Blade Lead and Lag

[Jmann841]

Thank you extrapilot for constantly explaining the fundamentals of aerodynamics. I always enjoy reading your posts and learn more from every single one. I give you a lot of credit for putting up with some of the misguided information and strong headedness of the "I know I am correct" crowd who do not possess any formal education in Aero theory.

I have learned a ton from your posts and appreciate your detailed explanations.

Nightstalker, I am in the Army and work in an Aviation briage. I work with Pilots and Mechanics of CH-47's, UH-60's, OH-58D's, and AH-64's. Many of them and friends of mine are former 160th pilots or mechanics. It is most certainly not a qualification of expertise in Aerodynamic theory. There is plenty of misinformation with this group as well, even the Army's doctrinal explination of lift is vauge at best.

[nightstalker]

You are not telling the truth jman

You do not know people from the 160th. Maybe a turd that got booted, but not pilots - definitely not like they're a dime a dozen.

Just like the other experts, you falsify your details to sound more credible. I'm not impressed

[Jmann841]

Originally Posted by nightstalker

You are not telling the truth jman

You do not know people from the 160th. Maybe a turd that got booted, but not pilots - definitely not like they're a dime a dozen.

Just like the other experts, you falsify your details to sound more credible. I'm not impressed

Falsify what details? I do not know people from the 160th? You must know me very well. I can not speak much more of this on an open internet forum but you do not know who I am and can not make such statement. Because you were a 160th crew cheif does not impress me when it comes to aerodynamic theory.

[Weule]

Interesting thread. Based on what has been presented I tend to agree with extrapilot.

Nightstalker,
Can you answer extra's question:

>>You say lead/lag is because of flapping. But, you also say our blades don’t flap. How do you explain the lead/lag shown in the video linked above? <<

As an unqualified but interested observer the video says a lot...

[nightstalker]

I can only assume the blade was moving up/down and back/forth. You have to understand that the blades won't lead/lag! They will only do this in conjunction with flapping on a fully articulated rotor head.

ONLY IF THIS WERE DUE TO DISSYMMETRY OF LIFT would the blades be considered flapping. Flapping is when the the blade "gives up" lift on the advancing side. The goal is to maintain a level fuselage in forward flight. Also, the flapping is built into the design. The chinook has a flapping hinge, the horizontal hinge. Teetering heads obviously teeter. I don't know the name of the hardware. My point in that is to illustrate the give that is built in. It's automatic.

To reiterate, ONLY fully articulated rotor heads flap AND lead/lag ( in addition to feather). Do not be mistaken by attributing those elements to RC 2 bladed rotor heads.
Lead/lag is only used on a fully articulated rotor head because there are 3 or more rotor blades that have flap hinges.

In order to get educated, you guys need to look up dissymmetry of lift and how it is managed through flapping and lead/lag. The most eloquent example is the fully articulated rotor head. You should realize that those two components work together on that system to do one thing only. Also, find where conservation of angular momentum is explained(why it has to lead/lag)

Riddle me this - where is blade flex built in to allow for flapping? Isn't flexibility in our blades variable, to give a different feel? The sab blades are stiff as a board! If RC rotors flapped, there would be a painfully obvious indicator - just think of the teetering rotor.

I already tried to explain lead/lag in terms of torquing the rotor with the blades in the same plane. This position is fixed on RC rotors. Any difference will be seen when using a blade with a different chord wise cg. Only then. If you're getting lead/lag - that's not good. Your blades are too loose. I'm not sure how much force would be required to lag them in flight . . . Which makes me think . . . That is the problem in the thought process of many of you. It's not about forcing the blades to do whatever that video shows. The force required to lead/lag is the difference in cg due to blade flapping. The force required to flap the rotor blade is the difference in lift in the advancing over the retreating blade. It's effortless and automatic.

