Does gyro on rudder work against aileron?
 

Guys need your help on this,

maidened my Carf Mig15 last week, it got an ACT Fuzzy SMM gyro on the Rudder/Nosewheel.
I observed that, during a turn to the left (and to the right) with only aileron input, the plane seem to slip to the opposite directiont;
I just wonder, can it be that because of no rudder (stick movement) at that moment de gyro is working against the aileron input?

Frank

RE: Does gyro on rudder work against aileron?
 

Sounds like classic adverse yaw which your gyro should correct but is not. See what the rudder is doing when you hold the jet in your hands and roll it, but I suspect it's not doing anything!

RE: Does gyro on rudder work against aileron?
 

It is most likely the gyro reacting against the turn.

Adverse yaw is a reaction to rolling and would cause the plane to yaw out of the turn but only during the roll in or roll out, not all through the turn. The rudder gyro would attempt to stop the adverse yaw. Holding the model in your hands and rolling it will not show any response from the rudder gyro (other than your imperfect holding of it!) since the yaw gyro will not sense roll.

A turn involves a yaw all the way through the turn, it is a fundamental part of turning. A yaw gyro senses this and tries to stop it by yawing the model out of the turn. It is a problem of the simplistic way in which we use a yaw gyro. The ideal yaw gyro will not react to the yaw of the turn but still react to the yaw of fishtailing, adverse yaw, turbulence, etc. Our gyros do not have that complex discriminatory ability.

The options are:

1. Reduce yaw gyro gain to the absolute minimum it needs, then it will do its least to oppose the turn.
2. Control the gyro gain from the rudder stick with a very steep fade-out so that a little move of the rudder stick cancels most of the gain and hold on that little rudder all the way around the turn.
3. Just accept that you have to hold on enough rudder all the way around the turn, to compensate for the gyro trying to stop the turn.

Start with option 1 and push it as far as you can, this may reduce the out of turn yaw to an acceptable level and then need no further action. Since 2 and 3 are pretty much the same and differ only by how much rudder you have to hold on, option 3 might be easier if you are not confident about programming a steep fade-out or do not have a channel to spare for the gain control. Also note that in 2 and 3 although you will be holding on rudder stick, the rudder will not have moved off centre in flight during the turn, as you and the gyro are balancing one another.

H

RE: Does gyro on rudder work against aileron?
 

Thanks for your answers gents,

adverse yaw was my first impulse too, so a little aileron differential would solve that until I realised the gyro might be the cause of this,

Maybe I can mix a few % rudder to the ailerons, so the gyro is not working during turns, to make it easier,

anyway there does not seem to be that much profit form installing a gyro so I tend to remove it, the only reason I put it in in the first place was because it was mentioned in the manual....
the plane does not show any 'fishtail' tendency's,

here a pic. of the Mig,

http://www.modelbouwforum.nl/forums/...testrun-16.jpg

Frank

RE: Does gyro on rudder work against aileron?
 

I'm not sure I agree with HarryC on this. The gyro should be damping the Yaw movements so, if you roll into a turn, the gyro will attempt to correct any adverse yaw. When established in the turn the gyro should simply damp down any deviations of yaw.

Are you sure that the gyro isn't in heading hold mode? This would cause the model to yaw out of the turn as it attempts to maintain its orginal heading.

RE: Does gyro on rudder work against aileron?
 

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RE: Does gyro on rudder work against aileron?
 

which is what I said


Exactly, and a turn is a constant deviation in yaw so the gyro will apply a constant opposite rudder all the way around the turn. Rate mode does not just react against the initial kick in yaw, it reacts to changing direction all the time, it only stops reacting when the heading change stops.

If you would like to see this in action, apply gyro to your rudder in rate mode not HH mode and turn the gain up to max so you can see it. Pick the model up, bank it as if in a turn and then slowly rotate on the spot to simulate it turning. Watch the rudder kick out of the turn and stay out of the turn for as long as you are rotating. As soon as you stop rotating the rudder will return to neutral but not before. the gyro does not react simply to the initial start of rotation, but to all rotation. It's the same reason that aileron gyros reduce your roll rate, they work against you the whole time and not just at initial acceleration.

H

RE: Does gyro on rudder work against aileron?
 

Harry,

I'm not sure your test is correct. When you turn the model like this, you are not accelerating it like it does in an actual flying turn. In a properly coordinated turn, there is no acceleration in the yaw axis, i.e., "the ball is centered." A gyro, if properly setup, should simply "step on the ball" and coordinate the turn if there is any adverse or proverse yaw. I've run gyros on rudder before and I've always turned the gain up until the model starts oscillating in yaw and then turned it down a bit. At this setting, I never had to reduce gain to get a coordinated turn.

I believe that siclick33 is correct on this.

Bob

RE: Does gyro on rudder work against aileron?
 

I would leave the gyro there for a while yet Frank. My F-86, similar layout, is fine in calm conditions but does fishtail in windy conditions. My guess is that its yaw damping is on the edge, and that the turbulence that comes with windy conditions upsets it and it takes some time to damp down by which time the next gust has upset it and so on in an endless cycle of being nudged again before the last one has damped down, rather like the escapement mechanism of a pendulum clock. As my first flights were done in mild conditions it seemed fine at first, it was only as I flew it later in stronger winds that the fishtailing became evident.

