Here is an Idea, In a paint program create a circle, color it black Next take a white paint brush and create a jagged blurry white line. One that kind of looks like a blurred propellor. You can use this as a transparency map. See where I am going. Take this a play with the gray levels until you have not a solid black background but almost. you want a nice yet defined gradient. Next create anothe circle the exact same size and fill it with a bright yellow. Next inside this circle make a smalle circle not much about a quarter inch around all sides and fill it with black. This is your color map of your propellor the black being the blades and the yellow the yellow painted tips. Create a cirular flat plane in bryce and apply the color map to difuse channel and the gray blurry line circle to the transparency channel. Play with the settings. The transparancy map should only allow the blurry part of the black and yellow disk to show making the rest transparent to the degree you wish by playing with the values. Should work well. What do you think?