Indicated vs calibrated airspeeds

Hi,

In the UK I have to install a back-up air speed indicator, amongst other things.

The POH lists these CAS speeds, in knots.

Stall Flaps Down (@ gross weight 1320 lb) (VSO) 41
Stall (@ gross weight 1320 lb) (VS) 45
Flap Operating Range (VSO ? VFE) 41-82
Normal Operating Range (green arc) 45-108
Maneuvering (VA ? blue line) 90
Maximum Structural Cruise (VNO) 108
Caution Range (yellow arc) 108-136
Never Exceed (VNE ? red line) 136

Any idea what I should mark on the ASI as equivalent "indicated" airspeeds?

Thanks...Keith

IAS vs CAS

The IAS is your CAS plus or minus the error in the system. By system I mean not only the error in the instrument but in how the static is placed, how the pitot is installed, etc. I doubt that any two aircraft are exactly the same, even type certified ones, let alone hand-built. I don't know of any way to measure those other errors on the ground. You can use a manometer to check the instrument itself. Do a search on the forum for that. Kevin Horton has some good information there and I think on his website, too.

You need to calibrate your airspeed indicator using your GPS. A two-way run is probably good enough if done carefully, but you can use a 3-way run and go here for a spreadsheet to do the math for you. You will want to calibrate several speeds from very slow to very fast and if the line looks smooth, extrapolate the rest.

Don't forget that you need to record your pressure altitude and OAT (which you have also checked, right?) for your GPS speed runs and then convert those True Air Speeds (TAS) to CAS by converting to standard density altitude. You can find a page on my spreadsheets (links below) which will do that math for you with good precision. Or you could just do your tests at sea level at 15 degrees Celsius and skip all the math.

The only way to obtain "calibrated air speeds" is by flight testing a particular aircraft.
For all practical purposes, you should use the airspeeds listed in the POH for your markings.

Thanks

Thanks for the quick response, guys. I'll go with the POH indicated airspeeds if the variation is negligible. I just wondered because I'm sure I saw slightly higher speeds listed elsewhere in the forum, but cannot now locate them.

As for calibrating my own plane, in Scotland, I'm always at sea level and 15 degrees (C) in August ;-)

Cheers...Keith

Since the RV-12 is...

..."sort of certified"... doesn't it come with the IAS/CAS error curve defined just like any US certified aircraft?

I presume the static port locations are completely defined (even pre-drilled?) for the -12, so they should be as consistent as a certified spam can...

Daft question, but how do you set up the same information on the Dynon? One presumes you will be using the same dynamic and static sources or are the LAA insisting on a completely separate system?

Same

As you surmise, I'm using the same static and dynamic sources.

Cheers...Keith