How to increase spacing between duplicated objects parented to a curve using *DupliFrame* ? I have a plane that has been duplicated along a curve:



I parented the plane to the curve (Ctrl+P) and then used DupliFrames on the plane to get the above scene. However, I want to increase the relative spacing between the planes. Any suggestions on that? 

Please don't ask me use to Array Modifier/Curve Modifier (I know that will solve my current problem but it will also prevent my planes from staying horizontal as they are currently since they will use the normal of the curve for their orientation). Is there a way to increase the spacing under the above setup? Maybe some curve property?
If you want to increase the spacing between all the instances evenly, that necessarily reduces their number, so reduce the number of frames in the curve's path animation panel.