Puranas (oz7832)

About this Plan

Puranas. Radio control aerobatic slope soarer.

Quote: "This aerobatic slope soarer was deigned for going fast and tracing graceful figures in the sky. With ist 53 in wing span, built as one-piece construction, it makes a very rugged and responsive performer. Puranas, by Mark Gumprecht.

You ease the stick back just a bit further, trying to gain a few more precious feet of altitude. A quick turn, and then back along the slope, diving for speed. The sound of the wind whistling over the wings rises in pitch as the plane accelerates in the dive. You level out at cliff-top level and do a long four-point, a gigantic loop, or a few snap rolls, whatever suits your fancy, then begin to climb for altitude once again. What will it be this time? Perhaps a few outside loops or maybe a beach run followed by a vertical roll. This is what slope aerobatics is all about!

Puranas is designed for slope aerobatics; for going fast, tracing graceful figures in the sky. With a wing span of 53 in, it's smaller than the average slope ship. The most important consideration in deciding on the size was the planned one-piece construction. A larger plane would be too hard to work on without using a removable wing. One-piece construction gives a more rigid airframe which is important for high speed flight. The ailerons are permanently connected, saving the trouble of making some sort of detachable linkage necessary when detachable wing panels are used. A removable one-piece wing would seriously weaken the fuselage because of the mid-wing design. From past experience, I have noticed that aileron response is too slow for good aerobatics with wing spans above 65 in. The pilot must remain in one place, and by the time three rolls can be executed, the plane is almost out of sight. This can be detrimental in contest flying, as it is difficult to keep maneuvers within the visual limits, except by flying farther away. I find it hard to do accurate maneuvers when flying far away. A wing span of 53 in gives the Puranas a fast roll rate, making it easier to do maneuvers closer in to the slope, but is not so small as to have that skitterish feeling of some smaller gliders. The smaller size also is nice when trying to fly on smaller, more confined slopes. The only disadvantage of smaller planes is increased sensitivity to turbulence, although it hasn't been much of a problem with this design. I've flown in some very strong winds with no problems, except the severe turbulence in the landing area where I fly.

The Puranas is designed along different lines than most slope stunters. I wanted to design a plane that was light and slippery, instead of heavy and fast. A heavy plane is more prone to breaking when it contacts the ground; a light, strong plane can take more bouncing about without getting hurt. Because I tend to build light planes, I decided to try a thinner airfoil of about 10-11% thickness. Thin airfoils can perform well at slow speeds if they are lightly loaded. Several other tricks can be used to make thin airfoils behave. I always use a tip airfoil that is slightly thicker than at the root, with a blunter entry. This gives a wing with very gentle stall characteristics; I can still control the plane while holding full up elevator. No washout is used as it would have the reverse effect while inverted.

Most slope soarers rely on weight to overcome the drag of thick airfoils and boxy fuselages. Streamlining is very important if you expect a small, light plane to go fast. Every effort was made to reduce drag, as a clean airplane will go faster at lighter wing loading, and carry speed better. I used a pod and boom fuselage because I believe it is the cleanest configuration, as evidenced by trends in full size sailplanes. A fibergalss boom was used and has proven more durable than a conventional wood tail - it bends instead of breaking - yet is sufficiently rigid in flight. Since I began using the pod and boom configuration, I have noticed a definite improvement in LID and speed, and no more broken tails.

Although not incorporated in this design, another plane of mine uses a fiberglass boom and plug-in rudder, mounted on music wire. This gives the ultimate in durability - there are no rigid joints to break. I have bounced the plane on its tail and sustained no damage. One disadvantage of a pod and boom fuselage, some will say, is lack of side area necessary for knife-edge flight. This is believed to be true because most planes have short, wide fins and need side area for good yaw stability. A tall, narrow fin with a good airfoil section is more efficient than a short, flat one - it generates more lift where it is needed - back in the tail, and eliminates the need for much side area. A flying stabilator, essential for good outside maneuvers, and a long tail-moment make for a smooth flying plane. The Puranas flies very well on ailerons and elevator alone, but I prefer to have rudder control for spins. snap rolls, and a little help on slow and 4-point rolls. I would recommend adding it if you feel you can squeeze in a third servo.

The Puranas has one surprising but enjoyable characteristic. With most slope designs, it is necessary to adjust the wing loading according to the lift conditions. This can mean adding up to a pound of weight in larger planes. In a crash, this weight can be very destructive. Because of the thin airfoil, large wing fairings, and pod and boom fuselage, the Puranas seems very happy in all conditions at a flying weight of 24-26 ounces, or about 12 oz/sq ft. It penetrates well even in very strong winds. I've never found it necessary to add ballast; which is good, because there isn't really much extra room for it...

http://www.rcgroups.com/forums/showpost.php?p=30163240&postcount=1532

Supplementary file notes

Article pages, text and pics.

Corrections?

Did we get something wrong with these details about this plan (especially the datafile)? That happens sometimes. You can help us fix it.
Add a correction