Propellors vs Rotors

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.

Peter Dohm wrote:

"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.

This makes sense to me now.

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.

Nit picking here, but the propellor is actually doing a lot of work
even when the aircraft is not moving. It just does not translate
into useful work on the airplane. You are thinking of the work as
thrust * velocity (airplane) which is correct from the viewpoint of
the airplane, but not the system. The prop is moving plenty of air,
dust, leaves etc. although that does not help you get where you are
going.

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

Yes it does. Thanks.

Don W.

The 300 hp in the helicopter is moving it's wing fast enough to lift the
Helo. The
fixed wing engine is also moving it's wing fast enough to lift the aircraft.
The Helicopter itself need not move forward, so the lift appears vertical,
but the wing is indeed climbing at a shallow angle, just like the fixed
wing.

Al


"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.