Flight-test feedback

December 2019

Q. After landing, a test pilot says the prototype jet was difficult to control in certain configurations and wanted to stall sooner than it should. He assesses “a Cooper-Harper of 7 to 8” at times. One of the engineers sighs: “Well, it was worth a shot, but we’d better put them back on.” What devices is the engineer most likely referring to, and why would it be desirable to leave them off?

Draft a response of no more than 250 words and email it by midnight Dec. 9 to aeropuzzler@aiaa.org for a chance to have it published in the January issue.

FROM THE NOVEMBER ISSUE

CANARD CONUNDRUM: We asked whether Rafale fighters and NASA’s X-59 low boom design have canards for the same reason. NASA’s Mark Guynn and Clint Balog of Embry-Riddle Aeronautical University helped us select the winning response:
WINNER: False. The Dassault-built Rafale fighter is a two-surface airplane which utilizes the canard for pitch stability and control. The X-59 is a three-surface airplane and uses the canard to redistribute the lifting surface area for the purpose of reducing the sonic boom made during supersonic flight. Most aircraft’s wings create lift aft of the aircraft’s center of gravity (CG) which results in a negative moment. Therefore, an additional lifting surface is required to balance the moments in the pitch-axis. In conventional aircraft, a tail is used to create negative lift and a positive moment. In the case of the Rafale fighter, the trailing-edge of the wing is already at the back of the plane. The designers utilize a lifting surface forward of the CG to balance the moments to maintain pitch stability while giving controllability in the pitch-axis with changing the canard’s incident angle.

The three-surface design utilized by the X-59 allows the wing to be smaller due to the canard providing lift. Also, the canard helps reduce the size of the horizontal tail because the forward-of-CG lifting surface would require a smaller aft lifting surface to balance the moments. By redistributing the cross-sectional area of a wing and tail along the fuselage, the designers may be able to achieve a quieter sonic boom. Loud sonic booms are a result of shock waves building up from different surfaces of an airplane. By redistributing the cross-sectional area, the shock wave buildup can be reduced.

Royd Johansen

AIAA member
San Jose, California
Johansen is an aeronautical engineer for Northrop Grumman.