VOR errors?

kfischer11

kfischer11

New Member
What causes VOR rodor modulation error? I know it has to due with specific prop settings but what exactly is affecting the signal from the VOR?
 
Propeller/rotor modulation error:

Certain propeller revolutions per minute (RPM) settings or helicopter rotor speeds can cause the VOR CDI to fluctuate as much as plus or minus 6°. Slight changes to the RPM setting will normally smooth out this roughness. Pilots are urged to check for this modulation phenomenon prior to reporting a VOR station or aircraft equipment for unsatisfactory operation.

Instrument Flying Handbook.

I cant find anything on the actual cause either... The VOR works off of phase differential. Maybe the prop/rotor produces a frequency that interferes at a specific RPM?
 
I can't believe I've never heard of this before.
'Oh i'm sure somebody told you sometime way back when..'
Nope.
This is wild.
 
The VOR signal is modulated 2 ways (FM and AM) at 30 Hz (1800 cycles per minute). Therefore, if you're running 1800 RPM and the signal is coming from in front of you (through the prop) that can throw off the signal (probably the AM portion of it, I would imagine the FM portion is relatively unaffected) resulting in wild CDI swings.
 
Roger said:
The VOR signal is modulated 2 ways (FM and AM) at 30 Hz (1800 cycles per minute). Therefore, if you're running 1800 RPM and the signal is coming from in front of you (through the prop) that can throw off the signal (probably the AM portion of it, I would imagine the FM portion is relatively unaffected) resulting in wild CDI swings.
Click to expand...

There was a day when I knew the RPM and frequency of the VOR. I don't recall ever hearing or reading this phenomena even though i have read the IFH cover to cover. That said, this is pretty cool and thanks Roger.
 
It's one of the things I remember from my Avionics class in A&P school. We actually learned a block diagram of a VOR receiver system, among other things. It was a fascinating class to someone like me who is good with electrical stuff. For many of the others, it was deadly dull and also difficult. The other thing I remember from it is that you use a 3/32 Allen wrench to remove most radios.
 
You know, I've noticed some fluctuations out towards the edge of the service volume, and I only really notice them when the prop deice is on...eeeenteresting.
 
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