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Mar 27, 2007, 11:32 AM
Old fart. VAAATSP
d_wheel's Avatar
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Idea

Roll Stability Augmentation using Standard Rate Gyro


There are some autopilot installations where the use of a wing leveler might not be the best choice. One such situation involves a fuselage with limited space. The Multiplex Easy Star is a good example. Being all foam, the fuselage walls are about 3/4 of an inch thick leaving precious little space inside for equipment. Even though a wing leveler such as the FMA Copilot is very small, it along with it's wiring are just too much to stuff into the little bird along with the autopilot and altitude hold board. On several similar occasions I have successfully used a standard rate gyro to augment stability enough to use "dumb" autopilots like the RCAP.

While at first it might seem that one would want to install the gyro in a position which senses roll, this is not the case. The gyro has no way of knowing when the wings are level so has no starting or ending point to use as reference. The proper position for roll stability is one that senses yaw. This has been done for many years in full size aircraft and while it does not hold the wings level, it does resist the tendency of an aircraft to turn unless commanded by pilot or autopilot. So in effect, we have a wing that is more or less level. The reason it works is this. If an aircraft is turning in one direction or the other, we can assume that it is banked it that direction. A rate gyro which has been placed in the yaw axis will sense this turning movement and correct in a direction that will resist or stop the yaw. If it senses no yaw, the wings must be level. I have never tried this with a “heading hold” gyro. Thinking about it, I don’t believe it will work. There is the problem of drift. Heading hold gyros tend to drift slightly over time. This might induce an unwanted turning tendency. Another problem is that this type gyro uses a “cumulative” error correction. If you want it to turn left, you give it a left command and it moves the control surface in that direction. If it is not turning at all or even if it is not turning fast enough it commands the control surface to move farther and farther until the desired turn rate is attained. I can see this causing the aircraft to go into an uncontrollable spiral if left uncorrected.

While testing the RCAP ver3, I again ran into the space problem so fell back on the rate gyro as a solution. The one used in this case is a Futaba GY240. It is small, light, and only has 2 wires so does not take up too much room in the fuselage. Besides, I had one in the supply drawer with no home at the time. The only draw back is that there is no way to switch the unit on/off in flight. If set up properly, this does not present a problem. While none of the other currently available gyros have been tested, I see no reason that any quality unit would not work just as well.

Setup:

Turn AVCS (heading hold) off.
Install flat in fuselage.
Set Gain to 50%.
Connect input of gyro to servo output of RCAP. Connect rudder servo to gyro.
Turn transmitter and receiver on.
After everything has initialized, rotate the fuselage either left or right. The rudder should move in a direction to cancel yawing motion. If you are turning fuselage to the left, rudder should deflect right. If rudder is moving in the wrong direction, change position of the Rev/ Normal switch and try again.
Make sure the gyro is not set so high that it over rides rudder input. To do this, hold the control stick to full right rudder. Rotate the fuselage to the right quickly and make sure the gyro does not move the rudder back past the center position.

You are now ready for a test flight. Before launching, move the roll trim full left and right. Note mentally how far the stick has to be moved to attain this same deflection. Center trims. Set the autopilot rudder throw control to give this same amount of deflection when commanding a turn. With the Easy star and RCAP ver3, this was about 10%. Set the gyro gain to a low position before the first flight. It is always best to have too little correction than too much. After launching and gaining sufficient altitude, move the control stick just enough to attain what would be a full left or right trim condition. If the aircraft maintains a smooth turn, you are all set. If it continues to roll past a safe bank angle, land and increase the gyro gain. In my case, on the Easy Star, I used full gain on the gyro. Now, fly by about 500 feet out and when even with the starting point, activate the return home mode of the autopilot. Be prepared to return to manual mode as soon as any unwanted condition occurs. Remember, it takes the RCAP about 1 second to return control to you after the switch has been moved. You should get a smooth turn toward yourself. If so, fly by in the opposite direction and try again. If both turns were smooth, try a more aggressive turn angle by flying away from yourself and activating the return home mode. If the aircraft makes a 180 degree turn without any bad tendencies you are all set. Flip into the waypoint mode and enjoy!

Since I have never tried , it should be fun and informative testing in Heading Hold mode. This might be disastrous so I need to make sure I have a back up airplane waiting in the hanger before starting. Will let everyone know how this works out.

