NASA tests Long-EZ
October 1981
Researcher Bruce Holmes and Research test pilot Phil Brown of NASA’s Langley Center recently visited RAF to study the Long-EZ. Of particular interest was the measurement of the extent of natural laminar flow on the flying surfaces. Also, the stall characteristics and departure susceptibility. They had previously measured extensive laminar flow on their full scale VariEze in the Langley 30 x 60 wind tunnel.
This was verified by flight tests at Langley of Bob Woodall’s VariEze. They found essentially textbook-predicted boundary layer transition locations are being achieved with the airfoils on the EZ despite the presence of wing sweep and canard impingement This is due to the stable contour that is achieved with our full-core composite construction. The Long-EZ main wing airfoil was designed by Richard Eppler.
It has a steep initial pressure gradient intended to provide a reasonable probability of laminar flow despite minor leading edge contamination. The Eppler computer code predicts 32% chord laminar flow on the upper surface for a perfectly smooth surface. As the photos elsewhere in this newsletter show, the sublimating chemical tests on Long-EZ’s N26MS and N79RA verify that the wings are achieving the full 32% chord laminar flow. Small insect remains on the leading edge forward of about 4% chord will not trip the boundary layer. Bugs aft of 5% chord will destroy laminar flow, as will the small bump of a paint stripe.
We now have documentation of the boundary layer characteristics on all surfaces and intersections of the Long-EZ. Also of interest to NASA was the departure-immune stall characteristics we have noted during our tests. NASA wanted to test the spin susceptibility to supplement the extensive data they have gathered on all types of general aviation aircraft.
Phil put the Long-EZ through all types of extreme stall entry conditions: Accelerated entries, vertical entries, etc. with all combinations of control inputs. He also alternated left and right rudder inputs at the Dutch roll natural frequency, combining opposite aileron to add adverse yaw effects, at the maximum attainable angle of attack. Despite all combinations of gross misuse of flight controls, and attaining over 45 degrees side-slip, he was unable to obtain a departure from controlled flight.
Phil left with the comment that he could find no way of inducing loss of control in the Long-EZ. This is a stark contrast to the general aviation aircraft configurations he has been testing at Langley, all of them being relatively susceptible to loss-of-control or spin entry from any aggravated stall entry. Many of them have unrecoverable spin modes that require wingtip rockets or anti-spin parachutes to effect recovery.