The World's smallest wind tunnel



RESTORATION - Presently the subject of a service and clean, this is the wind tunnel in the Museum in September 2023. The model used in the design of the Bluebird Electric 1, is seen in the foreground in brown modeling clay @ 1:24th scale. Over the years, the paint on the aluminium tunnel has crazed, where it was coated in epoxy resin, and then matt black, and the expansion coefficients in winter and blazing summer heat are different. The machine is still operable. The fan unit was tested on the 21st.







The MKII wind tunnel assembly, with the BE1 test model in the visualization chamber in the 1990s.




Not directly connected with the generating history of Herstmonceux Museum, though designed and constructed in the workshop garage, in Lime Park, as a way to visualize and measure the efficiency of a vehicle shape, in the ability to pass through air, with the least resistance  to movement.


This was the second compact re-circulating wind tunnel to be built in this location. Designed to be a desktop machine, it is that small. 


You do not have to spend a fortune to construct a very basic wind tunnel. The first was made of plywood, and recycled materials, including the fan and motor, costing something like £10 in materials. At hobby or school level, cardboard can be used, and cheap blades, where low air speeds don't matter. It is only when more accurate results are needed, and higher airspeeds, that it is worth moving to electronics.


Try putting your hand out of a car sunroof (only when safe to do so, and never a window), aiming your fingers into the wind. You can hold your hand there with no problems. Then put you hand flat to the wind, and feel the difference. Your hand will be blown backwards, much harder. Without any instruments at all, this simple experiment reveals the basics of all aerodynamic design.


Electronic strain gauges simply allow engineers to measure drag, lift and down-force (see pictures below) with more accuracy. In this case with a five element balance. A turn table was incorporated so vehicle models could be tested in side wind conditions. Air temperature and wind speed were also monitored and smoke could be trailed over models to visualize airflow, to highlight problem areas. This instrument measured just 60" x 40" x 15" (1524 x 1016 x 381mm). Being a closed circuit design it was not unbearably noisy in operation. 


This wind tunnel exhibit is a relatively complex piece of engineering, ideal for engineers working on a Masters Degree thesis, or other higher learning projects.


The Wright Brothers were the first inventors to use such a tool to compile lift and drag tables for various wing shapes. It is much more difficult to design a "balance" to measure lift and down-force on each wheel of a vehicle, than a tunnel to measure lift on a wing. When working at such a small scale, accuracy of measurement is essential. Fortunately, electronics come to the rescue, with compensated amplifiers providing multiplication of needle movement up to a thousand times the original voltage variation, as a resistive foil is stretched or compressed.






1:50th and 1:24th scale wind tunnel models - on display at Herstmonceux Museum in East Sussex



Some of the wind tunnel models on display at Herstmonceux Museum, the larger ones for use in the MKII measuring instrument were 1:24 scale. The smaller models were 1:50 scale. These were hand carved in wood, then painted to as near as possible present a surface finish representative of a full size vehicle. The rough grain of wood, would not have worked so well. These models were carved in the Lime Park workshops.


Having made a model of the vehicle you want to test, you need to work out the frontal area. This is easy in AutoCad. The frontal area should include all appendages (wing mirrors) and the wheels. A base card of the same frontal area is then used to calibrate the wind tunnel. Or, you can do this mathematically. Either way, the wind tunnel has to be calibrated. A flat plate is taken as 1.25 or thereabouts.





A wind tunnel is a research tool developed to assist with studying the effects of air moving over or around solid objects. The most famous early experimenters to build a wind tunnel were the Wright Brothers, Orvill and Wilbur. They used their tunnels to design wings, to calculate the lift of different airfoil sections, thus the wing area needed, to lift their biplane off the ground at Kitty Hawk at different speeds.


One of the main factors affecting top speed, or fuel consumption of a car, is the drag of the vehicle. This is determined by the shape of the vehicle's body. How slippery it is might mean a faster vehicle, such as in racing competitions, or how much it will cost to operate, to drive to the shops and back. Unlike the Wright Brothers, we don't want a car to take off, except in a James Bond movie. But how can we measure how well a car moves through the air, and the lift or downforce of a body shape?


