Why do some FBL heads have built in phase lag?

[myxiplx]

Originally Posted by Icanfly

Maybe 10% more blade efficiency (area/aspect ratio) could be thought of as having the effect of subtracting some same amount in degrees in phase lag, just a thought.

Utter gibberish I'm afraid, blade efficiency and phase lag simply do not work anything like that.

[Icanfly]

i said

Maybe

luvmyhelis is highly qualified to speak on this and I actually said to myself that he should reply next after my post was entered on the main forum, he will know. If people like him aren't tearing the info down what then? Nick Maxwell making recommendations on phase what then? uhh, no one said whatever was posted was totally accurate and I said it might need some objection where fit, MAYBE, so you tell us what "it" non 90* phase lag stems from.

I have spent a considerable amount of time studying this and associated phenomena for the past 3 years and by no means will that make me pass a University course on the topic, it helps understand what MIGHT be at work in the machine. My reply has allowed room for correction, you want to make the correction with substance not downright condemnation, gibberish, great, and we all learn when someone steps forth to really teach the cold hard facts, camon, is that you? no? you don't know? we're waiting, tell it like it REALLY IS.

[quinn]

Lol! Yeah I sort of agree. If someone wants to call gibberish, they better back it up. Personally, I can't say I really understand the matter, I just want to know enough to be able to fine tune it.

[myxiplx]

Sorry if I came across as a bit harsh, but you have a habit of posting things with a lot of technical phrases and numbers, and writing as if things are fact,but you're stringing together a whole bunch of things that simply don't work the way you think, and I don't want beginners thinking it's accurate.

"It takes a certain amount of compression to lift the heli". No, it's far more complicated than that. The lift generated depends on a whole host of factors, you can't state that a set amount of compression will always lift a heli.

"It takes up to 90* to finish... because for every action there's a reaction". Again, a gross oversimplification, you're quoting Newton's third law, and while that's a factor here you can't simply state that phasing happens purely because of it. You're mixing scientific buzzwords with pure guesswork.

"sending physical forces in the opposite direction as the air above the blade is also exiting the blade differently to each foil". Just no. Yes, the blade exerts an upwards force on the helicopter due to the motion it imparts on the air, but you can't say this is because the air is exiting "differently to each foil", that doesn't even make sense.

[Icanfly]

now you are much better understood and have at least held the notions as partly credible while clearly making sure they are to be understood as oversimplifications of the multitude of variables involved.

That last part, air passing over the back of a foil will speed up on it's way around the foil like a whiplash or more like the tack of a sailboat as it cuts a more acute right angle to the wind, it will speed up due to wind speed wanting to maintain momentum while working it's way across and around an object.

Different foils have different amounts/ratios of drag and lift and where they lift best along the chord, lead, center, and lag, the curved nature of the blade can cause it to lag or project forward in pitch events, like in an auto where they project forward in a flare as wind speed quickly increases across the topside of the blade sending it forward and into a early stall if pitched too much. Don't know if you understand this.

Was it finless bob who wrote a blog on the heli rotor and all it's effects, there's a topic out there with good info on phase, gyroscopic procession, flapping, etc, and sometimes you have to read it 2 or 3 times before it sinks in.

Anyway, after reviewing the first vid last Wednesday of the heli shutting off in flight I timed the un powered portion at 3 seconds of lightly descending flight to find orientation and then it came down fast and I flared at the moment of impact where it stalled hard while hitting the ground.The ultra wides have too much foil at the halfway point in the length and have an amount of drag greater than narrower ones. The TT blades went back on this aft and drag was less than the 70mm wides, foil thickness is also 2mm less on the tt 550's.

[desertstalker]

Originally Posted by Icanfly

it will speed up due to wind speed wanting to maintain momentum while working it's way across and around an object.

What?

A tip, if you find yourself personifying inanimate objects to make an explanation, re-phrase.

[Luvmyhelis]

As a few have commented, it is def a very complicated science to say the least. Few really understand it. I know I don't. Heli size, blades, HS, weight, even wind speeds and temps. Damps, head design, flight demands, controllers, and especially personal preference, ect ect ect ect.

[extrapilot]

For people who care- blades lag because they flap as pendulums. Look up pendulum math- the oscillation period in seconds is 2 * pi * sqrt(pendulum length in meters / acceleration in m/sec^2).

