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TAS/IAS verification with GPS?

airguy

Unrepentant fanboy
Sponsor
I'm suspicious my IAS/TAS may be reading a bit high, as I'm consistently seeing 162 KTAS at full power (IO360 and WW200RV prop) in my 9A, without gear pants or leg fairings. Today I took my second Phase-I flight and while I was up I did a 4-cardinal direction check and recorded TAS according to Dynon and GS according to GPS. Now I need the cute little Excel program to dump those numbers into to see if the TAS is legit - anyone have a link they can share for that?
 
Average the 4 GS and compare to average of 4 TAS? Why do you engineers have to make everything so complicated? :confused:
 
Average the 4 GS and compare to average of 4 TAS? Why do you engineers have to make everything so complicated? :confused:

Because doing it as you describe will give you the wrong answer. It isn't so complicated if youmare willing to read through the material and learn.
 
Average the 4 GS and compare to average of 4 TAS? Why do you engineers have to make everything so complicated?

While we're at it we should blame engineers for not making IAS and TAS the same all the time ;)

To airguy, If you have an IAS error, you almost guaranteed actually have a static error. Easy to test with a SkyView: Fly at cruise IAS, and note the GPS altitude and barometric altitude. Slow down to about 65 knots, and hold your GPS altitude. Your baro altitude should stay the same, but will likely go up as you slow down. This will cause your IAS to be high, and thus your TAS. Really common on RV's.

You can also do it the more fun way: Do a low pass over a runway with your hair on fire, and then one slow. Check the baro altitude on both passes.
 
Because doing it as you describe will give you the wrong answer. It isn't so complicated if youmare willing to read through the material and learn.

I will humbly use your spreadsheet and see if my TAS is accurate. :( I hope I don't have to rewrite my POH!:(

Says the guy that runs the oil rig, doing what the engineers tell him to do.... :D

Well, I cant badmouth engineers too much, my son is an engineer and a good one!.....Psst! I don't always do what they tell me at the rig, I just do it my way and then tell them their formula worked GREAT! :D

Seriously, I don't want to get into an argument with engineers on this site, I'm afraid I would be seriously out numbered!
 
To airguy, If you have an IAS error, you almost guaranteed actually have a static error. Easy to test with a SkyView: Fly at cruise IAS, and note the GPS altitude and barometric altitude. Slow down to about 65 knots, and hold your GPS altitude. Your baro altitude should stay the same, but will likely go up as you slow down. This will cause your IAS to be high, and thus your TAS. Really common on RV's.

You can also do it the more fun way: Do a low pass over a runway with your hair on fire, and then one slow. Check the baro altitude on both passes.

Good point - and I can actually do that right here at my desk this morning from the Dynon data I pulled from yesterdays flight, since I did some stall series and some full power engine break-in time.

I'm actually not a fan of high-speed low passes, below a couple hundred feet anyway. Too many unmarked towers out here and a good thermal or dust devil will purely spoil your day.
 
Ok - found some static error for sure - I graphed the difference between GPS altitude and pressure altitude for my flight yesterday, computed the delta between them, and graphed that against my indicated airspeed. I consistently show a difference of 100' on the static port between full cruise and near-stall. I show about 330' of delta between them sitting on the ground motionless, about 285' delta at stall speed, and about 185' delta at full cruise speed.

Now how much that will affect the IAS indication I'm not sure. How do you go about correcting this error?

To clarify - my pressure altitude comes UP as the airspeed increases - lower pressure on the port. I have dual ports, one on each side in the Vans recommended positions, and they are TEE'd together with equal-length hoses to prevent a side-slip pressure error.
 
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Followup on the question above of using an average GPS velocity for four tracks on cardinal headings versus the spreadsheet-calculated airspeed, corrected for wind. I just opened a couple of my spreadsheets in which I had calculated the TAS using the NTPS method, and then I additionally calculated the average GPS velocity for the four legs. It is surprising how close the mean GPS velocities are to the calculated true airspeed. A few tenths of a knot difference, with winds up to 30 kts. I thought the cross-wind correction would be a lot more. So, I stand corrected, at the speeds that we fly, Mike's methods of taking the average GPS ground speed of four legs, flown at headings 90 degrees apart, seems to get you pretty darn close to your true airspeed.
 
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My air was quite turbulent taking the GPS 4-course reading so I don't trust it a WHOLE lot, but the calculation give me a TAS of 150.5 which sounds a lot more believable than my displayed 162, considering I don't have wheel pants or gear fairing in place yet.

I'll rerun the GPS speed test on the next smooth-air day to get a little closer, and I'm also going to try to correct some of the obvious position source error I have on my static ports.
 
Now how much that will affect the IAS indication I'm not sure. How do you go about correcting this error?

To clarify - my pressure altitude comes UP as the airspeed increases - lower pressure on the port.

