Jonathan Livingston Sailplane (oz16130)

About this Plan

Jonathan Livingston Sailplane. Radio control Open Class sailplane model. Wingspan 12 ft.

Direct submission to Outerzone.

Update 24/6/2025: Added article thanks to CloudCruiser, from scans posted on RCGroups at: https://www.rcgroups.com/forums/showthr…

Quote: "The ultimate soaring machine with Freon-operated spoilers, a retractable tail wheel and some really sophisticated aerodynamics. Jonathan Livingston Sailplane, by Ken Bates.

Johnathan Livingston Sailplane is the third version of a ship originally built for the '73 SOAR Nationals. Johnathan I was built during a week's vacation before that event. I placed ninth overall that year.

Johnathan II used the prototype's wing with a new fuselage and tail. It maintains the same moments and areas that corrected a tailboom flutter which occurred at high speeds and on tow in Johnathan l's fishing pole tailboom. Johnathan III had the same moments and areas and sported a new fiberglass fuselage. The wing incidence was increased slightly to account for an unexpected increase in weight, due to a more sophisticated spoiler mechanism and faired-in control linkages.

Recently, Johnathan II was modified with the addition of flaps (shown as options on the plans). When added to Johnathan III, this will be Johnathan IV!

The original design concept evolved from a general dissatisfaction with the kit sailplanes on the market. Having built most of them, as well as several designs that appeared in magazine, I felt I had a good idea of what I did and didn't want.

The basic moments and areas were derived from information in Frank Zaic's Model Glider Design. However, these design parameters were evolved for free flight sailplanes, and some modifications were made to avoid a model that would tend to be too stable.

The desired flying qualities were good penetration, efficient turns (which is where I feel that most designs are lacking), and large size which, combined with the necessary weight, makes an effic;ent and stable airplane that is not at the mercy of turbulence.

Good penetration and gentle stalls were achieved by using a modified Eppler 387 airfoil. The modified airfoil actually is two different airfoils (one in the center section and the other out past the polyhedral break). The center section airfoil has a lowered leading edge center which, when combined with a small LE radius and 9% section thickness, contributes to good towing qualities and good, high-speed performance. The sharp LE seems also to turbulate the boundary layer at lower speeds, allowing the wing to slow down.

However, this airfoil has a sharp stall. To compensate for this, the section from the polyhedral break outward has an 11% section thickness and a blunter, higher center LE. This combination of airfoils resulted in the desired flying qualities, plus an even broader speed range than anticipated. (Incidentally, the fabulous Caproni A21 sailplane also uses two separate airfoils.)

The latest modification to the wings has been trailing edge flaps from the fuselage to the polyhedral break. These were added for additional landing control and, at this time, have not been extensively tested when used as a flight trim method. Flaps seem to work well for increased lift, as well as for penetration when reflexed. But, this is when they are used on an airfoil with a high LE center, so that the airfoil produced resembles a semi-symmetrical one. It is too early to tell for sure how they will work on a low center, sharp LE airfoil for flight trim. They are worth the extra effort for landing control, however.

Polyhedral was chosen for stability, with the break-out near the tip for two reasons. The first is to allow the largest possible portion of the wing area to be nearly flat for best towing performance. The second is to use the less efficient tip section (due to lower Reynold's number) to do most of the stabilizing out where it has a longer moment arm. Adding the polyhedral further in wouldn't gain much, since, as the moment arm shortens, the effectiveness is reduced.

The V-tail was chosen for several reasons. The first is that there is obviously one less surface to build. Also, wind tunnel tests have shown that the V-tail has 3/5ths the drag of a conventional tail. However, the V-tail does not damp out oscillations as rapidly as a conventional tail, so a larger tail moment and a forward CG (25-30%) are required for maximum stability. The prototype has been flown with a 60% CG and was found to have adequate stability in non-turbulent air. The V-tail also produces a seemingly more coordinated turn.

The main reason for the V-tail, however, was to eliminate the 'over-banking' tendency that most conventional designs show. That is, when a ship is in a turn and the controls are released, many will bank further and decay into a spiral dive. This necessitates putting in opposite rudder somewhere in the thermaling turn.

When a turn is initiated with a V-tail, the surfaces move as if they were rudders and elevators; ie opposite to the direction that ailerons would move (see sketch 1). Once the dihedral has banked the ship, the outer wing is moving faster than the inner wing, tending to cause the bank to steepen. The conventional method for counteracting this is to add more dihedral, necessitating a large rudder to turn the ship. This, in turn, can lead to dutch roll, ie the ship will oscillate in the yaw and roll modes.

If the dihedral is kept small, the torque effect of the V-tail (see sketch 1) occurs in the opposite direction from the bank, and can be used to cancel the over-banking. This also allows for less drag and less spanwise flow on the wing, increasing the airfoils' efficiency.

The aspect ratio of 16:1 was chosen as a compromise among span, area and Reynold's number. The 1440" sq area (10 sq ft) is about all the average winch can handle. The center area's chord of 10.5" keeps the Reynold's number in the 190,000 region (a Cirrus is about 40-80,000, depending on weight and airspeed). It still allows a reasonable aspect ratio to reduce drag (the smaller the tip chord, with respect to wing area, the less induced drag the wing produces). It was found in the first prototype that the airfoil didn't really start flying until the wing loading reached 9.6 oz./sq ft, or 6 lb up weight. The third version weighed in at 8 lb, or 12.8 oz/sq ft and, with slightly increased wing incidence, it had the same sink rate as the first ship. It also had a much higher speed while maintaining a reasonable sink rate.

The ability to use light lift is as good, but it is a bit difficult to see, since an 8 lb. ship doesn't exactly jump up and down and flap its wings when it cruises into light lift. The ship has been ballasted to 10 lb. (16 oz./sq. ft.) and, in 40 mph winds, had ample penetration at a surprisingly low sink rate. This can be seen when flying alongside my friend's light Cirrus, which comes in at about 8 oz./sq. ft. and has less than half the Reynold's number, or efficiency.

Johnathan can fly almost as slowly with an identical sink rate, but it can also fly twice as fast without a detectable increase in sink rate. Above about 20 mph (10 mph is slow flight for a Cirrus) the sink rate begins to increase. Johnathan has been timed at 70 mph, in what appeared to be a shallow dive at about 20°. This translates into excellent thermal-searching range.

Decide what sort of glide path control you want. The plans show conven-tional spoilers only, with optional dive brakes and landing flaps. If you don't intend to do a lot of contest flying, the usual, top-only spoilers are probably adequate..."

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