And with a pendulum, peak blade flap velocity is 90deg out of phase with peak displacement. A simple example is a kid in a swing. You push the kid a little bit at the base of the arc, and he hits peak displacement 90deg later. On the way back towards center, you add a bit more energy the other direction, and he reaches even higher displacement the other direction. Gravity is the spring which accelerates the kid back towards center. This is resonance, and it explains how a kid with very little energy can achieve very high displacement IF he puts in energy at the right time in the cycle.

Likewise, cyclic causes some flap force to be added- by generating a bit of uplift in one hemisphere, and a bit of down in the other. And as with a kid in a swing, it takes a few pushes to get large displacement- which is why it takes a rotor some time to react to cyclic changes at the swash (usually 1-2 rotations of the rotor before you see peak cyclic forces).

Where people get lost is in ‘why’ this works for a rotor that can spin at any RPM. And, the answer is just as simple. The g loading on the blade is a function of its RPM. When you change the RPM, you change the g load.

Examples:
Assume a 700-class with 62” rotor diameter. Assume dampers permit teeter/flap at the spindle center. 62” is 31” radius, or 0.7874m. So, the ‘pendulum’ here is 0.7874m long.

1000RPM at .7874m = 880.5g. 1g = 9.8m/sec^2, so acceleration is 8634.8 m/sec^2. Plug that into the pendulum equation. 2 * pi * sqrt(0.7874 / 8634.8) = 0.06 seconds. Convert that to frequency- 1/.06 = 16.6666hz. Convert to cycles per minute: 16.6666 * 60 = 1000. 1000 flap cycles per minute, at 1000 revolutions/minute.

1500RPM at .7874m = 1981g. 1981 * 9.8 = 19428 m/sec^2. 2 * pi * sqrt(.7874 / 19428) = 0.04 seconds. 1/.04 = 25hz/ 25hz * 60 = 1500 flaps cycles per min at 1500 revolutions/minute.

Doesn’t matter what RPM- the flap frequency stays locked to the headspeed because the restoring force (acceleration/centrifugal force) changes with RPM.

So why would phase lag ever be < 90? Because some helicopters do not have their flap hinge at the main shaft. With a DFC machine with locked dampers, there is no flap hinge- flap happens by bending at the blade root and grip/spindle. So now you have a shorter pendulum arm, but because the total radius is the same, you have the same acceleration, but it is acting on a shorter arm. That causes the flap frequency to increase.

Example:
Same machine, with a 4” grip offset- blade bends at the root.
Pendulum length = 31” – 4”, or 27”. That is 0.6858m. At 1500RPM, the acceleration is still the same as it was- because the radius is the same. The pendulum calc is 2 * pi * sqrt(.6858/19428) = 0.037s. 1/.037 = approx 27hz. This means the blade does not require 90deg to reach peak amplitude, so you lead it by less than 90deg. Here, that value would be approx 78deg.

[myxiplx]

Thanks extrapilot, that's what I understood although I don't profess to know enough to really apply the math here with any confidence.

I'm guessing though that it's very hard for any of us to know what the exact numbers are for any real world model. The designers may well have an idea, but since it seems it can also depend on the blades used (and the forces on those blades), as well as the head design, even the designers of our models may not have all the facts here.

Would I be correct in guessing that the head design is the dominant factor, and that blade flex only has a small effect, or is that an incorrect assumption on my part?

My suspicion would be that most rc helicopter models with phasing built in to the head have that as a result of feedback during the design phase from people testing and flying the models, or due to the designers experience with that type of rotor head. From an engineering perspective, on something this small it may well be easier to build and test than actually try to work out all the factors numerically.

I'm not experienced enough at flying or tuning helicopters yet to have tested out any phasing settings in my FBL, but I have read posts in the past by experienced pilots who needed to tune these settings to have a model flying accurately without any aileron / elevator interaction. So it wouldn't surprise me to find that the test pilots for these companies know enough to be able to feel when the phasing is off on a model just by observing how it flies, and it would be fairly easy for them to feed that input back to the designers to produce a model that needs no phasing adjustment for the average hobbyist.

[Icanfly]

excellent reply extrapilot, thanks, well put. That last part about what caused phase to subtract was not clearly associated with something physically tangible,

Same machine, with a 4” grip offset- blade bends at the root.
Pendulum length = 31” – 4”, or 27”. That is 0.6858m. At 1500RPM, the acceleration is still the same as it was- because the radius is the same. The pendulum calc is 2 * pi * sqrt(.6858/19428) = 0.037s. 1/.037 = approx 27hz. This means the blade does not require 90deg to reach peak amplitude, so you lead it by less than 90deg. Here, that value would be approx 78deg.

what exactly caused lag to decrease, if peak amplitude, was it reached earlier due to the distance of hinge point from rotor axis thus making the phase adjustment and what was the comparison factor involved here. this is very interesting. How exactly was 78* derived?

