To Unlock the UFO:
Beat target mode and get gold medals for each level.

To Get All Golds:
Get one complete color in numerical order.

100% in Target Mode:
You need to get one complete color in numerical order by flying through the centre of each target. Make sure you score 6 points with each target that breaks or else you wont get 100%.

To Get the Me-262:
Beat every mission in Adventure mode.

Autogyro XG-1:
Complete Target Mode by flying through all Red Rings.

To Get the Corsair:
Get 10 circled letter grades in adventure mode. To get circled letter grades, get lots of acrobatic points.

To Get the UFO Type Silver:
Beat adventure mode with an A ranking in each level.

To Get the Stealth Fighter:
Beat Adventure Mode by finishing each level in under 10 minutes.

How to Get The Extra Stuff:
There are extra planes called the Shinden and Shinden-kai, to get these planes, firstly, it helps to "buy" the special radar from target mode, then during free flight, you may notice landing areas in odd locations,

-For the Shinden,
for one of the wings, go to the labarynth level on arcadie island, from the starting piont go west of the entrance of the maze and you will see another landing strip land there to get the wing.

-Second wing, go to the first level and enter the ravine, at one point roughly mid-way through, you will notice a hole in the right hand wall, fly through it, navigate the maze, and you will find a tiny landing area, land here to reciece the other wing.

-Shinden Fusilade (the main part of the plane), go to the towers of terror level on courant island continue through the level, and at one point (it is hard to see) using the radar you will see a slightly lowered portion on the right canyon wall, fly over to find the last piece of the shinden.

-The shinden-kai, the jet engine, go to the great divide level on Blanc island, enter the canyon as normal, and just after the first split in the levels trail, you will see the landing strip with a little hut by it, land here to recieve the engine which fits the normal shinden.

Thanks to Revolution readers Enthasius and Eric Chernecki!