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

C. Beaty

Gold Supporter
The flapping illusion is caused by rotor disc tilt relative to the flight path; noseup in a gyro because its rotor must act in part as a windmill and extract its energy from the airstream whereas the rotor of a helicopter provides forward propulsion and flies nosedown.

The rotor of a compound helicopter where forward propulsion is supplied by a propeller and the rotor only provides lift can fly perfectly flat relative to the airstream. Cyclic pitch in such cases still does its job of equalizing lift between advancing and retreating blades but without the flapping illusion.
 

Jean Claude

Junior Member
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.
1144815
Both illustrations are correct, Bryan.
The illustration above shows that relative to the hub, there is a flapping movement without a change in pitch. But if you look good, you can observe the change of angle of attack in the air.

The bottom illustration shows that relative to the plane of the blade tips, only the change of the angle of attack exists.
In both illustrations, changing the angle of attack is necessary to balance the lift on each side, due to the differences in airspeed :

A blade section on the advancing side sees a higher air speed (Vc + Vt) than it retracting side symmetrical section (Vc-Vt), and its higher lift raises it at an angle a1 to the plane normal (longitudinal flapping angle) Its angle of attack then decreases (im - a1) while the angle of attack of the opposite section increases (im + a1) until the difference in lift disappears.
(with Vc: Circumferential speed, Vt: Translational speed, im: Average incidence on a revolution, a1: Tips plane angle with respect to the bearing plane)

Since the flapping moments are proportional to the angle of attack and the square of the air velocity on the section, the angle a1 is established at an equilibrium value such that (Vc + Vt) ² / (Vc-Vt) ² = (im + a1) / (im-a1)

So, with Vc = 100 m/s (section to .75R) , Vt = 30 m/s, and im = 5 degrees, we finds a1= 2.6 degrees
 
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kolibri282

Active Member
Progress in the discussion could, Imho, be made if everyone would clearly state which coordinate system he is using to describe the motion. In the upper picture of Javiers post #31 we see the motion of the rotor blade tip with respect to a reference frame that is rotating with the blade and which is perpendicular to the rotor shaft. Clearly the angle of the blade with respect to that reference frame (and the shaft) varies as it turns through one revolution. This is what the old timers called "flapping". In a reference frame that is moving with the blade but tilted such that it is lying in the plane of rotation of the blade tip, of course, there is no change in angle with respect to the Z-axis of the frame but we rather see the difference in angle of attack of the blade, which is called feathering. A long time ago Bramwell described the whole process very clearly saying:
Quote:
The above discussion illustrates the phenomenon of the so-called ‘equivalence of feathering and flapping’; the interpretation is a purely geometric one. If flapping and feathering are purely sinusoidal, the amplitude of either depends entirely upon the axis to which it is referred. In Fig. 1.12, aa′ is the shaft axis, bb′ is the axis perpendicular to the blade chord, cc′ the axis perpendicular to the tip path plane. If Fig. 1.12 shows the blade at its greatest pitch angle, bb′ is clearly the axis relative to which the cyclic feathering vanishes and is called the no-feathering axis. Similarly cc′ is the axis of no flapping. /Quote

Quote from #36: Well ...flapping is what a blade does in relation to air /Quote No! Flapping only takes place in a coordinate system perpendicular to the drive shaft in a helicopter or the mast in a gyro.

Quote form #32: 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. /Quote
For a rotary wing aircraft in straight and level flight the angle of the rotor shaft (or mast in a gyro) with respect to the oncoming air is constant.
The angle of the rotor blade with respect to the drive shaft in a helicopter or the mast in an gyro changes in forward flight as the blade rotates through 360°.
The diagrams in Javiers post #31 do not show any change in lift but if they did it would, of course, be the same for both of them. In the upper diagram that change in lift would result from the change in angle of attack with respect to the blade that is due to the fact, that the blade is moving up and down with respect to the shaft (mast) and which causes a velocity component perpendicular to the blade equal to the flapping speed of the blade.
In the lower diagram that same change in lift is brought about by the change in angle of attack by the blade feathering. So it is important to state whether you describe your blade motion with respect to the axis of no-feathring or the axis of no-flapping.
 

bryancobb

Junior Member
I surrender.! I am a Commercial Helicopter Pilot and CFI who was U.S. Army Trained in the mid 1980's . I have been flying helicopters for 30 years and I feel I have a significant grasp of rotary wing aerodynamics as the subject has been continuously taught for now-nearing 100 years. I know what flapping is and why it must occur. The FAA has voiced having reservations many times over the years with giving the gyro community full recognition and respect that is usually afforded most factions within the aviation world. This thread may be highlighting why that is the case.

I have read everything on this forum from..."Oh, those authors whom the FAA contracts to write training publications like the Helicopter Flying Handbook and the Army FM 1-203 Fundamentals of Flight, are merely hired writers with very little knowledge of the subject matter," to "real scientist know and can prove that flapping does not occur at all."

What I have been taught seems to more closely agree with the ingredients Arthur Young, Igor Sikorsky, Stanley Hiller, Newby O. Brantly, Chuck Kaman, and others have "baked-into-their-cakes" when they successfully conquered the quirky problems of helicopter rotors. There's a couple of guys on here that fly FAA Type Certificated Gyro's. THESE pilots do get the recognition from the FAA. You will never hear them say "I applied too much aft stick before my rotor RPM was up and I flapped my blades." Their rotor heads and takeoff techniques are nothing like those of the experimental gyros, flown by people who subscribe to the idea that flapping your blades hits and destroys the aft end of your aircraft. This is a meaning of the term that is not even in the FAA's vocabulary.
 

kolibri282

Active Member
Quote: flapping your blades hits and destroys the aft end of your aircraft. This is a meaning of the term that is not even in the FAA's vocabulary. /Quote

Having seen the film below I thought that the term isn't in the Army vocabulray either. If the blades move excessively they call it mast bumping, don't they?
 

bryancobb

Junior Member
Quote: flapping your blades hits and destroys the aft end of your aircraft. This is a meaning of the term that is not even in the FAA's vocabulary. /Quote

Having seen the film below I thought that the term isn't in the Army vocabulray either. If the blades move excessively they call it mast bumping, don't they?

Mast Bumping is not even in the same universe with what gyro guys call "flapping the blades." Its causes, its effects, and its degree of danger are not similar to a gyro's rotor blowing back because of low rotor RPM.
 

kolibri282

Active Member
Quote: Mast Bumping is not even in the same universe with what gyro guys call "flapping the blades." Its causes, its effects, and its degree of danger are not similar to a gyro's rotor blowing back because of low rotor RPM. /Quote
Very interesting, could someone please elaborate on the difference?
 
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