Now try this, why does phase lag move to 45* when a heli is resting on the ground?

desertstalker, glad you challenged that notion, it has just helped unlock a riddle on skin effect I've been fixed on for a few years. Odd, a vid I have of my little 300 in nose out sprint clearly shows some left ail was favored in forward flight. I'm now messing around altering the phase on all my helis where possible, I knew something didn't feel right as is, while something about 80* phase came to me two years ago, never pursued it until now.

[Xrayted]

Originally Posted by Icanfly

excellent reply extrapilot, thanks, well put. That last part about what caused phase to subtract was not clearly associated with something physically tangible,

what exactly caused lag to decrease, if peak amplitude, was it reached earlier due to the distance of hinge point from rotor axis thus making the phase adjustment and what was the comparison factor involved here. this is very interesting. How exactly was 78* derived? It was derived using the exact mathematical formulas posted in his reply and the part you you quoted in your post

Now try this, why does phase lag move to 45* when a heli is resting on the ground? It doesn't......this is just more made up gibberish. Why would phase lag have anything to do with the heli sitting on the ground or not?

desertstalker, glad you challenged that notion, it has just helped unlock a riddle on skin effect I've been fixed on for a few years. Odd, a vid I have of my little 300 in nose out sprint clearly shows some left ail was favored in forward flight. I'm now messing around altering the phase on all my helis where possible, I knew something didn't feel right as is, while something about 80* phase came to me two years ago, never pursued it until now
Once again more gibbersih...... Skin effect is a motor and electrical phenomena and has nothing to do with phase lag or anything anyone in this thread is talking about

[Icanfly]

this certainly is interesting

It was derived using the exact mathematical formulas posted in his reply and the part you you quoted in your post

can we hear from the man himself?

It doesn't......this is just more made up gibberish. Why would phase lag have anything to do with the heli sitting on the ground or not?

maybe it doesn't matter to you but it can if you have a scale model and it has wheels on it, or you land on soft grass and can't figure why your inputs for correction aren't doing what they are supposed to and the heli tips over.

Once again more gibbersih...... Skin effect is a motor and electrical phenomena and has nothing to do with phase lag or anything anyone in this thread is talking about

now something isn't right about you calling every fgging thing gibberish, ok so I used a term from electricity to explain "boundary layer" I'm freakin sawly, boundary layer, there.

This is what happens when your on the experimentation side of things and experts chime in while understanding the tangible world, unfortunately the heli rotor and dynamics of fluid are not an absolute science due to very small changes in foil, rpm, pitch, loading, exit point laminar flow, stall, etc, can affect all the others and blow the math off. How is it the birds have had for eons what it is we seek while only in the last 100 years has man actually understood fluid dynamics to the greater degree, even still many more improvements are being introduced as we speak, people at Naca (Nasa) are still experimenting with "X" craft utilizing ideas only added to the aerodynamics industry very recently.

the world is neither ruled nor owned by the math, Einstein reasoned energy equals mass times light/2 but he forgot to add reflective mass, we operate in a 3 dimensional world, not entirely e=mc2 (I discovered this only last week when it came to me in a conversation with a technical whiz, "I'm sorry but I have bad news, you didn't get 100% on the final test sir, the correct answer for the last question was not e=mc2, it was ?????" I said jokingly (this is where the squirrel came in and shut things down for the protection of innocent children around the world.) What if?

Laminar flow on a surface will cause drag and throw off the perfect geometry of phase lag as shown by extrapilot, every blade is different. Best we can do is adjust to our style and preference, if we knew we had that chance and understood phase lag is NOT absolutely 90* all the time.

Out of curiosity, what would phase be if the hinge point was at 50% of the length of throw from the axis point then? Take 1000mm, 1m, for simplicity sake why don't we? and explain why it would be so different from 90* phase lag then.

Interesting you guys cross this bridge now, it had come to mind to explore a new idea recently found on a component proposed for the military market, one I've been seeing for a little over two weeks outside my door.

[quinn]

So extrapilot, even when our helis have flexible dampers allowing the spindle to teeter at center, isn't there still gonna be some blade flexing going on during the cycle to affect the calculation? If the blades were absolutely rigid and it truly only teetered at center, then yes we could just call it 90 degrees exactly, but to me it seems that maybe it should still be slightly less than 90 degrees even with soft dampers.

