News:


  • March 28, 2024, 09:39:36 AM

Login with username, password and session length

Author Topic: Constant velocity or constant angular rate ?  (Read 5335 times)

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Constant velocity or constant angular rate ?
« on: January 12, 2022, 09:01:24 AM »
I'm working on an electric powerplant control / timer and I'm fairly certain that a near Constant Velocity could be achieved. I'll publish that work eventually. It has been the focus of much of the testing I have been doing with the exception of the spade exercise which was a valuable distraction. In the process of the analysis, I make the assumption (simplification) of Constant Velocity, CV, in order to simplify the math involved. We always talk about that being desirable but I'm not so sure. While flying the SV videos I would swear the velocity is nearly constant but when I do the math and watch the video for verification, I see the airplane decelerating on the 45 lines. Which begs the constant Angular Rate question.

A Constant Angular Rate would make the squares look uniform and the leg time would be the same basically. With a CAR the upper level line would take the same time as the lower level line. With CV the top leg would be short time while the bottom leg would be long time. When we watch objects moving we don't necessarily see the actual speed so much as we see the characteristic length coved per unit time. A 767 on final looks like it is flying much slower than a Learjet 23 does even though both have nearly identical approach velocities. My truck going down the road at 80 MPH feels much slower than the wife's LaCross doing 80 MPH.

So, the question opened for discussion is CV or CAR as it relates to perception of the quality of the maneuvers.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Online Bruce Shipp

  • 21 supporter
  • Trade Count: (0)
  • Commander
  • *
  • Posts: 240
Re: Constant velocity or constant angular rate ?
« Reply #1 on: January 12, 2022, 09:18:04 AM »
What is the difference in length between the top leg and bottom leg of a square?  What would be the airspeed difference between the two legs in order to get the times equal? How much slower would we need to fly the top leg and would the speed differential be great enough to cause stalling problems in the 3rd corner?

I’m assuming the CAR would apply to the vertical legs as well, accelerating (or increasing power) to keep the vertical up leg constant and slowing the down vertical leg accordingly.  The CV mode should already attempt or address the time differential in the vertical legs.  Do you think the CAR mode would be more effective than CV in the vertical?

Online Paul Walker

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1626
Re: Constant velocity or constant angular rate ?
« Reply #2 on: January 12, 2022, 09:31:52 AM »
When you say constant velocity, what is that relative to? The ground, or the air?

On squares, the top leg IS shorter than the bottom leg.  Don't know how you could get to the same time by either method.


Online Trostle

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 3338
Re: Constant velocity or constant angular rate ?
« Reply #3 on: January 12, 2022, 09:34:47 AM »

What is the difference in length between the top leg and bottom leg of a square?  (Clip)


Though the rules for the AMA square loops do not specifically state that the top and bottom legs are to be the same length, the diagram does show both the top and bottom legs to be the same length.  Since the rules do not require that the upright legs are to be perpendicular to the ground, the top two legs should be the same length given that the "Errors" section states that it is an error when the "Sides of loops are not equal".  Higher scores should be given to those squares where all four legs appear to be the same length, given the other factors are the same (including size, smoothness, sharpness of corners, etc).

(There could be an interpretation where the Errors section states "Sides of loops are not equal" could mean only the upright sides.  However, the upper side and the bottom side are also slides just like there is a left side and a right side.  Perhaps the Errors section statement should be written "All sides of the loops are not equal".  And here we are after just completing a three-year rules change cycle that had eleven change proposals, one would think the rules do not need more "adjustments".)

Keith

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #4 on: January 12, 2022, 09:41:05 AM »
When you say constant velocity, what is that relative to? The ground, or the air?

On squares, the top leg IS shorter than the bottom leg.  Don't know how you could get to the same time by either method.

Constant velocity of the aircraft. Airspeed. Since the square is a figure on a sphere, both top and bottom subtend the same angular change. For a constant angular rate the time it takes to transit that angle change would be the same. For CV the time on top would be less.

The question is semi hypothetical making an assumption of ability to achieve either.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #5 on: January 12, 2022, 09:46:23 AM »
What is the difference in length between the top leg and bottom leg of a square?  What would be the airspeed difference between the two legs in order to get the times equal? How much slower would we need to fly the top leg and would the speed differential be great enough to cause stalling problems in the 3rd corner?

I’m assuming the CAR would apply to the vertical legs as well, accelerating (or increasing power) to keep the vertical up leg constant and slowing the down vertical leg accordingly.  The CV mode should already attempt or address the time differential in the vertical legs.  Do you think the CAR mode would be more effective than CV in the vertical?

Initially, the question posed would be for the level section, however, yes, assuming on all legs of the sphere. An underlying element of the question as posed is assuming a powerplant capable meeting the requirement of either DCV or CAR.

I am a powerplant engineer by trade. This is like an exercise in requirements capturing.

To answer your last question I don't exactly know which is why I ask the question. What I can say is that the vertical legs present some difficult solutions in terms of thrust increase and braking.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #6 on: January 12, 2022, 09:58:14 AM »
Though the rules for the AMA square loops do not specifically state that the top and bottom legs are to be the same length, the diagram does show both the top and bottom legs to be the same length.  Since the rules do not require that the upright legs are to be perpendicular to the ground, the top two legs should be the same length given that the "Errors" section states that it is an error when the "Sides of loops are not equal".  Higher scores should be given to those squares where all four legs appear to be the same length, given the other factors are the same (including size, smoothness, sharpness of corners, etc).

(There could be an interpretation where the Errors section states "Sides of loops are not equal" could mean only the upright sides.  However, the upper side and the bottom side are also slides just like there is a left side and a right side.  Perhaps the Errors section statement should be written "All sides of the loops are not equal".  And here we are after just completing a three-year rules change cycle that had eleven change proposals, one would think the rules do not need more "adjustments".)

Keith

I presented the squares as an easy to comprehend way of thinking about the differences. There are a couple places in my videos where the deceleration can be seen but not necessarily perceived. One that is fairly evident is the approach to the outside squares. You might notice that the elevator control has a slight trend towards up elevator which is an indication of a loss of velocity. I don't yet have an airborne instrumentation package flying but I'm not far away from it. I've been using the videos for the understanding. Our club is going build several of Igor's indoor GeeBee  plane, and I was watching his plane on this same approach and it had the same characteristic.

My intent is not to question the rules in any way. That is not what I set out for the discussion. It may drift that direction but I'm not going to drive it there. I'm primarily interested in the powerplant control aspect.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Constant velocity or constant angular rate ?
« Reply #7 on: January 12, 2022, 10:16:18 AM »
... Constant Velocity could be achieved. ...

... A Constant Angular Rate would ...

Be careful with your definitions.

I think you mean a constant rate of rotation as projected on the ground.  That's one way of looking at things.

You probably don't mean the overall angular velocity -- that goes up in loops and especially in corners.

