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Coriolus Effect?


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#21 iChris

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Posted 16 October 2018 - 14:38

It seems some have problems dealing with this subject of dissymmetry of lift and flapping. They believe the advancing blade is flapping up in forward flight when it’s obvious the advancing blade is flapping down.

 

They were taught in regards to the early autogyro that used flapping as their method of correcting dissymmetry of lift; unfortunately, they were never taught that dissymmetry of lift can and is handled in another way in a helicopter by the pilot using the cyclic. Here’s a few quotes to set them on the right path:

 

“It isn’t any automatic flapping or feathering that equalizes the lift, it is mostly the pilot who stops the pitch and roll by applying the right amount of cyclic correction that balances the forces and moments.

 

Flapping helps relieve the stresses on the blade and hub, and keeps the structure lighter as a result. To a smaller extent, flapping does allow some automatic balancing of the forces across the disk, as does the blade's pitch-flap and flap-lag coupling (Delta 3 and Alpha 1 coupling).”

 

Nick Lappos, Question about flapping/feathering

 

 

Cyclic Feathering and Flapping

 

c. Correction of Dissymmetry of Lift for Helicopters. Two questions that seem to best encompass the subject of cyclic feathering versus flapping in helicopter flight are, "When does cyclic feathering correct dissymmetry of lift?" and "When does flapping correct dissymmetry of lift?"

 

(1) Cyclic feathering (cyclic repositioning) corrects dissymmetry of lift whenever a constant attitude is maintained by the aviator during changing lift patterns that occur during-

 

a. Acceleration.

d. Deceleration.

c. Rpm changes.

d. Collective pitch changes.

e. Transient gusts, wind shear, or turbulence.

 

(2) Blade flapping action corrects dissymmetry of lift whenever attitude change results from any of the conditions in (1) (a) through (e) above. When not prevented or corrected by the aviator, blade flapping action (blade flexing or hingeless rotors) will correct dissymmetry of lift in helicopters. Depending on whether airspeed is increased or decreased, this blade flapping action will cause a nose up or nose down attitude change.

 

(3) When cyclic feathering is preventing and/or correcting dissymmetry of lift, any action at or around the flapping hinge is due to nonaerodynamic causes such as- 

 

(a) Nose-high or nose-low fuselage due to existing C.G. 

(b Design shortcomings of rigging between rotor, mast, and fuselage.

 

(4) When action at or around the flapping hinge is due to nonaerodynamic causes, the aviator's concern is one of awareness for mast bumping, vibrations, C.G. management, and of shifting his item emphasis on daily preflight inspection.

 

5) Just as action around the knee joint of one's leg may involve kicking, this action could also be used for kneeling, sitting, stepping, or stooping. Therefore action at the knee cannot arbitrarily be labeled "kicking." Similarly, action around a "flapping hinge" should not be arbitrarily related to "dissymmetry of lift" and its correction.  

 

Ref: Army Field Manual, FM 1-51 Rotary Wing Flight, 1974

 

 

“If the pilot pushes the stick forward, the swashplate is tilted forward. Since the pitch arm from the blade is attached to the swashplate 90° ahead, the blade has its pitch reduced when it is on the right-hand (advancing) side and increased when it is on the left-hand (retreating) side. 

 

When the blade is over the nose or the tail, the forward tilt of the swashplate has no effect on the blade pitch. Cyclic pitch can be used for two purposes: to trim the tip-path plane with respect to the shaft, and/ or to produce control moments for maneuvering. 

 

In the first case, the pilot can mechanically change the angle of attack of the blades by the same amount as the flapping motion would have, thus eliminating the flapping. This adjustment can be used to eliminate all of the flapping, or to leave just enough to balance pitching and rolling moments on the aircraft—such as those due to an offset center of gravity.

 

In the second case, the pilot deliberately introduces an unbalanced lift distribution to tilt the rotor for maneuvering.

