I have a set of Don Smith B-17 plans and have found that it lacks a great amount of detail and contains many errors in several of his sheets. So I have decided to redesign the plane to correct some of these issues. The problem I am having is engine nacelle placement. The root airfoil section has 3.5 degrees of incidence with 2 degrees of washout at the tip. The center or thrust line of the engine nacelle in the front view appears to be going through the cord line on the LE. However the side view show the thrust line is offset from the LE and would appear to be going through the cord at some other point (ie. CG or M.A.C.). What is the correct location for the thrust line of the engine nacelles?

Thanks,
Mike

I've recently had to solve a similar problem on a personal project, a King Air B200.

On any side view of an aircraft that has washout, the designer has to decide what to show on the drawing, since there are actually an infinite number of sections along the span. So, they usually just show the root, or the root and tip. On a multi-engine plane, the nacelles are somewhere along that line, but at what angle...?

Dihedral is a factor, too, especially if the nacelles are not circular. Why? Because (at least on the King Air) the nacelles house the main landing gear, and in the front view, they are vertically aligned, although the wing passes through them going from low to high, inboard to outboard. (See the first image.) When I started thinking about how to solve this with precision, it gave me a headache, but eventually I came up with a solution.

If you have an airfoil plotter, it will be handy. The rest is "low-tech". The next series of images shows how I solved the problem on the King Air, which you may be able to adapt to your needs.

Ok, here's a front view which illustrates the problem. You have two "vertical" nacelles, at a known distance from the fuselage centerline, but the wing passes through them at an angle. To complicate matters more, there is a total of 3 degrees of aerodynamic washout in the wing.

The key here is (if the design allows) is to build the wing first... or at least enough of it to attach a pattern to. In my case, I'm making plugs for molds, so I went ahead and sheeted the wing center section.

First, find the point on the wing span-wise where the fore-aft centerline of the nacelle is. Measure the wing chord at that point, and print out an airfoil at that chord. Next, make a copy of the side view of the nacelle, and make a balsa pattern from it. Using your kit's side view as a guide, find the fore-aft centerline of the nacelle, and draw a line throught it. This automatically gives you the correct incidence line for your wing at that particular point.

Using the airfoil you printed out, align it on the nacelle pattern centerline, and cut that part of the nacelle out. (Leaving you with a balsa nacelle outline that's solid, but with the airfoil cut out.

Next, either slip the nacelle pattern onto the wing, or split in half (top to bottom) so that the two halves can be glued to the wing at the correct location. (Image #2) You can add some tabs, as I did, to elevate the wing off the table. Obviously, for the tabs on the outboard nacelles will be a little taller.

Before gluing anthing, check the alignment.

First, check that both wingtips are at the same incidence. Hopefully, you've already done that, if the wing center section is joined. (first image)

Also, make sure that your nacelle patterns are vertical. (next image)

Next step is to insure that your nacelle patterns are perfectly in alignment with the line of flight. (assuming that they're not offset)

Since I know that my wings are equal length, this can be done by measuring from the tip inward to the nacelle pattern, insuring that it's the same on both sides. (See images)

Now that everything is lined up, you can tack glue the nacelle patterns in place with some CA, and the rest of the process can be done normally, because the alignment is insured.

This is a pain... Much easier when the designer simply does all this for you and gives you precise patterns to use. But it works. I did all the steps above in a little over an hour, once I figured out how I was going to do it.