Bellcrank vs direct drive swash

[koenejet]

Oh and by the way extrapilot, you are wrong. The original question is "What are the pros and cons of bellcrank vs direct drive?"

Not upside-downside of bellcranks. He is asking bellcrank vs direct drive.

And as far as I'm concerned the answer is: direct drive is better, and bellcranks are used only when positioning the servo in a position for direct connection is not possible.

[extrapilot]

No mate- he is not asking if it is better to use bellcranks or direct drive. There is no answer to a question like that. There is no context. He is asking upsides and downsides relative to direct drive so that he can evaluate those factors in whatever context he defines.

Yes, people should disagree with you when you say we should fix what we can fix, when you define ‘fix’ as something that has, or may have in most contexts, a net negative impact.

You understand rotary aero, but you ask questions about it that cannot be answered… You write ‘600 rpm is plenty,’ when you don’t define core aspects of the blade parameters required to even consider the rotor performance. Simple actuator theory calcs to determine that in hover you have to accelerate approx 150 lbs/sec of air by approx 40mph, at a cost of about 20hp, means absolutely nothing; that calc doesn’t even include rotor RPM- just disc area and thrust, along with air density.

Even if you just assume a basic blade aspect ratio and a typical untwisted NACA0012 foil with a typical root cutout %, the pitch required for the thrust you define puts the rotor in stall in hover out of ground effect. Who cares about induced power when the machine cannot fly? To get even a basic sanity check here, you need to include a lot of data as regards the blades, the flight parameters, etc. But, you know all this.

[jessdigs]

Just waiting for these to start

Sent from my SM-G900V using Tapatalk 2

[desertstalker]

Originally Posted by koenejet

I have spent the last 20years in the aviation maintenance industry and have experience building and flying both rc and full size helicopters.

Oh dear

Besides, why the fixation on slop, both designs can be manufactured with negligible slop (it might be more expensive on a bell-crank setup as there are more linkages).

Thats another consideration, cost, bell-cracks require more parts and thus will (if manufactured to the same standard) cost more.

[koenejet]

When someone (like Pratt and Whitney) design a turbine engine, on paper it is designed to perform at a certain level. As the engines roll off the assembly line, each individual engine performs slightly different. So they include as part of each engine, an N1 compensation. That is usually in the range of .5% to 3%. It basically lies to the pilot, telling him that the engine is actually performing as it should when it actually isn't.

What's the point? Simple. The aerodynamics of a turbine engine is far more complex than a helicopter. The engineers can calculate it's performance down to just a few, to less than 1% accuracy. All these aerodynamic forces you talk about on a helicopter are calculable and predictable. If, for example a particular engine was found to need a 5% compensation on the test bench, and then an engineer noticed that the turbine inlet guide vane actuator linkage was a little too sloppy (yes I know that's a bad example), the head engineer would NOT say "don't worry about it, it'll only fix it by a couple percent".

Point being, slop should be fixed. And yes I agree, that a properly designed system won't have much slop. But the original question does ask "bellcrank vs. direct drive". And it is a simple question with a simple answer. Unless you over complicate it. Obviously these helis fly and they fly quite well. So we are basically arguing for no reason.

And yes, my questions are answerable even though I have not provided you with all the aspects. I'm not asking for an exact number. If I was, then you would be correct. But for you to say that there is no way to answer my question (and again I'm only asking for 'ballpark') with the info given, says to me that you know less than I do. AND I never said I know it all. If I knew everything, I wouldn't have to ask any questions. But you can't say I know nothing. If that were the case, then I could not have done the calculations in the first place. It's like my daughter asking me to double check her math homework. Does she know it all? No. That's why she asks me to double check. But if I see that she made a mistake and said that 2/3 plus 4/5 = 3/4, then I can tell it's wrong without redoing the math. There are plenty of 'rules of thumb' associated with helicopters. They are not exact, but close. That's all I'm asking for. If you don't know, then you don't know. So don't answer.

And why the face palm and 'oh dear' while quoting my experience as an aircraft technician? That was rather rude.

[R_Lefebvre]

Because we have seen lots of cases where people with technician or mechanic, or even pilot level education, thinking that they are an authority on rotary aero. This is usually made worse because their instructors have told them over-simplifications and outright lies about how helicopters work, and they take these things to heart.

I've been in discussions with navy helicopter pilots who believe gyroscopic precession is a significant factor in helo flight. Their minds are not open to the idea that they have been lied to all these years.

[Xrayted]

At the end of the day, would not the fact that we are using FBL gyros now negate the non linear movement from the servo to swash connections? Its fly by wire now not directly mechanical, so the gyro is told a specific angular rate of movement per second to accomplish and will do whatever is needed to accomplish that command regardless.

This also applies to the discussion of using expo in the TX, which in the past a small amount could be used to cancel out some of the non linear servo movement effect, but may have zero purpose today.

Im all for getting things set up as best as possible, but it just seems to me that any discussions of bell crank vs direct to swash connections, and the geometric effects or benefits of either setup are outdated now. I'm not sure it makes any real difference at all on our models today, and using one vs the other is just a way to be able to mount the servos in a wider variety of positions on the frame.

[koenejet]

Join Date: Nov 2010
Location: Canada



"Because we have seen lots of cases where people with technician or mechanic, or even pilot level education, thinking that they are an authority on rotary aero"


When did I say I was an authority? I said if I knew everything then I wouldn't need to ask the question. And also, pilots are not trained on the mechanics of an aircraft to any real technical level. Trust me. I as far as the mechanics of an aircraft go, technicians are trained at a much higher level than pilots. I have over eight years in the training industry. And not kids out of highschool getting A&Ps. Some of the people I train have been A&Ps for many years and are tech reps for the manufacture.

