Stall/Spin-Proof Your Plane
With Leading-edge Vortex Generators (LVGs)
Does your airplane like to drop a wing (aka “tip stall”), unexpectedly?
This could be fatal if you “lose it” at low altitude in the base to final turn. If one wing stalls before the other, the aircraft will be rolling and inverted in short order. I have had different models over the years with this issue. One was a big beautiful 81 inch span Bonanza that got into an un-recoverable spin all the way down to the pavement. The forward fuselage was destroyed, but fortunately the wing and empennage were relatively unscathed. I was able, years later, to rebuild it and it is flying today.
A Strong Vortex Will Keep the Airflow Attached
As you know, a wing stalls when the angle of attack (AOA) gets too high and the airflow separates from the wing, reducing lift and increasing drag. Leading-edge extensions, or strakes, along the forward fuselage of modern fighters, allow them to maneuver at high AOA without stalling. They generate strong vortices that keep the airflow attached over the wing. In the photo above, the vortices are made visible by water vapor condensation in the low pressure vortex cores.
Taking inspiration from this, I surmised that some sharp-edged shapes distributed along the leading edge of a straight wing airplane would have the same effect. In fact, it does! Here is my Bonanza before adding the leading-edge vortex generators (LVGs), and after:
To show the effect that the vortex generators have on eliminating stall, I put a GoPro camera in the Bonanza and had it look out over a wing that had tufts of thread attached for airflow visualization. Here is a video of the Bonanza:
Note that it takes only a blink of an eye, less than a second, between when the thread tufts all point forward, indicating complete flow reversal (stall) until the airplane is 90 degrees nose down!
In the next video of the wing with the LVGs attached, compare how the tufts of thread behave, and note that the wing never drops and control is maintained. The tufts wiggle around as you would expect while in a strong vortex, but never fully reverse across the wingspan. Full up elevator control is applied, and the airplane just mushes straight ahead:
The Bonanza isn’t the only model airplane that had issues with “tip stall” that got fixed with the LVGs. Here are others:
The model on the left has a 48 inch span, the one on the right has a 45 inch span, and the F-105 in the middle has a 21 inch span. The LVGs on the big Bonanza are 2.5 inches long, and the smaller models have 1.75 inch long LVGs. The LVGs are evenly distributed across the span of the wings. All modified airplanes can be maneuvered aggressively without fear of dropping a wing, rolling off into the ground!
Application to Full Scale Piloted Aircraft
Full size piloted aircraft also make use of vortex generators. “Micro” vortex generators are used to reduce stall speed, but don’t provide strong enough vortices to completely eliminate stall and spin.
The LVG concept should work for full sized airplanes to eliminate the deadly base-to-final-turn stall and spin accident. However, significant work would need to be done to determine the optimal size, shape, and distribution of the LVGs. FAA certification would be a major undertaking.
RC Airplane Innovations 