Propellors vs Rotors

Don W wrote:
Smitty Two wrote:

So what am I missing in this? I can see there might be some cost, ground
clearance, and possibly vibration issues involved in putting a twelve
foot diameter prop on my RV. But hypothetically speaking, are there
other, perhaps more important, reasons not to make the prop bigger and
the engine smaller and slower?

Smitty,

You just asked the $24 question that I am dying to find out the answer
to. Seems to me that thrust is thrust, and thrust is what makes
airplanes go fast, but I'll bet I'm missing something here, and someone
will set me straight.

Don W.

A larger, slower prop will provide better takeoff and
climb, but unless its blades are variable in pitch (as in a
constant-speed setup) your forward velocity will be lower in cruise.
You can't swing a large prop anywhere near fast enough to allow the
engine to develop its horses unless that prop has a low pitch, while a
smaller, higher-pitched prop will "slip" more easily at low forward
speeds. The smaller fixed-pitch prop compromises more easily.
WW2 fighters like the Corsair had huge props (14 feet or
so?) that turned at 1300 for takeoff and around 900 in cruise. The low
cruise RPM was necessary to keep the tip speeds within reason. A (tip
speed) squared plus B (forward speed) squared equals C (total tip
speed) squared. The prop's blade angles at the tip in cruise were more
than 45 degrees to maintain a workable AOA.

Dan

Don W wrote:

Hi Colin,

I was having a lot of fun in the Robinson until I made the mistake
of looking through the NTSB accident database. Wow! Those things
have a _much_ higher accident rate for the hours flown than
other helicopters. The main rotor loss of control accident rate
was 4x higher than the next worse helicopter (Bell 204).

(oddly enough, the Bell 206 had the lowest loss of control accident
rate for the hours flown at .015 fatal LOC accidents per 100K flight
hours.)

This is based on data taken from 1981 - 1994, and can be found
on page 12 of the following PDF:

http://www.ntsb.gov/publictn/1996/SIR9603.pdf

Scary stuff!!

Don W.


COLIN LAMB wrote:

Hi Don:

Yes, we are taught that if the rpm gets too low, we are dead. If the
rpm gets too high, the gearbox is blown. Keep the rotor in the green
or you may not walk away - and you have 1.75 seconds to drop the
collective when the engine quits in the Schweizer - even less in the
Robinson.

But, where else can you pay $200 per hour to move one foot away from
where you started and work up a sweat doing it, all while having a big
grin.

Colin




When you finally find out what's really going on,
helicopters are about the scariest machines ever made.

Some argue - second to "nuclear reactors built by the lowest bidder",
but that doesn't detract much from the claim...

If they were not so - unbelievably useful -
they would never be built.

- - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - -

Rotor rpm decay rate is a function of rotor inertia.

Robinson R-22 grosses 1370 pounds.

At an average 3300 pounds gross, the Bell 206 over twice as heavy.
And Bell makes special comment on the 206 high inertia rotor system.
Turbine time too.

Even a die-hard fixed wing fanatic has to admit,
that's a hell of an airplane.


Have fun having fun!


Richard

-------------snip------------

Better climb rates.

Quite possibly, but definitely steeper climb angles


A big slow prop should be good for a STOL airplane (Glider tug) and maybe
not so good for a cruiser. If I wanted maximum range on minimum fuel and
wasn't too concerned about cruise speed, I'd look at something like a
touring motorglider with a big, slow prop.


I agree completely,
Regrettably, I am still a bit of a speed freak.... ;-)

Peter

The Robinson requires more training time than the Schweizer. I think I
calculated that you have about 1.75 seconds to go into autorotation in a
Schweizer if the engine quits, but less than a second in a Robinson. With
practice, the 1.75 seconds is a piece of cake. I have never flown a 206,
but they sound wonderful.

Going through the FAA rotorcraft book (free on the internet), there are 8 or
9 easy ways to get into trouble with a helicopter. The gyrocopter, on the
other hand, only has a a couple - and those can be remedied. However, the
gyrocopter cannot hover.

Colin

Well ,for ****s sake, I thought ducks were made of depleted mercury!

