Hi,

My good mate HITC is busy working away on his new project, and every time I think of him in his workshop, I get more frustrated at not being able to work on the Razorback. Yesterday I drove out to the industrial site where my workshop is stored in a container, and the plug is under some trees with a tarp over it. Everything looks OK, but I'd like to get the plug into a new workshop ASAP. And get it finished.

In the meantime, I've been checking the design calculations again. It has proved to be quite a challenge to find reliable formulae which apply to tandem wing aircraft. But over the months I have ferreted out some really useful information. I have two sanity checks for the design. Inputting the data into X-Plane, and building a 1/4 scale RC model. X-Plane agrees almost exactly with my formulae in the spreadsheets. I'm still working on the RC model.

With the re-calculations, I have come to realise that with two equal sized wings, some of the traditional rules of thumb which apply to pure canard configurations don't necessarily apply. In a traditional canard design, for example, the main wing is aft, with the canard way up front and carrying significantly less load. All the grunt lifting is done by the aft wing. So it makes sense to work with the aft wing as the baseline, and simply increase the canard by a few degrees, ensuring it will stall first. Also, the downwash produced by the much smaller canard is significantly less than that produced by a full sized fore wing.

So, with a design which essentially has two equal wings fore and aft, it is not simple to get it right. First, calculating the angle of the fore wing downwash is critical. And for someone who is calculus-challenged, it was not easy to find a way to calculate this. But I finally did, with lots of help from the guys in the sailing world as it turns out... and surprisingly, the downwash angle at the aft wing is nowhere near as severe as I at first thought.

This angle is a function of both Cl and AR, so is greater at or near the stall than it is in cruise. Also it decreases with vertical separation and with distance aft of the fore wing TE. In the landing scenario, my rear wing is in a 2 degree downwash "shadow". I have chosen my airfoils carefully, so that the aft wing stalls at a full 2 deg later than the fore wing, giving me 4 degrees of safety in the final flare.

The next critical thing to calculate is the placement of the wings. The front wing actually carries 25% more load than the aft wing - due mainly to the downwash angle it induces. With my CG pretty much a given, the only way to ensure that the fore/aft wings' centre of lift falls suitably rearward of the CG was to move the wings fore and aft until the plane was balanced. Fortunately, my original calculations seem to have been reasonably in agreement with the new formulae. In fact, while there was an agreement between my spreadsheet predictions and X-Plane simulations previously, the degree of agreement was not always satisfactory. Now it is nearly perfect. The next step is to complete the 1/4 scale model and try it out.

So with the calculations re-done, minor adjustments made, and the planned move to a house with a shed out back, I may just be able to get the plug finished and start building the actual plane before too long.

Warm regards,

Duncan