How does an RMI really work?

Houston said:
The question is somewhat analogous to analyzing the workings of a thermos jug. When you put hot things in it, they stay hot. When you put cold things in it, they stay cold. Yet there are those will ponder how it knows which is which.
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A prime example of what my above post, regarding tgray and why he/we left, refers to. No offense houston, you played no part in it. Though you provide me with great example...sorry :)

It was this sort of thinking, and the personal attacks for his bold disagreements with such thinking, that was felt to be supported by the masses during the time of his departure. This is what drove him and myself out.

Seeking the why and how of everything is what makes the world exciting. Frowning on seeking such knowledge is analogous to ignorance; or the seeking of. Not something we, or anyone, should condone in my eyes. Especially in a field with such a variety of technical knowledge to be sought.
 
shdw said:
A prime example of what my above post, regarding tgray and why he/we left, refers to. No offense houston, you played no part in it. Though you provide me with great example...sorry

It was this sort of thinking, and the personal attacks for his bold disagreements with such thinking, that was felt to be supported by the masses during the time of his departure. This is what drove him and myself out.

Seeking the why and how of everything is what makes the world exciting. Frowning on seeking such knowledge is analogous to ignorance; or the seeking of. Not something we, or anyone, should condone in my eyes. Especially in a field with such a variety of technical knowledge to be sought.
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No offense taken (I think), although I did get lost in what you were trying to say.

The reference to the thermos jug is really an old, old joke and I just added it with the hope of bringing a little humor into the discussion. Apparently my intentions were misunderstood, and that would be my failure for not expressing myself clearly enough. Even with emoticons, it can be difficult to convey the twinkle in the eye or the wry smile that goes with an attempt at tongue in cheek humor when we are face to face.

If you look earlier in this thread you'll see my best effort at trying to make a serious response.
 
shdw said:
It works just like your ADF, it doesn't interpret the variable signal from the VOR. There isn't a need for it to do so. It simply points to the fixed transmitted signal, the same signal you use to identify the VOR.

The only reason you know what radial you're on is because of the directional gyro underlaid in the instrument; which is an advancement over the ADF in that you don't need to do the math. Though when I was taught/taught ADF interpretation the best way to interpret them is to artificially overlay them onto your DG. Also avoiding the math.

Wiki radio direction finder for a good read, though mostly geared toward the AM signals from NDBs and not the VHF (FM range) frequencies used by our VORs.
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That is not true, at least I do not think so for the majority of VOR RMIs. The reason i believe this is that I recall reading in an FAA publication (probably the instrument flying handbook) that an RMI tuned to a VOT will point at 180. This would be the case only if the RMI is translating the phase relationship rather than working as a DF.

Edit: found it. Instrument flying handbook page 7-16.
 
When I learned instruments in the T34 we didn't have a CDI or a HSI we had a RMI, later in the fleet we had a bdhi. Truth is it is not a dead technology an it is much easier to fly a dme arc with a RMI, imho than with a HSI/CDI. No need to twist ten turn ten yada yada yada. One of the habits I get into flying glass (green needles) or the G1000 is to bring up RMI needles on an hsi. You will never ever miss a turn at the FAF or lose situational awareness with that instrument. Remember you are at the tail of the RMI needle . There is an old radio nav trick to, that you can fly off radials fix to fix by using the pencil method on the RMI...

As far as the tech behind it, a VOR measure the phase difference which is based on the 360 degrees. So it works just like any other vor, and the ADF works like any gyro equipped movable card ADF.


Also imo it is easier to fly a vor approach using a RMI.
 
shdw said:
A prime example of what my above post, regarding tgray and why he/we left, refers to. No offense houston, you played no part in it. Though you provide me with great example...sorry :)

It was this sort of thinking, and the personal attacks for his bold disagreements with such thinking, that was felt to be supported by the masses during the time of his departure. This is what drove him and myself out.

Seeking the why and how of everything is what makes the world exciting. Frowning on seeking such knowledge is analogous to ignorance; or the seeking of. Not something we, or anyone, should condone in my eyes. Especially in a field with such a variety of technical knowledge to be sought.
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[offtopic] My only deal on this was time and place. There was a time and place for such detail, as it didn't apply in all operations at all times. Similarly, there was a time when such detail did have to be known, as there were certain times and places where it was required. It just depended. The problem was when people were staunchly one side or the other, without recognizing the need for both sides. [/offtopic]
 
Doug Taylor said:
Not at all. He was a good resource.

All people appreciate smart.

All people appreciate confidence.

