On the flapping and lead-lag blade oscillations (in German)

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)
After having to explain the Coriolis forces, to add that the simultaneous acceleration / deceleration of the opposite blades of a seesaw rotor makes it possible to remove the drag joint seems to you simpler, Bryan?
 

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
The important things a pilot needs to understand is, the helicopter or gyro goes the direction you push or pull the stick. Rotor RPM must be maintained at all cost. Heavy rotors (high inertia) loose and gain RPM slowly. Lighter (low inertia rotors) will loose or gain RPM very quickly and you have to pay more attention and be ready to act quickly if it trends too high or low, to keep from dying. Overloading your helicopter is a sure fire way to low rotor RPM and death or a wreck. That's it. The best switchboard operator in the world has no idea how a phone works. ;)
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,760
Location
Florida
Nice view of a rotor blade undergoing cyclic feathering with rotor disc running at an angle to the horizon. A camera mounted on a rotating bicycle wheel and aimed at the valve stem would look the same way if tilted at the same angle.

Do you suppose some people might be convinced that the bicycle wheel was flapping?
 

All_In

Gold Supporter
Joined
Apr 21, 2008
Messages
14,771
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
Nice view of a rotor blade undergoing cyclic feathering with rotor disc running at an angle to the horizon. A camera mounted on a rotating bicycle wheel and aimed at the valve stem would look the same way if tilted at the same angle.

Do you suppose some people might be convinced that the bicycle wheel was flapping?
No, only the creator of the term might?
 

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)
Flapping appears only in the distance from the edge of the picture

1144707
 
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)
The amplitude of the flapping may not be large, but that's probably due to the presence of a feathering input. Most of the dissymmetry of lift is compensated by the feathering, so flapping is reduced to a minimum...
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,760
Location
Florida
The center of lift is more outboard on the retreating side and more inboard on the advancing side of the rotor disc. The opposite is true for a gyroplane.

I expect this is the reason for most of the blade flexing in the video.
 

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
The amplitude of the flapping may not be large, but that's probably due to the presence of a feathering input. Most of the dissymmetry of lift is compensated by the feathering, so flapping is reduced to a minimum...
THREE things make-up the cyclic feathering.

The most apparent and intuitive is the pilot's control stick input toward the half of the rotor disk with the most lift. As your speed increases, most helicopters require more lateral cyclic.

The next most apparent and not quite as intuitive is the designed-in features and rigging of the rotor head that feather the blades without any pilot input as they climb and descend in relationship to the rotor mast and hub.

The hardest thing to see and understand is the varying vector sum, or "resultant" relative wind that is striking a rotor blade as the blade makes each 360 degree revolution. If a single blade is frozen at 360 points and analyzed at each, the resultant relative wind's vertical component at each point reaches max at 3:00 and 9:00 O'Clock, and tapers off to zero over the nose and tail as that vertical component changes from negative to positive. On the advancing half, blade climbing within the parcel of air results in an increasing, negative, vertical component of resultant relative wind and decreases lift. On the retreating half, blade descending within the parcel of air results in an increasing, positive, vertical component of resultant relative wind and increases lift. Even though this third part of cyclic feathering doesn't involve a blade actually twisting in its feather bearings like the first and second do, it is definitely a change in angle of attack so it is cyclic feathering by definition.

Now. To better understand how the climbing and descending of the blade compensates for dissymmetry of lift, and how all that acts on the helicopter, let's isolate the first 90 degrees of the advancing half of the rotor and look at it closer using some hypothetical, unitless lift numbers. Let's say 0 to 100. Over the tail and nose, not considering gyroscopic precession (for the moment), there is no lift force created by the rotor because the blade is at "zero-lift pitch." At 3:00 O'Clock, the blade has feathered to "minimum pitch" and the downward force is 100.

ALL OF THESE HYPOTHETICAL NUMBERS ARE AFTER THE PILOT INPUT AND THE OTHER TWO THINGS THAT AUTOMATICALLY FEATHER THE BLADES HAVE STOPPED THE PITCH-UP AND LEFT ROLL THAT IS CAUSED BY DISSYMMETRY OF LIFT (American CCW Helicopter).

