Moveable fins or canards for active stabilization?

[MidOH]

It's about time someone mentioned the rules. I can't believe all the support this is getting. I support research and such but this is inviting the worst kind of trouble from the alphabet agencies Bottom line, you cant legally steer a model rocket. What BPS is doing is something totally different.

Can someone quote exactly what laws prevent a model rocket from flying a programmed flight path? Up or down.

 

[MidOH]

Wouldn't a helicopter gyro and a servo with digital magnetic position sensing (not potentiometer), pretty much work right now?

 

[OverTheTop]

It's a little more complex than that, but yes.

 

[Richard Dierking]

Can someone quote exactly what laws prevent a model rocket from flying a programmed flight path? Up or down.

There's a pretty extensive thread on TRF and bunch of people participated in this thread including the TRA Board. We really covered it, and I wanted something to kind of put in my back pocket if this came up regarding active stabilization control. But, that was awhile ago. I do some searching.

You can't aim/fly at targets. You can stabilize a rocket and places in space are not targets.

My suggestion if you want to do more than stabilize a rocket to fly up (to increase safety) would be to join AMA and fly RC aircraft. Please don't try to do loops or something like that with an amateur rocket. Again, the intention should be to make the flight safer.

 

[Richard Dierking]

Can someone quote exactly what laws prevent a model rocket from flying a programmed flight path? Up or down.

I suggest you search for Active Flight Control on TRF from June of 2020. It's a bit of a long thread, but you could probably go through it quickly for the info you are interested in.

Btw, thinking about the Active Flight Control thread reminds me of the things I will miss about TRF. The conversation got heated at times, but I think everyone was respectful and listened to the opinions and concerns of others. Now, it's time for me to move on.

 

[schworer]

Wouldn't a helicopter gyro and a servo with digital magnetic position sensing (not potentiometer), pretty much work right now?

https://www.rocketryforum.com/threa...oll-stabilization-system.169756/#post-2212969
Did this a few years back with a R/C stabilizer and a single servo w/bevel gears. Sometime this year I will try it with a R/C heli gyro. Link will take you to the info, you may have to scroll up or down to find it. Bill

 

[diyaerospace]

Hello!
I am working on a design and I have made some mock-ups. I want to try to make this work with a 56mm airframe because the rocket doesn't need to be much wider. Anyway in my design, the servos hand out the side of the airframe by 11 mm. I will make some covers for them to reduce drag but I was wondering if having the canards not being flush with the airframe will be a problem?
Thanks for your help.

 

[OverTheTop]

The extra drag of things outside the airframe will affect altitude. Other than that I can't think of any problems.

Just make sure however you do it it is possible to be assembled!

 

[OzHybrid]

And it will move your CP forward.

 

[diyaerospace]

Hello
I finished my mockup and followed this tutorial to implement proportional control
https://toptechboy.com/9-axis-imu-lesson-25-proportional-control-system-for-self-leveling-platform/


When I upload the sketch and manually roll the rocket the fins start moving and keep moving until they reach the end of the servos actuation range. I think this is because when the rocket tries to correct its orientation by moving the fins, the roll doesn't change so the fins keep moving. This can make effectively testing the system difficult.
Does anyone have any suggestion on how to test/simulate how the fins will respond in an actual flight or something close to an actual flight?

I am also worried about how to join the servo arm to the servo shaft. Their connection is pretty weak and I think they would fly off at speed. I will epoxy the arm to the canard but I don't want my servo stuck to the canard so I will only be able to hot glue the shaft to the arm(hot glue dissolves with rubbing alcohol)
Does anyone have any solutions to this problem?


Thanks

 

[boatgeek]

Does anyone have any suggestion on how to test/simulate how the fins will respond in an actual flight or something close to an actual flIght?

The easiest way would be to put the AV bay on a lazy Susan and rotate it back and forth to simulate roll in flight. With a little practice, you can probably fake actual responses with greater angular acceleration/deceleration with greater fin angle.

 

[JimJarvis50]

Hello
I finished my mockup and followed this tutorial to implement proportional control
https://toptechboy.com/9-axis-imu-lesson-25-proportional-control-system-for-self-leveling-platform/


When I upload the sketch and manually roll the rocket the fins start moving and keep moving until they reach the end of the servos actuation range. I think this is because when the rocket tries to correct its orientation by moving the fins, the roll doesn't change so the fins keep moving. This can make effectively testing the system difficult.
Does anyone have any suggestion on how to test/simulate how the fins will respond in an actual flight or something close to an actual flight?

I am also worried about how to join the servo arm to the servo shaft. Their connection is pretty weak and I think they would fly off at speed. I will epoxy the arm to the canard but I don't want my servo stuck to the canard so I will only be able to hot glue the shaft to the arm(hot glue dissolves with rubbing alcohol)
Does anyone have any solutions to this problem?


