental results reported here we can conclude what we
could have figured out with a little thought: drag force increases with
length and wind velocity, and decreases with pitch. What is less clear
is how the drag force increases and decreases with these variables.
Does it increase linearly or quadradically with length and wind
velocity? When considering the pitch, does the windmilling drag force
also follow a cosine-squared curve? More accurate data are needed to
determine the characteristics of the crossover point. Does it depend on
wind velocity? Arguments both for and against rely on data that could
be drastically changed if just a couple of data points were moved.
Further work in this topic should begin either with an increase in the
range of the variables, or increasing the precision of the data.
Improving either one of these will help answer all of the questions
posed above.

The main goal of this investigation was simply to determine whether a
stationary or a windmilling propeller has more drag. The answer is
complicatedly simple: it depends. It is clear that it depends on the
pitch and length of the propeller, and it is probably independent of the
wind velocity. A crossover point was discovered where the drag forces
for the windmilling and stationary states were the same. This crossover
point is also dependent on the pitch, the length, and probably
independent of the wind velocity.

--
Duncan

In article ,
"Tim" wrote:

The point was how much do you gain by stopping a fixed pitch prop, as
opposed to letting it wind mill.

A lot.

You can actually do this experiment in a plane with a variable-pitch
prop. Idle the engine, and play with the prop control. I did this
years ago in a 182RG during dead stick landing drills. The difference
in glide performance between the two extreme prop settings was quite
dramatic, almost like having an extra set of very fast acting flaps.

There's also the conservation of energy argument. If the engine is
turning, the energy to overcome friction and compression has to come
from somewhere.

rg

In article ,
Ron Garret wrote:

There's also the conservation of energy argument. If the engine is
turning, the energy to overcome friction and compression has to come
from somewhere.

I don't think this works. In the non-spinning case, you're dissipating
all that energy into the air, and there's no real limit as to how much
that could be. Now, it would seem that the conservation-of-energy
argument gets you the right answer, but IMO not for the right reasons.

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon