The first time I first saw Maple in her southern-Michigan hangar that day in September 2018, I noticed that the GlaStar's four delta-wings had not been installed on the wings; Denny Lawrence, the builder, had left them assembled, but unpainted, in a box. Days later as I headed westward in my new plane, I figured the 36-hour trip home to the Pacific Northwest was long enough to learn how she'd handle without them. But by Colorado Springs I knew: she didn't need the delta-wings, she needed micro-vortex generators. See Why VGs? for general information about vortex generators.

Denny Lawrence assembled the delta-wing VGs, but left them unpainted and uninstalled.

To understand the scope of installation of VGs, I divided the project into the following phases, which have now been completed:

Phase 1: Record speeds in various flight configurations (no VGs) | Attach tufts to left wing
Phase 2: Fly with tufts installed | Record video | Analyze video and data
Phase 3: Install temporary VGs | Analyze video and data | Relocate VG positions | Retest as necessary
Phase 4: Analyze video and data | Set final location of VGs

As of March 2023 the following phases have not yet been completed:

Phase 5: Remove VGs | Prepare for painting
Phase 6: Paint VGs
Phase 7: Weigh and permanently install VGs
Phase 8: Flight test | Prepare Instructions for Continued Airworthiness | Update Weight and Balance/other documentation

* * * * *

PHASE 1: Record speeds in various flight configurations (no VGs) | Attach tufts to left wing

Goals:
* to establish a baseline for comparison
* to enable wind-flow analyses

First I flew Maple, with no tufts and no VGs, to establish a baseline showing airspeeds.


STALL-SPEED TEST RESULTS OVER MULTIPLE ATTEMPTS (no tufts, no VGS, wings level)

June 18, 2020
OAT: 16°, 4,000 ft AGL, max gross weight, full aft CG

PHASE 2: Fly with tufts installed | Record video | Analyze video and data

Goal: Discover where on the wing the boundary layer separates in various flight configurations

As previously discussed, the goal of the tuft test was to learn where the trouble spots are located on the wing, for subsequent placement of the VGs.

I installed tufts on the left wing in accordance with Jerry's VG installation instructions, then flight tested and recorded the movement of the tufts in the same flight configurations as used in Phase 1. Later, after flying multiple times with the tufts and viewing the video, I realized that while the wing stalls at different airspeeds in different configurations, airflow separation from the wing occurs in the same multiple locations regardless of configuration: power off, power on stalls, with and without flaps, slips, etc. All videos pointed to two major areas where the airflow separated from the wing at the slowest airspeeds and in stalls, in different configurations:

• an area spreading in a V-shape starting just aft of the leading edge near the strut, spreading quickly toward the wing root, trailing edge, and the flap-aileron junction
• subsequently, a second area spreading from near the wing root, then spreading back toward the trailing edge then outward

STALL-SPEED TEST RESULTS OVER MULTIPLE ATTEMPTS (with two cameras and tufts; no VGS)

July 10, 2020
OAT: 18°, 4,000 ft AGL, max gross weight, full aft CG

Stall speeds in power off and power on in all configurations with cameras and tufts were about the same as without. See Phase 1 Stall-Speed Test Results (supra).


PHASE 3: Install temporary VGs | Analyze video and data | Relocate VG positions | Retest

Goal: To find VG locations that provide maximum benefit

Jerry's vortex-generator installation instructions called for placing the first pair of VGs about seven percent of wing-chord (aft of the wing's leading edge) and eight to ten inches outboard of the wing root, and space subsequent VG-pairs at eight to ten-inch intervals on centre till reaching the flap-aileron junction, then reduce their spacing to six to eight inch intervals on centre for the remainder to the wingtip. However, after placing the VGs at seven percent aft and with the recommended spacing, post flight-test results were disappointing, both in terms of reduction of stall speed and elimination of the initial boundary-layer separation from halfway down the span of the wing.

This prompted a telephone call to Jerry for consultation. After some discussion, I asked Jerry, "why are the pairs of VGs so far apart? I see other aircraft with much closer spacing of VG pairs, perhaps four to six inches on centre." His answer was that spacing the VGs at eight inches to the flap-aileron junction, then six inches to the wingtip was to avoid drag. Indeed, it turned out that this is a general rule of thumb for certified aircraft.

Increased parasitic drag on my taildragging aircraft was not a concern (if I'd wanted speed I'd have bought a Mooney Acclaim); I wanted significantly slower stall speeds and improved low-speed handling.

Thus I began experimenting with changing the VGs on both wings, moving them closer to the leading edge and tightening VG-pair spacings, flight testing each iteration. The VGs were firmly attached to the wings with double-sided tape—but were generally repositionable once only—so most adjustments to VG positions required cleaning and application of fresh double-sided tape to the vortex generators. After completing a series of adjustments and eight test flights from August 5, 2020 to October 31, 2020, only one vortex generator was lost out of 108 installed.

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PHASE 4: Analyze video and data | Set final location

Numerous adjustments and test flights ultimately revealed the sweet spot of slowest stall speed and improved handling during very slow flight. The tufts showed that the boundary-layer separation at mid-wingspan had been eliminated, and that in stalls separation began at the wing-root. Best of all, the position of the VGs enabled a stall speed of 34 knots power-on, full flaps in level flight, a six-knot reduction from baseline stall speeds (see infra).

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FINAL STALL-SPEED TEST RESULTS (with tufts and VGs)
VG spacing at five-inches on centre from wing root, then six-inches on centre to wingtip


October 31, 2020
OAT: 10°, 4,500 ft AGL, max gross weight, full aft CG