Some notes on hovercraft

Reprinted from T TRAI4SACFIONS OF TEE (INco1u'Oi&Tht), _{Abbey House, Westminster, S}

141

Lab1

v1, Schepsou&.:J

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'6kgeschooJ

Deift

Monday October 3 1960.

E. G. MASSY COLLIER, President, _{in the Chair.}

"SOME NOTES ON HOVERCRAFT"

By W. A. CRAGO, B.Sc., A.F.R.AE.S, A.M.R.I.N.A.,

A.M.I.N.E.C.INST

1. _{THE FIELD OF TRANSPORTATION}

It is an all too familiar _{experience for the engineer to become so}

I

immersed in his own particular technical problems that he _{"cannot see} the woOd for the trees." _{Often it is evident that the}

technical specialist

in any given field has _{become so concerned with}

details that. it requires someone wth perhaps little or no interest in these details to appreciate the true overall nature of the_{problem and perhaps provide guidanëe.}

Today, nearly everyone is something of a specialist in the field of transportation, and, as a result, we are all in danger of not being able to see the wood for the trees.

Therefore, before dealing specifically with

the Hovercraft, it is well _{wOrth while to review the field of transport}

as a .vho1e.

To facilitate this review it is desirable to present salient_{characteristics} of as wide a range of forms of transport as possible_{on one sheet of paper} and in a readily assimilatable form.

, Gabrielli and von Kármán attempted

this first, in a Paper called

_{"What Price Speed"}

published in 1950,

and others have made _{use of comparable methods since.}

Recently a colleague of the Author attempted a similar exercise (ref. .1) and made use of two parameters to describe the _{major characteristics of}

vehicles,

which appear to be much more realistic and useful than any published

previously. _{The first of these parameters is,: -.}

Vehicle efficiencylb. of fuel per payload ton mile.

This parameter views the _{problem largely from a passenger or}

customer point of view and tries to tell us how much it will _{cost in terms}

of fuel to carry one ton of _{payload over one mile.'}

It does not take

into account the various _{costs of fuels (which in}

any case fluctuate from

time to time) or the type of

_{payload that has to be}

carried, and, in

particular, it does, not specifically _{distinguish between}

a slow and a fast transit time. 'However, it is obviously much more useful than _similaz parameters that employ the all up weight rather than payload.

The second parameter is more suited to the operator's_{point of view}

and is:

-Earning Capacity_{= Payload ton mile per year / capital cost.} Again this parameter has its _{defects, b't 'ahy attempt}

at

improve-ment leads to a considerable

increase in complexity and computation

time. _{In fact both parameters}

are sufficiently precise for our present

Figures 1, 2 and 3 show envelopes of values of these parametersfor

specified groups of vehicles plotted against speed.

Figure 1 shows vehicle efficiency for vehicles operatmg overthe sea,

and includes ships and aircraft. Figure 2 repeats the data of Fig. 1, but

also includes vehicles operating over, land. There appears to be a limit to efficiency defined by the heaVy dashed line which has been reproduced

in both the abOve flgiires There are no practical vehicles to the right of the line and it is not reasonable at the moment nor in the foreseeable future to expect the line to be crossed.

100 0.ot MAN SW IM?41N C

'I,

Id

if

U

z

0

- IL Ii. La

I

E

I'

:' \

_{\ 0TH0}

A1-'A 142 kELICOPTER5. Y SU PERS0N A/C I 10 100 1,000 - I 0,000

MILES PER HOUR

Fiouan 1. Vehicle efficiency for vhic1es operating overthe sea.

Figure 3 shows the variation of earning capacity with speed. The following conclusions may be drawn from these three figures: -Vehkle Effidency

1. Ships can be very efficient indeed. A bulk cargo ship or tanker,

for example, can be operated in the speed régime at the bottom

skin friction and wave drag is negligible. This results in a very

high efficiency, which is likely to remain uncontested in its Own speed range.

Practical requirements and convention seem to have set a limit to ship lengths of about 1,000 feet nd above speed length ratios

of V/ ..JL= 1.0 (V

ii

nots and L in feet) the wave making

resistance rises rapidly These facts have limited ship operation to speeds below 30 knots and they are thus found only on the left-hand side Of the diagram.

I 10 100

MILES PER HOUR

FiOnan 2. Vehicle efficiency for vehicles operating overland and sea.

Athongst ground-based. vehicles, trains _{are most efficient, but for} passenger traiisport. there is not much difference between_{road and}

rail.

Amongst aircraft the conventional passenger aeroplane is best, whilst the helicopter appears badly off. _{(The helicopter can, of} course, carry Out operations that other vehicles cannot, but _the parameter makes no allowance for this)

143

It is interesting to note the position of Man on

the graph. How

much of a man is payload is a nice theological question, but if he is regarded as havin a 100 per cent payload the inevitable

con-çlusion is that walking is to be avoided at all costs!

To the left of the dashed line and therefore presumably within

the reai±ns of possible achievement we have : -(a) Overland

A case for looking for faster road and, rail transport to fill the régime bounded by the letters A.B.C. in Fig. 2.

10,000 q 1,000

I

100 I0 144

Ig lao l0OO IOPCO

MILES PER HOUR

FIcURE 3. Earning capacity for vehicles operating over land and sea.

(b) Over Water

A case fOr looking for faster craft to fill the triangle marked by

the letters D.E.C. in Fig. 1. Since this triangle is larger than

A.B.C. then presumably the case forlooking for faster over-water craft is stronger than that for tile faster road and rail transport.

