No, no, no, Carlos. If you're going to weigh it multiple times, on trials 2 and 4 you have to turn the plane upside down, then add the negative readings to the positive readings from trials 1 and three to arrive at the true weight expressed in kilopascals, which is then divided by the wing area, correcting for the weight of the rubber bands. If there are no rubber bands, then you need to add some, and this is obviously why a plane that should weigh 5 # came out so heavy.

Hope this helps.