C.P. No. 983

MINISTRY OF TECHNOLOGY

AERONAUTICAL RESEARCH COUNCIL

CURRENT PAPERS

Low-Speed Wind-Tunnel Tests on a l/&h Scale Model of an

Air-Cushion-Vehicle (Britten-Norman Cushioncraft C.C.2)

by W. /. G. Trebble, B.Sc.

LONDON: HER MAJESTY’S STATIONERY OFFICE

1968

PRICE 11s 6d NET

U.D.C. No. 533.6.013.67 : 533.682

C.P. No. 983* December 1966

I,OW-SPEEE 7m-TUXQZ :X?TS OX A l/&h ECALX MODEL OF AN

AIR-CUSHIO,V-VEHICLE (BRITTZX-NORME CJSHIONCRAF!! C.C.2.)

by W.J.G. Trebble, B.Sc.

An investigation has been made into the effects of mainstream speed

cm the performance and stability of a model of an Air-Cushion-Vehicle. At

constant speed of the lifting fan, the lift increases both with reduction

in ground clearance and mcrease m forward speed. The craft has a large

drag whxh is mainly mtake momentum drag. Linnted speed could be available

from tilting the craft bows down, but au@entation fromppulexveunits would

be required for higher speeds.

Large nose-up moments occur at forward speed, reachmg a maximum when

the #in&ream dynamic head is comparable wxth the cushion pressure. At

higher speeds, the front air curtain breaks down with a resultant rearward

movement of the centre of cushion lift thus giving some reduction in the

nose-up moment. Pitch stiffness LS reduced by an increase in either ground

clearance or speed and IS reduced to zero when the front curtain breaks down;

at higher speeds there is some recovery 11~ stablllty. Attempts to improve

pitch control by means of a tallplane or by throttling the front Jet proved

inadequate.

* Replaces R.A. E. Technical Report 66383 - A.R.C. 29280

CONTrnS

1 INTROIxTCZ!ION

2 MODEL DETAILS

3 TEST j2QAlX3

4 RESULTS

4.1 Aerodynanncs of the baszc shape

4-2 Calibration of mass flow rate in model with optimised intake lips

4.3 Static hover

4.4 Intake modification

4.5 Effect of mainstream speed at zero lncldence

h6 Effect cf mncidence

4.7 Pressure distribution under the model

&i3 Directional and lateral stability

b9 Pzqulsmn mctors

&IO Tailplane

I,.11 Effect of sealing front peripheral Jet

5 CONCIXJDING -

Appendix Analysis of lift and drag forces

Table 1 Model details

Symbols

References

Illustrations Figures Detachable abstract cards

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9 10

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$4

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5 1 lXTROIXJC.lWN

The principle of the Air-Cushion-Vehicle (A.C.V.) has no# been well

, established end many types have been built both in this country and abroad.

So far they have only been operated over a speed range where the mainstream

dynsmic head is much less than the cushion pressure. Future developments will

obviously include increases in forward speed and it was felt that the balance

of -pressures at high speed might lead to a collapse of the front air-curtain

hence causing severe stability problems. A progrsnme of low-speed wind-tunnel

research was therefore called for to investigate A.C.V. behaviour at high speed.

The possibility of testing a full-scale craft above a ground-board in the

24 ft wind-tunnel was considered and rejected. This would have avoided the

problems of reproducing, on model scale, the full scale fan characteristics

and the internat duct losses but it would have meant that a craft would have been

unavailable for flight testing for a long period and any detailed modifications

suggested as the work proceeded would not have been easy.

Furthermore, there are great uncertainties in the mainstream speed when

ground effect tests are made on relatively large models in open-jet tunnels.

It was therefore decided to build a scale model of the Britten-Normsn C-C.2 for

testing in a closed tunnel at the R.A.E. Separate models for investigating the

effects of intake and exit flows independently had their attractions as the

complicated fan aero.Qnamics could then be removed but, in view of the urgent

need for results on a complete model to supplement full-scale research, a 1/6th

scale fan-powered model was constructed.

