Visual Display and Full Envelope Cueing
The Energy Management 101 article in the “how to use the AOA tone” section of the site explains basic energy management principles and how the tone logic provides a great Ps = 0 cue. If that equation doesn’t ring a bell, be sure to review the linked article before you press on with this blog.
We know that when the airplane is ONSPEED, thrust and drag are balanced for a given weight and power setting. Any “slow” tone, and the balance starts to favor drag over thrust and the airplane will go down, slow down or both unless the pilot does something (pushes the power up, reduces AOA or a combination of the two). Any “fast” tone and the pilot can pull harder (or reduce power), if desired. When the airplane is ONSPEED, turn performance over time is optimized: the best blend of turn rate (degrees per second) and turn radius (feet). Another way to think of this is if there is any slow tone, energy is negative (airspeed deficit), and if there is fast tone (or no tone at all), energy is positive (excess airspeed). To maintain aircraft control, there has to be sufficient energy AND the airplane has to be at a flying AOA (i.e., not stalled). Just avoiding a stall does not mean energy is positive, and it does not mean you won’t hit the ground at low altitude. The aural AOA logic makes this type of energy management about as simple as it can be: don’t pull harder than ONSPEED (unless you can accept energy loss or want to intentionally approach the aerodynamic [stall] limit of the airplane).
When we designed the visual display, we decided to depict this the same way most simple military displays do, with two chevrons and a doughnut. The lower chevron is a “fast” cue. It is pointed up, meaning you pull the nose UP to achieve and ONSPEED condition. The upper chevron is a “slow” cue. It points down, meaning you ease the nose DOWN to achieve ONSPEED. A green doughnut is ONSPEED. Pretty simple. We modified this 50-year-old design with three features: a trend line that moves up and down with actual AOA, an L/Dmax cue we call the “barbell” and a % lift display. The trend line is intuitive. When it’s aligned with the two balls outside of the lower chevron and looks like a barbell, the airplane is at L/Dmax. When it’s aligned with the center of the doughnut, the airplane is ONSPEED. We chose % lift in lieu of actual angle of attack for a couple of reasons. First, unless you’ve got good aerodynamic data for your airplane and have conducted instrumented flight test, a specific angle doesn’t have much meaning. Second, there are some universal % lift cues that work in every airplane. Namely ONSPEED (60%), L/Dmax (50%) and stall warning (90-95%). The good news is that actual AOA information is available, so if you prefer to read an actual angle (ala F-16), that’s in the art of the doable with some software modifications. One performance cue that’s currently missing is best range AOA, we are still knocking around ideas for that; but because it’s a bit outside the realm of basic maneuvering EM considerations, we haven’t put much of a brain cell on it yet.
The other two primary references for energy information are airspeed and G. I’m not always operating at low energy or near the aerodynamic limit. Sometimes when I maneuver, I’m more concerned with G limit or airspeed limits. The basic energy display shows me at a glance what each of the three parameters are that I need to know right now when I’m trying maneuver and “max perform” the airplane.
The biggest problem with a visual display is that, well, it’s visual—you have to look at. Our display is inexpensive, small and is designed to be mounted in a head-up location (glare shield, canopy bow, etc.) to be in the pilot’s visual scan when looking forward. But any good fighter pilot will tell you that your head is ALWAYS on a swivel (unless you’re IMC). If you fly an airplane like my RV-4 with a fantastic 360-degree bubble canopy, you probably won’t be looking forward any more than is necessary to manage your velocity vector (where the airplane is going). Unobstructed vision is one of the best features ever built into an airplane. Not all airplanes have it, granted; but any time you’re maneuvering, you are likely diverting your scan quite a bit. SO, that’s where aural cues are bloody brilliant (yeah, I’m biased, no excuse!)…
…which means it would be really nice to have aural cues for some other critical parameters besides alpha and beta (yaw)…
Recently, we incorporated 3D audio (stereo) capability to Gen 2 V3 to provide yaw cues (i.e., tell the pilot what to do with their feet) which just slews the basic AOA tone left or right with the ball; so you “step on the tone” to center it up in the sound field. We think that’s a really simply way to convey that message to the pilot.
We also decided to tackle the other two sides of the envelope: airspeed and G. For our first try at the G warning, we decided to modify the logic employed in the "overload warning system" in the F-15C. Since the F-15C has hydro-mechanical flight controls, it’s possible for the pilot to over-G and stall the airplane, so it employed a couple of cues to help the pilot out. The OWS in the Eagle uses a tone system; but that’s not really required for a straight-wing prop, since we can’t sustain G’s quite near the limits of the airframe like the jet can, nor are we using them in a dog fight (much ;). So, we are using a simple “G limit” call-out when you hit the limit. Because the V3 box has an IMU, it knows when it’s rolling; so, the logic has full symmetric and asymmetric capability. This is completely transparent to the pilot. All the pilot does is program the symmetric G limits of the airplane and the computer takes care of the rest. Stop pulling with you hear the “G limit” call-out. We have our first try at this software complete and plan to begin flight test soon.
To keep the airspeed logic as simple and intuitive as possible, we are using a gentle chime at very low repetition for Vno (maximum structural cruising speed) to gently alert the pilot that the airplane is in the yellow arc (after all, you may be there on purpose, so an obnoxious warning wouldn’t be appropriate) and a simple “airspeed” warning when you reach red line. Airspeed will change to yellow when operating in the yellow arc, and red if redline is exceeded. The G display will change to red when G limits are reached. At least that's the "going in" plan! We haven't implemented this software yet since we face some airspeed measurement challenges (OAT) with the box installed in a stand-alone configuration. More to follow as we get some time to experiment with this feature.
We think these capabilities allow the Gen 2 V3 to provide full flight envelope warning and energy cuing in a simple, intuitive manner. And, because some folks want a visual display, we have integrated color coding and a slip/skid indicator to provide the same functionality to the $45 “poor man’s HUD.” The visual display will also allow us to experiment with a graphic spin recovery display. There are a lot of ergonomic and KISS considerations when designing any cueing and warning logic, and definitely more than one way to skin a cat. We welcome suggestions or collaboration as we develop and test the logic.