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  • 5 years later...

I'm not sure what that has to do with Coriolis effect

Do you understand why a 2-bladed semi-rigid rotor is underslung? Isn't it to counteract the Coriolis effect which would normally force the advancing blade to rotate faster since flapping up would bring its center of mass closer to its axis of rotation if it wasn't underslung? I guess I'm not sure what that DOESN'T have to do with the Coriolis effect.

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"It would need a heavy, strong yoke to absorb the forces."

 

I'm not sure what that has to do with Coriolis effect

 

All rotors generate vibrations as a function of the in-plane forces. Coupled flap-lead-lag motion (Coriolis) is an additional in-plane blade force along with the blades inertial force, centrifugal force, and aerodynamic force. These forces and vibrations can become a problem unless somehow suppressed, balanced, damped, absorbed, or eliminated.

 

The in-plane stiffness at the hub and blade root has a pronounced effect. If the in-plane flexibility is tuned above one-per-rev (1.2 -1.5 per-rev) we can attempt to duplicate the effect of appropriate underslinging. We could also reduce flapping thus Coriolis effects by placing the pitch link/pitch horn connection so it’s offset from the flap hinge axis. This is similar to what’s used on tail rotors to avoid having to use lead-lag hinges, delta-3 hinge offset.

 

In other words, as in Eric Hunt’s post above, we’re likely adding on more weight and cost and the oscillatory outcome may still be less than underslinging.

Edited by iChris
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So what would it be like to fly a two bladed helicopter that wasn't "underslung"?

 

Most likely an unacceptable rotor vibration levels would be the case; however, unless you were a test pilot, most any helicopter that you’ll get your hands on, underslung or not, has already passed certification were vibration levels have been deemed acceptable. Whatever you do you’re not going to completely eliminate rotor vibrations.

 

Back in the 1920’s, Cierva’s early autogyro blades suffered high stresses and even structural failures resulting from the drag and inertia loads during each blade revolution until he solved the lead-lag problem, in his case, with a vertical hinge. You’ll normally find one of the three, underslinging, in-plane stiffness, or vertical hinge

 

AC 27.251. § 27.251 VIBRATION.

 

a. Explanation.

 

(1) Each part of the rotorcraft must be free from excessive vibration under each

appropriate speed and power condition (rule statement).

 

(2) This flight requirement may be both a qualitative and quantitative flight

evaluation. Section 27.571(a) contains the flight load survey requirement that results in

accumulation of vibration quantitative data. Section 27.629 generally requires

quantitative data to show freedom from flutter for each part of the rotorcraft including

control or stabilizing surfaces and rotors.

 

(3) Review Case No. 70 (reference FAA Order 8110.6) contains a policy

statement concerning compliance with this rule. This policy statement is condensed

here for convenience:

 

“The rotorcraft must be capable of attaining a 30° bank angle (turn), at VNE,

with maximum continuous power (maximum continuous torque) without encountering

excessive roughness/vibration. The FAA/AUTHORITY requires the maneuver

demonstration to provide the pilot with some maneuver capability at VNE and further to

provide the pilot some margin away from roughness when operating in turbulence.”

(This maneuver may result in a descent or a climb.)…

 

REF: 27-1B - Certification of Normal Category Rotorcraft

 

Edited by iChris
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AkAr said:

 

 

Isn't it to counteract the Coriolis effect which would normally force the advancing blade to rotate faster since flapping up

 

Well, the advancing blade isn't flapping up, it is flapping down, which is why the disc is tilted forward. People seem to get hung up on the "flapping to equality" bit, which only happens if the disc experiences a gust of wind, and there is no cyclic input. That makes the front of the disc tilt back, and the aircraft will move backwards. Now the relative wind is from the rear, the disc flaps forward, the aircraft reverses direction, and in one or two more of these cycles, it crashes. Feed in some forward cyclic to (a) stop the flapback and (b.) keep the aircraft moving forward, and there is no longer any flapping to equality - killed off by cyclic input.

 

The problem with Coriolis begins on the retreating side, which IS flapping up, and (also because of coning) is "apparently" getting shorter, when looked at from vertically above the mast. Shortest is at the back, longest at the front.

 

Conservation of angular momentum will make the "shorter" blade across the back of the disc want to turn faster, and the "longer" blade want to turn slower, putting both blades on the right side of the disc. Bad news.

