Human Powered Hovercraft :: Steam Boat Willy

technical.

 

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ASSUMED POWER REQUIREMENTS

Lift Fan
This requirement is constant
17 cu ft/sec at 4 lb/ft^2 at efficiency of 0.61 provided by 0.9 lbft torque at 1200 rpm(20 rps, 125 rads/sec) which is 112 ft lb/sec

Propeller
These are the figures at hump speed , this requirement is less at lower and higher speeds.
13 lb thrust at 11.5 ft/sec at efficiency of 0.70 provided by 8.5 lbft torque at 240 rpm(4 rps 25 rads/sec) which is 214 ft lb/sec
Total Pedal requirement at hump is 326 ft lb/sec at cadence of 60 rpm

 

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PROPELLER

Diameter 2.5 meter = 8.2 ft Two blades Design Operating Conditions :- Forward speed 3.5 m/s = 11.5 ft/sec ( this is the hump speed of the hovercraft ) Thrust 57 Newtons = 12.8 lb Advance ratio 0.111

But also it is required to be efficient at lower speeds including zero forward speed. The pitch is adjustable from 30 degrees finer than nominal to 23 degrees coarser. Tests so far have shown that at the lowest pitch, the hovercraft is driven backwards. No measurements yet, all we know is that it has done all we have asked of it. Shape In this table, each line defines a cross-sectional shape. The columns are :- distance from hub as a proportion of radius, local chord ( width ) of each blade and local angle of section. Look at the line half-way down This line tells us that a section 0.725 of the radius from the centre, the chord is 0.118 of the radius and it is set at an angle of 0.347 radians. Each other line likewise. Note that the first 11 lines have less decimal places. In this region, near the hub, Simon edited the Xrotor output to produce a practical shape. When Chris ran these modified figures back in Xrotor he found that it predicted that the performance would be negligibly affected. In fact in some conditions (of speed and thrust) it would be marginally improved.

The aerofoil section is Eppler 193. Near the root, the section has been thickened. For those using Xrotor, the file header is :- 30 2 1.226000 340.0000 1.779999E-05 1.250000 3.500000 0.1114084 0.0 0.0 0.0 6.280000 1.500000 -0.5000000 1.300000E-02 0.5000000 4.000000E-03 200000.0 -0.4000000

Location

The propeller is at the top of a swivelling pylon. At the initial design stage, we had to consider that a large diameter had the effect of the thrust being appreciably above the resultant drag of the craft, leading to a nose-down pitch. Also that a large prop would be wider than the beam of the hovercraft, which we considered as excessively hazardous. Thus the small diameter compared to what we would really like for efficiency.

Construction

The stub is a tapered carbon-fibre tube 12 inch long each side fixed to the shaft. The spar of each blade, another tapered carbon-fibre tube telescopes into place on this. The core of the shape is blue Styrofoam. The surface is two layers of 1/2 oz (# gsm ) glass cloth. This is a barely tough enough surface, and there has been some local damage from handling. The bearings at root and tips of stub are cold-cast bronze (epoxy resin with bronze powder filler) inner race, and Delrin outer race.

Power Transmission

is by 6mm twisted chain. A (rather heavy) off-the-shelf freewheel unit is incorporated into the final drive shaft. The slack side of the chain has two spring-loaded tensioners, one near the pedals, the other near the shaft. For more details see [Transmission].

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TRANSMISSION

There are three chains. One from the pedals to a layshaft, (The 'Pedal Chain') one from the layshaft to the fan, (The 'Fan Chain') and one from the layshaft to the propeller. (The 'Prop Chain') They are all 6mm pitch chain. (0.236inch pitch. Beware this is not the same as 1/4 inch pitch 6.35mm pitch). Pitch is the LENGTH of each link. (Not the width as in bike terminology.)

Chain Twist

Both the fan chain and the prop chain twist over their length. This has been done many times before. There just has to be enough length between sprockets. On the SBW, when the pilot steers, the pylon swivels, and so the twist in the chain as it goes up the pylon alters. When central, the twist is, of course, 90 degrees. The mounting of chain guides had to be thought out carefully for this. See Guides below.

Development History

Our first chain was H.P.C. Gears brand. This stretched after being used awhile, and suffered as a result of abuse from jumping off teeth as described below. We replaced this with 6mm pitch Renold Chain. Fan Freewheel Added. As initially built, the transmission did just about fulfil its function. The pilot pedalled and this caused the fan to rotate and the propeller to rotate. However, if you stopped pedalling suddenly, the chain came off. To obviate this, we added a freewheel to the fan. Now when you stop pedalling, the fan keeps spinning. The propeller had a freewheel at the outset.

Sprung Tensioners Added

The other problem we encountered was that because of the flexibility of the structure, the chains became very slack on the slack side when there was tension on the drive side of the chain. This caused them to sometimes jump a tooth. We tried various methods of chain tensioning. The system we have now, which is proving satisfactory is to have spring loaded swivelling guide sprockets similar in principle to the small sprockets on the derailleur gear of a bicycle. There is one of these at each end of the prop chain, and one on each of the others. These are mounted at an angle to suit the angle of the chain at that point, as it twists along its length. We take the tension off the springs when the craft is not in use.