It is suggested that the hovercraft or the ground effect principle

may be adopted with success in both instances and in fact is the only potentially successful way of filling the gap. Presnt projected 100 ton

hovercraft designed to avoid momentum resistance alreadyappear-. to be

possible at point Y. Earning Capacity

It will be appreciated that hidden subsidies, particularly on passenger

aircraft, can invaEdate this anaiysis to a marked degree. However, the following comments can be made:

Since operators Will all bcrrow capital at roughly the same interest

rates the scatter of data will tend to be less:

The expected earning capacities of projected and future possible hovercraft X and Y in Fig. 3 are reasonable.

Having established that there is practically an uncontested place for the application of the ground effect principle in the transport field and

CATEGORY I CATEGORY II

Non-viscous flow _{Predominantly viscous flow}

1

w

Plenum chambertype FIGURE 4. 145 "Levapad' on a rail

that the associated earning capacity appears reasonable, we can now

proceed to consider the principle in more detail.

2. THE GROUND EFFECT PRINCIPLE

The feature which characterises all "ground effect machines

(GEM's as they are now generally called) is the deliberate generation of a cushion of air by some means between their bottom surfaces and the

ground, whether the "ground" be land or water. This cushion of air has a pressure higher than that of the atmosphere and therefore is

asso-ciated with a lift force which counterbalances most of the GEM's weight.

Thus the GEM in its basic form has no physical contact with the ground although practical and economic limitations on the cushion generation mechanism imply that it will be relatively close to the ground. Since

the basic craft is air borne, its propulsion and control will most likely be aerodynamic.

The precise definition and classification of GEM's have been the subject of some legalistic controversy and as soon as some progress in clarification has been made, a new form, or a cross between a hovercraft

and some other type of vehicle is suggested which immediately invalidates the definition or classification system. Presumably definitions and

classi-fications wifi become quite important in the future when the hovercraft principle has a wider application and one will wish to know just what form of licence to apply for!

In seeking to classify the various types of GEM one criterion which divides possible designs into two groups is whether the generation of

FIot1iB 5. SR.Nl Hovercraft. 14

the cushion is predominantly a viscous flow phenomenon or not. Figure 4 llustrates the two categories.

The SR.N1 hovercraft illustrated in Fig. S falls in the first category, whilst the air bearipg can be considered as an application of the ground effect principle falling in the second category.

- Another method of categorisation is to consider whether or not the

craft can operate at least to some degree over rough or unprepared

ground. In practice this provides the same division of principles given

by the previous suggestion.

In general the first category corresponds to an application over

water whilst the second category has a more obvious potential in con-nection with faster road and rail transport, where the air cushion acts as a low frictiOn lubricant.

This Paper is concerned with vehicles in the first category and in particular with ' Hovercraft" which have been called a "momentum curtain" type of GEM.

3. HOVERCRAFT PERFORMANCE

It is not the purpose of this Paper t present a precise mathematical

analysis of the mechanics of hovèrcraft performance. Therefore the

Author has taken the

liberty of employing approximate theory to derive simple basic formulae which illustrate the major performance

characteristics.

It is logical to consider first how the relatively small thrust from the

peripheral nozle is augmented to carry the total all-up weight under

conditions of zero forward speed, and how this augmentation is dependent on hoverheight. Next we will consider the resistances to forward motion

and their relative irnportanàes. We may then go on to deal with the

power required both for hovering and also to obtain forward motion. Subsequent sections of the Paper will very briefly deal with the stability and sea kindliness of a hovercraft.

3.1. Static Lift

Consider an elementary hovercraft consisting of a single peripheral jet supplied from a .plenun chamber and an intake fan (Fig. 6).

)

Frouiz 6 147

Let t be the jet width,

V the mean jet velocity, its angle tO the horizontal, h the hoverheight,

p the cushion pressure, the craft weight,

S the cushion pressure area

and 1 its periphery.

The rate of change of momenti.im in the jet in a horizontal p1ae may

be equated to the cushion pressure acting over the periphery of the cushion so that :

-pV2lt (1 + cos 1

The craft weight is approximately the integral of the cushion pressure over its bottom.

That is:

-2

COmbining 1 and 2 and eliminating p we have

L_-pV2 St (1+cos8)

.3

Ii

Consider now the case where vertical jets only are employed to produce

lift. We have :

-Lj=pV2 It where L is the lift genrated by the jets

Dividing 3 by 4 we have

/L=(1+cos9)

(1+cos 0)

or Z/L1=

_{4 (h/D)}

where D is the effective diameter of the cushion pressure area.

This equation is an approxima.tion, but it clearly illustrates certain important points. The first is that the term z/L1 (Which is called the augmentation) will, easily exceed unity if S is large and h is small, as is

the case with the hovercraft. With a helicopter or vertical take off

machine the augmentation /L must obviously be less than unity and thus for equal loads the hovercraft will require a muchsmaller lift force from the jets than a helicopter or vertical take offcraft.

Another feature revealed by equation 5 is that once a value of

hover-height is chosen to clear obstacles or waves then the larger the size. of the craft (S or- D) the larger becomes the augmentation. Also the best value of augmentation for. a given hoverheight is realised when SI I is largest, which is obviously achieved with a ci±cularplanform.

In the case where the hovercraft operates over water, the water

surf.ce becomes distorted because of the cishion pressure and becausç of the impact of the jets. The jets themselves also produce spray. These

facts, together with others we need not consider here, tend to reduce the accuracy of the reasoning given above.

148

4

'1

An interesting demonstration of the hovercraft principle that can be repeated under the kitchen tap is illustrated in Fig. 7. The paper cone

has been varnished to render it waterproof and is held by a vertical

tube with a hole drilled in its side near the top. If water is allowed to flow

over the cone it forms a near vertical curtain as shown in the left hand

photograph. If air is now blown into the space beneath the cone the curtain direction is changed, as shown in the second photograph, and the configuration is stable until the extra pressure beneath the cone is

Ficui 7.

allowed to escape. _{The crude water curtains are thus able to maintain}

a pressure under the cone higher than atmospheric.

