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  #11  
Old 01-08-2022, 05:18 PM
Vac Vac is offline
 
Join Date: Jul 2009
Location: Niceville, Florida
Posts: 511
Default 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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Mike Vaccaro
RV-4 2112
Niceville, Florida

Last edited by Vac : 01-10-2022 at 07:42 AM. Reason: Added note regarding incorrect stall warning alpha setting.
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  #12  
Old 01-10-2022, 12:07 PM
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chepburn chepburn is offline
 
Join Date: Apr 2006
Location: Ottawa , Canada
Posts: 237
Default

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
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  #13  
Old 01-10-2022, 12:23 PM
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Taildragon Taildragon is offline
 
Join Date: Jun 2019
Location: Key Largo
Posts: 9
Default 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 donít have the warning system that you have installed. Routine stall practice should be practiced by all.

Much appreciated,
Beau
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  #14  
Old 01-10-2022, 02:44 PM
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Dan 57 Dan 57 is offline
 
Join Date: Sep 2018
Location: LSZF
Posts: 963
Default

Thanks for the work and sharing Mike!
Fascinating to see flow separation at work
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  #15  
Old 01-10-2022, 06:02 PM
Vac Vac is offline
 
Join Date: Jul 2009
Location: Niceville, Florida
Posts: 511
Default Data

Chris,

Drop me an email at vac@flyonspeed.org. 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
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RV-4 2112
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  #16  
Old 01-14-2022, 08:07 AM
Vac Vac is offline
 
Join Date: Jul 2009
Location: Niceville, Florida
Posts: 511
Default 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:



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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Mike Vaccaro
RV-4 2112
Niceville, Florida

Last edited by Vac : 01-16-2022 at 03:19 PM. Reason: Added links to edited videos and adjusted verbiage appropriately.
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