Do a search, there are lots of posts about static error and how to correct it.

First order, each 100' of static error is 7 knots at 150 knots. It's only 0.05 PSI, but it matters. Lower pressure on the static will lead to a higher than real airspeed, so this matches your experience.
 
tas

Followup on the question above of using an average GPS velocity for four tracks on cardinal headings versus the spreadsheet-calculated airspeed, corrected for wind. I just took a look at a couple of my spreadsheets using the NTPS method, calculated the average GPS velocity for the four legs, and was surprised at how close they are to the calculated true airspeed. A few tenths of a knot difference, with winds up to 30 kts. I thought the cross-wind correction would be a lot more. So, I stand corrected, at the speeds that we fly, the average GPS ground speed of four legs, flown at headings 90 degrees apart, seems to get you pretty darn close to your true airspeed.

andy:good on you. i like your above method.
 
Greg,
I would start on the ground, verify that your static system has no leaks into the cabin.

Agreed. Many builders find that their aircraft has a static leak. Static leaks in RVs typically cause the static pressure to read about 100 ft too high, which makes the IAS read roughly 10 mph too high.
 
Agreed. Many builders find that their aircraft has a static leak. Static leaks in RVs typically cause the static pressure to read about 100 ft too high, which makes the IAS read roughly 10 mph too high.

I'll be flying again this weekend, I'll check for leaks before messing with the ports. I hadn't even considered that a leak to cabin pressure could be the root cause, but it would certainly present those symptoms.
 
Did they do a static leak check when you had your transponder checked?

Transponder has not been checked yet - the aircraft has only made 2 flights and I haven't taken it from my private strip yet, trying to get a few hours on the engine before I venture outside glide range. I've got a good avionics shop 20 minutes away, that will be next week sometime.
 
To close the loop on this - I think I found my culprit today. I have dual static ports, one on each side of the fuse in the Vans-standard location, with a Tee connection between them in the middle to eliminate side-slip error. I sealed off one side today and kissed my plane, trying to draw a vacuum on the static port. Right side, I get a bunch of leakage but obviously some restriction. Left side, I got zero restriction, was just open flow. I had noticed on my last flight that opening my alternate static to the cockpit gave a distinct difference in altitude/airspeed, so I was hoping that I didn't have a cockpit leak, but it turns out I did.

After raping the interior out of the airplane, laying some 2x's in the aft fuse and crawling back there, I found and fixed the tygon tubing that had come off the pop-rivet static port on the left side. I should get to fly again this coming weekend, we'll see how it works.
 
After raping the interior out of the airplane, laying some 2x's in the aft fuse and crawling back there, I found and fixed the tygon tubing that had come off the pop-rivet static port on the left side. I should get to fly again this coming weekend, we'll see how it works.

Did you put a little fillet of sealant around the base of the line after re-installing it? (I believe it is specified in the static kit instructions)

If that is done the line will not come off.
 
Did you put a little fillet of sealant around the base of the line after re-installing it? (I believe it is specified in the static kit instructions)

If that is done the line will not come off.

Yes, I put a bead of RTV around the line to hold it in place - same as I had before, yet it came loose from the original install.
 
To close the loop on this - after fixing the line that had come off one of my static ports, my Dynon-calculated TAS agrees with the 3-leg GPS calculated TAS within 1.4 knots at cruise speed and better than that at slow flight, close enough for the girls I go out with... :D

I'm showing 143 knots TAS at 67% power and 7500' now, without wheel pants and gear fairings.

I'm also showing 140 knots TAS at 46% power and 17,500' burning 6gph - love this 9A wing!!
 
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Thanks for closing the loop on this. I'm happy you found the problem, and that it was an easy fix.

There are quite a few folks that are patting themselves on the back for having built a very fast RV who really just have a static system leak, which is causing the IAS to read too high.
 
Thanks for closing the loop on this. I'm happy you found the problem, and that it was an easy fix.

There are quite a few folks that are patting themselves on the back for having built a very fast RV who really just have a static system leak, which is causing the IAS to read too high.

I guess that works just fine, as long as you simply never get your pitot/static system checked.
 
Easy check

First, I subscribe to the belief that the NTPS 3-way spreadsheet is best. Kevin Horton gave us that, I think. Thanks, Kevin.

However, GRT says to calibrate their instrument to TAS by finding a heading where you are going straight up wind or as close as you can find it. GPS max and min are the way to measure that. Then you can average them and get a pretty accurate answer while in flight. When I did some simple trig on this I found that if your course is less than 5 degrees off the wind the error will probably be less than 1 mph.

Any time you are suspicious of your readings you can check them immediately this way and then do the 3-way for the rest, later. I suppose you could put that spreadsheet on a smart phone or tablet now, but we did n't have them when I was in phase one.
 
