J

Jrskygod

Well Known Member
Patron
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?
 
Last edited:
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)
 
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).
 
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.
 
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
 
Last edited:
Jrskygod said:
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.
Click to expand...

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.
 
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.
 
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.
 
units of measurement?

Kevin, perhaps you should remind folks that many of those formulae work ONLY in Canada, where airspeed and headings are in centipedes per kilojoule!...unless east of the border de la belle province, where all bets are off!:D n'est ce pas?
 
Jrskygod said:
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.
Click to expand...

In principle, assuming you are using an accurate method to calculate TAS from IAS, and Dynon has done their work correctly, the TAS calculated either way should be the same. In any case, you don't need that - the spreadsheet handles that conversion.

You need to record the following data during the box pattern:

OAT
Pressure altitude (should be the same on each leg, plus or minus 100 ft or better)
IAS (should be the same on each leg, as close as humanly possible)
GPS track
GPS ground speed

Static leak check - 45' drop in one minute at 18,000 ft is extremely good. The standard check is done with enough suction to raise the altimeter 1,000 ft above the starting elevation. 18,000 ft would put a lot more pressure differential across the connections, so is an even more severe test.

Note: 18,000 ft of suction on the static system would also cause the ASI to see a delta pressure equivalent to about 520 kt (assuming sea level), unless the suction was also applied to the pitot line. 520 kt worth of delta pressure would not be good for many analog ASIs, but the Dynon should handle it OK.
 
Flew again today

Kevin I flew it again today and here is the data collected from my Skyview display: I flew the course twice to verify the findings.

Indicated altitude = 5500
Density altitude = 7361
OAT = 70 deg f
RPM = 2450
MP = 22.1

Track 1 = 360
Mag Hdg1 = 004
Ias1 = 152
Tas1 = 169
Gs 1 = 155

Track2 = 270
Mag Hdg2 = 276
Ias2 = 152
Tas2 = 169
Gs 2 = 184

Track3 = 180
Mag Hdg3 = 179
Ias3 = 152
Tas3 = 169
Gs 3 = 190

Track4 = 090
Mag Hdg4 = 086
Ias4 = 152
Tas4 = 169
Gs 4 = 162

It still looks like I have IAS error as the difference from the Skyview TAS of 169 and the spreadsheet 4 way calculation of 173.6 is roughly 5 MPH.

If I remember correctly adding crescents in front of the static port will bring the displayed Tas more into line.

Am I looking at this correct?
 
That data looks better. I calculate 174.4 TAS with a much smaller std deviation of 1.3. Still looks like your indicating about 5 low.

This is using the NTPS spreadsheet.

TJ
 
Last edited:
Jrskygod said:
Kevin I flew it again today and here is the data collected from my Skyview display: I flew the course twice to verify the findings.

Indicated altitude = 5500
Density altitude = 7361
OAT = 70 deg f
RPM = 2450
MP = 22.1

Track 1 = 360
Mag Hdg1 = 004
Ias1 = 152
Tas1 = 169
Gs 1 = 155

Track2 = 270
Mag Hdg2 = 276
Ias2 = 152
Tas2 = 169
Gs 2 = 184

Track3 = 180
Mag Hdg3 = 179
Ias3 = 152
Tas3 = 169
Gs 3 = 190

Track4 = 090
Mag Hdg4 = 086
Ias4 = 152
Tas4 = 169
Gs 4 = 162

It still looks like I have IAS error as the difference from the Skyview TAS of 169 and the spreadsheet 4 way calculation of 173.6 is roughly 5 MPH.

If I remember correctly adding crescents in front of the static port will bring the displayed Tas more into line.

Am I looking at this correct?
Click to expand...

Using the NTPS spreadsheet, and converting mph to kt, and deg F to deg C (those are the units used by that spreadsheet), I get an IAS error of about 3.4 kt, or 3.9 mph. That would give a TAS error of about 4.5 mph. These values assume that you were at 5500 ft pressure altitude (i.e. with altimeter setting 29.92). If the 5500 ft was with a different altimeter setting, it will make a small difference. In that case, tell me what the altimeter setting was, and I can correct the altitude.

My calculation assumes a 0.85 recovery factor on the OAT probe - i.e. it recovers about 85% of the ram temperature rise from the TAS. The recovery factor of your probe may be lower than that. My OAT probe has a recovery factor of about 50%, probably because it is mounted in a NACA scoop, and is partially shielded from the ram air. If I assume a recovery factor of 0.5, that lowers the calculated IAS error to about 3.1 kt, or 3.6 mph, or a TAS error of about 4.1 mph.

We can estimate the recovery factor of your OAT probe if you go to some low altitude, and stabilize at max speed until the OAT stabilizes. Record OAT, IAS and pressure altitude. Then slow to the lowest possible speed at the same altitude, and wait for the OAT to stabilize again, and record OAT and IAS. If the OAT reads the same at all speed, the recovery factor would be zero. If the recovery factor is greater than zero, the OAT will read higher at high speed than at low speed. Send me the data, and I can calculate an approximate probe recovery factor.

There are several possible sources of this error:
  • Pitot leak, which would lead to IAS reading low,
  • OAT error (a small effect, unless the OAT error is huge)
  • EFIS ASI instrument error (the instrument error is likely small, unless your EFIS is unserviceable, which is possible). This can be checked using a water manometer.
  • Static system position error (the likely cause of most of the error).

Questions:
  • Have you done a pitot system leak check? If you do one, keep in mind that many pitot tubes have a small moisture drain, and you must tape over this when doing the leak check.
  • Does the indicated OAT match reasoably well with what you would expect?

Assuming that static system position error is the root cause, you need a change in geometry that leads to lower pressure at the static ports. Classical ways to do that are a crescent in front of the ports, or raising the surface of the ports a bit higher above the skin somehow, perhaps by glueing a round disk with a hole in the centre onto the external surface of the protruding part of the port.

If you had an analog ASI, ASI instrument error would be a very possible factor. I've seen ASIs with more than 5 kt instrument error when delivered from the factory.
 
Flying again! said:
That data looks better. I calculate 174.4 TAS with a much smaller std deviation of 1.3. Still looks like your indicating about 5 low.

This is using the NTPS spreadsheet.

TJ
Click to expand...

I also calculate a std deviation of 1.3, but only if I put his heading values in the track column.

If I put track values in the track column (as you should), I calculate a std deviation of 0.6, which is a reasonable quality test point. Ideally, you'd prefer std deviation less than that, but it is a matter of luck to do much better. It is easy to do worse too.
 
You must log in or register to reply here.