H

RE: Does gyro on rudder work against aileron?
 

Bob, if there is no yaw there is no turn. A turn is made by banking to incline the lift and the plane simply starts to travel sideways, on its own the bank does not change the direction the plane is pointing. The change of angle of airflow over the fin causes it to yaw into the new direction of travel. It's a constant smooth process all the way round the turn. Without yaw you have partial knife edge flight on a constant heading.

RE: Does gyro on rudder work against aileron?
 

In all the Mig's I have flown over the years I have never seen this if the gyro is set up correctly. It could have too much gain in the air or possibly is backwards with very low gain. I have seen both of these situations with gyros that were backwards or with way too much gain.

RE: Does gyro on rudder work against aileron?
 

Self contradicting! It is the yaw that centres the ball, it is the rate of yaw that provides the turning circle that acts at right angles to gravity and pulls the ball back up the slope. If you bank and use opposite rudder to prevent the turn and maintain a constant heading, e.g sideslip, where is the ball? Down at the bottom of its tube. To get the ball centred you must allow the plane to continually yaw around the turn.

Gyros on rudder do not produce balanced turns, what a slip ball measures is something different.


H

RE: Does gyro on rudder work against aileron?
 

A gyro is not a rudder command !

A gyro only stop angular speed variation, it is not an anti-sideslip device !

The gyro is made to containt a yaw movement when aileron are applied and adverse yaw appear => modification of angular speed value. When turn is well established, the aileron are back to neutral. No more adverse yaw, so angular speed is only the plane turn rate, and is constant as bank, and pitch actions, are constant.

Angular speed constant => gyro is at neutral or very low correction. (Gyro is not on heading lock mode)

Check this :
- Are you sure the gyro is doing the corrections the right way ??? :D (yes, dude solution, but who knows...)
- Are you sure you are not in heading lock mode => I think no, you plane should be crashed at this time.
- Does the gain is not too low for your plane => the gyro does not react as firmly as it should be when you are applying ailerons.
- Does your differential rate don't need to be tuned ?

As with aerobatic planes, you can perform a perfect sideslip pass for all runway long. Altitude constant, bank constant, sideslip constant, etc... That equal = your plane is perfectly stable = the gyro do nothing... Gyro is not an anti-sideslip device...

It is natural that all planes, gliders, and even copters, need small appliance of rudder during stabilized turns to have the "turn and sideslip indicator" stay to neutral (even with differential perfectly tuned), thus indicating you are performing a perfectly coordinated turn.

RE: Does gyro on rudder work against aileron?
 

HarryC did the same response at the same time... :D


Frank : Do you know that the photo of your Mig is just AWESOME !!!! [X(][8D]

RE: Does gyro on rudder work against aileron?
 

See http://www.flightlab.net/Flightlab.n...ngDynamics.pdf
page 3
"Banked turns are combinations of yaw and pitch
(Figure 2). Coordination (keeping the velocity
vector on the plane of symmetry) means
establishing both the yaw rate and the pitch rate
appropriate for the bank angle."

RE: Does gyro on rudder work against aileron?
 

I know that there is yaw in a turn - you are correct in that this (and pitch rate) is what produces the heading change. My issue is with what the gyro responds to and the magnitude of its response. I know that they are (when not in HH mode) "rate gyros," but I'm not sure that they produce an accurate representation of yaw rate in a coordinated turn. That is why, in an INS, you need a magnetometer in addition to the rate gyros to produce an accurate indication of heading.

Try this with a helicopter. Fly around with the gain set at a certain level in heading hold mode. You'll *have* to apply rudder commands during turns or the helicopter will be flying sideways, and eventually backwards - in spite of the (minimal) side force from the fins. Now switch to "rate mode" at the same gain setting. You can fly around all day long with the rudder at neutral and the tail will follow the direction of flight just because of the (again minimal) side force of the fins. Even at maximum gain (below oscillation), in "rate mode" the gyro will not significantly alter the yaw angle in a coordinated turn.

If setup properly, I have never experienced a "rate gyro" on the yaw axis commanding enough rudder control to un-coordinate a turn. Now I have, (unintentionally) flown a plane where the builder put the gyro into HH mode. That *did* produce uncoordinated turns (to a point, even with HH mode, the tail eventually followed the general direction of flight) and significant "crab" angles during straight flight after a turn.

Bob

RE: Does gyro on rudder work against aileron?
 

The gyro will be moving the rudder to oppose the turn. It cannot, not. In most cases the gain will be quite low so the maximum travel that the gyro can move the rudder is limited, and the rate of yaw in most turns is somewhat below the rate of yaw in fishtailing. The combination of low gyro gain and low rate of yaw in a turn compared to fishtailing means the rudder movement by the gyro in a turn will normally be very small, it may not be noticeable, but it will be there. If it isn't then the gyro is not working! If the gain is high, the rudder travel in a turn may produce a noticeable yaw outwards. That is why in my first post on this thread I gave option 1 as being to turn the gain down to the lowest value that works to prevent fishtailing as this may produce an out yaw too small to notice.

