Part 1
In 1907, Orville and Wilbur discussed an “angle of incidence” device to assist with energy management…’course that discussion predates the term “energy management” or even bolting an airspeed indicator into an airplane (that happened in 1912 at Farnborough). So, it appears the roots of some of the discussion in this thread may have been started about 111 years ago…
AOA and airspeed are complimentary concepts. Either airspeed or AOA (or a combination of the two) will work as a reference if properly engineered, tested, and applied by a knowledgeable, trained pilot. In a dynamic, maneuvering environment, AOA can be an easier to use and a more precise energy reference, depending on how accurate the information is, how it’s conveyed to the pilot and what the airplane’s energy state is (e.g., if I'm going fast, I'm probably more concerned with G or airspeed limits).
In the Boeing 777 I fly at work, there is a nifty computer program we call the Aircraft Performance System (or “APS”) that is full of data for all phases of flight. APS derived or cross-checked airspeed for takeoff, approach and landing are entered into the flight management system by the pilots (note: the system requires the input of the primates in the cockpit). These airspeeds are then displayed on the airspeed tape. The utility of the display is further enhanced by a dynamic “foot” that shows what the energy (airspeed) margin is when we maneuver (bank the airplane). It also has a nifty cue at the top end of the airspeed band we call a “zipper” that lets us know where the Mach limit is (or the flap speed limit if those are deployed). Airspeed data are derived from extensive performance engineering and validated by thorough flight test and displayed in a user-friendly format on the primary flight display. The airplane is equipped with calibrated pitot, static and AOA sensors installed and maintained in a certified configuration. Precise data are available for static source pressure error, etc. It doesn’t matter what the gross weight; density altitude or CG condition is—there are reference airspeed data for everything that’s within the usable flight envelope. We fly the airplane by reference to IAS/mach. The 777 has an AOA gauge as well. We use
that to make sure that we didn’t screw up programming the airspeed cue for approach, since approach AOA is always the same value, regardless of other parameters (no cockpit math or pilot input required!). We learned to cross-check the “always the same approach AOA” on final after we almost lost an airplane when the crew accidentally mis-programmed the gross weight of the airplane, which provided an erroneous speed cue. AOA cues provide a great “trust but verify” of the programmed airspeed cue. Boeing academics and design philosophy regarding the use of AOA cues can be referenced here for folks that want to learn more:
http://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html. Some EFIS systems available for EAB airplanes have similar airspeed displays, and
if accurate flight test data is available, can provide much of the utility of the Boeing airspeed display logic
I also have some experience flying upside down and exploring the G and aerodynamic limits in fighters equipped with hydro-mechanical flight controls…that means that the only “limiting device” was the pilot—which is the same as flying an RV. All of the flight control logic resides between the pilot’s ears. Turns out, if I can fly some key AOA’s in my RV-4, properly modulate power and G; and respect airspeed limits, I can operate throughout the usable envelope of the airplane in any attitude and any energy condition, even if I don’t have extensive performance, static source pressure error or CAS correction data available for the airplane. If I can listen to the AOA, this becomes a caveman simple, “how hard to pull on the pole" cue, and I don’t even have to be looking in the cockpit to do it. One reason so many of us washed-up fighter pilots fly RV’s is because, well, they fly like a fighter (on a beer budget
). It turns out, if you adapt some handling techniques that work well in fighters, they also work well in RV’s.
The key AOA’s that we care about when we fly a straight-winged propeller driven airplane are stall, ONSPEED, L/Dmax and Carson’s Speed. If we have a properly calibrated system and the pilot can either easily see or (better yet) hear these AOA’s, we can use them for
many energy management tasks in an RV.
Let’s start by using AOA to land an RV…since we all have to land, regardless of what kind of flying we like to do in our RV’s.
