First, "Dumbed down notes" may be a bad approach. As an instructor you are trying to explain something to a student that is probably new to them. You need to explain it in a way that they can make the leap from their knowledge base (area of knowledge) to yours. They are not dumb, they lack specific knowledge at this time. Later you can decide if they are dumb or not...
Imagine in a dictonary to define: Airplane - is an airplane. You can see this won't work because the student still doesn't know what an airplane is. If you define an airplane as: A machine that moves through the air supported by fixed wings, propelled by the thrust of an engine. They may have to look up specifically what "wings" and "engine" are but, it is a better deffinition because they (hopefully) know what a machine is, what air is, what wings are, and what an engine is.
You can also explain by an example. First you need to find out what background a student has. If you have a 50 year old mechanical engineer as a student, the mechanical explanations may come easier. If your student is a 40 year old house wife, you may need more domestic examples to find their knowledge base. They may have a wide background which makes it easier for you to find common ground for an explination, but you need to find this out early. A student that has no background makes it more of a challange to you. In your spare time read the book "Seven Kinds of Smart". It can give you ideas to look for common ground.
But this is not helping you write your book. The examples given in eariler posts in the thread are good. Direct drive is when the propeller is directly connected to the crankshaft. And taking them to a hanager to show an example is very benificial to the student. (make friends with the mechanic early and always ask permission, and stay ot of his way when they are working.) I suggest you give an example of another kind of drive to aid in the students understanding. A geared propeller is:... to show the difference so the student can understand the differences and not just a fact. It all depends in what level of learning you are trying to teach; Rote, Understanding, Application, or Correlation.
This is a long rant but, I just was concerned that you were using the word "dumb" with "student". The level of your book may be at the Rote level, the student may not be dumb.
Remember that to be an "Instructor" you only have to be ahead of the class by a couple of pages in the book. To be a good or great instructor requires much more.
I'm getting off my soap box now.
JAFI