Help....INS giving me headache!!

[avrodamo]

Im into my revision for my Navs and when covering instruments it's become really clear that i don't really have a grip on INS systems.What is Schuler tuning all about. It just does not seem to make any sense. Does anyone have any tips, know any good text books that would explain it clearly, know anything that might help ???
How much emphasis and depth is incorporated into the exam in relation to INS ??
Any help much appreciated <img src="rolleyes.gif" border="0"> <img src="confused.gif" border="0"> <img src="rolleyes.gif" border="0"> <img src="confused.gif" border="0">

[GoneWest]

Damo...I came to read this message because I thought you were having trouble getting a visa!!

[redsnail]

All I bothered remembering for the exam was that Schuler tuning equates to a bounded error with a time period for the oscillation of 84.4 minutes. Strictly speaking pertains to the INS but they have the Schuler period incorporated into the IRS.

For more info I would read Pallet's Avionics and Associated Instruments. ( I can't remember the exact title. It's in the UK and I am in Oz)

[MorningGlory]

Good advice from Reddo, the Instruments and GenNav exams were packed with INS/IRS when I did them.

[JJflyer]

I´ve gotten away using INS systems for the last 4 years with out ever needing to use that information. <img src="confused.gif" border="0"> Is this for some JAR stuff you are studying for?

[Paul Hickley]

This is not an easy subject to explain in a couple of paragraphs on a website like this but, in broad terms, you need to know about bounded errors (ie, Schuler tuning) for the JAAs. At a practical level, on older INS systems (not usually modern IRSs) it can be helpful in understanding fix error growth patterns over a period of time, but is usually unnecessarily sophisticated for the average working pilot (not intended as a patronising remark - simply a statement of the truth).

Broadly, the reasoning is this: You have to keep the INS accelerometers level with respect to Earth gravity because, if you don't, they will pick up a small component of gravity and misinterpret this as lateral acceleration. You can't use gravity to reference the vertical and keep the platform level because you're trying to use it to measure accelerations - so you can't use as a datum the thing that you are trying to measure.

So you have to keep the accelerometers level to Earth gravity by using gyros. The trouble is, gyros are rigid with respect to inertial space, not to an Earth-based frame of reference. As you travel over the Earth, the direction between you and the centre of the Earth (ie, the gravity vector) changes. So you have to tell the gyro this. It's called the vertical component of Earth rate (or Profile Rate, in some books). The way you correct the platform/gyro is to have a feedback circuit from the velocity integrators which keeps the gyros level to Earth. It's called the V/R loop (for latitude) or the U/R loop (for longitude).

However, if the initial conditions leading to the calculation of the velocity are in any way in error (which they are on nearly every flight, to a very small extent), then the V/R loop oscillates - at a period of 84.4 minutes, giving you a position error (usually up to a couple of miles or less) which will never increase to beyond its maximum amplitude, and then repeats itself in a perfectly sinisoidal pattern every 84.4 minutes.

Incorporating a V/R feedback loop into the design of an INS/IRS to keep the platform level is called 'Schuler tuning' - a fancy buzzword which, in my opinion, tends to confuse.

I hope you didn't find that too difficult. Post again if you didn't understand any of it.

All the best,

Paul

[ 30 December 2001: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]

[ 30 December 2001: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]</p>

[nice_beaver]

Redsnail is absolutely correct, plus the chance of a question on INS in the instrument exam is very very low!

[Paul Hickley]

Not true, actually, Nice_Beaver. Check the feedback. You tend to get about 2 -3 INS /IRS questions in each instruments exam. You tend to get about 3 pure INS / IRS and about 6 - 8 overall on INS / IRS / FMS in Gen Nav.

I don't teach this stuff to make me look good or sound clever. I teach it to get my students through the JAA exams (and be more knowledgeable pilots later). If it weren't what was required, I'd spend the time on something that was required. At Oxford we check the feedback and find out what they're actually asking. They ask this stuff.

All the best

Paul

[ 30 December 2001: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]</p>

[The man formerly known as]

Thanks Paul Hickey, I never managed to understand Schuler Tuning until I read that explanation.

Luckily I managed to answer enough of the other questions correctly to get away without it.

I still don't understand why the oscillation is exactly 84.4 minutes. How is the error so precise if it is an error. Does 84.4 minutes correspond to a pendulum length of the diameter of the earth??

