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Mar 29, 2019, 11:27 AM
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Kerswap


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This is one of many versions of Kerswap, a free flight model originally conceived by Gil Morris in 1941. The version in the screen grab here is a Texaco competition model for .049 engines published by Bob Isaacks in Model Builder in 1985. A similar model was published in Flying Models in 1994, again by Bob Isaacks. An Outerzone link is here:

https://outerzone.co.uk/plan_details.asp?ID=8971

There is also a .020 version. And a beautifully made electric version, here:

http://samchamps.org/3%20Day%20Kerswap.htm

The CAD model shown on this blog was rendered in Rhino3D.
Here is a stern view:
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Kerswap may be so enduringly popular because of its simplicity, relative to FF aircraft of the same era. Note the pylon, which became a typical FF feature in the very late 30s and early 1940s.
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In this 1985 Texaco aircraft, the pylon is basically a 3/8" sandwich, consisting of two 1/8 inch balsa cheek plates and a 1/8" plywood core. The plywood core is extended as a blade to the bottom of the fuselage's frame, so the pylon is very solid in the vertical plane. The wing is undercambered so the pylon cap is quite noticeably curved. This corrugated cap produces a strong T-joint at the top of the pylon where the wing is joined to the fuse.
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The thin profile of the pylon is nicely streamlined and simple to build. Contrast the Kerswap's pylon with the compound curves of one of its contemporaries, Henry Struck's "New Ruler."
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The New Ruler pylon is chromed, in CAD, as a trick to reveal any flaws in fairness or texture.

The original pylon was crafted with silk covering over a balsa frame.
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A major production, and although the finished pylon is voluptuous, it presents a large frontal area compared to Kerswap's.

Why pylons? As I understand it the pylon was adopted for two reasons.

First, it puts the drag of the wing high above the thrust line. Thus, while the engine is running, there is a strong tendency for the ship to pitch up and climb. At the top of its ascent the engine stops, and in that instant the forces that were pushing the nose up are lost. The plane levels out for a nice long glide.

These planes were designed before r/c, and the design itself specifies a flight path: Steep ascent followed by a long slow glide. The essential element is the high center of drag relative to the thrust line. This could be accomplished with cabin planes, but the pylon elevated the wing without the frontal area and extra structure of a full cabin.

Here is one other approach, Folly II, which was designed by Rod Doyle for the California State Fair competition of 1937.
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He elevated the wing by mounting it on 1/8" diameter piano wire. This may have made it easier to change the angle of attack and/or raise and lower the wing (and its drag) to optimize the plane's rate of climb under power. But wires are draggy. By 1941 pylons had become common, and the familiar "flying hatchet" shape of FF fuselages like Kerswap's had become well established.
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A second advantage of pylons: It was thought in that epoch that the pylon acted in a favorable way, perhaps as a vane, upon the spiraling propwash. It is easy to imagine two spiraling vortices trailed by the prop tips -- double superhelices. But it is not so easy to guess the exact effect on the propwash of a particular pylon. The modeling science forum has some fascinating commentary on the problem, and on empirical measurements of the effects of straightening out the propwash.

But it remains a puzzle to me whether or how an alignment or deflection of the propwash by a pylon could be advantageous to a FF model.

Does it make sense to radio control one of these beautiful antiques? Yes. A minimal rc is insurance the plane will not disappear over the horizon. But rc makes the pylon design somewhat superfluous. An elevator under radio control enables us to willfully dial in and trim the steep ascent that was originally programmed-in by the pylon. Put another way, if we have rc installed, there isn't much point that I can see in building a pylon. The Kerswap I have modeled in CAD has hand-adjustable control surfaces but no radio.

A couple of short cuts in constructing the CAD model:

In a boxy, symmetrical stick model like this one, it is not necessary in 3D CAD to actually measure and cut individual sticks. All the sticks in the fuselage are 3/16", so I started by making a solid model, 3/16" thick, of half of the fuse.
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Working from the SIDE view of the plan, I traced the "holes" between the sticks and longerons, and extruded the tracings into a cookie cutter. The red lines indicate the original tracings.
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When the cookie cutter is driven through the solid fuselage half-frame, the assembly looks like this.
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From here, the cookie cutter literally cuts away all the side material that isn't a stick. When you erase the debris you have a ladder-type half fuselage. In Rhino3D this structure can be mirrored to create the other half fuselage, and the two halves can be joined.

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Working from the TOP view of the fuse, the cookie cutter technique is used once again to simultaneously remove all of the top and bottom squares and rectangles and triangles of balsa that intervene between the sticks. When the debris (including the cookie cutter) is erased, you have a completed fuselage. Rhino3D "thinks" the fuse consists of just two parts -- a right half and a left half.

Another short cut was the Cox .049 Black Widow, which was simply downloaded and screwed to the firewall. The screws were also downloaded. The CAD/CAM forum, especially in the thread "CAD Parts Bin" has a collection of glow engines, electric motors, links to motors and props, servos and servo horns, hinges, etc.

I noticed in the online story about the electric "3-day Kerswap" that the expert builder departed from the original plan, notably at the tail, which is built light. I tried to follow his example.
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The rudder is lightened up, and the sub-rudder, aka the skid, is modeled as laser-cut plywood. (The original skid was solid). I think both components are important to the vertical stabilizer and should be covered.

It often helps to scan a plan and build a "model of the model" from it in CAD before cutting wood. Any dimensional or other errors or problems in the plan become apparent before you have committed to balsa.
Last edited by mcg; Apr 06, 2019 at 11:10 AM.
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