Airdata accuracy

[Rocketboy]

My plane indicates airspeed about 3 percent low. Any way to adjust the Dynon yo make it indicate accurately?

 

[khorton]

My plane indicates airspeed about 3 percent low. Any way to adjust the Dynon yo make it indicate accurately?

This has been an oft requested feature, but it does not yet exist.  Where are your static ports?  If they are aft fuselage, are they flush, or protruding?  It may be possible to modify the static ports to correct the airspeed.  

Keep in mind that if the error is from the static system it is also affecting the altimeter.  So it is better to try to fix it at the source, if possible.

I'm assuming you have checked for pitot and static system leaks.

 

[Rocketboy]

Kevin,

Yep checked for leaks, flew alongside a buddy with a dynon RV10 and our instuments were within 20 feet and 1 knot, but I'm pretty sure it is reading low. Any ideas on how to make my flush static ports increase the airspeed?

Bob

 

[dynonsupport]

All the Dynon EFIS units are fully calibrated at the factory, and that's the only place we can adjust the airspeed. We always calibrate to the correct value though- X PSI = Y Airspeed, so we can't adjust for a custom error in a plane. If you do become convinced that it is the EFIS, we'll always happily check it and recalibrate it if needed.

Generally you fix flush static ports by building up a small dam behind them.

 

[Rocketboy]

OK, I'll try that. I've seen others add a washer around the static port. Anyone know what effect that has?

Bob

 

[khorton]

Any ideas on how to make my flush static ports increase the airspeed?

I've seen several reports of success from people who have glued something with a hole in it over the outside of the static port, to give it a profile that is similar to what you would have from Van's recommended pop rivet.  This causes the sensed static pressure to decrease, which increases the IAS.  Some folks drilled a hole in a solid rivet, then cut off the shank and glued the head to the static port.  Another guy made some sort of thin disk with a hole in it to glue on the static port.  Either of these fixes would be an iterative approach, as you would need to experiment to find the correct height.

As Dynon support said, a dam would work too - a dam behind the static port increases static pressure, and reduces the IAS.  A dam just ahead of the static port will reduce the static pressure and increase the IAS.  With a dam, you could tailor the magnitude of the effect by adjusting dam height and distance to the static port.

 

[Rocketboy]

Thanks

 

[plapthorne]

For what it is worth.

I've run air data checks on 3 dynon D-100 systems in the last month.

Checks on the ground with a calibrated Barfioeld tester showed the Dynon was accurate in Airspeed, Vertical Speed and Altitude within 1% on all three aircraft. This was with the aircraft stationary, so there were no dynamic errors due to airflow across the ports. I would put my money on the hints posted here already, rather than on an error in the Dynon.

Cheers

 

[Rocketboy]

Yep Thanks. I believe the dynon is quite accurate as well.

Bob

 

[Ken_Kopp]

What you are all talking about it is Ps (P sub S) or static position error. Yes, you can place a dam, or washer, or pop rivet over or around your static port but you'll still not eliminate Ps. You can't as reading ambient pressure is much more difficult than you might think as it is not only a function of location on the aircraft but also changes as a funtion of AoA, power (in prop planes) and othe factors. In the ideal - you run a test profile to determine your CAS using one of many valid methods; such as timed course, GPS box patterns or if you have the equipment using a trailing bomb. Ps is determined by the difference between CAS and IAS at each Airspeed increment. If we could program to process our Ps curve our EFIS then you would display CAS and not have to mess around with your static port. You can make your IAS do anything you want by "tricking" the ports but won't change what the airplane is really doing. If however, after you've determined your Ps is simply way off in most flight regimes and depending if it is + or - you then could apply a static dam. However, if your Ps changes sign (which does in fact happen in many airplanes) all you do is correct the sign you compensated for and make the other worse. What I did in my BD-4 was connect a second static port on opposite sides of the fuselage. This really smoothed out my Ps curve and brought my CAS to within 3 mph of IAS throughout all but the extremes of the flight envelope where it grows to +6mph at the fast end and -4 mph at the slow end.

safe flying!

CDR Ken "Spanky" Kopp
NAVY Test Pilot
XO HSM-70

 

[PilotKris]

This topic was also covered extensively in this thread:

http://dynonavionics.com/cgi-bin/yabb2/YaBB.pl?num=1166320957/45#45

I've even dubbed Kevin the "Static Source Position Error Crusader" and he's right. Fix the root problem (SSPE) and all of the other "errors" will go away.

