Flew another tuft flight on 11 January. Whole video may be viewed here, and I've included edited portions below:
The objective of this flight was to adjust stall warning to FAR 23 standards of Vs + "not less than 5 kts" and then test the result in a series of 45 deg banked gliding turns (which is optimum bank angle for a turnback maneuver) to determine the aerodynamic margin. Aerodynamic margin is plotted in a previous post and is the difference between actual airspeed and stall speed adjusted for G and actual AOA and stall AOA. I screwed up some previous testing where I had warning set to 115% Vs. The AOA system in the RV-4 allows me to adjust the actual alpha for stall warning real-time, via Wifi (I use my iPhone to make changes). So, the methodology is to derive 1G Vs at test conditions, then fly a trim shot at Vs + 5 kts and re-adjust warning AOA.
Flying an airspeed only turnback maneuver is higher workload than using an accurate AOA cue. The hypothesis we have been working on this month is that if only airspeed was utilized in the conduct of a turnback maneuver, use Vref (as computed and tested to FAR 23 standard) and honor the stall warning. This proved to be doable in the RV-4; but is more of an instrument than visual maneuver. Maneuvering flaps help.
We typically think of Vref as Vs x 1.3. This is actually very close; but the flight test requirement to validate this takes G load into account--specifically a 45 degree bank angle. The short answer is that the airplane has to be able to pull sufficient G to fly a 45 deg banked turn without tripping stall warning. In this condition, there is a desired 3 kt aerodynamic margin. A couple of examples of simulated engine failure followed by a 210 degree turnback maneuver can be viewed here if you are interested in tuft behavior under those conditions:
Rick Marshall was nice enough to put this math in tabular form. The "alpha" reference on the top line of the table simply refers to the variable "a," not angle of attack. It's actually bank angle:
If you listen to the tape, you can hear me suffer thru the cockpit math real time...Also several mis-speaks--I have a bad habit of referencing stall as "breakdown of directional stability." This is generally not correct. I really mean "loss of longitudinal stability" if I can't hold the nose up and "loss of lateral stability" when I lose roll control during the accelerated stalls. I need to fix this, no excuse.
What's cool about the tufted wing, is that the inboard/aft tufts begin to show signs of separation at 2-3 MPH/kt prior to the stall. Here's a short video showing these two tests (note the difference in behavior of the inboard/aft tufts between a flaps 0 and flaps 20 condition): https://youtu.be/W9Zr-Gd4IoI
If you fly airspeed only without a stall warning system, and have your Vref dialed in IAW FAR 23, and pull 1.4 G's to capture final, this is your margin--which is to say, not much. Hence old rules of thumb like Vref + 5, Vref + winds, etc. That will give you more margin, but will also mean excess energy when you roll out. These physics are why us knuckle-draggers like to fly alpha for approach and landing--overall just a whole lot easier and more consistent. No math.
Interestingly, in the Boeing I fly at work, the airspeed indicator is smart enough to compute a G-required airspeed and as you maneuver the airplane, you can see the "foot" rise and fall real-time. Part of the display shows airspeed margin, and part shows actual stall speed adjusted for conditions. That's a neat way to depict the aerodynamics intuitively, but requires looking inside of the cockpit and is still harder than just flying the tone.