TAS/IAS verification with GPS?

[airguy]

Originally Posted by rvbuilder2002

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.

[airguy]

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...

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!!

[Kevin Horton]

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.

[airguy]

Originally Posted by Kevin Horton

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.

[hevansrv7a]

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.

[Doug]

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...y/TAS_FNL4.pdf

[rightrudder]

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.

[mark960]

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

[GalinHdz]

Originally Posted by mark960

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.

[mark960]

Originally Posted by GalinHdz

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.

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