Revised: 6/18/2012 Subject to Revision
The prototype wind waggon box in the blog story (see other blog links) is 24 feet long and 10 feet wide and some 8 feet tall. The overall height is about 30' 10" tall including the sail turntable. The prototype wind waggon is a fictional machine featured in a fiction blog being written by the same author as this blog. The idea for both is develop the story and the model in parallel so that design aspects of the fictional prototype are practical features developed for the model.
The author intends that the model be working and controlled by radio control equipment developed for model airplanes, etc. The RC equipment would permit the model to be operated in a rear prototype manner to evaluate how well model design features operate while the model is running. The scale for the RC model needs to be determined to provide adequate volume for both the model running gear of scale belts, pulleys, gears, levers and such, but the RC servos, receiver and batteries as well. Modern RC servos, receivers and batteries are very small, however, it is conceivable that perhaps one or two larger size servos may be required to operate control levers in the model.
Should a scale of 1/2"=1' be used the model box would be 12" long, 5" wide and 3.5" high. Granted there will be considerable scale mechanisms for wheel drive involved, but perhaps sufficient space would exist for several RC servos, receiver and battery. This seems a bit tight.
An alternate that would provide more space would be to use a scale of 3/4"=1' which would result in a model 50% larger. That would give a main box of 18" long, 7.5" wide and 6" tall, approximately. The overall height with the sail turntable would be 23.125". In the story the actual size has not yet been finalized, although a notional design has been presented by the lead designer. During concept design detailing by thane wind waggon team in the story, the design will be subject to change.
It might work best to detail the equivalent model features and then migrate those to the story so that various operational features will work both in the model and in the prototype. In the ongoing development, the model design will pace the story line and the prototype sizes of various details will be driven by the more detailed model design being done in parallel. The model and prototype designs will be done using Aibre 3D CAD software. Insofar as practical, the model will be built using the same materials as the full size prototype. The model design will be such that it can directly be scaled up to prototype size and look correct there. Features such as parts made of various materials would be prototypical insofar as possible.
The society in the story has fairly primitive metal working capabilities, perhaps analogous to those that existed in the 16th and 17th centuries and equivalent woodworking skills. They could not make large metal parts such as long shafts suitable for the turntable mast, but could make pulleys, gears, reinforcement hoops, short shafts, longer rods, threaded bolts, etc. They could work in various metals such as brass, aluminum, iron or steel, etc. They could also make castings up to several feet in size and machine those parts with drills, mills and planing tools. For wood they could make parts quite long up to near tree length, bind, glue, mortise, join, drill, chisel and many other operations.
The model will take these capabilities and limitations into account and design the larger structures as combinations of wood and metal taking best advantage of each. The wood for the model will probably be largely oak or other similar hard wood. The metal will largely be aluminum.
First Parts
The first parts to be considered are the rotating sails and turntable. Both in developing the model and the prototype in the story those are drivers to the balance of the design.
Sails
The sail in particular is revolutionary. It is a rigid structure built on a largely wooden frame covered with sail cloth. The direction given the development team in the story is that the structure be built in sections so that broken parts can be replaced from spares if need be and that the canvas sails be attached with hooks or bolts so it too can be replaced. Each such sail is some 16' tall and perhaps 4' wide. It is mounted on bearings so it can freely turn. No ropes or other devices are used to move the sail. It is more like a lightweight panel than a canvas sail typical to a ship. On the model each sail panel would be 12" tall and 3" wide. The trick will be to design the wood frame for suitable strength on the prototype and design it to be held together with bolts for the model. The sail cloth will probably be some fine weave cloth such as silk for the model, or perhaps some other material such as nylon or rayon. The trick for the cloth will be to devise suitable reinforced holding points for bolt or hook attachments.
A wind pressure calculator on the web indicates that the pressure on a prototype wind sail of 64 square feet orthogonal to a wind would be 0.56 pounds per square foot (psf) with a 20 mph wind (35.84 pounds total) and 2.24 psf with a 40 mph wind (143.36 pounds total), 5.02 psf at 60 mph (321.28 pounds total), 8.9 psf at 80 mph (569.6 pounds total) and 13.96 psf at 100 mph (893 pounds total).
Turntable
The turntable is a complex structure holding six sail panels evenly spaced around their mounting diameter. The vertical forces on the turntable are not large considering that the sails are relatively light weight. The larger forces to be reckoned with are the wind forces that push the sail panels to rotate the turntable. The problem is to adequately couple the sail panel side forces to the turntable shaft.