Do you really want to believe you see this while doing pitch pumps and beating and banging? Is that where you guys think you see this? Have any of you flew straight and level and considered how much the blades are flapping with relatively little force on them? Seriously. Where's this happen- if not due to dissymmetry of lift? Experts?

[Raz0rSh4rp]

I understand that flapping is primarily of use in FF. I understand flapping is a function of rotor head design.

I also understand that most RC helis have soft dampers or a teetering spindle. Sometimes both. This damping allows the spindle and grips to tilt allowing the blade tips to go out of plane. This causes the blades to speed up and slow down, ariculating in the grips. How this all ties in together I have yet to fully digest.

Sent from my SCH-I535 using Tapatalk

[nightstalker]

Well, I just thought about that Raz0r - it's called a feathering shaft. Because it feathers the blades. If it were called a teetering shaft, then we would have to take it for what it is.

Soft dampers are not to allow for flapping. The answer lies in the newer models that benefit from very stiff dampers. You wouldn't put very stiff dampers in a raptor 30, nor would you put soft dampers in a DFC T rex. This should allow you to see that those variables are distinct to how the model is designed. Mostly to either allow for more stability or to take advantage of more power and precision.

Also, the blades never speed up or slow down relative to each other. Do you know what would happen if one sped up or slowed down?

[Raz0rSh4rp]

What do you think would happen? I don't know.

BTW, any chance this flapping argument may stem from terminology. Maybe we are using the same word to describe different things?

[extrapilot]

It is absolute nonsense to suggest the blades lead/lag at the same rate at all times. In fact, they almost never do for machines with lag hinges in FF. What happens? You get dynamic unbalance (read: vibration).

If you care to learn about how these machines work, do yourself a favor- look up any of the Google Books previews from any of the majors in this area. Some were referenced in post #15. You are conversing with a guy who says a rotor blade that has no flap hinge cannot flap. Flap is one of the most basic concepts in rotary aero, and it is covered in explicit detail in every real rotary text.

Just google ‘wayne johnson "effective offset”’and look at page 234 (Section 5-15 Hingeless Rotor). Likewise, ‘Leishman “Effective hinge”’, page 201-202. These two men are absolute authorities in rotary aero. They have something to say about hingless rotors (which flap- like all rotors)…

And no- this is not some confusion in terminology. Flap is flap- the blade bounces up and down as it orbits. It doesn’t get any more simple/clear.

Kind regards

[R_Lefebvre]

From Helicopter Flight Dynamics, Gareth Padfield, page 42:

The Bo105 and Lynx were the first hingeless rotor helicopters to enter production and service.
On these aircraft, both flap and lead–lag blade motion are achieved through elastic
bending, with blade pitch varied through rotations at a bearing near the blade root. On
the Puma, the blade flap and lead–lag motions largely occur through articulation with
the hinges close to the hub centre. The distance of the hinges from the hub centre is a
critical parameter in determining the magnitude of the hub moment induced by blade
flapping and lagging; the moments are approximately proportional to the hinge offset,
up to values of about 10% of the blade radius. Typical values of the flap hinge offset are
found between 3 and 5% of the blade radius. Hingeless rotors are often quoted as having
an effective hinge offset, to describe their moment-producing capability, compared
with articulated rotor helicopters. The Puma has a flap hinge offset of 3.8%, while the
Lynx and Bo105 have effective offsets of about 12.5 and 14% respectively.

nightstalker, what exactly is your educational resume, other than turning wrenches in the army?

[nightstalker]

What is yours, other than posting irrelevant quotes?

[Raz0rSh4rp]

Again you deflect. Please, contribute or go.

Sent from my SCH-I535 using Tapatalk

[nightstalker]

What is the appropriate response to a rhetorical question?
If you're not mistaken, I have contributed a lot to this thread.
It is not within my power to enlighten you to the helicopter; you have to be able to see it. I'm really trying to get across the real deal. All my posts reiterate the same thing.

