#### 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.

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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#### bryancobb

##### Junior Member
Thanks J.C.. Your diagrams are 100% informative for me now.

#### 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?

#### bryancobb

##### Junior Member
Correct me if I'm wrong. I not a gyro pilot and I'm only basing what I say on what I have learned on this forum in the last decade or so. When a gyro pilot speaks of "flapping the blades," they seem to be discussing the results of improper takeoff technique involving low rotor RPM plus too much aft cyclic that result in the rotor disc being suddenly tilted significantly aft. It usually results in the blades striking the empennage, causing significant damage.

In my understanding, the aircraft is always on the ground and constrained laterally about the longitudinal axis by the main wheels, while it is free to pitch along the lateral axis as it rolls along. This means the nosewheel may be on the ground or the pilot may be balancing on the mains using fore/aft cyclic.

The rotor would probably be supporting some, but not all of the aircraft's weight.
___________________________________________________________________________________________________________________________________________________________
NOW FOR HELICOPTER MAST BUMPING
When a helicopter pilot speaks of mast bumping, they are talking most often about PREVENTING a fatal event where the mast snaps into and the rotor system flies away on its own and the fuselage plummets to the ground killing all on board.

Although mast bumping can happen on the ground, it usually involves high speed flight. On the ground, it could happen during rotor engagement (slow-turning-rotor) when a big wind gust comes or another helicopter hovers close-by. It can also happen on the ground when a part breaks or fails or when a ham-fisted pilot makes an accidental extreme cyclic input or lets go of the controls.

The rotors may not strike the tail and it may not even be felt if not severe. Hueys have a "witness hole" in the teter stops that gets deformed when a mast bump happens and acts as an event recorder. If a pilot sees that hole looking flat on one side during post flight, a mast bump has occurred.

Imagine a Huey at a constant 90kts and 50' above the highest obstacle. This is known as "low-level" or "contour" flight in Army lingo. Now imagine doing this 50' above an open field and crossing to a 50' above a heavily wooded forest area with 30' pine trees. This requires a slight application of aft cyclic well in advance of the tree line, followed by a gentle application of forward cyclic when 50' above the treetops is reached.

Now imagine, as you near the treeline, in a well-planned, slight cyclic climb...a previously unseen powerline appears. It follows the treeline and is about 80' above the trees because it sags between two mountains on either side. You instantly react by tripling the amount of aft cyclic to clear the powerline. This puts G's on the aircraft and the rotor supports a lot more weight. The blades cone more and the fuselage hangs almost "normal" toward the ground in a direction dictated by the sum of gravity, g's created by the curved climb path , and sideward thrust of the tail rotor at a point that does not lie vertically on the rotor disc.

After your 4G pullup, you must push over to intersect your height of 50' above treetops. This pushover is where mast bumping is going to become an issue. During the pushover that is likely to approach momentary weightlessness, all weight supported by the rotor diminishes toward ZERO and creates a dangerous, possibly deadly situation.

The sideward tail rotor thrust is the only significant force that remains to act on the fuselage. Since the tail rotor is rarely mounted at the same height as the main rotor, that force causes the fuselage roll to the left. The pilot applies right cyclic to stop the roll. Keep in mind, the rotor is supporting almost zero weight and flight controls are still connected and working. For that reason, the rotor still responds to control inputs and reacts much quicker since it is not carrying much weight. The pilot is making a significant cyclic input to the right and the tail rotor thrust keeps rolling the helicopter (AND TILTING THE ROTOR MAST) to the left.

BOOM!!!!! The rotor hub strikes the mast on the right side every time a blade passes by. On a Huey, this is about 600 times per minute or 10 times in the two seconds it takes the pilot to realize what's happening and take corrective action. Ten hits on the right side of the mast during this event will snap the mast into if significant enough. Everyone on board falls into the 30' tall pine forest at 90kts and 32Ft/sec/sec. Death is certain.

That my friends, is why Helicopter Mast Bumping and Flapping The Gyro Blades are two different animals.

#### Vance

##### Gyroplane CFI
Correct me if I'm wrong. I not a gyro pilot and I'm only basing what I say on what I have learned on this forum in the last decade or so. When a gyro pilot speaks of "flapping the blades," they seem to be discussing the results of improper takeoff technique involving low rotor RPM plus too much aft cyclic that result in the rotor disc being suddenly tilted significantly aft. It usually results in the blades striking the empennage, causing significant damage.

In my understanding, the aircraft is always on the ground and constrained laterally about the longitudinal axis by the main wheels, while it is free to pitch along the lateral axis as it rolls along. This means the nosewheel may be on the ground or the pilot may be balancing on the mains using fore/aft cyclic.

The rotor would probably be supporting some, but not all of the aircraft's weight.
The way I use "blade flap event" related to gyroplanes is when someone is going to fast for the rotor rpm with the disk too far back, and the teeter range is exceeded causing a lot of feed back in the cyclic. It extreme cases the retreating blade stalls and the advancing blade runs it into the empennage or the propeller.

I prefer to think of it as a divergent rotor event because a two blade semi rigid rotor won't work without flapping so flapping is normal.

#### bryancobb

##### Junior Member
The way I use "blade flap event" related to gyroplanes is when someone is going to fast for the rotor rpm with the disk too far back, and the teeter range is exceeded causing a lot of feed back in the cyclic. It extreme cases the retreating blade stalls and the advancing blade runs it into the empennage or the propeller.

I prefer to think of it as a divergent rotor event because a two blade semi rigid rotor won't work without flapping so flapping is normal.
By your definition Vance, can it happen in flight or is it only an "on-the-ground" possibility?
The cyclic feedback that you mention, is that from metal striking metal?
If a gyro pilot feels metal to metal parts of the rotor head THUMPING together during a "blade flap event" is the aircraft grounded until the parts are dye-pen, eddy-current checked, or x-rayed for damage?
By your definition, can a gyro pilot have a blade flap event just by incoming wind or relative wind blowing back the rotor disc or must the retreating blade stall?

In the helicopter world, only the type flapping you mention in your last sentence is trained.

#### Vance

##### Gyroplane CFI
By your definition Vance, can it happen in flight or is it only an "on-the-ground" possibility?
The cyclic feedback that you mention, is that from metal striking metal?
If a gyro pilot feels metal to metal parts of the rotor head THUMPING together during a "blade flap event" is the aircraft grounded until the parts are dye-pen, eddy-current checked, or x-rayed for damage?
By your definition, can a gyro pilot have a blade flap event just by incoming wind or relative wind blowing back the rotor disc or must the retreating blade stall?

In the helicopter world, only the type flapping you mention in your last sentence is trained.
Several people have tipped over their gyroplane by bringing the cyclic back at low rotor rpm and too high indicated air speed. It may briefly lift into the air but the flight is very short.

That is the stops striking the stops. They can be made out of anything.

Most people would not ground their gyroplane after a minor flapping event. If the rotor hits something there is a lot of energy to dissipate and the gyroplane needs to be grounded and lots of parts closely inspected. Some parts are more bullet proof than others.

In my experience because the retreating blade is at the higher angle of attack it is the first to stall. In my opinion on some gyroplanes you can hit the stops without stalling a blade. It is hard to know at what point the blade stalls.

In my experience I can have a divergent rotor event as the rotor slows down from wind gusts.

Usually it is just too much indicate air speed for the rotor rpm. Most gyroplanes will tilt the rotor back sixteen degrees so if the rotor is not going fast enough something had to give.