[Xrayted]

Originally Posted by Icanfly

this certainly is interesting

can we hear from the man himself?

maybe it doesn't matter to you but it can if you have a scale model and it has wheels on it, or you land on soft grass and can't figure why your inputs for correction aren't doing what they are supposed to and the heli tips over.

now something isn't right about you calling every fgging thing gibberish, ok so I used a term from electricity to explain "boundary layer" I'm freakin sawly, boundary layer, there.

This is what happens when your on the experimentation side of things and experts chime in while understanding the tangible world, unfortunately the heli rotor and dynamics of fluid are not an absolute science due to very small changes in foil, rpm, pitch, loading, exit point laminar flow, stall, etc, can affect all the others and blow the math off. How is it the birds have had what it is we seek for eons while only in the last 100 years man has actually understood fluid dynamics to the greater degree, even still many more improvements are being introduced as we speak, people at Naca (Nasa) are still experimenting with "X" craft utilizing ideas only added to the aerodynamics industry very recently.

the world is neither ruled nor owned by the math, Einstein reasoned energy equals mass times light/2 but he forgot to add reflective mass, we operate in a 3 dimensional world, not entirely e=mc2 (I discovered this only last week when it came to me in a conversation with a technical whiz, "I'm sorry but I have bad news, you didn't get 100% on the final test sir, the correct answer for the last question was not e=mc2, it was ?????" I said jokingly (this is where the squirrel came in and shut things down for the protection of innocent children around the world.) What if?

Laminar flow on a surface will cause drag and throw off the perfect geometry of phase lag as shown by extrapilot, every blade is different. Best we can do is adjust to our style and preference, if we knew we had that chance and understood phase lag is NOT absolutely 90* all the time.

Out of curiosity, what would phase be if the hinge point was at 50% of the length of throw from the axis point then? Take 1000mm, 1m, for simplicity sake why don't we? and explain why it would be so different from 90* phase lag then.

Interesting you guys cross this bridge now, it had come to mind to explore a new idea recently found on a component for the military market, one I've been seeing for a little over two weeks outside my door.

Icanfly, I can assure you this will be my last reply to you moving forward.

People are not just just busting your balls for fun of it. The problem, which has been pointed out to you numerous times by many others as well is you constantly present yourself as some sort of "expert" on all of these topics you chime in on. You post all have some "well I've been studying this concept for 30 years" or "I've been working in fabricating airfoils for 30 years" etc, but you post clearly show that you don't even understand the most basic aspects of the topics you claim to be an expert in.

This does neither you or anyone else trying to learn here any service at all. Its just causes confusion for new members who think the things you post are actually correct and God forbid try and apply them in real life. One would also think it would cause embarrassment for you when you are told over and over by many that your post make absolutely no sense at all, yet you continue to post the same crazy things over and over again.

You claim to be an expert in airfoil design for many years, yet every single post you made in a recent thread on the subject was complete nonsense and was detailed as to why, yet you continued in other threads with more of the same while still posing an an expert on the subject.

You claim to be a motor expert and have the image in your avatar, yet every single post you made in a recent thread on the topic I was following was completely wrong. How can you claim be an expert in something and offering advice to others if you don't even understand the very basic concepts yourself?

Now, here we are yet again on another subject that you claim to have "been studying" for some time now, and posting random nonsense as if fact again and just stringing big words together in your post whether they have anything to do with the subject or not, and you are surprised when people question you on it?

We are all here to learn. Maybe spend more time reading and learning yourself before posting a lot of nonsense as facts on topics that you don't really have any real grasp of yet yourself.

[myxiplx]

Those are some of the first sensible things you've said, and I totally agree: This stuff is far too complicated to work out using math and theory, for the most part we're better off relying on the experience of others, or observing what actually happens in the real world.

It's good to understand the theory as it helps you work out why some of these odd behaviours are seen, but even understanding the theories is difficult, let alone applying them.

I afraid can't comment on your question regarding 45° phase lag, I think I saw you post on another forum that it's something you saw on a 300 size model? I guess there's a chance that peak amplitude happens at a different point in the cycle at very low headspeeds, but that's nothing more than a guess on my part. I suspect that as ever it will be due to a host of factors, and may even be model or situation specific.

[extrapilot]

Originally Posted by quinn

So extrapilot, even when our helis have flexible dampers allowing the spindle to teeter at center, isn't there still gonna be some blade flexing going on during the cycle to affect the calculation? If the blades were absolutely rigid and it truly only teetered at center, then yes we could just call it 90 degrees exactly, but to me it seems that maybe it should still be slightly less than 90 degrees even with soft dampers.

Quinn

There are other things that affect lag.