I'm telling you! I'm telling anyone who wants to know!
If there were any flap - the heli would fly like garbage. Flap is basically the blade moving independent of the fuselage. This is important in full size aircraft to deal with dissymmetry of lift. It allows the fuselage to remain level by tilting the disc instead. All of the above examples are full size aircraft that carry people. It's obvious that a full size helicopters would incorporate elements that make for a smooth ride. In contrast, the RC rotor head is very primitive. There is no one in an RC helicopter. When was the last time anyone bought blades based on how well they flap? It a non issue in this hobby. Full size helicopters are big and slow. They only really fly forward; and that's hard enough. RC helicopters go wherever they want. It doesn't matter. Do you REALLY think a simple tic toc would be possible with a flapping element in the rotor system?? Why would you want a weakness in the rotor system? Like I said a moment ago, you have to see it. Flapping is a weakness built into a rotor system to deal with dissymmetry of lift.

Please!!! Think about this! The flapping hinge on a chinook rides on bearings. It's free to move. The teetering hinge on say a uh-1 is free to move. To mention the BO 105 from the above quote, even it has elastic bearings. All designed to keep the blades moving independent of the fuselage. Now, take your RC helicopter and mimic that freedom of movement. Position a blade over the side. Put your finger close to the cg and push upwards. Look at the amount of movement in the rotor system before the fuselage moves. It's negligible isn't it. The blade may flex some but you should notice that even if you tilt the model up, off one skid, the blade wants to stay straight. There is no weakness built in. And if you consider the disc at full rpm - you should realize that there will be far less flexing than any you could perceive on the ground.

[extrapilot]

He is not mistaken- you have contributed less than nothing. You have posted a slew of ‘facts’ which are not just wrong- they are dead wrong.

You know nothing about this subject. Your repetition of this same nonsense does not make it correct- nor will it- ever.

There are mentions in this thread to the books which serve as reference to the industry. Dr Johnson is responsible for CAMRADII- the helicopter simulation software used by every major full scale manufacturer to evaluate rotor/machine performance at the design phase. He has undergrad/grad degrees in aero engineering, and a PhD in fluid mechanics, all from MIT. Likewise with Prouty, Bramwell, Padfield, Leishman, and others all with similar qualifications and/or experience. And, you disagree with these people.

A video was posted here of a head mounted camera on an RC heli which shows the blade tips climbing/descending, and also leading/lagging, in flight. And you disagree that this is happening.

Phase lag is the result of flap- if you have infinitely-stiff blades/head, you have no phase lag. We have phase lag- which means, we have flap. And you disagree with this.

You can put ANY of our machines on a fixed bench so that the frame cannot move- run up the machine with collective at 0, and apply large cyclic input. You will see the rotor tip path plane move- because the blades flap. Doesn’t matter if it is constrained teeter, DFC, whatever. But you disagree with this.

How in the world do you think we have boom strikes if the blades don’t bend/flap, or lead/lag? The boom bends up and is hit by the rotor?

If you would just put down whatever coloring book the Army gave you, and read a few chapters of Johnson, or Bramwell, or Leishman, you would be much better off for it.

[nightstalker]

Why are you name dropping? Pointless here

Why would you strap down the helicopter to demonstrate flapping? That is more proof that you are ignorant to what a helicopter is.

It did just figure it out though. None of you fly RC helicopters. I'm sure you own them and move them around. But you don't fly them to accentuate the capabilities of the machine.
I say this because I do and that is why I know about RC rotors. I study them! Not books that I can quote dr. Whoever regarding full scale theory. If you could wrap your head around that detail, you'd be a little closer to some enlightenment.
Even my urging to look at the model on the ground to see that the blades don't move,
and referencing truly flapping rotors, that move effortlessly and independent of the cyclic/ fuse went way over your head. That is so basic, clear and obvious.
You want somebody to strap it down and MAKE it do something?? Why? That makes no sense. Do you need to do this on full size helicopters to see flapping? (rhetorical)

It's funny too because you ( extra) try to slip in bending/flapping as the point. Perhaps you are already aware that I am right, but don't understand how yet. You are trying to broaden the terms to fit what you want to "be right." You are very defensive because you place a high regard in what you know and contribute to this forum. I can't fault you for that. You did display an immediate arrogance based on what you think you know, rather than receiving some good information. That's your fault and you have to own it. I won't ask for any apologies for going against what I'm saying- but don't expect me to apologize for being an a-hole.