You probably also don't mean the angular velocity around the airplane's nominal yaw axis*, because if you do the math you'll find out that's exactly equal to the velocity in an earth-centered frame of reference, just as a consequence of geometry.  Then if you actually think about how a plane flies, you'll find out that the actual angular velocity about the actual yaw axis isn't a good proxy for the nominal yaw axis velocity.

* By "nominal yaw axis" I really mean the axis that's perpendicular both to a line from pilot to plane and to the plane's velocity vector.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #8 on: January 12, 2022, 10:41:59 AM »
Be careful with your definitions.

I think you mean a constant rate of rotation as projected on the ground.  That's one way of looking at things.

You probably don't mean the overall angular velocity -- that goes up in loops and especially in corners.

You probably also don't mean the angular velocity around the airplane's nominal yaw axis*, because if you do the math you'll find out that's exactly equal to the velocity in an earth-centered frame of reference, just as a consequence of geometry.  Then if you actually think about how a plane flies, you'll find out that the actual angular velocity about the actual yaw axis isn't a good proxy for the nominal yaw axis velocity.

* By "nominal yaw axis" I really mean the axis that's perpendicular both to a line from pilot to plane and to the plane's velocity vector.

Definitely an engineer within this one. My thinking is more the flight path and not the aircraft axis'. 
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Constant velocity or constant angular rate ?
« Reply #9 on: January 12, 2022, 12:15:10 PM »
When you say constant velocity, what is that relative to? The ground, or the air?

On squares, the top leg IS shorter than the bottom leg.  Don't know how you could get to the same time by either method.

   Paul, as usual, beat me to it - with respect to ground/inertial space, or air. I would add - I would not count on any accelerometer you can fit in a stunt plane to be adequate to integrate the acceleration over an entire 6.5 minute stunt flight. So if you really mean WRT inertial space, you are going to need some absolute velocity sensor to measure it. I can think of two likely candidates, but one measures the airspeed and the other measures the groundspeed.

    For the record, my own design (hypothetical only) tries to control the inertial velocity using the Y axis accelerometer as a measure of the inertial velocity for relatively slow corrections to the integral of the acceleration, then uses a PI controller to drive the motor (which is presumed to be a 1/s plant).

     Brett
« Last Edit: January 12, 2022, 12:38:45 PM by Brett Buck »

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #10 on: January 12, 2022, 01:00:12 PM »
   Paul, as usual, beat me to it - with respect to ground/inertial space, or air. I would add - I would not count on any accelerometer you can fit in a stunt plane to be adequate to integrate the acceleration over an entire 6.5 minute stunt flight. So if you really mean WRT inertial space, you are going to need some absolute velocity sensor to measure it. I can think of two likely candidates, but one measures the airspeed and the other measures the groundspeed.

    For the record, my own design (hypothetical only) tries to control the inertial velocity using the X axis accelerometer as a measure of the inertial velocity for relatively slow corrections to the integral of the acceleration, then uses a PI controller to drive the motor (which is presumed to be a 1/s plant).

     Brett

So what about the observer perspective? This input is what you're doing doing and closing loop on acceleration. Most all of the approaches if not all of the approaches to the motor control take on the architecture of an autopilot system running an auto throttle. The filtering necessary to make an IMU work and the powerplant response create a fairly large lag between the onset of speed droop and thrust correction leading to overshoot and oscillation. Given that a corner of a square is not very long, any thrust lag is significant.

So, the question is really what is it we desire? If we could create a control that could fly at a desired mode.

Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Constant velocity or constant angular rate ?
« Reply #11 on: January 12, 2022, 01:44:53 PM »
So, the question is really what is it we desire? If we could create a control that could fly at a desired mode.

I could not tell you the search terms for that, but if you do some digging you may find some discussion about that headed up by Igor Burger.

The Burger timer is not the first iteration of his attempts at electronic control of speed regulation for electric stunt -- he describes going through some iterations including, but not limited to, controlling the motor like a piped engine (i.e. using a speed control with an unstable zero in the disturbance loop, so that the motor would go faster under load), yaw rotation rate, and, finally, y-axis (outward from center of circle) acceleration.

He settled on y-axis acceleration because that worked best for him.

If you can find it, read it -- it's a very good map of the potential potholes on the road to joy for any would-be engine regulator designer (like -- me, if I ever get back to it).

Personally, having read what Igor has said, and users of Fioretti timers, and hearing rumors about some of the top US folks, I'm thinking that a speed control needs to widely tunable, and given that the trick will be to find a way to tune it so that you don't need to be a control systems engineer to make the plane behave the way you want.

Also personally, I'm very curious of what it would be like to fly a plane that's servoed very close to constant speed in inertial (ground-referenced) space.  I know that if you hand something like that to an experienced pilot they're unhappy because the pacing of the maneuvers is just all wrong.  But I'm wondering if you deliberately go there and then practice up to it if you can't do a better pattern after the (probably) year or two of acclimating yourself to the plane's behavior.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Scott Richlen

  • AMA Member
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2084
Re: Constant velocity or constant angular rate ?
« Reply #12 on: January 12, 2022, 01:54:11 PM »
Quote
   what is it we desire?   

What I desire is a power plant that actually decelerates when heading down hill but maintains (but does not accelerate) speed going uphill.

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Constant velocity or constant angular rate ?
« Reply #13 on: January 12, 2022, 02:29:23 PM »
I was just wondering if Mark is aware of the résumés of the participants in this conversation, not to mention the cumulative man-years they've spent just on square-maneuver-corner thrust profile.
The Jive Combat Team
Making combat and stunt great again

Online Paul Walker

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1626
Re: Constant velocity or constant angular rate ?
« Reply #14 on: January 12, 2022, 03:12:40 PM »
I have been working with the Igor control system for 9 years now.

I first saw it at the 2012 world champs. I was amazed by the amount and rate of the changes in motor speed on his and several others. My first thought was that I was not sure it helped the corners. I purchased a system from Igor and was off experimenting with all the options. And options abound! I fell into the same trap everyone else who has had one does. That being I wanted to hear and feel it do its thing by speeding up and slowing down. I clearly won two Nat's because of the system, and feel I also gave away one, clearly, and likely a second one also. It is easy to go overboard, as is very addictive. For the longest time I have wondered why mine does not function like Igor's.  My conclusion is that it comes down to the trim of the plane. Igor's yaws noticeably, and his system, as has been pointed out already, responds to Y axis accelerations. His plane yawing about thus responds more than my plane which I trim to give very steady line tension. I have tried every imaginable combination to make it work, including his "pipe" mode.

Saying all that, I have found that using too much brake is a sure fire way for me to bounce corners. The system just does not respond fast enough to get back on the power to keep the corner flat. My solution has been to reduce the sensitivity, but not eliminating it, to "keep" the power on to keep the corners flat. Even with the low sensitivity, it doesn't run away in the wind coming downhill. My sensitivity is enough to know it is there, but not enough to hear. It is my feeling that this is optimal for my style of trim and piloting.

