RBS, precession & LZ
 

This thread has begun at the suggestion of Lu (see vote thread).

I have seen many of Lu’s posts, and I believe that his search for understanding needs to start back with first principles, so I will have a go at retreating blade stall whilst (hopefully) demonstrating to Lu that he needs to go back to basics first…………………

To Lu: you need to grasp first aerodynamic POF before you can hope to grasp the explanations offered by so many in various threads. You particularly need to understand how FLAPPING TO EQUALITY, achieves EQUALITY OF LIFT. These concepts were ably explained to you in the last thread by Arm out the window, with almost no acknowledgement from you, but once you understand these basic POF concepts, most of the explanations offered to you will become more easily understood. After all, your stated aim is to understand.

Why am I going down this path? Some examples….

You have persisted with the notion that the rotor disc has different lift in different areas: You said: >> If it were pure aerodynamics the helicopter would roll to the right because of the differential of lift across the disc<<. This is NOT TRUE. The DISSYMETRY OF LIFT (not differential) is equalised by the blades FLAPPING TO EQUALITY, thus we have the steady state situation of lift being equal across the entire disc from entirely aerodynamic processes.

You said: >>When a helicopter is in the flight regime that will result in retreating blade stall the blade does not stall like a wing on an airplane. Most people think that when the blade stalls it drops out of the tip path and strikes the tail boom this is not true. What happens, is the left side of the rotor system although it has higher pitch it is generating less lift than the right side of the disc. This differential of lift will cause the perturbing force to be on the right side and like the rotor on a gyro the disc will tip up over the nose and down over the tail<<.

NOT TRUE

A rotor blade stall is entirely consistent with a wing on an aeroplane that enters a stall. BECAUSE A ROTOR BLADE IS A WING (maybe that’s why they call them “rotary wing” aircraft). The blade does NOT become >>suddenly unstable<< as you have asserted. It does NOT have a higher pitch angle. It does not suddenly fall out of the plane of rotation. It does not even lose all lift. The stall is caused by the blade exceeding its critical angle of attack (AoA), as opposed to pitch angle, due to the airspeed/AoA relationship, just like a fixed wing. As the blade approaches the critical AoA, it is still generating significant lift, but the drag is beginning to increase dramatically. Further increase in the AoA produces the buffet where airflow begins to break its laminar flow over the aerofoil and lift dramatically decreases whilst drag dramatically increases.

At this point in the process, a DISSYMETRY OF LIFT situation is created because of the rapid loss of lift from the retreating blade side and the blade is unable to flap to equality from this situation. Accordingly FOR PURELY AERODYNAMIC reasons, the aircraft will roll toward its retreating blade side. The stalled blade begins to reduce its upward movement (remember the retreating blade is normally climbing), but due to rotation, the airflow changes and the blade quickly becomes unstalled, and resumes its climb toward the highest point of the disc over the tailboom. Because the stall interrupts the blade’s climb, it can no longer reach its pre stall height over the tail boom, hence the tip path plane is lower at the rear when in retreating blade stall. Accordingly, the aircraft will pitch nose up. How gyroscopics applies to this is beyond me. So what the pilot feels and sees is:
A number of blades per rev vibration (ie a two per in a B212, a four per in a S76) rapidly building in severity, a pitch up and roll moment toward the retreating blade side.

By lowering the collective, the pilot can bring the blade out of the stall and recover the aircraft. BUT here is the crunch: when it recovers this way, there is a restoration of flapping to equality and lift is again equalized across the disc and therefore the disc DOES NOT SUDDENLY ROLL BACK TOWARD THE ADVANCING SIDE. So why des the nose try to gently pitch down if there are no GYROSCOPIC roll moments applied? Because the blade again climbs to a higher high point over the tail (the upward movement around the retreating side is no longer disrupted by the stall) and the aircraft will pitch VERY gently nose down (if you are brave enough to let it). IE an aerodynamic situation.

There is more:

You said: >> I won’t challenge your statement that you allude to as pure crap as that would take too much time but I will address one point. I made the comment about the Bell blades not having equal lift when the blades passed over the longitudinal centerline. I caught a lot of flack on that so; I contacted the that showed what causes the two per rev. At approximately 5-7 degrees ahead of the lateral centerline the advancing blade will cause an upward movement. Just like the explanations of Nick and Frank. He explained that this upward movement was the cause of the two per rev. Now as I addressed the movement of the transmission on the 214 it moved not only up, it moved down by the same amount. that if the advancing blade made the transmission move up then what makes it move down. is the blade loses lift at some point and the transmission falls. When the next blade comes around the trannie moves up and this happens at twice the rotor speed. Based on this, I would like to hear some of your opinions.<<

My opinion? You caught a lot of flack because what you are saying is NOT TRUE.
The Facts? See Nick Lappos’ excellent description of the causes of the vibration. If you look back over your statement, you will see the key to understanding what people are trying to tell you……

You said: >>[The]senior aerodynamicist at Bell….sent me some engineering diagrams. I didn’t fully understand the diagrams. My mind tells me. The only thing I can think of.<<

When you don’t understand, you do not ask, you create a theory in your own mind and reject explanations that do not accord with your theory. I believe that if you understood equality of lift as achieved by flapping to equality, you will be equipped with the basic tools you need to help you understand the rest of the discussions presented to you. Good luck in your search for understanding Lu.

