Propeller Efficiency

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, 3:11*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, 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

Not trying to invalidate the point you're making, but sometimes we
simply get lazy or our brain goes asleep. I got some useful help here
after having asked what turned out to be something easily found had I
been better at searching.

However, trom a technical perspective going from an 80% efficient
prop to 100% will most likely not do much to change a design from
fantasy to 'realizable'. There's a hierarchy of problems associated
with design, and this particular one, prop efficiency, would be pretty
far down on most designers' lists if they were looking to design an
innovative GA airplane that converts to a car.

It would be nice to save 20% of one's fuel, though.

Sometimes those not skilled in the art make breakthroughs, most times
not. If you saw the potential cancer treatment talked about on 60
Minutes last weekend you were were treated to something that most
likely will fall into the "not" group.



. On Apr 14, 10:59*pm, Jim Logajan wrote:
"Morgans" wrote:
Jim, have you forgotten?

Le Chaud Lapin is a MX sock puppet, or so most think.

I recognize "Le Chaud Lapin" from previous threads. I also recall that he
eventually stated it was a mistake to have posted a question on
aerodynamics to a discussion group having to do with piloting. I'm sure he
remembers his own writing.

On Apr 15, 10:17*am, Tina wrote:
Not trying to *invalidate the point you're making, but sometimes we
simply get lazy or our brain goes asleep. I got some useful help here
after having asked what turned out to be something easily found had I
been better at searching.

That's actually what happened in my case.

However, trom a technical perspective *going from an 80% efficient
prop to 100% will most likely not do much to change a design from
fantasy to 'realizable'. *There's a hierarchy of problems associated
with design, and this particular one, prop efficiency, would be pretty
far down on most designers' *lists if they were looking to design an
innovative GA airplane that converts to a car.

A hieararchy indeed. One would have to solve many problems at once.
I'm already spending too much time thinking about this, but if I were
to have a go at it, right now, I would aim for:

1. One seat-only, initially.
2. Structural symmetry throughout, whenever feasible.
3. Inexpensive USB-based COTS sensors everywhere.
4. Inexpensive USB-based COTS controls everywhere.
5. Elimination of conventional ICE and prop. [Biggest impediment to
flying car, in my not-sufficiently-educated opinion].
6. Even weight distribution. Ideally, the aircraft would have a box-
like structure.
7. Glass-cockpit everything with marginal cost of $1000 for
commodity CPU. *No more* Garmin.
8. Pressurized cabin.
9. Computer assisted take-off, computer assisted approach, computer-
assisted stabilization, computer-assisted tracking.
10. Ultralight components (no pun intended). I see no fundamental
reason that a 100kg man should ride in 1000kg vehicle. Use plastic and
other frilly components if doing so does not compromise structural
integrity or pilot safety.
12. Abnormally-scary dependence on fly-by-wire. If it can me made
electronic instead of mechanical, make it so.
13. Basic safety features (parachute, auto-oxgen, auto-extinguisher,
auto-pilot when computer senses that pilot is incoherent)
14. Convenient means of entry an exit. Grandma should not have to
mount the wing.
15. Efficiency - all that heat lost by ICE, plus 20% loss due to prop
twisting air, plus unnecessary weight from all those mechanical
components that could just as well be made of plastic actuators.
15. Leather seats.
16. Luxury sound system including digital radio.
17. Video-games (including Microsoft Flight Sim).
18. Inter-aircraft communication using WiMax (or something similar).
Proximity detectors, etc.
19. Pre-heating and pre-cooling of cabin.
20. Three-liter water tank with spigot on dash.
21. Air conditioner.
22. Integration of all instruments into computer monitors with few
exceptions (backup compass, backup altimeter, backup etc.)
23. USB camera mounts around the aircraft
24. Electronic megaphone for voice communication to those in immediate
vicinity of aircraft.
25. Real-time capture of all flight data in minutest detail onto
sealed hard disk for when it crashes.
26. Elimination of rudder control with foot pedals. Computers should
make this unnecessary, right?
27. Significant reduction in sound pollution.
28. Rear-mounted fuel-tank.
29. Trash bin.
30. Order of magnitude more control over the orientation of
aerodynamic surfaces. [IMO, this represents and *enormous* opportunity
reduce requisite skill in flying aircraft].

