Hello,
I'm at this moment reading a book : Basics of RC Model Aircraft Design of Andy Lennon,
There are some units however i don't know (and then can't understand): The engine disp. (cid) and Power loading (oz./cid). I know what is a power loading but not the term "cid". Could someone help and tell me what does "cid" means and what is the oz./hp equivalent of oz.cid?

I can tell you that "C.I.D." stands for "Cubic Inch Displacement" , this is a measure of the swept volume of an engine's piston and is the way model engine's size was established way back when US engine manufacturing dominated the industry. For instance, an OS engines "FS 56" is a four stroke engine of .56 Cubic Inch Displacement. Nowadays a lot of our model engines are measured in "CCs" (Cubic Centimeters) rather than CID, for instance a "DLE 100" is 100 CCs .

I don't think there is a direct correlation between "oz./hp" VS "oz./cid" because I take "oz./hp" to mean how much horsepower vs how many ounces , a power to weight ratio in other words, whereas "oz./cid" appears to refer to Cubic Inch Displacement VS weight in ounces , a ratio of physical size against the weight of the engine that does not specifically note the actual power of that particular engine.

Thanks a lot. You are a great help.
I understand now. Indeed, it seems to be not possible to have a direct relationship between these two parameters (cubic inch displacement and horsepower). It does necessitate more pieces of information.
Have a great day.

I'm happy to have been able to help, and have a great day too

Could the OZ per HP refer the the amount of power required to fly a model of a certain weight. An example would be the often used 100 watts of power per lb in reference to electric power. Being that watt output is easy to measure on an electric setup. Glow engines output is not so easily measured. Usually it gets down to making an experience call.

We'd need more context to know what the author was talking about there.
To further complicate matters, Horsepower is a deceptive figure. Max HP is often made at an RPM that you'd never actually see with a useful prop with a torque production that's incredibly low. True HP/Torque curves are much more useful but rarely published. RPM numbers with various useful props are the best we usually get.