I have an F-100 which tends to veer strongly from side to side in the early part of the take-off run. I have a few things to try yet to calm it down but am thinking a gyro on the noseleg steering might be worth having anyway just as a final back-up. It's not something I have any experience of. Does it really work? Do you use a rate or a heading hold gyro? Can a cheap £10 gyro do the job or are there reasons for buying a more expensive brand name gyro?
Any advice welcome
Harry

My 100 tract well, check alignment of mains, no binds, hard tires and no slop nose wheel.I could turn around in about 10 feet.
Darryl Usher

this will work fine http://dreamworksrc.com/catalog/prod...oducts_id=1043

also check for slop in your stearing linkage/servo set up

Just a thought on alignment: You probably already are aware that a slight toe-in on the mains is helpful?

I tried all the suggestions above on my JHH F-4. Toe in the mains, tightening linkage, different expo setups, etc. Nothing helped. It had a bad tendency on rollout to magnify any steering input causing it to go astable and spinout. Not sure if it was due to the dual-wheel on the nose or what. I could taxi but anytime under power (i.e. taking off) the plane was not reliably ground controlled.

A gyro fixed all of this! It tracks straight as an arrow now. Get a normal gyro, DO NOT use a heading lock gyro. Place it as close to the CG as possible. Make sure it is mounted properly, that is if you rotate it to the left the steering corrects to the right. Start off with 1/4 -1/2 on the gain setting. You can use a cheap piezo gyro, but mine was close to the turbine since that is the CG point. So I opted for a more expensive Futaba 240 gyro since they are less temperature sensitive. You could try a normal piezo gyro, I never got around to it. I figure since you are only using it during short periods of time the temperature drift shouldnt be an issue as it would be in a heli. I ended up with around 1/2 gain setting. Too much and it will oscillate, too little gain and the corrections will be too late to help (sluggish). Do some taxi runs and dial in the gain needed until she is stable. Then you shouldnt have to touch the gyro, just let it do its job...

Jim

May I ask why you insist on placing it close to the CG? Angular rate is identical whether at center of mass or at far extremities of the model.

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This debate has come up time and time again, especially in the heli segment. There are guys that fly gyros in the nose, the tail boom, near the main mast, on the tail fin etc.. As far as the gyro is concerned the angular velocity/acceleration measured will be identical anywhere on the rigid member, that is, the aircraft.

As long as the gyro experiences the angular acceleration it can react fast enough. Actually for small deviations (like in steering) if you put the gyro up near the nose it doesnt see true angular motion - it sees more of a tangential component that the gyro will not respond to. The distance between the point of rotation and the gyro location is too large and angular acceleration looks linear for small angles. It's simple geometry. It may work in some planes but you will have to dial up the gain quite a bit.

I first mounted my gyro near the nose and it didnt work well at all. Once I put it at the CG it started working. That is the point where the plane rotates about and where the gyro reacts best given its feedback loop and the small angles involved in a steering compensator.

First of all, when a plane is sitting on the landing gear/wheels, the CG doesnt really have anything to do with where the center of rotation is, does it? It sitting on the gear... what has CG have to do with it?

Secondly, do the geometry... the gyro senses a change in angular movement, which is identical anywhere on the airframe, as was stated. Think of it this way... pick up the plane, and change the "heading" 90 degrees. Now place the gyro ANYWHERE on the airframe, and the gyro also sees a change of heading of 90 degrees. How much translational movement it experiences while rotating is not relevant... only how many degrees per second it is moving - which remains constant regardless where it is located.

AJC

Since we are talking steering direction and not flight, the plane rotates about the WHEEL BASE where ever that is. The CG isn't a consideration. http://en.wikipedia.org/wiki/Gyroscope

Uh, well my plane rotated on its CG when it spun around. It certainly didnt rotate about the nose wheel. So yes, the CG has everything to do with the plane on the ground. Its a point where all the weight can be centralized. I better rotate about that.

And I suppose by your dissertation radius of curvature has nothing to do with angular acceleration? I hardly agree.

Plus, you have some incorrect notions. If you change heading the gyro doesnt sense the 90 degree positional change. It senses the acceleration and only reacts to that. You would have to double integrate to even know the position change.

If we envision placing the gyro on a stick, at the front, the middle and then the end of the stick In all cases if the stick changes direction of its Y axis the gyro will see a change. Thats whats important.

