Coriolis...is it a force, or an effect?

gyromike

Administrator
Staff member
Joined
Oct 29, 2003
Messages
3,740
Location
Abbeville, Louisiana
Aircraft
Bensen B8MG
In the discussion of forces, real and imaginary, in the thread On the flapping and lead-lag blade oscillations (in German) Coriolis is mentioned.

Is Coriolis a real force, or is it an effect that depends on frame of reference.
I have always understood it to be an effect, but the more I think I know the more uncertain I become.
 

Jean Claude

Junior Member
Joined
Jan 2, 2009
Messages
1,978
Location
Centre FRANCE
Aircraft
I piloted gliders C800, Bijave, C 310, airplanes Piper J3 , PA 28, Jodel D117, DR 220, Cessna 150, C
Total Flight Time
About 500 h (FW + ultra light)
Coriolis explains by an imaginary force an anomaly of movement in a reference whose acceleration has been omitted.
 

bryancobb

Junior Member
Joined
Aug 2, 2010
Messages
5,009
Location
Cartersville, GA
Aircraft
Owned Brantly B-2b/Fly Kitfox III/Mini-500b
Total Flight Time
1350
Mike,
Several "elders" in the fields of rotor aerodynamics and physics, some of whom are members of this forum, emphasize that a rotor system that is in forward flight is compensating for dissymmetry-of-lift by tilting of the tip-path plane, mostly caused by cyclic feathering (pitching) of the blades. This can be actively done by the pilot making a cyclic input with the stick, or it can be done passively by the way the head is designed and rigged. These elders stand firm on the idea that no acceleration or deceleration of individual blades occurs.

Chapter 2 of the Rotorcraft Flying Handbook (https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch02.pdf) presents the topics of Coriolis Force, Flapping, Compensation for Dissymmetry of Lift and lead/lag in the more traditional way. The frame of reference the discussion is based on is THE SURROUNDING AIR PARCEL that is pumped or accelerated by the rotor to create lift and thrust or keep the helicopter level laterally. A blade that is climbing and descending within this parcel of air IS, by definition, flapping in the traditional sense. I agree with the elders that no flapping is happening if the frame of reference is the tilted axis the rotor system rotates about, but aerodynamically, that axis is of little importance. What IS of major importance is the AIRFLOW. The upward and downward flapping of a blade (in the air parcel), and the lead/lag of that blade (in the air parcel) creates forces that would not not be present if the flapping and lead/lag were not occurring.
 

XXavier

Member
Joined
Nov 13, 2006
Messages
1,027
Location
Madrid, Spain
Aircraft
ELA R-100 and Magni M24 autogyros
Total Flight Time
579 gyro (March 2020)
(...) I agree with the elders that no flapping is happening if the frame of reference is the tilted axis the rotor system rotates about, but aerodynamically, that axis is of little importance. What IS of major importance is the AIRFLOW. The upward and downward flapping of a blade (in the air parcel), and the lead/lag of that blade (in the air parcel) creates forces that would not not be present if the flapping and lead/lag were not occurring.
Of little importance...? It's the axis of the plane that the blades really sweep when rotating...
 

WaspAir

Supreme Allied Gyro CFI
Joined
Oct 21, 2006
Messages
4,502
Location
Colorado front range
Aircraft
Bell 47G-3B-1 / A&S 18A / Phoebus C, etc.
Total Flight Time
stopped caring at 1000
I am of the opinion that the term "Coriolis force" is remarkably unhelpful when instructing helicopter students, if applied to rotor systems, and too often leads to horrendous confusion for the poor student. Everything addressed by that term is more easily understood when described in terms of the simple basic physics principle of conservation of angular momentum, and I find explanations based upon that to be much clearer and cleaner.

I only use the C word when discussing weather systems, and the rotational direction of large airmasses around a high or low pressure area. A fixed eastward velocity when seen at different latitudes produces a different rate of longitudinal progress because of the Earth's different radius from the polar axis at a given latitude, and that is easy to show on a globe or a map without engendering undue confusion.
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,756
Location
Florida
Here’s an example, Mike:

When a long range artillery projectile in fired in a N-S direction, the Earth’s rotation makes it appear to a ground based observer that the projectile is following a curved path.

But Newton, in his 1st law says the projectile travels in a straight line unless acted on by an unbalanced force.

French scientist Coriolis came up with an imaginary force to explain the imaginary curvature of the artillery projectile without violating Newton’s 1st law.
 

kolibri282

Active Member
Joined
Aug 2, 2009
Messages
2,822
Location
Duesseldorf
I think Wasp and Jean-Claude combined give the best answer:
Quote: Coriolis explains by an imaginary force an anomaly of movement in a reference (frame) whose acceleration has been omitted. /Quote
I have added the word "frame" to Jean Claude's answer, I think it was lost in the translation. Coriolis is indeed a sort of virtual force needed when describing motion in a moving (rotating) reference frame.

