irs ring laser gyros

[flybubba]

Question is do all modern irs systems use some form of ring laser gyro or fiber optic gyro? I believe these systems also have solid state piezo-electric accelerometers, but I thought they also need the rlgs.

[M2dude]

Every modern IRU, ADIRU or SAARU that I've come across uses either RLG's or Fibre Optic LGs.
They are all strapdown units, with NO stabilised platform or any such thing.

Dude

[welliewanger]

They have two types of component, ring laser gyros and accelerometers. One of each for each axis. The RLGs sense the attitude of the plane and the accelerometers sense which way (and how fast) it's accelerating. From these six parameters the IRS extrapolates attitude, position and speed.

[Obi Offiah]

Assuming an aircraft flies into a head wind, it's airspeed will increase but it will actually be decelerating. Does this pose any problems for any of the aircrafts systems?

[M2dude]

welliewangler
errr yes, you'll need accelerometers as well. On a per axis basis a single integration WRT gives you velocity and a second integration gives you displacement. And as a matter of point, all RLGs and FOGs are essentially rate gyros also.
What really is fascinating is that the Honeywell ADIRU fitted to the Boeing 777 has the equivalent integrity of a triple IRS but instead of using 9 RLGs and accelerometers it has only 6 of each. The integrity comes from having non-orthogonal axis', so in theory even the loss of more than one axis has no operational effect. Amazes the hell out of me.
Dude

[welliewanger]

Dude, there's gonna be some clever maths going on in a system like that! Very interesting.
/geek_mode

[flybubba]

So basically, the RLGs sense angular or rotational accel, alpha, while the linear accel is sense by the accelerometers (piezo, or otherwise)?

[ChristiaanJ]

flybubba,
The gyros sense angular rate, the accelerometers sense acceleration.
In a basic system, that's three gyros and three accelerometers for each of the X, Y and Z axes.
Whether they're ring l@ser gyros, fiber optic gyros or the good old spinning tops is only a matter of technology, and totally irrelevant to the principle; same for the accelerometers.

CJ

[M2dude]

The Delco Carousel 4A system did not use rate gyros and I am fairly certain that the pre-Type 82 Litton systems did not either. RLGs and FOLGs are rate gyros in concept (the outputs can only ever be proportional to angular rate) but what is basically required for an inertial system is angular DISPLACEMENT and not rate. The IRU/ADIRU software integrates the rate output to obtain the required angular displacement.
The accelerometer principle however I agree is identical.

Dude

[flybubba]

So the RLG measures angular rate or rotational rate (not accel) and then one must integrate to get angular displacement (or angle change). This is in contrast to accelerometers which one must do the double integration to get distance.

[M2dude]

Yes that is 100% correct sir. Actually we are looking at displacement from an angular datum.

Dude