 

Underslinging helps to reduce this effect by moving the rear blade further away from the mast, and bringing the front blade closer in to the mast.

Edited by Eric Hunt
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  • 2 weeks later...

 

The rotor system tilts about the point (P) which is offset beneath the rotor plane, or point (Q). This results in an "overslung" rotor system that requires less rotorshaft length and which produces less shaft movement than a comparable "underslung" system.

 

https://patents.google.com/patent/US4580945A/en

 

I am curious about the tradeoffs involved there. Not much info on "overslung" rotor systems.

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Up relative to the "disc" as you call it, not relative to the fuselage.

 

So, what are you trying to say here? Do you still think the blades are "flapping to equality", and the advancing blade is flapping up?

 

If it is flapping up, why is it at its lowest at the front? Why is the swash plate taking pitch AWAY from the advancing blade, and adding it to the retreating blade?

 

It is to stop the blade from flapping up, and to tilt the plane of rotation, and the Total Rotor Thrust, forward to make it go forward. The pilot uses cyclic to make the advancing blade flap down, and the retreating blade flap up. If no cyclic is used, yes, the advancing blade flaps up in the relative wind, the plane of rotation / disc / disk / frisbee tilts back, and the aircraft moves back. Until the relative wind from the back changes the flapping, and it does a couple more cycles and crashes.

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The rotor system tilts about the point (P) which is offset beneath the rotor plane, or point (Q). This results in an "overslung" rotor system that requires less rotorshaft length and which produces less shaft movement than a comparable "underslung" system.

https://patents.goog...t/US4580945A/en

I am curious about the tradeoffs involved there. Not much info on "overslung" rotor systems.I am curious about the tradeoffs involved there. Not much info on "overslung" rotor systems.

 

You’re a bit confused about an "overslung" rotor system. Go back and read that patent again. It’s a *soft-in-plane type design were each blade is attached at its root to the hub by a pair of flexbeams. It is essentially a 4 blade fully articulated rotor system incorporating elastromeric type bearings and flexbeams, instead of mechanical hinges. The fact that it is overslung is of little consequence other than what is already claimed - “Rotor system that requires less rotor shaft length and which produces less shaft movement than a comparable "underslung.”

 

You shouldn’t over think the design. Underslung/overslung, all we’re trying to do is eliminate, as much as possible, unwanted acceleration forces at the hub, governed by the law of conservation of angular momentum, by ensuring the distance between the center of mass of each blade and the rotor mast remains constant, regardless of rotor tilt.

 

*Also, check out a post back in Jan. 2011: Soft-in-plane Rotor System. What?

 

 

wlxcyTG.jpg

Edited by iChris
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Yeah, I realized just after posting that how silly an "overslung" hub would be. Like if you inverted the normal underslung hub so the tettering hinge was below the plane of the rotor disc.

 

You'd basically have a flexible shaft then, and the "correction" applied when the blades flap would be in the wrong direction. The shaft would want to whip and the rotor would tend to whirl about the shaft axis pretty vigorously. I don't think you'd be able to fly a helicopter with a hub like that.

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So, what are you trying to say here? Do you still think the blades are "flapping to equality", and the advancing blade is flapping up?

 

If it is flapping up, why is it at its lowest at the front? Why is the swash plate taking pitch AWAY from the advancing blade, and adding it to the retreating blade?

 

It is to stop the blade from flapping up, and to tilt the plane of rotation, and the Total Rotor Thrust, forward to make it go forward. The pilot uses cyclic to make the advancing blade flap down, and the retreating blade flap up. If no cyclic is used, yes, the advancing blade flaps up in the relative wind, the plane of rotation / disc / disk / frisbee tilts back, and the aircraft moves back. Until the relative wind from the back changes the flapping, and it does a couple more cycles and crashes.

 

Do you have a copy of Principles of Helicopter Flight? Chapter 12, Forward Flight, directly contradicts you. Particularly the section under the heading "Eliminating Dissymmetry of LIft". Thoughts?

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Principles of Helicopter Flight? Chapter 12, Forward Flight, directly contradicts..

 

Particularly the section under the heading "Eliminating Dissymmetry of LIft". Thoughts?

 

Read it again, lots of confusion on this, you need to go back and really understand that assumption. That description does not represent a helicopter in sustained forward flight at a constant airspeed and altitude.

