In a previous post, I mentioned that I didn't think my airspeed indicator was reading properly and it had me scratching my head. I even mentioned a 11-12 knot difference between it and my GPS groundspeed. After some research, digging around and plain old memory refreshment, I have to confess I feel like a complete idiot. Once again, since I always want to be completely honest with you about my mistakes, here's a doozy for ya.
I was confusing my indicated airspeed (IAS) with my true airspeed (TAS). I knew, from my pilot training, that TAS was IAS adjusted for non-standard conditions (temp and altitude). That wasn't a mystery...what was a surprise to me was just how much it was different at the altitudes we fly at here in the Denver area (my home airport is at 5600'). Compounding the IAS vs. TAS spread is the fast speeds of the RV, which also magnifies it. In my previous post, I said I thought the IAS and groundspeed (with zero winds aloft) should match. TAS is what I should have said; (not IAS) those should match for that particular condition. Another lesson learned...they just keep coming, I do enjoy learning new things.
Not to make excuses, but I have very little time flying behind a modern glass cockpit, and none recently. All of my flight training was in a round gage 30 yr old Skyhawk. I have never had much experience with TAS being spit out of my EFIS for me to view in real-time during a flight. I suspect many of you can relate. I had to break out the ol' E6B flight calculator to get TAS back in those days. This is a whole new world to me to have all of this flight data at my fingertips in the cockpit...and now I'm learning what to do with all of that information. Again, this illustrates the point of not only honing your stick and rudder skills prior to first flight...but also your glass cockpit familiarity, its not trivial. I could've done a better job with that in my preparation.
By the way, in my research, I came across this handy little tool for calculating TAS. Its an estimate, but should be pretty close. From this screenshot, you can see that 160 kts IAS at 6000' translates to 179 kts TAS, almost 20 kts difference! So, that's my so called airspeed indication problem. Its working just fine is my guess.
One of the really cool things about these modern EFIS's is their "black box" capability. The SkyView will spit out all of your flight and engine parameters to a file that you can download after your flight to a thumbdrive. Its incredibly useful to analyze your flights during flight testing and beyond. Above, I have isolated the TAS and IAS values for comparison and plotted them on an Excel spreadsheet (click to enlarge). This gives you an idea the spread between the two in reality (from my last flight). I was doing a little bit of climbing and diving at various points in this sequence, which is why you see quite a few spikes and valleys...straight and level is boring!
First and foremost, a much belated congratulations on your accomplishment. Your immediate issues with airspeed caught my attention, and while part of me wishes you would have troubleshot and determined the line problem while on the ground, I also found it interesting how you verified the problem while airborne.
ReplyDeleteI have also had an actual airspeed failure in flight, so I know exactly what that feels like. In my case I had no GPS or other means to determine airspeed other than power settings and attitude of the A/C. Kudos for maintaining control during the event, and successfully solving the problem.
I thought I might help provide some additional insight to your IAS vs. TAS confusion. Iguess it's the CFI in me, and I just can't resist:
IAS - Indicated airspeed read directly from the airspeed indicator based on dynamic pressure differences fed to the instrument
CAS - Calibrated airspeed which is IAS corrected corrected for instrument and position errors. In most cases the difference between IAS and CAS is within a few knots, but CAS is considered to be the more accurate speed to use when calculating TAS, not IAS.
TAS - True airspeed,which is actually based on CAS corrected for non-standard temperature and pressure. (NOT IAS as you had suggested)
Ground Speed is TAS corrected for wind
The large differences you see between IAS and TAS are mostly based on differences in air pressure and temperature at different altitudes. The air pressure entering the pitot tube at altitude is generally less than at lower altitudes. Less pressure entering the pitot tube and less static pressure acting on that ram air pressure means a lower reading on the airspeed indicator. This same scenario also means a higher TAS due to less dense air/temperature/pressure at altitude.
As a result, as you have discovered, the differences are quite remarkable between the IAS and TAS the higher and faster that you fly. The challenge that I am most interested in is how you are determining CAS (figuring out the effects of instrument and position error). Does Dynon provide methods for figuring that out?
Congrats again and happy and safe flying! Enjoying your honest and informative posts as always.
Bryan
Awesome stuff Bryan, thanks for the feedback. To be honest, I don't know how SkyView derives CAS? I will look into this. I also wonder if the TAS is actually derived from GS on the SkyView...not CAS as you mention? Learning as I go...
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