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AOA units - Angle of Attack

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AOA units - Angle of Attack

Old 25th Apr 2019, 17:41
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AOA units - Angle of Attack

OK Prune, what is the difference between AoA in degrees and AoA in units, why are units used? What are Units?

yours etc
DroneDog is offline  
Old 25th Apr 2019, 17:59
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‘Units’ represent a scaled value of AoA deg.
The range of AoA angles in flight is relatively small, or at least those which have meaningful value. Thus degrees are converted to units, with using expanded scaling (ratio) as appropriate for the display medium and intended use

Any aircraft using non-linear scaling ? (Civil aircraft transposing AoA for EFIS airspeed overlay).
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Old 25th Apr 2019, 18:41
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The AoA can vary over different parts of the aircraft structure and the aerodynamic surfaces and are not always linear across the performance envelope. Often what is claimed as AoA in degrees isn't and units may or may not have a direct correlation with true AoA or may use another term entirely that has a rough correlation with actual AoA. The scaling factors have been explained by safetypee and vary by aircraft type.

There is a thread on this page covering an F-15C accident and is a case in point. This aircraft type displays alpha as Cockpit Units (aka CPU) and are presented to the HUD and helmet in degrees. The CPN units are around 10 degrees higher than actual AoA but the offset varies slightly over the performance envelope. The head-down displays / MFT use the alpha symbol and Angle-Of-Attack nomenclature on the Spin Recovery Display, but are also CPU values displaying up to 45 degrees of CPU.

This is just a gross simplification of a rather big subject.
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Old 25th Apr 2019, 21:58
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Often "units" may be used instead of degrees for simplicity and robustness of a display.

To take another example, on aircraft where the trimmable stab range varies over the positive and negative ranges, someone intending to set +5 and actually getting -5 will be seeing "-5" on the display instead of " 5". That could be missed.

If instead stab position is displayed as a always-positive scale, there is reduced scope for confusion and not seeing that small, but important, minus sign.

For the F-15 example above, the fact that the units display is always high by a magnitude which is probably close to a limiting negative alpha of -10 degrees suggests some of the same "never display a negative" thinking played a part in the design.
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Old 25th Apr 2019, 22:16
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Always bothered me on the A-7D displays, but the VIper HUD and instruments showed actual AoA as used by the computers. The F-101B AOA gauge was in some kinda "normalized" scale, but it had a needle for your AoA and another one that said "don't go past here". I only had one hop in the T-38 to get currency for the Viper and it had a display like the VooDoo, Remember, I did not fly the T-38 in pilot training and was in the last coupla T-33 folks.

Oh well, as long as they have that barber pole doofer, I can handle it.

Gums sends...
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Old 26th Apr 2019, 06:45
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I think it's the Russians who have a combined G-meter and AoA gauge? For max perf in the turn, pull to the +G limit until that corresponds to the max AoA limit as IAS bleeds off, then maintain that....

Playing with the Aggressors at Alconbury in an F-4, I once went vertical, looked back over my shoulder and suddenly felt the rudder pedals shaking... I had rather more AoA units than IAS . Fortunately the recovery drill worked fine without any dramas, but the Aggressor chasing me later told me that the contrail looked rather like something launched from Cape Canaveral!
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Old 26th Apr 2019, 08:36
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Like quite a few other things in aviation, as long as the limits are adhered to it probably doesn't make too much difference what the gauge is calibrated in.
Another example: should rpm gauges show actual numbers or a percentage?
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Old 26th Apr 2019, 19:52
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I was told that for production aircraft, limits should only ever be given in terms of what the pilot sees on instruments. In development that may not be a fixed rule but a calibration would usually be provided to help the Flight Testers with scheduling. Even that can be problematic these days. From memory Typhoon has corrections for things like foreplane angle where proximity to the probes is a fairly transient condition.

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Old 1st May 2019, 16:21
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In the USN carrier landing environment units AOA are always used. When in the groove making a carrier approach "On speed" is actually specific units AOA, usually 7.4 to 8.8 regardless of aircraft type. This angle is critical not only to fly the correct approach airspeed for that aircraft, but also to achieve proper hook to deck angle to engage the wire. USN aircraft also have an AOA indexer on the glare shield. The indexer has no units, but instead three symbols in different colors. "On Speed" AOA symbol is an amber circle. "Low Speed" indexer (which corresponds to high AOA) shows as a green down arrow, and "High Speed" indexer (which corresponds to low AOA) shows as a red up arrow. In the Hornet the HUD also displayed AOA as an AOA bracket. "On speed" on the indexer corresponds to 7.4 to 8.8 units on the bracket. Also, on USN aircraft, the nose gear has a series of lights that correspond to the lights on the pilot's indexer. Thus when the aircraft is slow (too high AOA) the LSO (Landing Signals Officer) sees a green light on the nose gear, when its on speed he sees an amber light, and when its fast (too low AOA), he sees a red light.

Some USAF tactical aircraft have an indexer similar to the USN's, but of course the colors are different. The down arrow is red, the circle is green, and the up arrow is amber. Go figure.
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Old 1st May 2019, 21:26
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Easy to figure, KenV: too fast is too dangerous for the deck, so red makes sense, whereas for the air force red would display the more traditional "too slow" environment.
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