It would be nice to save 20% of one's fuel, though.

Sometimes those not skilled in the art make breakthroughs, most times
not. If you saw the potential cancer treatment talked about on 60
Minutes last weekend you were were treated to something that most
likely will fall into the "not" group.

What do you think of Steve Moller? I have seen respectable
commentators laud his work, but...

-Le Chaud Lapin-

Le Chaud Lapin wrote:

26. Elimination of rudder control with foot pedals. Computers should
make this unnecessary, right?


How does the computer know when you *want* to slip?

On Apr 15, 12:08 pm, Le Chaud Lapin wrote:

I'm already spending too much time thinking about this, but if I were
to have a go at it, right now, I would aim for:

1. One seat-only, initially.

Mm. Utility.

2. Structural symmetry throughout, whenever feasible.

Symmetry often means simple which in turn often means heavy.
Bridges are complex affairs to save weight so they don't collapse
under their own mass. The lightest and most useful airplanes are
usually rather complex structurally.

3. Inexpensive USB-based COTS sensors everywhere.
4. Inexpensive USB-based COTS controls everywhere.
5. Elimination of conventional ICE and prop. [Biggest impediment to
flying car, in my not-sufficiently-educated opinion].

So, electric, which is really heavy and has short range, or a
turbine, which makes the money saved using COTS sensors look tiny
indeed. Or were you maybe thinking nuclear?

6. Even weight distribution. Ideally, the aircraft would have a box-
like structure.

Drag, big time. Corners, even corners aligned with the flight
path, create drag. Even weight distrubution will mean CG problems, or
spin recovery issues.

7. Glass-cockpit everything with marginal cost of $1000 for
commodity CPU. *No more* Garmin.
8. Pressurized cabin.

Weight. Lots of weight. The systems to control it weigh more,
too. A fuselage strong enough for pressure is considerably heavier
than its non-pressurized counterpart.

9. Computer assisted take-off, computer assisted approach, computer-
assisted stabilization, computer-assisted tracking.

Might as well leave the pilot on the ground. Why bother
learning to fly? Besides, there are already too many people driving
airplanes that know too little about flying.

10. Ultralight components (no pun intended). I see no fundamental
reason that a 100kg man should ride in 1000kg vehicle. Use plastic and
other frilly components if doing so does not compromise structural
integrity or pilot safety.

Already been done. Leeon Davis designed a single-seat airplane
that weighed 177 lb and was powered by an 18 HP Briggs industrial
engine. Clocked well over 100 mph.

12. Abnormally-scary dependence on fly-by-wire. If it can me made
electronic instead of mechanical, make it so.

What are you saying? Fly-by-wire is scary but we'll make it so
anyway?

13. Basic safety features (parachute, auto-oxgen, auto-extinguisher,
auto-pilot when computer senses that pilot is incoherent)

Parachutes weigh something and take up considerable room.
Oxygen tanks are heavy, too and take up more room. Where is the pilot
supposed to go in this little airplane? And what is he doing in it
when he's incoherent? And what happens if the computer incorrectly
decides he's incoherent and takes over just when the pilot, who sees a
danger approaching, decides to avoid that danger and the computer
decides NO?
The Piper Arrow had an automatic gear-extension system to
prevent the pilot from landing gear-up. It sensed pitot pressure and
dropped the gear below a certain airspeed. Trouble was that this
"safety feature" killed a few folks when the pitot tube iced up and
the system thought airspeed had dropped, so it lowered the gear,
adding drag and another ice-catcher just when the pilot was struggling
to stay airborne long enough to get out of the ice. These automatic
systems sound nice until the unforeseen occurs. Those unforeseen
things are why it's harder to get a pilot's license than a driver's
license. You have to know what's going on.