If your indicated airspeed is off because of SSPE, the indicated (and mode C) altitude is also off negating the benefits of all the various collision avoidance systems.

PilotKris

 

[Ken_Kopp]

true - but your Ps would have to be significant and well outside acceptability to impact the accuracy of baro-altimeters or xpdr in mode c (well within the 100 ft increments reported in mode C). A +-10mph in Ps is within 10-20 feet in altitude. I have a curve of the effect of Ps on my altitude for my BD-4. I'll try to dig it up to confirm. I do remember it being insignificant for general aviation flying.

 

[khorton]

true - but your Ps would have to be significant and well outside acceptability to impact the accuracy of baro-altimeters or xpdr in mode c (well within the 100 ft increments reported in mode C). A +-10mph in Ps is within 10-20 feet in altitude.  I have a curve of the effect of Ps on my altitude for my BD-4. I'll try to dig it up to confirm.  I do remember it being insignificant for general aviation flying.  

It depends on how fast your aircraft goes.  At low speed, 10 mph error should equate to quite a small altitude error.  But at the speeds of some of the popular amateur-built aircraft, 10 mph can start to equate to significant altitude errors.  E.g., at 200 mph CAS, an IAS 10 mph too high implies an altitude error at sea level of about 140 ft.  This is certainly significant for IFR flying, and probably important for VFR too, given the number of other aircraft that have TCAS.

I agree that mucking about with the static port can only hope to minimize static source error in a range of speeds.  I suspect most people would choose to minimize the error in the range of speeds normally seen in cruise, and then have to accept whatever static source errors remained at other conditions.

I too would love to have a means to correct for static source errors in the EFIS.  But there seems to be little appetite at Dynon to do that.  The most they have hinted at is a way to fudge the TAS, but any errors in altitude would remain uncorrected.

 

[PilotKris]

I too would love to have a means to correct for static source errors in the EFIS. But there seems to be little appetite at Dynon to do that. The most they have hinted at is a way to fudge the TAS, but any errors in altitude would remain uncorrected.

Since our entire ATC system and every collision avoidance device on the planet depends on the accuracy of the mode C altitude, I reiterate my objection to giving end-users the ability to monkey around with the reported altitude until such time as those "corrections" can be verified during the bi-annual pitot/static/transponder check. I suspect that the FAAs position on the matter is the same.

Play with TAS all you like, but don't mess with the altitude encoder.

PilotKris

 

[Ken_Kopp]

Being able to correct for ERRORs throughout the speed range of the aircraft by applying a correction to Ps will in fact make the ATC airsystem safer as our airplanes will actually be flying at both the airspeed and altitude we are reporting, at least to within the instrument error tolerances (of course a seperate instrument error correction could also be applied by Dynon at the factory). Or after the Bi-annual we could also enter corrections from the correspondance test and be dead nuts on in both what we see in the cockpit and what ATC see's as our reported PA.

As an experimental aircraft builder I certainly hope you are not suggesting we are not capable enough to figure out how to make the correct - corrections??? I can think of a 100 other things far more dangerous than screwing up a Ps correction. However, to alleviate your fear, placing a strict limit upon the magnitude of the correction would prevent ridiculously erroneous altitude reporting. I'd very much like to see this as an added feature in a future release.

 

[PilotKris]

.
As an experimental aircraft builder I certainly hope you are not suggesting we are not capable enough to figure out how to make the correct - corrections???

I'm not suggesting that at all... I'M FLAT OUT SAYING IT!

Verifying and "adjusting" the altitude being reported by the transponder is one of the very few things that an experimental aircraft builder can not do. The FAA has mandated that it must be done by a certified technician, with equipment that has been tested and certified accurate.

PERIOD! END OF DEBATE!

It's thier sandbox and if we want to play in it, we have to follow their rules.

PilotKris

 

[Ken_Kopp]

It is not the end of the debate by any means....

The FAA only directs the TEST of correspondance between altitude reporting systems and the integrity of the pitot/static system be conducted by certified technicians. I am absolutely free to modify that system in anyway I choose provided it meets the required standards during the bi-annual check. As an example...I added a second static source and a static reservoir to my BD-4. In your view I was wrong huh? Nope...totally within my rights as a repairman of my EXPERIMENTAL airplane. If builders want to place static dams around the static ports they are free to do so. Having the ability to add a Ps correction in software on the EFIS is merely another method to accomplish what I have done mechanically.