Well, apart from a wind tunnel, you also need an accurate model of the vehicle you are proposing to build. Small enough to fit inside the measuring chamber of any tunnel. Hence, the size of your models, determines the size of your wind tunnel, and vice versa, the size of your tunnel, determines the size of your models. The bigger the model, the bigger the tunnel. Most car makers use full size vehicles, in very large wind tunnels, the size of a giant car park. Imagine the cost of that. It is many £millions of pounds.


The wind tunnel above cost around £8,000 to develop in the 1990s, about £30,000 adjusted pounds today (30 years later). It took a little over 6 months, including designing the frame, fan housing, ducting, and measuring instruments. Three months conceptualizing and drafting, and three months actually building the machine, welding the mild steel frame, cutting and welding the alloy sheet, some of which work was completed at Filching Manor Motor Museum, near Polegate. Mostly, the plexiglass chamber was fitted in Filching. The electrics and electronics were wired up in Herstmonceux.


The design was an improvement on a smaller plywood version, with an oil bath float, on which the smaller model vehicles were mounted, facing into the wind, only measuring drag. The machine above measures lift and downforce on each wheel of a model, and the subject vehicle can be rotated to simulate sidewinds. Unfortunately, the original wooden version was scrapped. A real shame for the Museum, to be able to show progression.


The other major component of vehicle drag, is the rolling resistance of the tyres, including the driveshaft and wheel bearings. Meaning that thinner wheels and tyres, and higher pump pressures, will all reduce rolling resistance, and improve MPG. The road surface is also important. Smooth tarmac is much easier to travel over than sand. Concrete increases rolling resistance, compared to tarmac. Many land speed records are run and records set on dried salt. Such as the famous Bonneville Salt Flats, Utah, USA.








5 ELEMENTS - This is the business end of the wind tunnel, a five element strain gauge (beam or balance) arrangement, that measures drag, lift and downforce on each wheel. The pushrods of the assembly are not shown in this picture. There are four stainless steel cylinders below each wheel gauge. The cylinders are oil filled, to damp the vibrations, helping to stabilize readings. The pushrods go from a piston in the cylinders, through the wheel element beams, to a Plexiglas pad in the visualization chamber. The whole unit is adjustable on screw threads, with a built in level, and may be revolved. The model being tested has steel inserts in the wheels, that clamp to the pushrod pads electro-magnetically. It's a bit rusty, but not bad for 30 years unpainted. Copyright photograph © Herstmonceux Museum, 28 September 2023.






Air is blown or sucked through a duct equipped with a viewing port and instrumentation where models or geometrical shapes are mounted for study. Various techniques are then used to study the actual airflow around the geometry and compare it with theoretical results, which must also take into account the Reynolds number and Mach number for the regime of operation. For example:

*Threads can be attached to the surface of study objects to detect flow direction and relative speed of air flow.

*Dye or smoke can be injected upstream into the air-stream and the streamlines that dye particles follow photographed as the experiment proceeds.

*Probes consisting of a Pitot tube can be inserted at specific points in the air flow to measure static and dynamic air pressure.





A key component of any wind tunnel is the fan assembly. A fan should be able to generate high wind speeds, sufficient for the vehicles being tested. Take a look at the propeller and motor to the left. A 1hp motor is mounted on eight springs to reduce vibration, rubber damped. The whole fan module is then positioned on rubber mounts, which in turn channel vibration through a weighted plywood frame. The net effect is to pass the vibration through materials with different natural frequencies, so acting as a vibration filter, much like coils and capacitors are used to filter out unwanted frequencies in Hi Fi speaker systems. Lead weights are also employed to give mass to the mild steel base frame unit.




Air temperature and wind speed were also monitored and smoke could be trailed over models to visualize airflow, to highlight problem areas. This tunnel could fit comfortably on an office desk. It measured just 60" x 40" x 15". Being a closed circuit design it was not unbearably noisy in operation. 

The plexiglas chamber provided superb all round visibility, which is so important.



Strain gauges provided measurement of drag, lift and down-force. A rotating table was incorporated so that vehicle models could be tested in side wind conditions.

It is much more difficult to design a balance to measure lift and down-force on each wheel of a vehicle. When working at such a small scale, accuracy of measurement is essential.


