Franchot Rotary Engine: Sliding-Wing Mechanism and Animation

How the sliding-wing mechanism works

The rotor constrains the wing to turn with the shaft while allowing it to slide across the rotor. The shaft sits off-centre within the casing, so the wing projects farther on one side at some angles and farther on the other side at others.

The casing must have a special profile: measured along any straight line through the shaft, the distances to the two opposite walls must add to the same total. That total is the wing length. A generic oval does not necessarily satisfy this condition.

Main parts

• Fixed casing: provides the internal surface followed by the wing ends.
• Slotted rotor: rotates about the shaft and guides the wing sideways.
• Rigid wing: spans the casing and slides through the rotor without changing length.
• Shaft and bearings: locate the rotor axis and carry rotation to an external load.

For steam to drive the engine, a pressure difference must act across the exposed wing surfaces. The changing chamber shapes alone do not determine that pressure difference: admission and exhaust must be timed to the rotation. The animation shows the motion, without simulating steam flow.

What you can explore with this mechanism

The arrangement is useful for studying how a casing profile can guide a sliding part without a separate connecting rod.

• Geometry demonstrations: follow both wing tips through a complete turn.
• CAD motion studies: test the relationship between shaft location, casing shape and wing length.
• Rotary-machine comparisons: compare a single through-wing with separate sliding vanes.
• Sealing studies: identify the tip, side-face and slot interfaces that separate working spaces.

These are ways to study the mechanism, rather than operating specifications for a production engine. Hiscox’s short description and drawing do not give a rated speed, output or service clearance.

The geometry and the power comparison

Constant wing length

Let r(θ) be the distance from the shaft centre to the casing along the wing direction. A rigid wing can touch both opposite walls when:

r(θ) + r(θ + π) = L_w

Here, L_w is the fixed tip-to-tip wing length. This condition concerns opposite distances from the shaft; it is not the same as a constant-width curve measured between parallel tangents.

Profile used in the animation

The drawing uses the illustrative polar profile r(θ) = R − e sin(θ), with e/R = 30/104. Its opposite radii add to 2R, so L_w = 2R. The wing’s midpoint slides by −e sin(θ) relative to the shaft. The circular rotor has radius R − e. These proportions demonstrate the motion; they are not manufacturing dimensions from Hiscox.

Double-acting piston-power comparison

The calculator uses a separate piston model so you can explore the effect of pressure, size and speed on indicated power:

A = πB²/4; IHP = P_m × (S/12) × A × 2N / 33,000

• B: piston bore, inches.
• S: piston stroke, inches; S/12 converts it to feet.
• P_m: mean effective pressure, psi.
• N: speed, revolutions per minute.
• IHP: indicated horsepower, before mechanical losses.

The two piston faces are treated as equal areas; piston-rod area is neglected. The calculator also gives indicated torque as 5,252 × IHP/N in lb-ft, mean piston speed as 2(S/12)N in ft/min, and swept volume per revolution as 2AS in cubic inches.

To calculate work for the rotary engine itself, use the pressure-volume loop of each chamber: W = ∮p dV. Sum the chamber work over one shaft revolution and multiply by revolutions per second to obtain indicated power. Actual shaft power is lower by the mechanical losses. Bore and stroke from the piston comparison do not supply those rotary chamber data.

Worked example: comparing a double-acting piston cylinder

Take a hypothetical piston cylinder with a 4-inch bore, 5-inch stroke, mean effective pressure of 48 psi and speed of 220 rpm.

1. Piston area: A = π × 4²/4 = 12.566 in².
2. Stroke in feet: 5/12 = 0.4167 ft.
3. Indicated power: 48 × (5/12) × 12.566 × 2 × 220 / 33,000 = 3.35 hp.
4. Indicated torque: 5,252 × 3.35/220 ≈ 80.0 lb-ft.
5. Mean piston speed: 2 × (5/12) × 220 = 183 ft/min.
6. Swept volume per revolution: 2 × 12.566 × 5 = 125.7 in³/rev.

These results describe the hypothetical piston comparison. They are not a measured output or a rating for a Franchot engine. At fixed pressure and dimensions, doubling the speed doubles indicated power in this calculation; it leaves indicated torque unchanged.

Design considerations

• Casing profile: the opposite-radius condition must hold throughout the turn. Otherwise a rigid wing will leave a gap or bind.
• Wing guidance: the rotor slot must allow sliding while controlling angular motion.
• Sealing: tip contact, axial side clearances and the slot all affect leakage between working spaces.
• Friction and wear: the wing slides against guides and casing surfaces; contact force and lubrication affect the losses.
• Steam distribution: pressure history depends on port geometry, timing and operating conditions, not just rotor position.

The animation treats the parts as rigid and shows ideal contact. It does not calculate thermal expansion, seal loads or operating clearances.

Frequently asked questions

Reference

Gardner Dexter Hiscox, Mechanical Movements, Powers, Devices and Appliances (1901), Steam Power, page 90, Figure 326. Read the original drawing and description.