Cam

"ELIPPSE" wrote in message
oups.com...
Well, the way I hears it, the low pressure on the wing is produced by
the Coanda effect as air travels up and over the wing, not because of a
change of velocity.The velocity change hypothesis has been disproven
over and over! The mass of air displaced downward is the result (read:
reaction) to the air having clung-to and been moved around the surface
by the Coanda effect. Lift is caused by a pressure difference, not the
downwash. There is no such force as suction, and downwash doesn't
generate lift.

When You look up, does gravity push?

Cam

GET BACK IN THERE! 'YA DAMN WORMS!

Yea, rotor wash is real. I've been in it... under an AS Puma slow
orbiting just over wet pine trees, 100 ft. from a Bell 412, 212, 214,
206, 222, Boelkow 105C, AS AStar, AS Twinstar, Hughes 500C getting out
of a Bell 206-B3, Robinsin R44, Bell 206-L1, and numerous other
encounters that I can't quite remember over the years. My Dad was a
helicopter A&P.

Yea, a significant portion of the air surrounding a hovering helicopter
is moved downward, just as fixed-wing props throw back a pretty good
blast. So I guess it is possible that fixed wings do deflect some air
downward, though I've never felt it while flying hang-gliders. And
that's still not what makes wings work. Its one of the things that
makes them less efficient.

Wilbur Wright struggled with this very issue for months while
attempting to develop his propeller theory. And Wilbur's theory is that
propellers are not "screws", no or they fans. They're airfoils and not
rotating "aero planes". Not that you can't propel a plane or SkyCar
with a ducted fan, but thats reaction-thrust from my understanding. A
propeller is a wing traveling in a corkscrew path through air. Some of
the energy consumed by the prop makes lift and pulls the aircraft
forward. Some of the energy consumed by the prop pulls the air
backward. Developing a prop that puts enough energy into pulling the
plane forward and not just swishing the air around is the trick. Its
kinda like trying to turn a bolt with a wrench in space. Your arm can
turn the wrench in reference to you, or it can turn you in reference to
the wrench. In reality, space arms and propellers are pretty good at
doing some of both.

Wilbur and Orville used the largest props that would fit on their
airframe. In 1903 those were 8' 6" each and turned between 300 and 350
rpm depending on how hot the engine was. At an average of 8.56hp (the
engine only made 11.78hp for a few seconds dead cold), the twin props
produced an average of 96 lbs of thrust. or 11.22 lbs of thrust per hp.
Not bad on the first try.

Getting back to the original issue, here's another experiment. Hold a
peice of paper vertically. Grasp its lower edge with your thumb and
forefinger. Now let the paper drape over your wrist so that the free
edge hangs down and away from you. Now blow along the upper surface of
the paper. DO NOT let any part of your breath blow under the paper. See
what happens. Hmmmmmmm. What's holding that paper up? All of the air
that it, the sheet of paper, is throwing downward, all on its own,
because it instinctually "knows" that this is the correct behavior for
good little sheets of paper that get blown on? Hold your other hand
under the paper as you blow. Any air moving downward? And what's the
paper doing? Hmmm?

BTW, addressing my previous statement about AOA, some planes can
definately climb nose-down in upright flight. Amazing, but the B-52 is
one of them. I was reminded of this 2 days ago while watching the
Hitler Channel. Looks goofier than hell.

The original Wright 16" wind tunnel did not survive history. However,
the original balances and test airfoils did and are currently at the
Franklin Institute in Philly. Orville stored them in a box for years
and almost threw them out once. Thanks for shaking that box Orv. There
are numerous reproduction wind tunnels in museums. I'm planning to
build one myself. Nick Engler had blueprints for one on his website
http://first-to-fly.com/Adventure/Wo..._and_drift.htm

Harry "rotor-ramp-rat" Frey

Hi Harry,

wright1902glider wrote:
GET BACK IN THERE! 'YA DAMN WORMS!

