DI accuracy
Join Date: Jan 2002
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Just out of interest, how does a slaved HSI work? Does it have its own internal mag. compass?
I ask because some of the Warriors I fly have these and some don't. I always turn them to manual because I 'don't trust' the automatic ones. I'd rather align it with the compass myself, as per my training. Perhaps understanding how it works would help me overcome this. I asked an instructor once but he didn't know.
I ask because some of the Warriors I fly have these and some don't. I always turn them to manual because I 'don't trust' the automatic ones. I'd rather align it with the compass myself, as per my training. Perhaps understanding how it works would help me overcome this. I asked an instructor once but he didn't know.
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I think an HSI uses a magnetometer as a reference? (edit: a quick search refers to a "flux valve" - not sure if this is the same thing or not 
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Why do it if it's not fun?
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Knobby,
It basically uses some very sensitive coils, usually placed in the wing-tip because that's where they get the least interferance, to detect the earth's magnetic field. The detector is held in a "hook joint" which enables it to remain vertical as the aircraft banks and pitches.
The DI is connected to a similar set of coils, which are connected to the detector in such a way that a current is produced if the two are not aligned. This current is used to turn the DI. When it aligned with the detector unit, no current is generated and so the DI remains stationary. As it drifts (which all DIs will do, as we've discussed) a current is produced and it synchronises itself again.
The DI is normally aligned at a rate of 2 degrees per second. However, there is a "fast" mode for initial alignment when it is first turned on (or after it topples). I also recall something about the detecting unit being ignored at high angles of bank, and it basically becomes a standard DI, which then re-syncronises itself with the detecting unit when the aircraft is rolled level again - but I can't find anything about that in my notes.
Hope that helps!
FFF
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It basically uses some very sensitive coils, usually placed in the wing-tip because that's where they get the least interferance, to detect the earth's magnetic field. The detector is held in a "hook joint" which enables it to remain vertical as the aircraft banks and pitches.
The DI is connected to a similar set of coils, which are connected to the detector in such a way that a current is produced if the two are not aligned. This current is used to turn the DI. When it aligned with the detector unit, no current is generated and so the DI remains stationary. As it drifts (which all DIs will do, as we've discussed) a current is produced and it synchronises itself again.
The DI is normally aligned at a rate of 2 degrees per second. However, there is a "fast" mode for initial alignment when it is first turned on (or after it topples). I also recall something about the detecting unit being ignored at high angles of bank, and it basically becomes a standard DI, which then re-syncronises itself with the detecting unit when the aircraft is rolled level again - but I can't find anything about that in my notes.
Hope that helps!
FFF
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Join Date: Apr 2003
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Keef
There was something wrong with that slaved HSI/DO you had if it could not follow heading changes quickly.
Normally these are gyro instruments so they respond like any gyro DI to heading changes, i.e. immediately. The slaving part of it merely updates the heading very slowly (a few degrees per minute or so) from a fluxgate magnetometer (a crude form of solid state compass) mounted usually in a wingtip.
I think the total cost of a slaved HSI system is nearer to 10k than 5k. But once you have a slaved HSI you can add a slaved RMI for a mere 4k
The same heading signal can also be used to rotate a gps and a stormscope display, etc.
This stuff is ludicrously expensive for what is actually in there.
There was something wrong with that slaved HSI/DO you had if it could not follow heading changes quickly.
Normally these are gyro instruments so they respond like any gyro DI to heading changes, i.e. immediately. The slaving part of it merely updates the heading very slowly (a few degrees per minute or so) from a fluxgate magnetometer (a crude form of solid state compass) mounted usually in a wingtip.
I think the total cost of a slaved HSI system is nearer to 10k than 5k. But once you have a slaved HSI you can add a slaved RMI for a mere 4k
The same heading signal can also be used to rotate a gps and a stormscope display, etc.This stuff is ludicrously expensive for what is actually in there.
Easiest way to think of a HSI is as a 'standard' DI that gets given an automatic slight 'nudge' to keep it aligned. The nudge coming from an electronic compass mounted somewhere remote from interference.
Add an automatic fast nudge to get the thing aligned in the first place & a means to disconnect the nudge.
Once disconnected it's very like a standard DI, only you use an electric switch to set alignment instead of a mechanical knob.
Other bells & whistles include an integral VOR display.
Add an automatic fast nudge to get the thing aligned in the first place & a means to disconnect the nudge.
Once disconnected it's very like a standard DI, only you use an electric switch to set alignment instead of a mechanical knob.
Other bells & whistles include an integral VOR display.
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Join Date: Apr 2001
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IO540 - it must've been worn out. It would certainly align itself pretty quickly after "power up", and was fine in normal use. But a quick 180 on the ground after "spin up" would "topple" it every time.
We didn't worry too much, because we didn't do a lot of that. We didn't have that particular Arrow for long anyway - the engine blew up over the Solent and the insurance wrote it off.
It's certainly a nice bit of kit, but I think I'd go for a GNS430 before I'd go for one of those.
Don't reckon I'll be in that league again, being a poor retired fella with a part-time job that doesn't pay anyway ;-)
We didn't worry too much, because we didn't do a lot of that. We didn't have that particular Arrow for long anyway - the engine blew up over the Solent and the insurance wrote it off.
It's certainly a nice bit of kit, but I think I'd go for a GNS430 before I'd go for one of those.
Don't reckon I'll be in that league again, being a poor retired fella with a part-time job that doesn't pay anyway ;-)
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Which is the expensive bit? The plane I usually fly already has a 'remote indicating compass' (rather an old presentation, vertical card like a DI but the needle revolves instead of the card, so you 'fly the needle' to take up a new heading. It suffers from dip but does seem more accurate than the one on the coaming. Is this using a flux-gate? If so, is the HSI or RMI then quite cheap to add?
Tim
Tim
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tmmorris
I am responding without reference to current price lists, but to install a slaved HSI you need
- the HSI itself, e.g. KI525A
- the fluxgate magnetometer assembly, e.g. KM112
- remote directional gyro, e.g. KG102A
- various bits e.g. auto/manual switch/meter (KA51B)
- installation of the whole thing
- possible approvals from the CAA
You can contact your local avionics show for a quote but you will need a coffee beforehand, and another one afterwards
This stuff is mostly early 1980s but is still current fit in higher end brand new planes.
I don't believe you will get any change from £10k. There are far cheaper alternatives available for U.S. experimental category planes...
I am responding without reference to current price lists, but to install a slaved HSI you need
- the HSI itself, e.g. KI525A
- the fluxgate magnetometer assembly, e.g. KM112
- remote directional gyro, e.g. KG102A
- various bits e.g. auto/manual switch/meter (KA51B)
- installation of the whole thing
- possible approvals from the CAA
You can contact your local avionics show for a quote but you will need a coffee beforehand, and another one afterwards
This stuff is mostly early 1980s but is still current fit in higher end brand new planes.
I don't believe you will get any change from £10k. There are far cheaper alternatives available for U.S. experimental category planes...
