HUMAN-POWERED AIRCRAFT

An attempt to increase their versatility and usefulness

Not another push-pull pedal system?”

This is a schematic presentation of an idea for a pedal-powered propulsion system that has shown itself to be capable of producing up to 1500 rpm, before the addition of the propeller(s) and drive system.

One of the most common means of applying powerful, continuous force is the gear rack, as used on canal lock gates, for example. The essence of the present design turns this principle around so that two gear racks apply force tangentially, via one-way ratchets (freewheels), to the driving gear of a 9:1 gearing-up system. The leverage of greatly-extended pedal levers overcomes the increased resistance. The force-over-distance objection that obviates the use of such long levers in bicycles, for example, is less relevant to a pedal-driven propeller system, because propulsion is achieved indirectly, by the reaction force between the angled propeller blades and the air, rather than directly between wheels and the ground.

My ambition is to make a contribution towards the construction of HPA craft that will enable leisure-flying by pilots of average fitness and weight, even in conditions of light to moderate winds. Without this capability, HPA craft have little potential outside their current niche. The astounding achievement of the Daedalus flight from Crete to Santorini had taken place 33 years previously, at the time of writing, and no equivalent feat has been accomplished since. HPA craft require a very high level of human fitness, and squall-free conditions. Fragile construction, low airspeed and extremely wide, high aspect ratio wings aggravate the difficulty of maintaining stability and control. The difference in airspeed between the wing tip on the inside of a turn and that on the outside is considerable, and exists very close to the ground-effect.  Current designs demonstrate a very high level of skill and exist because their creators had the determination, knowledge and ability to succeed.

Whilst I accept the potential for failure in this present idea, I would mention that significant advances in aviation are always precipitated by more effective methods of propulsion. 

More versatile machines could lead to the creation of an Olympic event. Unfortunately, as things are, it is difficult even to guarantee a successful flight at the appointed date and time, unless a suitable indoor arena could be provided.

Reducing weight and drag continue to be important but the present proposal places more emphasis on increased thrust and manageability. 

A biplane or triplane with slightly swept wings and pusher-propeller/propellers is proposed. The multi-wing layout reduces the wingspan, the intention being that the increased drag be overcome by increased thrust. Such layouts also combine strength and low weight.

The principle

The rotary pedal system conventionally used has many advantages that account for its continued use. Despite challenges from other designs, it has consistently shown itself to be the best method of transferring reciprocating motion to rotary motion. There is a cycle of effort and rest with each crank rotation and the mass of the pedals and legs possesses angular momentum. 

Disadvantages are: 

  1. power is produced in a series of spikes because pedal force reduces as the levers pass the horizontal position. Attempts to achieve constant power output with rotary pedals require the leg in the ‘rest’ period of each rotation to exert a lifting force. Many cyclists do not practice so-called ‘circular’ cycling, using toe clips and pull-ups, because there is no benefit overall. 
  2. a pilot’s seated position fails to take advantage of the fact that human leg muscles, in the standing position, are capable of producing over 700W, when ascending stairs briskly, for example. A standing pilot will not increase frontal area beyond that already necessitated by the layout of some existing designs.

The pilot could be seated once cruise altitude has been attained.

Objective

The objective is to increase thrust with a system that requires slower, smoother, more powerful pedal operation, thereby also extending the pilot’s endurance. Practical assessment of ergonomics using a full-size, stationary test bed will be the next stage of development. 

When pedalling, human legs of average length can comfortably travel through a maximum vertical distance of around 2 feet. Accommodating this limitation and using 40 inch long pedal levers with a 10 inch gear rack appears to be the optimum layout. Legs do not have to move the full distance to achieve high thrust. In the Lego® schematic, below, a familiar application of 24 and 8 teeth gears achieves a final drive ratio of 9:1. The addition of one more 24 x 8 gear set increases this ratio to 27:1. Simplifying the design by removing one gear set gives a final drive ratio of 3:1. The usual maximum ratio attained with bicycles is 5:1. 

A conventional Chainwheel/sprocket arrangement would require a 72 teeth chain wheel with an 8 teeth sprocket to attain a 9:1 ratio. In the present arrangement, the heavy load on the teeth, at the point of contact, caused by the high ratio, is ‘shared’. The pressure points on all teeth will also be wider.   

