Windfreak (oz5725)

About this Plan

Windfreak. Radio control sialplane model. Flying wing layout.

Quote: "Finally - an advanced flying wing sailplane design that will match the duration of the best conventional designs, while maintaining the legendary maneuverability of flying wings.

Flying wings have long intrigued me, but I have been very puzzled by their absence from the usual flying field. Furthermore, I have never seen a truly competitive design used in competition. The reason for this state of affairs becomes rather obvious when one watches the flight performance of typical flying wings. They tend to have very poor glide slopes which result in poor duration, they seem to have to fly fast, have strange control behavior, and tend to be suicidal on tow.

At the same time, it is apparent from theory that flying wings should offer magnificent maneuverability and have an outstanding glide slope. Now, greater maneuverability and glide slope sounds ideal for a sailplane - why aren't people flying them? The answer is that nobody has developed a flying wing that would match or exceed the performance of conventional sailplanes.

I do not know why better flying wings haven't been developed, but felt that there was no good reason that one couldn't be developed. I have designed a long string of conventional salplanes, and some of these were not good performers, but some of them were outstanding. I felt that I had determined what would produce outstanding flight performance. Since flying wings are subject to the same laws of flight as conventional gliders. I believed that I could apply my experience and knowledge to a flying wing with excellent results.

The Windfreak was inspired by Ken Bates' Windlord, which it superficially resembles (Model Aviation, March 1978). However, the Windfreak is a totally different aircraft except for the same general wing planform. The important aerodynamic parameters are quite different; the airframe is grossly stronger and, probably most important, a much different airfoil is utilized. While I obviously feel that the Windlord can be improved upon, I nevertheless feel that Ken Bates' design was a giant step forward in the design of flying wings. The name Windfreak was partly derived from Windlord out of appreciation for Mr Bates' design. The 'freak' part of the name seemed appropriate because the ship is certainty the most unique ship one is likely to see at your local flying site.

In this article I will attempt to give some insight into how I go about designing a sailplane, some of the parameters involved, what the various parameters do, and the various compromises involved. Before we can design a sailplane, however, we must know what we want it to do, I, therefore, developed several performance goals I hoped to accomplish. These goals used Lee Renaud's Olympic II (oz5116) as a yardstick. I doff my hat to Mr Renaud as I feel his designs are the finest performing ones on today's soaring scene. The Olympic II has, in my opinion, the flattest glide slope, the best manners, and greatest maneuverability of the commercially available standard class gliders. I fully agree with Mr Renaud's design theory as outlined in his article on the Olympic II in RCM, June 1976. However, I felt that a flying wing should be able to outperform it. My goals were as follows:

1) The glide slope must be as good as, or better than, an Olympic II.

2) The maneuverability must surpass an Olympic II.

3) Behavior on tow must be stable, predictable, and result in launch altitude at least equal to an Olympic II.

Obtaining the first goal, a good glide slope is deceptively easy. All that is required is to have a good lift to drag ratio (L/D). Since lift is a compromise of various factors and is fixed by the airfoil used. what we are really talking about is reduction of drag. Drag reduction is a very furtile hunting ground. Much can be done to reduce it. The effect that drag reduction has on flight is startling, Virtually all gliders have more drag than they might have, including the Olympic II. It was by reducing drag and improving the lift characteristics of the airfoil that I hoped to exceed the glide slope of the Olympic II.

There are three types of drag: induced, parasitic. and profile. Induced drag has been called the 'price of flight'. It is produced in exchange for lift, and it is determined by the aspect ratio of the wing. In short, long skinny wings have less induced drag than short wide wings. As usual, there are compromises here. It involves Reynolds Numbers, wing loading, towing characteristics, wing strength, and turning performance. Without getting into these things in detail, let me simply state that, as a general observation, it can be seen that low aspect ratio wings exhibit better Reynolds Numbers, have lower wing loadings, tow higher, have greater wing strength, and turn better than high aspect ratio wings. Therefore, one must trade off drag for some of the other desirable characteristics..."

Hi Steve - Here is Rodger Sanders' Windfreak from Radio Control Models magazine issue 11-78.

Direct submission to Outerzone.

Update 16/3/2022: Added full article pages (parts 1 and 2), thanks to RFJ.

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