Fortunately, electronics come to the rescue, with compensated amplifiers providing multiplication of movement (lift/downforce) up to ten thousand.










Electrical signals, as variable voltages, are amplified and sent to moving-coil, needle instruments in a purpose built instrument console.


These gauges measure micro amperes, to provide a visual indicator of how a vehicle is performing in the wind tunnel. it is possible to see attitude changes from lift and ground effect. Apart from measuring simple drag.







The Power Supply Unit (PSU) shows the current and operating voltage of the main fan motor. The speed of the fan motor may be infinitely varied. There is also an hours run meter and temperature gauge.


Design took 3 months, with build of the wind tunnel approximately 3 months more. 6 months from start to conclusion, at a cost of around £8,000 in 1988 (around £30k today. A bargain research tool.


The wiring of the PSU's various components look like spaghetti junction, being a custom prototype build, or one-off. Even with some PCBs. A large high output toroidal transformer is used with multiple tappings, taken at 18volts. The AC supply is rectified to DC, with a heat sink. There is a 12v DC cooling fan and heat sensor to cut off the mains power, in the event of overheating. Thirty years later, stored in damp and cold conditions, and the unit is still operable.





Five RS strain gauge amplifiers gave separate channels of information to provide a range of measurements to include overall drag, + lift and down-force for each wheel.



Each amplifier is fed information from a (Wheatstone) bridge of four foil resistors bonded to each element of the balance, two on each side. The balance comprises of five elements, consequently, quite a few (20) strain gauges were needed, and some patience during the epoxy bonding and positioning. The marking out must be exceptionally accurate for consistent results.



The components used in the making of this tunnel, were quite expensive if you are on a small budget, but more importantly, with suppliers such as Radio Spares (RS) they are at least widely available to professional scientists and amateur enthusiasts, schools and universities.





Here is a close up of the wiring of one of the five steel elements against which the slight bend, fractionally stretched one side of the plate (increasing resistance) and compressed (lowered resistance) the other side of the plate, alters resistance. The sensitivity (voltage) multiplication effect of such a bridge is well known.

The completed assembly is coated in silicone for environmental protection. Making it look a little messy.






The drag coefficient Cd is equal to the drag D divided by the quantity: air density r times reference area A times one half of the velocity V squared.

Cd = D / (.5 * r * V^2 * A)

This slide shows some typical values of the drag coefficient for a variety of shapes. The values shown here were determined experimentally by placing models in a wind tunnel and measuring the amount of drag, the tunnel conditions of velocity and density, and the reference area of the model. The drag equation given above was then used to calculate the drag coefficient. The projected frontal area of each object was used as the reference area. A flat plate has Cd = 1.28, a wedge shaped prism with the wedge facing downstream has Cd = 1.14, a sphere has a Cd that varies from .07 to .5, a bullet Cd = .295, and a typical airfoil Cd = .045.





NASA on the shape of objects, as affects the drag                    A table of shapes, with approximate drag figures



Table approximations, one from NASA. These are assumptions, guesstimations that can be made, while you work out the details of your own method of calibration. Ideally, you should test a flat plate, sphere and airfoil. All of the same frontal area. That should give you 3 relatively reliable datum points on your meter display. Be sure to use the same airspeed when taking measurements.






English military engineer Benjamin Robins (1707–1751) invented a whirling arm apparatus to determine drag. The Englishmen Wenham and Browning did air flow experiments in a wind tunnel in 1871.

The Wright Brothers, working with Octave Chanute invented and built a simple wind tunnel in 1901 to study the effects of airflow over various wing shapes while developing their revolutionary 'Wright Flyer.' The Wright wind tunnel was used more recently to test modern low-speed fliers, such as the human-powered "Albatross".

Subsequent use of wind tunnels proliferated as the science of aerodynamics and discipline of aeronautical engineering were established as air travel and power were developed.

Wind tunnels were often limited in the volume and speed of airflow which could be delivered.

The wind tunnel used by German scientists at Peenemünde prior and during WWII is an interesting example of the difficulties associated with extending the useful range of large wind tunnels. 