3.2. Forward Speed

The effect of forward speed is to modify the lift generated and to produce a resistance to motion.

The forward speed achieved is determined by _{a balance of forces on}

the craft as shown in Fig. 8. _{These forces are as follows:}

-The Thrusts

T0 the longitudinal component of the integrated _{cushion pressure.} T the propulsion thrust derived from airscrew _{or other convenient}

source.

The Resista'ices

Rm the momentum resistance associated vith theloss- in momentum of the free stream entering the fan intake and being ejected all

around the peiiphery of the craft. Since- a hovercraft has. a

large mass flow requirement this term will be relatively important R the effective aerodyna.mic profile resistance.

Thus we. have :

-To+TpRM+Rp

6

T

T

JIouR

8.-The water resistance does not. appear specifically in this equation and

to obtain this we have to consider the equilibrium of the cushion itself

when over water as illusfr4ed in Fig. 8. The forces ate as follows:

-The Thrusts - .

-TE the cross flow thrust due to air escaping from the cushion in

an assyipetric manner. The Resistances- . . - -

-R0 the aerodynamic profile resistance of the cushion

T0 already define4. - . - -.

w the wave. m2king resistance.

Thus:

-= R0 + T0 +

Eliminating- T0 between eqiations 6 and 7 we have + = RM + R ± + R

150

R

7

Typical values of the terms in the righthand side of equation 8are shown

in Fig. 9.

It will be noted that the wave making component of the total

resistance has a peak at low speed and then falls away to_{a negligible}

I

I')

MILES PER HOUR

Ficjijpn 9. _{Typical breakdown of the resistance to motion of a Hovercraft.}

D=22ft. i=7,5OOlb. 151

Soc

J I

000

) U I 2 3

4

V/r-I 1 5

7

AERODYNAMIC

4

R+

500

WAv _{RESISTANCE R}

value. The height o the peak is goveriled mainly by the cushion

pressure whilst the speed at which it occurs is determined by the length

of the cushion. In general, the wave making resistance will only be of

significant importance in a craft designed to run at low speed length

ratios of the order of VKI /D=30 or less and, in the Author'sopinion, hovercraft designed to operate in this region appear to have little to offer.

The effective profile resistance varies as speed squared, as would

be expected, and thus becomes increasingly important as speed is

increased. It follows that there is an obvious case- for "streamlining"

the fast hovercraft.

The momentum resistance varies linearly with speed and in the case

considered in Fig. 8 is of major importance from, say, 15 m.p.h, to 60 m.p.h. Its reduction is clearly worth while. Reduction can be

achieved by directing the peripheral jets in an aft

direction or by

attempting to use the same air over and over again in a closedcircuit.

3.3. Power Requirements for Static Lift

The power (FL) required for hovering at zero forward speed may be obtained from the nozzle total pressure, the volume flow and suitable-factors to allow for pressure losses and fan efficiency. We will therefore

write : -= cOnstant V' Ii 9 Equation 3 gives us V3 as h ,3f2 V3=

[pst(1±cos8)J

10

Substitution of the value of V8 from equation 10 in equation 9and some

rearrangement gives :

-PL=constan 1

11

S [i/h (1+cosO)]

It can be shown by means of a more precise theoretical approach that for all simple hovercraft at zero forwar4 speed t /h. (1 + cos 0) reaches

a unique optimum whilst (1/S+) is a simple geometric parameter

deter-mined by the planform of the hovèrcraft. We may therefore simplify

equation 11 to:

FL = constant x h

A D'sJS

and z/S may be taken as the cushion pressure.

It may be remarked at this point that whilst the method of deriving

equation 12 is open to criticism the equation itself is substantially correct.

Equation 12 indicates that for minimum power the hoverheight should

be kept as small as possible commensurate with clearing waves or

obstacles and the structure weight should be kept to aminimum. 3.4. Power Requirements at Forward Speed

When forward speed (V) is introduced, the cushionlift is reinforced to a small degree by aerodynamic lift generated by the hovercraft itself,

152

and the effect of ram pressure and the _{pressure field around the craft}

will have their own repercussions_{on the power required for lift.}

The power required for propulsion can be computed from resistance

curves similar to those shown in Fig. 8. _{The power to overcome}

momen-thin resistance varies as V, whilst that to overcome the aerodynamic

resistance varies as V3. _{Water resistance is uSually negligible except at}

low speeds, but may _{cause a local hump in the curve which can be}

inconvenient.

1000

SHP/t 100 H.P. Ton-' 10

000I

hID

FIGURE 10. _{Typical variation of power per ton with hoverheight.}

Figure 10 shows how the total power per ton varies with hID fo! a particular hovercraft having various values of _{/S. In this case the}

curve for zero forward speed and also for 80 knots

_{may be taken as}

roughly the same. _{For comparison it is interesting}

_{to note that the}

following values of

_{power per ton are achieved by other means of}

transport:

Hypothetical 50 knOt planing craft 80 ft. long 86 HP / ton

"Queen Mary" at 30 knots

... ... 2 HP/ton

Typical aircraft at 350 m.p.h. ... ... ... 200 HP/ton 153

Motor car t 80 rn.p.h. ... . ... 35 HP/ton

Man, at 4 m.p.h...

..

.10 HP/ton

if we use anhID of 0.08, and a /S of 30 lb.. ft.