BTW...you kinda lack tact.

 

[Ken_Kopp]

By your reasoning we probably shouldn't be able to self adjust our kolsman window either since all the incapable pilots, yourself excluded obviously, might make a mistake and disrupt the harmonious ATC system with pilots flying randomly all over the sky. You are ridiculous and woefully misinformed if you think we as builders and/or repairman of experimental aircraft are not authorized to "Experiment" with absolutely every system installed in our aircraft at any time for any reason - including engines, avionics, props, wings, tires, wheels, and autopilot systems without out-of-trim indicators. We are however required to meet the applicable standards of safety, inspection and documentation when we do tinker with those system. In this example, we have to pass the bi-annual checks to fly IFR. Shoot, I don't even have to have a transponder in the first place to fly VFR so what check would I be subject too if I had access to a Ps correction? If my static leak checked passed before I added a correction to the EFIS then it will pass afterwords too. I'm not bound by Part 43 in anyway except Appendix D as part of my condition inspection and the applicable parts if I want to fly IFR - at least that is what my FAA written and signed operating limitations say. If I want to build my own airspeed and altimeter using rubber bands, a paper clip, and a balloon I'm free to do that too.

You PilotKris are not qualified to make a statement as to who is and isn't capable of such work and fortunetly for the rest of us...YOUR OPINION doesn't matter any more than mine does. Its the law.

I'm happy with the regs - they allow many of us very qualified and innovative folks to learn new and better ways of doing things. Some of us become qualified in the process, some of us come to it already qualitifed. Some make stupid decisions and pay the price. Doesn't matter one bit to me. Its a damn free country! Then again I'm not affraid to take responsibility for the changes I make...I guess you are. Knitting sweaters is a really safe hobby - you might try that - watch out for those needles though I hear they might poke you in the eye.

CDR Kenneth G Kopp, US NAVY
Helicopter Maritime Strike - 70
Repairman, Builder, CFI, Graduate of US NAVY TEST PILOT SCHOOL Class 120, Test Pilot, Weapons Tactics Instructor/Evaluator, and Future Commanding Officer, BS in Control Systems Engineering, MS in Aeronautical Engineering. Logged over 4500 hours in 77 different military and civilian aircraft - Go ahead and tell me again how I'm not smart enough..... J.A.

 

[Brantel]

Go ahead and tell us how you feel CDR...

That was one good vent! ;D

By the way, thanks for your service to our country!

 

[PilotKris]

It is not the end of the debate by any means....

...BTW...you kinda lack tact.

My Dear Ken Kopp,

I respectfully must point out that your interpretation of the Federal Aviation Administration Regulations (re: transponder and encoder certification) is flawed.

A test of the system is required not only on a "bi-annual" basis but also after the performance of any "maintenance" of any component of the system. Any change (even the disconnecting and re-connecting a static line) necessitates another check of the system by an appropriately certified technician.

While you, as an appropriately anointed repairman of your personally built experimental aircraft ("anointed" meaning in possession of a FAA issued repairman certificate for your aircraft) may perform said "maintenance", including "adjusting" the altitude reporting device, but the accuracy of said "adjustments" still must be verified (and certified) by an appropriately certified technician prior to turning on the altitude reporting system.

So, if you still wish to pick nits, you are correct (assuming you have a Repairman Certificate for your aircraft) that you may "adjust" your altitude reporting device. But you can not use said altitude reporting device (and therefore the transponder) until the system is checked by an appropriately certified technician.

You are also incorrect in stating that only the correspondence and integrity tests must be completed by an appropriately certified technician. Those are only 2 of the tests that are mandated during the Bi-annual test. Not all of the individual tests are required after maintenance, only the ones specific to the maintenance performed.

To answer your specific questionn: Yes, you were wrong.

While it was legal for you to modify the static system of your BD-4 (assuming you possess a Repairman’s Certificate for the BD-4) but it is not legal for you to use the transponder or fly IFR until the system is tested by an appropriately certified technician. Assuming you made no "adjustments" to the altimeter or encoder since the last (current) Bi-annual test, the test would only need to be for integrity of the system.

Your arguement that, in the veiw of the FAA, an electronic "Ps" correction is the same as relocating a static dam is also wrong (check with your local FSDO).

Respectfully,

PilotKris

(Tactful enough for you?)