As Wasp stated conservation of angular momentum is the basis in describing blade motion if you want e.g. to understand why a rotor driven by an engine needs a lead-lag hinge.
 

WaspAir

Supreme Allied Gyro CFI
Joined
Oct 21, 2006
Messages
4,502
Location
Colorado front range
Aircraft
Bell 47G-3B-1 / A&S 18A / Phoebus C, etc.
Total Flight Time
stopped caring at 1000
And now, we can discuss whether water drains backwards when you switch hemispheres . . .
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,756
Location
Florida
A 3-blade rotor not driven by an engine also needs a lead-lag hinge where the hub axis is not aligned with the tip plane axis as exemplified by the Cierva C-30 with tilt head cyclic control or the A&S-18A with swashplate cyclic control.

"As Wasp stated conservation of angular momentum is the basis in describing blade motion if you want e.g. to understand why a rotor driven by an engine needs a lead-lag hinge."






Like Reply

Report
 
Last edited:

XXavier

Member
Joined
Nov 13, 2006
Messages
1,027
Location
Madrid, Spain
Aircraft
ELA R-100 and Magni M24 autogyros
Total Flight Time
579 gyro (March 2020)
A 3-blade rotor not driven by an engine also needs a lead-lag hinge where the hub axis is not aligned with the tip plane axis as exemplified by the Cierva C-30 with tilt head cyclic control
(...)
(...)
If I get it correctly, the lead-lag hinges in such a rotor work only when the cyclic control is acted on, with the blades orbiting to find their new equilibrium plane, but in normal, s/l flight, the blade tips never depart their rotation plane, so the lead-lag hinges don't need to work...
 

kolibri282

Active Member
Joined
Aug 2, 2009
Messages
2,822
Location
Duesseldorf
Very good point, Chuck, so let's put it that way:
"As Wasp stated conservation of angular momentum is the basis in describing blade motion if you want e.g. to understand why a rotor driven by an engine or a free wheeling rotor with more than two blades needs lead-lag hinges."
If we assume that one blade is pointing athwart ship (3 or 9 o'clock) where the center of mass has the greatest distance form the axis of rotation, then the two remaining blades form an angle (usuallay designated a1*cos(psi) + b1*sin(psi) ) with the axis of rotation which brings their center of mass closer to said axis of rotation which results in said blades speeding up (remember the figure scating girl moving her arms closer to her body and spinning faster, ok.... works for blokes as well...I just prefere to imagine a nicely figured girl.......;-)

The bending moment resulting from this increase in angular velocity is relieved by said lead-lag hinges.
 

kolibri282

Active Member
Joined
Aug 2, 2009
Messages
2,822
Location
Duesseldorf
Quote: the lead-lag hinges in such a rotor work only when the cyclic control is acted on /Quote
As pointed out in #11 lead-lag hinges go to work in every revolution. Just today I tried to clarify this with a chap of our (steam turbine) rotor department. Hope I got it right...
 

XXavier

Member
Joined
Nov 13, 2006
Messages
1,027
Location
Madrid, Spain
Aircraft
ELA R-100 and Magni M24 autogyros
Total Flight Time
579 gyro (March 2020)
Quote: the lead-lag hinges in such a rotor work only when the cyclic control is acted on /Quote

As pointed out in #11 lead-lag hinges go to work in every revolution. Just today I tried to clarify this with a chap of our (steam turbine) rotor department. Hope I got it right...
But in unaccelerated, s/l flight, the blade tips trace a flat circle, with no flapping motion with respect to the axis perpendicular to that tip-path circle. That being the 'real' rotation axis, in the absence of blade flapping, lead-lag hinges shouldn't be working at all...
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,756
Location
Florida
I don’t understand why some people make rotor motion so complicated by getting bogged down in imaginary motions and forces.

Rotating masses, whether rotorblades or rocks on strings, rotate at uniform angular velocity in a simple circle; they don’t speed up, slow down or flap.

In the usual situation where the hub axis is not aligned with the rotational axis of the blades, hinges, both flap and drag, forming a universal joint, are a kinematic necessity permitting the blades and hub to rotate about different axes. Aerodynamic force is negligible compared to inertial forces in determining the path of the blades.

Floating hub rotors where the rotor hub is connected to the rotorhead by some sort of universal joint or in the case of tip jet propelled rotors, by a spherical bearing, don’t need flap/drag hinges, examples being the Doblhof WNF-342 tip jet helicopter and the Doman LZ-5 shaft driven helicopter.

I agree that viewing the blades from the fixed hub may look like flapping but also, viewing the motion of the Sun as it rises in the East, moves across the sky and sets in the West makes it look as though the Sun is rotating around the Earth. Galileo was convicted of heresy for suggesting that the Earth might be rotating around the Sun.
 