.webve:-Hovercràft at 80 knots ... ... 95 HP/ton .3.5. Stability

Clearly a hovercra.ft must be statically stable in pitch and roll and

there is no apparent reason why a simple system 'such as is shown

in Fig. 6 should be other than neutrafly stable at best. In fact, there

are a number of secondary effectSwhich do in fact provide three

diñien-saonal simple hovercraft with a measure of stabthty for h /D values less

than about 004. Above this value special steps have to be taken to

.154

10

Priod of encounter _._ Increasing

Effective naturtl'period = Tuning factor

provide stability and the simplest is associated with _{compartmentation}

of the cushion either by solid walls or by jets.

In this case the jet

with the lowest ground clearance will produce the _{highest cushion}

pressure (as shown by equation 1). and hence the lowest compartment will have the highest lift. _{Such a system is obviously self sfabilising.}

3.6. Sea Kindliness

The Author's own physiological experiences in motor torpedo boats and passenger hydrofoil craft in bad sea cnditio.ns have convinced him of the. need for good performance _{over waves and it is of interest to} consider the factors affecting this feature.

There is no basic difference between the mechanism of the motion of a simple hovercraft and that of a ship and Fig 11, which shows the respouse diagram for a ship could also apply equally well for a

hover-craft. _{Hovercraft speeds are of course higher than those of a ship and}

the tuning factor defined in Fig. 11 is therefore likely

_{to be much}

lower and the resultant responses relatively small.

One feature of the hoveicraft performance which _{is particularly} important in waves is its "stiffness" and damping _{both in heave and}

pitch. A hovercraft with inadequate stiffness tends

_{to "teter about"}

and when running in a seaway eventually gets its bows _{well into a wave.} Stiffness, i.e. good stability, may be provided aerodynamically in the jets and cushions or perhaps hydrodynamically by means of

hydro-foils. _{In fact the marriage of the hovercraft and hydrofoil or planing}

craft is particularly attractive when water propulsion

_{with its high}

efficiency is considered.

One point which is well worth stressing is that there _{cannot be a}

sea-going craft which is absolutely infallible and which is perfectly

satis-factory in all sea states. _{What can be done, however, is to produce a}

FREQUENCY OF OCCURRENCE OF VARIOUS STEEPNESSES FOR WAVES OF A GIVEN HEI

The Hovercraft can negotiate waves of low steepness easily. The Table shows that steep waves are relatively rare.

FREQUENCY OF OCCURRENCE OF WAVES OF SPECIFIED HEIGHTS

(MiDAmnc) FIGURE 12. 155 Wave height

ft.-Percentage of waves steeper than

1:20 1:30 1:40 _{1:50 1:60.} 3 1 13. 32 57 70 6 3 24 59 80 90 9 8 46 76' 89 96 12 9 61 83 93 99 0-6 ft. 6-12 'ft. _{12-20 ft.} 68% _20%' 10%

craft which extends the boundaries of acceptable operation and makes

bad motion statistically more unlikely.

In the case of a hovercraft, the waves over which it is to pass decide the value of hoverheight and hence the power and economic characteris-tics of the craft. Some statistics dealing with waves are shown in Fig. 12.

It should be borne in mind, of. course, that although it is not im-reasonable to design a hovercraft to operate in "average " wave

con-ditions for its operational area, there will always be an occasion when a really steep wave will be encoüntçred however statistically unlikely it

is.

It is desirable so to design the craft that when

this happens

although it may be uncomfortable or even suffer structural

damageit

will not dive into the wave. In other words there should be no obvious

hydrodynamic limitations to the craft's operation in a rough sea.

A feature of hovercraft performance in waves .which has caused some discussion is the response sustained should the craft be forced to

,flôat on the water. In fact, tests have indicated that flat-bottomed hovercraft perform far better under these conditions than might have

been expected,

4. REFINEMENTS TO THE SIMPLE HOVERCRAFT

So far we have, considered fairly basic hovercraft systems. The

reader will not be suiprised. to learn that there are many refinements that can be made, and "tricks" that can be played in order to improve

efficiency or sealdndliness, etc. Many of these refinements are currently

the subject of intensive research and must remain shrouded in secrecy for obvious reasons. Some of the simpler refinements are discussed

below.

4.1. Jet Systems

4.1 1. Total Loss System

For comparison Fig. 13 illustrates the simplest hpvercraft system in which the jet is directed inwards and subsequently turned outwards

all around the craft periphery by the cushion pressure. The jet stream

is cOmpletely lost and hence the momentum resistance is high.

FIGURE 13. Simple total loss system.

4.1.2. Total Loss Systm zoith Reduced Momentium Resistance

In this case deflectors are put in the jet to direct the jet air aft to

produce thrust (or perhaps, more correctly, to reduce the momentum resistance). Figure 14 illustrates the prindiple.

AT

SEc-rso.j T&JE-r

Fiou 14. Total loss system with reduced momentum resistance.

4,1.3. Recirculation System

With this design the momentum resistance can theoreticaliy be

reduced to zero by using the air over and over again. Figure 15 shows. the system This is a particularly attractive idea especially_{if the fans} can be located between the exit and recoveiy ducts _{so that the duct}

losses are minimised. Also the spray will obviously be reduced bythis configuration.

- FIGUBE 15. Rècirulation system.

4.1.4: Articulated Jet System

By articulatmg the jets (not necessarily only in the manner shown in Fig 16) certain important advantages accrue _{First the power} re-quirements will be low since the value of h/Din equation 12 will be

reduced. Second,

he articulated jet will "give"

_{to the crests of}

waves and the resultant craft motion in a seaway will be _smoother. Of course it will be appreciated that these four examples by no means exhaust thelist of possible jet systems.

Each of the configOiations considered so far require horsepower to

punp the air and consideration of this fact leads tothe question as to

*het1er the jet curtain could not be partially replaced by a system

that does not require this horsepower e±penditure.

