Bird of Time (oz2093)

About this Plan

Bird of Time. Radio control sailplane model.

Quote: "The indomitable and intrepid Dave Thornburg strikes again with the final version of a competition sailplane that has made its mark in contest circles. Despite Dave, the Bird of Time helped put Steve Work in the number one slot on the American FAI team.

Here's the 'final' version of a sailplane that's done fairly well on the west coast contest circuit for over three years, and helped put Steve Work, of New Mexico, in the number one slot on the new American FAI team. Steve flew his Bird just one-tenth of a second slower than the fastest time recorded at the team Finals, and did it at only 9.4 ounces per square foot, while the other competitors were having to ballast up to over twelve just to stay in the race.

What was his secret? Besides very smooth flying, which Steve is master of, it took a few simple tricks of aerodynamics: a thin airfoil (9%) with a lot of Phillips' Entry, a rearward CG, and a clean machine. All these characteristics (except the piloting skill) are built into the Bird of Time. The design is based on the simple principle that full scale glider folk stumbled onto way back in the fifties: the only thing more important than minimum sink is maximum go.

The reasoning behind this principle is that virtually anything - hang gliders, bumblebees, Ugly Sticks, magazine editors - will soar like an albatross in good lift; the real challenge is to develop planes that are clean enough and fast enough to outrun the sink! Once you find your thermal, you're home free - what you need is a plane that will get you to it with maximum speed and efficiency.

Well and good. So you need a nice fast cruise speed, then. It sounds like all you have to do is add lead to your 'Whistler 900' and presto! you have a modern design. And that works pretty well - until.

Until you try to slow it down for a precision landing. Then you begin wishing you could spray your ballast out in a fine mist, the way the big boys do. Or until that early morning contest round when you have to ride 6 ft diameter bubbles down a 16 mph wind for seven long, cold minutes! Then you wish you could drop your ballast in the lift, and pick it up again when it's time to move back upwind!

So raising your wing loading isn't the all-round solution - as anyone who's flown the Graupner Cumulus can tell you. (The Cumulus was one of the most efficient designs to come out of the sixties, an airplane that 'flew better than it oughta' - but its 11 oz/ft wing loading finally dragged it out of the skies, as far as US competition goes.)

We need a higher cruise speed without a sacrifice in weight. then. So what does the Bird of Time do about this quandry? Just what the big boys did: it goes after a better airfoil and a cleaner profile.

The cleaner profile comes easy: get rid of excess frontal area, square corners, protruding dowels and skids and switches. Find a wing lip shape that's still quiet at 50 mph. Move the stab up out of the wing wash as far as practical, because down wash characteristics vary radically with airspeed, and nobody knows what the hell really happens to the air around our wings under actual flying conditions.

The 'better airfoil' is something else. I've suspected for a long time that our sailplanes generate far more lift than they need. (What?! Heresay! Too much lift? Thornburg's finally gone bonkers!) But even the slide rule disciples among us will admit that you pay a tremendous penalty in drag for every ounce of lift you extract from an airfoil - that's one of those rare points on which full scale theory and modeling practice seem to agree.

And if, like me, you're naive enough to think that, in normal sport or contest conditions, the difference in sink rate between a high-lift airfoil eg the Windrifter (oz6700) section and a relatively low-camber section eg the modified 374 of the Windfree (oz1637) is small enough to ignore, then you're ready to take the Two Giant Steps to a better sailplane airfoil:

(1) Build it as skinny as possible. In the thirteen aircraft built so far in the 'BoT' series, I've been down as low as 8% thickness. This is hardly radical: the Legionaire flies at 8%. But the 'BoT' shown on the plans is back to 9% - I just couldn't 'feel' any real advantage to the thinner wings, and the sacrifice in structural rigidity wasn't worth it to me.

(2) Raise the entry point of the airfoil. This is the big secret to the airplane that turned 12.8 seconds in the FAI speed run at the 1976 LA Semi-finals, while loaded to only 9.6 ounces/ft. It's the basis of the famous 'Miller Mod' to the Aquila (oz5136) airfoil.

Raising the entry point (adding Phillips' entry) effectively lowers the mean camber of the airfoil. You do this, and you're going to sacrifice some lift. agreed. But you get a tremendous decrease in the induced drag, particularly at very low angles of attack. In short, you get a wing that moves quick, even at very low wing loadings. And that ain't a bad banana.

You still fretting about that lost lift? Then take another look at the lift formula: lift varies with the square of velocity. And I just gave you an increase in cruise speed, right? So you actually got back more lift than you threw away. I told you we had Lift to burn.

So much for esoterics. People ask about the appearance of the Bird. Why does it look so funny? The fact is, if your aerodynamics are sound, you can wrap most any shape you choose around them. The Bird of Time is a kind of copy from memory of a Nordic glider from the early fifties called Big Time (oz11786). It appeared in Model Airplane News, but my ragged copy had its plans pirated by a razor blade decades ago.

The wing shape was originated by Germany's most famous pre-war soaring pilot and designer, Wolf Hirth (who invented thermals, on a trip to America in 1930). The Hirth wing shape was popularized in the US by Frank Zaic, unquestionably the dean of model soaring in the States. I could tell you what I suspect the trailing edge shape does to reduce interference drag at the polyhedral joint - but I promised an end to esoterics.

So I'll just say this, instead: Build yourself a Bird of Time, and trim it to the rearward CG position shown on the plans. I think you'll find it to be the quietest sailplane you've ever flown. And that's the only measure of efficiency we currently have.

CONSTRUCTION: Wing: I usually begin with the wing, because once the ribs are cut, the plane is half built! There are a number of ways of duplicating the ribs: Xerox them off the plans, spray-glue the Xeroxes to plywood or aluminum and cut masters (this if you can foresee building more than one plane. or have friends who might want one). Or spray-glue the Xeroxes directly to balsa and cut the ribs out two at a time. Or lay the balsa under the plan and 'pinhole' the outlines onto it. Or use carbon paper. Or cut up the plan..."

Update 31/03/2014: Added supplemental eBIRD wing plan (modern alternate high performance wing), thanks to DavePayne. [see more pics 004, 005] An alternative high performance wing design.

Quote "Steve, Here is my plan for a high performance wing (non-polyhedral) for the Bird of Time. It has ailerons and flaps so 'crow' or 'butterfly' settings can be used. I would like to contribute it for those people who would want to build an advanced version of the BOT or add another wing option to their current BOT. DXF files for laser cutting will also be available. Here is my build log with photos: http://www.rcgroups.com/forums/showthread.php?t=2125318 The design has been tested and performs beyond my expectations. Using slight amounts of flap adds to the camber for still air and really improves the glide ratio. Also negative flaps helps to de-thermal a ship that is in a monster thermal. Landing with 'crow' makes precision spot landings a breeze. I am the author of the design. DavePayne"

Update 12/12/2014: Replaced this (the main) plan with a clearer version, thanks to theshadow. Also added alternate version of same plan as a supplement (another good clean scan) thanks to Thermaler.

Update 04/01/2016: Replaced Article file with a more complete copy (fixed missing text on p5) thanks to RogerClark.

Update 03/07/2020: Added kit instructions (from an original Thornburg kit) thanks to ChrisD.

Supplementary file notes

Article, thanks to Thermaler, hlsat.
Alternate version of the fullsize plan, thanks to Thermaler.
eBIRD wing plan.
Instructions.

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