First test flight with Stolspeed VGs.

3 flights, total time about 2.5 hrs flown with instructor Michael Peare

Density Altitude: 2,600'
CG: Close to forward limit
Weight: 490 kg

Test 1 – VGs half span
Test 2 – VGs full span
Test 3 – VGs full span but leave gap of 80 cm at wing root

POWER OFF / POWER ON – 3000 RPM
(Flap setting in degrees: 0 / 15 / 48)

No VG
Power Off: 45 / 40 / 36
Power On: 44 / 38 / 35

Half Span VG
Power Off: 43 / 36 / 36
Power On: 43 / 37 / 34

Full Span VG
Power Off: 41 / 36 / 34
Power On: 38 / 36 / 34

Stall speed reduced (kts)
No VG: 4 / 4 / 2
Half Span VG: 6 / 2 / 1

Airspeeds in knots – Dynon and ASI both calibrated

No VGs
Aircraft breaks and drops the right wing. Significant wing drop with power on. Little buffet just before stall. Ailerons ineffective close to stall. Needs some rudder to correct wing drop. Ailerons a little heavy.

Test 1 – VGs half span and VGs on vertical stabilizer
Ailerons lighter, more effective. A lot of buffet warning.
Power Off: no break, no wing drop. Stick full back, aircraft nose high and buffets.
Power On: mild wing drop, quick recovery. Can be rolled 30 left or 30 right a few knots before stall. Stable.
Ailerons can be used to roll level after stall. Can fly 40-degree banked turn a few knots before stall. Aircraft buffets but does not break. Small reduction in stall speed, except power off reduced 4 kts at half flap.

Test 2 – VGs full span
Ailerons same as Test 1. Reduced buffet.
Power Off: mild wing drop, quick recovery. At zero flap, no break, descends 500'/min.
Power On: break and mild wing drop, quick recovery. Significant reduction in clean stall speed, power on/off. Aircraft can be trimmed to fly at 38 knots full flap with very high nose angle. Stable.

Test 3 – Removed VGs at the wing root (approx span of the elevator)
This was to try to get the strong buffet seen in Test 1 with half span VGs. Made no difference.

Notes
Factory stall warning gave good warning under all tests. For example, power off, full flap, the stall warn sounds at 42 kts, buffet at 37 kts, and stall at 34 kts.

Right wing drop even from left turn with left rudder: 34 kts, 3,800 RPM.

Increasing RPM to 4,000 full flap, stall speed same, 34 kts. Wing drop not significantly worse with more power (very different from standard configuration with no VGs).

Stall speeds in POH: 43 – 39 – 35 kts.

An interesting test might be to fit the VGs closer spaced over 75% span to combine the low stall speeds of full span but large buffet and reduced wing drop of half span.

Overall
The VGs on the tail did not give a noticeable difference in handling. Results are broadly similar to many aircraft, i.e., large difference in clean stall speed, less so in approach configuration. Half VGs gave the best handling but a small reduction in stall. Full VGs gave good reduction of stall speed but less buffet and more wing drop than half span.

The Allegro has powerful 48-degree flaps. The inboard span stalls long before the outboard, and the VGs cannot help much. Big difference in aileron response. Much nicer to fly. Although the Allegro has an 11 m wingspan, it feels light in roll. Surprisingly, removing the VGs close to the wing root did not bring back the buffet that was present with half span VGs. These VGs were a simple and inexpensive way to improve safety and handling.

Regards,
Paul