The length of the 10 inch gear rack divided by Pi = 3.18 inches. Using 3 inch diameter, compound spur gears providing, say, a 9:1 final drive ratio, and the full downwards stroke of both pedals in the present system occurring @ two per second, 2 x 60 seconds x 9 = 1080 rpm results. A full turn of the driven gear can not be achieved because the full length of the gear rack can not be utilized but, on the other hand, each pedal can be operated at a greater rate than 1/second. In fact, much higher rpm was achieved see  illustration below: 

 

The pedal levers operate via freewheel, or ‘ratchet’, gears with the result that pedal movements can be overlapped, even to the point of using both pedals simultaneously. Constant power is possible throughout the combined pedal strokes. The freewheels also allow the propeller(s) to ‘windmill’ to reduce drag in momentary power-off phases. The pilot could pull up on the ‘handlebar’ control to increase pedal force when required.

Plastic and carbon-fibre materials

Plastic rack gears, spur gears, bevel gears and other lightweight materials are used. The transference of torque through 90° to the propeller shaft is achieved by plastic bevel gears. See below: “Alternative layout of the gear sets’).

Commercial possibilities

Even when times are hard, people still find money to pursue their passions. Bicycles can cost over £10,000 (*$14,000). Versatile, transportable and easily assembled HPA craft capable of being operated by relatively inexperienced people would have great commercial potential.      

*At the time of writing 

(The essence of this design, that of a gear rack applying force tangentially, with extended pedal levers, was the subject of a successful patent application. The patent, was allowed to lapse in 2006 because the renewal could not be afforded. This prior disclosure would prevent the present idea from being patented unless the GB Patent Office could be persuaded that the present system is new, which is unlikely.)                                       

The Lego® schematic

The basic 3×3 gearset/bevel gear
rpm achieved within the limitations of the model. 1345 rpm was briefly attained. The Lego in the picture is used to hold down the test button as the rig is operated
The test rig

The above results would be accomplished with an acceptable level of effort in a full-size version. Much less vigorous action is needed than is required with rotary pedals. 

The spacing of holes in the Lego bricks in these schematics governed the choice of 24 and 8 teeth gears. This factor would not affect the size of gears and the ratios chosen for a full-size version.

My home-built plastic ratchet gears were unable to cope with attempts to apply the maximum force of the levers. I am fairly confident that 1500 rpm could be achieved prior to the addition of the propeller(s) and drive system. 180 rpm is typical with existing designs.

The relationship between the length of the gear rack and the circumference of the ratchet gears in the above schematic could be overridden to enable longer pedal levers whilst maintaining a manageable layout.

Propellers

Raising the rpm of a given propeller eventually results in thrust levels falling, as propeller blades increasingly stall, therefore propellers would be designed accordingly, in the present system, to achieve a suitable compromise. See below: ‘Variable pitch propellers’

It may be that, despite the urgent need to save weight in HPA applications, heavier propeller blades or the addition of a flywheel might actually be advantageous. The increased angular momentum, equivalent to the ‘rolling gate’ of translational motion, might contribute to the more continuous propeller function sought with this design. The increased torque reaction of the greater propeller mass/faster rotational speed is countered by the pilot until airspeed increases, which happens with powered aircraft, although the ‘corkscrew’ propeller-wash problem is alleviated by the pusher-propeller layout, if canard control surfaces are used. See below: ‘Twin propellers’.

Twin propellers

There is a powerful torque-reaction penalty to pay in the present design which can be eliminated by  using a twin-propeller (counter-rotating) design:

The gear racks engage with the ratchet gears indicated in the above schematic.

Here again, the use of plastic rack, spur and bevel gears, and carbon-fibre shafts and other components, saves weight. A sandwich of polystyrene foam and carbon fibre sheet is suggested for the bulkheads and base of the system, which will require some degree of strength. I have so far failed to find a source of existing plastic ratchet gears.

Adding another propeller, with its related friction, will further increase the starting load but with two, smaller propellers will be used.

Please note:

Each pedal drives both propeller shafts all the time so that a yawing motion is not imparted to the airframe resulting from uneven pedalling or by small differences in applied-power efficiency and friction. 

 Alternative gear set layout (twin propellers layout shown)

Turning the gear sets through 90°, and probably angling them, also, to maintain a comfortable space between the pedals, obviates the use of bevel gears. It will result in a lighter and simpler design and reduce friction. Again, the two propellers are counter-rotating. Gear sets are located between parallel bulkheads to constrain angular distortion of the gear sets resulting from the powerful pedal strokes.

