Airspeed help

[Jrskygod]

For some reason I just can't get Kevin Horton's spreadsheet to work. perhaps I don't have the correct program on my computer to run it or ????? Perhaps someone can help out and maybe educate me along the way.

From a test flight yesterday in nice calm air here is the data collected from my Skyview display:

Indicated altitude = 5500
Density altitude = 7525
OAT = 71 deg f
RPM = 2450
MP = 21.6


Hdg1 = 360
Ias1 = 151
Tas1 = 168
Gs 1 = 201

Hdg2 = 270
Ias2 = 151
Tas2 = 168
Gs 2 = 178

Hdg3 = 180
Ias3 = 151
Tas3 = 168
Gs 3 = 144

Hdg4 = 090
Ias4 = 151
Tas4 = 168
Gs 4 = 174

What I'm trying to determine is if I have an error in my indicated airspeed. Am I going about this the right way?

[Flying again!]

With the data you have I show an average TAS of 175.6 (mph?) and a std dev of 3.5

[rolivi]

You're in a -9.

What engine and prop?

Most readers will be able to opine on with that. (I'm a noob and it looks cool to me so far)

[rvbuilder2002]

Your calculated average ground speed is 174.25

Your stated TAS is 168 (you didn't say whether this is calculated on an E-6B or computed by an EFIS system) If the data is valid and your pitot/static system is working properly the TAS and average ground speed should be about the same.
If you calculated the TAS, it could be a calculation error. If it is what was displayed by an EFIS, I would say your airspeed reads about 6 low.

For accurate flight testing, upper level winds of 30 (kts or MPH?) is pretty strong.
Try again on another day when the air mass is stable and air is calm (early in the morning).

[rv7charlie]

I assume the -9 needs static port 'bumps' like the -4, right? Does yours have them?

[Jrskygod]

The true airspeed was calculated by the Skyview and the speed is indeed in MPH. It is my understanding that the skyview or even calculated true airspeed should be the average of the four ground speeds. If so that indicates it to be low by 6 mph. Previously I had a Dynon D10A installed with Van's static ports and I would routinely see 180 mph true at the same settings and altitude i was testing yesterday at. I changed my static ports to Safe-Air ports along with a complete Skyview system. In doing so I apparently lost some true airspeed due to the ports. From memory I think I need to add some crescents in front of the ports to get the speed back.

[humptybump]

Hi Ted,

I'm pretty sure we are talking of the same thing but the calculation is not an "average". There is a spreadsheet which takes 3 (or 4) GPS headings and GPS ground speeds and calculates TAS, wind speed, and direction. Based on your data, the calculated TAS is 17.6, wind speed of 29, and wind direction of 86 degrees.

For this calculation to work it's important to use GPS track and not magnetic heading.

http://www.ntps.edu/information/downloads.html

[rvbuilder2002]

Originally Posted by Jrskygod

I changed my static ports to Safe-Air ports along with a complete Skyview system. In doing so I apparently lost some true airspeed due to the ports. From memory I think I need to add some crescents in front of the ports to get the speed back.

I had the understanding that the ports sold by Safeair mimic the Van's supplied pop rivet static port.
If you have ones that do not (the port should not be flush to the skin surface), then you likely have a static system error.

[Kevin Horton]

The spreadsheet I normally use requires the track for each of the four runs. Did you happen to record that too?

Crunching your data using four different mixes of the three headings, using the "three legs with headings 90 deg apart" method, I get quite variable results depending on which three legs I use. This suggests that perhaps one of the ground speeds was misrecorded (or mistyped in VAF), or perhaps one of the IAS or headings was off the mark during one of the legs.

Bottom line - I can't make any reasonable sense of that data either. Something doesn't add up somewhere.

[Jrskygod]

Thanks for the reply Kevin. I was using magnetic headings and not gps track. In retrospect I can see with the wind at altitude that day how an error can occur. I will fly the same test pattern using track and report back. Also when I run the test again should I calculate the true airspeed from indicated or rely on the number Skyview shows.

I also checked my static system and it lost 45' in a minute at 18000'. If I recall correctly the allowance is 100' per minute at that altitude.

The Safe-Air ports I installed are not flush. They protrude from the skin, however they are not the same shape as the Vans ports. I'm sure paint thickness, installation method and location, manufacturing tolerance and other factors can affect their performance.