 

For example, if the helicopter is hovering and the pilot wishes to tilt the nose down to go into forward flight, he pushes the stick forward, causing the swashplate to tilt down in front. The pitch of the blade on the right side is decreased as the left side is increased. 

 

The resultant lift unbalance accelerates the right-hand blade down as it moves toward the nose and the left-hand blade up on its way to the tail. The rotor flaps down over the nose and up over the tail— tilting the rotor-thrust vector forward to produce a nose-down pitching moment about the aircraft’s center of gravity.”

 

Ray W. Prouty, Helicopter Aerodynamics, “Blade Flapping and Feathering”

 


Edited by iChris, 17 October 2018 - 11:03.

  • Wally and octagon like this
Regards,

Chris

#22 iChris

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Posted 16 October 2018 - 19:29

Army Field Manual, FM 1-203, Fundamentals of Flight, 1988

 

Army Field Manual, FM 1-51 Rotary Wing Flight, 1974

(Note: Original Army FM 1-51, 1974, not the abbreviated, in technical content, ASA civilian reprint)

 

 

Untitled.jpg

XyvrxQB.jpg

 

“It isn’t any automatic flapping or feathering that equalizes the lift, it is mostly the pilot who stops the pitch and roll by applying the right amount of cyclic correction that balances the forces and moments.”

 

As seen above as the pilot trims the helicopter with increasing right cyclic as airspeed increases, thus the pilot’s application of corrective cyclic, cyclic feathering, correct dissymmetry of lift.


Edited by iChris, 13 November 2018 - 01:39.

Regards,

Chris

#23 Eric Hunt

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Posted 16 October 2018 - 21:00

Bear in mind that fig 1-51 shown above is for a 4-bladed system, not the teetering heads most are used to. In 1-51, it uses a lead angle of 45 degrees on the pitch horn and then tilts the swash plate 45 degrees in advance, to give the lead angle of 90 degrees most are used to. However, it shows in a confusing manner that the whole swash plate is tilted at 45 degrees from the path of travel.

 

In most 2-blade systems, the swash plate tilts in the direction of travel, and the pitch horns get their input from 90 degrees ahead of the blades, not 45 degrees.



#24 octagon

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Posted 28 October 2018 - 15:52

Figure 2-33 is very interesting, so I found a copy of FM 1-51 from 1974 and bought it online. Excellent purchase but unfortunately there is no in-depth explanation of that diagram in the FM.

 

I'm wondering about the lateral displacement of the cyclic in the diagram. In a hover some left cyclic is needed to correct for the translating tendency caused by the tail rotor thrust, and at low forward airspeed left cyclic must also overcome the rolling tendency caused by the transverse flow effect, so that part seems straightforward. At higher speeds it seems reasonable that transverse flow effect is negligible and the vertical stabilizer reduces the need for tail rotor thrust, so left cyclic is no longer needed. But why is right cyclic needed at 90kt? 



#25 iChris

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Posted 29 October 2018 - 12:23

Figure 2-33 is very interesting, so I found a copy of FM 1-51 from 1974 and bought it online. Excellent purchase but unfortunately there is no in-depth explanation of that diagram in the FM.

 

I'm wondering about the lateral displacement of the cyclic in the diagram. In a hover some left cyclic is needed to correct for the translating tendency caused by the tail rotor thrust, and at low forward airspeed left cyclic must also overcome the rolling tendency caused by the transverse flow effect, so that part seems straightforward. At higher speeds it seems reasonable that transverse flow effect is negligible and the vertical stabilizer reduces the need for tail rotor thrust, so left cyclic is no longer needed. But why is right cyclic needed at 90kt? 

 

The right cyclic stick action is required by the pilot for control of attitude, for changing attitudes, and for the prevention and correction of dissymmetry. If not for the right cyclic to counter the dissymmetry the helicopter would roll left toward the retreating blade. 

 

Remember, we're talking about forward flight here. Note the quote in post #21:

 

It isn’t any automatic flapping or feathering that equalizes the lift, it is mostly the pilot who stops the pitch and roll by applying the right amount of cyclic correction that balances the forces and moments.