Later;

D.W.
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Mar 27, 2007, 03:41 PM
Registered User
You might also be interested in the following article discussing the use of a gyro to increase spiral stability in RC sailplanes by Blaine K. Beron-Rawdon 1999.

http://www.rc-soar.com/tech/spiral.htm

Excerpt:
Conventional 6-servo radio controlled thermal sailplanes require active pilot roll input to maintain a constant bank angle in a thermal circle. On their own, the models tend to tighten up the circle and spiral in – they lack "spiral stability" due to the typical low-dihedral wing. An electronic rate gyro is used to sense yaw rate and drives the ailerons to counter this spiral instability. New gyros control the ailerons via two channels, preserving the option of aileron camber changing and "crow". A third input to the gyro provides in-flight adjustment of gyro gain so that fine-tuning is easy.
<snip>
Mar 27, 2007, 08:37 PM
Old fart. VAAATSP
d_wheel's Avatar
Thread OP
Thanks! Interesting reading.

Later;

D.W.
Mar 29, 2007, 07:17 PM
Registered User
D.W
I have a BTA autopilot. It works on the same principle of roll stability by sensing yaw. But it seems that it is more complicated. I opened the box and it contains a motoroal microprocessoe and many small IC's
The manufacturers say that it is patent.
It also contains a pressure sensor. I think that the Ic's found inside compose a hardware differentiator. In this case the signal achieved will be rate of climb.
Now you have two rate signals which are used to stabilise the airplane.
I think that other should enhance this discussion since it is great to add stability augmentation to those who have RCAp's. No need then for a copilot.
All stuff will be internal in the fuselage.

D
Mar 30, 2007, 11:54 AM
Registered User
When the plane rolls The down going wing will be subjected to more drag then the up going wing.
This will cause yaw in the airplane attitude,
Therefore in order to sense roll and oppose it we should place a rate gyro to sense the yaw .
If you are at maximum aileron stick my plane will roll to maximum 30 degrees and stay there.
I wonder how the BTA can do this?
D
Mar 31, 2007, 01:22 PM
Registered User
For bank angle less than approximately 30 deg., and assuming no significant sideslip, it can be mathematically shown that bank angle and yaw rate are related by:

bank angle = yaw rate * Airspeed / g

Where:
bank angle (radians)
yaw rate (radians/sec)
Airspeed (ft/sec)
g = 32.174 ft/sec^2

When you use a gyro to sense yaw rate you are making a very simple bank angle estimator based on the above equation. The steady-state bank angle is a function of airspeed and yaw rate only, so for a fixed airspeed the rate gyro output (in a steady turn) is a direct indicator of your bank angle. Feeding this signal into the ailerons will stabilize or control the aircraft's bank angle. A system such as the BTA autopilot uses an airspeed input and software to limit the maximum yaw rate to give a 30 deg. steady state bank angle.

Steve
Apr 02, 2007, 11:59 AM
Registered User
Steve,

The BTA does not have a sensor to measure speed. It only has a barometric pressure sensor.
The other sensor is a gyro.
Look at this patent for explanation.
http://www.freepatentsonline.com/4964598.html
D
Apr 02, 2007, 08:28 PM
Registered User
David,

I thought some versions of the BTA had airspeed, but it isn't necessary to enable the autopilot to limit bank angle. If you set the maximum comanded yaw rate to a fixed value this will give you a fixed bank angle. For example, if I set the command limit to 13 deg/sec and I happen to be flying around 50 mph, then the bank angle will never exceed 30 deg. The limit will be different at other flight speeds, but the angle will still be limited.


Steve
Apr 02, 2007, 09:17 PM
Professor of Wood
kd7ost's Avatar
There are no current versions of the BTA unit with airspeed control. There was an early version with true altitude lock but it's not around anymore. The current 07 and its predecessor, the 06 only use a gyro for roll control, (It looks at the Yaw axis to do this) and a small BP sensor for pitch control. It resists altitude changes through this pitch sensor but it’s not an altitude lock. It’s like the co-pilot in its operation even though it functions differently. It will sink or rise while resisting pitch changes.

Dan
Apr 02, 2007, 11:01 PM
Registered User
Question: What happens if your plane or glider has zero or slightly negative dihedral (so that there is zero yaw roll coupling). Will the BTA or similar auto-pilots still work?