For others.....have I grasped retreating blade stall? It has been a long time since I have reached into these memory banks.

AAARRRGGGGHHH my helmet is on fire……………..

[ 04 August 2001: Message edited by: helmet fire ]

I think it's about right!

It needs to be borne in mind that the blade cannot be thought of as a single wing like that of an aeroplane because of the variance of airspeeds seen by the blade from root to tip (very high speed near the tip, very slow near the blade root). That is why the blade is designed with twist and sometimes with profile and size variations along its lentgh.

As soon as the helicopter moves away from the hover, the blade on the retreating side is experiencing a reduction in airspeed and the inner portions / elements will very soon reach a stalled condition folowed by a reversal of airflow i.e. negative airspeed.

To put some simple figures on that (the only type most pilots like):

Rotational velocity = circumference of circle times rotor rpm = 2 x "pi" x R x N

"pi" = 3.142, R = distance from centre of rotor hub and N = rotor rpm.

A blade element 1 foot out sees 14.3 mph

A blade element 2 feet out sees 28.6 mph

A blade element 20 feet out sees 286 mph

It should be seen that the inner part of a rotor may be stalled or very close to it even in the hover! Once the helicopter moves into forward flight the stalled area expands outwards. An area of not only stalled blade but with a reversed airflow follows it outwards from the centre as the forward airspeed increases. Retreating blade stall is very definitely not a sudden phenomena.

It is of even more complicated than this! The airflow is of course modified by the induced flow which affects the angle of attack of each blade element, and at low speeds this is significantly different from "front to back" of the disc with regard to the direction of travel (looks like we are into inflow roll and flap-back next!). :eek:

Just thought I'd remind folks of this.

ShyT

[ 04 August 2001: Message edited by: ShyTorque ]

To ShyT:
Thanks for that excellent info. You reminded me that the blade does not stall along its entire length, rather it starts with the hub and expands outwards and fore/aft of the 9 oclock position (American helos) as the stall deepens. Thanks.

To LZ:
RBS is not about precession per se, thats why it was addressed seperately to Dave Jackson's thread. Perhaps the topic title should have excluded it, but it entered this discussion because you attributed the nose up pitch experienced in RBS to gyroscopic precession. Hopefully, I have shown you an alternative explanation.

I note your comments about putting up theories for discussion, but arguing with known principles is not achieving anything. Your stated aim was to gain understanding and I am suggesting the road to that goal. The above explanation of a relatively straight forward situation is intended to help you grasp first principles so that your deeper discussions can be more benefitial to you, and less anoying for others. For example, there is no point discussing the effect of the moon on tidal fluctuations if you do not first accept that the earth is round and the moon orbits the earth. In our case, you need to understand flapping to equality before you can grasp phase lag, delta hinges, 2 per vibrations, flapping in a Bell teetering head, cyclic feathering, etc.

[ 04 August 2001: Message edited by: helmet fire ]

Lu,

Why is it you always start the majority of your posts with a thumbs down?

You can't take it, but you can sure dish it out mate.

You can disagree all you like with me, or I can disagree with you, but you cant fight science, maths and the laws of physics, which you seem to do very well, I endorse the above posts about UNDERSTANDING helicopter priciples of flight.

You however don't. I recall when you tried to comment on the mixing box of the CH-47 a few months ago and got trashed by very experienced pilots and instructors on that aircraft. You tried to pass off your theories off and up until that point you had everyone convinced that you were an expert on the Chinook, then you end the thread by saying, "thats my theory but I could be wrong". You also said that when a pilot is executing an autorotation "The pilot then lowers the collective to effect touchdown"

To which I replied" Please tell us that was a typo Lu"

To which you failed to respond.

You really don't understand fundamental helicopter aerodynamics, so you put theories forward to this forum to make yourself out to be an aerodynamic guru.

Concepts of understanding are way different to what you have been putting forward on your posts, you just don't understand.

If the above posts help other to understand the concepts of helicopter aerodynamics thats great!

Don't throw your stuff off as theories, they are set in concrete, and no matter what keeps you up at night be it a Robinson or an Airbus, your theories don't cut it.

