Propeller Efficiency

On Apr 13, 8:20 pm, WingFlaps wrote:

Does anyone have any idea of the ratio between thrust power and churn
power?

Less than 80%. Look in Wiki for discussion of losses.

Wiki isn't so accurate. The figure for max efficiency is in the
range of 85 to 87%, depending on AOA and a bunch of other stuff. The
Wright Brother's propeller on their Flyer had an efficiency of 83%
because they understood that it was a rotating airfoil rather than
some sort of paddlewheel.
For prop math, see this: http://www.epi-eng.com/propeller_tec..._propeller.htm

Sure, the air will swirl around some as it leaves the prop. It
has to, since there is no such thing as a drag-free propeller. But
it's manageable. Anyone who thinks he can design a better propeller or
airplane or anything else is well advised to do his research first so
as to avoid spending vast sums of money making the same mistakes
dozens of other guys have already made. If the OP, who is a PPL
student and has been known to "know better than the experts" in the
past, wishes to design and build himself a phenomenally new and
successful airplane or flying car, he'll have a pretty hard time doing
it. There are hundreds, maybe thousands worldwide, of aeronautical
engineers who know the limits of the physics and materials involved
and they are often employed at very good salaries by huge aircraft
manufacturers who wish to save even a few percent on fuel consumption,
drag, safety risks and other costs just to give themselves a
perceptible advantage over the competition. Any large improvement at
this point is going to require some new technologies that don't exist
yet. Better to spend the time discovering those new technologies.
There are many garages and barns and landfills full of pointless
efforts at designing a new airplane. Most successful new designs are
variations on the same old theme we've had for a long time now.

Dan

On Apr 14, 10:11*pm, wrote:
* * *Sure, the air will swirl around some as it leaves the prop. It
has to, since there is no such thing as a drag-free propeller. But
it's manageable. Anyone who thinks he can design a better propeller or
airplane or anything else is well advised to do his research first so
as to avoid spending vast sums of money making the same mistakes
dozens of other guys have already made. If the OP, who is a PPL
student and has been known to "know better than the experts" in the
past, wishes to design and build himself a phenomenally new and
successful airplane or flying car, he'll have a pretty hard time doing
it. There are hundreds, maybe thousands worldwide, of aeronautical
engineers who know the limits of the physics and materials involved
and they are often employed at very good salaries by huge aircraft
manufacturers who wish to save even a few percent on fuel consumption,
drag, safety risks and other costs just to give themselves a
perceptible advantage over the competition. Any large improvement at
this point is going to require some new technologies that don't exist
yet. Better to spend the time discovering those new technologies.
There are many garages and barns and landfills full of pointless
efforts at designing a new airplane. Most successful new designs are
variations on the same old theme we've had for a long time now.

It is true that I am still a student PPL. And it is true that I tend
to fish in ponds long deemed to be devoid of fish. But I know that I
know less than others. However, in areas of science that I am
passionate about, I feel that it is better to not accept stocks answer
that smell fishy.

On the matter of flying cars, yes, someday I would like to take a shot
at design some kind of flying vehicle. It would be a daunting task to
say the least, but that would not deter me. Though it is true that
1000's of people have sought to make flying cars and failed, if you
look at their designs, many of them are cars with wings on them. I
doubt that this is the right way to make a PAV.

In the early days of flying, there were many things tried by many
people that we now know with certainty could not possibly work lest
they violate basic physical principles. Hindsight might be 20/20, but
forethought and more rigorous paper analysis could have preempted many
of these attempts, but people tried them anyway. They tried them
perhaps because they could not contain their passion and desire to
make a breakthrough. This is where I get my encouragement from, not
from thinking I know better. I learned a while back that discplined
thought, the kind that requires doing nothing but sitting still and
thinking, can be an inexpensive way to solve a problem.

I do have an idea about propulsion, which, ironically, was derived
from my initial exploration into whether backwash could cause lift.

*IF* my suspicions are correct, there would exist a new type of
propulsion system that would have very desirable attributes as far as
flying cars are concerned. It would, indeed, require a restatement of
the explanation of aerodynamics above the wing. But as I have no idea
whether it is correct, so I cannot yet say either way. I have tried
little paper models at home, which all seem to confirm my suspicions,
but needless to say, paper models do not constitute proof, and in any
case, I do not understand the physics well enough to be able to
explain them to someone else, even though I am convinced that what is
happening is _not_ entirely explained by prevailing aerodynamic
theory.

So if I were to begin fiddling with this problem, the first thing I
would do is focus on the parts that matter, to see if there is
anything worth pursuing. If it turns out that I am wrong, I would
abort.

I would _not_ spend years tweaking some aspect of the vehicle until I
finally squeezed 3% more efficiency out of it. As you noted, there
are many people who are much better than I ever will be at that.

