A Form 1 windmill is a horizontal-axis wind machine in which four cloth-covered sails mounted on an inclined windshaft drive a vertical millstone or pump shaft through a brake wheel and wallower gear pair. The sails are the defining component — they convert wind kinetic energy into shaft torque by presenting a twisted lifting surface set at roughly 5° to 25° across the sail length. The mechanism exists to extract mechanical power from wind without fuel, historically to grind grain or lift water. A working post mill the size of Outwood Mill in Surrey produces around 3 to 6 kW at the stones in a 7 m/s wind.
Windmill Form 1 Interactive Calculator
Vary sail RPM and brake-wheel to wallower ratio to see the resulting millstone speed and gear-train tradeoffs.
Equation Used
The calculator applies the worked example gear relation for a Form 1 windmill: the slow windshaft speed is multiplied by the brake wheel to wallower step-up ratio to estimate millstone RPM. The inverse of the same ratio shows the ideal torque fraction remaining at the faster vertical shaft.
- Ideal gear mesh with no slip.
- Gear ratio is the brake wheel to wallower step-up ratio.
- Torque fraction is the inverse of speed ratio, ignoring friction losses.
How the Windmill (form 1) Actually Works
Wind hits four sails set at a small angle to the rotation plane. Because the sail surface is angled, the wind force splits into a useful component that pushes the sail tangentially around the shaft, and a wasted component that just tries to push the whole mill over backwards. The tangential component spins the windshaft. That windshaft tilts upward toward the tail by 5° to 15° — not a mistake, deliberate. The tilt keeps the sails clear of the mill body when they sweep past the bottom and stops rainwater tracking down the shaft into the brake wheel bearing.
On the inboard end of the windshaft sits the brake wheel, a large face gear typically 2.5 to 3 m in diameter with 60 to 80 wooden cogs. It meshes with the wallower, a smaller lantern pinion on the vertical main shaft. Step-up ratio is usually 5:1 to 8:1, so a windshaft turning at 12 RPM drives the millstone at around 80 RPM — which is exactly the speed range where a 1.4 m French burr stone grinds wheat cleanly without scorching the flour. If the gear ratio is wrong, you either lug the stones at 30 RPM and produce coarse meal, or overspeed at 140 RPM and burn the flour brown.
What goes wrong? Sail twist set incorrectly is the classic failure — if the outer end of the sail is at the same angle as the inner end, the tip stalls, the mill struggles to start, and you get half the rated torque in a 6 m/s breeze. Cloth tension is the other one. Common sails use canvas spread over a wooden lattice, and a slack cloth flutters and shakes the entire mill cap. Heritage millers re-tension every morning. The tail pole or fantail must keep the sails pointed within about 15° of the wind direction, or yaw misalignment kills output by a factor of cos²(angle) — at 30° off-wind you've already lost 25% of your power.
Key Components
- Sails (Stocks and Whips): Four wooden lattices mounted on two crossed stocks bolted through the poll end of the windshaft. Each sail is twisted along its length from roughly 25° at the root to 5° at the tip — this twist matches the increasing local wind angle as you move outboard, exactly like a modern wind turbine blade. Span is usually 8 to 11 m on a working post mill.
- Windshaft: The main rotating shaft, traditionally oak or cast iron, set at a 5° to 15° upward tilt. Carries the sails on the outboard end and the brake wheel on the inboard end. Bearings are typically a brass or lignum vitae neck bearing at the front and a thrust bearing at the tail.
- Brake Wheel: A large face gear, 2.5 to 3 m diameter with 60 to 80 hardwood cogs (apple or hornbeam). Doubles as the brake drum — a wooden brake band wraps the rim and clamps via a long lever to stop the mill in high winds. Cog wear at more than 3 mm out of round causes audible clatter and tooth-skip.
- Wallower: A lantern-pinion gear with vertical staves running between two discs, mounted on the top of the main vertical shaft. Engages the brake wheel above. Step-up ratio of 5:1 to 8:1 sets the millstone speed.
- Main Vertical Shaft: Transmits torque from the wallower down to the stone nut, which drives the runner millstone. Typically 200 to 300 mm diameter oak or cast iron, 4 to 6 m long in a post mill.
- Tail Pole or Fantail: Yaw mechanism that keeps the sails pointed into the wind. On a Form 1 post mill the entire body rotates around a central post — the miller pushes the tail pole by hand, or a fantail (small auxiliary wind rotor at right angles to the main sails) drives the body around automatically. Yaw error above 15° measurably reduces power.
Industries That Rely on the Windmill (form 1)
Form 1 windmills powered the rural economy across Northern Europe from roughly 1180 to 1900. Today they survive in heritage operation, demonstration milling, and a handful of working production sites where the wind resource and grain demand still line up. The same kinematics also appear in modern reconstructions for living-history sites and educational rigs.
- Heritage flour milling: Outwood Post Mill in Surrey, England — built 1665, still grinds stoneground wheat for local bakeries on demonstration days using its original brake wheel and 1.4 m French burr stones.
