The ornamental design for a lightfixture for a model airplane, as shown and described.
FIG. 1 is a front view of a light fixture for a model airplane;
FIG. 2 is a rear view of the light fixture;
FIG. 3 is a left side view of the light fixture;
FIG. 4 is a right side view of the light fixture;
FIG. 5 is an enlarged top view of the light fixture;
FIG. 6 is an enlarged bottom view of the light fixture; and,
FIG. 7 is a right side view of the light fixture and a left side view of the light fixture secured to wingtips of a model airplane.
The broken lines showing the airplane in FIG. 7 is for environmental purposes only. All other broken lines represent parts of the article not claimed. The broken lines form no part of the claimed design.