IAS Calibration

[Xavier]

Hi all,

 

 

 

Since I started flying my Savannah 9 months ago I felt that the IAS wasn't very accurate so this morning I have done some testing.

 

 

 

I flew around a 4NM square at 1500 feet maintaining 5000Rpm and an IAS of 71 knots (70-72 average)

 

Temperature was around 28 Deg Celcius.

 

Wind was 5-10kts N-NW (early morning)

 

 

 

I flew the square twice and this is the data collected :

 

 

 

Legs - Tracking - Dist (NM) - ASI (Kt) - TAS(Kt) - GS (Kt)

 

South - pass 1 ------- 3.7 ----------- 71 -------- 74.6 -------- 88

 

East - Pass 1 ---------- 3.4 ---------- 71 -------- 74.6 -------- 85

 

North - Pass 1 -------- 3.5 ---------- 71 -------- 74.6 -------- 77

 

West - Pass 1 --------- 3.2 ---------- 71 -------- 74.6 -------- 79

 

South - Pass 2 -------- 3.1 ---------- 71 -------- 74.6 -------- 88

 

East - Pass 2 ---------- 3.3 ---------- 71 -------- 74.6 -------- 84

 

North - Pass 2 -------- 3.3 ---------- 71 -------- 74.6 -------- 78

 

West - Pass 2 --------- 3.5 ---------- 71 -------- 74.6 -------- 79

 

Average --------------- 3.375 -------- 71 -------- 74.6 -------- 82.25

 

 

 

GPX file is attached (just unzip "ASI test.GPX.zip")

 

 

 

I thought that on the North and West legs, the IAS would have been less than the GS

 

 

 

What do you make of those numbers ?

 

 

 

Cheers,

 

 

 

Xavier

 

 

ASI test.GPX.zip

 

ASI test.GPX.zip

 

ASI test.GPX.zip

[Kyle Communications]

but was the wind actually NW at 1500 ft?

 

I know at Caboolture I can have NW at 5 kts on the ground and takeoff then crikey at 500 ft I have wind from the south east at 20kts

 

 

[pmccarthy]

it does suggest the ASI is under reading.

 

 

[Xavier]

Its hard to say exactly because it wasn't windy at all on the ground, at 7:30am NNE 1kt gusting 2kts. ( http://www.bom.gov.au/products/IDD60801/IDD60801.94105.shtml )

 

Darwin was North 4kt gusting 5kts ( http://www.bom.gov.au/products/IDD60901/IDD60901.94120.shtml )

 

Batchelor was CALM - NNW 1kt gusting 3kts ( http://www.bom.gov.au/products/IDD60901/IDD60901.94125.shtml )

 

Based on the GS it looks like it was a NNW

 

 

[facthunter]

If your distance is correct and times also, you have a G/S on each leg. I have no idea why any other reading means anything. Your instrument is clearly under reading. Nev

 

 

[Nobody]

It isnt quite as simple as averaging the ground speeds to determine the true airspeed. You need to work out the wind vector and subtract that(in a vector subtraction sence). For a small amount of wind relative to the ground speed. The method is described in the links below.

 

Method description

 

http://www.ntps.edu/images/stories/documents/gps-pec-method.doc

 

Link the Spreadsheet

 

http://www.ntps.edu/images/stories/documents/gps-pec.XLS

 

Assuming that your directions above were ground tracks and not headsings when I put your numbers into the spreadsheet get a true airspeed (based ont eh gps numbers ) of 82.4 knots and hence would conclude that you ASI is out by approx 7.8 knots at that speed. It seems like a pretty big error.

 

 

[facthunter]

There's no reason to believe the error will be constant over the range either. It could be the calibration of the instrument or the pitot and plumbing. static pressure port (s) location . Nev

 

 

[Xavier]

Thanks for your prompt replies, I just realised I made a mistake on the legs, East should be West and West should be East.

 

This doesn't change the end result.

 

the correct numbers are :

 

I flew around a 4NM square at 1500 feet maintaining 5000Rpm and an IAS of 71 knots (70-72 average)

 

Temperature was around 28 Deg Celcius.

