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Does gyro on rudder work against aileron?

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Does gyro on rudder work against aileron?

Old 07-14-2011, 12:19 AM
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lozza1965
 
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Default RE: Does gyro on rudder work against aileron?

Hey Curzon get and do some work, thats what your paid for
Old 07-14-2011, 12:22 AM
  #27  
HarryC
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Default RE: Does gyro on rudder work against aileron?


ORIGINAL: lozza1965

Hey Curzon get and do some work, thats what your paid for
What RIAT ticket?
Old 07-15-2011, 08:58 AM
  #28  
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Default RE: Does gyro on rudder work against aileron?

ORIGINAL: HarryC

ORIGINAL: cactusflyer
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.
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.
OK So how come my coffee stays steady in the cup and there is no perception of a turn in a coordinated turn, but in a slip or skid the brew sloshes around to one side of the cup or the other and I can feel the yaw rate? I no nothing of the "non-existent" axis...............Fifth Dimension maybe?

I think you are confused about what the yaw axis is and you are using it to describe the rate of turn. The yaw axis is perpendicular to the pitch axis and therefore varies with bank. The airplane can be in a turn with no yaw rate sensed in the cockpit and hence sensed by the gyro.

Tailwinds,

John
Old 07-15-2011, 10:32 AM
  #29  
HarryC
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Default RE: Does gyro on rudder work against aileron?

It's you who is confused John. If you read my post it is quite clear I gave a very powerful differentiation between rate of turn and yaw rate, showing how yaw rate can fall close to zero in the tightest of turns. Your claims about balanced turns are about a totally different thing since it involves the balance between centripetal and gravitational acceleration.
H
Old 07-16-2011, 06:35 AM
  #30  
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Default RE: Does gyro on rudder work against aileron?

Harry C - I know as Catusflyer has stated that if I am in an airplane and in a cordinated turn I feel no yaw in the seat of my pants. I have no experience with model gyros but I think the gyro is mounted such that it also would not sense any yaw in a corridinated turn.
Old 07-16-2011, 08:11 AM
  #31  
HarryC
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Default RE: Does gyro on rudder work against aileron?

You are mistaking yaw with yawing out of balance. Yaw is any rotation about the vertical axis. Clearly when a plane changes heading there is a rotation about the yaw axis. Yaw is not just when yawing out of balance. Also you are confusing yaw with side force, the "seat of your pants" feel is sideways acceleration due to slipping or sliding whilst not yawing enough or yawing too fast.

H
Old 07-16-2011, 09:59 AM
  #32  
MX240
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Default RE: Does gyro on rudder work against aileron?


ORIGINAL: HarryC

ORIGINAL: rhklenke
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.
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.
I agree with Harry C.
I have used a rudder gyro on my warbirds for twenty years when you use to hear the tiny gyro spinning away inside the black box.
When I make a LEVEL turn with the gyro still on everything is normal.
When I happen to let the nose drop in a turn with the gyro on I can see the gyro giving me TOP RUDDER and the turn feels like st. This is why I turn it off after takeoff and back on for the landing. On some birds I seem to be able to have a low rate of gain on all the time...some others I can't.
And I never use heading hold.
Jim
Old 07-18-2011, 02:36 AM
  #33  
HarryC
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Default RE: Does gyro on rudder work against aileron?

There seems to be some major misconceptions about what a yaw gyro does, in particular a belief amongst some of you that it will balance turns. It will do no such thing, indeed in many cases a yaw gyro will try to put the model out of balanced flight, and keep it out of balanced flight.

Imagine your model flying along, in balance. You add left rudder to skid the model left. You release the rudder. The fin tries to yaw the model to the right, back into the airflow. The yaw gyro senses this and applies left rudder to stop it. So the gyro tries to prevent a return to balanced flight. Now add a little bit of left bank. The model starts to slip to the left and the fin tries to yaw the model so that it is pointing into the new direction of travel. That is a turn, wings change the direction of travel and a combination of yaw and pitch change the direction of pointing. As soon as the model starts to yaw to point the same way it is going, the yaw gyro senses this and applies opposite rudder. It thus tries to put any turn out of balance. Next imagine being hit by a gust from the left. The model should yaw slightly to the left to keep the airflow head on. That would keep the slip ball or yaw string centred, and thus in balanced flight. But the yaw gyro will try to stop that, resulting in the gust causing a sideways acceleration which will be shown by the slip ball or yaw string moving away from centre, thus once again the yaw gyro is causing unbalanced flight.

Why should it try to unbalance the flight? Simply because it is not designed to balance the flight! Balance is matching the angle of bank to the resultant of the 2 translational accelerations of gravity and any horizontal motion in the plane of the lateral axis. A gyro is an angular motion sensor. An angular motion sensor can’t sense translational effects, can’t sense accelerations, can’t sense angle of bank. Since a gyro can’t sense any of the requirements of balanced flight, it is not going to create balanced flight.

Since the yaw gyro will try to yaw the model out of the turn, and swept wings have a strong dihedral effect, the yaw gyro will have the secondary effect of reducing the roll rate and even trying to level up the model. Therefore you need to use the lowest gain that is required to stop the fishtailing, or to reduce it to an acceptable level. A higher gain is wasteful and increases the unwanted effects.
Old 07-18-2011, 03:18 AM
  #34  
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Default RE: Does gyro on rudder work against aileron?

