Propellors vs Rotors

In article ,
Don W wrote:

Alan Baker wrote:

Don W wrote:

Can someone explain to me why 300HP applied to a large rotor
at ~700 RPM is enough to lift a 2000lb helicopter straight up,
but the same 300HP applied to a smaller diameter propellor
at ~2600 RPM can not even come close to allowing a 2000 LB
airplane to climb vertically?
Don W.

snip


So if you go from rotor moving x mass per second at y speed to a
propellor moving x/2 mass per second at 2y speed, then your power goes
down by half from the change in mass, but *up* by four from the change
in speed. IOW, move half the mass to achieve the same force and you need
to use twice the power.

Does that help?


Yes Alan, it does. I had just not thought of it that way. Now that
you point it out, it makes perfect sense. This is what I think you
said:

force = d (mv)/dt =
force = d (m)/dt * d(v)/dt = force = d(m)/dt * (v1-v0)

In English: force is equal to mass flow rate times the difference
in velocity before and after the propellor.

Ek= 1/2 (mv^2) -and-
Power = d(Ek)/dt = Power = d(m)/dt * (v1-v0)^2

In English: Power is the rate of change of the kinetic energy of
the airflow which is equal to the mass airflow times the square
of the difference in velocity before and after the propellor.

Is that correct? If so, it says that for fuel efficiency you
want as big a prop as you can fit turning slow. That also makes
sense because the parasitic drag on the prop goes up as the
square of the blade velocity as well.

big grin

I think something fundamental just just clicked.

Don W.

Looks correct to me, Don. Glad to have helped.

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

wright1902glider wrote:

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

Consider it opened ;-)

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.

I'm not familiar with the particular experiment, although I have seen
the wind tunnel you are referring to. It is at the Air Force museum
in Dayton Ohio.

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.

Rotorway's Exec 162F main rotor turns 520 RPM at 100%. The tail rotor
is turning 2600 RPM at 100%. I tried to find info on the Robinson R22
and R44 but didn't find it. IIRC it was in the same range. I would
expect that larger helicopters would use larger main rotors turning
somewhat slower to avoid supersonic tip speeds.

Harry

Don W.

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

Or, as a very good aero guy told me

"It is better to annoy a lot of air a little than to annoy a little air
a lot."

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?

Calling things which don't agree with your own notions "fairy tales" as
Denker does doesn't make it wrong. It's always easier to invoke
name-calling rather than to be able to demonstrate how something is
wrong; we have enough of that in politics! Anyone who has worked with
the Coanda effect can easily show, mathematically and demonstrably, how
all of what takes place in lift occurs, even the upwash ahead of the
wing. This whole idea that explaining lift based on lift's reaction,
downwash, rather than the action which caused the downwash, is the real
"fairy tale". So we have lift from Bernoulli, longer distance above to
below, circulation, and on and on.

This is a little like the reporter asking President Lincoln how long a mans
legs should be. "Long enough to reach the ground", the Civil War president
replied.

So, what are the problems with a big prop?

High tip speeds limit cruise performance.
Long landing gear legs.
Larger "P" factor.
Requirement for a PRSU if the engine is to turn fast enough to produce
reasonable HP.

On the upside:

MUCH greater eficiency
Lower noise - if the RPM is low enough.
Greater acceleration during ground roll leading to better short field
performance.
Better climb rates.

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.

Bill Daniels

"Smitty Two" wrote in message
news
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?

ELIPPSE wrote:
Calling things which don't agree with your own notions "fairy tales" as
Denker does doesn't make it wrong. It's always easier to invoke
name-calling rather than to be able to demonstrate how something is
wrong; we have enough of that in politics! Anyone who has worked with
the Coanda effect can easily show, mathematically and demonstrably, how
all of what takes place in lift occurs, even the upwash ahead of the
wing. This whole idea that explaining lift based on lift's reaction,
downwash, rather than the action which caused the downwash, is the real
"fairy tale". So we have lift from Bernoulli, longer distance above to
below, circulation, and on and on.


There is a minor problem withe the "Longer on top" theory...


http://jef.raskincenter.org/publishe...da_effect.html

scroll down to OTHER PARADOXES

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

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.