The McDonnell F-4 had a nifty, aural AOA system that allowed you to hear the angle of attack when operating around L/Dmax and slower…it had a “solid tone” that indicated when you were ON SPEED, or flying optimum AOA, let you know when you were slow and really got your attention when you were approaching a stall. ON SPEED was especially helpful during approach and landing as well as maneuvering flight; and the logic allowed the pilot to make fine corrections between slightly slow, ON SPEED and slightly fast when desired. The F-4 didn’t have the greatest handling characteristics, and the system helped the pilot avoid “pulling on the pole” too hard and making bad things happen. I’ve been privileged to have flown lots of airplanes, including fighters and jumbo jets; and I’ve found the F-4 aural AOA system to be the best I’ve used in any airplane.
Recently, I met a couple of smart engineers and fellow RV'ers that have helped to build a prototype system that allows me to program the F-4 AOA tone logic for use in my RV-4 to demonstrate the concept to folks that might not be familiar with it. The “tone generator” is actually a small computer that processes the basic serial output message from my Dynon DY-10A, which includes AOA information from a standard Dynon AOA pitot tube I recently retro-fitted to the airplane. The tone is pumped into the intercom system and the volume is pilot-adjustable (except for the slow tone and stall warning). In the demonstration videos, the camera is receiving raw audio output and it’s much louder than what I’m hearing in the headset; and, to be clear, this is not the Dynon programmable AOA tone.
The AOA tone allows me to hear the “back side of the drag curve” or what you might have learned as the region of reverse command, i.e., where it takes more power to go slower. Optimum AOA lies in this region. At speeds above or AOA’s less than L/Dmax, there is no tone. The way it works, as the airplane slows down (or AOA increases) to L/Dmax, a slow, low frequency beep starts. This reference can be handy, since lots of good things happen performance-wise at L/Dmax. As the airplane slows down more and AOA increases to optimum, the pulse rate of the low-frequency tone increases and changes to a solid tone when you reach “ON SPEED,” or optimum AOA. If you keep pulling and increasing AOA, then the tone transitions to a high frequency and starts to beep again at a slow rate, letting you know that you’re “slow.” And if you insist on stalling, the pulse rate will increase until about 15% or so before the stall a loud, high-frequency quick pulsed tone (that sounds a lot like a stick or pedal shaker) lets you know that the bottom is about to fall out. Here's a picture that illustrates the AOA tone logic:
. Here's what that picture sounds like in the cockpit as the airplane slows down from L/Dmax all the way to the edge of the stall: https://youtu.be/S9H6T_tOLe4. The easiest way to conceptualize how this works is that the tone essentially tells you that you are "fast" (low frequency), "on speed" (steady tone), or "slow" (high frequency) when operating at lower airspeeds or higher AOA. The pulse rate allows you to fine tune pitch inputs using this basic logic.
Here’s what an ON SPEED base and final to a low-approach looks like: https://youtu.be/p7PXlOgJUJs. The video starts with the airplane rolling out on downwind and slowing down to begin the base turn. There is about 20 knots or so of over-shooting crosswind at pattern altitude, but only a few knots of right crosswind down near the runway. Prior to starting the base turn, the airplane is configured for landing (Flaps 40) and trimmed for ON SPEED. Throughout the base turn, AOA (tone) is controlled with pitch adjustments. During the turn, you’ll hear some instances of “slightly fast” (a fast-pulsed low frequency tone) and “slightly slow” (a slow-pulsed high frequency tone) as I explore the envelope a bit! Typically, until rolling out on final or approaching the roll-out point, power is at idle for a normal-sized pattern (traffic permitting); but in any case, glide path is controlled with power. Typically, a bit of power is added rolling out on final as the airplane settles “into the groove” for a stabilized approach. ON SPEED is maintained until the flare for a normal landing and you’ll hear a slow tone as I round-out during the low-approach, which is normal. It’s not necessary to look in the cockpit to hear the tone, so the pilot’s eyes are freed up to look at the runway environment and scan for traffic with an occasional cross-check of the airspeed indicator to make sure everything is working as intended. If conditions warrant, then it might be more appropriate to fly a “slightly fast” turn and approach. For example, if conditions are gusty and turbulent or you wish to carry a bit more energy for a wheel landing, etc. Similarly, if stopping distance is critical, establishing a “slightly slow” condition on final might be appropriate.