Thanks

What is going on in the video is not what you want for rockets. I think you can make a simple change to the program to say that the servo position should be proportional to the error term pitchservoval = G pitcherror. G is the proportional constant that is referenced when we talk about proportional control for rockets.

Jim

 

[diyaerospace]

I have finished a PCB that I will fly on my model rocket this weekend to try to record roll, pitch, and yaw angles along with velocity, altitude, and other data. I am still worried about connecting the servo arm to the shaft as the fit is pretty weak and will probably come off at speed.
Does anyone have any suggestions to improve the fit?
Thanks

 

[OverTheTop]

Have a look here at my Mk1 VTS.
https://forum.ausrocketry.com/viewtopic.php?f=56&t=5324

 

[Kelly]

Does anyone have any suggestion on how to test/simulate how the fins will respond in an actual flight or something close to an actual flight?

How about this:
https://www.rocketryforum.com/threads/electronic-gyro-rocket-roll-stabilization-system.169756/

 

[diyaerospace]

Hello,
I have a rocket built and my proportional control algorithm written. I was wondering how to center the canards before the flight. Will I have to use some sort of jig or can I use software to center the canards? Right now I am just using two canards to control roll, they are directly driven by the hitec hs-5055mg. Thanks for your help. Hopefully, I will be able to fly in the next few weeks.
Thanks

 

[OverTheTop]

You should be able to fine-tune the PWM out of your system to put the canards in a neutral stance.

 

[schworer]

In the roll control system I built the solution wasn't elegant but it works. Basically I stand the rocket vertical and power the system on, the servo will move to a neutral position and stop. With the electronics and servo still powered on I then manually align the canards in parallel with the tube. I can do that because each canard axle goes into a coupler with set screws that engage the 45 degree bevel gear output shafts. Once everything is zeroed that way the system can be powered off and re powered on the rail, even if the unpowered servo/canards have moved a bit during transport they will recenter on the rail. I guess you could use a template for allignment, but I've found that eyeballing it probably gets to within a couple of degrees of deviation from vertical if careful in setting them up because you can tell if the canard is parallel to the rocket body by looking at both at the same time (take any blended focus glasses off when doing so as they can distort the image).

I dont know what your project looks like, but I recommend putting a downward looking mini recording video cam above the canards so you can visually review what they were doing in flight.

The thing to remember is once launched the velocity of the air flowing over the canards is going to change over time on the way to apogee, so forces on the canards when exerting corrective moments are also different. I've found that small deflection angles, like maybe 5 degrees, are sufficient to keep my particular rocket from rolling much (the residual roll I believe is due to the R/C plane stabalizer drift and that input into the servo, the next flight will be with a high speed heli pizzo gyro as the servo input which is much more sensitive to heading changes). One test I increased the canard deflection angle 50% and when the rocket was going around 450 mph at the end of the 8 second burn, the correction movement for a very slow roll resulted in "stripped" canards in their shaft coupling, even though the canards were balanced on the axles with equal surface areas for and aft. I suspect that at that speed the center of pressure on the canard had moved and when that happened the canards were unbalanced on their axles. The stripped couplers were the intentional weak spot in the system as I didnt want to put excessive pressure on the gears or the servo. When I find time this summer I'll research on CP for control surfaces vs. speed later this summer.

 

[OverTheTop]

The center of lift is somewhat further forward than mid-chord for the canards. Have a look at double-wedge cross section if you want something easily calculable and well-defined over larger speed regimes.

Keep in mind that the moment of inertia in roll is quite small and it is easy to have too much gain and induce oscillations.

 

[JimJarvis50]

Hello,
I have a rocket built and my proportional control algorithm written. I was wondering how to center the canards before the flight. Will I have to use some sort of jig or can I use software to center the canards? Right now I am just using two canards to control roll, they are directly driven by the hitec hs-5055mg. Thanks for your help. Hopefully, I will be able to fly in the next few weeks.
Thanks

You can get a DPC-11 programmer. This will allow you to center the canards and to set the left and right end points (i.e., the gain of the servos). Other things, such as direction of turn, can also be adjusted. I use a little jig over the servos to determine when they are aligned straight along the body tube.

Jim

 

[diyaerospace]

Thanks for the replies!
I will look into the DPC-11 that Jim suggested. I was wondering how straight canards really need to be to have a good flight because the control algorithm could handle the misalignments.
For example, one canard is at 92 degrees instead of 90. When the rocket launches the misalignment will cause the rocket to roll, this will create an error which is when the flight computer commands the canards to turn. The flight computer will keep moving the canards until the rocket is at its target roll of 0 degrees.
Unless I am missing something I think a couple of degrees of misalignment won't be a problem?

BPS space mentions using an integral controller in this video to deal with the trust vectoring mount being misaligned. Will something like this work for rockets with canards?




Thanks for your help!

 

[OverTheTop]

Unless I am missing something I think a couple of degrees of misalignment won't be a problem?

I would be aiming to get them as close to neutral on launch as practicable. You want the rocket to go as straight as possible to make sure it is getting further away from spectators before the controls kick in.