In the above layout the two spur gears connected to the propeller shafts are in contact, so that either or both pedals drive both propellers, as in the previous design. In this respect, the problem of unacceptable starting loads and the optimum length of levers in both ideas needs to be assessed.

The driven gears in contact with the racks are one-way ratchet gears or, if plastic ratchet gears cannot be obtained or constructed, metal freewheel gears will share a common shaft.

Some means of creating the required space between the two propeller shafts to accommodate two side-by-side propellers is needed that would obviate undesirable universal joints. 

A solution, once more using plastic components, is illustrated below (pusher-propellers layout):

The whole drive unit, including propellers, is comparitively short, perhaps only around 5/6 feet long. 

Ergonomics would govern the final layout.

Although the appearance of this mechanism appears impossibly massive for the present application, the use of a carbon-fibre/foam sandwich for the bulkheads, and plastic spur and rack gears, is intended to render the mechanism practicable. Metal brackets and metal plain bearings serving the pivots for the pedal levers might be necessary because of the considerable strain imparted by the pedals as they overcome the very high starting loads resulting from each one driving both propellers and gear sets all of the time. 

This is intended to be an aircraft that is available to the general public, so that adjustable mountings between the drive unit and airframe will be necessary to enable final positioning of the pilots to be made according to their weight and the need to maintain the correct C.G. position. 

Variable pitch propellers

An idea used with the cam wheel in automatic valve timing advance/retard mechanisms in some car engines is worth consideration. The propeller blades are attached to the hub via a very coarsely-pitched helical spline whereby centrifugal force, operating against return springs, as the blades move in and out, twists the blades automatically, without attention from the pilot.

Ducted fans/shrouded propellers

The advantages/disadvantages of enclosed fans/propellers are well known and researched but in the present application, that of a ready-to-assemble leisure craft, operated by amateurs, enhanced safety on the ground makes shrouds more attractive. Shrouds, rather than ducts, have possibilities in applications where the rpm figure is low.

Wing construction

With more compact, lower aspect ratio wings, in a biplane or triplane, consideration might be given to the use of polystyrene foam wings, cut with a hot-wire. The wing is shaped in manageable sections, delineated by the blue dotted lines, below, threaded onto a tubular carbon fibre spar, glued together and covered in laminating film. Such a wing could possess considerable strength, especially if the film is applied span-wise. Wings will be much quicker and cheaper to replace and spare wings can be carried: 

Colour film enables attractive livery designs for commercial craft.

Practical experiment and observations based on adjustable dimensions and locations

Even with the most careful calculations, trial and error still has a part to play. With the above wing construction, practical experiment and observation can easily be carried out, with modular components, involving the trial of wings possessing greater or lesser wing span/area, possibly with the upper and lower wings of a biplane design having different wingspans. It would be possible to produce a triplane layout fairly easily, resulting in an even more compact, manageable aircraft.

As changes are made anywhere in a design, the facility of rapidly adapting its other aspects saves valuable time and effort. (It is because of the difficulty of predicting certain results theoretically that we use wind tunnels.) Trial and error flexibility, of adjustable dimensions, is useful in all areas, especially those connected with the centre of gravity and pilot interaction – seating position, controls, etc.

Wing section

In conclusion, I wonder if the trade-off between lift and drag, with under-cambered aerofoil sections, might need to be re-assessed with the present design. A symmetrical section will reduce drag and increase airspeed. The advantage of increased air velocity over the control surfaces will be felt in increased controllability and predictability and a more rapid response to pilot corrections.

I recognise that the latter proposal involves a considerable departure from methods that have been successfully tried and tested over many years.

Although the volume/mass increases resulting from an increase in the size of an object are cubed, I believe that the effects of the present model would scale conformally. 

The availability of dependable plastic freewheel gears capable of accepting the heavy loads experienced with the present design, reliably and over a long period of time, is being researched at the time of writing.   

I hope the ideas presented in this document prove to be of interest. 

John Morton

8 Elmbridge Way, Sedgley, Dudley DY3 1SH

United Kingdom 

perform@blueyonder.co.uk  07531 207219                                                                                    July 2021                                                                                                                           

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