ASI correction method

Mike G

Junior Member
Joined
Jun 16, 2005
Messages
1,704
Location
Lillebonne France
Aircraft
Owned Magni M16 now ELA 07
Total Flight Time
550FW + 500 gyro
One very important parameter for my Gyro Warning System (GWS) is True airspeed (TAS). The internal ASI in the GWS has to be calibrated to correct for particular pitot/static errors in each gyro model. To do this we use a version of the “cloverleaf method” as outlined in the Society of Flight Test Engineers Handbook which uses an excel sheet by Doug Gray of NSW Australia. His method is detailed here http://contrails.free.fr/temp/TAS_FNL4.pdf

This excellent method gives you TAS from ground speed, however we needed to know the Calibrated Air Speed CAS because the GWS is constantly measuring an IAS (from its internal ASI) and needs to calculate the actual TAS from the altitude and OAT (in other words the DA) and to do that it needs the CAS.

There are two ways of doing the cloverleaf method, one is to fly 3 headings at a constant IAS and to use a GPS to determine tracks and ground speeds. The other is to fly 3 tracks at a constant IAS and use a stop watch or GPS to determine the ground speeds.

For manufacturers who will install the GWS we’ve developed a simplified method using the constant track method over a pre-determined triangle. The tester draws a triangle on his map (Google earth is a good tool) with legs about 2 - 3 km long. Using Google maps, you can measure the length of each leg and its direction. Here’s an example:

1657178642140.png

Each turning point should be clearly visible from the previous one to make it easier to fly the chosen track. The pilot then does a long press of the GWS mute button and the GWS starts recording, at the next turning point a short press and the GWS tells the pilot the time for the leg, the average IAS, the air pressure and OAT. If the pilot repeats this for the 3 legs and notes the values for each leg he simply has to put them into a supplied spreadsheet that calculates the correction factors to be entered into the GWS for that particular model.

We recommend doing 3 triangles at different airspeeds the first being as slow as reasonable. The critical factor here being maintaining a constant airspeed and correct track, maintaining altitude also helps. This is best done on a calm day.



I then realised that this method could be used by anybody wishing to calibrate their ASI using a stop watch and some method of measuring OAT in flight.

Here is what the excel sheet would look like if I’d done it without the GWS recording the data for me. This assumes that I measured the time for each leg with a stop watch and noted the OAT, altimeter setting and height.

1657178595850.png

You can see that the correction is CAS = IAS x 0.95 +2.82. You could probably ignore the 2.82 kph, it’s only 1.5 knots.

This spreadsheet also calculates the wind speed and direction for each triangle flown, you can see that the 3 are very close and similar to the predicted on the day which allows you to check if the calculation makes sense. If there were very different wind speeds/directions then you know that something is wrong with either your data or flying.

The graph shows a straight line through the 3 points and could be used as a correction card for the ASI. If the 3 points didn’t align it would be worth flying more triangles at different speeds to confirm the data or draw a curve.

It seems I can't attach the spreadsheet so pm me if you want to try this.

Mike G
 

Mike G

Junior Member
Joined
Jun 16, 2005
Messages
1,704
Location
Lillebonne France
Aircraft
Owned Magni M16 now ELA 07
Total Flight Time
550FW + 500 gyro
I realise that the spreadsheet data isn't very clear so here's a zoom.

1657179217216.png
 
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