One solution is the introduction of sidewalls.

FIGURE 16. Articulated jet system.

4.2., Sidewall Systems

4.2.1. Simple Sialez.all Configuration

Figure_17 illustrates the system. Clearly sidewails that project mto the water must he parallel to the craft s direction of motion and any one of the jet systems previously mentioned may be employed at the fore and aft ends.

This system is attractive, but the following potential defects must

be borne in mind:

The potentially large hydrodynamic resistance penalty of the

side-walls at high speèd particularly in waves due to water skin

friction.

The existence of directional, roll and pitch stability difficulties.

4.2.3. jetted Sidewalls

-This is an obvious development of the single sidewall system in

which the sidewalls themselves are fitted with jets. The clearance

be-tween the jets and the water surface can be kept low without the

danger of heavy wave impacts. Another possibility is to use jets of

air, not to maintain the cushion but to interpose a thin film of air betveén the sidewalls and the water and thus to reduce the sidewall

skin frictiOn.

5. FU1URE DEVELOPMENT

The possible exploitation of any new transport system- falls into

either or both. of two groups, uiz.:

View on 'A'-'A'

159

Fxoug 17. Sidewall HOvercraft

COnventional Transportation

Transportation in special environments.

In the first group any new development in transportation may either rapidly supersede already ex]stmg methods, it may for a long while bein

competition with existing methods or it may prove not able to compete at all. Present projected hvercraft will certainly not rapidly supersede

existing methods of transport, but over water they do appear to be

competitive with certain régunes of conventional transportation _over relatively short distances and i reasonably sheltered conditions This

fact is tremendously significant m the Author s view smce the hovercraft

has been tinder development for only two years. To have achieved

anything like parity with any branch of contemporary transportation

; such a short time suggests that the potential achievements of hovercraft

are considerable. It is significant in this respect that the hovercraft is

competing against well developed systems where a very considerable effort is required to increase efficiency even a small amount.

In this first group it is unlikely that a hovercraft designed to compete

seriously with existing transportation will be less than 100 tons and per-haps more than all other forms of transport, the hovercraft offers rapid

increases of efficiency with size. This implies that very large hovercraft may be built one day. In the immediate future, however, intermediate vehicles are essential as stepping stones to larger craft. This explains the design and cOnstruction of such craft as the Saunders-Roe SR.N2 illustrated in Fig. 18, which will have an all-up weight of 35 tons and

a payload between 12 and 15 tons. The installed horsepower is only 3,300 H.P. and the cabin space is large enough to carry 59 passengers or 110 standing troops at a cruising speed of 70 knots and ahoverheight of one foot. A maximum hoverheight of 25 feet will be possible at light

load. Again, in the immediate future, research work may be expected

to continue, aimed particularly at increasing the hoverheight without

having to suffer a large increase in power requirements. To this end

the use of recirculation, articulated jets and sidewalls will undoubtedly be tried. With increase in hoverheight will come the ability to operate

in rougher water and this will allow the hovercraft to compete with a wider range of orthodox transport systems.

The hovercraft is attractive from the cargo or passenger handling

point Of view. Assuming that it is aerodynamically propelled and has

no projections underneath, there is no reason why

it cannot run up

a beach or ramp at the end of its over-water travel and be unloaded on land with rudimentary facilities. With reasonable values of hoverheight

to craft length ratio the requirement of being able to run up a slope

merely involves the provision of sufficient thrust to overcome the craft's weight times the sine of the angle of the slope.

The facility of land unloading has particular attraction for military purposes and commercially the construction and maintenance of deep-water port facilities would be rendered unnecessary and offset only by the necessity of clearing a rough slope.

For certain limited operationsgiven multi-engine installations,

hovercraft could eventually do without any buoyancy chambers such

as are now considered necessary and the structure weight could be reduced commensurately. The argument for this would be substantially the same

as that which has led to the almost exclusive use of land based aircraft

to fly over water.

As with all types of transportation it is interesting to consider the

effect on structure weight caused by the "square cubed" law. It has

been suggested that this law really explains the end of the development of wooden sailing ship transportation and that the introduction of iron

hulls and engine propulsion was a providential revelation which just

saved the situation. An informed but crude guess as to how large hover-craft might become, before structure weight becomes embarrassingly

pre-dominant has been made and suggests somewhere., between 800 and

1,000 tons. However, one is also conscious that not so long ago it was 160

Photograph of a model of the SR.N2.

Fiourta 18.

161

confidently and expertly predicted that the speed of aircraft could never

reach 100 mp.h.

In the second group, where special environments are considered,

the hovercraft has the undoubted advantage that it can operate with

equal facility over land and water or combinations of both, and when operating over land it does not require an elaborately prepared roadway.

There is therefore a clear requirement for hoyercráft to be cmpioyed

where conventional forms of transport are virtually inoperable. The

development of the Sahara, the Arctic and various parts of Africa, Asia, South America and Northern Canada may be helped tremendously by the employment of hovercraft designed tO be particularly rugged and simple to operate, possibly at some expense in design sophistication and

resultant efficiency. This lack of efficiency would be more than offset

by operational flexibility. One British company is already preparing to market a series of" Hovertrucks," whilst another has already "flown"

a cralt specifically designed for the carriage of bananas from plantations

in the Southern CamerOons. Vehicles in this group also have a military application which is too ol)vious to need cpmment.

It will be appreciated that, regardless of which group of hovercraft is consideted, an essential prerequisite to successful design, construction and operation is practical know how, and in this respect the more

hover-raft that are actually built the better. A film wi]l now .be shown to

illustrate the story of the SR.N1 hovercralt.