[Raz0rSh4rp]

Extra, I think I will start reading some of those references that you have stated. Thank you.

[R_Lefebvre]

Originally Posted by nightstalker

Why are you name dropping? Pointless here

Maybe because these people are actually educated in helicopter aerodynamics. They educated the people who go on to actually design the aircraft.

You are a nobody. A guy who turns wrenches in the army, who for some strange reason thinks he knows more than the people who engineered the machine he works on. It's a very common affliction, which I've seen before in the auto industry.

All of the above examples are full size aircraft that carry people. It's obvious that a full size helicopters would incorporate elements that make for a smooth ride. In contrast, the RC rotor head is very primitive. There is no one in an RC helicopter. When was the last time anyone bought blades based on how well they flap?

There are no people on board. But sometimes there are cameras.



When you can get vibration free HD video from a helicopter, come back here and tell us that a smooth ride doesn't matter. This was done using the softest dampers available, despite the fact the machine has an AUW of 16.5lbs and swings very heavy blades. Because it matters.

t did just figure it out though. None of you fly RC helicopters. I'm sure you own them and move them around. But you don't fly them to accentuate the capabilities of the machine.
I say this because I do and that is why I know about RC rotors. I study them! Not books that I can quote dr. Whoever regarding full scale theory. If you could wrap your head around that detail, you'd be a little closer to some enlightenment.
Even my urging to look at the model on the ground to see that the blades don't move,
and referencing truly flapping rotors, that move effortlessly and independent of the cyclic/ fuse went way over your head.

You're right. I don't know anything about how helicopters fly. I only program a full featured autopilot for them. Basic flybarless controller, self-leveling, bailout, automatic Return to Home on RC Failsafe, auto takeoff and land, waypoints, etc.

[Jmann841]

Originally Posted by nightstalker

Why are you name dropping? Pointless here

Why would you strap down the helicopter to demonstrate flapping? That is more proof that you are ignorant to what a helicopter is.

It did just figure it out though. None of you fly RC helicopters. I'm sure you own them and move them around. But you don't fly them to accentuate the capabilities of the machine.
I say this because I do and that is why I know about RC rotors. I study them! Not books that I can quote dr. Whoever regarding full scale theory. If you could wrap your head around that detail, you'd be a little closer to some enlightenment.
Even my urging to look at the model on the ground to see that the blades don't move,
and referencing truly flapping rotors, that move effortlessly and independent of the cyclic/ fuse went way over your head. That is so basic, clear and obvious.
You want somebody to strap it down and MAKE it do something?? Why? That makes no sense. Do you need to do this on full size helicopters to see flapping? (rhetorical)

It's funny too because you ( extra) try to slip in bending/flapping as the point. Perhaps you are already aware that I am right, but don't understand how yet. You are trying to broaden the terms to fit what you want to "be right." You are very defensive because you place a high regard in what you know and contribute to this forum. I can't fault you for that. You did display an immediate arrogance based on what you think you know, rather than receiving some good information. That's your fault and you have to own it. I won't ask for any apologies for going against what I'm saying- but don't expect me to apologize for being an a-hole.

Wow, this amount of arrogance is unprecidented. I have never seen something like this from any soldier I have worked with, in the convential army, SF community, and other organizations both air and ground. Every person from the 160th I have worked with has been incredibly humble and professionals in their craft. This is amazing to me to meet someone who claims to have been part of that organization with such arrogance.

