Angle of Attack Instrumentation

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Angle of Attack Instrumentation

Postby MoonlightVFR » Thu Nov 08, 2018 2:54 pm

From time to time I think of AOA instrumentation in the C170.

Do our small airplanes need AOA?

If we have any visual reference outside to the horizon aren't we somewhat protected?

Do many of our forum visitors have instrumentation?
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Re: Angle of Attack Instrumentation

Postby rupertjl » Thu Nov 08, 2018 5:05 pm

I purchased the iLevel BOM to install that gives me NORSEE approval and provide ADS-B in, AOA, and AHRS data to run on my ipad. I just created the install bracketry and hope to have it calibrated and flying on the plane by then end of the year. I'll report back what i find.
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Re: Angle of Attack Instrumentation

Postby wabuchanan » Thu Nov 08, 2018 11:49 pm

Rupert,
I will be interested in your report and looking for it.

I currently already use the iLevil ADSB portable in, so have been paying attention to the BOM solution. My other option is the Garmin GDL 82, so costs are similar between the two by the time all is said and done, but get more useful functions with the BOM.

==Bill
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Re: Angle of Attack Instrumentation

Postby lowNslow » Fri Nov 09, 2018 12:03 am

Looking at there website the BOM is ADS-B in only, the Garmin 82 is ADS-B out. Very interesting configuration though.
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Re: Angle of Attack Instrumentation

Postby ghostflyer » Fri Nov 09, 2018 7:51 am

Please bear with me on this one but I am on a learning curve here. Why do I need a AOA in a Cessna 170 . I have a ASI , and a host of other instruments that tell me if the stall margin is close plus experience in flying this machine . I have fitted a couple of AOA instruments for clients in a Cessna 210 and and a Cessna 177rg. What a pain in the U know what. I was flying these aircraft and at the same time and we were trying to calibrate these instruments . When flying and calibrating , ATC called me up and asked why were we doing aerobatics in a Cessna 177rg. We were in a control zone but with clearance to do aerial work [testing].clearance to operate from sea level to 4500ft . A couple of times I could tell the wave height . No doubt the transponder was sending out different heights .[its the only time I have ever done a tail slide in a Cessna 177rg. ]
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Re: Angle of Attack Instrumentation

Postby rupertjl » Fri Nov 09, 2018 1:11 pm

With our venturi systems, I know my airspeed indicator is a little bouncy at slow speeds, and AOA tells you when your wing will actually stop flying. Normal everyday flying I'd expect to not really need AOA, but I'm starting to test some backcountry style short strip approaches where AOA would be another tool in the toolbox to fly a more precise approach and provide the lowest energy landing I can achieve in the plane. I'm also a gadget guy and like playing around with these things. Definitely just a personal preference!
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Re: Angle of Attack Instrumentation

Postby gahorn » Fri Nov 09, 2018 3:30 pm

Aviation Safety magazine published an article on this subject in the Oct 2018 issue. I'll see if they'll allow me to scan/post it.
But in a nutshell...(and we have plenty of nuts around here, heh-heh)…
..
There is NO SUCH THING as a single "stall speed". But we all know that. What we DON'T KNOW is..."What is the speed at which this airplane will stall in THIS attitude?" and "What about NOW?"

In other words, the stalling speed published in the Owner's Manuals, POH's, etc., are applicable to an airplane in the landing configuration (usually) and that speed is applicable ONLY in that configuration...and ONLY at gross-weight, ...and only when the speed is decaying at the rate of approx. 2 kts-per-second... which is what the test pilots observed when your airplane was NEW. (and actually not YOUR airplane, but an airplane typical of your model.)

We all know for example, that increasing the bank angle will increase the stalling speed... right? But what if you want to know RIGHT NOW in THIS BANK...where the airplane will suddently stop flying? AOA will tell you when that wing is about to quit at any weight (yes, that affects things), at any load-factor, at any bank, at any aircraft configuration other than the limited one published in the POH.


There's more to it than that short description, and most of us don't really care in our daily flight operations where our airplane will actually stall.
But if you're operating in the narrow-margins, either due to operational necessity... or because you're crazier than most pilots who fly with conservative margins...(I'm one of the latter)... and you want to REALLY KNOW when your wing is about to "let go" of it's lift-capability in all situations of bank, flap/aileron configurations.... then ANGLE of ATTACK will more reliably provide that information than an airspeed indicator will.


I am old-school and learned to fly by airspeed and seat-of-the-pants and don't like flying "on the edge" where I can get hurt...so I feel an AOA is not worth the investment for the type of flying I do in light-planes. But the AOA was definitely part of the instrument-scan in the jets in which I'm qualified, and the AOA was "helpful" when flying aerobatics (which I do much more rarely these days anyway, so I won't be buying one.) However, the AOA is becoming a tool for the flight-bag and is being taught much more commonly these days to newbies.

Hope that helps.
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Re: Angle of Attack Instrumentation

Postby ghostflyer » Fri Nov 09, 2018 11:44 pm

It’s sounds like I am old school type. But as I said before it’s only a “170” where we have fun . Part of the fun is knowing your limitations and the aircraft limitations . At the moment we are fitting a Garmin auto pilot to a early Cessna 172 n series . The owner stated to all its has a button on it if you get the aircraft in a unusual attitude and in cloud or other just press this button and the aircraft will right itself . I asked him what are you doing in cloud and the aircraft is is only a VFR and you are only VFR rated . Bloody moron. It’s all about limitations and the laws of physics,you can’t change that .
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Re: Angle of Attack Instrumentation

Postby gahorn » Sat Nov 10, 2018 5:46 pm

Aviation Safety Magazine provided this article as a courtesy. (Their website provides subscription service links http://www.aviationsafetymagazine.com/ ):
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Re: Angle of Attack Instrumentation

Postby TFA170 » Mon Nov 12, 2018 12:30 am

gahorn wrote:... and you want to REALLY KNOW when your wing is about to "let go" of it's lift-capability in all situations of bank, flap/aileron configurations.... then ANGLE of ATTACK will more reliably provide that information than an airspeed indicator will.


A true AOA system will do all of that. Unfortunately, most of these add-on systems can only be set up for one config. I would assume most would set it for landing config (probably most useful), but perhaps some have a reason for something else?

Regardless, once it is fully and properly understood, as George says, it will keep you out of trouble far better than airspeed alone because it provides your actual AOA more directly than airspeed can. Airspeed is a pretty good approximation, and since we generally fly with a buffer of 1.3 or thereabouts, it's usually good enough.

If you understand the assumptions/conditions that go into a given stall speed, you're ahead of the game compared to someone who blindly believes the ASI will keep them safe.

I learned on AOA while flying in the Air Force and believe it to be a very useful tool because things get complicated quickly, people get distracted by a variety of things, and a properly installed AOA gauge can't really lie to you like an ASI can. I find it amusing we all learn that a wing stalls when it exceeds its critical angle of attack, and yet we go out and rely on airspeed to keep us safe...then wonder why things happen when we're not in the 'book' config. I'd love to have a real AOA on my 170, but for now, it's a nice-to-have vs a need-to-have
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