[Paul Hickley]

To: The man formerly known as - Yes, it does correspond to the length of an Earth pendulum (radius, actually, not diameter) - but, on its own, that's not much of an explanation - although that's usually all you get in most ATPL textbooks.

If you do a mathematical analysis of the V/R and U/R feedback loops, you come up with a differential equation that defines the feedback term. Since you're using the radius of the Earth, that's one of the constants, and another is the value of gravity. If you know something about control engineering, you can show that the oscillation is a form of Simple Harmonic Motion and that the period can be calculated from the equation:

T = 2 pi x Square root (R/g)

where R is the radius of the Earth (3437.7 nm) and g is 32.2 feet per second per second.

Substitute that lot into the above equation and you'll find it comes to 84.36 minutes.

It so happens that a pendulum is also an example of SHM and has the same defining equation. Some people often explain platform levelling carried out by the V/R feedback loop in term of a pendulum analogy, though I think that, unless it's done well, it generally tends to confuse.

The pendulum analogy explanation goes as follows:- Imagine your INS platform bolted at right angles to a pendulum which runs from your aircraft to the centre of the Earth. Then, as you travel over the Earth, the pendulum bob will stay in the centre of the Earth, so your platform will stay horizontal to the Earth's surface. That's V/R loop levelling. But if you start with the pendulum out of true, it will swing with a pendulum period of:

T = 2 pi x square root (L/g)

But in this case, L is Earth radius, 3437.7 nm - so a pendulum would behave exactly like an electro-mechanical V/R loop.

I actually prefer to tell my students the proper explanation. Personally, I think it's easier.

Don't get the idea that I think you need this level of depth to be an airline pilot. I don't. Nor is this kind of mathematical approach right for every student. But you did ask.

All the best

Paul

[ 01 January 2002: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]

[ 01 January 2002: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]

[ 01 January 2002: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]</p>

[spitfire747]

just a quickie, i am intending to start the ATPLs this year. I am a PPL but reading this has blown my brains out and made me think, how the bl**dy hell can i understand this, will I ever understand this, CAn i ever understand this.

Please tell me that it all makes sense when it is taught to you over a period of 6 months, PLEASE <img src="smile.gif" border="0">

[The man formerly known as]

Paul,

Thanks again, I think I understand now.

I agree with you its a bit OTT to expect pilots to understand all of this.

Spitfire,
This is one of the more obscure bits of the ATPLs I could never understand and Paul is the first person I have come across who understands it. The rest of the stuff you need to learn is more obvious but it is detailed. The way I look at it is if you have a PPL and you want to pass the ATPLs then you probably will.

Good luck.

[spitfire747]

Thanks alot, got a bit worried it was all like that, phew..... :-)

[avrodamo]

Many thanks for your replies. Me thinks i may be reading a bit tooooo much into this section. I can calculate the error at the destination, and i understand how it works. The Schuler bit just freaked me out a bit.
Spitfie: Myintention was not to make you scream and run!!! Its hard going, but appears to me to be the huge volume of information, and not the difficulty. It is hard, hard work though. Rewarding though when it starts coming together !!

[The man formerly known as]

Paul,

One more question, who was Schuler????

[Paul Hickley]

Dr Maximilian Schuler was a German scientist and engineer who published a paper in 1923 on the accelerations experienced in ships' gyrocompasses due to the motion of the sea and showed that they could be eliminated by the use of an electro-mechanical analogue of an Earth pendulum. When the new science of Inertial Navigation emerged in the early 1950s, the concept of Schuler feedback,which had been developed thirty years earlier, was incorporated into the first INs.

Paul

[David Webb]

Paul is quite correct.

Maximillian Schuler was a German naval engineer who was involved in engineering theory and experiments as to how to keep a ship's gimballed compass (the ancient brass and heavy systems that were used from about the beginning of the eighteenth century) level despite rolling and pitching movements of the ship. He reasoned that if the compass suspension system was hooked to a pendulum (mass not calculated) the mass of which was placed at the centre of the mass of the earth, then the gimbal and compass system would remain level. Unfortunately it proved impossible to place a pendulum mass at the centre of the earth! Still, 2pie root l/g is about 84.4 minutes at earth average radius. Modern Schuler frequency curves account for distance above earth surface using, normally, baro-inertial height loops

Happy new year, David