It would seems that if the plane could roll without yawing (such as with an aerobatic model) it would not allow the BTA to work. Perhaps I am missing something?

-John Elliott
Apr 02, 2007, 11:24 PM
Professor of Wood
kd7ost's Avatar
Good question. I don't know that answer. You're talking about a plane that is dynamically roll unstable in the case of anhedral to the point that yaw input causes a plane to roll outside. It's not an optimum set up by any means. It's best to have yaw cause a coordinated roll or a flat turn but not beyond that.

I do know if I taxi a plane on the ground with the BTA 06 or 07 unit, that when I steer around and put in some yaw during ground handling, it will deflect the ailerons to prevent the roll even though the plane isn't rolling. Only moving forward and putting in yaw.

It has been used on pattern type aircraft and works just fine.

Dan
Apr 02, 2007, 11:33 PM
Registered User
John,

Gyro wing-levelling does not require dihedral to work. If your model has no dihedral and you add yaw rate-to-aileron gyro feedback it will feel like flying a trainer with lots of dihedral.

The basic idea of how it works is pretty simple. Any aircraft in a coordinated turn will have some measureable yaw rate and some corresponding bank angle. The gyro senses the yaw and moves the ailerons to make that yaw rate go to zero. The airplane's turn dynamics require that if the yaw rate is zero then the bank angle must also be zero because you can't have one without the other (unless there is sideslip). The relationship between turn rate and bank angle is independent of dihedral. The relationship only assumes you are in a steady turn without sideslip. If you draw a picture of an airplane in uniform circling motion with fixed bank angle and speed, you should be able to derive the yaw rate that this motion produces. If you assume small angles you'll get the relationship I posted previously.

Steve
Apr 03, 2007, 02:49 AM
Registered User
d wheel

Are you using ailerons or just rudder control. I'm sick of calibrating the co-pilots on my Magie AP.

Also I fly close to factories so the co-pilot picks up the heat from the buildings and causes the plane to be unstable.


I am installing the Alti and RCAP 3 in my Magpie Ap and have a spare gyro.

The Magpie only has rudder control so there is no way of aileron correction.

Do you think this will work in the Magpie.

Your thoughts greatly appreciated.


Crash Pilot
Apr 03, 2007, 08:16 AM
Old fart. VAAATSP
d_wheel's Avatar
Thread OP
Quote:
Originally Posted by Crash Pilot
d wheel

Are you using ailerons or just rudder control. I'm sick of calibrating the co-pilots on my Magie AP.

Also I fly close to factories so the co-pilot picks up the heat from the buildings and causes the plane to be unstable.


I am installing the Alti and RCAP 3 in my Magpie Ap and have a spare gyro.

The Magpie only has rudder control so there is no way of aileron correction.

Do you think this will work in the Magpie.

Your thoughts greatly appreciated.


Crash Pilot
I have used it both ways in the past. On the EasyStar, it is on rudder. It doesn't work as well as ailerons because the autopilot and gyro are sort of fighting each other. Setup takes longer because you have to make sure the two are "balanced". I have the gyro gain turned all the way up, and the RCAP servo throw all the way down to get stable turns. The turns are very slow and the airplane searches back and forth during straight legs. I am in the process of cutting ailerons into another set of the wing and will move the gyro to them when finished. In this configuration, it works much like a CoPilot in that the gyro attempts to keep the wings level and the rudder makes more or less flat turns. Tighter turns can be accomplished without a tendency to drop the nose. The gyro will probably have to be turned down and the RCAP servo throw increased quite a bit.

I am not familiar with the Magpie, but if it is stable in roll you should be able to get it to work with patient fine tuning. Just remember, it will not be as stable as the CoPilot, and you will not have pitch stabilization. The EasyStar is not very stable in roll so if it will work on rudder only, anything should.

First, get the gyro to work correctly without the autopilot connected. Start with low gyro gain and work up. After you can make stable turns using trim only, and the airplane levels out when trim is re centered, you are ready to test with autopilot. Start with very low autopilot output, no more than the rudder moves when you have full trim, and work up until the airplane seems to turn too tightly, then back off a little. Remember to be ready to disengage the autopilot at the first sign of trouble because it takes about 1 second to get control back after flipping the switch.

Later;

D.W.
Apr 03, 2007, 08:26 AM
Registered User
d wheel

Thanks heaps for your advise.

I will setup and test this weekend.

Crash Pilot


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