NASA Technical Paper 2256 - CAFE Foundation

This report presents the results of several NLF wind-tunnel and flight experi-
ments recently conducted by NASA to determine the maximum Reynolds number, Mach num-
ber, and sweep-angle ranges over which the smoothness of modern, practical airframe
construction techniques fail to meet requirements for NLF in favorable pressure gra-
dients. These expeciments were designed to address the issues of achievability and
maintainability of NLF on production-quality airframe surfaces in typical operating
environments. The significant factor that distinguishes these _ecent flight experi-
ments from those of the 1930's and 1940's is the difference in preflight preparation
of the surfaces tested. The recent experiments were conducted on production-quality
surfaces, that is, on surfaces which received no modification by filling and sanding
to meet the airfoil contour or waviness requirements for NLF (minor exceptions are
described in the text).
Full-scale flight and wind-tunnel experiments were conducted with the following
specific objectives:
I. Examine the effect of increasing Reynolds numbers and Mach numbers on laminar
flow for production airframe Surfaces.
2. Observe transition locations on a large variety of aerodynamic surfaces
(including wings, fuselage nose, wheel fairinqs, horizontal and vertical
stabilizers, and propeller spinner and blade airfoil surfaces), and, where
possible, correlate measured transition with empirical predictions.
3. Measure the effect of total loss of laminar flow (fixed transition due to
transition grit Or simulated rain) on airfoil behavior, airplane perfor-
mance, and stability and control.
4. Observe the effect of a propeller slipstream on laminar flow.
5. Document the effect of flight _%rough clouds on transition.
6. Observe the practical effects of wing leading-edge sweep on spanwise contamination
of the attachment line.
7. Investigate the nature of insect contamination on an NLF airfoil in flight.
Eiqht different airplane types were used in-the fligh t experiments: two high"
aspect-ratio canard, pusher-propeller configurations (the Rutan VariEze and L0ng-EZ);
a large negative-stagger biplane, tractor-propeller-confiqurati0n (the Rutan Biplane
Racer); a business jet (Gates Learjet Model 28/29 Longhorn); two low-wing airplanes
(Beech 24R Sierra and Bellanca Skyrocket II); and a high-wing single-engine general-
aviation airplane (Cessna P-210 Centurion). The eighth airplane, a Beech T-34C, was
equipped with laminar-flow gloves on which experiments were conducted to provide
additional data to support the findings of the other flight experiments. The wind-
tunnel experiments used only the VariEze airplane.
Based on the results of these flight and wind-tunnel experiments, this paper
provides a new appreciation for the achievability and maintainabillty of NLF on mod-
ern airplane surfaces at chord Reynolds numbers representative of business and com-
muter transport aircraft. The implications of these results to further airplane
designs, flight testing procedures, and further studies are also discussed.
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