There are simple ones (i.e. spindle can setback in soft dampers due to blade drag, where it physically is not parallel with the head block, and the headblock is phased to (or drives) the swash). DFC’s claim to fame was that it eliminates this component. Unfortunately, it adds another.

There are complex ones. There can be a coupling between blade flap and blade pitch, where the flap itself alters the blade pitch, not the other way around. That coupling is termed Delta-3, and it is sometimes used in heads to increase stability of the blades in turbulence. Every degree of Delta-3 that exists in a head alters phase angle by the same number of degrees. Some heads have Delta-3 designed in, in obvious ways. For many designs, it is not obvious, though often it exists in some form to a small degree.

Blade flex in the form you are talking about would not change phase angle in anything like normal conditions- it would change the amplitude, but the peak angle- whatever value that becomes- would still be seen X degrees later, where X is the value you would have with infinitely-stiff blades. If the blade bend causes blade twist, that is also Delta-3, and would change the phase angle.

The point in all this is not to define the lag angle for a particular machine generically. It is to explain why lag happens for a rotor operated in air (it is not simple gyro precession, else lag would always be 90deg).

You don’t have to understand the physics of it to dial a knob in the FBL software and see how she behaves. But some people like to understand what is going on under the hood. Most just prefer to turn the key and not sweat the details.

Enjoy the day mate-

[extrapilot]

Originally Posted by myxiplx

I afraid can't comment on your question regarding 45° phase lag, I think I saw you post on another forum that it's something you saw on a 300 size model? I guess there's a chance that peak amplitude happens at a different point in the cycle at very low headspeeds, but that's nothing more than a guess on my part. I suspect that as ever it will be due to a host of factors, and may even be model or situation specific.

No- rotor phase lag is an aero/mechanical phenomenon between swash, head/grips/hinges, and blades. Being on the ground doesn’t change anything as regards phase lag.

[Icanfly]

that 45 degree shift was experienced by a guy who used to land his inverted heli and hold it at the top of a stick. It happened to a 500 I was asked to set up and convert to fbl and put a scale body on, so I had to look at the concept more closely. I often land nose in, controls shifted on the ground. The accompanying photo and story in the thread helped me get a simple understanding phase shift was already known.

By no means at any time have I come out as an expert with credentials, much of my experience comes from making deductions based on my own and other persons experiences, I make necessary adjustments and only reason what and where I can, I EXPECT to be corrected when getting off track or am somehow making a claim that might be off track. These are things I have been involved with for a long time though never investing as much intense study as that in recent years. Like a high polished Diamond I'm trying to grasp every facet.

Like a blind person who can hear better than most, a sighted person who conditions to adjust to the environment like darkness, a person who plays poker, you can't always win and you can't always repeat the last hand, everyone makes mistakes too, don't assume my answers are expert and or see everything. I take a gamble coming out to put my 2c's in.

Now, to denounce everything as gibberish, what does that tell anyone, don't listen to this guy? because he makes a few mistakes everything he says is worthless too? Then one day he comes along with something totally phenomenal? part of a calling ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, don't say you weren't warned.

anyway, real people know the world is never perfect, we all make a few mistakes, I confess to be human too.

Do you understand "DIFFERENTIAL ROTATION" I witnessed it yesterday and was curious as to why the body of water I watched was revolving at all, everything is in a state of revolution.

Do you believe phase lag and adjustment is something everyone can clearly understand and put to use? I feel we have the math and some proof of how it is achieved, but what about guys that need a radical example to substantiate proof in the lesser adjusted example? Maybe we need pictures to demonstrate to the eye how it is done, you see, the doubting Thomas's of the world only understand in visuals.

[myxiplx]

Originally Posted by extrapilot

No- rotor phase lag is an aero/mechanical phenomenon between swash, head/grips/hinges, and blades. Being on the ground doesn’t change anything as regards phase lag.

I don't think it was about being on the ground, but was mentioned as being seen during spool down, with control becoming very difficult as the blades slowed down.

It's something I don't understand, but I take reports of phenomenon like this at face value. I suspect that if it happens its nothing to do with phase lag and may be something else entirely.

Loving this thread though, I'm learning a lot.

[Atomic Skull]

Originally Posted by extrapilot

There are simple ones (i.e. spindle can setback in soft dampers due to blade drag, where it physically is not parallel with the head block, and the headblock is phased to (or drives) the swash). DFC’s claim to fame was that it eliminates this component. Unfortunately, it adds another.

And a teeter hub in the center of the headblock is a simpler and more effective solution to that anyway.