To land using AOA: slow to ONSPEED, configure the airplane and maintain ONSPEED until landing. That’s it. There is no gust correction (not required), and if you change bank angle (G load), gross weight or density altitude, it doesn’t matter; ONSPEED AOA is
always the same. If you are ONSPEED, you have sufficient energy to land (i.e., not too fast or too slow, no math required). Now the Lieutenant Neanderthal lobe of my brain still has a ballpark 1G IAS reference for approach and landing, so I always “trust but verify” proper AOA operation before I go eye’s out (just like I cross-check airspeed in the 777 using AOA or an F-22 pilot verifies the accuracy of the displayed approach AOA cue using IAS). But once I’ve got a warm fuzzy the system is working, it’s all about aim point and AOA. In small airplanes, it’s a good idea to be stable 10-12” prior to touchdown. It actually takes longer to read this paragraph than it does to demo how to fly the tone in flight
(See post 13 in this thread)
Some AOA rules of thumb: L/Dmax for maximum range glide, maximum endurance (time aloft), and best rate of climb. ONSPEED for approach/landing, best angle of climb on takeoff, maximum endurance glide and optimum turn performance. Since L/Dmax is pretty slow in RV’s, I like to use Carson’s for holding (best MPG) and as Vz reference for optimum climb performance. If you know actual AOA for L/Dmax (flight test, engineering math, or wind tunnel test), then ONSPEED occurs at 1.73 x L/Dmax AOA and Carson’s occurs at .58 x L/Dmax AOA. If you know CAS for L/Dmax, then ONSPEED occurs at .76 x Velocity L/Dmax and Carson’s occurs at 1.32 L/Dmax. For our purpose, we adjust the ONSPEED cue to be about +/- 1 degree, which works out to about a 5 Knot speed band in an RV. We bias ONSPEED slightly to the right of max power available on the thrust/power required curve for reasons that are a bit too long to get into here, but I’m happy to discuss in a PM, email or phone call. Of note, at a 1G stable (no bank) condition, ONSPEED is about 1.3 x stall speed (which should sound pretty familiar).
We’ve built a demonstration system that uses the calibrated AOA output from a commercial EFIS to generate a simple tone pattern that lets the pilot hear L/Dmax, ONSPEED and stall. Our first-generation system, for which we received the EAA Founder’s Innovation Award this year, works at up to 2G’s per second onset rate and is damped to mimic the response characteristics in similar systems equipping fighters. It is possible to “beat the system” if the pilot applies 3 G’s per second or greater, but the system catches up prior to secondary stall. Here’s a video demonstration of a rapid pitch input exceeding system capability:
https://youtu.be/DLtamTAh-Is. Our “next generation” system is equipped with indigenous pressure sensors and samples at a rate of 50Hz with an accuracy of ¼ to ½ degree. This eliminates the need for a source of AOA signal and allows installation in any airplane. One of our objectives with the new design is to increase high-gain input response rate to handle an onset rate of 3-4G’s per second in addition to simplifying AOA calibration with the new design. This ability to accurately process AOA data and provide a pilot-usable cue that is appropriately damped is why no gust correction is used flying AOA for approach. If the pilot wants extra energy during approach, the low inertia and rapid power response rate of RV’s make flying a “slightly fast” cue easy until it’s time to slow to ONSPEED for landing transition.
Here’s a short, five-minute overview of the aural logic in flight:
https://youtu.be/48ZgOYDQUfk
Here’s a long “how to” video that discusses flying AOA using aural cues for ONSPEED, L/Dmax and stall:
https://youtu.be/-kbA6NxMpmQ
Here’s an unedited and unnarrated software validation test flight video that has numerous closed patterns and air work using aural AOA cues as a primary reference:
https://youtu.be/-iudL-gAL5E
Detailed discussion about how to fly AOA in an RV in different phases of flight as well as pilot aero academics are included in the RV Transition Training Manual (reference the sticky at the top of the VAF Safety page for a link to the most current version).
Full disclosure: I don’t have a PhD in aeronautical engineering, but here’s a great paper by someone that does that explains the science behind our AOA development work:
https://drive.google.com/file/d/1WELUD--STAb77zxNegIGNXtFioI3IFmX/view?usp=sharing and here’s the short, three page version that summarizes all of the basic relationships:
https://drive.google.com/file/d/12dCY5b0jFgEn3G-J8d0ZgIwf7IJB1tci/view?usp=sharing. The author, Dr. Dave Rogers, also graciously helps with our AOA project, making sure that we are always coloring between the lines.