XXavier

Member
Joined
Nov 13, 2006
Messages
1,027
Location
Madrid, Spain
Aircraft
ELA R-100 and Magni M24 autogyros
Total Flight Time
579 gyro (March 2020)
Ah, now I get it... In a fully articulated rotor head such as the C-30, the flap and drag hinges are working all the time, since the universal joint provided by those articulations in each blade allows the blade tips to trace a perfect circle... There is no flapping or lead-lag motion at all, but the hinges do work continuously...

Having said that, I still don't see any 'conservation of angular momentum' issue, since the blades are rotating regularly and uniformly...
 

kolibri282

Active Member
Joined
Aug 2, 2009
Messages
2,822
Location
Duesseldorf
My understanding is a bit different, Xavier, since the case I had in mind is where the tip path plane is not perpendicular to the axis of rotation. This happens due to fuselage or horizontal stabilizer moments. That angle is usually designated a1s and is the difference between the flapping angle a1 and the longitudinal cyclic control angle B1. Prouty gives a formula for a1s on page 468 in "Helicopter Performance, Stability and Control". With a1s not equal to zero the blades move closer to the axis of rotation depending on their position from the zero datum, i.e. the rear and will thus speed up or slow down, according to the conservation of angular momentum, which requires the lead-lag hinge
 

XXavier

Member
Joined
Nov 13, 2006
Messages
1,027
Location
Madrid, Spain
Aircraft
ELA R-100 and Magni M24 autogyros
Total Flight Time
579 gyro (March 2020)
I see...
But (if I'm understanding you correctly) that isn't the case in non-perturbed, unaccelerated, s/l flight, where the axis of rotation is indeed perpendicular to the tip-path plane...
 

bryancobb

Junior Member
Joined
Aug 2, 2010
Messages
5,009
Location
Cartersville, GA
Aircraft
Owned Brantly B-2b/Fly Kitfox III/Mini-500b
Total Flight Time
1350
Rotating masses, whether rotorblades or rocks on strings, rotate at uniform angular velocity in a simple circle; they don’t speed up, slow down or flap.
Can you at least concede, Chuck...That your rock on a string which is traveling in a smooth, uniform circle that is on a 45 degree tilt to the horizon...and is being done so outside a car window that has a road speed of 80 MPH... IS DEFINITELY EXPERIENCING A NEGATIVE VERTICAL COMPONENT of RELATIVE WIND DURING THE HALF OF THE CIRCLE WHEN THE ROCK'S HEIGHT ABOVE THE ROADWAY IS INCREASING, AND A POSITIVE VERTICAL COMPONENT OF RELATIVE WIND DURING THE HALF OF THE CIRCLE WHEN THE ROCK'S HEIGHT ABOVE THE ROADWAY IS DECREASING?

Can you also concede...that as the rock travels through the advancing half of the disk, drag on the rock increases greatly? As the rock passes through the retreating half of the disk, drag decreases significantly. My argument is the rock's velocity is constantly changing due to varying drag.

My argument is, your rock is FLAPPING within the parcel of air it's traveling through, and it is also leading and lagging due to drag.
 
Last edited:

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,756
Location
Florida
Of course a rock being twirled on a string by hand would be affected by holding it out of a car window. However, if the rock was traveling at typical rotor tip speed, say 500 fps (340 mph), the influence produced by a moving car would be negligible.
 

All_In

Gold Supporter
Joined
Apr 21, 2008
Messages
14,752
Location
San Diego, CA. USA
Aircraft
Piper Archer, Aviomania G1sb
Total Flight Time
Not sure over 10,000+ logged FW, 260+ ultralights, sailplane, hang-gliders
Can you at least concede, Chuck...That your rock on a string which is traveling in a smooth, uniform circle that is on a 45 degree tilt to the horizon...and is being done so outside a car window that has a road speed of 80 MPH... IS DEFINITELY EXPERIENCING A NEGATIVE VERTICAL COMPONENT of RELATIVE WIND DURING THE HALF OF THE CIRCLE WHEN THE ROCK'S HEIGHT ABOVE THE ROADWAY IS INCREASING, AND A POSITIVE VERTICAL COMPONENT OF RELATIVE WIND DURING THE HALF OF THE CIRCLE WHEN THE ROCK'S HEIGHT ABOVE THE ROADWAY IS DECREASING?

Can you also concede...that as the rock travels through the advancing half of the disk, drag on the rock increases greatly? As the rock passes through the retreating half of the disk, drag decreases significantly. My argument is the rock's velocity is constantly changing due to varying drag.

My argument is, your rock is FLAPPING within the parcel of air it's traveling through, and it is also leading and lagging due to drag.
Accelerating over the ground when your rotors are not up to speed causes blade flapping. That is what I would expect in your example. Now speed up the blades/string and no problem. Right?
 
Top