 

Wind estimate is 5-10kts N-NE (early morning)

 

I flew the square twice and this is the data collected :

 

Legs - Tracking -- Dist (NM) -- ASI (Kt) - TAS(Kt) -- GS (Kt)

 

South - pass 1 -------- 3.7 ------------ 71 --------- 74.6 -------- 88

 

West - Pass 1 ---------- 3.4 ----------- 71 --------- 74.6 -------- 85

 

North - Pass 1 --------- 3.5 ----------- 71 --------- 74.6 -------- 77

 

East - Pass 1 ----------- 3.2 ----------- 71 --------- 74.6 -------- 79

 

South - Pass 2 --------- 3.1 ----------- 71 --------- 74.6 -------- 88

 

West - Pass 2 ---------- 3.3 ----------- 71 --------- 74.6 -------- 84

 

North - Pass 2 --------- 3.3 ----------- 71 --------- 74.6 -------- 78

 

East - Pass 2 ----------- 3.5 ----------- 71 --------- 74.6 -------- 79

 

Average --------------- 3.375 -------- 71 --------- 74.6 ------ 82.25

 

GPX file is attached (just unzip "ASI test.GPX.zip")

 

I thought that on the Northern and Eastern legs, the IAS would have been less than the GS

 

What do you make of those numbers ?

 

I put the numbers in "Nobody"'s spreadsheet and I am getting a true airspeed between 81.6 and 82.5 knots.

 

 

[Happyflyer]

I had the same problem with my airspeed indicator. Replaced it, problem fixed. If you flew your legs accurately the average figure should be your airspeed. Wind doesn't matter as long as it was constant. See if you can borrow an ASI and see if it fixes the problem. If it doesn't check your pitot and static lines for leaks.

 

 

[Nobody]

The average of the speeds is not correct. It is close when there is little wind but is wrong at higher wind speeds.

 

Take for example an aircraft with a perfect ASI. Say it can fly at 100 knots indicated air speed and the owner goes out to do the calibration on a day where the wind is blowing at 50 knots from the north. When he flys north he will have a ground speed of 50 knots. When south he will have a ground speed of 150 knots. To fly directly east he will have to fly at a heading of 060 and have a ground speed of 86.6 knots. To fly west he will have a ground speed of again 86.6 knots and need to fly at a heading of 300.

 

The average is (150+50+86.6+86.6)/4=93.3.

 

Now I accept that this is only a 7% error for what is quite a high wind but even with more normal wind at the slower speed that raaus aircraft fly this can be significant. The other advantage of the spreadsheet method above is that you do not need to be exactly on track and they don't need to be the cardinal points. Any ground speed and track works as long as they are in different directions. Greater accuracy comes when they are evenly spaced around the compas.

 

 

[Geoff13]

I have done extensive testing on my ASI.

 

Firstly you actually need to Calibrate the ASI itself. This can be done using a manometer which is very easy to build.

 

You then need to calibrate the pitot system and test for leaks.

 

Then test for leaks in the static.

 

These all need to be done before getting into the air. Just because the system is accurate on the ground does not mean it will be accurate in the air, but you do need to test it against a known GPS.

 

There are several suggested methods, but I have found the 3 leg method to be fairly accurate.

 

In my case my system is accurate up to 60 knts but above the it drops off. At a reading of 100 knts IAS I am actually only doing 95 knts.

 

I can change the readings simply by connecting the static in different locations. It has taken me approx 15 hours of testing and I still do not have the system accurate over the whole range.

 

Knowing that it is accurate to 60 knts gives me some comfort as that lower end is critical to safe flight in the danger zone of takeoff and landings. The top end at Vne it is reading 5 knts high so that gives me a safety buffer at the top end as well. Works for now but I still am not happy with it and am open to suggestions.

 

For my GPS testing I fly 3 legs of 10 mins each 120degrees apart and take a reading every minute. Far simpler to do with a passenger to watch the clock and gps and take notes leaving me to fly a consistent circuit maintaining height track and speed.

 

 

[DrZoos]

geoff thats how i thought you do it, but there is no need to make eaxh leg so long...

 

You just need a few readings in each direction at different speeds for both ias and gps

 

http://www.challengers101.com/CalASI.html