Actually this was my original paper on the subject. It certainly would have gone unnoticed without Kevin Horton who introduced it to other test pilots during a conference, the NTPS picked it up from there. I had submitted it to Kitplanes but was rejected.

My objective was to get the idea across as simply and as clearly as I could.

http://www.kilohotel.com/rv8/rvlinks/doug_gray/TAS_FNL4.pdf
 
Wow, great economy at altitude, Greg! Maybe an O2 setup will be my Christmas gift to myself...or possibly sooner. :) I'm used to 5500'-6500', so right now 8500' is a whole new experience.

I'm glad you found the leak and the error is very low...well done.

My IAS reads about 3 percent low, so static system is probably sealing well. The shop on the field is booked up on pitot/static checks for a while, but I'll get it done eventually. After the initial RTV seal on the static ports (Van's rivet style), I applied mini volcanoes of extra RTV on the outside...none too pretty, but I'm sure they'll stay put.

 
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IAS Low

OK from what I read, a static problem typically causes the airspeed to read too high. Mine is the opposite. It reads LOW. I checked the static system for leaks. Found and fixed a minor leak. But it is still reading LOW by about 5-10 knots. I have BOTH a steam airspeed and a Dynon airspeed (D-100) They both read the same +-1 or 2 knots. IF, in flight, I reach up and disconnect the static from the Dynon, both airspeed jumps up approx 10 knots. Kinda of like using the Alternate Static Source. (And the altitude goes up by about 150 feet )

I have the flat static ports installed both sides about 4' aft of the rear bulkhead. I'm thinking that it must be something about the ports acting wonky in that location.

Thoughts????

Mark Klepper
N1075
 
OK from what I read, a static problem typically causes the airspeed to read too high. Mine is the opposite. It reads LOW. I checked the static system for leaks. Found and fixed a minor leak. But it is still reading LOW by about 5-10 knots. I have BOTH a steam airspeed and a Dynon airspeed (D-100) They both read the same +-1 or 2 knots. IF, in flight, I reach up and disconnect the static from the Dynon, both airspeed jumps up approx 10 knots. Kinda of like using the Alternate Static Source. (And the altitude goes up by about 150 feet )

I have the flat static ports installed both sides about 4' aft of the rear bulkhead. I'm thinking that it must be something about the ports acting wonky in that location.

Thoughts????

Mark Klepper
N1075

Sounds like your static port is in an area of "HIGH" pressure instead of the more typical "LOW" pressure area. Does the error increase the faster you go? This is a quick and easy way of verifying a static port location error. If so then your can either move your static port to a better location or put a wedge in FRONT of the current static port. This lowers the pressure sensed at the static port. Opposite of what we typically find.
:cool:
 
Sounds like your static port is in an area of "HIGH" pressure instead of the more typical "LOW" pressure area. Does the error increase the faster you go? This is a quick and easy way of verifying a static port location error. If so then your can either move your static port to a better location or put a wedge in FRONT of the current static port. This lowers the pressure sensed at the static port. Opposite of what we typically find.
:cool:


Yes it appears to be a larger error, the faster I'm going. Your thought makes sense. I like the idea of using a wedge. Maybe I will need to move the static ports, but I hate to move or add new ports til I'm sure that will solve the problem.


Mark Klepper
N1075
 
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FYI, you can use electrical tape to create a temporary dam in front of the port. Simple to add and take away layers till you get it right, then make a permanent piece the exact thickness of the final tape layer.
 
OK from what I read, a static problem typically causes the airspeed to read too high. Mine is the opposite. It reads LOW. I checked the static system for leaks. Found and fixed a minor leak. But it is still reading LOW by about 5-10 knots. I have BOTH a steam airspeed and a Dynon airspeed (D-100) They both read the same +-1 or 2 knots. IF, in flight, I reach up and disconnect the static from the Dynon, both airspeed jumps up approx 10 knots. Kinda of like using the Alternate Static Source. (And the altitude goes up by about 150 feet )

I have the flat static ports installed both sides about 4' aft of the rear bulkhead. I'm thinking that it must be something about the ports acting wonky in that location.

Thoughts????

Mark Klepper
N1075
Flat static ports are quite possibly the cause of the problem. The bump formed by the rivet head on the recommended static ports causes the air to accelerate as it goes up and over the bump. The pressure is lower in the accelerated air, and this lower pressure is sensed by the static system.

Several other builders have had the same problem, and fixed it by drilling a hole in the centre of a 2 AN470 rivet heads, then cutting off the heads and gluing them on top of the static ports to make a bump.
 
Kevin,

Here is the applet you used to have on your flight-test website:

http://reacomp.com/true_airspeed/

If I remember correctly, you stated on your website it would work okay. Is it accurate? It seems almost too easy...

By the way, the applet won't run on the Chrome browser.
 
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