REFERENCE

1. An unpublished report by. Mr. D. I. Hardy, Saunders-Roe Div. Westland Aircraft Ltd.

APPENDIX

This appendix present a list of GEM's of vrious kinds.. In the literature on GEM's it is sometimes difficult to distinguish between

actual working craft and projected machines not actually built, but as

far as possible all those in the list have actually flown and carried a

man, or else are serious projects which should materiaiise in terms of

"hardware" in the near future.

The list is not exhaustive.

Some of the machines listed

are "backyard" types built by

enthusiasts, but at this

early stage in the history of GEM's their

inclusion is deemed to be right.

DISCUSSION

The President said he was perfectly certain that everyone would

agree that the Paper they had heard and the film they had seen had been of extreme interest.

He had two questions to ask Mr. Crago. He had noticed that in the artist's drawing of the model of. the proposed new hovercraft, it

appeared that this was not circular; ifthat were so, it rather seemed

to cut across Mr. Crago's original explanation as to the most efficient

shape, from a weight-lifting point of view. There might be other more

important considerations.

The second point was, in the hovercraft of the same design, where it had a hoverheight of one foot and a maximum Of two-and-a-half feet,

when crossing over water, with wave action; from where was that

measured? Was it from the crest or from the trough, or a mean? He appreciated that for very small wave action, it probably made very

little difference, butwhat was the effect on the bovercraft if it encountered

fairly heavy seas but with comparatively short distances between the

waves themselves?

Mr. Crago, in reply, said that the President was quite right, and

the equations did lead one to expect that maximum augmentation was to be obtained from a circular planform.. but maximum augmentation was nOt everything. It would be readily appreciated that a circular

pla.nform craft could have a high a.erOdynamic drag, and it did not necessarily lend itself to good performance over a seaway so that a

compromise had to be reached. In the case of the SR.N2, this com-promise resulted in an Ogival planform so that the craft was pointed

at both ends.

With regard to the point about running over waves, under these

circumstances the average hoverheight was higher (because of the wave

motion) than it would be in calm water. It had been found, as a general iule, in short crested seas, a hoverheight of one foot would give satis-factory characteristics in waves of the order of two feet high. Whether

that was going to be so when larger craft were constructed remained to

be seen and cOuld only be proven practically by going ahead and building them.

Mr. E. S. Waddington first congratulated the Author for one of the most interesting Papers he had heard for a number of years.

Secondly, he wanted to ask a few. very elementary questions, because,

in that type 'of engineering, his experience and knowledge was nothing.

He hoped the Author would excuse the questions if, they were a bit

obvious and if be himself had not seen the answers in the Paper.

He asked what would be the effect of vibration? Was there . a

large amount of vibration in the hovercra.ft due to the large fans

running?

Secondly, was it essential to use the propeller type -of fan or could the turbine or drum type of fan be used?

Thirdly, although in the artist's drawing, passenger accommodation was provided, it appeared to be extremely cramped. He was thinking

in terms of a Channel car ferry, where, not only would facilities for

the storage of cars have to be provided,, but also for the receiving .of passengers, and it would probably be desirable to provide some form of

accommodation.

-What would be the effect on the hovercraft if it was going over smooth, or comparatively smooth, sea, and an extremely squally con-dition was struck where the wave height increased from, say, two feet to five or six feet, suddenly?

Mr. Gago, in reply, said that there was no evidence to suggest that vibration was critical in any way with hovercraft. Excessive vibration had not been noted and did not appear to constitute a design hazard.

With regard to the propeller type fan, it was in fact quite efficient at

its design point, but on the new craft currently under construction a

fan system with a considerably higher potential would be employed. With regard to the accommodatipn, he stressed that the new craft being built was still an experiment. It was only 35 tons and was still a stepping-stone. However, the accommodation was more roomy than usually provided in aircraft; and in any case, since the speed was high, a passenger would not have to stay in it for very long.

On larger craft travelling over greater distances, more cubic feet

per passenger would have to be provided, and comments such as Mr. Wadington had made were being borne in mind on the advanced designs which were in the project stage.

With reference to the over-wave performance, it should be noted

that at worst, -the hovercraft could come down on to the waves and

become a surface craft. In this way the SR.Nl had been successfully

operated in waves 8 feet' high. Whilst running on the surface was

to be avoided if possible, it was by no means disastrous if sea conditions were met which were worse than the design case.

Mr. R. Thompson asked if -Mr. Crago could give an indication of what the natural period of vibration would be when the hovercraft was actually in movement across the water surface. It would not act truly as a free body, but was being suspended by a column of air, which

would be assimilated to a spring. How would the natural frequency be

calculated?

Mr. Crago, in reply, said that the computation was a relative easy

one. The equation of motion, for example in heave, was simply made up of the vertical acceleration, the nass of the craft and the vertical force

actingthe latter consisting of the difference between the weight and

the--lift as computed from the equations already given in the Paper.

Mr. G. N. Swayne associated himself with Mr. Wàddington's

remarks and congratulated the Author on putting forward a Paper which was certainly one of the most interesting he had ever heard.

He had also heard the hovercraft on the Thames. It seemed to him that one of the problems not mentiOned by the Author was the

question of noise. He fully realised that the hovercraft was experimental

at the moment. The hovercraft could manceuvre in very small spaces near to seaside towns and it seemed to him that noise would be some-what of an objectiOn to their use near towns.

Mr. Crago, in reply, said that he lived in Cowes and was all too

- well aware of the problem of noise. It was the engine that made the noise;

it was not the hovercraft principle, so it was just a question of silencing

an engine. In the SR.N2, the engine room was at the rear of the craft and the engines were turbines, so there was no reason at all why they cOuld not be silenced -to bring the noise level down very much more

than in the S.R.N1.