14. Convenient means of entry an exit. Grandma should not have to
mount the wing.


Good luck. Structural nightmare. The Cessna Cardinal successfully
addressed this back in 1968, but the weight penalty was significant.

15. Efficiency - all that heat lost by ICE, plus 20% loss due to prop
twisting air, plus unnecessary weight from all those mechanical
components that could just as well be made of plastic actuators.

Fantasy. Like I said, find those new technologies. As far as
plastic goes, the Boeing 787 is mostly plastic but its control are
metal. Plastic does not do well handling hot hydraulic fluid.

15. Leather seats.

Weight.

16. Luxury sound system including digital radio.

More weight. And a distraction. Stay at home in your living room.

17. Video-games (including Microsoft Flight Sim).

Now there's an intelligent thing. Flying while pretending to
fly.

18. Inter-aircraft communication using WiMax (or something similar).
Proximity detectors, etc.

Weight. Complexity. Expense.

19. Pre-heating and pre-cooling of cabin.

With what? Heaters and air conditioners weigh a lot.
Especially air conditioners.

20. Three-liter water tank with spigot on dash.

Another eight or nine pounds.

21. Air conditioner.

See above.

22. Integration of all instruments into computer monitors with few
exceptions (backup compass, backup altimeter, backup etc.)

Been done.

23. USB camera mounts around the aircraft
24. Electronic megaphone for voice communication to those in immediate
vicinity of aircraft.
25. Real-time capture of all flight data in minutest detail onto
sealed hard disk for when it crashes.
26. Elimination of rudder control with foot pedals. Computers should
make this unnecessary, right?

Someone is lazy.

27. Significant reduction in sound pollution.

Mufflers. More weight.

28. Rear-mounted fuel-tank.

So fuel splashes forward over everything when the airplane
ccrashes, and so that the CG wanders all over the place as fuel is
burned.

29. Trash bin.

The whole design should go into this trash bin.

30. Order of magnitude more control over the orientation of
aerodynamic surfaces. [IMO, this represents and *enormous* opportunity
reduce requisite skill in flying aircraft].

OK. Design an "airplane" with all those goodies, and see
just how heavy it will be. It'll have a stall speed in the range of
120 MPH. Even the 1000 kg airplane that carries the 100 kg man, the
airplane you think is inefficient, already exists and has some of the
above goodies. That's why it weighs so much and can carry so little.

You forgot de-icing systems.

Dan

On Apr 15, 1:21*pm, B A R R Y wrote:
Le Chaud Lapin wrote:

26. Elimination of rudder control with foot pedals. Computers should
make this unnecessary, right?

How does the computer know when you *want* to slip?

When you move the controls in such a way to indicate that you want
slipping.

I would design first for mechanism, leaving policy 100% in the
software domain. For example, I imagine that there are many "hard-
coded" feedback systems in basic GA aircraft, mechanical controls for
which aerodynamicists have spent years determining the optimal
interdependencies, then effecting these interdependencies through
mechanical interlocks. I would break all of these interlocks, and move
all actuation to software control (even external light systems).

So if you move the joysticks to indicate that you want slipping, the
software would do whatever is necessary, at that particular instant,
based on set of formulas, to achieve the effect you seek.

-Le Chaud Lapin-

On Tue, 15 Apr 2008 12:21:07 -0700 (PDT), Le Chaud Lapin
wrote:

On Apr 15, 1:21*pm, B A R R Y wrote:
Le Chaud Lapin wrote:

26. Elimination of rudder control with foot pedals. Computers should
make this unnecessary, right?

How does the computer know when you *want* to slip?

When you move the controls in such a way to indicate that you want
slipping.

If you flew, you'd know how silly that comeback is.