The F-4 AOA system essentially provides the pilot with a couple of options as to how he or she would like to “listen” to the tone. If the pilot is interested in “max performing” the airplane, then the entire back side of the drag curve is presented, with the solid tone indicating optimum AOA and the ability to easily differentiate between a “slow”, ON SPEED or “fast” condition. If, on the other hand, all the pilot is concerned with is progressive stall warning (or just the “slow” indication), then it’s possible to adjust the volume to minimize or eliminate all tone for ON SPEED or faster conditions—in other words, the only the 1600 Hz high frequency tone is present and begins when 70% of available lift is used up. This function is similar to progressive stall warning audio currently available in other AOA systems such as those provided by Dynon or Garmin. If adjusted in this manner, then the technique used for flying ON SPEED for approach and landing is to simply slow to ON SPEED and then adjust pitch to occasionally touch the slow tone; so instead of a constant solid tone in the headset, no tone is heard unless the AOA increases into the slow range. Like all AOA systems, the F-4 system provides excellent stall warning, although at high G/AOA onset rates, there is some lag. In other words, if you pull hard enough, fast enough, you can “beat the system” and generate a stall before a stall warning tone is heard (although you will get some slow tone, even at high onset rates). This lag at high onset rates is best illustrated in the accelerated stall demonstrated in a previous post. In that example, the first stall occurs before stall warning is heard, but you’ll note be the second stall in the sequence, the system has “caught up” with pitch rate.
Let’s take a look at two very basic stalls to see how the stall warning portion of the tone works. The first thing to note is at the start of the slow tone, there is still 30% of the total lift available, so if a maximum performance recovery is desired, it is only necessary to break the angle of attack sufficiently to restore flying AOA, and the tone will catch up. In other words, the tone is simply a performance indication under these conditions (as it is in most, but not all, cases).
First we’ll look at a simple, power-off stall demonstration that begins at airspeed approximate to L/Dmax (the bottom of the drag curve) and uses a nominal 1 kt/sec deceleration rate:
https://youtu.be/-2nBsFPnpT0. Keep in mind, that if the volume was turned down, only the slow tone would be present. Note that during the recovery, following the initial “unload” (reduction in AOA) to an ON SPEED condition, a momentary slow tone is heard since there is still “lift reserve” available and the objective was to minimize altitude loss following the stall (i.e., the type of maximum performance recovery that would be appropriate if the ground was a factor).
Next we’ll take a look at a basic power-on stall. I’m adding power carefully to control the deceleration and pitch prior to the stall to make things as stable as practical for the demonstration:
https://youtu.be/QE0UoBK981k. Like the power-off stall, this demonstration uses a 1KT/sec deceleration rate. I’m cheating a bit end-game with some climb to control the deceleration rate to compensate for the addition of power. In this case, you’ll note the initial recovery is right to the edge of the stall warning (20% lift remaining point). In a power-on condition, it takes a bit longer for the AOA to stabilize during the recovery. So, once again, the AOA is providing performance feedback that must be compared with pitch, power and airspeed.
There are a few key points to understand about any AOA system. First, the system is only going to be as good as its calibration. Second, it is incumbent upon the pilot to understand the system and indications whether they are aural or visual (or a combination of both). Third, AOA indications should, generally, be used like any other “performance” vs a “control” indication. A simple mantra might be “pitch, power, AOA/airspeed” (or swap airspeed/AOA if you wish). However, under stable conditions (e.g., base turn, maximum performance sustained turn or catching ON SPEED during maneuvering, etc.) the AOA can become a de facto “control” indication for fine-tuning pitch inputs. During basic stall recovery, the pilot must use a combination of aircraft feel, airspeed increase and AOA indications to obtain a maximum performance/minimum altitude loss recovery. Some pilots may be familiar with an advanced instrument display called a “pitch limit indication” (PLI) which is a computer generated indication on the attitude display that looks at the pitch/power/airspeed/AOA conditions and does this math for the pilot. The MD-11 I fly at work does not display AOA on the primary flight display, but it does have a PLI, which can be helpful when flying out of a stall. If the ground isn’t a factor, it’s always possible to unload to a low g condition (push to make the tone go away), allow airspeed to increase and AOA to decrease, and then smoothly pull to ON SPEED for recovery (provided that part of the tone volume is turned up sufficiently to be heard).
Some folks have commented that the tone pattern is too complex, too annoying, or distracting in the headset. Unfortunately it’s extremely difficult to convey the utility of this type of system with a simple video demonstration; but for folks that are interested, I’ve compiled some of our test video to put together a brief 5 1/2 minute demonstration of how the system works in various flight conditions:
https://youtu.be/5VUbTaeTLAM. With proper training and experience, using a system like the F-4 aural AOA becomes second nature, but it’s possible without that experience it may be difficult to appreciate the utility it offers. And, with all of the options available on the market, it really boils down to a matter of personal preference. The single most valuable contribution of any aural AOA system is that it frees up the pilot’s eyes so they can be out of the cockpit. If it’s properly mechanized, it provides valuable performance cues, especially in airplanes lacking easily interpreted aerodynamic (e.g., buffet) cues.
As we continue to test and develop the tone generating software, we’ll continue to look at ergonomic improvements; and if anyone has any suggestions, please post, drop a PM or e-mail. It would also be insightful for folks with experience with other systems to compare and contrast those systems with some of the examples we’ve posted—in other words, add a post to this thread with thoughts (pro and con) and/or experience with any of the many different options we RV’ers have when selecting an AOA system.
Fly Safe!
Vac
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