You are not an Engineer,
You are not an Aerodynamicist
You are not a Pilot
You are not involved in rotor design
You are by experience an unlicensed A&P Mechanic, I think you should stick to that.

If you are trying to understand helicopter principles of flight by way of this forum and not expousing your theories, then all the other posts you have made are therefore, by definition, a misunderstanding of helicopter theory.

Thats fine, but don't pass your own theories off as an expert, you definately are not one.

Anyone who reads your posts should understand this.

Anyone whos interested should go to the top of the page, click forums for last 100 days,go to page 8, find the Subject of "Chinook Flying" then click on page 2 and read the stuff from Lu, then read the responses by the experts.

Lu, you just can never accept the experts

[ 05 August 2001: Message edited by: sling load ]

no response as usual

SLING LOAD 1

LU ZUCKERMAN 0


Very interesting reading on the Chinook posts

All this nastiness is making me very upset!!!
Must be time for a group cuddle!!!

[ 08 August 2001: Message edited by: Too Cloudy ]

Lu,
I have never stated I was right, my position is that of Nick Lappos, your position is to argue with everyone, and Im not talking about concepts of understanding, Im talking about theory.

You argue and try to knock theories, or convince people to beleive you are an expert, I have never tried to convince anyone im an expert, you however have.

Im a Helo driver with an ATP and over 3000 hrs m/e ifr sar/ems exp, Mon and Tues i beleive i was at University doing my Aerospace Engineering studies.

Lu, you are obviously trying to impress me with your reliability and maintenance work on the Rolls Royce Tay engine, hey, like i said, stick with what you know, thats great, but don't advise helicopter pilots how to fly if youre not one, and don't tell engineers theyre wrong if youre not one, and if you are trying to pursue a concept of understanding thats good too, if you are trying to be different for the sake of it, you are obviously trying to be noticed.

Trouble is Lu, youre being noticed for the wrong ideas.

All the Chinook drivers and instructors blew you out the water, while you offer your "thats my theory but I could be wrong ", up until that point you tried to convince everyone that you were an expert, sorry mate, you aren't. And neither am I an expert in Chinooks, but I have got quite a deal of flying behind me and some heavy aerodynamics studies ive done, but I AM IN NO POSITION to dish out criticism of a proven rotorhead design like you do, IM NOT QUALIFIED TO DO THAT LU.

Contrary to the opinion expressed by Mr Thumbs Down, it's not a good idea to lower the collective as you're sliding along the ground at the bottom of an auto.

Keep the collective where it was when you finished cushioning on until the aircraft stops moving, then lower it, otherwise you'll probably have the blade flapping all over the place and cause damage to various bits of the airframe.

The thing about the blades stalling out and dropping is a crock!

Lu,

What I was saying was that the collective shouldn't be lowered until all movement stops. This is because at very low RPM such as will be experienced at the bottom of an auto, you don't have much in the way of direct response to your cyclic inputs. Therefore, any blade flapping may lead to exaggerated disc movement and possible contact with parts of the airframe as you are bouncing along.

The collective should be held where it was when you finished cushioning on to the ground, definitely not lowered, as that would increase the likelihood of the blades getting low enough to strike the airframe.

The 'blade stalling and dropping' idea that you expressed above doesn't apply - even though they may not be flying very efficiently at these low speeds, positive pitch on the blades will always help to hold them up until you're ready to lower them (gently).

I was concerned because what you said in the post a few above seems to suggest that the collective should be lowered before the blades get slow enough to stall and drop quickly, allowing them to strike the tailboom or whatever.

That is what I was referring to as being a crock, and instead I would encourage pilots to definitely not lower the collective until all sliding or rocking motion at the bottom of an auto has ceased.

Thanks Arm Out the Window,

I went over this point exactly on another thread after Lu posted this rubbish.

SL

I must admit I am not a regular on rotorheads but as a CFS (H) Sqn staff chap with quite a bit of POF experience, I thought the explanation at the top of the page was brill summing RB Stall in one quick post. This chap Lu is WRONG, he says we are all entitled to an opinion, yes we are, he is WRONG however. Gyroscopic Shymoscopic. Let him deny all day, there are people in this world who will not accept they can be WRONG. Don't get stressed and slag him off because he is WRONG, he is entitled to his input. We all make mistakes,don't we?

Jayteeto,

Absolutely agree, you probably missed however, this Lu started a thread name "18 degrees Ask Nick Lappos the ultimate arbitration".

