To answer your question, yes friction affects the measurement, weight of the plane affects friction, as does the quality of your wheels, wheel size, etc. Whether the plane is moving (rolling) while you take the measurement affects the contribution of friction. Grass will give you pretty unreliable measurements. Pavement is better. For kicks, you might compare the load while pulling the plane slowly back, with and without power on. The difference, theoretically, is thrust net of friction. You might find in your case the friction is really small - or not. A more accurate approach would be to suspend the plane in a craddle, which would offer less friction than the ground. Probably the best experiment would be to mount the plane on a plate with high quality slides, or better yet, do that with an engine test stand.