One great disadvantage in testing complete .models at low Reynolds number

is that the internal flow may not be truly representative of that in the full-

scale A.C.V. For this reason it would be imprudent to assume that the results

oan necessarily be scaled up in detail though the general trends should apply.

2 MODEL DETAILS

A 1/6th scale model of the Britten-Nom C.C.2 (Figs.l-4) was msnufactured

from wood structural members covered with an outer skin of aural sheeting. The

model construction was similar tc that used by the firmfor their own dynamic

model but the extra weight and rigidity required measure-

ments was prcvided by a solid wooden cabin containing the heavy variable-

frequency three-phase electric motor used to drive the lifting fans while heavier

gauge metal sheeting was used throughout. Thus the basic model weight was

increased from 20 to 80 lb.

A pair of Heba 100 type fans (F&k(a)), were fitted in the model. Origi-

nally the fan design incorporated rather sharp intake lips (Fig.&(b)), but alter-

native lips were also provided to give lip radii of 5% and 1% of the inlet throat

diameter. The fans exhausted through a plenum chamber to a peripheral blowing slot which was inclined inwards at 9 to the horizontal.. AdclitiWlallongitudinal blowing slots (Pig.J), were fitted on the full-scale craft to provide pitoh and

roll stability. These could be represented on the model, if required, and guide vanes were avialable to give a vertical Jet or C30° deflection fore and aft.

On the full-scale vehicle, the fans rotate in opposite directions but, for simpli-

city of construction, the model fens bothrotatedin the same direction, clockwise

when viewed from abwe. This may have caused some difference in the velocity

distribution around the cirour&erence of the blowing slot.

Behind snd outboard of the rear intake, a pair of fixed fins could be

mounted on the craft and a rudder capable of deflections up to 5o'was cut in

each fin (Fig.3). Furthermore, a tailplane could be mounted in a high position

between the two fins (Fig.2). Provision was also made for the installation of a

pair of propulsion motors ahead of the cabin with eaoh motor drivang a two-bladed

propeller (Fig.1 and 2).

3 TESTDETAILS

The tests were made in the No.1, 114 x @ ft low-speed wind-tunnel at

B.A.& Farnborough during 1963 and 1964. The model was suspended in an upright

position from the overhead 6-component balance, using a conventional V-wire rig

on which the wire lengths could be adJusted so that different hover heights could

be represented. The 'ground' oonszsted of a 2 in thiok elliptic-nosed wooden

board spanning the tunnel. It split the tunnel cross-sectional area approximately

in the ratio 2 : 1 snd extended 2 ft ahead of and 4 ft behind the model extre-

mities. The wind-speed at the model position had been determaned, before the

model was installed, from measurements made with a suitably located pitot-static

tube for a series of values of the pressure differential between the usual two

reference stations in the tunnel carsuit. Experience has shown that a model of

normal size has little or no effect on the total mass flow rate through the

tunnel, so that a corrected value of wmd-speed at the model could be obtained

by measuring the changes of the mean velocity in the air passage below the

ground board. No constraint corrections have been applied except for this

velocity re-distribution.

5

Measurements were m3de at four hover heights of the lift, drag and pitchulg

moment of the unyawed model, both with and without stability Jets, at mainstream

speeds up to 75 ft/sec over an incidence range that was limited solely by the

bow or stern touching the ground. Usually the chosen fan speed was that required

for dynamic similarity (1288 rpm) though some tests were also made over a range

from 1030 to 1800 z-pm. The fan speed was measured by counting pulses on a tachw

generator in the motor and could be controlled quite easily to within too.@ of the

desired value. At a hover height representative of 14in full scale (h/t = 3.57),

the directional and lateral stability were investigated for a range of values

of the ratio of nC.nstream dynamics head to cushion pressure. Tests were also

made with a pair of propulsive motors installed end their performance was exarrrined

for a range of propeller blade settings.

Lift end moment contributions frcen the air cushion were obtained from eleven

static PressureoriPicesfitted along the centreline of the under-surface of the

model and a further tenorifioesdistributed over the port half of that surface.

