The Impossible Turn...

[Vac]

Prop, partial power and rudder coordination

Prop considerations:

Great point regarding the type of propeller fitted. That will have a huge impact on glide performance. We haven't discussed drag associated with controllable types. The modeling in the app accounts prop drag to the extent the "about right" solution is correct. There are two drag "tweek" calibration features the user inputs.

The entire turnback from failure to touch down is less than a minute (admittedly, seems a LOT longer with temporal distortion when the adrenaline kicks in). The "I'd maintain aircraft control, analyze the situation and take the appropriate action" matrix is so condensed, that the "maintain aircraft control" part is going to take up most of the brain bytes. If the pilot exercised good checklist discipline prior to takeoff and is operating with a sufficient supply of clean fuel, there isn't much troubleshooting to be done. There is certainly nothing wrong with a couple of bold face (memory) items...boost pump, ignition and gas are obvious contenders; but not at the expense of aircraft control.

The app will model the drag of a windmilling propeller at high RPM (fine) pitch, so that means that condition is accounted for in the physics. I'd never fault a re-start attempt, but if I was in that situation, my boldface would be to reduce prop RPM and maneuver for landing. As we pointed out previously, CAFE testing has shown that there is no tactical advantage to trying to stop the prop to reduce drag for a power-loss event below 3000' AGL. Similarly, there is a minimum altitude below which troubleshooting is likely to channelize attention and reduce overall maneuvering SA (i.e., maintaining aircraft control). Obviously, the later will vary from pilot to pilot and situation to situation. It's a good place to establish a personal minimum. Mine are pretty straight forward: I'm below emergency bail out altitude, so my only option is to fly the airplane to the crash, and I prioritize accordingly.

Partial Power Loss:

When I was a young CFI in the late Paleozoic, I lost a jug in a Piper Cherokee shortly after takeoff when a valve exited the cylinder head through the valve cover. The remaining three continued to run until all of the oil leaked out--a good couple of minutes. Sufficient to maneuver to low-key for an alternate runway. The engine flamed out on final, but we had sufficient energy to land and clear the runway. By the same token, we lost an F-15 pilot in an RV-6 after a partial loss that turned into a complete loss of power. He maneuvered to low key in attempt to return to the departure runway in a low energy state, almost made the airport, but departed controlled flight at low altitude trying to stretch the glide end game and impacted the ground in 90 degrees of bank.

The takeaway is there is no one right game plan. It's probably best to assume complete power loss at any time and maneuver accordingly. A basic hip pocket will be to turn base for an alternate runway or maneuver to low key for the runway just departed. The app will actually do the math for the runway departed, and indicate when you are at low key with sufficient energy to make the runway.

For folks not familiar with the terminology, "low key" is the point on downwind that the airplane can glide to the runway out of a continuous base turn. The place your instructor would pull the throttle and say "just maneuver to the runway and land."

Rudder coordination:

Everyone that points out the need for proper training is spot on. Fail to coordinate, exceed critical angle of attack and the airplane is going to depart controlled flight. With an unintentional skid input it happens fast: https://youtu.be/cLg_LGjpL9Q. Likely non-recoverable at low altitude. If the airplane is properly coordinated and inadvertently stalled at low altitude, a recovery may be possible: https://youtu.be/BPD5xk1wgOw. This is why we move the AOA tone with the ball in the sound field (i.e., you hear it when the airplane isn't coordinated properly) and put a slip/skid ball on the visual display. It's critical for the pilot to have SA, especially in an airplane like an RV that doesn't provide much (or any) buffet cuing at low G.

Having said that, if the critical angle of attack isn't exceeded, then the airplane can't depart; so ultimately it's AOA awareness that is the key to maintaining positive aircraft control when you are maneuvering near the aerodynamic limit.

v/r,

Vac

 

[Vac]

Additional Graphics

The engineers converted some of our high-speed GNSS/INS data to .kml format and then used Google Earth to develop a nice 3D graphic of a couple of turn back tests . This compliments the academics in Charlie’s briefing in the first post of this thread and cockpit video well to illustrate what’s happening throughout the maneuver.

Here is the 25 Oct test (previously posted): https://youtu.be/F_RFKlUhdeE

If we plot that, the flight path looks like this:

I ran another test with updated software which fixed the glide capability graphic and voice warnings on 12 Dec. I manage to smartly screw this test up! Notice that I count to three and then pull the power after the "turnback possible" cue. This is backwards. No excuse, but there are still some good lessons to be had. Here is the cockpit and iPhone video from that test: https://youtu.be/zXjkHjFdLAA.

Here is the test plotted in .kml format from takeoff to landing:

It’s pretty apparent why I’m only able to test a right-hand turn back at my home field.

In the Google Earth pictures, you can see wind effects on the geometry of the turn circle. Also apparent in both pictures is that climb angle is steeper than glide angle.

I deal with the bit of extra altitude (a good problem to have) after each initial turn back using different techniques. In the first video, you hear my say “square corner,” as I perceive the need to lose some energy by flying a maximum instantaneous turn. Maximum instantaneous turn rate occurs at the aerodynamic limit of the airplane (i.e., right before stall). Using this technique, I pull the nose around as quickly as possible AND increase drag simultaneously. This allows me to touchdown with sufficient distance to roll-out. Notice that after “bleeding” the excess altitude, I transition back to on speed for touchdown.

In the 12 December test, you can see a similar sight picture after turning back to point at the runway, and initially I mention a “square corner,” but then elect to slip the airplane during the alignment turn instead. If you watch the AOA indexer, you can see that I transition to L/Dmax (vs on speed) in the slip. This increases descent rate and provides a bit more maneuverability. It does, however result in excess airspeed that I have to lose during the transition to the flare. An on speed slip may have been a better option, but things happen fast during a turn back and both techniques employed in the video resulted in flight path management sufficient for touchdown in accordance with “training rules” (i.e., enough runway left to allow for roll-out and stop with a 10-knot tailwind). The latter is a personal limit and determined pre-takeoff using ambient conditions. I avoid quartering tail winds with a tail wheel unless it is necessary for flight test. Just like an actual emergency, the key is to have figured this out before takeoff, not during the heat of the moment when I’m sucking up the seat cushion.

Flying the simulated power loss during initial climb and return to the runway for an opposite direction landing is one of the most difficult maneuvers I can imagine in a civilian flying environment. I'm not skilled enough to "eye ball" a turnback without help from the computer (TLAR app) and some pre-flight planning, nor am I skilled enough to max perform the airplane this close to the ground without the "eye's out" trend information provided by the AOA tone. I’ve labeled the AOA cues I’m using on the graphics and included indexer (visual AOA display) in the video. My hat's off to anyone that can fly with this level of precision using conventional instrumentation: https://youtu.be/OwcstKP3DVQ. It's a privilege to fly and work with such talented folks.

Fly safe and best wishes for a Merry Christmas and Happy New Year,

Vac

 

[rv8ch]

... Also apparent in both pictures is that climb angle is steeper than glide angle.
...

Really important point you have raised, Vac. The two angles are dramatically different for your aircraft, and I guess most RVs. We don't glide well, and we are typically over-powered, whatever that means.

I guess if you are going straight out on the runway heading, this is the fundamental condition that must happen for a successful turnback.

Any aircraft with an anemic climb rate would be wise to stay in the airport "safety cone" until getting to altitude.