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Posted (edited)

I'm with you on all that. I understand that it doesn't work in a vacuum (heh) but the basics don't change.

 

In my head, to make it all make sense, I imagine it like vectors. Forward airspeed is causing flapping of x amount, and cyclic pitch is causing flapping of -x (to compensate) plus y. But in order to not confuse things, I don't imagine flapping as something I directly control. I understand that in the grand scheme of things, in order to change the tip path plane, I AM controlling flapping, but I personally think that it just muddies the waters to combine them. (Edit: I feather, it flaps in response. But it can also flap without me feathering. Which I guess means I don't *directly* control it, but I can affect it.)

 

I think the biggest thing to me, is that we can't ignore the fact that the retreating blade is flapping down. We're just forcing it back up. Personally, that's what got my attention, and would be like saying "Transverse flow effect doesn't happen, because I put in left cyclic and didn't roll right." ("The retreating blade doesn't flap down, because I put in forward cyclic, and didn't start flying backwards.")

 

Does any of that make sense? Am I inherently wrong with any of my assumptions/thoughts/understandings?

 

As far as the basic aerodynamics, that’s all that is required. All you basically need to know is ‪Dissymmetry of lift is compensated for by blade flapping that changes the blades AOA across the rotor disk to equalize lift. (That takes care of the PTS requirements).

 

As an operator of the helicopter, all the “how questions” are sometimes purely academic.

 

However, when taken to task, you need to know the rest of the story.

Edited by iChris
  • Like 1
Posted (edited)

 

As far as the basic aerodynamics, that’s all that is required. All you basically need to know is ‪Dissymmetry of lift is compensated for by blade flapping that changes the blades AOA across the rotor disk to equalize lift. (That takes care of the PTS requirements).

 

As an operator of the helicopter, all the “how questions” are sometimes purely academic.

 

However, when taken to task, you need to know the rest of the story.

 

Again, I agree with you on this.

 

My issue, and why I commented on this thread at all, was Eric Hunt made posts much earlier that seemed to imply 'dissymmetry of lift was compensated for with up flapping on the retreating blade, and everyone who said otherwise was obviously incompetent'. I couldn't imagine that was *actually* the case, which is why I tried to back everything up, ask basic questions with basic answers, and figure out exactly what he was saying, and why (for everyone's benefit, especially random "new guy" that doesn't yet understand those basics, and now may all of a sudden think that flapping up increases the angle of attack). Unfortunately, any effort to have a meaningful discussion on that front has tended to be met with derision, and attacks.

 

Sometimes, we need to talk about the basics first, and expand on them from there. For some reason, some people seemed to be implying the basics didn't happen, or aren't still happening, simply because other things *also* happened. I think we all understand that it's not as simple as any one thing happening, with nothing else affecting it.

 

I'd like to think that I do know the rest of the story, or enough of it anyway, and if it seems like I have that story wrong, please let me know; I'm not above learning something new. But just because I'm only talking about the basics so that everyone involved can be on the same page/understand each other doesn't mean I don't get it, or that I've been "taken to task".

Edited by CharyouTree
  • Like 1
Posted

Helonorth, tell me then how the pilot can make the disc tilt in any direction without it flapping?

 

Remember that the definition of flapping is movement up and down about its flapping/teetering hinge.

 

Feathering puts pitch on the blade, which makes it flap. Even in nil wind.

 

In a no-wind CG "sweet spot" hover (no stick input) …and uniform feather (blade pitch is the same on all blades) then there is no flapping going on (no dissymmetry of lift). Only symmetrical coning.

 

And therefore, no leading or lagging or underslung lateral shifting is occurring either. No coriolis effect (conservation of angular momentum). The disk in concentric with shaft centerline.

 

And that’s why they call it the “sweet spot”. One has to adjust CG get it dead on.

 

Get out of the sweet spot...then it's all happening.

Posted

Mr Tree says:

 

 

I think the biggest thing to me, is that we can't ignore the fact that the retreating blade is flapping down. We're just forcing it back up.

So if you are forcing it back up, wouldn't the end result, the steady unaccelerated state, be that "The retreating blade is now flapping up."

 

Sure, it wants to flap down, no argument about that,and it would do that if we didn't interfere with it by moving the cyclic. But to achieve level flight, with the disc tilted forward instead of being blown back, that blade has to move UP to get from the front to the back. There is absolutely no way of justifying saying "it is flapping down", because it is flapping UP about its flapping (or teetering) hinge to get to the back. Look at the pictures.