-Le Chaud Lapin-

On Apr 15, 3:11*pm, wrote:
On Apr 13, 8:20 pm, WingFlaps wrote:

Does anyone have any idea of the ratio between thrust power and churn
power?

Less than 80%. Look in Wiki for discussion of losses.

* * * Wiki isn't so accurate. The figure for max efficiency is in the
range of 85 to 87%, depending on AOA and a bunch of other stuff. The
Wright Brother's propeller on their Flyer had an efficiency of 83%
because they understood that it was a rotating airfoil rather than
some sort of paddlewheel.
* * * For prop math, see this: *http://www.epi-eng.com/propeller_tec..._propeller.htm

There's nothing like theoretical efficiency calculations to impress.
I'll say it again, real props struggle to achieve 80%. Now Dan, before
you jump down my throat, note that of these calculations in your ref.
did not include vortex tip losses and most don't even consider
friction and never compressibility (which is major problem as the tip
goes near or supersonic). Basing efficiency purely on slip doesn't
work for real airscrews and the washout is nearly always _wrong_.

Cheers

On Apr 15, 3:53 am, WingFlaps wrote:
On Apr 15, 3:11 pm, wrote: On Apr 13, 8:20 pm, WingFlaps wrote:

Does anyone have any idea of the ratio between thrust power and churn
power?

Less than 80%. Look in Wiki for discussion of losses.

Wiki isn't so accurate. The figure for max efficiency is in the
range of 85 to 87%, depending on AOA and a bunch of other stuff. The
Wright Brother's propeller on their Flyer had an efficiency of 83%
because they understood that it was a rotating airfoil rather than
some sort of paddlewheel.
For prop math, see this: http://www.epi-eng.com/propeller_tec..._propeller.htm

There's nothing like theoretical efficiency calculations to impress.
I'll say it again, real props struggle to achieve 80%. Now Dan, before
you jump down my throat, note that of these calculations in your ref.
did not include vortex tip losses and most don't even consider
friction and never compressibility (which is major problem as the tip
goes near or supersonic). Basing efficiency purely on slip doesn't
work for real airscrews and the washout is nearly always _wrong_.

Cheers

Those calculations are more than theoretical. We know, in foot-pounds
per minute, what an engine produces, and we can take that directly to
the acceleration of the airplane or its cruise speed versus drag, and
come up with an efficiency figure.

Dan

On Apr 16, 3:05*am, wrote:
On Apr 15, 3:53 am, WingFlaps wrote:





On Apr 15, 3:11 pm, wrote: On Apr 13, 8:20 pm, WingFlaps wrote:

Does anyone have any idea of the ratio between thrust power and churn
power?

Less than 80%. Look in Wiki for discussion of losses.

* * * Wiki isn't so accurate. The figure for max efficiency is in the
range of 85 to 87%, depending on AOA and a bunch of other stuff. The
Wright Brother's propeller on their Flyer had an efficiency of 83%
because they understood that it was a rotating airfoil rather than
some sort of paddlewheel.
* * * For prop math, see this: *http://www.epi-eng.com/propeller_tec..._propeller.htm

There's nothing like theoretical efficiency calculations to impress.
I'll say it again, real props struggle to achieve 80%. Now Dan, before
you jump down my throat, note that of these calculations in your ref.
did not include vortex tip losses and most don't even consider
friction and never compressibility (which is major problem *as the tip
goes near or supersonic). Basing efficiency purely on slip doesn't
work for real airscrews and the washout is nearly always _wrong_.

Cheers

*Those calculations are more than theoretical. We know, in foot-pounds
per minute, what an engine produces, and we can take that directly to
the acceleration of the airplane or its cruise speed versus drag, and
come up with an efficiency figure.

And the answer is TADA less than 80%....

Cheers

On Apr 15, 3:11*pm, wrote:
On Apr 13, 8:20 pm, WingFlaps wrote:

Does anyone have any idea of the ratio between thrust power and churn
power?

Less than 80%. Look in Wiki for discussion of losses.

* * * Wiki isn't so accurate. The figure for max efficiency is in the
range of 85 to 87%, depending on AOA and a bunch of other stuff. The
Wright Brother's propeller on their Flyer had an efficiency of 83%
because they understood that it was a rotating airfoil rather than
some sort of paddlewheel.

83%? BS. Even if it had reached optimal speed it would have struggled
to get 70% (note the CFD calcs do not include surface roughness
losses):

http://www.fluent.com/about/news/new...i2_fall/a2.htm

It was lucky that Orville knew from tests a bit about about props (he
estimated 66% efficiency) or it might not have flown at all. That of
course was not such great insight on his part as the theory of
propellor design was well known from naval architecture.

Cheers

On Apr 15, 4:02 am, WingFlaps wrote:
On Apr 15, 3:11 pm, wrote:
The
Wright Brother's propeller on their Flyer had an efficiency of 83%
because they understood that it was a rotating airfoil rather than
some sort of paddlewheel.