I'm with Hyperdyne on this one. I do not know the math behind the workings but I do know from empirical data that every time I have placed a gyro towards the nose of the aircraft they dont work well at all-when I have moved the gyro to the center of yaw (which is close to the CG) they magically work wonderfully. Scotty

In the last issue of Contrails, I did a little write up, just a brief one, on gyros that I have used. I am just about finished with my Skymasters L-39. I installed a GWS brand gyro on the nose gear. It is mounted, on it's side, about 7 inches forward of the CG. I will test run the L-39 for the first time tomorrow, as I bench tested the gyro, it is very effective where it is mounted. No need to buy the big dollar gyro's as where the Gws brand sells for $35.00

GWS PG-03 Piezo gyro.

Mark

Well, if I remember my physics correctly (over 20 years ago...) If angular velocity (and therefore acceleration) wasn't the same at every point of the airplane, the plane would break, right?. Imagine your canopy at 1 rpm and your nose at 2 rpm... not possible right?. And if you started from 0 rpm and increased it to 10 rpms in 10 seconds, the angular velocity would be the same accross the whole airframe at any given time during those 10 seconds, which means that angular acceleration was the same accross all the airframe (2pi radians/sec*sec), thus, the gyro would read the same acceleration no matter where you put it. Therefore, distance from center is irrelevant, is not a variable in the angular position, velocity and acceleration equations. Does this make any sense????

Now, all this is thinking of regular gyros... I don't have the slightest idea of how a heading hold gyro works... maybe for those the center position is critical, but that is not what we use for steering


Now, back to the subject, nose gear gyros work perfect. To me, take off run is probably the closest you can be to crashing your plane, so it is where I want the most help. It also helps to hold some up elevator so your mains are solid against the ground. Otherwise, some planes tend to lift the mains before the nose (specially when using flaps) making them harder to control.



Jack G

This whole gyro issue is like a brain teaser to me and I wish there was a diagram or simpler explanation of some sort. Jack G's explanation using the roll axis seemed to make sense until I considered a gyro placed out on the wingtip. Wouldn't it have a greater "angular acceleration" than one on the centerline? I darn sure know it will have more "g's" on it. I think I'm confusing terms here but just can't get it clear in my mind.

Anybody out there who can make this issue understandable to us non-physics major types?

Craig

Hyperdyne, youre post leads me to believe you dont understand what has been said - ie, when the plane is sitting on the wheels, the CG has nothing to do with how the plane turns or moves, only the wheels are acting to do this. If you load the back of your pickup truck with 1/2 ton of gravel, does the steering change? No, but the CG point certainly did.

Now, simply put - the gyro units we use sense only a change in angular velocity. That means they can sense if there is a ROTATIONAL movement, and how fast that movement is happening (in degrees/second). They do not sense translational movement whatsoever. If you hold the gyro in your hand, and slide it one way or the other so its purely translational and nothing rotational, the gyro doenst act on the servo. AT ALL.

What matters - and what has been said here more than once, is that EACH AND EVERY point on an airframe is rotating at the same ANGULAR VELOCITY. Right at the center of rotation, to the furthest point from it.... its all about degrees per second. NOT about how far the gyro is moving as well.

Maybe in an infinitely large plane, with the gyro sitting at an infinite point away from the center of rotation, will the gyro now sense no angular motion... but we dont live in an world with infinitly large model aircraft do we? Nothing would matter then anyway because the gyro would be moving faster than the speed of light

Seriously, I think poor gyro installation has more to do with this, than misunderstanding of how these units work, and why they can be installed anywhere on the aircraft and function the same.

AJC

gyros are good but it you set the nose wheel up properly and put some expo in it it should not need a gyro. I have never needed one yet. In some planes you may need one on the flying surfaces. on most of the jets that I have seen there was to much steerig I don't use alot of seering movement just enough to steer it. I use approx 10 mm and it is enough to turn the plane around on a 7 metre wide runway. I just find to much steeing causes the hassales

Jack thank you for helping me freshing up my physics .

you are correct. gyro no mater where placed in the aircraft it will be rotate always with the aircraft with no angle change.


guys ....can i make a question about the nature of the sensor...

the only difference in the placement we talk (center of the aircraft or nose) is the side G force the gyro will experience.

do you think this has any effect on the sensor?any kind of friction maybe?

thank you

I think that is the key AJC, the difference between angular/rotational velocity and translational movement. In my mind it confuses the issue to think of the movement as being about an axis as opposed to orientation on a plane. A good example to me is a magnetic compass...it senses angular rotation from a fixed heading regardless of where it is placed on the plane or how it moves on the plane. This is an easy explanation to me of why it does not matter where the yaw (rudder/nosewheel steering) gyro is placed on an aircraft, only that it is aligned (or is it perpendicular?) to the yaw plane.