 

 

 

"Transverse flow effect doesn't happen, because I put in left cyclic and didn't roll right."

No, I would never say that. Try this: "transverse flow effect does happen, so I put in left cyclic, and didn't roll right."

 

 

 

"The retreating blade doesn't flap down, because I put in forward cyclic, and didn't start flying backwards.")

Exactly right. I stopped it from flapping down, and now my disc is tilted forwards instead of backwards.

Posted (edited)

Mr Tree says:

So if you are forcing it back up, wouldn't the end result, the steady unaccelerated state, be that "The retreating blade is now flapping up."

 

Sure, it wants to flap down, no argument about that,and it would do that if we didn't interfere with it by moving the cyclic. But to achieve level flight, with the disc tilted forward instead of being blown back, that blade has to move UP to get from the front to the back. There is absolutely no way of justifying saying "it is flapping down", because it is flapping UP about its flapping (or teetering) hinge to get to the back. Look at the pictures.

 

No, I would never say that. Try this: "transverse flow effect does happen, so I put in left cyclic, and didn't roll right."

 

Exactly right. I stopped it from flapping down, and now my disc is tilted forwards instead of backwards.

 

The only difference is Transverse flow effect is only during certain airspeeds.

 

If the blade wasn't flapping down (or...didn't want to flap down), the tip path plane would be further inclined at a given airspeed and cyclic position. The effect of the flapping down is still there... you need more forward cyclic than you "should".

 

On the left is your cyclic input, flapping up. On the right (center) is blade flapping due to forward airspeed. Far right is the sum total that the blade flaps up. There's still a flapping down "component".

 

^ ^

| |

| |

| + = |

| |

| |

V

Edited by CharyouTree
Posted

When a blade tries to flap out of plane, and is forced back into plane with feathering it causes bending and twisting along the span of the blade. Many pilots who have not seen what happens to a blade under a variety of flight regimes would probably not want to fly quite as much if they did see it.

  • Like 1
Posted (edited)

If the blade wasn't flapping down (or...didn't want to flap down), the tip path plane would be further inclined at a given airspeed and cyclic position. The effect of the flapping down is still there... you need more forward cyclic than you "should".

 

On the left is your cyclic input, flapping up. On the right (center) is blade flapping due to forward airspeed. Far right is the sum total that the blade flaps up. There's still a flapping down "component".

 

^ ^

| |

| |

| + = |

| |

| |

V

 

Are you still trying to get that retreating blade to flap down?

 

This reminds me of the Lift Theory/ Bernoulli’s Theorem Post back in 2012, schools/CFIs teaching around Bernoulli’s theorem as the source of lift.

 

Link: Helicopter Lift Theory - Equal Transit, Skipping Stone & 1/2 a Venturi - all wrong...

 

 

There’s no “net” downward flapping component with respect to the retreating blade.

 

There’s many forces and moment that must be overcome to obtain and maintain equilibrium, like weight/gravity and drag. However, the net control forces and moment must dominate any resistance. Any change in the amount of forward cyclic required, is due to the helicopters Speed Stability or Static Longitudinal Stability (post # 84 & #98 §27.173).

 

In forward flight the net control forces and moment must dominate any resistance and the blade flapping is from a low point over the nose to a high point over the tail. In other words, the advancing blade flaps down toward the nose and the retreating blade flaps up toward the tail.

Edited by iChris
Posted (edited)

Below are two examples of how schools are teaching dissymmetry of lift and how to compensate for the dissymmetry.

 

The first doesn't include anything about the pilot’s cyclic inputs or cyclic feathering. Leads you to believe that uncontrolled flapping is all that’s needed.

 

The second talks about cyclic feathering and flapping (in isolation) but doesn’t tie them together or connect their relationship with the pilot’s cyclic inputs.

 

However, both are good enough to meet the requirements of the commercial and CFI PTS.

 

Again, as an operator of the helicopter, all the “how questions” are sometimes purely academic. However, when taken to task, you need to know the rest of the story.

 

http://youtu.be/pDBjD3I4qH0

 

http://youtu.be/5zj_JtleHlM

Edited by iChris
Posted

Interesting how the first school says that the lift equation includes "half Pressure" and shows a P instead of being "half density" with a greek Rho.

Posted (edited)

i thought i was done with this thread, up until i saw that girl say that 2 cars at 50 mph on a head on impact is actually 100 mph? holy moooo'n cow

 

i hope you don't actually believe that chris

Edited by pokey
Posted (edited)

Interesting how the first school says that the lift equation includes "half Pressure" and shows a P instead of being "half density" with a greek Rho.