Some can't handle smart + confident.
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smart + confident = listo (clever) bordering on listillo (Cliff Clavin Clever)
also
smartass =listillo

Fine line
One that I consciously have to fight over the years, or only my mother would like to talk to me.

I am not talking about anyone in particular, just pointing out an annoying trait.
 
Houston said:
If you look earlier in this thread you'll see my best effort at trying to make a serious response.
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I did read it. :) Like I said, it wasn't meant to attack you. Merely an example of the type of post that was targeted with malicious intent and not as a joke some time back when tgray departed this forum.
 
Roger said:
That is not true, at least I do not think so for the majority of VOR RMIs.
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True if the RMI is set up with a VOR receiver. Though I'm not sure if they are ever installed without both a VOR and ADF receiver. Maybe you can answer this? I was commenting to an RMI with the assumption that only an ADF receiver/antenna combination was being used. In the later case it can still be tuned to a VOR, but the ADF is not designed to interpret the phase angle such that the VOR receiver does. The ADF can still read the 30 Hz reference signal and point to it, assuming you could tune to it.

Is the OP looking to know how the phase angle is interpreted? If so, some more information can be found here: VHF omnidirectional range - Wikipedia, the free encyclopedia .. particularly section 1.3 and, well beyond my knowledge now, section 2.
 
shdw said:
True if the RMI is set up with a VOR receiver. Though I'm not sure if they are ever installed without both a VOR and ADF receiver.
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I think that is the default setup, although I never used one.

bdi.jpg
 
shdw said:
True if the RMI is set up with a VOR receiver. Though I'm not sure if they are ever installed without both a VOR and ADF receiver. Maybe you can answer this? I was commenting to an RMI with the assumption that only an ADF receiver/antenna combination was being used. In the later case it can still be tuned to a VOR, but the ADF is not designed to interpret the phase angle such that the VOR receiver does. The ADF can still read the 30 Hz reference signal and point to it, assuming you could tune to it.

Is the OP looking to know how the phase angle is interpreted? If so, some more information can be found here: VHF omnidirectional range - Wikipedia, the free encyclopedia .. particularly section 1.3 and, well beyond my knowledge now, section 2.
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I think your understanding is flawed.
Your typical RMI has a VOR needle (usually tuned through Nav1, though sometimes selectable between Nav1 and Nav2), and an ADF needle, tuned through the ADF head. The VOR needle is driven by the phase difference between the fixed and variable phases, as evidenced by the fact that if tuned to a VOT it will always point to 180. How the electronics work to actually point the needle, I don't know, but I suspect that somewhere in the process the difference in phase angle gets rectified into a DC voltage. It does NOT work by standard VHF DF, as it would then point towards the VOT. The ADF portion works the same as any ADF.
 
Roger said:
Your typical RMI has a VOR needle and an ADF needle
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There is my error, well assumption anyway. I've only ever used a single needle RMI; that is, it only had an ADF needle. I knew of the dual needle, apparently typical, setup. I've just never flown with it.
 
shdw said:
There is my error, well assumption anyway. I've only ever used a single needle RMI; that is, it only had an ADF needle. I knew of the dual needle, apparently typical, setup. I've just never flown with it.
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It can, as a matter of fact even have a TACAN needle. Perfect instruments for approachs with DME ARC or near perfect SA around a VOR/DME or TACAN navaid. What you described I would casually call a movable card ADF. What I don't understand, if you have never flown an aircraft equipped with one, and your breadth of experience is limited; Why o why would you intentionally flame a guy with beacoupe experience for not explaining a basic instrument in inane detail?
 
granlistillo said:
What I don't understand, if you have never flown an aircraft equipped with one, and your breadth of experience is limited; Why o why would you intentionally flame a guy with beacoupe experience for not explaining a basic instrument in inane detail?
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How to use it: Need to know (if using one)

The minutia of how it physically operates within the instrument itself: Nice to know.
 
SpiraMirabilis said:
Seriously. It is no more a dead technology than the HSI is. In many cases doing a VOR approach with the RMI is a lot easier in any case.
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Fully agree. The RMI is an amazingly easy and useful instrument; one of the reasons being as you point out above, as well as what gran points out about DME arcs.

Dead technology it is not.
 
RMI Training USN/USAF: Fly Directly from the 090R at 10 DME to the 030R at 25 DME. Completion standard 1 NM graded with RNAV
RMI Training JAA: Please be prepared to design, manufacture and service a RMI. Please plot the phase differences between the two signals for a given radial. Be prepared to listen to the Captain on how to fly a DME arc with a RMI.
RMI Training FAA: WTH is an RMI?
 
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