It used some large natural caves which were increased in size by excavation and then sealed to store large volumes of air which could then be routed through the wind tunnels. This innovative approach allowed lab research in high speed regimes and greatly accelerated the rate of advance of Germany's aeronautical engineering efforts.

Later research into airflows near or above the speed of sound used a related approach. Metal pressure chambers were used to store high pressure air which was then accelerated through a nozzle designed to provide supersonic flow. The observation or instrumentation chamber was then placed at the proper location in the throat or nozzle for the desired airspeed.

Computational fluid dynamics has augmented, and is starting to replace, the use of wind tunnels. For example, the experimental rocket plane SpaceShipOne was designed without any use of wind tunnels. (However, on one test flight threads were attached to the surface of the wings, performing a wind tunnel type of test during an actual flight in order to refine the computational model.)

An area that is still much too complex for the use of Computational fluid dynamics is determining the effects of flow around buildings and bridges. Boundary layer wind tunnels are the state of the art method to test such structures. These wind tunnels are also used to simulate and measure wind characteristics at the pedestrian level and exhaust gas dispersion patterns for laboratories and other emitting sources. 

Wind tunnel tests in a boundary layer wind tunnel allow for the natural drag of the earth's surface to be simulated. For accuracy, it is important to simulate the mean wind speed profile and turbulence effects within the atmospheric boundary layer. Most codes and standards recognize that wind tunnel testing can produce reliable information for designers, especially when their projects are in complex terrain or on exposed sites.


Alongside, the world's smallest water basin, the Museum is home to some interesting technological exhibits, many world record contenders.







THE WORLD'S SMALLEST WATER BASIN - Project director, Chris Close, after a day testing the SeaVax in an open air water basin. On this day in 2016, the 29th of July, the proof of concept model, cleaned microplastic from the test tank, collecting the litter in her hydro-cyclonic filters. The total cost of building the 2.1 meter (fully functional) model was around £120,000 pounds. Test tank costs, added to the project cost, but not prohibitively.






There are several innovative vehicles and vessels on permanent display at Herstmonceux Museum, including:


1. Art Gallery - Collection of paintings, pictures, graphics, sculptures, wooden carvings & exotic glassware

2. Archives - Historic documents library, patents, trademarks, copyright, films, catalogued legal papers & letters

3. An Edwardian ice well, throwback to the days before refrigeration

4. A large underground (condensation/cooling) and water storage chamber for ice making

5. The world's smallest water basin, test tank for model boats & ships to 1:20 scale

6. World's smallest wind tunnel, vehicle drag measuring instrument using electronic strain-gauges

7. Three PV boat models, Navigator, SWATH & 2 cats + route map prior to Swiss PlanetSolar

8. Seavax, the ocean cleanup proof of concept prototype from 2016

9. AmphiMax, radio controlled (working) beach launching & recovery vehicle for SeaVax

10. Anthony the most dangerous giant Australian bulldog ant, 300 times normal size

11. EV - FCEV refueling station model in 1:20 scale

12. The only working (fully functional) water well in Herstmonceux village

13. The fountain of youth, supplied from natural well water drawn on site

14. Second World War, 'Anderson Inspired,' bomb proof shelter constructed by Major Charles de Roemer

15. City sports FCEV-BEV, hydrogen gull wing proof of concept DC50 electric car

16. Land speed record car: Bluebird-Electric BE1 (original 1st) with battery cartridge exchange

17. Land speed record car: Bluebird-Electric BE2 (original 2nd) with cartridge exchange

18. A complete mummified squirrel, found when re-roofing the Museum June 2017

19. A fully operational, and restored VW Kombi van dating from 1978 (historic vehicle)

20. BMW i3, battery electric vehicle hybrid, with onboard generator range extender

21. Solar panel, sun tracking system, with battery storage

22. A hornet's nest found on site & preserved in 2016 (reported as [Asian] invasive species, to be safe)

23. Three sewing machines, including an antique Singer and a Brother industrial.

24. Adventure climbing frames for children (back to nature) Swiss Family Robinson

25. 'Elizabeth Swann' proof of concept model 1:20 scale hydrogen powered trimaran

26. Holm oaks, planting and growing trees from acorns on site, re-wilding in Sussex














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