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Coriolus Effect?


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#1 eagle5

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Posted 08 September 2013 - 16:23

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



#2 aeroscout

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Posted 08 September 2013 - 16:44

Or "pre coned" ?



#3 Eric Hunt

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Posted 08 September 2013 - 18:49

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



#4 hanzSchmidt

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Posted 26 September 2018 - 06:47

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



#5 AkAr

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Posted 26 September 2018 - 23:50

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.

#6 AkAr

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Posted 26 September 2018 - 23:56

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


The Coriolis effect has to do with the conservation of angular momentum. Does that help or do you need another hint?

#7 iChris

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Posted 27 September 2018 - 02:01

"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, 14 October 2018 - 23:20.

Regards,

Chris

#8 iChris

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Posted 27 September 2018 - 12:07

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, 03 October 2018 - 04:35.

Regards,

Chris

#9 Eric Hunt

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Posted 01 October 2018 - 18:23

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, 01 October 2018 - 18:24.


#10 octagon

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Posted 10 October 2018 - 18:20

I really enjoy Mr. Schramm's explanation:

 

The whole series of Helicycle engineering videos is pretty great.



#11 octagon

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Posted 11 October 2018 - 12:22

 

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.



#12 AkAr

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Posted 12 October 2018 - 00:27

 
Well, the advancing blade isn't flapping up, it is flapping down, which is why the disc is tilted forward.


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

#13 Eric Hunt

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Posted 12 October 2018 - 18:39

 

 

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.



#14 iChris

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Posted 13 October 2018 - 01:35

 

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, 13 October 2018 - 13:17.

Regards,

Chris

#15 octagon

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Posted 13 October 2018 - 12:32

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.



#16 AkAr

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Posted 13 October 2018 - 23:58

 

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?



#17 iChris

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Posted 15 October 2018 - 12:17

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, 15 October 2018 - 14:43.

  • Wally likes this
Regards,

Chris

#18 AkAr

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Posted 15 October 2018 - 14:21

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.



#19 Eric Hunt

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Posted 16 October 2018 - 13:47

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.



#20 iChris

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Posted 16 October 2018 - 14:25

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 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, 16 October 2018 - 14:29.

Regards,

Chris




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