First off, engineers do not have any monopoly on knowledge or understanding. Some know a lot about something other than what you are interested. Let's just say you want help from knowledgeable folks. We got those, and some are engineers, others experienced in airplane configurations, etc.
Standard advice is to stick to the plans unless there is a known issue and a known good fix for the issue, then stick to the plans for the known good fix. Deviating from the plans Is entering the world of airplane design, experimental fabrication, and test pilot work in new airplanes. Most builders, even the ones on this forum are simply not prepared for this… Build to plans and you know what you will get. Start altering the plans and who knows what demons you will excite. If you just gotta do it, well, do it with your eyes open.
Now that I have the warning out of the way, let’s talk about the small angles on the engine and what going zero-zero does.
Small offset angles have been experimented with and are widely used because they produce less drag in cruise and less rudder input over the operating range than a 0-0 alignment.
“Oh, but doesn’t that adversely affect prop efficiency.” Maybe… None of us should worry over efficiency of a single item, but instead worry over efficiency of the entire airplane. When you optimize the prop but make the airplane less efficient, you have made a poorer airplane. Ideally you want the prop perpendicular to the flow through it. A 2 degree angle to the prop is so small that you will have a hard time measuring its effect on thrust as compared to 0-0, so the losses are just not big enough to worry over. Basically the efficiency scales with the cosine of the angle. Cosine of zero degrees is 1. Cosine of 2 degrees is 0.9993, or really close to 1. The energy that might be recovered with a zero angle engine is miniscule. The wing causes a gentle up wash in the air ahead of the wing, so a small tilt down is usually a good thing for prop efficiency. Then the prop causes air to spiral coming out of the prop disc. This is the energy wasted in a prop, and we want to minimize it’s total effect on the airplane. While it might seem to make the prop more efficient to have its yaw axis align with the long axis of the bird, the 0-0 prop requires rudder input to make the airplane go straight at cruise and usually more rudder input over the entire operating range. Rudder angle produces drag too, and at cruise, rudder is sometimes thought to cause more losses than the tiny loss from the angled prop...
So, fool with it if you must, but you will probably have a better airplane if you build with the angles of the plans.
Billski