If
1) Lift over the tail is 0
2) 10 degrees later lift is 10 downward
3) 20 degrees later lift is 22 downward
4) 30 degrees later lift is 34 downward
5) 40 degrees later lift is 46 downward
6) 50 degrees later lift is 58 downward
7) 60 degrees later lift is 70 downward
8) 70 degrees later lift is 82 downward
9) 80 degrees later lift is 93 downward
10) 90 degrees later at 3:00 O'Clock, downward lift is max at 100

Now if we apply gyroscopic precession (reaction is 90 after the applied force) to see how the rotor actually RESPONDS to those forces.
1) No force up OR down
2) A miniscule left roll of 1, and small nose-down force of 9
3) A small left roll of 7, and a larger nose-down force of 15
4) A larger left roll of 14, and a larger yet nose down force of 20
5) About equal left roll and nose down forces of 23 and 23
6) About equal left roll and nose down forces of 23 and 23
7) A larger left roll of 45 and a smaller nose down force of 25
8) A larger still left roll of 70 and a nose down force of 12
9) A left roll force of 90 and a tiny nose down force of 3
10) A large left roll force of 100
Totals
Left Roll Force = 373
Nose Down Force = 260

As I said earlier, these forces are needed to equalize lift on the advancing vs. retreating halves of the rotor disk.

This is what keeps the helicopter level. Similar but opposite forces are present on the retreating side.
 

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)
(...)
(...)

The hardest thing to see and understand is the varying vector sum, or "resultant" relative wind that is striking a rotor blade as the blade makes each 360 degree revolution. If a single blade is frozen at 360 points and analyzed at each, the resultant relative wind's vertical component at each point reaches max at 3:00 and 9:00 O'Clock, and tapers off to zero over the nose and tail as that vertical component changes from negative to positive. On the advancing half, blade climbing within the parcel of air results in an increasing, negative, vertical component of resultant relative wind and decreases lift. On the retreating half, blade descending within the parcel of air results in an increasing, positive, vertical component of resultant relative wind and increases lift. Even though this third part of cyclic feathering doesn't involve a blade actually twisting in its feather bearings like the first and second do, it is definitely a change in angle of attack so it is cyclic feathering by definition.

(...)
It's indeed hard to see, but the pitch of the blades does really change cyclically...

The blades do really feather... It's a mechanical effect due to the existence of the flapping hinge. In the only real-world plane where the blades revolve, the tip-path plane, there's no flapping motion at all. Only feathering.
Of course, the perception of motion, of any motion, is frame-dependent. If you are Ptolemaic-minded (or flapping-minded) you can describe the planetary (or the rotor blade) motion in a given way. But, in the real world, the planets revolve around the Sun, and the tip-path trajectory is the real way the blades move. Please find attached two revealing pictures, due to Chuck Beaty and JC. The top one is for Ptolemaic-minded people, for whom blades flap:

1144806

But the second one is the real thing, for Copernicans, since the plane of rotation of the blade tips is the only real plane of rotation... The blades don't flap, but feather. That feathering compensates the dissymmetry of lift caused by the forward, edgewise motion of the rotor through the air mass.
 
Last edited:

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
But, in the real world, the planets revolve around the Sun, and the tip-path trajectory is the real way the blades move. Please find attached two revealing pictures, due to Chuck Beaty and JC. The top one is for Ptolemaic-minded people, for whom blades flap:

But the second one is the real thing, for Copernicans, since the plane of rotation of the blade tips is the only real plane of rotation... The blades don't flap, but feather. That feathering compensates the dissymmetry of lift caused by the forward, edgewise motion of the rotor through the air mass.
Both of your illustrations are impossible.

Your top one depicts a rotor blade that has climbed and descended through the parcel of air without any variation in angle of attack or variation in lift produced.

Your bottom one depicts a rotor blade with constantly changing angle of attack and constantly changing amount of lift produced, but it never climbs or descends.

The planets revolving around the Sun is a poor illustration too. Space does not create any lift or aerodynamic force on any of the planets like air does.
A helicopter rotor operating in space would behave just like your second illustration.

