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Tuft Jig and Stall Behavior

Vac

Well Known Member
Benefactor
I recently built this extremely high-tech, fully digital, solar-powered tuft jig for some stall testing:

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I scored the top of the 2x4 with a miter saw, and held the yarn with two fingers whilst slicing with a razor blade...used standard blue tape to lay out the tuft lines:

3c039a_79bc51964c8349dcab3bb9baef7c8972~mv2.jpg


And used purple "no mar" Scotch tape to hold the individual tufts in place:

3c039a_58248d9bc95e40ba9e09752b4037ccc5~mv2.jpg


Built a new mount to raise the oblique camera above the canopy skirt to improve the field of view of the tufts during flight:

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The purpose of this drill is to document separation behavior when flying near the stall and stalled. The constant thickness, no-twist, no-taper RV planform tends to stall "all at once." If the airplane is in a gliding turn (say, trying to maneuver after a loss of power during initial climb phase), this breakdown in lateral stability (fancy way of saying "stall") manifests itself as a sudden wing drop. If you plot AOA and airspeed margin (how close the the aerodynamic limit you are) in a gliding turn at the edge of the envelope, it looks like this:

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The valleys in the airspeed and AOA are actual stall (wing drop) where you can see the margin (difference between actual speed and AOA and stall speed and AOA) is zero.

Here's another plot of the same maneuver showing angle of bank (roll) and AOA. Note that as the airplane stalls, sudden uncommanded roll (wing drop) occurs:

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Fly safe,

Vac

P.S. The indicated stall speed increases as a function of the square root of G load. A 45 degree banked gliding turn in the RV-4 averages 1.3 G's. Assuming you want a safety buffer, you can multiply indicated stall speed by the square root of 1.5 to determine stall IAS during that maneuver. Keep in mind, stall IAS will vary with gross weight. Easier for us mathematically challenged types to reference AOA when we are slower than corner velocity/maneuvering speed if the objective is to avoid the aerodynamic limit.
 
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Thanks for posting this. That's some interesting data you collected.

Would you happen to have video of the tufts movement entering and during a stall?
 
There is no indication if a loss of directional stability, as noted in the X axis of the 2nd plot. That would require measurement of sideslip and rudder angle. You must be referring to lateral stability.
 
Scott,

You are correct. Thx! Changed directional to lateral above.

Vac
 
A little trouble locating the tuft tape . . .

And used purple "no mar" Scotch tape to hold the individual tufts in place:

Thanks for another great and well done test!!

Scotch seems to make a million tape types. Is this what they call "wall safe" with a purple splashed package?
 
Bill,

Agree, I'm not sure Scotch has ever made the same tape twice (they must have hired some folks from Lycoming ;)). I found this "purple" tape in the aviation isle at my local Ace. Does a good job and no issues with the paint. I did learn it's probably a good idea to dip the bitter end of the tuft in some Elmer's or something similar to keep it from unraveling...20/20 hindsight, of course.

Mike,

Here is the raw video from yesterday's sortie. Some interesting things to note. Prop wash extends out to the second wing bay (just outboard of the no-skid pad). Looks as though there is a decent vortex generated by the stubby, low-aspect wing (as would be expected) that keeps the outboard bay or two of the wing energized during a stall. This may explain why the aileron remains effective during a stall (I'm not advocating for use of aileron at high alpha, just an observation). The other observation is how fast "reattachment" occurs when you east the stick to recover--darn near instantaneous and impressive to watch. For these tests, I have stall warning (high pitched, rapid beeps) adjusted to FAR23 115% Vs. I normally fly with less warning (personal preference) to have more useful "slow" tone available for maneuvering. RV's essentially fly until they don't, stall is abrupt when it occurs; and recovery is prompt.

https://youtu.be/pwzrWaOpy_o

In the video, you hear me reference the deceleration display. For years we've always used a nominal 1Kt/sec deceleration rate for stall testing; but nothing in the cockpit shows you acceleration or deceleration rate. Since one of our objectives for the AOA system is to develop a calibration "wizard" we wanted to give the pilot an easy visual indication to fly when decelerating to a stall, so Lenny developed a nifty graphic that allows you to easily make 1/2 kt adjustments. That display is also super handy for flight test work.

