A CFII applicant I was talking to wanted to know exactly how an RMI works. I realized that I have no idea how an RMI actually works in-depth, I just know how to use it! I've found a few installation diagrams from Rockwell Collins describing the systems that input information to the RMI, but haven't found anything telling me how the instrument interprets all the various data inputs. Does anyone older and wiser than me have anything better?
How does an RMI really work?
- Thread starter pwttogfk
- Start date
CoffeeIcePapers
Well-Hung Member
Are you talking about the salved gyro portion or the VOR and ADF portion?
Houston
Well-Known Member
I cannot give you a good technical answer such as you are seeking. However, I would suspect that the VOR receiver senses the current radial and presents is on the RMI. With the standard VOR head, the VOR receiver is looking at the OBS and responds with the CDI to indicate "this is it" or "this is close to it" or "we're nowhere near it". So, the RMI is merely a different presentation device for the same information the VOR receiver has.
A related subject is the ARC (Automatic Radial Centering). With that device, the pilot can pull or push on the OBS knob to have the OBS spin around until the CDI is centered TO or FROM. Again, this shows that the VOR receiver is sensing what the correct radial is, and just using another method for display.
If you visit an avionics shop and ask the question, I'm sure you'll get a much better and more specific answer. For the rest of us, it's more a matter of just believing in the magic.
A related subject is the ARC (Automatic Radial Centering). With that device, the pilot can pull or push on the OBS knob to have the OBS spin around until the CDI is centered TO or FROM. Again, this shows that the VOR receiver is sensing what the correct radial is, and just using another method for display.
If you visit an avionics shop and ask the question, I'm sure you'll get a much better and more specific answer. For the rest of us, it's more a matter of just believing in the magic.
gotWXdagain
Polished Member
NAV Radio = Receives signal from VOR
OBS Gauge = Interprets signal received by NAV Radio to decipher your location relative to a preselected course
RMI Needle = Just displays what radial the NAV RADIO is receiving. Basically, its a NAV RADIO display in raw data form.
That is, unless you have a G1000, then its super scientific hampsters spinning wheels in the aft avionics bay.
OBS Gauge = Interprets signal received by NAV Radio to decipher your location relative to a preselected course
RMI Needle = Just displays what radial the NAV RADIO is receiving. Basically, its a NAV RADIO display in raw data form.
That is, unless you have a G1000, then its super scientific hampsters spinning wheels in the aft avionics bay.
Mike H
Well-Known Member
BWAHAHA, everybody knows there aren't any moving parts in the G1000 other than the knobs & buttons. Those hamsters are just along for the ride
gotWXdagain
Polished Member
The same way an OBS gauge does? or to put it more simply... who cares?
Crockrocket94
Well-Known Member
What happened to him? Did he just get fed up with bs or ban?
Douglas
Old School KSUX
Honestly I don't remember the whole deal now, I think there was some static coming his way and he put it down and walked.
Not at all. He was a good resource.
All people appreciate smart.
All people appreciate confidence.
Some can't handle smart + confident.
Der_Meister
Well-Known Member
Phase Shift, that is how it knows. Just like a CDI with a VOR. The RMI just Points TO the station on what ever Radial it is receiving; kind of like a ADF.
R
Roger, Roger
Guest
There was a thread about this a while ago, I think we sort of came to a conclusion... Don't know if I can dig it up.
Bite your tongue you SNAP.
A multitude of people got on him over being overly technical and his nit picking being unnecessary Which, incidentally, is absolutely necessary in the sciences and technicalities of any field. He wasn't the only one that fell off the map after that incident, I just don't have the brain power to be remembered for; as that man has.
That said, and I've seen this trend continue as I have been still surfing these forums ever since, there is too much opinion and ego without enough hard evidence. Apparently this forum used to be about something different, namely factual evidence and a general attempt to gain knowledge. For some of us, that is the only reason we participate. Without it, why bother?
As for the question..."Direction finding (DF) refers to the establishment of the direction from which a received signal was transmitted."
An RMI is an A/RDF (automatic/radio direction finder) overlaid onto a directional gyro. 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.