As far a a square maneuver being the same length on all four sides, use the engineers favorite check by taking things to the limit.  A square maneuver by definition has vertical sides that are perpendicular to the ground.  Now, consider a 90 degree tall square loop. It is 90 wide. Follow those sides vertically to 90 degrees, and what do you get? They intersect, forming a three sided square. Oops..three sides?  If the "square" is limited to 45 degrees, the top horizontal leg IS shorter than the bottom level flight leg.  If you use timing, to do your square, it will not be correct.  Gravity against you going up, shorter leg on the top, gravity with you going down, and the bottom leg longer than the top.  All contribute so one shouldn't use your internal metronome for this maneuver.

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Constant velocity or constant angular rate ?
« Reply #15 on: January 12, 2022, 03:43:10 PM »

Saying all that, I have found that using too much brake is a sure fire way for me to bounce corners. The system just does not respond fast enough to get back on the power to keep the corner flat. My solution has been to reduce the sensitivity, but not eliminating it, to "keep" the power on to keep the corners flat. Even with the low sensitivity, it doesn't run away in the wind coming downhill. My sensitivity is enough to know it is there, but not enough to hear. It is my feeling that this is optimal for my style of trim and piloting.

    For reference, I found the same issue with incorrect IC engine setup, if it backs off too much coming down the hill, it almost guarantees that it will hop out of the corners. Before I lost the bubble completely (as noted the weekend before Golden State) I was adjusting that with oil content, more oil, less "backoff".  I think the mechanism is pretty clear, it suddenly backs off the power, reduces the control loading, which causes you to go harder over with the controls than you were expecting, hop. I think you need it to maintain some reasonably consistent control pressure.

    BTW, the same thing was what struck me about my first electric flight with Bobby's Genesis, the control loading was positive, and consistent, in every corner, instead of some of them being soft and just "falling" into the corner, and having to put in positive force to get it to go. This was with no feedback at all. just the RPM regulator - so no backoff at all.

  Of course, at the NATS and our pre-Golden State practice session, I had the opposite problem, it was charging in the corners, making it nearly impossible sometimes. Had I left that alone and then tried to fly it in the 25 MPH wind, I would certainly have stuck it in the ground -  I was barely making it at 15. Switched setups, back to good again for the real contest.

     Brett

Online Brent Williams

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1260
    • Fancher Handles - Presented by Brent Williams
Re: Constant velocity or constant angular rate ?
« Reply #16 on: January 12, 2022, 03:51:35 PM »
Paul, I am curious on your opinions regarding the governor-only type timers (Hubin FM-9/Castle esc) when compared directly against the lowered sensitivity parameters you have used with the Igor system. 

As a thought exercise, compare as if these systems were used in same plane, same conditions.  What/where/how do you feel the governor-only system would be lacking.  Which maneuvers would suffer or gain the most?  Ect.? What/where/how would the Igor type system be easily superior? You have a distinct perspective from your experience testing and competing to comment on the differences. 
Laser-cut, "Ted Fancher Precision-Pro" Hard Point Handle Kits are available again.  PM for info.
https://stunthanger.com/smf/brent-williams'-fancher-handles-and-cl-parts/ted-fancher's-precision-pro-handle-kit-by-brent-williams-information/

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Constant velocity or constant angular rate ?
« Reply #17 on: January 12, 2022, 03:56:14 PM »
So what about the observer perspective? This input is what you're doing doing and closing loop on acceleration. Most all of the approaches if not all of the approaches to the motor control take on the architecture of an autopilot system running an auto throttle. The filtering necessary to make an IMU work and the powerplant response create a fairly large lag between the onset of speed droop and thrust correction leading to overshoot and oscillation. Given that a corner of a square is not very long, any thrust lag is significant.

   So, are you suggesting/proposing some sort of feedforward (presumably from the bellcrank or Z axis of the accelerometer), to get more lead?

    the problem that seems to be common with all of these systems is that you are still dependent on the motor controller/throttle bandwidth, which sets a upper limit on the acceptable feedback control. You appear to be suggesting a feedforward of some sort to torque the motor directly rather than send a signal to a separate controller and allowing it to adjust the RPM. That is at least similar to my (completely notional) idea, where the controller *is* the accelerometer, there is no separate RPM-regulating loop that you are adjusting up and down.

Quote
So, the question is really what is it we desire? If we could create a control that could fly at a desired mode.

   That is the critical question, as Paul notes, you might not necessarily want a perfect controller, and using the nonlinearities to adjust the ideal response might make a perfect controller a moot question.

       Brett

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Constant velocity or constant angular rate ?
« Reply #18 on: January 12, 2022, 03:59:52 PM »
... I fell into the same trap everyone else who has had one does. That being I wanted to hear and feel it do its thing by speeding up and slowing down. ...

Soooo, an Igor timer plus a speaker playing engine noises, that sound more labored when the 'lectric motor is being asked for more power?

It could be done!

edit:

I think you could even do that with all-analog electronics, or mostly-analog.  That'd be -- interesting.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Constant velocity or constant angular rate ?
« Reply #19 on: January 12, 2022, 04:05:51 PM »
The problem that seems to be common with all of these systems is that you are still dependent on the motor controller/throttle bandwidth, which sets a upper limit on the acceptable feedback control.  ...

A continuing frustration for me is that I don't have time to design an ESC for this.  I've got the control systems and motor drive chops, there's open-source designs out there that just* need software.  I think that -- possibly at the expense of needing a custom tuning for each motor/prop combination, or an adaptive controller -- you could drive the bandwidth way higher than what ESCs do now.

Or (and this isn't my original idea) a constant-speed ESC and a variable-pitch prop driven by a fast servo.

* It's never "just software" -- but it's so easy to say!
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #20 on: January 12, 2022, 05:48:16 PM »
What I desire is a power plant that actually decelerates when heading down hill but maintains (but does not accelerate) speed going uphill.

That is doable. A variable pitch propeller is necessary to achieve it.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #21 on: January 12, 2022, 05:48:43 PM »
I was just wondering if Mark is aware of the résumés of the participants in this conversation, not to mention the cumulative man-years they've spent just on square-maneuver-corner thrust profile.

I am.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #22 on: January 12, 2022, 06:09:09 PM »
A continuing frustration for me is that I don't have time to design an ESC for this.  I've got the control systems and motor drive chops, there's open-source designs out there that just* need software.  I think that -- possibly at the expense of needing a custom tuning for each motor/prop combination, or an adaptive controller -- you could drive the bandwidth way higher than what ESCs do now.

Or (and this isn't my original idea) a constant-speed ESC and a variable-pitch prop driven by a fast servo.

* It's never "just software" -- but it's so easy to say!

I'm not certain a fast servo is even required.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Constant velocity or constant angular rate ?
« Reply #23 on: January 12, 2022, 06:32:49 PM »
I'm not certain a fast servo is even required.

Overall, I don't either.

But if you want to keep up with the plane's loss of velocity in a corner -- yes, it is.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Constant velocity or constant angular rate ?
« Reply #24 on: January 12, 2022, 07:09:41 PM »
That is doable. A variable pitch propeller is necessary to achieve it.