I'm still not sure I understand the concept that gyros "do not sense translational movement whatsoever" because I know if you apply a force to a gyro (such as a spinning top) it will react/move 90 degrees to the direction of that force. Does that have anything to do with this conversation?

Craig

Craig,
No it doesnt. The gyros we used to use (mechanical, with an actual spinning mass) are free to move in one axis only. The newer piezo gyros with no "moving parts" also detect angular motion in one axis only as well. That why they have to be placed correctly, ie, for rudder, to sense a change in yaw.

I say it again, if you guys who dont believe me want to try it... take a gyro and servo. Hold the gyro in your hand and rotate it about the axis by turning your body. watch the servo. Now hold it out at arms length and spin your body around at the same rate and watch the servo - same thing.

Now, still holding the gyro, move it sideways back and forth, or fore and aft - being carefull not to apply any rotational motion. Move it fast, move it slow - the rudder servo stays put!

Translational (straight line motion) doesnt have any effect. Only angular/rotational movement does. Thats why these things work.

AJC

ps to see how this works you cant do much better than take an old mechanical gyro apart and see how the rotor sits, and how it is free to move, and then it is easier to understand IMO. A piezo gyro just isnt as good a learning tool.

Exaclty the same happened to me. Although I have not yet moved the gyro close to the C.G., I noticed that I needed to increase the gain with respect to the ailerons gyro that is positioned in front of the rudder/nose wheel gyro but in the center of the roll axis.

Although in flight, the first time I tested it, without gyros, my turbine jet (Skymaster F-16 1/8) under cross wind flight, she had wagging tail and wagging wings.

In the next pass, I turned on the ailerons gyro and wing wagging stopped but tail wagging continued.

In the next pass I turned on the rudder/nose wheel gyro and the jet flew like a rock.

My gyros are ACT Fuzzy Pro airplane gyro.

My problem moving the rudder/Nose wheel gyro back close to C.G. is that the cables are not long enough so I would need Heavy Duty (and maybe amplified) servo extensions.

For now, I think it is working and this jet is easily handled on ground while in take off.... I have nose wheel expo at -40% for each side (Futaba expo is negative while JR is positive I guess).

See pictures

And watch this video

<a href="http://www.youtube.com/watch?v=D2Nu5NVn-J8">http://www.youtube.com/watch?v=D2Nu5NVn-J8
</a>
Sorry for the 0.57 minute censored video (some guy was yelling bad words....) and for the landing censored (hard landing that broke the legs suports and made the guy on the camera run away. He was too close to the runway side line), but anyway, watch minute 2.15 to 2.20 aprox. and you can see the wings and tail wagging and in minute 3.18 to 3.24 you can see that in flew like a rock.....

BRG
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The key is that we're talking about and using gyros here, not accelerometers or associating IMUs.

Translational movement have no effect on gyroscopes but do affect accelerometers. For accelerometer platforms, the offset to center of rotation is relevant and depending on application, can compensated by algorithm.

Hi Andrew:

Please read the attached .pdf file regarding the ACT Fuzzy Pro Gyro: Page 5 (English)

<div></div><div><span style="color: rgb(0, 0, 255); ">"English</span></div><div><span style="color: rgb(0, 0, 255); ">FUZZY SMM</span></div><div><span style="color: rgb(0, 0, 255); ">Instruction manual</span></div><div><span style="color: rgb(0, 0, 255); ">Description</span></div><div><span style="color: rgb(0, 0, 255); ">The FUZZY SMM is used for the stabilisation of model aircrafts. Its intelligent microprocessor</span></div><div><span style="color: rgb(0, 0, 255); ">controlled electronics, coupled to a Piezo sensor, monitors the rotation of the model around the gyro/s</span></div><div><span style="color: rgb(0, 0, 255); ">axis. The FUZZY SMM Gyro is specially designed to be used in an airplane with features airplane</span></div><div><span style="color: rgb(0, 0, 255); ">flyers require.</span></div><div><span style="color: rgb(0, 0, 255); ">The FUZZY SMM monitors all movements around it&rsquo;s axis much faster than the pilot can, and</span></div><div><span style="color: rgb(0, 0, 255); ">responds with different modes."</span></div>
I know that your conclusions are supported by the test you mentioned above but in this gyro type, doesn't it mean that if you place the gyro sensor in the nose of the plane, its rotational axis is different than that if you place the gyro near the plane yaw axis?

And BTW, the angular velocity is detected in AVCS mode only? (it do not detect angular acceleration??)


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