And as I have also said before, this little project I'm playing around with is my first attempt at designing a rotary wing. That's why I asked for a little bit of a double check. I never claimed to be an expert. I'm not the one who is vomiting all over these threads with a wealth of knowledge in an obvious attempt to make everyone think I'm as smart as I wish I was. I could though. I can cut and paste as well as the next guy. But if anyone wants to prove that you are as smart as you come off, then I would be happy to give you the chance to do so. Let me know and I will provide all the numbers, dimensions, and factors and you can then redo all my work to see if my calculations are correct.

[koenejet]

Also, Xrated, I do believe you are absolutely correct with your last post, and I couldn't agree more.

All of these discussions would have made more sense 20 years ago. Like I said, these helis do fly quite well, so obviously the engineers know what they are doing, so we are, again, arguing over nothing.

[R_Lefebvre]

Originally Posted by Xrayted

At the end of the day, would not the fact that we are using FBL gyros now negate the non linear movement from the servo to swash connections? Its fly by wire now not directly mechanical, so the gyro is told a specific angular rate of movement per second to accomplish and will do whatever is needed to accomplish that command regardless.

Yes, exactly. This is why one particular manufacturer who is still selling machines with mechanical mixing "to prevent control interactions" really leaves me shaking my head.

[extrapilot]

Originally Posted by koenejet

Join Date: Nov 2010
Location: Canada



"Because we have seen lots of cases where people with technician or mechanic, or even pilot level education, thinking that they are an authority on rotary aero"


When did I say I was an authority? I said if I knew everything then I wouldn't need to ask the question. And also, pilots are not trained on the mechanics of an aircraft to any real technical level. Trust me. I as far as the mechanics of an aircraft go, technicians are trained at a much higher level than pilots. I have over eight years in the training industry. And not kids out of highschool getting A&Ps. Some of the people I train have been A&Ps for many years and are tech reps for the manufacture.

And as I have also said before, this little project I'm playing around with is my first attempt at designing a rotary wing. That's why I asked for a little bit of a double check. I never claimed to be an expert. I'm not the one who is vomiting all over these threads with a wealth of knowledge in an obvious attempt to make everyone think I'm as smart as I wish I was. I could though. I can cut and paste as well as the next guy. But if anyone wants to prove that you are as smart as you come off, then I would be happy to give you the chance to do so. Let me know and I will provide all the numbers, dimensions, and factors and you can then redo all my work to see if my calculations are correct.

Mate

No one here needs to prove anything to anyone here. This is not a democratic process. It is aero engineering. You are the one asking for help.

The aerodynamics of a normally-operating turbine are simple compared to rotary wing flow. That is why the performance can be calculated so precisely for turbines, and why CFD models are accurate. It is also why it is not presently possible to model heli rotary flow accurately outside of very limited flight regimes- because in normal flight you have little control over a slew of important variables. We don’t have inlet ducting to deliver a nice, subsonic flow perpendicular to the rotor face. We don’t have blades with ideal twist/taper. We don’t have blades which remain in plane. We don’t have blades with very low tip loss. We don’t know anything about blade vortex interaction, since that depends on flight speed, thrust, disc angle. Etc etc.

You have done all this number crunching. OK, what is the inflow angle at 30% span for your 6-bladed rotor at 600RPM at 300lbf thrust in still air hover? What is Ct/Sigma for the rotor in this same condition? These are simple questions for someone who has crunched any basic numbers on a rotor design.

You have access to flight data for light machines like the R22, and to ESC log data from 450-800 class machines. I found it in 30 seconds searching here and in the R22 POH. Why do you need an aerodynamicist to tell you how to divide nominal power by nominal flight mass across this spectrum, to see that they are all in line as regards hover power/mass? (They are- typical value is about 80W/lb for still wind hover).

I don’t believe anyone here is talking about benefits of slop, mate. ‘Fixing’ non-linear servo travel on these machines may well be a net negative for the reasons described several times in this thread. It comes down to variables that are not defined, and test. And the result will be specific to an install/flight context.

[koenejet]

For someone who says nobody needs to prove himself, it sure sounds like you are trying to prove your self.

Bottom line is this. I have been building for years. I want to try designing. I found a book with all the equations. I plugged in the numbers for the size heli I would someday, maybe, like to build. Since this is my first time calculating these types of numbers, I just thought there was someone with enough experience on large helis to give me an idea of if I was close. Obviously you can't do that, but I believe it is very possible for someone with the right experience to look at my numbers and just say "yes that sounds about right" or " no you're way off".

And by the way, I have built Kurt Shrecklings (I know, I spelled it wrong) engine, and it runs fine. For a simple engine, the design is fairly complex. Not as complex as a heli, but when you look at large turbines with several stages of axial flow compressors, and centrigual flow impellers, and multiple stages of turbine blades, and variable inlet guide vanes; the physics and aerodynamics behind all of that stuff (and don't forget the stators), is far more complex than any model heli. Just the workings of the fuel control unit is more complex. And yes I have read the books on the designs, and it may just be my opinion. If you are correct and it's impossible to determine certain factor about rotary flight, then remind me to never fly on an helicopter again. All the engineers are getting really lucky that all there guess work was correct enough for the machine to fly. And some of them actually fly pretty well.

[npomeroy]

I was trying really hard not to keep the handle turning in this ("my") thread but...

If you guys and koenejet are going to argue about knowledge and design w.r.t his large scale project, could you do it in his thread here https://www.helifreak.com/showthread.php?t=682908.

He did mention it in passing in his post #37 here which did set some guys off.

[koenejet]

Thanks. You beet me too it, and you are correct. wrong place for the conversation. Sorry.