Always consider how to make the test flights safe. As a minimum I would be treating it as a complex rocket and giving it twice the distance to the flight line. Consider where you turn on stabilisation. I do that after boost in my system so the bird is a long way in the air and traveling away from the people.

Answering your question as to whether a couple of degrees is a problem, at the start of the flight there is almost no airflow over the fins and they will exhibit minimal effect. The control system should bring them to where they are needed during flight, if you have the control system adequately designed.

BTW, treat every flight as a test flight IMHO.

 

[diyaerospace]

Hello,
I am almost ready for a test flight of my actively stabilized model rocket. I have a flight computer together and integrated. I am using two Hitec hs-5055mg servos connected to 3d printed canards. These will try to control roll!
My current problem is the stability margin on my rocket. According to rocksim, it is about 1.2, and my mass budget is relatively low so I am looking for another way to make my rocket more stable. I think adding 3 smaller fins in between the main ones in the back of the rocket would do the trick. The only problem is that the rocket will be asymmetric as a result because I need to leave space for the launch rail. Will this be a problem?
I attached a screenshot of the base view of my rocket.

Thanks for your help!

 

[boatgeek]

Hello,
I am almost ready for a test flight of my actively stabilized model rocket. I have a flight computer together and integrated. I am using two Hitec hs-5055mg servos connected to 3d printed canards. These will try to control roll!
My current problem is the stability margin on my rocket. According to rocksim, it is about 1.2, and my mass budget is relatively low so I am looking for another way to make my rocket more stable. I think adding 3 smaller fins in between the main ones in the back of the rocket would do the trick. The only problem is that the rocket will be asymmetric as a result because I need to leave space for the launch rail. Will this be a problem?
I attached a screenshot of the base view of my rocket.

Thanks for your help!

Just curious--why do you want it more stable than 1.2 calibers fully loaded? That's a completely normal stability margin unless you're headed for Mach.

 

[schworer]

I dont know what you have for a motor, but as the propellant burns the weight at the aft end it going to lighten and shift the CG forward. I've flown the 4" diameter roll control rocket twice with about 1 caliber of stability at lift off and it was fine. Like Boatgeek said, if you are reasonably less than mach that stability margin should be just fine.

 

[OverTheTop]

I fly my Vertical Trajectory System with similar stability.

 

[diyaerospace]

In my experience, a rocket with canards and a stability of 1 caliber will often appear less stable than the same rocket without canards. I don't know why this is, but I have many flights that demonstrate this. I advise designing for a static stability of not less than 1.5 calibers. The rocket will be less unstable, and can still be moved around.

 

[JimJarvis50]

I stand by that statement. I'm using Rocksim9 with the Rocksim stability equations.

I don't know the sizes of your fins or canards, but you might have an easier time just reducing the span of the canards a little. It wouldn't take much.

It may be more trouble than you want, but the technique I use to adjust stability is to add fin tips. Just a little basswood or balsawood to increase the span of the primary fins. I do this on pretty much every flight.

Jim

 

[diyaerospace]

Hello,

I just flew my roll control system with... interesting results. I put a camera on my rocket to look up at the canards that shut off mid-flight for some reason. During the ascent, the rocket rolled positive at a rate of 500-1000 degrees per second. The onboard footage and data show that the canards moved to their maximum deflection angle (which is roughly 5 degrees) to try and reduce the roll. Despite this, the rocket kept rolling. Do you know what could be causing this? The fins were not grossly misaligned and I don't think there were any surfaces on the vehicle that would induce this roll. I attached a video showing the flight and onboard video.

Thanks for your help!

 

[JimJarvis50]

Hello,

I just flew my roll control system with... interesting results. I put a camera on my rocket to look up at the canards that shut off mid-flight for some reason. During the ascent, the rocket rolled positive at a rate of 500-1000 degrees per second. The onboard footage and data show that the canards moved to their maximum deflection angle (which is roughly 5 degrees) to try and reduce the roll. Despite this, the rocket kept rolling. Do you know what could be causing this? The fins were not grossly misaligned and I don't think there were any surfaces on the vehicle that would induce this roll. I attached a video showing the flight and onboard video.

Thanks for your help!

It sounds to me like you encounted control reversal. If the canards were indeed trying to turn the rocket opposite of the actual roll, then that would be my guess. This is caused by vortices from the canards interacting with the main fins. It can cause the rocket to roll in the opposite of the intended direction.

A video of the flight where I encounted this is attached. The control system was between the booster and sustainer. While the booster was attached, the rocket was in the control reversal mode. As soon as the booster fell off, roll went to zero. This flight was why I developed my spin can. When the vortices hit the fins on the spin can, the fins just turn instead of turning the rocket.

I would suggest you go back to four fins (remove the small ones), and then arrange for the canards to be between the gaps in the lower fins instead of aligned with the fins (I'm not sure how you have them now). Then, make the canards smaller and further reduce their travel if need be.

Jim