Fly safe,

Vac
 
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Vac, hats off to you. I know how long this takes and the amount of resources required (which is the reason very few go after it). Thanks for sharing this with our community.
 
Slow Motion Video

Thanks Norman and Axel. My pleasure.

I had a chance to do a bit of video editing and add some slow motion sequences. It's interesting how fast things happen, even in slow motion! The videos show the maneuver real-time, then show tuft behavior during stalls in slow motion:

Here's a basic 1G, wing's level, flaps zero stall:

https://youtu.be/Lce_4oA_UBI

Here are a couple of accelerated stalls during a gliding turn:

https://youtu.be/j2H-ssR83_g

Power on stalls:

https://youtu.be/XBW1Y3ugTdo

Takeoff and departure stall to simulated turnback:

https://youtu.be/pcNarYGHIUg

Fly safe,

Vac
 
Slow Flight

I flew some slow speed trim shots at speeds from stall warning (set to 115% Vs) down to Vmin. The airplane exhibits a perceptible nose slice (inability to control heading or breakdown in directional stability) at about Vs + 2 MIAS. And, like all RV's, the airplane abruptly transitions from flying to not flying, back to flying (if AOA is reduced) when encountering a stall. In other words, natural buffet cues are lacking; but the airplane starts flying as soon as AOA is reduced.

In the video, you'll hear me referring to indicated airspeed. This is read off my primary EFIS in MPH. These numbers are specific to my airplane. Unless airplanes are identical and pitot/static systems are configured identically; any reference to a specific IAS number only applies to that airplane under similar conditions.

'Course AOA is constant from airplane to airplane of the same type, doesn't care about G, gross weight or density altitude :). If you look at the video carefully, you'll see the AOA (alpha) vane deflected downward on the boom at high AOA. Minus upwash, that's a visual depiction of angle of attack. At 21:00 minutes elapsed, you can see the test boom, vane and horizon line. The difference between the boom and horizon is pitch. In a stable condition (level, unaccelerated flight), pitch (plus incidence angle--1/2 degree for the RV-4) is equal to geometric angle of attack--any difference you observe between the AOA vane and the horizon in a stable condition is upwash. Neat to see that outside of a wind tunnel.

Here is the raw video:

https://youtu.be/iPl8CIRKcUo

Fly Safe,

Vac

10 Jan 22 Addendum: Tape monologue refers to "FAR 23 standard stall warning set to 115% Vs." This is incorrect. FAR 23 required stall warning is "not less than 5 kts." For the RV-4 with an EFIS MIAS stall speed of 51 MPH at 1G and test weight, stall warning should have been set to 56-57 MIAS. For this test, stall warning was set to 59 MIAS (115% Vs).
 
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Great work Mike,

Did you happen to time sync your video with your instrumentation? It would be interesting see a plot of AoA at a given tuft time point. I would like to correlate the tuft behavior pattern against a 3D viscous CFD model I am playing with for the RV. I don't expect it to be even close, but hey, it will still be interesting :) Especially the root stall progression pattern as the wing let go.

Regards,
Chris
 
Great Stuff Mike!

Thanks Mike for sharing this info and the videos. Pretty sure my Harmon Rocket II has the same wing as your RV4. I found the stall characteristics also to be very defined(sudden). I don’t have the warning system that you have installed. Routine stall practice should be practiced by all.

Much appreciated,
Beau
 
Data

Chris,

Drop me an email at [email protected]. Happy to share the data files (and full res video) with you. I'll check with the flight test instrumentation engineer (better known as Bob) to see how we can marry things up. Data is recorded at 50Hz and oblique camera is recording at 30 FPS; so I'm thinking we should be able to get things pretty close.

Cheers,

Vac
 
FAR 23 Stall Warning Lessons Learned: New Video

Flew another tuft flight on 11 January. Whole video may be viewed here, and I've included edited portions below:

https://youtu.be/SLjISNBneno

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:

https://youtu.be/9S4XP0wMACc

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:

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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.

Fly safe,

Vac
 
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