   Yes, and that is a much more direct way of controlling the thrust than changing the RPM. In that case the prop inertia helps you rather than hurts you. Igor tells us that the prop inertia is a driving problem for the conventional controller design, presumably because of finite torque/finite current or current slew rate. I think that came up in the thread about folding props, it came up in the "electronic controls" rule discussion, and several similar SSW threads.

   The actual numbers matter (which I don't have)  but I would expect that the pitch control servo could achieve much higher Fdot (rate of change of the thrust) than trying to spin the prop up or down.
 
     Brett

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #25 on: January 12, 2022, 07:23:44 PM »
   So, are you suggesting/proposing some sort of feedforward (presumably from the bellcrank or Z axis of the accelerometer), to get more lead?

    the problem that seems to be common with all of these systems is that you are still dependent on the motor controller/throttle bandwidth, which sets a upper limit on the acceptable feedback control. You appear to be suggesting a feedforward of some sort to torque the motor directly rather than send a signal to a separate controller and allowing it to adjust the RPM. That is at least similar to my (completely notional) idea, where the controller *is* the accelerometer, there is no separate RPM-regulating loop that you are adjusting up and down.

   That is the critical question, as Paul notes, you might not necessarily want a perfect controller, and using the nonlinearities to adjust the ideal response might make a perfect controller a moot question.

       Brett


Since Howard brought up resumes, I feel compelled to share a bit of mine pertinent to the conversation.  I am aware of the level of talent here.

I am after all a powerplant engineer and I've been involved with Full Authority Digital Engine Controls, FADEC, since Allison built their first lunch box computer version in 93. I've worked on two tiltrotors, several helicopters, a bunch of jets, some power stations, a boundary later blower for a flying boat, a twin engine pack single propeller and some off the wall things. Shaft engines from 50 Hp to 4,500 Hp and thrust engine from 400 pounds to 26,000 pounds thrust. I have a little insight. I'm not and likely never will be the stick I am currently in the midst of. But I can make a better powerplant. That I am ultimately confident in. One of those projects is the RAH66 Comanche helicopter. It required the chase vehicle to be a Corvette in order to keep up with its side wards acceleration. That takes some doing from the powerplant control system perspective and it can't accomplished using a droop based control loop. That was one of the vehicles I had a hand in.

Bare with me. I have not yet reached a complete analysis in order to present here on what I am working on. This is something I started last year and it included running a variable pitch propeller which is why I did the hinged propeller testing which precipitated that precession discussion. I'll refrain from using constant speed because that is not what is necessary for constant CV AR or other given criteria. Think of variable pitch as a way of shifting the efficiency around for a given velocity and power requirement. We'll trek that road another day.

When Howard posted his data, there were two elements that drew my attention one of them was the thrust response which can be seen in the x axis data. More importantly the lag and subsequent following rate error. The very first part of the chart the x accel is showing a bit of a negative level, likely from some yaw. Shortly the pitch rate ramps up from the control input and is seen in the x axis deceleration.  I made a couple thin black lines to show the offset and the droop. Granted this is acceleration but the speed surely followed as the drag set in. At a point the x axis droop turned around and increased. It's hard to know exactly what is transpiring and it would be good to have motor command in this plot. I drew a green line to represent what I think is going on in terms of thrust response. I have a couple Fiorotti timers and have worked with them enough to know they aren't much different. This lag is inherent in the filtering required to get a low noise signal from the IMU whether it is rolling average or a Kalman filter or some other method.

I'm a powerplant controls guy. I'm gonna state this. The droop governor is for steady state operation. This chart is an example of why we have other control loops for transient operation. Maneuvering is not steady state.

So, let me pose this question. What would happen if we could match the thrust output to the thrust required to overcome the drag of maneuvering? What if it could be done without over shooting and minimal phase lag?

Part of the difficulty of trimming the power output of the G or rate sensing systems I'm seeing is that he gain is linear. At least the Fiorroti seams to be when I play with it on the bench connected to my Oscope. I'm not certain how much I'm going to continue to mess with it beyond flying some with the data logger.

Drag is not linear with G. So, when the gain is g = kG there will be times when there is overthrust or under thrust for a given condition. This is impossible to trim without having lots of velocity variation. My active timer is turned way down. Given the phase lag of the error development and the thrust errors this system won't ever meet the dynamic demands perfectly during maneuvering.


Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #26 on: January 12, 2022, 07:31:23 PM »
   Yes, and that is a much more direct way of controlling the thrust than changing the RPM. In that case the prop inertia helps you rather than hurts you. Igor tells us that the prop inertia is a driving problem for the conventional controller design, presumably because of finite torque/finite current or current slew rate. I think that came up in the thread about folding props, it came up in the "electronic controls" rule discussion, and several similar SSW threads.

   The actual numbers matter (which I don't have)  but I would expect that the pitch control servo could achieve much higher Fdot (rate of change of the thrust) than trying to spin the prop up or down.
 
     Brett

Yeah I had some conversation with Igor on that. Apparently the active timer has better performance with higher spin motors due to the lower resistance. My propeller testing has been on hold as the test airplane was involved in a catastrophic landing. A controllable pitch propeller would be the cat's meow in terms of performance  which has been my journey for the last year and some change. That is, in fact, the end game of what I'm working on.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Constant velocity or constant angular rate ?
« Reply #27 on: January 12, 2022, 08:02:13 PM »

Since Howard brought up resumes, I feel compelled to share a bit of mine pertinent to the conversation.  I am aware of the level of talent here.


   I wasn't the one discussing resumes and I wasn't challenging your assumptions, just trying to find out what you were talking about and maybe offering some suggestions - so I am not sure why you are responding to my post. As I mentioned the other day, conveying information inherently demands asking questions, and those questions are not necessarily challenges or "assertions in the form of a question". I am very happy to discuss it with anyone with an idea - if it is good idea, it doesn't matter where it came from. In any case, I am sure Howard's CV looks better than mine; if nothing else his would be mostly unclassified.

   I noted that you could use feed-forward to reduce/eliminate the phase shift inherent in the closed-loop control. You can make it a function of anything you want. My notion from a few years ago would be to use the Z axis acceleration/load factor as either an additional lead element, or, as a feedforward, since it more-or-less measures the drag for you. You would need a band-pass filter to block DC and high frequency noise, but otherwise you could make the feedfoward signal proportional to the absolute value of load factor, the square of the load factor or whatever else you think it should be.

    But your first question is still the most critical - what is it that we are trying to control? Paul's observation, I think, relates to excessive/excessively aggressive/excessively nonlinear/out of phase control of the airspeed, where the pitch sensitivity changes rapidly. I think it is the same issue from the "what does 'penetration' mean?" thread. So I might be inclined to start on the premise you are trying to control the body frame X acceleration, that is, the "orbit rate", for lack of a better name for it, and using the Z acceleration as a feedforward and maybe the Y acceleration as a lead element in some sort of a lead-lag control system.

