Propellors vs Rotors

BTW, a 700 rpm rotor is pretty small and may be really inefficient--even by
helicopter standards!

The Schweizer 300 (formerly Hughes 269C) has a rotor rpm power on of 442 to
471 rpm. Power off range is 390 to 504 rpm. Esceed these limits and you
are quite likely to break something.

The Schweizer has 190 hp and gross weight is 2050 lbs.

Colin

Hi Colin,

I cant remember what the max rotor rpm on the R22 Beta is although
I have a few hours in them. I just make sure that both guages
stay in the green arc and that little light and the annoying horn
don't come on ;-).

Don W.

COLIN LAMB wrote:

BTW, a 700 rpm rotor is pretty small and may be really inefficient--even by
helicopter standards!


The Schweizer 300 (formerly Hughes 269C) has a rotor rpm power on of 442 to
471 rpm. Power off range is 390 to 504 rpm. Esceed these limits and you
are quite likely to break something.

The Schweizer has 190 hp and gross weight is 2050 lbs.

Colin

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

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

At the risk of opening up a huge can of worms, I have 2 questions and
one statement:

1. If a helicopter makes lift by displacing air downward with its
rotor:

Rotor blades are airfoil shaped (I've seen 'em) just like airplane
wings. Therefore airplanes fly by displacing air downward with their
wings? There's something wrong with your logic Sir Maxim. It would
seem that we killed this theory about 104 years ago with Will & Orv's
little wind tunnel. Recall, the flat inclined surface displaced more
air than any of the airfoil surfaces as measured by the vane balance.
However, it also made less lift than any of the airfoil surfaces at a
similar AOA. Ergo, an airfoil makes lift not by displacing air
downward, but by producing a condition where the air flowing across its
upper surface travels faster, and therefore has less pressure, than the
air flowing under its lower surface. Therefore, an airfoil wing does
not "fly" by displacing air downward, but rather exploits a zone of
differential pressure caused by a difference in the speed of the
airflow. And since a helicopter rotor blade is a long skinny wing
flying around in a circle, it produces lift just the same as an
airplane's wing does. I can only think of 2 machines that fly by
displacing air downward. Those would be ballistic rockets/missles, and
the Harrier jet in vertical or hovering flight.

2. A helicopter glides forward on an inclined cushion of displaced air:

A helicopter flies in a chosen direction due to the cyclic change in
rotor blade pitch impatred by an inclined swash plate. What's a swash
plate do? Well, imagine a doughnut smashed between 2 dinner plates. The
dinner plates are fixed to the fuselage and do not rotate. The doughnut
rotates at the same rate as the rotor head. When you tilt the dinner
plates, you also tilt the doughnut. Now if the doughnut is attached to
the rotor blade pitch-control horns by rotor blade pitch-change links,
the links will go up and down relative to the fuselage as the tilted
doughnut spins. This pushes and pulls on the rotor-blade control horns,
constantly changing the pitch of the blade as it flys around in a
circle. If you tilt the dinner plates forward, the blade flys at a
lower AOA in the front 1/2 of the rotor disk than it does at the back
1/2. Since its producing more lift in the back 1/2 than in the front
1/2, the blade flies higher in back. Stay with me here. As the blade
flies higher, its coning angle relative to the rotor head increases to
a greater angle than it does in the forward 1/2 of the rotor disk..
Therefore, its line of thrust relative to the fuselage is not vertical,
but is actually inclined forward. A helicopter "pulls" itself forward
through the air, more or less.

3. Rotor blades turning at 700 rpm vs. a prop turning at 2600 rpm.

Well, helicopter rotors don't turn that fast. Most are somewhere in the
300-350 rpm range. A Boeing Vertol CH-47's rotors only turn at 255 rpm,
or so I've heard. If I'm not mistaken, Hughes once built some kinda
giant tip-thrust powered test-freak that had a rotor speed of about 16
rpm. I've seen the videos, but I can't recall the name.

I could of course be completely and totally wrong about all of this. It
might just be fairies and Leprachauns.

Harry

"Therefore, an airfoil wing does not "fly" by displacing air downward, but
rather exploits a zone of differential pressure caused by a difference in
the speed of the airflow."

Does this mean that airplanes cannot fly upside down, or does the shape of
the wing change when the airplane rolls over?

Colin

Does this mean that airplanes cannot fly upside down, or does the shape of
the wing change when the airplane rolls over?

Colin

Good question. In most aircraft, the airfoil shape does not change.
However, assuming that an airplane maintains level flight at +5 degrees
AOA relative to the horizon, and then rolls over upside down, what is
its new AOA? That would neg. 5 degrees. Gravity aside, the plane would
fly in a downward direction because the air flowing over the "upper"
surface of the airfoil would still be traveling at a higher rate
relative to the "lower" surface.