If you are a helicopter driver with a lot of experience, you KNOW that blades flap and you know it because the old timers learned it very early-on and learned to design helicopters with nearly perfect design features that passively cancel out dissymmetry of lift. You know it because you can clearly SEE a delta flapping hinge on a 2-blade tail rotor and SEE that pitch is removed on the advancing half and added on the retreating half. You know it because you can see in a tail rotor video that way the tip path plane is perpendicular to the output shaft in a calm wind hover and as the ship goes faster and faster in forward flight, that same tip path plane tilts more and more away from perpendicular to the output shaft, with increasing speed. A rotor flaps within the parcel of air. You know blades flap because no one dares to fly a helicopter that does not have features other than the cyclic stick to null out dissymmetry of lift. You know blades flap because every agency on Earth that issues pilot licenses teaches it, expects helicopter pilots to learn it, and will not issue a licence unless the pilot can intelligently discuss the concepts related to it and why it's so important.
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,760
Location
Florida
The idea of flapping is so firmly embedded even among technically trained people in the helicopter industry that progress has been nearly non-existent.
As I’ve mentioned before, my friend Martin Hollmann used to stop by my office whenever he was in Tampa while employed by Martin Aircraft in Orlando and argue rotor dynamics.
Martin had a degree in aeronautical engineering and had taken all the available courses about rotorcraft. He was also living in the shadow of his father’s fame, Hans Hollmann, a well-known German physicist who had been an early radar pioneer in the 1930s-40s, developing a multicavity magnetron.
Martin was so thoroughly convinced that flapping was real that I built a floating hub rotor without flap or drag hinges just to prove it was imaginary. It was simply a triangular aluminum plate with a feathering bearing attached to each apex with normal cyclic pitch control, attached to the rotor head by a rubber “U” joint.
This was in the early 1970s before I had heard about the Doman rotor, there being no Internet as it exists today with nearly everything Man has ever known online.
First flight took place at a Bensen Days flyin. Martin had gathered up an audience and was explaining to them how it was going to fling itself asunder just as I came motoring sedately past. That was related to me a member of Martin’s audience.
Igor Bensen had also witnessed the first flight and was familiar with floating hub rotors, having been the project engineer for General Electric’s evaluation of the Doblhof tip jet helicopter. He sent me a copy of the GE report he had written.
The hingeless rotor of the Doblhof machine was attached to the airframe by a double row spherical roller bearing.
 
Last edited:

All_In

Gold Supporter
Joined
Apr 21, 2008
Messages
14,771
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
Bryan appears more semantics than substance. Your describing the same things in different terms is all I see. Tomato or Tamoto it is the same.
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,760
Location
Florida
Here’s an illustration that I drew for an article that was published in the PRA magazine back in the days when it was printed on paper. All that tilting the rotorhead can do is rotate the blades about their feathering axes.

When the rotorhead is running at an angle to the tip plane axis as is usual during forward flight, the variation of blade pitch is exactly the same as Javier’s (XXavier) illustration in post #31.
Also, viewing the blades from the rotorhead axis shows the imaginary flapping as depicted in the same post; there can be no cyclic pitch variation relative to the rotorhead axis without feathering bearings for cyclic control.





Fig 1A.jpg
 
Last edited:

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
Well ...flapping is what a blade does in relation to air. After all...air makes lift, and angle of attack determines how much if the rotor RPM is constant (the absolute goal in helicopters). Take Chuck's plank and a rope. Make an upside down "Y" bridle that straddles the chord of the plank and attaches at the midpoint, so that the plank's chord stays parallel to the horizon. attach a 6" long yarn "tuft" about every 6" along the plank's span, on the trailing edges. Now give it a whirl on a 45 degree tilt and film it as it rotates.

On the half of the "rotor" disk that is flapping upward, the yarn tufts will be deflected downward by the Resultant Relative wind. This proves a lessening of angle of attack caused by flapping that decreases lift on that half.

On the half of the "rotor" disk that is flapping downward, the yarn tufts will be deflected upward by the Resultant Relative Wind. This again proves a greater angle of attack caused by flapping, that increases lift on that half.

Even in the case of the rigid rotor of the BO-105 in my video, you can clearly see the blades flapping upward above the horizon on one half and downward below the horizon on the other. If yarn tufts were installed, you could see where Resultant Relative Wind is from. The blades on the BO-105 are designed to flap by flexing because they have no flapping hinge. I am not familiar enough with a BO-105 to know how lead/lag (mismatch in rotational speed of the mast vs. the blades) accommodated.
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,760
Location
Florida
The valve stem of a tilted, rotating bicycle wheel rises and falls with respect to the horizon; is it flapping?