 

Turn the page and read the text under “Blow-Back (Flap-back).” Then you should begin to understand that Eric’s post is correct, it flaps down not up in sustained forward flight. In those next few pages you'll see the following quotes:

 

“Thus while flapping acts to eliminate the problem of dissymmetry of lift, it introduces an undesirable blow-back reaction that interferes with the helicopter’s airspeed.”

 

“As soon as the disc experiences blow-back, the cyclic must be moved forward yet again to overcome the problem. As airspeed becomes progressively higher, blow-back becomes stronger and further forward movement of cyclic is called for. Strictly speaking, there comes a speed where the cyclic stick is on its forward limit and the helicopter cannot fly any faster.”

 

in other words, the pilot must do something in order to sustained forward flight at a constant airspeed and altitude.

 

Let us know what you find/learn. Also, this reaction is documented in Ch. 21 under “Longitudinal Stability” or aka "Speed Stability."

 

IXcCh3w.jpg

Edited by iChris
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Turn the page and read the text under “Blow-Back (Flap-back).” Then you should begin to understand that Eric’s post is correct, it flaps down not up in sustained forward flight. In those next few pages you'll see the following quotes:

 

 

Does figure 12-5 depict sustained forward flight?

 

 

 

in other words, the pilot must do something in order to sustained forward flight at a constant airspeed and altitude.

 

 

I think you're misinterpreting that. If I'm flying straight and level at 90 kts, will I run out of fuel or forward cyclic movement range first? Assuming full tanks at takeoff.

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To start moving forward, you have to feed in a little forward cyclic. it starts to move, but then flapback wants to raise the nose - you add a bit more cyclic to hold it down. You are now at a steady 10kt.

You want to go faster, lower the nose a little (forward cyclic) and it accelerates, but again the nose wants to rise - hold it down with more forward stick, and now you are at a steady 30kt.

 

For each speed change, there is an initial forward cyclic movement, and then another forward movement to hold it there, stopping the flapback. The faster you go, the more forward cyclic you need, not just to accelerate, but to hold the speed.

 

Eventually you would reach the cyclic limits, it will accelerate to a higher speed but flapback will then raise the nose and you slow back to your previous high speed, as you have none left to hold it there.

 

 

 

Does figure 12-5 depict sustained forward flight?

No, it shows the initial movement after some forward cyclic has been fed in and then not moved again. It shows that the advancing side has more lift than the retreating side, and if the pilot does not correct it, flapback will occur, the nose pitches up, and the dynamic instability in pitch thing happens.

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Does figure 12-5 depict sustained forward flight?

 

I think you're misinterpreting that. If I'm flying straight and level at 90 kts, will I run out of fuel or forward cyclic movement range first? Assuming full tanks at takeoff.

 

No, figure 12-5 doesn’t depict sustained forward flight. We know that from the figure’s description stating what it assumes:

 

“Assume that the pilot moved cyclic forward initially to gain airspeed, and that beyond that first movement, the stick was held still.”

 

Since the helicopter possesses static longitudinal stability, if that is accomplished as described, the helicopter we react as seen in the photo below (figure 12-6). That’s the flap-up that figure 12-5 depicts. Finally, as a result of the cyclic being held still, the helicopter will soon come to a stop. This is due to the helicopter’s longitudinal stability (aka speed stability) which works to return the helicopter to its prior state of Equilibrium.

 

In order to enter forward flight and sustain such flight at a given airspeed, the pilot must act by moving the cyclic forward to overcome the blowback, which is a product of the helicopter static longitudinal stability. This is exactly what’s shown by figure 12-3 and described on page 95:

 

“Thus while flapping acts to eliminate the problem of dissymmetry of lift, it introduces an undesirable blow-back reaction that interferes with the helicopter’s airspeed.”

 

“As soon as the disc experiences blow-back, the cyclic must be moved forward yet again to overcome the problem. As airspeed becomes progressively higher, blow-back becomes stronger and further forward movement of cyclic is called for. Strictly speaking, there comes a speed where the cyclic stick is on its forward limit and the helicopter cannot fly any faster.”

 

AKAr, we had a prior post over this same subject on flapping in Aug 2014. It went 8 pages and has 17,911 views, link below:

 

Question on Retreating Blade Stall, Aug. 2014

 

nQ2OAki.jpg

Edited by iChris
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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
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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
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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.

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  • 2 weeks later...

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?

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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
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