83%? BS. Even if it had reached optimal speed it would have struggled
to get 70% (note the CFD calcs do not include surface roughness
losses):

http://www.fluent.com/about/news/new...i2_fall/a2.htm

It was lucky that Orville knew from tests a bit about about props (he
estimated 66% efficiency) or it might not have flown at all. That of
course was not such great insight on his part as the theory of
propellor design was well known from naval architecture.

Better see this: http://www.memagazine.org/flight03/propwr/propwr.html

Wright estimated an efficiency of 66%. Later, more
sophisticated tests on the Flyer's prop design gave an efficiency of
82%.

A quote from the article:

"These data show that the 1903 Wright propeller had a maximum
efficiency of 82 percent.
"Based on Wilbur Wright's notes on the fourth flight of Dec. 17,
1903, the Flyer had an estimated forward speed of 31 mph during the
steady flight portion of its path and the propellers were turning at
379 rpm, which yields an advance ratio of 0.85. Hence, the 1903 Wright
propellers were operating at a mechanical efficiency of slightly over
75 percent during steady flight.
"This was a remarkable feat, considering the state of propeller
knowledge prior to World War I.
"Since Wilbur estimated their propeller performance to be 66
percent in March of 1903, we found the results of our experimental
tests to be quite surprising. Using Wright bent-end propeller
reproductions as our reference test case (there are several well-
preserved sets in existence), we have subjected these propellers to
multiple wind tunnel tests. We recalibrated the instrumentation used
in the propeller tests and we subjected the bent-end geometry
propellers to a full Navier-Stokes equation computational fluid
dynamics analysis in order to affirm our test results. The bent-end
propellers had peak efficiencies of nearly 87 percent. The overall
comparisons between the numerical predictions and the test results
agreed. To our surprise, we learned that the Wrights' bent-end
propeller twist distribution (a variation of pitch angle with radius)
was in nearly exact agreement with modern computer-based designs over
the outer two-thirds of the propeller blade."

How's that?

Dan

On Apr 16, 3:11*pm, wrote:
On Apr 15, 4:02 am, WingFlaps wrote:





On Apr 15, 3:11 pm, wrote:
The
Wright Brother's propeller on their Flyer had an efficiency of 83%
because they understood that it was a rotating airfoil rather than
some sort of paddlewheel.

83%? BS. Even if it had reached optimal speed it would have struggled
to get 70% (note the CFD calcs do not include surface roughness
losses):

http://www.fluent.com/about/news/new...i2_fall/a2.htm

It was lucky that Orville knew from tests a bit about about props (he
estimated 66% efficiency) or it might not have flown at all. That of
course was not such great insight on his part as the theory of
propellor design was well known from naval architecture.

* * * *Better see this: *http://www.memagazine.org/flight03/propwr/propwr.html

* * * *Wright estimated an efficiency of 66%. Later, more
sophisticated tests on the Flyer's prop design gave an efficiency of
82%.

* * * * A quote from the article:

* * * "These data show that the 1903 Wright propeller had a maximum
efficiency of 82 percent.
* * * "Based on Wilbur Wright's notes on the fourth flight of Dec. 17,
1903, the Flyer had an estimated forward speed of 31 mph during the
steady flight portion of its path and the propellers were turning at
379 rpm, which yields an advance ratio of 0.85. Hence, the 1903 Wright
propellers were operating at a mechanical efficiency of slightly over
75 percent during steady flight.
* * * *"This was a remarkable feat, considering the state of propeller
knowledge prior to World War I.
* * * "Since Wilbur estimated their propeller performance to be 66
percent in March of 1903, we found the results of our experimental
tests to be quite surprising. Using Wright bent-end propeller
reproductions as our reference test case (there are several well-
preserved sets in existence), we have subjected these propellers to
multiple wind tunnel tests. We recalibrated the instrumentation used
in the propeller tests and we subjected the bent-end geometry
propellers to a full Navier-Stokes equation computational fluid
dynamics analysis in order to affirm our test results. The bent-end
propellers had peak efficiencies of nearly 87 percent. The overall
comparisons between the numerical predictions and the test results
agreed. To our surprise, we learned that the Wrights' bent-end
propeller twist distribution (a variation of pitch angle with radius)
was in nearly exact agreement with modern computer-based designs over
the outer two-thirds of the propeller blade."

* * * *How's that?


Very romantic. You'd think that good propellors had never been built
before... The only uncertainty they had to deal with was the actual
RPM their engine would generate -they already knew the drag numbers
for the flyer by kite testing the design. BUT, as I said, the
optimal effiiency is almost never reached so less than 80% is the
correct ball park figure for props. I'm also suspicious that the CFD
calculation quoted in that article was a lot higher than that reported
by researchers I gave you the ref. for...

Cheers