   If you are going to control the "orbit rate" which is probably what you want with the "constant angular rate" idea, you *do* have to have some other system to sense it directly, hence, Y acceleration (which at low frequency is proportional to the integral of the X acceleration due to our constraint).

    But that is just notional, I am not sure if it is well thought through.  The original idea was to use the X acceleration to replicate a Fox 35, which is kind of how this all started.

     Brett

p.s. this (and most similar) threads point out an even more basic engineering issue - there are tremendous gaps translating the engineering to the subjective handling qualities that most of us think are the goal. Put another way, we have never defined "good" VS "bad" in an engineering sense, even to first approximation. Paul's example (and my similar example) illustrate that in droves. Paul knew exactly what he changed, and I had at least some idea,  but why that improved the cornering performance is at best educated guessing. In Paul's case, very highly educated, but nonetheless, a guess). This is a problem with full-scale aircraft, too. This is the basis of the Cooper-Harper Handling Qualities ratings - a subjective scale used to determine how difficult an airplane is to fly. Of course, the pilot has to be qualified to provide these ratings, and while Paul is probably the best possible evaluator, any of the more experienced/accomplished stunt pilots probably give reasonable evaluations.

    But it *is* subjective. I have flown two flights 10 minutes apart, one where the airplane would have rated a 2 (good, negligible deficiencies), and another where it maybe rated a 5 (moderately objectionable). The differences - same airplane, same engine, same conditions, absolutely no trim changes, just altering the oil content of the fuel by 4%. Do the same the next weekend in different conditions, and I might get the same ratings - for exactly the opposite change!

   Another example, a 9 (Major Deficiencies, intense pilot compensation required to retain control) to about a 2. Again, the same airplane one minor trim change, and one minor engine setup difference.  Several people were witness to me nearly crashing my airplane at the bottom of a triangle in 15 mph wind, same airplane with 1/2 ounce of weight on the tail and a slight pipe length change, it handled 25 with not much problem.

    Point being, while I definitely try to figure out what causes these differences, we don't really have any way to translate that into engineering parameters, so it is very difficult to do more than guess at the right answer to your original question, and then try it and see. Electric helps because there is at least some hope of knowing what changed in reasonable engineering terms, why it mattered is still subject to speculation.
« Last Edit: January 12, 2022, 08:34:33 PM by Brett Buck »

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #28 on: January 12, 2022, 08:55:49 PM »

   I wasn't the one discussing resumes and I wasn't challenging your assumptions, just trying to find out what you were talking about and maybe offering some suggestions - so I am not sure why you are responding to my post. As I mentioned the other day, conveying information inherently demands asking questions, and those questions are not necessarily challenges or "assertions in the form of a question".

   I noted that you could use feed-forward to reduce/eliminate the phase shift inherent in the close-loop control. You can make it a function of anything you want. My notion from a few years ago would be to use the Z axis acceleration/load factor as either an additional lead element, or, as a feedforward, since it more-or-less measures the drag for you. You would need a band-pass filter to block DC and high frequency noise, but otherwise you could make the feedfoward signal proportional to the absolute value of load factor, the square of the load factor or whatever else you think it should be.

    But your first question is still the most critical - what is it that we are trying to control? Paul's observation, I think, relates to excessive/excessively aggressive/excessively nonlinear/out of phase control of the airspeed, where the pitch sensitivity changes rapidly. I think it is the same issue from the "what does 'penetration' mean?" thread. So I might be inclined to start on the premise you are trying to control the body frame X acceleration, that is, the "orbit rate", for lack of a better name for it, and using the Z acceleration as a feedforward and maybe the Y acceleration as a lead element in some sort of a lead-lag control system.

    But that is just notional, I am not sure if it is well thought through.  The original idea was to use the X acceleration to replicate a Fox 35, which is kind of how this all started.

     Brett

That wasn't exactly intended for you, sorry. And I don't feel attacked or otherwise by you Brett. You guys are very talented individuals. However when that is blatantly thrown in to my face I feel compelled to respond in kind. That it appeared to be directed towards you, wasn't meant to be. Just a knee jerk reaction on my part.

My original question is subjective and is about what is it we are really after. A constant velocity, for instance, would cause some interesting visual appearances and the model would look to be hauling a$$ up high. Which brings the alternate question, would a constant angular velocity be better. Both the upper and lower sides of the square have the same angular displacement. So, flying at constant angular velocity or close would maybe have a better appearance. That is my question. It wasn't intended to go down the road of controlling a motor. My bad for stating that I came up with this question during an analysis I am performing.

As is what I have stated, I began this with a notion of what would it take to truly make an airplane fly constant velocity and consequently I worked up the basic needs in terms of physical hardware. A motor controller and variable pitch propeller. I'm not convinced that attitude is necessary for good performance. Given the correct flexibility of the powerplant without a constant velocity or angular velocity constraint requirement I'm 85% confident it isn't.

For instance take my question. If the powerplant can provide the correct thrust to compensate for increased drag during maneuvers then the velocity variation becomes simply a function of the change in gravitational potential. ie Delta V from level flight would be the square root of two times the change in height.  That can be improved some fairly simply I think and I haven't yet finished working through that portion. Which will need some inertial computations to achieve. This is why I asked the question the way I did was to be able to decide whether to pursue that aspect or not.

I've actually made some comments in the past about the approach because it is derived from how we use anticipation and acceleration power control in the FADECs. I think that the discussion on the actual control and gain strategy should be a separate thread.



Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Constant velocity or constant angular rate ?
« Reply #29 on: January 12, 2022, 09:22:45 PM »
That wasn't exactly intended for you, sorry. And I don't feel attacked or otherwise by you Brett. You guys are very talented individuals. However when that is blatantly thrown in to my face I feel compelled to respond in kind.

   As noted the other day, I think you are reading far more into this than was intended. Noting other people's capabilities does not mean that he was questioning yours.

    In any case, whether it was intentional or just perceived, getting wound up about it doesn't help you, me, or anyone else advance the state of the art. You have interesting observations and ideas, don't derail your point on side issues .

     Brett

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #30 on: January 12, 2022, 10:01:07 PM »
   As noted the other day, I think you are reading far more into this than was intended. Noting other people's capabilities does not mean that he was questioning yours.

    In any case, whether it was intentional or just perceived, getting wound up about it doesn't help you, me, or anyone else advance the state of the art. You have interesting observations and ideas, don't derail your point on side issues .

     Brett

I will do my  best. Apologies Howard.