Having followed this thread from the beginning certain things have become apparent.

Harry is a very knowledgeable guy with an excellent grasp of theoretical and practical aerodynamics but, from where I sit does not appear to be getting to the root of the problem. I.e. a suggestion to eliminate yaw hunting or dutch roll.

My practical answer is to remove the gyro from the rudder and just allow it to control the nose wheel. Second is to fit a gyro to the aileron control because from practical experience with my DeHavilland 110 which has a swept back wing like the Mig and has suffered this ugly dutch roll for a long time has been completely transformed following the fitment of an ACT Wing gyro, the yaw hunting has completely disappeared.

I did consider a yaw gyro as watching the plane fly it definitely appeared the problem was yaw induced but, as the other jets I fly all have roll damping I thought I would try that first. It may well be I have cured the second problem first but because the model can no longer roll at will the tail cannot wag the dog.

m

Old 07-18-2011, 03:41 AM
  #35  
HarryC
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Default RE: Does gyro on rudder work against aileron?

Mick, in the other great thread about gyros. I think it was entitled the ABCs of gyros, I said that although the problem is started in yaw, and stopping the yaw will stop the rolling action, the roll reaction can be much higher than the yaw action and thus it can be more profitable to start with an aileron gyro and stop the very noticeable rolling reaction, when the fishtailing is of low amplitude. With very highly swept wings, the gain from yaw to roll can be high so the slightest fishtailing which may be below noticeable level, can produce a noticeable wing rock. In those situations it can be hard to eliminate the fishtailing sufficiently to stop the wing rocking. If only fishtailing is obvious use a rudder gyro, if wing rocking is obvious use an aileron gyro first, and then only consider a rudder gyro if a noticeable fishtailing remains after the wing rocking has stopped.

The full size use yaw dampers which is a sophisticated version of our rudder gyro to kill dutch roll. But when you sit in a full size you feel any slight fishtailing and it is not comfy, whereas slight fishtailing on a model that is too small to be noticed from the ground is not a problem to us, hence for us the cosmetic value on the flightpath of the aileron gyro has more appeal.

H
Old 07-18-2011, 06:19 AM
  #36  
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Default RE: Does gyro on rudder work against aileron?


ORIGINAL: HarryC

Mick, in the other great thread about gyros. I think it was entitled the ABCs of gyros, I said that although the problem is started in yaw, and stopping the yaw will stop the rolling action, the roll reaction can be much higher than the yaw action and thus it can be more profitable to start with an aileron gyro and stop the very noticeable rolling reaction, when the fishtailing is of low amplitude. With very highly swept wings, the gain from yaw to roll can be high so the slightest fishtailing which may be below noticeable level, can produce a noticeable wing rock. In those situations it can be hard to eliminate the fishtailing sufficiently to stop the wing rocking. If only fishtailing is obvious use a rudder gyro, if wing rocking is obvious use an aileron gyro first, and then only consider a rudder gyro if a noticeable fishtailing remains after the wing rocking has stopped.

The full size use yaw dampers which is a sophisticated version of our rudder gyro to kill dutch roll. But when you sit in a full size you feel any slight fishtailing and it is not comfy, whereas slight fishtailing on a model that is too small to be noticed from the ground is not a problem to us, hence for us the cosmetic value on the flightpath of the aileron gyro has more appeal.

H
Interesting thread, had to think a few seconds before I got it. Harry C does understand the dynamics. At first glance the yaw 'seat of pants' effect is counter intuitive. But physics is physics! Using an INS in an autopilot the software would have to take care of this since yaw and pitch start to mix during bank. If 90 deg bank was possible without departing level flight pitch would be yaw and yaw would be pitch in world coordinates?
The dutch roll effect and yaw roll coupling would determine how severe the "yaw" effect is I suppose.
Wonder if the likes of ACT gyros could be set to respond to only the rate of rate-change? That way a prolonged turn would not be affected, only oscillations?
Old 07-18-2011, 06:42 AM
  #37  
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Default RE: Does gyro on rudder work against aileron?

ORIGINAL: Moerig
If 90 deg bank was possible without departing level flight pitch would be yaw and yaw would be pitch in world coordinates?
In "world co-ordinates", yes. But it gets confusing trying to switch back and fore between the aircraft and the world, so we stick to using the aircraft's frame of reference. As mentioned earlier, the overall change of heading is broken into the two components in yaw and pitch, the component in the yaw axis is cosine angle of bank and the component in the pitch axis is sine angle of bank. The rate of angular motion seen by the yaw gyro is therefore rate of turn times cosine angle of bank. At 60 degrees of bank cosine is 0.5 so the gyro sees a rate of rotation only half of the rate of the aircraft's rate of turn. This in effect gives a change in the ratio of fishtailing to turning, since any fishtailing continues in the yaw axis but the rate of turn seems to the gyro to be less and less as bank increases, so it maintains its full fishtail sensing but very little turn sensing.

The point at which the rudder gyro will oppose you the most is in small angles of bank, with high turn rate which means slow speed. In other words times like turning finals are most likely to encounter the rudder gyro trying to counteract the turn with opposite rudder.

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