 

A little bit off on the difference between pressure and density.

 

 

i thought i was done with this thread, up until i saw that girl say that 2 cars at 50 mph on a head on impact is actually 100 mph? holy moooo'n cow

 

i hope you don't actually believe that chris

 

This is a bit off too, two cars colliding head-on each at 50 mph being the same as one car colliding into a wall at 100 mph. The closure rate between cars is 100 mph, but the two head-on cars at 50 mph equaling one at 100mph into a wall, incorrect.

 

Both of the head-on cars (assuming both are identical) would share in dissipating the sum total of their kinetic energy.

 

KE(total) = {½mV2 + ½mV2} = mV2

 

For the one car at 100 mph into the wall, which is twice the speed of the head-on car, the one car would have to dissipate:

 

KE(total) = {½m(2V)2} = {½m x 22 x V2} = {½m x 4 x V2} = 2mV2

 

The 100 mph car into the wall would have to dissipate twice the energy.

 

However, what is equal, is since the two head-on cars share in dissipating the sum total of their kinetic energy ( mV2), we can divide mV2 by 2 or {½mV2}.

 

That gives us what each car dissipates {½mV2}.

 

That takes us to what is equal:

 

What is equal are two cars colliding head-on, each at 50 mph receiving the same damage as one car colliding into a wall at 50 mph. The three cars would all receive the same amount of damage since they all dissipated the same amount of kinetic energy.

Edited by iChris
Posted

I have no problem with the fact that you can get much further in depth with just about any topic regarding helicopter aerodynamics. Of course you can, it's a complicated subject. However, it's not fair to act like people should have this much knowledge about the subject at an early stage in their development of knowledge. Those of you posting who know the "deeper level of knowledge" on RBS, answer this question honestly:

 

Did you understand it this well when you were first learning, or even when you first became a CFI?

 

I highly doubt that you did, and I think that some of you might be forgetting that you started with the basics too, and acquired the "deeper level of knowledge" as your career progressed.

Posted

 

 

Did you understand it this well when you were first learning, or even when you first became a CFI?

Whiteshadow, I studied aerodynamics at university before stepping into a helicopter, so the answer is "Yes".

 

There is only one aspect of this whole thread where I said that people's knowledge was too shallow, and that was the bald-faced untruth that :

 

"The retreating blade is flapping down and the advancing blade is flapping up", which is aerodynamically wrong, and blindingly obviously wrong to any onlooker.

 

Chris has also shot you down over this, but nobody seems to doubt Chris's word - which is good.

 

When an instructor only has 100 hours more than the student, there will be some loss of knowledge, forgotten by the instructor and not understood by the student, who then passes the same disinformation on to his own students 100 hours later.

 

Simply correct your statements, "The retreating blade is flapping up, because the cyclic forces it up" and you have broken that cycle.

Posted (edited)

EH: Whiteshadow, I studied aerodynamics at university before stepping into a helicopter, so the answer is "Yes"

 

 

Yeah, well I stayed at a Holiday Inn last night, too. Studying is one thing, practicing the discipline quite another. What level of study did you achieve? Thought so...

 

Sorry, a couple of guys opinion on the internet and a paragraph from an unknown army manual doesn't amount to much. Pardon me if I don't swallow the line your selling. So yes, I doubt iChris' word. He may very well be right: it only took about 50 years for the "equal transit time" thing to be scrapped out. Good luck with it.

Edited by helonorth
Posted

Whiteshadow, I studied aerodynamics at university before stepping into a helicopter, so the answer is "Yes".

 

 

 

Hmm, I only went to a junior college, but it seems to me that there's something grammatically incorrect about that sentence? Something that someone who studied at a university should know? :blink:

Posted (edited)

Sorry, a couple of guys opinion on the internet and a paragraph from an unknown army manual doesn't amount to much.

 

So yes, I doubt iChris' word.