I posted an article in a separate thread walking through the derivation of an open loop gain control. Bottom line that drag is a fairly simple equation 1-Cos X which lends itself readily to a simple gain. An input using a pot or hall effect sensor to pick off control position is all that is necessary to set thrust. We use this type of thing in the turbine engines when we loose a sensor or we're providing some acceleration anticipation for load. The loop closes naturally with the pilot in the loop. No need for an outside measurement. This initial article is without any inertial input. In order to improve the performance getting a G vector is necessary simply to know if the airplane is upright or inverted and turning upward or downward. In that case a trimming gain would be added to the primary gain. I don't believe that is necessary because this should get the airplane through to corners with most of it's original energy depending on the response of the motor. Clearly using this approach running a propeller has huge potential. The real question is whether that is necessary or not. Having flown the Fiorotti now for a couple hundred flights, my jury is out on continuing the variable pitch effort. My current effort is building the instrumentation package followed by finishing the airplane design. This timer control was on hold for a while but Howards data provoked me to progress it further at this time since that data gave me a piece of information I was looking for.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Constant velocity or constant angular rate ?
« Reply #31 on: January 12, 2022, 10:24:14 PM »


I posted an article in a separate thread walking through the derivation of an open loop gain control. Bottom line that drag is a fairly simple equation 1-Cos X which lends itself readily to a simple gain. An input using a pot or hall effect sensor to pick off control position is all that is necessary to set thrust.

   1-cos(x), where the 1 is a normalized bias representing the parasitic drag and the cos(x) is the induced drag, all multiplied by V^2 somewhere else?  In any case, I think you want to filter out the DC components, so maybe you don't need the bias/parasitic drag.

   So, the answer to my earlier question is that you are using the control position as the feed-forward signal, the theory being that if the controls deflect, this will inevitably lead to the generation of induced drag, slowing the airplane - open-loop compensation (on top of anything else), around any closed-loop control.

     It is a very interesting idea, and there is some experience with that sort of system from the mists of time - Scott Bair's modified ST46 with an exhaust throttle actuated by bellcrank position. This was entirely mechanical, if the bellcrank deflected either way, it opened the exhaust throttle. This worked more-or-less OK compared to any of the similar systems using a carburetor for the throttle, because the exhaust throttle seemed to work much faster.

   Using it for electric is likely to work a lot better, for the same reason - less lag. The limiting factor is likely to be frying the output transistors in the motor controller and/or running out fo current, because you probably want to run your feedforward right to the base of the driver transistor.

   My alternate idea was to do something similar using the Z acceleration as a direct measure of the load factor/induced drag. 

  In either case, make sure you can adjust the transfer function from the bellcrank to the delta-current easily with software changes because I think, as noted above, that "perfect" compensation might not be the best practical solution - which is again a restatement/the implication of Paul's comment. More than just the gain parameter, the entire transfer function (in case it wants to be linear, or a square root function, or something else, when you test it).

   Excellent thought process, and definitely food for thought and grounds for further investigation.

     Brett

Online Paul Walker

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1626
Re: Constant velocity or constant angular rate ?
« Reply #32 on: January 12, 2022, 10:32:22 PM »
. My propeller testing has been on hold as the test airplane was involved in a catastrophic landing.

Yeah, I hate when that happens!

 A controllable pitch propeller would be the cat's meow in terms of performance  which has been my journey for the last year and some change. That is, in fact, the end game of what I'm working on.

This sounds like a great idea. I would like to make a point here. If you have tried one of Igor's hollow props, you know how light they are. I commented to Igor how much I liked them because they reduced the gyroscopic precession significantly. His response was that he didn't make them for that reason, but to reduce the inertia the motor had to overcome to speed up and slow down. For me, it was one of the quantum changes that improved my flying and the planes performance. In building a variable pitch propellor, I am concerned that the gyro effects will get worse, not better. Not sure how you could build that arrangement for 16 grams. A higher mass prop in the corners concerns me because of the yaw. Something to consider.

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #33 on: January 12, 2022, 10:39:42 PM »
   1-cos(x), where the 1 is a normalized bias representing the parasitic drag and the cos(x) is the induced drag, all multiplied by V^2 somewhere else?  In any case, I think you want to filter out the DC components, so maybe you don't need the bias/parasitic drag.

   So, the answer to my earlier question is that you are using the control position as the feed-forward signal, the theory being that if the controls deflect, this will inevitably lead to the generation of induced drag, slowing the airplane - open-loop compensation (on top of anything else), around any closed-loop control.

     It is a very interesting idea, and there is some experience with that sort of system from the mists of time - Scott Bair's modified ST46 with an exhaust throttle actuated by bellcrank position. This was entirely mechanical, if the bellcrank deflected either way, it opened the exhaust throttle. This worked more-or-less OK compared to any of the similar systems using a carburetor for the throttle, because the exhaust throttle seemed to work much faster.

   Using it for electric is likely to work a lot better, for the same reason - less lag. The limiting factor is likely to be frying the output transistors in the motor controller and/or running out fo current, because you probably want to run your feedforward right to the base of the driver transistor.

   My alternate idea was to do something similar using the Z acceleration as a direct measure of the load factor/induced drag. 

  In either case, make sure you can adjust the transfer function from the bellcrank to the delta-current easily with software changes because I think, as noted above, that "perfect" compensation might not be the best practical solution - which is again a restatement/the implication of Paul's comment. More than just the gain parameter, the entire transfer function (in case it wants to be linear, or a square root function, or something else, when you test it).

   Excellent thought process, and definitely food for thought and grounds for further investigation.

     Brett


It's all software based and a basic Arduino nano can host it. The actual control gain looks like gain = K1*(1-K2*Cos (k3 * theta)). Pass that through a dead band Gain out = abs(gain) >Kfilt will filter most of the noise and any that gets through isn't much of a problem since the powerplant has inertia. The output from the control / timer is a simple PWM to the ESC so there is some filtering there too. While it is open loop to the motor, the actual loop is control input, gain, motor, airplane, pilot. If the airplane is slow the elevator will be deflected inputting a command for power causing the airplane to accelerate. Think of the control deflection being the error signal generator.

I posted my incomplete evaluation:

https://stunthanger.com/smf/engineering-board/electric-motor-control-severe-geek-speak/msg629304/#msg629304
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Constant velocity or constant angular rate ?
« Reply #34 on: January 12, 2022, 11:02:02 PM »
Quote
My propeller testing has been on hold as the test airplane was involved in a catastrophic landing.

Yeah, I hate when that happens!

 A controllable pitch propeller would be the cat's meow in terms of performance  which has been my journey for the last year and some change. That is, in fact, the end game of what I'm working on.

This sounds like a great idea. I would like to make a point here. If you have tried one of Igor's hollow props, you know how light they are. I commented to Igor how much I liked them because they reduced the gyroscopic precession significantly. His response was that he didn't make them for that reason, but to reduce the inertia the motor had to overcome to speed up and slow down. For me, it was one of the quantum changes that improved my flying and the planes performance. In building a variable pitch propellor, I am concerned that the gyro effects will get worse, not better. Not sure how you could build that arrangement for 16 grams. A higher mass prop in the corners concerns me because of the yaw. Something to consider.

   I am sure even you can't build it for 16 grams. But as noted above, with the variable pitch setup, a higher propellor inertia actually helps you as far as controlling the speed goes, because it will tend to mitigate the prop slowing down/speeding up as you change the pitch. Whether it is better enough speed-control wise to make up for the extra precession, only one way to find out.