Of course, if a pilot rolls her plane over and then pushes forward on
the stick, that would produce a positive AOA relative to the horizon.
How much would be enough to increase the flow over the "lower" surface
of the wing (now sunny-side up) than on the "upper" surface? I dunno.
Push the stick forward till we quit flying down toward the dirt.

Some planes, and some helicopters, have semetrical airfoil wings. They
achieve a differential airspeed/pressure by flying at a positive AOA.

So why is AOA so important? If the leading edge is up, and the trailing
edge is down, doesn't that mean that the wing is still forcing air down
as it travels through the air? Maybe somewhat, but that's not where the
magic is. Its all about the relative difference in airspeed and
therefore air pressue. Since AOA directly effects airflow over the
wing, is it not reasonable to think that it alone could produce enough
of a difference in speed/pressure to sustain flight?


Old Regallo wings could/would change shape. Google "luff-dive"
sometime. Things get ugly in a hurry when your airfoil reverses its
loveley curved shape and slams you into the ground with over 300 lbs of
force. Been there. Done that. Lived.

By the way, I'm not professing to know much more about aerodynamics
than what was discovered in the Wright wind tunnel. But I'm pretty well
convinced by those results that most airplanes do not stay in the aloft
by forcing air downward.

Now here's a simple 19th Century way to prove the point. Attach a
length of yarn to the "lower" surface of a slow-flying plane. Watch the
yarn and see what direction it takes in flight. Is it straight back? Or
is it down? If air is indeed being forced downward by the wing,
shouldn't we be able to see the results in the yarn?

Harry

wright1902glider wrote:

Now here's a simple 19th Century way to prove the point. Attach a
length of yarn to the "lower" surface of a slow-flying plane. Watch the
yarn and see what direction it takes in flight. Is it straight back? Or
is it down? If air is indeed being forced downward by the wing,
shouldn't we be able to see the results in the yarn?

Harry


Thank you, Harry!

----------------------much snipping-------------

If I'm not mistaken, Hughes once built some kinda
giant tip-thrust powered test-freak that had a rotor speed of about 16
rpm. I've seen the videos, but I can't recall the name.


Fairchild-Hiller once built something like that, supposedly with ram jets in
the tips, and Hughes may have as well; although I am fairly sure that the
rotor speed was within the normal range for helicopters.

There was also a propane fueled pulse jet powered single blade helicopter of
the strap-onto-the-pilot variety several years ago. ( I even had a "blurb"
about it--and a related APU for gliders--which may still be buried among
other books and catalogs.) Although I don't know if it ever flew out of
tether...

Peter

P.S.: As to the original post: the pilot shops at most airports have
passable texts to introduce the theory of helicopters, and a lot of
accomplished helicopter pilots and mechanics used to hang around on
"rec.aviation.rotorcraft: so that lurking over there could pay dividends...

This pushes and pulls on the rotor-blade control horns,
constantly changing the pitch of the blade as it flys around in a
circle. If you tilt the dinner plates forward, the blade flys at a
lower AOA in the front 1/2 of the rotor disk than it does at the back
1/2. Since its producing more lift in the back 1/2 than in the front
1/2, the blade flies higher in back. Stay with me here. As the blade
flies higher, its coning angle relative to the rotor head increases to
a greater angle than it does in the forward 1/2 of the rotor disk..
Therefore, its line of thrust relative to the fuselage is not vertical,
but is actually inclined forward.

Except the swash plate is not inclined forward in forward
flight. It's inclined to the right in forward flight, if the rotor
turns counterclockwise as seen from above, as in most American
'copters.
The rotor is a gyroscope, and trying to tilt one edge of it
will result instead in tilting the edge 90 degrees away in the
direction of rotation, like any other gyro. The rotor blades reach
their maximum pitch on the left side of the machine, and the blades
reach their maximum flap at the rear, tilting the disc forward. The
advancing blade on the right has minimum pitch and the front of the
disc is lowest.This is a happy coincidence, since we also need a lot
more AOA on the retreating blade to partly make up for its much lower
airspeed through the air compared to the advancing blade on the right.
Assymetrical lift has to be dealt with or the machine will roll over as
soon as it move forward, so the retreating blade's higher pitched AOA,
the blade's small downward flap approaching the retreating side and
rising flap approaching the advancing side also contributes to AOA
changes, and the lead/lag hinges on many rotors allow the blades to
accellerate on the retrating side and decellerate on the advancing side
at and therefore reduce some of the airspeed difference. Symmetrical
airfoils are used to minimize vibration caused by center of pressure
changes with AOA changes.
Helicopters are a lot more complex than they seem.

Dan