- Living-history museums: Bourn Windmill in Cambridgeshire, one of the oldest surviving post mills in Britain (circa 1636), preserved as a working example of the Form 1 layout.
- Polish heritage sites: The post mills at the Sanok open-air ethnographic museum in southern Poland — relocated 18th-century koźlak post mills running demonstration grinds for visitors.
- Educational engineering rigs: Scaled 1:10 working post mill models at the Technische Universiteit Delft for undergraduate wind-energy coursework, demonstrating sail twist and yaw control.
- Craft milling: Sturminster Newton Mill in Dorset (a tower mill cousin running similar sail and gearing kinematics) supplies stoneground flour to local bakers and the National Trust shop.
- Water lifting: Reconstructed drainage post mills in the Norfolk Broads driving Archimedes screw pumps for marsh dewatering on heritage estates.
The Formula Behind the Windmill (form 1)
The shaft power a Form 1 windmill extracts from the wind scales with the cube of wind speed and the square of sail span. That cubic relationship is the single most important number to internalise — doubling the wind from 5 m/s to 10 m/s multiplies output by 8, not 2. At the low end of the typical operating range (around 4 m/s) a heritage post mill barely turns the stones. At the high end (10 to 12 m/s) the miller is reefing sail cloth or shutting down because shaft loads climb past what the wooden gearing can survive. The sweet spot sits around 7 to 9 m/s, where the sails extract useful power without overstressing the brake wheel cogs.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| P | Shaft power delivered to the windshaft | W | ft·lbf/s |
| ρ | Air density (≈ 1.225 at sea level, 15°C) | kg/m³ | lb/ft³ |
| A | Swept area of the sails (π × R² for a four-sail rotor) | m² | ft² |
| v | Free-stream wind speed | m/s | ft/s |
| Cp | Power coefficient — fraction of wind power captured (0.10 to 0.20 for traditional cloth sails) | dimensionless | dimensionless |
Worked Example: Windmill (form 1) in a heritage rye-milling post mill in Friesland
A Frisian heritage trust is restoring a 17th-century post mill near Workum to grind rye for a local rye-bread bakery. Sail span is 22 m tip-to-tip (so R = 11 m), giving a swept area A = π × 11² ≈ 380 m². The site has a measured average wind of 7 m/s with regular gusts to 10 m/s and lulls down to 4 m/s. They want to know the shaft power they can plan around for sizing the rye stones. Assume cloth-sail Cp = 0.15 and ρ = 1.225 kg/m³.
Given
- R = 11 m
- A = 380 m²
- vnom = 7 m/s
- Cp = 0.15 —
- ρ = 1.225 kg/m³
Solution
Step 1 — compute the swept area from the sail radius:
Step 2 — compute shaft power at the nominal site wind of 7 m/s:
That is the design point. At 12 kW shaft power and a stone speed of 90 RPM the trust can plan to drive a single 1.5 m rye burr stone comfortably with margin to spare for the sack hoist.
Step 3 — at the low end of the typical operating range, 4 m/s, the cubic term collapses output:
2.2 kW will only just keep the stones turning under load — the miller will hear them lugging and the flour will come through coarse. Below about 3.5 m/s you cannot grind at all, the stones simply stall.
Step 4 — at the high end, a 10 m/s blow:
35 kW is more than the wooden brake wheel cogs were designed to transmit — apple-wood cogs typically rate to around 20 kW continuous in this size. The miller must reef cloth (fold sail canvas back along the lattice) to drop effective sail area by half, bringing power back down to 17 kW. Above 12 m/s you furl completely and apply the brake.
Result
Nominal shaft power at 7 m/s is roughly 12 kW — exactly the right figure to drive one 1. 5 m rye stone at 90 RPM with margin for the auxiliary hoist. The cubic wind law means this same mill produces only 2.2 kW in a 4 m/s breeze (stones lug and grind coarse) but theoretically 35 kW in a 10 m/s wind (well past the wooden gearing's safe limit, requires reefing). If the trust measures only 7 or 8 kW at 7 m/s instead of the predicted 12 kW, three causes dominate: (1) sail twist set too shallow at the tip so the outboard quarter of each sail is stalled, (2) cloth tension uneven across the lattices causing one sail to flap and dump energy as drag rather than lift, or (3) brake wheel cog wear letting the gear ride high on the wallower staves so 10 to 15% of the torque is lost to slip and friction instead of reaching the stone nut.