Harry

RE: Does gyro on rudder work against aileron?
 

MEMS rate gyros use Coriolis forces to measure angular rates about an axis, so you are correct in maintaining that a rudder gyro will command opposite rudder, even in a coordinated turn. However, as you also pointed out, the yaw rate in a turn (and the corresponding gyro-rudder command) will be significantly lower than in a yaw oscillation. The yaw rate in a turn is also affected by the bank angle - to the extent that in a 90 degree banked turn, there is 0 yaw rate - all of the heading change is due to pitch rate, so your statement that "if there is no yaw there is no turn" is technically incorrect.

From a practical standpoint, I, personally, have never seen a yaw (rudder) gyro who's gain is set below the point of oscillation, cause noticeable (from a ground-based observer's perspective) adverse yaw in a turn on a jet model. If simply reducing the gyro's rate solves the OP's problem, then I will stand corrected...

Bob

RE: Does gyro on rudder work against aileron?
 

Thanks again for all your input guys,

first I put the Mig on a 'test' stand to check everything, :)

http://www.mijnalbum.nl/GroteFoto-SKCP3B8Z.jpg

Everything looks alright: no heading lock, as I turned the plane left and right on the yaw axis, the rudder moves in the right direction.

But when I checked the gain setting it was a bit high: like 50% while the advice is to start with 20%

With this 20% setting I will make a couple of flights (probably next week on 'Jets over Pampa' in Belgium..) and if it still occurs; add some aileron differential


again, thanks for your help everybody, as a 'reward' another pic. of the Mig [8D] ;

http://www.modelbouwforum.nl/forums/...testrun-14.jpg

Frank

RE: Does gyro on rudder work against aileron?
 

I did wonder if someone would come up with that fallacy! In a level, balanced turn a bank of 90 degrees is physically impossible. Bank in a level balanced turn must always be less than 90 degrees so a yaw vector must always be present, no matter how small!

H.

RE: Does gyro on rudder work against aileron?
 

That is one ugly pilot ... looks like my mother!

RE: Does gyro on rudder work against aileron?
 

If it in fact level and balanced - both of which are near impossible to determine from a ground-based perspective. The point is, it *is* possible to change heading without any angular rate about the yaw axis - and the angular rate about the yaw axis vs. the rate of heading change *is* variable with bank angle...

Bob

RE: Does gyro on rudder work against aileron?
 

Ok Harry, I have a couple vodkas onboard, I will bite on this one! if there is ALWAYS yaw in a turn. Can you explain the yaw relationships in a coordinated turn, side slip and a skid?......Assuming it's an airplane turning and not a Mini Cooper or boat.

Tailwinds,

John

RE: Does gyro on rudder work against aileron?
 

Because of the moustache you mean? :)

http://www.modelbouwforum.nl/forums/...1-afbouw14.jpg

Frank

RE: Does gyro on rudder work against aileron?
 

A level turn is a combination of rotation about pitch and yaw axes. Imagine looking at your model from behind while it is banked over some amount and turning. There is no axis of rotation pointing vertically up at some angle through the model while it is banked, yet the turn is a rotation about that non-existent axis. If the turn was purely yaw around the yaw axis, you would see the nose go down due to the angle of bank. If the turn was purely pitch around the pitch axis, you would see the nose go up. So somewhere in between those is the turn, it is a combination of yaw and pitch rotations. In a very shallow bank it is almost purely rotation around the yaw axis. In a very steeply banked turn it is almost purely rotation about the pitch axis. If you want to picture it more precisely, the yaw fraction is the cosine of the angle of bank and the pitch fraction is the sine of the angle of bank. So as bank angle increases, the yaw fraction starts high and falls towards zero, the pitch fraction starts at zero and heads towards 1.

At 90 degrees angle of bank, the yaw fraction is zero and the pitch fraction is 1. But a level balanced turn at 90 degrees angle of bank is impossible, in reality it is impossible beyond some number quite well below that, so the cosine of the angle of bank, which is the yaw element, never reaches zero in a balanced level turn. However beyond 60 degrees angle of bank the yaw fraction is becoming small and falling rapidly. A yaw gyro will sense only the rate of the yaw fraction of the turn, not the entire rate of turn. So a very rapid turn can be a high rate of turn but if it is very steeply banked the yaw gyro will see very little to counteract. For example, suppose a little park flyer and a fast model jet are flying together and both make a rapid turn at the same rate around the same 360 degrees. The circles will be very different sizes due to the speeds but the yaw gyro does not sense that, only the yaw rate. Suppose both models are flown to turn the circle in the same time, so the park flyer is a shallow bank and the fast jet is banked very steeply. Now, both have the same rate of rotation around the circle. But the yaw gyro in the jet will record a low rate of rotation whereas the yaw gyro in the park flyer will record a very large rate of rotation. Both are rotating in the circle at the same rate, but the yaw gyro sees the rate multiplied by the cosine of the angle of bank. Therefore the gyro in the park flyer will kick far more against the turn than the one in the more steeply banked jet, despite both turns being the same rate.

PS. Frank, that is a super model.

H.