If you are looking to hit 70mph...... I am pretty sure you will not be disappointed

Once the gear is up on the phantom it is fairly quick.

Just as a reference, this video is an AKModels SU-27 with 90L power. It is about the same size, maybe a little bit lighter, but it has about the same frontal cross section area (drag from the hole it much punch in the air). [link=http://www.jettengineering.com/video/su27baker.wmv]Rich Baker video[/link]

There are a few other projects for reference on here too: http://www.jettengineering.com/hall/hall-propjets.html

Always happy to be of help

Bob