Henry Cole's Stratosphere

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Bob H
Posts: 183
Joined: Tue May 12, 2009 10:46 pm

Henry Cole's Stratosphere

Post by Bob H » Fri Jun 12, 2009 8:52 pm

Henry Cole's "Stratosphere !"

The June 1941 issue of Model Airplane News featured an elegant 36" wingspan rubber freeflight design by Henry Cole.
Henry Cole's prototype of 1941.
The name "Stratosphere" evokes some of the mystery and danger surrounding early attempts at setting high altitude records. Cole's model offers a number of interesting features. It has a highly streamlined cabin fuselage of circular cross section and Cole provided an oddly shaped windscreen that suggests to me some of the pioneering attempts at cabin pressurization. The stabilizer is raised above the wing's downwash by mounting it to the fin. The top-surface of the leading edge of the wing is sheeted to maintain a truer airfoil section between ribs, and the model is driven by a respectable amount of rubber. A spinner completes the picture. By building and flying the model, I have learned some things, I've enjoyed the challenge, and I'm pleased with the results.

Your author's 2009 version of Stratosphere
Stratosphere 018AA.jpg


Typical for it's time, the original magazine plan omitted many details, and some other details were not drawn full scale. It also appears that the currently available copy of the plan, has not been reproduced directly from an original master, but probably from a 3rd or 4th generation paper copy and then scanned into a jpeg. Dimensional distortion appears to have accumulated with each generation. Using CAD, I was able to re-scale the jpeg to match the reference dimensions that appear on the jpeg. Precise rib shapes were generated anew from the airfoil library provided by Profili, then imported to the CAD drawing. By this process, a new distortion free, true orthogonal, full scale, and complete CAD master drawing of the Stratosphere has been created.

A link to a full scale copy of this drawing in .pdf format is offered later in this article, in case you would like to build the Stratosphere.

A typical cut-sheet
The Stratosphere4.gif
On my CAD drawing, all piece-parts were drawn full scale and arranged on 3" wide "cut-sheets". Each "cut-sheet" can be printed full scale on paper. Using 3M77 spray adhesive, the paper cut-sheet can then be laminated to a 3" wide balsa sheet of the appropriate thickness. The parts are then cut out using a Dremel saw (scroll saw). The paper template can be easily peeled off of each finished balsa part. If necessary, a hot air gun can be used to hasten release.

Where multiples of certain parts are called for, these are grouped on the cut sheet according to their quantities. Multiple balsa sheets can then be stacked together for sawing the multiple parts in one pass of the saw blade. To maintain alignment of the stacked balsa sheets during sawing, a light dusting of 3M77 is applied to one side of each sheet when making up the balsa sheet stack. The paper cut-sheet is laminated to the top balsa sheet only. The photo shows my Dremel Saw. A magnifying glass mounted on a swing arm and a (blue) air-blast nozzle mounted on the saw, help to improve sawing accuracy. Pieces can be produced very quickly and accurately this way to result in a light interference fit that is ideal for assembly.

Dremel saw set up with magnifying glass and air-blast.
Stratosphere 018A.jpg
The fuselage has no flat external surfaces for alignment purposes, an assembly fixture can be made, as seen in the photo below. Details of this assembly fixture are also shown on the new drawing.

Fuselage on the Assembly Fixture
Stratosphere 017A.jpg
Having no dethermalizer ("D/T"), Cole reported that one of his flights lasted for 35 minutes. Even our generous southern Ontario sod-farm flying fields won't accommodate 35 minute free-flights, so a D/T is a necessity. The stabilizer of Cole's original model was glued solidly to the fin, and pitch-trim was adjusted by shimming the wing's incidence angle. My version of the Stratosphere tilts the entire stabilizer when D/T-ing, and also incorporates a stabilizer trim mechanism. In the photo below, the carbon fibre axle for the stabilizer halves can be seen protruding through the fin. Each stabilizer half is retained on its' axle by a tiny 0-80 bolt used as a cross-pin. The stabilizer halves can then be easily dismounted for storage, transport or repair purposes.