Lu has been bashing the R 22 design for a while, he asked Nick Lappos a highly regarded Sikorsky Test Pilot his opinion, eventually Nick got his point across, but as usual, Lu had to throw in his two cents back, at which point Lu started a slanging match because Nick did not agree, like we all, don't agree that the R-22 is "dangerous". Lu started a thread that went to about 100 posts and Lu decided to remove it, because of the caning he copped I supect, however Lu is still active telling all of us helo pilots that all we know about aerodynamics is really precession, and other "theories".

Go to his old posts, and you will make a judgement probably like the rest of us.

[ 09 August 2001: Message edited by: sling load ]

Naaah ....

It's pretty much the entire planet, but the ISS astronauts haven't been polled yet, you could hold out for Jim Voss but ...

You haven't accepted anything since this started long ago, you fail to realize that no one will accept an "Alternate theory" that is plain wrong, and is not supported by FACTS. No one here has countered your drivel with a theory, just facts.

Several personnel have tried to prove your theory, something you yourself have refused to do on your own, and they all have reported back: What you predict doesn't happen.

When a theory is not proven, it is discarded.

You are the only one who seems to have a problem with that. But then As I have repeated so many times, if you didn't have that one theory to banter on about endlessly, you would no longer have a reason to be here, for the Robbie Ranting seems to be your only calling.

Your theory is not correct, it has not been proven by numerous people in the know. It has been discarded by everyone but yourself, the only reason it continues here is you.

Your answering everyone else is just from the FACT that you either want to have the last word (which is still meaningless in light of the flawed theories), or from the fact that you still will not accept that you are just plain wrong.

My thought is both, and as in the other threads you have started or busted into, you will now dredge up any number of sorry sad excuses to continue, make other personal observations of me and others (the pond education division theory was a nice try, but just who you kidding here?, no one is falling for it)

[ 10 August 2001: Message edited by: RW-1 ]

Lu Zuckerman said:
I can absolutely guarantee that anyone that is from OZ or the UK or any country allied to them in language and government will say Lu Zuckerman is full of crap and so are his theories.

Nick sez:

Oh well, I really like the US, but considering how I feel on this issue, I guess I have to move to Oz or the UK now. How's the beer, mate? :D

Nick, the beer's nice, cold and tasty. Or, if you're going to the UK, warm and tasty. ;)

I assume Lu's done a comprehensive world survey to support his obviously well-founded claim that he can personally guarantee what everyone in the UK or Aust helicopter fraternities thinks.

[ 09 August 2001: Message edited by: Arm out the window ]

Nick,

Are you sure you want to move? You would be made very welcome but rotor blades go the opposite way over here in UK and even worse, helicopters fly upside down in Oz..

Wait a minute - maybe that's the whole problem. Perhaps gyroscopic precession only works one way round....

ShyT

:D

Lu,

I do believe that aerodynamic forces are largely responsible for the motions of rotor blades in response to pitch and airflow changes, and that the 'gyroscopic precession' explanation is just a convenient teaching tool

However, I also think that because rotors are large heavy whirling masses, I would be a mug to discount entirely the effects of gyroscopic forces on how they behave.

So yes, I subscribe to the 'aerodynamics' school of thought on the subject, but I'm sure it's more complex than any of us can understand or describe.

My two objections to your posts above are:

1. You seemed to be telling people that they should lower the collective after an auto to prevent the blades stalling and dropping suddenly. I strongly believe that is bad advice, for the reasons I gave before, and I know from experience that your reasoning is flawed in regard to that point.

2. Your statement about the aerodynamic vs. gyroscopic theories and how they apply to different countries and groups of people is probably broadly true, in that we tend to repeat the things we were taught, but I didn't want to let your sweeping generalisations go unchallenged.

Read back over your statement - how can you possibly guarantee, as you did, that everyone in those groups think as you say they do?

I like reading some of your stuff, but I don't think it's right to pass off your opinions as fact.

LZ, it's not just Oz and UK pilots that disagree with you. Remember the "Just Helicopters" forum? Most of the contributors to that are from North America, and they told you where to go with your theories in no uncertain terms, to the point that you vowed to stop posting. Virtually the next day you popped up on Pprune and started all over again. You got the same reaction, only with a lot less vitriol.

Lu,

I, like Arm out the window, believe the aerodynamic theory whilst not entirely discounting gyroscopics because I do not fully understand some of the more complex dynamics. BUT, I note that you have focussed on gyroscopics in this thread, despite the fact that it is an attempt to help you understand RBS. See the original post, and my response to your first post on this thread.

So rather than just go >>on and on<< as you said above, how about reponding to the topic at hand....RBS, and your need to grasp flapping to equality in order to apply fundamental background logic to your theories? If you can describe them in terms of this, you will have many more followers than detractors. I reiterate my above comments on having to understand that the earth is round before discussing tidal fluctuations.

You will never understand any one's reply to you until you grasp flapping to equality.