[Revenant759]

I'll preface this with saying I don't know all the physics and theory that go with flying helicopters, but I'm definitely interested and have learned a lot since really getting into the hobby.

Every single one of my helicopters has discernible play in the head, even when brand new, including my 550 DFC with super-hard dampeners. It's not much of an amount of play, but it's absolutely there. If I push one blade down, the other rises, even if by a small amount. This is with the fuselage secured. It's exceptionally visible on the helis with much softer dampening. My friends 450x has such soft dampeners it's hilarious to move one blade tip in a circle and see the other follow suit at an inverse. I have seen myself while balancing micros, the whole rotor, what I can only imagine, flapping as I hold it still and the gyro drifts (130x and MCPX BL) where the whole disk seems to "tilt" while the fuselage stays where it was... Is that not an instance of flapping? From my understanding, a feathering shaft, or teeter, is a configuration of allowing the blades to flap, correct? The blades move independently of the fuselage.

I'm fully comfortable saying that I do *not* have the authority, education, or experience to speak confidently of these things, but from my understanding of what I've seen, that proves our helicopters are able to flap, right? Is there actually a difference between how the flapping is achieved, whether it be by a fully articulated head, or a teetering mechanism that uses dampeners to hold the grips/blades in plane?

[extrapilot]

Rev

Any time you have some reservation about ‘facts’ posted anywhere on this subject- consider looking to one of the 3-4 major books referenced in the thread. These books are written by the authorities in the industry, and their texts have withstood the test of time- thousands of aero engineering PhD students, etc who leverage these works as foundation, etc. There is very little in them that is not baseline for full scale helis built in the past 20 years.

Your observations are correct here. Any time a blade is bouncing up and down as it orbits, it is flapping; the blade has a periodic vertical component of velocity. Your eyes see a disc that leans about as your alter cyclic- and that is just the blades flapping.

I think people have difficulty understanding how a DFC head with a fairly solid spindle can see large flap, when the blades seem so stiff. The answer there is pretty simple- have a look at the Tacoma Narrows bridge videos on Youtube. You have a massive bridge which is stiff, and yet it flaps around like wet spaghetti because its resonance frequency is excited by the wind.

Pendulums behave that way- and blades behave in flap very much like pendulums. Look at a 700-class machine. With a rotor radius of 62.5”, you have a pendulum length of the rotor radius- 31.25”. At 2000rpm, the G load is huge- 3481 G at the tip. When you put that into a pendulum calculator, you will get a pendulum frequency of approx 33hz (rounding error), where at 2000rpm, the rotor blade has a rotation frequency of 33.33hz. You can see that the flap period = the rotation period. That is a recipe for resonance. Try it for a few different RPMs- the frequency stays locked.

Because the flap period is tied to RPM, when you add a little cyclic lift/bias at the same place in the rotation over several rotations, the blade flap amplitude grows- cyclic is exciting its flap resonance.

You don’t need a flap hinge for this to happen. Think about a park toy for a kid- a horse supported by a large vertical spring- and the kid can rock back/forth on it. Not only does this show some resonance, but- where is the hinge? It cant possibly bend without a hinge! Well, that is nonsense. And if you look at the toy in operation from a distance, you can see where a hinge might be averaged if one existed- that is the concept of an ‘effective flap hinge’ in a heli rotor which does not have a flap hinge. Blade still flaps- but the location of the hinge is virtual.

One last thought. As with pendulums, swings, springs, etc- these resonant systems have a 90deg phase lag. That is, the peak velocity is out of phase with the peak amplitude by 90deg. Same with the rotor; it has phase lag which is related to the difference between peak flap velocity and peak flap amplitude, where peak amplitude follows peak velocity by some amount (90deg for teetering, a bit less for rigid and system with offset hinges, etc). Understanding basic flap is important if you want to better understand things like head design, phase lag, lead/lag, etc.

Regards