Mr. Swayne said that the one which

_{came up the Thes.had a}

jet engine for forward movement. A jet engine _{was difficult to silence.} Mr. Crago said that the SR.N1 was anexperimental machine built to prove a principle. _{Subsequently it was found desirable to increase} the speed potential above that originally considered. A jet engine had

been simply mounted on its after deck as the most simple way of achieving

the end of higher speed. The criticism that the engine was noisy was

accepted.

Squadron-Leader R. W. White said that he also would like to

compliment the Author on his Paper, but wondered if Great_{Britain. still}

led in this field of research. _{Had Mr. Crago any knowledge of the}

advances made by other countries?

There were very large air intakes in this craft and Mr. Crago had mentioned the suitability of the hovercraft for work in the Sahara. He

seemed to recall the trouble experienced at the Farnborough _{Air Show} when loose grass was sucked in and wondered_{what had been done to}

overcome this problem.

Mr. Crago replied that, as far _{as international standing was} con-cerned, Great Britain was very definitely in the lead at the moment.

Howeer, the Americans were pouring a tremendous amount of money into research and unless we did the same, the inevitable conclusion was that they would pass Great Britain when they reaped the reward of their

investment. At the moment Great Britain had the full _{scale know-how}

which the Americans lacked.

He suggested that Mr. White _{was confused between the Vertical}

Take-off Aircraft and the Hovercraft. _{The V.T.O. machine indeed had} to stop its demonstration because of the grass sucked in. In contrast

the hovercraft had sucked in all manner of things and its _performance

remained unaffected.

Mr. L. G. Culleton asked whether, in passing at a slow speed over loose fine sand, which might be encountered in _{the Sahara, or having}

to land for any reason on such .a loose sand surface the _hovercraft might not blow away the resisting surface to such

_{an extent that it}

would dig itself in?

Mr. Crago, in reply, said that that thought had been _considered.

The hovercraft had .been tried over loose sand and it did not dig itself in.

But he understood that there were conditions in the Sahara where the sand was very loosely packed and wheeled _{vehicles could almost sink.} What would happen to the hovercraft in similar conditions he did not

know. _{Over normal sand, it certainly did not dig itself in.}

Mr. J. B. Martin, following up the point of

the hovercraft's

versatility on land, asked whether it would land in such a position so that it came to rest spanning a ditch or wide opening with, say, six feet of air beneath most of the ôraft? _{This being so, would the hovercraft} be able to take off again?

Mr. Crago, in reply, said that there was always a condition with

any machine when practical operatipn had to be curtailed.

However, the opinion was that, in general, Mr. Martin's suggestion

did not represent a very realistic condition because, unless the

hover-craft was deliberately driven into it, it would generally be passed over fairly quickly.

Mr. MartIn said he had been thinking of army vehicles.

Mr. Crago said the hovercra.ft had been used for landing exercises

in connection with the army up pretty steep and rough beaches. So far it had behaved remarkably well.

But it had never got into the

position suggested.

Mr. M. Langley said that particulars so far supplied indicated that there were two separate systemsin the vehicle, one a lifting principle, which was the hover principle, and the other the propulsion principle, which provided the forward speed. This reminded him of the early

days of rotary wing aircraft where the hover principle was supplied by

the overhead rotor and the propi4sion principle was a rotating pro-peller at the front end. He was thinking of the Cierva autogiro.

Had the possibility been considered of combining the two, as was now done in the straightforward helicopter, by tilting

the rotor, or by

providing flaps or vanes at the outlet of the lifting principle?

Mr. Crago, in reply, said that the thrust component of the cushion

pressure did not push the hovercraft along fast enough. It was often

contended that with the helicopter the forward component of the rotor lift also did not provide enough thrust.

In order to go faster with helicopters, hybrid machines were proposed

and built which employed auxiliary propulsion, and this waswhat had

been done with the SR.Nl. A separate propulsion system was also

employed with the SR.N2.

Mr. J. C. D. Cotes said that thedescripion and film of the hovercraft

had described the SR.N1 withthe main engine only installed.. He

under-stood that subsequently a small jet engine had been fitted tothis vehicle to give additional forward speed.also it was noted that the wodel of

the SR.N2 was fitted with propellers for the same reason. He asked

whether the information given on hovercraft power, economics and effi-ciency compared with other forms of transport applied to vehicles with

or without the additional devices to improve forward speed.

The hovercraft was said to be successful so far in relatively calm

conditions: was it considered that, in time, the hovercraft would be

able to operate in more inclement conditions?

Mr. Crago replied that the graphs he had given definitelyincluded total power; otherwise they would be unrepresentative.

-It was considered that

hovercraft could be designed

forand

operated over specified routes and certainly deal satisfactorily with the worst weather conditions that could occur on these routes. It was, of course, not considered that hovercraft could operate, in mid-Atlantic

166

gales and the mid-Atlantic1 in any case; 'was not considered a suitable

régiin _{in which to operate such craft at the moment.}

The President proposed _{a heaiiy vote Of thanks to the Author,} which was carried by acclamation, and the proceedings thenterminated.

APPENDIX I

Name Designer- DateofFirst

Flight O.A.L. Beam Weight Power Hoverheight Speed

Type Photograph Available

Ilen Karl Weiland

(Swiss) 13 2 60

-

-10 000 lb 630 H P 18cm 43 5 mph Labyrinth seal Flight 26260

SR Nl

(Hovercraft) C S Cockerell &Saunders Roe Ltd 11 6 59 30 0"

24 0" 7 500 lb 435 H P 15 in 45 Knots

(cruising)

Hovercraft Photographic

Aircar

Model 2500 Curtiss Wright

-28 0" 8 0 2 800 lb 300 H P 10 in 60 m p h Plenum chamber with

flexible skirts

Awatzon Week 21 9 5 Aeroplane 1112 Cushioncraft Britten Norman

Ltd

660

-18 -18 10 18 10 2 240 lb (empty)

170 B H P 12-15 in

(estimate) (estii ate)40mph

Annular jet Aeroplane 246 0

I Air Car Crest Bed) Curtiss Wright .