Currently, I’ve got the system programmed to wake up at 10 knots on takeoff, which lets me hear “what the wing is thinking” as the airplane accelerates; so it’s fairly easy to transition to an ON SPEED takeoff (which approximates maximum performance) for the initial climb segment when it’s necessary or advantageous to do so: https://youtu.be/OtLObb2b_us.
I'll add to this post as we experiment and learn more.
Fly Safe,
Vac
Recently, I met a couple of smart engineers and fellow RV'ers that have helped to build a prototype system that allows me to program the F-4 AOA tone logic for use in my RV-4 to demonstrate the concept to folks that might not be familiar with it. The “tone generator” is actually a small computer that processes the basic serial output message from my Dynon DY-10A, which includes AOA information from a standard Dynon AOA pitot tube I recently retro-fitted to the airplane. The tone is pumped into the intercom system and the volume is pilot-adjustable (except for the slow tone and stall warning). In the demonstration videos, the camera is receiving raw audio output and it’s much louder than what I’m hearing in the headset; and, to be clear, this is not the Dynon programmable AOA tone.
The AOA tone allows me to hear the “back side of the drag curve” or what you might have learned as the region of reverse command, i.e., where it takes more power to go slower. Optimum AOA lies in this region. At speeds above or AOA’s less than L/Dmax, there is no tone. The way it works, as the airplane slows down (or AOA increases) to L/Dmax, a slow, low frequency beep starts. This reference can be handy, since lots of good things happen performance-wise at L/Dmax. As the airplane slows down more and AOA increases to optimum, the pulse rate of the low-frequency tone increases and changes to a solid tone when you reach “ON SPEED,” or optimum AOA. If you keep pulling and increasing AOA, then the tone transitions to a high frequency and starts to beep again at a slow rate, letting you know that you’re “slow.” And if you insist on stalling, the pulse rate will increase until about 15% or so before the stall a loud, high-frequency quick pulsed tone (that sounds a lot like a stick or pedal shaker) lets you know that the bottom is about to fall out. Here's a picture that illustrates the AOA tone logic:
Here’s what an ON SPEED base and final to a low-approach looks like: https://youtu.be/p7PXlOgJUJs. The video starts with the airplane rolling out on downwind and slowing down to begin the base turn. There is about 20 knots or so of over-shooting crosswind at pattern altitude, but only a few knots of right crosswind down near the runway. Prior to starting the base turn, the airplane is configured for landing (Flaps 40) and trimmed for ON SPEED. Throughout the base turn, AOA (tone) is controlled with pitch adjustments. During the turn, you’ll hear some instances of “slightly fast” (a fast-pulsed low frequency tone) and “slightly slow” (a slow-pulsed high frequency tone) as I explore the envelope a bit! Typically, until rolling out on final or approaching the roll-out point, power is at idle for a normal-sized pattern (traffic permitting); but in any case, glide path is controlled with power. Typically, a bit of power is added rolling out on final as the airplane settles “into the groove” for a stabilized approach. ON SPEED is maintained until the flare for a normal landing and you’ll hear a slow tone as I round-out during the low-approach, which is normal. It’s not necessary to look in the cockpit to hear the tone, so the pilot’s eyes are freed up to look at the runway environment and scan for traffic with an occasional cross-check of the airspeed indicator to make sure everything is working as intended. If conditions warrant, then it might be more appropriate to fly a “slightly fast” turn and approach. For example, if conditions are gusty and turbulent or you wish to carry a bit more energy for a wheel landing, etc. Similarly, if stopping distance is critical, establishing a “slightly slow” condition on final might be appropriate.
Currently, I’ve got the system programmed to wake up at 10 knots on takeoff, which lets me hear “what the wing is thinking” as the airplane accelerates; so it’s fairly easy to transition to an ON SPEED takeoff (which approximates maximum performance) for the initial climb segment when it’s necessary or advantageous to do so: https://youtu.be/OtLObb2b_us.
I'll add to this post as we experiment and learn more.
Fly Safe,
Vac
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