Fin with Axle for Stabilizer Halves, and in foreground, the Stabilizer Trim Jacking Bolt
Stratosphere 015A.jpg
The stab halves can be assembled in one piece, then cut apart.
Stratosphere 016A.jpg
The D/T release cable and the Rudder Trim Jacking Bolt are seen here.
The Rudder Trim Jacking Bolt also serves as the anchor point for the D/T actuating rubber band.
Stratosphere 011.jpg
(Photo above) To remain true to the streamlined intent of Cole's design, my D/T release-cable is routed from the fuse-snuffer-tube located under the wing, internally through the fuselage, and then internally through the fin, exiting to the stabilizer. The RH stabilizer's trailing edge is yoked to the LH stabilizer's trailing edge using a tiny 0-80 Allen-head-bolt. Thus the RH stabilizer follows the driven motion of the LH stabilizer. I use these bolts extensively on all my models, since they are small, relatively lightweight, and have an Allen head which prevents the wrench from slipping and damaging the model.

How to make wheels: Cut some balsa wood discs, and then laminate them with alternating grain directions. Drill a hole through the laminated stack of discs and insert a 4" length of aluminum tubing through the hole. The tubing should protrude equally from both faces of the wheel, as shown in the photo. C/A glue the tubing to the balsa laminate. Cut a piece of 1/16 dia. music wire 4-1/2" long and then insert the wire into the tubing to stiffen the tubing. The wire and the aluminum tubing together, now can be used as a mandrel for gripping the wheel in a drill chuck. But first, clamp a block to the drill press table, and drill a 1/16 hole in the block. Then insert the 1/2" of music wire that protrudes from the aluminum tubing into that hole, and grip the opposite end of the tubing in your drill press. Tighten the chuck until it crushes the tubing into contact with the internal music wire. You've just made your drill press into a lathe for turning your wheel to shape.
Turn on the drill press and sand the wheel to the correct profile. Paint the wheel to suit your own tastes. Remove the music wire, and cut off the excess lengths of the tubing. The tubing left within the wheel becomes the wheel bearing. There, yer dun.

Wheel shown before excess length of aluminum mandrel is cut off.
Stratosphere 013A.jpg
I also modified Coles design by building a split wing, rather than a one piece wing. This simplifies storage, transportation and repair. The carbon fiber wing joiner protrudes from the LH wing and the wooden alignment dowel is seen to protrude from the RH wing (photo below)

Wings rubber banded bottom to bottom for transportation and storage
Stratosphere 010.jpg
To improve his model's glide by reducing drag, Cole's original model featured a propeller brake and folding propeller. I converted Cole's propeller design to a freewheeler, with blades hinged to fold only on impact. The blades are restrained against folding during flight. A flexible freewheeler of this extreme diameter survives landing impacts better than a rigid freewheeler.

The Front End... Blade in foreground has its restraining rubber band removed to demonstrate hinge action under shock loads.
0-80 jacking bolts provide tracking adjustment for the extended blades.
Stratosphere 012.jpg

Flight Testing

Flight tests were conducted at FAC's 2009 and 2010 events in Geneseo, New York. Flight testing has also been done at the Southern Ontario Glider Group's sod farm flying site near Mount Hope, Ontario. Around 100 flights have been made to date July 31, 2010.

Ready for flight-testing.
Stratosphere 020B.jpg
Particularly with this model, washOUT on either wing should be avoided as it will lead to intermittent spiral dives all the way to the ground. In accordance with Cole’s recommendation, my model has been trimmed for a RH turn under power, followed by a RH glide. A degree or two of wash-IN on the RH wing doesn't hurt, and some believe that it improves a model's ability to latch on to passing thermals.

I am using 8 loops of 3/16 rubber for the motor. The motor loop length is 1 ½ x the distance from the propeller shaft hook, to the rubber peg (i.e. 1 1/2 x 22.7" = 34"). This 50% 'excess' length of the motor represents additional energy storage capacity. Motor-braiding is a technique that is used to reduce this length, so that when the motor is installed in the airplane no slack rubber exists. Slack rubber could move around causing the aircraft's centre of gravity to shift forward or backward. Each time the motor is installed in the model, it is divided into two bundles of 4 loops each. Each bundle is wound 80 turns in the normal winding direction. The two bundles are then recombined and allowed to unwind, forming the braid. The motor is then ready to use.

