banktoturn

Dustflyer,

You raise a whole bunch of points. I'll offer a couple comments:

1) The 'formulas' can give some useful predictions, if you have accurate data to use in applying them. In particular, if you know the prop pitch and the best engine speed you can get, you can predict the best possible airspeed, just because you can't outrun your prop in level flights. This kind of prediction can't be expected to predict your actual speed, because it does not account for drag. Also, applying that 'formula' entirely ignores the possibility that your engine may not be able to spin that prop at that speed. This is why the speed planes that use high pitch to go fast must use smaller diameter. As the prop diameter increases, the power needed to spin it at a given speed increases really fast. If you choose the prop pitch needed to fly 200 MPH, and decide that it should be 15" in diameter, your engine will never be able to spin it fast enough. If you have already chosen your engine and have a target speed, you need to pick a pitch that is high enough to make that speed possible, and then pick a diameter that gives your engine a chance to spin it fast enough. If the engine simply can't make enough power, you won't make it. Larger diameter is not generally the way to get max speed though. Any 'formula' that doesn't include the CD for the airframe cannot be relied on for an accurate prediction of max. speed.

2) Forget about induced drag. Diamond Dust does have a fair amount of drag, but at high speed, induced drag is a very small component of it. Induced drag is high when the CL of the wing is high. When a plane is flying fast, the wing is operating at a low CL, and induced drag is small. Many modelers seem to assume that planes with low aspect ratio suffer from high induced drag, and it is not always the case. Diamond Dust has a lot of drag because the engine, pipe, servos, linkages, etc. are jutting out into the airflow. If you want to reduce the drag, get all that stuff inside fairings. Take a look at a control line speed plane. Even if you had to add a long 'bubble', or make the root of the wing thicker, it would be a huge win to get that stuff faired over. I would take a look at running the elevon linkages through a narrow fairing and through channels in the fins, or something like that. With all that junk smoothed over, Diamond Dust does not look all that draggy, overall. There is a fair amount of wetted area, which is hard to do much about. If you don't mind losing some of the turning performance, you could chop the wingspan down, but I would worry a bit about losing elevon area.

3) Some of the props being run may be too small in diameter, but I don't think 10 inches is going to be the answer. If you're really looking for top end, send Dub Jett an email about trying a single bladed prop. I bet he would have some good advice. If the balance problem can be solved, this is the most efficient solution.

If you get out your balsa and glue, and fair all the junk over, you can give Syd a nasty surprise the next time out.

banktoturn