Back when I was building my -8, I thought quite a few times about installing an Angle of Attack indicator, but when it came time to make priority trades, it always came in just below the line. Later on, when I could have picked up a system, I didn?t want to go cutting into my wings (or wingtips) for pressure taps, or replace my pitot tube and run more plumbing?just lazy I guess! But still?.flying AOA is a really nice way to get the most out of your airplane when operating close to the edge, and I have kept my eyes open for a system that might be adaptable. And sure enough, good things DO come to those who wait!
GRT has been talking for awhile about a ?derived? AOA indication for their EFIS. Derived AOA means that it is computed using parameters from the flight computer rather than being directly sensed through a special probe or probes. While it can be argued that Derived AOA is not quite as accurate in a rapidly changing situation as sensed AOA, the difference is probably in the noise for our type of operations if the derived data algorithm is properly designed. (Derived AOA is, in fact, used in some very high speed aerospace vehicles that operate in regimes where probes would melt right off ? the Space Shuttle uses derived AOA all the way through entry until the Air Data Probes deploy about Mach 3.) Anyway?. I recently got the chance to start doing some testing of the GRT AOA system, and I must say that I am impressed!
Their system works on the HX EFIS displays, and looks much like a high end AOA indexer. It only appears (next to the airspeed tape) when AOA gets into a region of interest, such as approach, or high G loading. This is nice, as it doesn?t clutter up the display in normal cruising flight. The display gives you a filled green circle when you ae on approach AOA ? fall below it, and you get yellow Chevrons pointing up, telling you to pull, and if you get to high, you get red chevrons pointing down telling you to push. (Get high enough, and the word PUSH appears!). In addition, there is a red pitch limiter bar that appears in the center of the PFD and moves down towards the pitch lubber line as you get close to the stall. (After a couple of flights, I am very comfortable with the AOA display ? the pitch limiter and I are still working things out?.)
The system needs to be calibrated before you can use it, but this is very simple. You establish gliding flight with the power pulled back to idle and the flaps out. Pull back to a stall, recover, then slow down again, and when you get a knot or to above where you just stalled, you give the system a ?mark? by pushing a soft key, and it knows where the critical angle of attack is. You can then set the indexer up to maintain a factor above stall for approach ? I am using 1.3 Vso for now ? it allows 1.0 ? 1.4 I think.
After calibrating the system, I messed around in slow flight, and it was easy to see the system at work. Rolling into a turn and keeping the deck angle constant, the system showed an increasing AOA, just as you?d expect. Drop the nose, and you went back in to the green. Accelerating with power, the system showed decreasing AOA, and finally disappeared. A sharp pull towards an accelerated stall brought the indexer back up on the PFD, and showed the AOA increasing. I tried to get it to appear with an accelerated stall from a steep turn starting at about 130 knots, but hit 4.5 G?s without a buffet, and without a chute or G-suit, I decided I didn?t want to go further on the initial flight. Wince I got no buffet, I wasn?t surprised that I saw no high AOA indication.
Right now, this is preliminary software, but I think it shows real promise. When you consider that a good sensed AOA system can cost a bit shy of a thousand dollars, this is quite a benefit for folks with a GRT EFIS ? it will just come along with the new software when it is ready! No sensors, no plumbing, no wiring ? no sweat! The only problem I see so far with the current plan is that you sort of have to have your head ?down? to see it on the EFIS screen, at a time when you want to be head?s up. I mentioned to the guys at GRT that a cool accessory would be a little standalone display that could hook up to a serial line from the EFIS, and mount on the glare shield to display AOA ? that would be the Cat?s Meow! Even for a couple hundred bucks, it would be cheaper than a sensed system.
I am sure there is more development time to come on this, and I know that GRT doesn?t put stuff out there until they are sure it is ready and working, but it is fun to see what is coming down the line. Those folks who already fly with AOA systems know how useful they can be ? and getting one for free along with your EFIS ? that?s hard to beat!
Paul
GRT has been talking for awhile about a ?derived? AOA indication for their EFIS. Derived AOA means that it is computed using parameters from the flight computer rather than being directly sensed through a special probe or probes. While it can be argued that Derived AOA is not quite as accurate in a rapidly changing situation as sensed AOA, the difference is probably in the noise for our type of operations if the derived data algorithm is properly designed. (Derived AOA is, in fact, used in some very high speed aerospace vehicles that operate in regimes where probes would melt right off ? the Space Shuttle uses derived AOA all the way through entry until the Air Data Probes deploy about Mach 3.) Anyway?. I recently got the chance to start doing some testing of the GRT AOA system, and I must say that I am impressed!
Their system works on the HX EFIS displays, and looks much like a high end AOA indexer. It only appears (next to the airspeed tape) when AOA gets into a region of interest, such as approach, or high G loading. This is nice, as it doesn?t clutter up the display in normal cruising flight. The display gives you a filled green circle when you ae on approach AOA ? fall below it, and you get yellow Chevrons pointing up, telling you to pull, and if you get to high, you get red chevrons pointing down telling you to push. (Get high enough, and the word PUSH appears!). In addition, there is a red pitch limiter bar that appears in the center of the PFD and moves down towards the pitch lubber line as you get close to the stall. (After a couple of flights, I am very comfortable with the AOA display ? the pitch limiter and I are still working things out?.)
The system needs to be calibrated before you can use it, but this is very simple. You establish gliding flight with the power pulled back to idle and the flaps out. Pull back to a stall, recover, then slow down again, and when you get a knot or to above where you just stalled, you give the system a ?mark? by pushing a soft key, and it knows where the critical angle of attack is. You can then set the indexer up to maintain a factor above stall for approach ? I am using 1.3 Vso for now ? it allows 1.0 ? 1.4 I think.
After calibrating the system, I messed around in slow flight, and it was easy to see the system at work. Rolling into a turn and keeping the deck angle constant, the system showed an increasing AOA, just as you?d expect. Drop the nose, and you went back in to the green. Accelerating with power, the system showed decreasing AOA, and finally disappeared. A sharp pull towards an accelerated stall brought the indexer back up on the PFD, and showed the AOA increasing. I tried to get it to appear with an accelerated stall from a steep turn starting at about 130 knots, but hit 4.5 G?s without a buffet, and without a chute or G-suit, I decided I didn?t want to go further on the initial flight. Wince I got no buffet, I wasn?t surprised that I saw no high AOA indication.
Right now, this is preliminary software, but I think it shows real promise. When you consider that a good sensed AOA system can cost a bit shy of a thousand dollars, this is quite a benefit for folks with a GRT EFIS ? it will just come along with the new software when it is ready! No sensors, no plumbing, no wiring ? no sweat! The only problem I see so far with the current plan is that you sort of have to have your head ?down? to see it on the EFIS screen, at a time when you want to be head?s up. I mentioned to the guys at GRT that a cool accessory would be a little standalone display that could hook up to a serial line from the EFIS, and mount on the glare shield to display AOA ? that would be the Cat?s Meow! Even for a couple hundred bucks, it would be cheaper than a sensed system.
I am sure there is more development time to come on this, and I know that GRT doesn?t put stuff out there until they are sure it is ready and working, but it is fun to see what is coming down the line. Those folks who already fly with AOA systems know how useful they can be ? and getting one for free along with your EFIS ? that?s hard to beat!
Paul
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