Propellors vs Rotors

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?

This is really bugging me. BTW, does anyone have any idea
what the thrust produced by the propellor of the hypothetical
300 HP (say LYC-IO540 powered) airplane would be? Obviously
the thrust produced by the 300HP helicopter exceeds 2000 LBs.

TIA,

Don W.

In article ,
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?

This is really bugging me. BTW, does anyone have any idea
what the thrust produced by the propellor of the hypothetical
300 HP (say LYC-IO540 powered) airplane would be? Obviously
the thrust produced by the 300HP helicopter exceeds 2000 LBs.

TIA,

Don W.

It's got everything to do with the amount of air they move and the
difference in efficiency between moving a little air at high speed or a
lot of air at lower speed.

Let's look at this qualitatively.

In order to lift an object by moving air, you need to create enough
force. Force is equal to a change in momentum with respect to time. That
is, you can think of force as being equal to changing the momentum of a
constant mass at a constant rate of acceleration (F = ma), *or* you can
think of it as applying a constant speed change to a flow of mass (F =
m/s * v). But as long as multiplying the two together gives you the same
total force, it does matter from a momentum perspective.

But! From and energy and power perspective it matters a lot.

Kinetic energy is proportional to the mass being moved but also
proportional to the *square* of the speed you move it at.

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?

--
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."

"Alan Baker" wrote in message
...
In article ,
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?

This is really bugging me. BTW, does anyone have any idea
what the thrust produced by the propellor of the hypothetical
300 HP (say LYC-IO540 powered) airplane would be? Obviously
the thrust produced by the 300HP helicopter exceeds 2000 LBs.

TIA,

Don W.

It's got everything to do with the amount of air they move and the
difference in efficiency between moving a little air at high speed or a
lot of air at lower speed.

Let's look at this qualitatively.

In order to lift an object by moving air, you need to create enough
force. Force is equal to a change in momentum with respect to time. That
is, you can think of force as being equal to changing the momentum of a
constant mass at a constant rate of acceleration (F = ma), *or* you can
think of it as applying a constant speed change to a flow of mass (F =
m/s * v). But as long as multiplying the two together gives you the same
total force, it does matter from a momentum perspective.

But! From and energy and power perspective it matters a lot.

Kinetic energy is proportional to the mass being moved but also
proportional to the *square* of the speed you move it at.

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?

--
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."

Good explanation.

This is why I've been trying to get glider tug guys interested in gearing a
small engine to a large slow prop.

Bill Daniels

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?

Think of the airplane as a boat. Take a 30 foot boat and put in 300 hp
motor and a 6" propellor. It will not move the boat much. But put on a 2
foot prop and low rpm and you will move the boat quite nicely. Air is like
water.

Colin

In order to lift an object by moving air, you need to create enough
force. Force is equal to a change in momentum with respect to time. That
is, you can think of force as being equal to changing the momentum of a
constant mass at a constant rate of acceleration (F = ma), *or* you can
think of it as applying a constant speed change to a flow of mass (F =
m/s * v). But as long as multiplying the two together gives you the same
total force, it does matter from a momentum perspective.

Alan:
Does this mean that a helicopter in hover is continuously pushing
down a mass of air equal to the weight of the aircraft? The corollary
is that a rotary wing or fixed wing aircraft in unaccelerated flight is
displacing a mass of air equal to its weight, i.e., it is not flying
because of low pressure above the wing but because of the upward force
on the airfoil from the displacement of air downward. True?
Russell Thorstenberg
Houston, Texas

"Don W" wrote in message
. net...
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?

This is really bugging me. BTW, does anyone have any idea
what the thrust produced by the propellor of the hypothetical
300 HP (say LYC-IO540 powered) airplane would be? Obviously
the thrust produced by the 300HP helicopter exceeds 2000 LBs.

TIA,

Don W.

I am not a helicopter guy, so please don't expect my to carry this thread
very far; but I'll try at the most basic level.

Lift as generated by throwing air downward in order to maintain the
altitude, or the constant rate of ascent or descent, of an object is based
upon a momentum equation--rather than an energy equation. Therefore,
throwing twice as much air downward half as fast will support the same
weight; but will require about half as much energy per unit time, or about
one half the horsepower. Remember that horsepower is a measure of energy,
or work, per unit of time.

The helicopter is thus supported on the downwash from its rotor, producing a
vertical thrust at least equal to its weight (actually more when hovering)
and literally glides forward in response to tilt.

I stopped for a moment to dress in my flame retarding coveralls in
anticipation of the response to my use of the word "glide"; however that is
what it does. It even recovers a little efficiency, compared to its
hovering condition, by virtue of continuously transitioning onto new and
undisturbed air. At its most "efficient" speed, a helicopter might be more
than half as efficient as a really atrocious airplane.

OTOH, in the case of an airplane propeller, we need to make the energy
equation work--while the wings deal with the momentum equation. We can
choose a wingspan appropriate for the intended weight and cruising speed and
a wing area to meet our stall speed requirements, determine the expected
drag in cruise, choose a propeller disk area and number of blades
appropriate for reasonable efficiency in cruise, and match the result to an
engine, and possibly a PRSU since the propeller disk area determines the
diameter and the maximum RPM. Finally, determine that the available power
can supply sufficient thrust for take-off and climb. Traditionally, small
airplanes produce a maximum static thrust on the order of one fifth of their
weight when tied in place, and much less in cruise. The propeller, of
course, constantly transitions into new and undisturbed air and its
efficiency improves from zero at the start of the take-off roll to an
acceptable figure in cruise.

One size does not fit all.

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

I hope this helps.

Peter

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

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