Gear Ratio and Cadence

There never has been the facility to "change gear" as one does on a bicycle. We did, of course think of this, but considered it unecessary. The fan always needs to rotate at the same speed, so we saw no point in a gear change in the drive to the fan. The propeller has variable pitch which, from an ergonomic point of view, amounts to more or less the same thing. Low pitch feels like being in low gear, high pitch like high gear. However, some pilots now feel that if a pilot-operated-gear-change could easily be incorporated then it would be worthwhile. Alexi Halkola, for whom the craft was tailored asked for a cadence of 60rpm and the sprockets were selected so that this would be the cadence at which it would hover. We must have done all the calculations right, both for the aerodynamic fan calculations and counting the numbers of teeth, because this was indeed the cadence needed to hover. However, other pilots preferred a higher value for going forward, and the sprocket on the layshaft from the pedal chain was enlarged to increase the cadence to 70rpm.

Sprocket Wear

All the sprockets were initially Delrin, specially ordered from H.P.C.Gears Ltd. Some of these were straight out of their catalogue. Some, such as the larger ones, were specials for them. We find dealings with this firm entirely satisfactory.

After abuse from chain jumping, the three smaller ones that actually carry load, rather then just guiding, were showing wear, and were replaced with stainless steel sprockets from the same firm.

Things Getting Caught

The only problem we now encounter is things getting caught in the chain(s). When operating from a sandy beach, sand in the chain, on one occasion part of a pilot's garment, and once a camera-strap.

Construction

We use standard bicycle pedals, cranks and an off-the-shelf bicycle bottom bracket bearing. The bearing is lashed to the frame with strands of carbon-fibre in epoxy-resin. All the other bearings are 'Swiss' skateboard bearings, 10mm i/d 22mm o/d.

Mounting of Sprockets on Bearings.

We have developed a method of mounting a Delrin sprocket to a skateboard bearing. The sprocket has a 22mm central hole which locates it. Also a ring of small holes around this. Kevlar roving is threaded in and out of these holes to create a key to resin-filler mixture. We have made a mould into which the sprocket ( with its Kevlar), and the bearing are placed. The resin-filler mixture is then trowelled in. When set, we extract from mould and check for eccentricity and fill any bubbles. Occasionally, we have to hack off the resin and restart.

Shafts

10mm steel rod is used where there are the main loads, otherwise aluminium alloy tube. These shafts are lashed to the frame with strands of carbon-fibre in epoxy-resin, as most things are on the SBW.

Guides

Where there is a long length of unsupported chain, there are tubular guides. We found some empty plastic cotton-reels that were the right diameter and had just the appropriate bell-end. Amazingly they function excellently as guides for 6mm pitch chain. In the main the chain does not change direction at these guides, except for two on the pylon which have to accomodate a change in chain direction when the pylon is swivelled for steering.

Tooth Numbers

Pedal Chain 97 teeth to 27 teeth.
    Hence the layshaft rotates at 97/27 times pedal revs
Fan Chain 97 teeth to 23 teeth.
    Hence the fan rotates at 97/23 times 97/27 = 15.2 times pedal revs
Prop Chain 23 teeth to 27 teeth.
    The propeller rotates at 23/27 times 97/27 = 3.06 times pedal revs
Guide-Sprockets at bottom of pylon Taut-Side 35 teeth, Slack-Side 23 teeth
Tensioners mostly 21 or 23 teeth

Would we do it like this again

Probably. But we would incorporate sprung-tensioners and freewheels at the start. If you arrange that the layshaft rotate at exactly twice the pedal speed then you can incorporate a "Bradshaw" spring to alleviate the effects of cylcic variation in torque from pedalling. Shaft drives are lighter, put less load on the structure and are more reliable. But, they need a gearbox for a change in direction, and you can't just replace one sprocket to alter the ratio.

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WEIGHT ANALYSIS

	Weight lb	Mass lb	Weight N	Mass Kg
Hull	56	56	249	25
Enclosed Air	0	15	0	8
Pilot	154	154	685	70
Passenger	56	56	249	25
TOTAL	266	281	1183	128

 

Weight Analysis of Hull from measurements in April 2008.

	lb
Structure	35.0
Cloth components	5.7
Moving Parts	15.3
TOTAL HULL	56.0

 

Weight Analysis of Structure from measurements in April 2008.

One measurment

35 lb

 

Weight Analysis of Cloth components from measurements in April 2008.

	lb
Bag	2.8
Deck	1.4
Fingers	1.5
TOTAL	5.7

 

Weight Analysis of Moving Parts from measurements in April 2008.

	lb
Pedals & Cranks	2.4
Tensioners & Idlers	1.4
Pylon & Prop-Shaft	4.2
Propeller	1.9
Chains	2.5
Fan+Mounting+Dome	2.5
Tiller Rod	0.4
TOTAL	15.3

 

 

 

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