    Can't help you directly on the precession - however, an earlier thread also notes that in addition to controlling the overall pitch, you could *hypothetically* vary the pitch up and down through 1 rev - just like collective and cyclic control on a helicopter, and use that to generate both pitch torque in the direction you want to go, and also, provide a yaw torque to compensate for the precession.

    Effectively you would be using the cyclic control to reorient the prop angular momentum vector instead of forcing it around with the rest of the airplane. To first approximation, it wouldn't matter how heavy/how much moment of inertia the prop might have - which is fortunate, because the swashplate and other mechanics would make it even heavier. There are several very obvious flaws with this idea, not the least of which is trying to pitch the blades up and down at 400 Hz and this pounding the motor mount, swashplate, swashplate follower, prop hub, etc, to dust.

  Either way sounds like a lot of pretty precise machining, which more-or-less lets me out of acting on any of my grand ideas.

    Brett

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Constant velocity or constant angular rate ?
« Reply #35 on: January 12, 2022, 11:04:03 PM »
In any case, I am sure Howard's CV looks better than mine; if nothing else his would be mostly unclassified.

Not even close.  Brett is a national stunt champ and—um—a responsible person.

My notion from a few years ago would be to use the Z axis acceleration/load factor as either an additional lead element, or, as a feedforward, since it more-or-less measures the drag for you. You would need a band-pass filter to block DC and high frequency noise, but otherwise you could make the feedfoward signal proportional to the absolute value of load factor, the square of the load factor or whatever else you think it should be.

The square: synthetic induced drag.  You could find out experimentally what the sign should be.  It might be a function of wind.
« Last Edit: January 13, 2022, 12:49:36 PM by Howard Rush »
The Jive Combat Team
Making combat and stunt great again

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #36 on: January 12, 2022, 11:06:21 PM »


Well, except when you put a hinge at the root of the propeller, then the precession is nearly eliminated. A pinned joint can create no moment. A pinned and hinged blade creates no moment and therefore doesn't precess. I made a brash statement of such without a good demonstration and created a real sit storm. Probably deserved some of it. I have been on a very long road towards this power system. So, I was worried about fatigue from bending of the blade from the continuous precession of the propeller during flight with the CL model. Each blade undergoes a bending cycle every revolution during flight and it doesn't take long to get a few million bending fatigue cycles. Most applications don't undergo this much continuous load.

If the blade spindle is a small bolt it must carry that bending. So, the idea came to me that one of my F1C folding propellers had a hinge and that would eliminate the bending. I did a test to see if the propeller could follow sharp corners. When I did, I realized there was little precession evident. I asked a question, in the wrong tone, about how significant is the precession for the CLPA airplanes, meaning is it something worth spending time and effort on. Well... Flight testing there is a noticeable improvement for some of the planes I tried it on. While others not so much. The low TVCv airplanes really tell you about it. My future plan is to use the Fuse cam and fly the prop on another airplane. I'm also working on the datalogger which will have an IMU, throttle control system load cell and position for ubergeekdom data generation. I don't have a version of the prop yet created for the large airplanes. I'm waiting on some more observation flights before I make that commitment.

Here's a link to my discussion on the hinged propeller:

https://stunthanger.com/smf/stunt-design/propeller-precession/msg620424/#msg620424
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #37 on: January 12, 2022, 11:16:43 PM »
This sounds like a great idea. I would like to make a point here. If you have tried one of Igor's hollow props, you know how light they are. I commented to Igor how much I liked them because they reduced the gyroscopic precession significantly. His response was that he didn't make them for that reason, but to reduce the inertia the motor had to overcome to speed up and slow down. For me, it was one of the quantum changes that improved my flying and the planes performance. In building a variable pitch propellor, I am concerned that the gyro effects will get worse, not better. Not sure how you could build that arrangement for 16 grams. A higher mass prop in the corners concerns me because of the yaw. Something to consider.


   I am sure even you can't build it for 16 grams. But as noted above, with the variable pitch setup, a higher propellor inertia actually helps you as far as controlling the speed goes, because it will tend to mitigate the prop slowing down/speeding up as you change the pitch. Whether it is better enough speed-control wise to make up for the extra precession, only one way to find out.

    Can't help you directly on the precession - however, an earlier thread also notes that in addition to controlling the overall pitch, you could *hypothetically* vary the pitch up and down through 1 rev - just like collective and cyclic control on a helicopter, and use that to generate both pitch torque in the direction you want to go, and also, provide a yaw torque to compensate for the precession.

    Effectively you would be using the cyclic control to reorient the prop angular momentum vector instead of forcing it around with the rest of the airplane. To first approximation, it wouldn't matter how heavy/how much moment of inertia the prop might have - which is fortunate, because the swashplate and other mechanics would make it even heavier. There are several very obvious flaws with this idea, not the least of which is trying to pitch the blades up and down at 400 Hz and this pounding the motor mount, swashplate, swashplate follower, prop hub, etc, to dust.

  Either way sounds like a lot of pretty precise machining, which more-or-less lets me out of acting on any of my grand ideas.

    Brett

I'm not sure of the camming portion of this you imagine. Placing a hinge in the blade root eliminates 90%+ of the precession. Much lighter than a swash plate. I actually have a 10x6 version that has been flying. I had two before the incident. Fatigue is a huge concern when it builds up at 400 hz.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Constant velocity or constant angular rate ?
« Reply #38 on: January 12, 2022, 11:33:14 PM »
Using it for electric is likely to work a lot better, for the same reason - less lag....My alternate idea was to do something similar using the Z acceleration as a direct measure of the load factor/induced drag. 

Wouldn't one of these tend to goose the throttle at the bottoms of round loops and the other at the tops? 
The Jive Combat Team
Making combat and stunt great again

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Constant velocity or constant angular rate ?
« Reply #39 on: January 12, 2022, 11:35:07 PM »


   Any resume would have extensive gaps and blank records for bonuses, promotions, etc. All my records from 1983 to 2002 are completely blank - because they were in my paper file, and they didn't bother to transfer it for the electronic system. The others have a bunch of dollar values, awards,  and nondescript "promotions" for no adequately explained reasons.  I just got semi-roped into another stretch - but the end is in sight.

   Of course, this all supports my jet-set lifestyle, I am an International Hollywood Playboy, just like Jethro on the Hillbillies.
   
  "Will perform conformal mapping for food."
« Last Edit: January 13, 2022, 03:17:03 PM by Brett Buck »

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Constant velocity or constant angular rate ?
« Reply #40 on: January 12, 2022, 11:54:01 PM »
Wouldn't one of these tend to goose the throttle at the bottoms of round loops and the other at the tops?

   The low-pass filter/estimator would have to be carefully defined. In this hypothesized system you are just using it to correct for the drift you get from integrating the X acceleration.   Despite Marks earlier comments, the very last thing I would use is a variable-gain filter, and in practice, it's running at a constant frequency with all the input continuously available, the gains would rapidly wind up at steady-state values anyway.