There is some blade flex of the BO-105 in forward flight but most of the rotor’s tilt is due to the tilt of the airframe as a result of the hingeless and stiff rotor. The BO-105’s CG is located as near to the rotor as possible by locating the heavy stuff such as engines and gearbox on top of the cabin in order to minimize blade flex.

If the rotor blades weren’t stiff, it would be impossible to fly the BO-105 upside down.
 

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
The valve stem of a tilted, rotating bicycle wheel rises and falls with respect to the horizon; is it flapping?

There is some blade flex of the BO-105 in forward flight but most of the rotor’s tilt is due to the tilt of the airframe as a result of the hingeless and stiff rotor. The BO-105’s CG is located as near to the rotor as possible by locating the heavy stuff such as engines and gearbox on top of the cabin in order to minimize blade flex.

If the rotor blades weren’t stiff, it would be impossible to fly the BO-105 upside down.
The horizon has nothing to do with flapping, so no, the valve stem is not flapping. A helicopter diving straight toward Earth is still experiencing dissymmetry of lift solely because of blade speed and angle of attack must be varied (flapping) to compensate. Flapping has everything to do with the air parcel an object is moving through. Where is the valve stem's relative wind striking it from and is the angle of impact changing? I suspect that has never been analyzed but my GUESS is if the turbulent flow could be eliminated, the relative wind striking the valve stem parallel to the stem's tip-path-plane. Again, no flapping.

The airframe of the BO-105 needs to fly laterally level in forward flight to be comfortable for occupants. The video I posted at the beginning of this thread shows clearly that the tip-path-plane is tilted while the fuselage and mast are not. The BO-105 Technical Manual, published by Eurocopter was written by engineers on the design team. It says the blades are specifically designed using biased fiber-reinforced glass so that the blades are elastic in the desired directions to enable them to FLAP and lead/lag.

When I read your words " but most of the rotor’s tilt is due to the tilt of the airframe," my jaw hit the floor! If this is truly your feeling, considering your decades and decades of study, it does not indicate simply a semantics discussion here where a renowned expert is looking from a different viewpoint than 90% of the rest of the world. This statement would indicate a true misunderstanding of the fundamentals of helicopter design. I'm assuming you just typed that without thinking much about what you were saying.

If the fuselage and rotor were tilting as one to compensate for dissymmetry of lift, the the tilt would need to be continuously occurring to make the advancing blade continuously rise in relation to the incoming air and make the retreating blade continuously descend in relation to the incoming air. Said in another way...the helicopter would be constantly doing a left aileron-roll about the longitudinal axis to create adequate flapping. Surely you do not believe this Chuck?
 

Attachments

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)
There's a misunderstanding here... In a rotor where the axis of the tip-path plane is not parallel to the rotor axle (all present-day helicopters and gyros. The exception is the floating-hub rotor of Doblhoff and Doman) the flapping and lead-lag hinges (real or virtual, as in hingeless rotor heads like the BO-105) are working continually while in flight (well, except in vertical autorotation, when axis & axle are parallel). But the blades do not flap... They move smoothly in a circle...

The positions of the planets, in pre-Copernican times, were correctly calculated, and perfectly good predictions were made within the Ptolemaic, Earth-centered-universe theory. In a similar way, flapping can be considered as real in order to calculate and predict the behaviour of rotating blades. However, in the real world, the Earth is not the center of the universe, and rotor blades do not flap...
 
Last edited:

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,760
Location
Florida
What would be dissymmetry of lift while in translational flight is accommodated by cyclic pitch control. It has nothing to do with the flapping illusion.
While in forward flight for instance, the advancing blade has a decreased angle of attack and the retreating blade has an increased angle of attack. It is irrelevant whether cyclic pitch is accomplished by tilting the rotorhead in the case of tilt head gyros or by tilting the swash plate in the case of helicopters and gyros with feathering bearings and swashplate control.
Cyclic pitch control via feathering bearings and swash plate isolates the rotor from rotorhead tilt. The rotorhead may be tilted at any angle relative to the tip plane so long as flap/drag hinge travel limits are not exceeded
 
Last edited:
Top