When wound to 600 turns, the 8-loop motor is near its practical energy storage capacity. In dead air with optimum trim, six hundred turns produces a moderate power burst and a climb to about 200 feet. If 'lift' is present, much higher climbs under power are possible, and the model has shown itself to be a respectable soarer.

For initial trimming, the number of motor turns was increased very gradually. Frequent repeat-tests made sure that each adjustment was satisfactory. Thrustline adjustments were made progressively, aiming to keep the circle radius during the motor-run slightly larger than the circle radius during glide. This strategy maximized the margin of stability during the hazardous portion of the power burst.

The use of tiny 0-80 jacking-bolts for the adjustment of rudder trim, stab trim and thrustline make frequent, minute trim adjustments easy, fast and precise. The model’s propeller blade hinges have also proven to be an engineering success. We fly on short grass at a sod farm. Including 3 or 4 times that I've knocked the wings off, the propeller and propeller shaft have not experienced any damage so far. Another success has been the dethermalizer system which seems to be rugged enough and reliable.

I found it necessary to carefully realign the wing to the fuselage before each flight. If the wing is not properly centered in the v-shaped wing saddle, it causes the wing to tilt relative to the horizontal tailplane. A tilted stabilizer produces yawing force which adds to or subtracts from the effect of rudder. Perhaps some future redesign could include a flat saddle for the wing, rather than the existing v-shaped saddle.

A Good Day at SOGGI
Stratosphere in flight 017B.jpg
Stratosphere in flight 012C.jpg
Stratosphere in flight 013B.jpg
When freewheeling during the glide, those large propeller blades do cause a lot of drag. To reduce drag, I used an exacto knife to "field modify" Cole's original blade-length and blade-chord. A higher glide speed resulted and the glide angle improved noticeably, leading to improved flight duration. My CAD drawing shows the revised propeller geometry.

In hindsight, Coles propeller that folded during flight appears to have been the best idea. Cole's 1941 Model Airplane News article concluded that the Stratosphere's best altitude and overall duration were available from a 10 loop motor of 3/16 rubber. A 10 loop motor could theoretically store 25% more energy than my 8 loop motor. I may eventually convert the model to a 10-loop motor with a Montreal Stop (propeller brake) and folding propeller.

I have completed a spinner for the model which adds 2.6 gm to the nose. Without that spinner, a similar amount of nose-weight would be required to offset the rearward weight of a 10 loop motor.

Looking kind of Stratospherey ... Fibreglass spinner weighs 2.6 gm incl 0-80 mounting bolts and nuts
Stratosphere 001.jpg
Full-size Downloadable Drawings
The accompanying drawings are offered free-of-charge for the personal use of fellow modelers. If you open any of the following links and then wish to return to this page, use your browser's BACK command.

You will need to have Adobe Reader installed on your computer before you can open the drawing files. The no-cost Adobe Reader can be obtained from:

The first link below, opens a CAD drawing depicting details of my as-built model. It provides full size piece-part cut-sheets, full size assembly views, plus the model's trim set up optimized for a freewheeling propeller. The second link shows a fuselage assembly fixture, and for reference, the original magazine plans. Open either link, and then use your Adobe Reader's <File><Save as> command to save a copy of it to your own computer. If you provide a copy of that file to your local blueprint shop by e-mail or on a CD, they can produce a full-size paper copy so you can build the model.
Stratosphere 'As modified and Built' Drawing
The Stratosphere 'as modified' drawing.pdf
Henry Cole's original drawing, an Fuselage Assembly Fixture and Various Details
Stratosphere Supplementary Drawing.pdf
I'd like to thank my friends Warren Kelley for encouraging me to proceed with this project in the first place, and Sam Burke for some crucial observations about trimming the model. I'd also like to thank the membership of the Southern Ontario Glider Group Inc.(SOGGI), an R/C sailplane club that graciously shares their flying sites with our small rubber freeflight fraternity.

I hope you have found this article entertaining. I would enjoy hearing from any other Stratosphere builders.

Bob H
FAC Tigertown Squadron / Southern Ontario Glider Group Inc.
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Last edited by Bob H on Fri Jan 01, 2010 12:50 pm, edited 52 times in total.

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