    Note that this is a strictly hypothetical idealized situation - my expectation is that by the time you put in enough limiters and saturations to keep it out of trouble, you are going to be riding one or the other of them almost continuously.  You don't necessarily even want perfectly controlled speed no matter which speed you decide to control, as Paul notes.

  I like the idea of the feedforward, you might just use the existing governor system, feed Mark's signal forward, and use the same signal into the governor to keeo them from fighting each other. So, more-or-less, Igor's system with an additional feedforward signal.

    Brett

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #41 on: January 13, 2022, 06:27:30 AM »
   The low-pass filter/estimator would have to be carefully defined. In this hypothesized system you are just using it to correct for the drift you get from integrating the X acceleration.   Despite Marks earlier comments, the very last thing I would use is a variable-gain filter, and in practice, it's running at a constant frequency with all the input continuously available, the gains would rapidly wind up at steady-state values anyway.

    Note that this is a strictly hypothetical idealized situation - my expectation is that by the time you put in enough limiters and saturations to keep it out of trouble, you are going to be riding one or the other of them almost continuously.  You don't necessarily even want perfectly controlled speed no matter which speed you decide to control, as Paul notes.

  I like the idea of the feedforward, you might just use the existing governor system, feed Mark's signal forward, and use the same signal into the governor to keeo them from fighting each other. So, more-or-less, Igor's system with an additional feedforward signal.

    Brett

That's a true statement. Probably the biggest trouble is the liner gain with G. The gain function I wrote would just as appropriately work with other systems. Simply inserting the 1-Cos into the gain loop would make a significant improvement in performance. The phase shift would still be there though.

In the turbine engines we use the same kind of strategy with the exception that the gain is derived from a look up table or a function several points along the curve. Look at my other thread and the CL v AOA op line is essentially how we drive the inlet guide vanes. In those controls we compute multiple gains continuously include this particular anticipation gain and they all are being feed to a highest or lowest wins gate. So in normal operation the control is operating on the "droop governor" but when a large control input is made the "anticipation gain" wins the gate and takes over. The ESC governors actually work quite well and the output of the timer control actually works as a reset for that governor. 
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Online Paul Walker

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1626
Re: Constant velocity or constant angular rate ?
« Reply #42 on: January 13, 2022, 01:01:59 PM »
Well, except when you put a hinge at the root of the propeller, then the precession is nearly eliminated. A pinned joint can create no moment. A pinned and hinged blade creates no moment and therefore doesn't precess. I made a brash statement of such without a good demonstration and created a real sit storm. Probably deserved some of it. I have been on a very long road towards this power system. So, I was worried about fatigue from bending of the blade from the continuous precession of the propeller during flight with the CL model. Each blade undergoes a bending cycle every revolution during flight and it doesn't take long to get a few million bending fatigue cycles. Most applications don't undergo this much continuous load.

If the blade spindle is a small bolt it must carry that bending. So, the idea came to me that one of my F1C folding propellers had a hinge and that would eliminate the bending. I did a test to see if the propeller could follow sharp corners. When I did, I realized there was little precession evident. I asked a question, in the wrong tone, about how significant is the precession for the CLPA airplanes, meaning is it something worth spending time and effort on. Well... Flight testing there is a noticeable improvement for some of the planes I tried it on. While others not so much. The low TVCv airplanes really tell you about it. My future plan is to use the Fuse cam and fly the prop on another airplane. I'm also working on the datalogger which will have an IMU, throttle control system load cell and position for ubergeekdom data generation. I don't have a version of the prop yet created for the large airplanes. I'm waiting on some more observation flights before I make that commitment.

Here's a link to my discussion on the hinged propeller:

https://stunthanger.com/smf/stunt-design/propeller-precession/msg620424/#msg620424

Sounds interesting. I will be watching for you flight test results.

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Constant velocity or constant angular rate ?
« Reply #43 on: January 13, 2022, 02:50:59 PM »
So, more-or-less, Igor's system with an additional feedforward signal.

Maybe Igor's system canceled during a corner.  Igor adds throttle with +X acceleration.
The Jive Combat Team
Making combat and stunt great again

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Constant velocity or constant angular rate ?
« Reply #44 on: January 13, 2022, 03:00:14 PM »
This sounds like a great idea. I would like to make a point here. If you have tried one of Igor's hollow props, you know how light they are. I commented to Igor how much I liked them because they reduced the gyroscopic precession significantly. His response was that he didn't make them for that reason, but to reduce the inertia the motor had to overcome to speed up and slow down. For me, it was one of the quantum changes that improved my flying and the planes performance. In building a variable pitch propellor, I am concerned that the gyro effects will get worse, not better. Not sure how you could build that arrangement for 16 grams. A higher mass prop in the corners concerns me because of the yaw. Something to consider.

Suppose you could reduce the mass of the rotating machinery by a factor of (I'm just pulling an integer out of the air) 4.  That would reduce moment of inertia by 16.   4 of those little masses might be equivalent in power to the original rotating machinery, but with 1/4 of the moment of inertia, even if they rotate the same way. 
The Jive Combat Team
Making combat and stunt great again

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Constant velocity or constant angular rate ?
« Reply #45 on: January 13, 2022, 03:13:41 PM »
One thing to keep in mind, and some of you do, is that some stuff varies with the square of airspeed and some stuff remains constant with airspeed.  I imagine a doodad hooked up to the flap control horn with a bobweight-actuated slider to work the throttle.  Details left as an exercise.   
The Jive Combat Team
Making combat and stunt great again

Online Paul Walker

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1626
Re: Constant velocity or constant angular rate ?
« Reply #46 on: January 13, 2022, 05:24:03 PM »
Suppose you could reduce the mass of the rotating machinery by a factor of (I'm just pulling an integer out of the air) 4.  That would reduce moment of inertia by 16.   4 of those little masses might be equivalent in power to the original rotating machinery, but with 1/4 of the moment of inertia, even if they rotate the same way.

Now why would you be mentioning this??🤣🤣🤣🤫🤫🤫🤫

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #47 on: January 13, 2022, 05:33:19 PM »
Now why would you be mentioning this??🤣🤣🤣🤫🤫🤫🤫

I think because it would be entertaining. If I wasn't already doing several other projects, I might would do it just for the entertainment value.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Online Paul Walker

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1626
Re: Constant velocity or constant angular rate ?
« Reply #48 on: January 13, 2022, 07:49:18 PM »
I think because it would be entertaining. If I wasn't already doing several other projects, I might would do it just for the entertainment value.

I believe that Howard got his stick out, sharpened it, and poked me with it based on what we talked about last week.

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Constant velocity or constant angular rate ?
« Reply #49 on: January 14, 2022, 12:07:41 AM »
I believe that Howard got his stick out, sharpened it, and poked me with it based on what we talked about last week.

Ah. I was back one post and totally missed it. I was thinking about the bobweight doodad which I have some flyweight governor work in my history. Before FADECs there were mechanical systems which are very interesting. That input almost provoked me in to setting my brain on to working out the solution.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman


Advertise Here
Tags:
 


Advertise Here