How to get the correct intake size for your craft

While building planes you might have come across the problem of your engines having an intake efficiency under 100%. You solution likely was, to make the intakes of your craft larger, which might make it look worse. You might also have found, that intakes don’t need to be as large as the engine, when you place the intake with auto resize on the engine. How much smaller you can make the inlet seemed to be a guessing game.
This guide should give you the knowledge on how to make your inlets just the right size for your engines.

1. Only the intake end exposed directly to the airstream counts.
   The other end of the inlet and any following ones can be whatever size and shape you want them to be.

2. How intake air ‘flows’ through the craft is still a mystery.
   Certain parts seemingly don’t allow intake air to flow through them, but most parts do. If you place your inlet on a fuselage you should get the best results possible. An example of intake air not flowing through some parts is, that an intake placed on an engine on the tip of a wing doesn’t contribute to an engine placed on the core of the craft.

3. Shape matters.
   A round intake intakes less air than a square one of the same size. The less rounded a corner is, the more intake air the intake produces.

4. Size matters, a lot.
   Size directly effects the area of the inlet.

5. Area is everything.
   The area of an inlet is what determines how much air it intakes. To get the required total area for all your inlets you first need to know the area of the front of your engines. This is relatively simple as the ‘Size’ slider of an engine basically gives you it’s diameter in meters, you just have to convert from percent to real numbers(simply divide by 100). After this you calculate the area of a circle with that diameter. You do this for every engine on your craft and add them up, if applicable (see point 2). Now comes the fun bit.

6. Calculating your inlet size.
   It all boils down to the fact, that the sum of the area of all inlets has to be greater than or equal to the sum of the fronts of your engines (you calculated that in point 5) multiplied by a factor to get 100% inlet efficiency. That factor is the square root of 0.75, or expressed differently it is the square root of three divided by 2.
   If you use as many circular inlets as you have engines and no other inlets, you just multiply the diameter of your engine by the factor mentioned above to get the diameter of your inlet.
   For more complicated shapes you need to do the whole calculations. To do these I highly recommend advanced math tools like Wolfram, GeoGebra and similar things. You need to calculate the four quarters of the inlet area individually.
   A quarter with 0% roundness is a rectangle, with the sides being halve the length of the width/depth.
   A quarter with 100% roundness is section of an ellipse, of which you need to calculate the area. The semimajor axis is halve the length of the longest side of the inlet and the semiminor axis is halve the length of the shorter side. To get the area of the quarter you then divide the area of that ellipse by four.
   For any roundness in between I can’t tell you the formula, you just have to find it out yourself.
   Optimising the sizes of your inlets is also up to you, as this is just a guide to help you find out if your total inlet area is large enough.

7. Occlusion, your enemy.
   Occlusion is very simple if your inlet is occluded only at the plane of the inlet end by a single fuselage. You simply determine what area of the fuselage intersects with the inlet and subtract it from that inlet.
   Otherwise it probably is complicated. I currently don’t know if angle of attack has an influence, but it doesn’t seem like it, as flying backwards has no impact on your engine performance. Also wings block airflow. Parts further away block airflow too, but by how much is unknown.
   If you don’t want to deal with occlusion just enable the ‘Occlusion: never’ option in the tinker panel. Be aware that there might be side effects to this.

Lastly I want to state that this isn’t an authoritative guide and that everything stated above could very well be untrue. Just try it yourself and make your own findings. Any constructive feedback is appreciated.