 

Make sure you’re in the right FM 1-51. The original Army FM 1-51, 1974, not the abbreviated, in technical content, ASA civilian reprint (The original has 11 chapters) . The explanation starts on page 2-18, section 2-33. It starts as follows:

 

2-33. Cyclic Control Stick Position Versus Airspeed Relationship

 

The cyclic control stick plot (fig 2-33) is an engineering graph made by a stylus placed on the cyclic stick. This permits a graphic plot and a record of stick positions required to maintain various steady-state airspeeds. The stick plot may cover the entire flight envelope, starting from the hover and continuing on to "velocity never exceed ("NE)" airspeeds, and perhaps blade stall. The stick plot was originally used to record cyclic travel for initial certification of newly designed or extensively modified helicopters.  

 

Figure 2-34 also graphically illustrates the extent and importance of the cyclic stick role in the correction of dissymmetry of lift. Without cyclic pitch correction for dissymmetry of lift, practical helicopter flight would be impossible.  

 

y7vTsix.jpg


Edited by iChris, 29 October 2018 - 17:58.

Regards,

Chris

#26 octagon

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Posted 29 October 2018 - 12:34

My understanding of the effect of dissymmetry of lift was that an imbalance between the advancing and retreating blades, for example as airspeed is increased, results in fore-and-aft tilting of the rotor disc (flap back), which is corrected by forward cyclic. The pilot increases pitch of the retreating blade relative to the advancing blade by applying forward cyclic, not right cyclic -- is that not correct?

 

My understanding of the need for left cyclic when transitioning through ETL is due to an imbalance in lift production between the forward and aft halves of the rotor disc (inflow roll/transverse flow effect), which like the above case causes a response from the rotor disc 90 degrees later.

 

I can't think of an effect that would require right cyclic as speed increases, and even more confusing is that it appears to be required at 90kt but not at 100kt...



#27 iChris

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Posted 30 October 2018 - 20:48

My understanding of the effect of dissymmetry of lift was that an imbalance between the advancing and retreating blades, for example as airspeed is increased, results in fore-and-aft tilting of the rotor disc (flap back), which is corrected by forward cyclic. The pilot increases pitch of the retreating blade relative to the advancing blade by applying forward cyclic, not right cyclic -- is that not correct?

 

My understanding of the need for left cyclic when transitioning through ETL is due to an imbalance in lift production between the forward and aft halves of the rotor disc (inflow roll/transverse flow effect), which like the above case causes a response from the rotor disc 90 degrees later.

 

I can't think of an effect that would require right cyclic as speed increases, and even more confusing is that it appears to be required at 90kt but not at 100kt...

 

You answered your question in your first sentence. Keep in mind these are cyclic trim requirements you probably don't even notice. Most pilots correct subconsciously to these seemingly small needs for trim. If you're an R22 pilot, you have a trim knob for this.

 

That the backward S pattern that's sometimes called the cyclic snake.  it's a regular S on helicopters with clockwise turning rotors from above.

 

it's called the snake because the cyclic trim position kind of snakes around the hover position as you push forward and gain airspeed. Also, coning roll increases as more thrust is produced.

 

XyvrxQB.jpg

 


Edited by iChris, 13 November 2018 - 01:39.

Regards,

Chris

#28 AkAr

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Posted 31 October 2018 - 14:43

Thanks for taking the time to explain all this repeatedly. After much reading and re-reading here, FM 1-51, and the Wagtendonk I see the light and it seems obvious now that cyclic action is what compensates for (or maybe more accurately prevents) dyssemetry of lift when the cyclic is used to gain airspeed.

I wish my flight school would have at least mentioned it. They spent so much time making sure we understood dyssemetry of lift and flapping to equality when airspeed is >0, but didn't touch on cyclic blowback and the ramifications. Again, obvious once you've considered it but the way it was presented was 0 airspeed = no flapping, >0 airspeed = flapping.




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