When to Use a Windmill (form 1) and When Not To
Form 1 post mills are not the only way to extract wind energy at this scale. Tower mills, smock mills, and modern small wind turbines all compete for the same niche. Here is how they line up on the dimensions a heritage trust or off-grid builder actually cares about.
| Property | Form 1 Post Mill | Tower Mill (brick) | Modern Small Wind Turbine |
|---|---|---|---|
| Typical shaft power at 7 m/s | 8 to 15 kW | 15 to 40 kW | 3 to 10 kW (electrical) |
| Power coefficient Cp | 0.10 to 0.20 | 0.10 to 0.22 | 0.35 to 0.45 |
| Yaw mechanism | Whole body rotates on central post | Cap rotates on curb rail | Tail vane or active yaw motor |
| Cut-out wind speed | ≈ 12 m/s (manual reefing) | ≈ 14 m/s (manual reefing) | 25 m/s (automatic feathering) |
| Build cost (heritage rebuild) | £300k to £600k | £500k to £1.2M | £15k to £40k installed |
| Service interval (sail cloth) | Re-tension daily, replace every 2-4 years | Same as post mill | No cloth — composite blades, 20-year life |
| Suitable application | Heritage stone milling, demonstration | Higher-output heritage milling | Off-grid electrical generation |
| Mechanical complexity | Moderate — wooden gearing, manual yaw | Higher — curb rail, cap rotation | High — electronics, but no internal gearing visible to user |
Frequently Asked Questions About Windmill (form 1)
Starting torque is dominated by the sail twist at the inboard quarter, not the outboard tip. If the inner end of each sail sits at less than about 20°, the wind has nothing to bite on at low rotational speeds and the sails just flap. The tip can be at 5° because once the mill is spinning the local wind angle there is shallow, but the inner sections need that steep angle to break stiction.
Check the angle at the root of each whip with a digital protractor. If it reads under 18°, re-set the sail bars. The other common cause is a tight neck bearing — lignum vitae bearings need to run wet, and a dry bearing can add 200 to 400 Nm of breakaway torque, which is enough to keep the whole mill stationary in a light breeze.
Common sails give you a higher peak Cp (around 0.18 to 0.20) because the canvas can be set perfectly for the day's wind. Patent sails with hinged shutters give you 0.12 to 0.15 peak but self-regulate — the shutters open in gusts and dump excess wind, so you do not have to climb out and reef in a squall.
If the mill runs once a week for demonstrations with a miller present, stay common — the period is right and the output is better. If the trust wants the mill turning unattended on weekdays for visitor display, go patent. The conversion was historically common from about 1807 onward (Hooper's roller-reefing and later Cubitt's patent of 1813), so it is period-appropriate on most surviving British post mills.
Hot stones mean the runner is turning too fast for the feed rate, or the tentering is set too tight. On a post mill, stone speed scales directly with windshaft RPM through the fixed gear ratio, so in a strong wind the stones overspeed unless you reef cloth. If the cloth is fully spread in 10 m/s wind you are pushing the stones to 130 RPM or more, well past the 80 to 100 RPM range a 1.4 m French burr is happy at.
Fix one of two ways: reef sail area to bring the windshaft below 14 RPM, or open the tentering screw to lift the runner stone slightly and reduce grinding pressure. Dark flour means starch is scorching, which means heat above about 60°C at the stone face — that always traces back to speed or pressure, never the wind itself.
Two reasons. First, the formula assumes uniform wind across the entire swept disc — in reality the sails see a wind gradient, with the lower tip in slower air close to the ground and the upper tip in faster air aloft. On a 22 m sail span this gradient alone knocks 10 to 15% off the theoretical figure.
Second, Cp for cloth sails is genuinely low — 0.15 is a good day. Modern wind turbines hit 0.40 because composite aerofoils have proper lift-to-drag ratios; cloth on a wooden lattice is closer to a flat plate than an aerofoil. If you measured 8 kW where the formula predicted 12 kW, you are probably running at Cp ≈ 0.10, which is normal for a mill with cloth that has not been re-tensioned recently.
Power loss with yaw error follows roughly cos²(θ) for a rotor — at 15° off-wind you have lost about 7%, at 30° you are down 25%, and at 45° you have lost half. The miller can usually feel the difference by 20° because the mill body shudders unevenly as each sail passes through different parts of the off-axis flow.
On a Form 1 mill without a fantail, the miller pushes the tail pole by hand whenever the wind backs more than about 10°. Mills with a fantail hold yaw error to within 3 to 5° automatically. If your output suddenly drops mid-run on a steady wind day, walk outside and check tail-pole alignment first — it is the single most common cause of unexplained power loss.
A 1.2 m runner stone grinding wheat at 90 RPM needs roughly 4 to 6 kW at the stone. Allow 20% drivetrain loss, so 5 to 7.5 kW at the windshaft. At a typical site wind of 6 m/s and Cp = 0.15, that puts you at A ≈ 130 m², or R ≈ 6.4 m, meaning a sail span of about 13 m tip-to-tip.
That is a small post mill by historical standards — most surviving Dutch and English post mills run 18 to 24 m spans because they were sized for two pairs of stones plus auxiliary machinery. If your site averages closer to 5 m/s you need to go bigger, fast — the cubic wind law means you need almost double the swept area to compensate for a 1 m/s drop in average wind.
References & Further Reading
- Wikipedia contributors. Post mill. Wikipedia
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