Great Discussion
I love VAF...this is the kind of discussion that makes this site (thanks Doug) an absolute gem in the EAB world. It's not my intention to turn this into an AOA discussion (or bar fight of AOA vs airspeed), just offering some observations based on experience and lots of instrumented testing of the RV-4...
Some points to ponder in no particular order:
1. The "impossible turn" is actually a low-altitude target re-attack. This is something that air to ground guys are generally reasonably good at. Turn rate/radius and energy management is the key. In this case, you have come off target at a relatively low energy state, but the FAC (forward air controller) wants you back in ASAP on a reciprocal axis. The "target" is the TDZ, and the attack parameters are to be in a three point attitude at an energy state commensurate with touch down over the TDZ.
2. Only two factors affect turn performance: radial G and TAS. A steep, high-rate turn is more productive when overall energy is low (i.e., not very high or fast); but it can be eye watering if you are not used to maneuvering relative to the ground (think crop duster or A-10). Absolutely possible to manage turn performance using airspeed and G, but both will vary considerably depending on conditions.
3. Using AOA as primary reference greatly simplifies ANY maximum performance maneuvering in any airplane (think 1 v 1 combat or coming in to land). There is one sweet spot AOA that results in maximum sustained turn performance under any conditions: ONSPEED.
4. An airplane cannot stall ONSPEED. An airplane lands ONSPEED.
5. AOA is not effected by aircraft gross weight, G load or density altitude. It doesn't care about bank angle. If I can hear the AOA, I don't have to look inside the airplane when I'm maneuvering.
6. When maneuvering close to the ground, some use of flaps may be beneficial, depending on the airframe and pilot skill level. It's already been mentioned in this thread, but a flap setting that produces more lift than drag can be useful. L/Dmax decreases with flaps deployed and marries up nicely with ONSPEED. This isn't optimum glide performance (that happens at L/Dmax and flaps up), this is an optimum COMPROMISE of energy bleed and turn performance. If range is the ONLY consideration, maneuver in reference to L/Dmax AOA. In the video, I'm using half flaps (20 deg) in the RV-4. I'd prefer 10 degrees, but my manual flaps have only two settings.
6. Keep ground track in the cross-check (axis of attack, i.e., if landing on runway 27, heading of 270 degrees). You have to
offset the turn circle from the desired ground track, and an easy way to do that is to remember to "lean into the wind." This will compress the size of the turn circle relative to the ground. You should have a pretty good idea of wind direction if you don't have a cockpit read-out since you just took off.
7. A turn-back isn't a 180 degree turn. It's at least 270. If you know how much altitude you lose in an optimum power-off 360, that will put a little extra in the bank. If you don't have a properly calibrated AOA, it's gonna' be a
lot harder; but you can develop some good rules of thumb regarding Vref and bank angle.
8. Keep any altitude while you establish an optimum glide condition. It goes without saying that you don't want to stall; but if you're listening to your AOA on takeoff, it's pretty straight forward to hear the airplane decelerate to an ONSPEED condition when you begin to maneuver.
9. No slower than ONSPEED to the crash, wherever that may be.
10. There is no one size fits all answer, other than managing turn rate, radius and energy. Easier to do that when you've got some sort of energy cuing. That can be aural AOA (obviously I'm biased), but there are some good energy displays being developed by universities and NASA that will likely find their way to EAB first. The EAA "crowd sourcing" experiment for data is an awesome idea.
11. Experiment. Doesn't matter what kind of airplane. Participate in the EAA program if you are so inclined. Also mentioned above: use a three-count or something similar at the start of the drill, since there will be a MUCH larger pucker factor real-life. Work from high to low, i.e., experiment at altitude. Don't exceed your comfort level. If you aren't trained and plan to crash straight ahead or within thirty degrees of the runway centerline, amen. Be respectful in the bar fight and remember there isn't one right answer
12. The flying in the demonstration video isn't hard to do--I'm not that good. All I'm doing is deploying lift flaps, maintaining ONSPEED and doing whatever it takes to get to the TDZ. The airport in the video is surrounded by farm fields on the east end, so that is a tactical advantage for maneuvering.
13. Prop drag is a huge factor in glide performance. Not much use trying to stop the prop below 3K' AGL unless you are very familiar with the technique in your airplane. Harder to do with a controllable prop.
14. Fixed-pitch props produce residual thrust in IDLE. Therefore, if you do your practice in IDLE, remember actual engine-out performance will differ. Add wind to that equation and you can see that each occurrence has the potential to be unique; but that doesn't mean that you can't practice and "what if" it to develop a gameplan for your airplane.
15. If you are going to maneuver, do it NOW. Energy will NOT improve with time if the engine isn't working.
And here is the mighty RV-4 sitting mid-field after the carburetor decided to stop participating in the combustion process recently:
An engine failure is never a non-event, and this one occurred during return for landing, not immediately after takeoff; but the point is that it's nice to have a good energy reference to fly when you want to max perform the airplane. Since 1979, this engine non-cooperation event number five, so my list of willing passengers is getting smaller (two, however, where in twin engined fighters--lower pucker factor when you've got a spare with lots of thrust).
Fly safe,
Vac
Here are a list of some technical resources discussing the topic:
Should You Turn Back?
https://www.nar-associates.com/technical-flying/impossible/possible.html
The Feasibility of Turn Back From a Low Altitude Engine Failure During the Takeoff Climb Out Phase.
https://www.nar-associates.com/technical-flying/jett/jett_wide_screen.pdf
The Possible "Impossible" Turn.
https://www.nar-associates.com/technical-flying/impossible/impossible_wide_screen.pdf
Estimating the Turn Back Altitude.
https://www.nar-associates.com/technical-flying/impossible/EstimatingTurnbackAltitude.pdf
The Penalties of Non-optimal Turn Back Maneuvers.
https://www.nar-associates.com/technical-flying/impossible/nonoptimalcost_screen.pdf