As was pointed out in the story, the lead designer estimated that the each sail panel on the prototype could experience about 893 pounds of side force coupled half and half to the upper and lower sail panel bearings that attache the panels to the turntable radial arms. Each radial arm would experience 446.5 pounds of side force at a point about 4 feet from the turntable shaft, a moment arm force of 1786 pounds applied towards rotating the turntable shaft. Each radial arm would need to be strongly anchored at the shaft end and braced so that it would not twist out of alignment with the shaft.
The primary problem with the radial arms is to safely couple the high rotational torque to the shaft and keep all radial arms in proper position. Those radial arms are about 5' long and made of hardwood. The shaft is also hardwood. Coupling the high force torque through two pieces of oak at right angles is the problem. The design will most likely need to use metal joints to do so such that the wood radial arms couple their force into a metal channel that in turn couples the force through a right angle channel around the central shaft. The channels that contain the wood parts would use bolts to keep the wood parts firmly in place. The portion around the central shaft would be a large piece of metal that channels top and bottom for the turntable shaft and six radian channel for the radial arms.
The long 16' central shaft of the turntable between the bottom and top of the sail panels carries half of the overall sail force. In the story the lead designer indicated that the expected forces would be 1.35 times that of one panel at 4' radius or 2411 foot-pounds torque force. The central shaft below the turntable going down into the box where the power take-off would exist would need to handle the entire 2.7 times the force of one panel or 4822 foot-pounds torque force. Additionally, the turntable shaft would need to handle perhaps 75% of panel force as side bending force of about 904 pounds as the sails will not travel around at wind speed so as in a ship sail considerable side force will be applied to the central shaft as well. All the above figures are for the very unusual case of a 100 mph wind, an extreme case in a severe storm. The lead designer is very conservative and wants the design to hold up even in a severe storm. As the story evolves, a sort of sail throttle will evolve that will free the sails to rotate most of the time and only rotate them at an angle to the wind in order to deal with strong winds.
A much larger problem is that the rotating sail panels will likely only be useful with fairly brisk winds of 20 to 40 mph. Twenty mph winds only provide a miserly force of 96.8 pounds at the 4' sail panel radius or 387 pounds torque total for instance. Depending on the drive ratio this may be sufficient. The estimate made in the story is that the turntable rotates at 10 to 20 revolutions per minute which it probably could do with no load. Loading depends on the drive ratio and overall load due to waggon weight and whether going uphill, etc. During story evolution a drive transmission of pulley ratios is contemplated to permit driving with various wind speeds, waggon weights and driving conditions.
Waggon weight and wheel drag conditions have not been determined and as the design evolves the ratio of transmission pulleys may need to be revised to permit waggon progress with relatively low wind velocities. It may also be necessary to increase panel size or even resort to two turntables or possibly to revise the sail panel arrangements on the turntables. An alternate configuration using airfoil shapes with flaps to orient the airfoils relative to the wind permits four or more airfoils to pull together increasing the number of elements that extract wind power. In the interests of simulating a new revolutionary design in the story, flat sail panels were selected.
It is expected that the turntable shaft below the top of the waggon box will extend down to the inside lower deck at the bottom of the box. Bearings at the box top and bottom will need to hold the turntable shaft in position vertically while allowing the shaft to freely rotate while sustaining the side forces due to the wind. The waggon box will need to be reinforced to bear the forces applied. At the bottom bearing plate below the bottom of the waggon box a large pulley wheel will be used to couple the turntable rotational torque to the main transmission assembly.
The design will attempt to located all of the drive transmission below the waggon box bottom inside a covered area between the drive wheels. At this point the main drive pulley is expected to rotate from 10 to 20 times per minute on the prototype. Based on the lead designers calculations, the force applied to the drive transmission pulley will be of between 64.5 foot-pounds per second available to drive the prototype vehicle in a 20 mph wind. That is not much power. One hp is 550 ft-lb / sec, so the effective motive power is 0.117 hp. Not very practical if the calculations are correct.
The model sail panel is 0.0039 time the area of the prototype and have a surface area of about 1/4 square foot. A twenty mph wind would apply 0.56 x 1/4 = 0.14 pounds force for one orthogonal sail panel. The three panels absorbing wind power would be 2.7 times that or 0.378 pounds. The sail force would act at a radius of 3" which is 1/4 ft. The expected torque level would be ~ 0.0945 ft-lbs. If the turntable were to rotate ~ 10 times per minute the effective power would be 0.01575 ft-lb/sec. That is 0.000029 hp. That seems incredibly small so the vehicle would move very slowly. It seems that their needs to be much more sail area to produce a reasonable amount of horsepower.