 

Oh, so you wanted to know were to find this stuff, now you know:

 

U.S. Army Field Manual: Rotary Wing Flight, FM 1-51

 

U.S. Army Field Manual: Fundamentals of Flight, FM 1-203

 

U.S. Army Field Manual: Fundamentals of Flight, FM 3-04.203

 

Rotorcraft Aeromechanics, Wayne Johnson, NASA

 

Principles of Helicopter Aerodynamics, J. Gordon Leishman

 

Helicopter Performance Stability, and Control, Raymond W. Prouty

 

Helicopter Aerodynamics Vol 1 & 2, Raymond W. Prouty

 

The Art of the Helicopter, John Watkinson

 

Cyclic & Collective Art and Science of Flying Helicopters, Shawn Coyle

Edited by iChris
Posted (edited)

Helonorth says:

 

Yeah, well I stayed at a Holiday Inn last night, too. Studying is one thing, practicing the discipline quite another. What level of study did you achieve? Thought so...

Since you asked,

Bachelor of Science (BSc) with majors in Mathematics and Physics, 1970

1 year postgraduate studies in Aerody, Meteorology and Aircraft Engineering

First solo (airplane) Jan 1969 after 11 hrs

First solo helicopter May 1973 (11 hrs to solo, in turbine helicopter)

 

How about you?

 

 

 

 

 

 

Oh yeah, gave flying lessons to Tom Cruise too.

Edited by Eric Hunt
Posted

You got a BS in physics 45 years ago, soloed in airplanes and helicopters after 11 hours AND taught Tom Cruise to fly? Why didn't you say so in the first place? Since it took me longer to solo and I haven't taught any movie stars to fly yet, I will defer to you.

 

Attention all flight instructors, student pilots and everybody else: tear out all the pages in any book that states that blade flapping neutralizes dissymmetry of lift and that the retreating side flaps down. Then burn them.

 

ichris, it only took 92 posts for this one lonely and uncredited outside reference. Excuse me if I didn't burn all the other books earlier.

 

110. Cyclic feathering compensates for dissymmetry of lift (changes the AOA) in the following way. At a hover, equal lift is produced around the rotor system with equal pitch and AOA on all the blades and at all points in the rotor system (disregarding compensation for translating tendency). The rotor disk is parallel to the horizon. To develop a thrust force, the rotor system must be tilted in the desired direction of movement. Cyclic feathering changes the angle of incidence differentially around the rotor system. Forward cyclic movements decrease the angle of incidence at one part on the rotor system while increasing the angle in another part. Maximum down flapping of the blade over the nose and maximum up flapping over the tail tilt the rotor disk and thrust vector forward. To prevent blowback from occurring, the aviator must continually move the cyclic forward as velocity of the helicopter increases. Figure 1-56 illustrates the changes in pitch angle as the cyclic is moved forward at increased airspeeds. At a hover, the cyclic is centered and the pitch angle on the advancing and retreating blades is the same. At low forward speeds, moving the cyclic forward reduces pitch angle on the advancing blade and increases pitch angle on the retreating blade. This causes a slight rotor tilt. At higher forward speeds, the aviator must continue to move the cyclic forward. This further reduces pitch angle on the advancing blade and further increases pitch angle on the retreating blade. As a result, there is even more tilt to the rotor than at lower speeds.

Posted (edited)

Helonorth says:

Since you asked,

Bachelor of Science (BSc) with majors in Mathematics and Physics, 1970

1 year postgraduate studies in Aerody, Meteorology and Aircraft Engineering

First solo (airplane) Jan 1969 after 11 hrs

First solo helicopter May 1973 (11 hrs to solo, in turbine helicopter)

 

How about you?

 

 

 

 

 

 

Oh yeah, gave flying lessons to Tom Cruise too.

This is probably the most hilarious and yet pathetic posts (in a turbine helicopter!) I've seen in a least two weeks.

 

BTW, it's been quite awhile since I saw "Top Gun" but Maverick sure knew how to handle that jet and save us from the commies, too. Now I know who to thank!

Edited by helonorth
Posted

Since the dissenters insist on referencing it, I might as well show what the FM actually states. From FM 3-04.203 - Fundamentals of Flight, pages 1-13 and 1-14.

 

Huh, it says that in directional flight the advancing blade is flapping up and the retreating blade is flapping down. That's funny, who woulda thunk it?

post-33066-0-64690500-1408297899_thumb.jpg

post-33066-0-37837400-1408297911_thumb.jpg

Posted

 

good ole Juan , interesting history (as i have stated earlier)

Posted

Let's keep the hilarity coming, Helonorth, put up your educational background.

You too, Pokey, your "30 years around helicopters" appear to be somewhat unverified.

 

UH-60, keep reading and get to the page that Chris referenced, where the pilot puts in forward cyclic to stop that flapping and continue in forward flight.

 

And I only taught Tom in a helicopter, he already knew how to fly planks, having his own P-51 Mustang.

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