-

21 0 8 0 1 000 lb (empty) 85 H P

-

-Plenum chamber with

flexible skirt

Aviation Week 6 7 59 Flight 16.10.59

GEM 1 (FASS) Nat Res Assoc

College Park, U.S.A.

Late 59 14 7

-

1 000 lb (gross) r 80 B HP 9-15 in . 38-58 m.p.h. Hovercraft Aviation Week 16 11159 P GEM

(Princeton X 3) Dept of Aero EngPrinceton Univ

U.S.A.

18 1059 20 0" 20 0 350 lb

(empty) 3 H P with5 H P for

-Propulsion

12-14 in 20-2.5

m p h

Plenum chamber Aero Space I

Engineering

460

Hydrostreak Hughes Tool Co

U.S.A. 1960 21 0

-

-

engine fOr80 H P cushiOn

-

-

Sidewll Hovercraft with water jets

Aviation Week 27 6 Gyrodyne 55 Gyrodyne Co of U.S.A.. - 10 59 9 1" 6 0 600-800 lb. gross 72 H P

--.

-

Hovercraft Space/Aero-nautics

-.660

Air Scooter X 2 Dept of Aero Eng

Princeton Univ U.S.A.

-

8 0 8 0 750 lb (gross)

...

5 H P . 4-Sm 6 in 9 ntp h 40 m p h :. . Hovercraft Hovercraft Aeroplane 1112160 Aviation Week 7 60 -Aeromobile W Bertelsen Neponset, Ill., U.S.A. 1959 8 5 5 II . 583 lb (gross) 408 lb. (empty) 72 H P I

Levacar Mk I Ford Motor Co

U.S.A. .

458

7 10 4 6 450 lb . -15 H P with H.P. for Propulsion (External to craft) Film of Air . 20 m p h -Levapad Aeroplane 111260

Skimmer David Taylor

Model Basin, U.S.A.

-. 7 0 . 7 0" 300 lb Compressed Air 2 Ib./sec -, (External) 2-4 in

-. Hovercraft Flight 19 5 59 Fletch Aire

GliceMobile C FletcherSparta, N.J.,U.S.A.

-

.

-

-

243 lb 72 H P 4-6 in 26 m ph.

-

Aviation Week 15J2 60

Turtle Wind Hisao Ogiwara

Japan

-

-

-

-220 lb 12 H P

-. ' 19mph .

-

Aviation Week _{21J5 60} I

_HovermobileHClisby,.Adelside,

Australia

---.-7' 0" .7' 0"

-

- '-30H.P..-,- . -.--6in.-

... ---.--

...--.

. . ..- .

--

Spacetronics

-

--

-

-

12 H P 4-5 in

-

Plenum Chamber Aviation Week 1211 59

-Toiro J Kaaerio Finland TOiroJ. KaaiO,-Finland 1935 1949-1950 6 0" 8' 0" - . 8 0" 10' o"

-.

--

-

-

12 knots

'-Ram wing Ramwing

-Flight 196.59

_-Kiddie Kushion K & R Wernicke

(Bell Engineers) 1959 5 0" 3 4" 67 lb (gross) 1 2 H P 1 in Walking Speed Plenum Chamber (cardboar..d inaskingtape) Aeroplane 111259 DennyBrothers

-,.

-

-c-

-

---

-

Hovercraft

-

Vickers Armstrongs

-

-

-

25 ton

(Payload)

-

-

-Hovercraft Flight 2 8 60

:.

z

I-o O Z SR.N2 Saunders-ROe Ltd. .

-

62' 6" 29' 6" 35 tons (A.U.W.) 3,100 H.P. . 15 in.

-. Hovercraft

-,

o---..

-

Spacetronics .

-. 50 0 .... 32 0" I I 15000 lb (empty) 55000lb (gross) 1 350 H P

--

100 m ph Plenum Chamber -

-

-.

---.27-1780 Curtiss-Wright.;

---

87' 0" 36' - 0" I I I. 14,000 lb. (Payload) .

--

.-.

---_.---.

-

..---

-..

30 rn.p.h. (land) 28m.p.h. (Water) Sidewall Hovercraft

..

...

Space Aeronautics

-.6.60

GEM

(Hovertruck) FollandAircraft

-.

(Payload).5on

. ..

-. Vickers Armstrongs .

-

-

-

4 ton (Payload)

-

.

-

-

Hovercraft . Flight 6 8 60

-

Spacetronics

-

30 0 24 0

-

270 H P

-

100 m p h Plenum Chamber

-Pegasus El . National Research Association, College Park, U.S.A.

-

--

-

-170 H P 4 ft 0 in

-

-

-Pegasus E National Research Association, College Park, U.S.A.

-

14 7 8 0 .

-

130 H P 3 ft 0 in . -

-

-.

-. A.S.W.Killer NationalResearch AssociatiOn, College Park, U.S.A.

-

25' 0" . 50' 0" .

-

-

-

-A S W Killer National Research

Association, College

Park, USA

-.

-

-

1 600 lb 80 H P 20 in

-

Plenum Chamber

-Boll Air Scooter

-Bell Helicopter CorporatiOn, U.S.A. - -: -An elec-trically powered prototype has flown 7' -1" 4' 5" 160 lb. (empty) . 12 H.P. . . 25 in. , 25 m.p.h. -Plenum Chamber -Space/Aeronautics

-.6.60