First you need to know what you want to achieve. Therefore it is important to know what is necessary in a certain type of airplane. What sort of airplanes do we have? Here follows a crude summation.

Here we will try to list the typical demands of the types. If you know more than listed below, give us a sign. Any help is welcome to complete this list, especially I could use some help in the glider-section (not my speciality).

Such a glider needs to have a large speed range. Slow speeds are needed inside the thermals and higher speeds are needed to get as fast as possible from one thermal to another. I do not know other demands yet.

I heard that these gliders needed to "shave" (flying from left to right and back again) a ridge. They fly mostly into the wind at an angle (wind in their sides). From this point I think that a ridge glider needs to be very stable, even autos table if possible. They fly most of the times "crosswinds" (wind in the sides) and they can be hit by a ridge-wind-gust. Forward sweep doesn't seem to be the ideal thing here unless you have a very large tail to react quickly. I have no idea if there are any speed demands. But like I said: gliders are not a speciality.

Motor gliders have an engine to get them in the air. And that is all the engine has to do. So you don't need a large engine as in an ultra light. There are some known models that have the possibility to restart their engines in the air. That would be nice, but not necessary in a basic model. Restarting an engine requires an electrical starter and a battery. They sound like a lot of weight to me. Gliders have mostly a good glide ratio. The better the glide ratio and the lower the weight, the smaller an engine has to be. So here again we see that the engine may be smaller than an ultra light. Most ultra lights have a glide ratio like a brick... Although there is a good evolution in that area.

Always keep in mind that this sector is mostly used by persons without the means to buy a sport plane or without the will to complete schooling to become an authorised pilot (ultra light-pilots don't need a pilot license like the pilots of larger sport categories, they do need a (less expensive) schooling in a ultra light club). What do we want to make clear: don't make your design to expensive. What would you choose if you had the money, a tiny ultra light or a small sport airplane at the same cost? My choice is quickly made.

So no need to make retractable landing gear, fit in a megablaster or an ILS-module. Just the basics! A closed cockpit should be the maximum.

Every category has his typical restrictions by law. Like with the ultralights we advise you to contact your local department of aviation to get the correct list of restrictions. Since there are a lot of categories I will not list them (if someone knows a site where you can find this info. We will place a link)

I don't think that an employee of Boeing or Northrop or any other large firm will need to be explained what a large aircraft has to do. So I leave this section untouched. I only will give you some quotes.