;-)

The original Wright 16" wind tunnel did not survive history. However,
the original balances and test airfoils did and are currently at the
Franklin Institute in Philly. Orville stored them in a box for years
and almost threw them out once. Thanks for shaking that box Orv. There
are numerous reproduction wind tunnels in museums. I'm planning to
build one myself. Nick Engler had blueprints for one on his website
http://first-to-fly.com/Adventure/Wo..._and_drift.htm

Harry "rotor-ramp-rat" Frey

I misunderstood what I was seeing at the Air Force museum. You
are correct that it is a 3/4 scale replica constructed under Orville
Wright's guidance sometime before WWII.

http://www.centennialofflight.gov/wb...ind-tunnel.pdf

This link also refers to the balances and experiments you are talking
about.

Thanks for correcting me.

BTW, I think that the discussion about the way wings work
is fascinating although slightly off the original topic.

IIRC the diplacement of the air molecules around a wing can be
conclusively demonstrated by multiple smoke streams in a wind
tunnel, or by mutiple dye streams in a water tank. I remember
such demonstrations in the lab back at good ole Wichita State
U.

I've only got one such photo available right now, and it is on page
141 of "Fluid Mechanics" 5th edition by Ray Binder. It shows a
symmetric airfoil at approximately 20 degrees AOA. In the photo
_all_ of the smoke streams (e.g. both the ones above the airfoil
and the ones below) that are disturbed by the airfoil end up
lower than they started out. This shows that a symmetric
airfoil at positive AOA pushes the air below it down, and _also_
pulls the air above it down. The net result is a slight downward
displacement of a _lot_ of air including air that is ~2x the chord
away from the airfoil.

I'll try to see if I can find similar photos on the web because I
think it will enliven this discussion to the general benefit of all
the participants (including me).

Don W.

wright1902glider wrote:

GET BACK IN THERE! 'YA DAMN WORMS!

Hi Harry,
snipped in places...


Wilbur and Orville used the largest props that would fit on their
airframe. In 1903 those were 8' 6" each and turned between 300 and 350
rpm depending on how hot the engine was. At an average of 8.56hp (the
engine only made 11.78hp for a few seconds dead cold), the twin props
produced an average of 96 lbs of thrust. or 11.22 lbs of thrust per hp.
Not bad on the first try.

96 pounds of thrust from 11 horse?

What did that whole rig weigh?



what happens. Hmmmmmmm. What's holding that paper up? All of the air
that it, the sheet of paper, is throwing downward, all on its own,
because it instinctually "knows" that this is the correct behavior for
good little sheets of paper that get blown on? Hold your other hand
under the paper as you blow. Any air moving downward? And what's the
paper doing? Hmmm?

But you Cheated!

Very localized pressure field resulted above the paper, and so what?

You sped up the air above the paper by blowing it.
(cheater)



BTW, addressing my previous statement about AOA, some planes can
definately climb nose-down in upright flight. Amazing, but the B-52 is
one of them. I was reminded of this 2 days ago while watching the
Hitler Channel. Looks goofier than hell.

BIG Lift Fairies!

under the paper as you blow. Any air moving downward?

None directly below the paper. There *is* air moving down
behind the paper

Here is another interesting way to think about air and lift - stolen
from basic Chemistry.

Don't think in terms of flow but of "vibration" of the individual air
molecules. At any temperature above absolute zero there will be
movement, or "vibration" of the fluid molecules. With no "flow" the
probability of each molecule of air being in a particular place can be
visualized as a sphere of decreasing density. Around a solid body at
rest the impacts of these air molecules will be at a given even rate
all about the surface. The "pressure" exertided is the same at all
points. Move either the surface or the air molecules (induce flow) and
the relative shape of that variable density sphere will change with
respect to the surface and the average impact vector of each will also
shift . If there is a net difference in the "flow" about the solid
then there will also be a corresponding change of impact, and impact
average vector, of the air molecules about the solid . This leads to
an imbalance that results in varying pressure at each point and the
resulting movement of the solid.

Thinking this way it's possable to see how both Newton and Bernouli
explain exactly the same thing, just with different, but equal,
mathmatical models. Kind of like the different ways of mathmatically
expressing a simple line.

Thinking this way helps to "see" how density and temperature change
lift, how skin drag works, how the boundry layer is formed and even how
things like humidity and viscosity change things.

I've probably done a really poor job of describing how I visiualize
this so I may have confused things much more than I intended.
================
Leon "Brownian Motion" McAtee