Mass flow rate through the tie1 and the exit momentum were estimated crudely

from measurements cfthe total and static pressures at seven peripheral stations

in the port half of the nozzle (Fig.&); two pitot tubes vrere fitted at each station whilst the static pressure was obtained. at crlficcs in the inner and outer

walls of the slot. Similarly the flow through the stability Jets 1'789 obtained

from single pitot and static tubes at seven stations along the length of the port

Jet.

4 RESULTS

In order to facilitate more gcnerai use of the information obtained frcnn

these tests, the results are mainly quoted o.n a non-dimensional form in which the

forces and moments have been snde ncn-dimensional by dividing by the lift (L)

and also for the pitching moments by the craft length (0); lift itself has been

non-dimensionalised by dividing by the hover laft (Lo) at zero incidence for the

appropriate hover height. In addition a velocity parameter 90 u--) PC

has been

0

formulated where q, is the mainstream dynamic head and Pc is the mesn value of 0

cushion pressure at hover. The yawing moments, sideforces and rolling moments have all been non-dimensionalised in the standard aerodynamic way.

It should be remembered that these tests were msde at very low Reynolds number (about 1/15th of full scale) and thus it is unlikely that the internal

6

flow regime was truly representative of that apertainingto the full-scale craft,

particularly as regions of separated flow were observed. Consequently it smst not

be assumed that the results necessarily apply quantitatively to the full-scale

craft thcugh general trends would be expected to be similar.

4.1 Aerodynamics of the basic shape

The aerodynamics of the basic craft with intake-lips removed and intakes and

exits sealed. are given in Fig,5(a)-(i). The forces end moments are here given

in the usual aerodynamic coefficient formwith the measured values divided by the

mainstream dynarmc head (~0) and plan area (3) and also, for the moments,by the

overall length of the model (c) or width (b) as appropriate. As expected, the

lift slope increased as the tie1 was brought nearer to the ground but there was

negligible lift at aero incidence (CL E 0.02). Integration of the pressure

measurements on the under surface of the craft at zero incidence gave the negative

values of lift coefficient indicated in the table below; the difference between

these values and the balance measurements gave lift external to the cushion area

which varied between CL-values cf 0.15 and 0.20 over the tested height range.

r ~~~

'CL ' cL cL h/t Overall Cushion area toE~~~t;"i$ I

1.95! +0.02 -0.18 +a20

3.57 +0.02 -0.16 +018

b.95 +0.02 -0.15 +O.A7

7.921 +o.Ol I -0.14 +0.15

Frcm the pitching moment curves (Fig.5(b)), it could be deduced that the

aerodyne&c centre was at about one sixth of the overall length at zero incidence

but moved back towards the rmd-point as the incidence was increased. The proflle

drag (CD z 0.05) was virtually independent of ground clearance (Fig.5(c)) though 0

there was the usual reduction in induced drag as the ground was approached.

B'lthout fins the model was directxndlly unstable though tie chosen fm

design gave stability at sideslip angles up to 17' (Fig.5(g)).

I+.2 Calibration of mass flow rate m model with optimised intake lips

An exploration of the peripheral Jet with pltot- end static-pressure tubes

a.n the plane of the nozzle exit indicated that, even in the hover, there were

variations in the Jet velocity (VJ) end total head around the periphery (pig.6).

Increases in the mainstream speed reduced the mass flow rate through the front

sections of the nozzle but this loss was more than compensated for by increased

flow through the remainder of the nozzle; similar variations were &so cbserved

in the stability jets. Thus overall some benefit appeared to be derived from the

mainstream ram pressure but flow separations from the front portions of the

intake lips (Section 4.4) precluded any further gains once the mainstream speed

hadbecome comparable with the intake velocity.

l+3 Static hover

The effect of hover height on the static lift is illustrated in Fig.7 where

the results have been converted to lb and ft full scale. opening the stability

Jets led to some loss in lift at moderate hover heights as this imposed some

decrease in the flow-rate through the peripheral Jet but close to the ground

the stability jets had little effect as the large back pressure of the cushion

then reduced the flow-rate through them to a negligible emount. The maJority

of the lift could be attributed tc the cushxon pressure (Po) acting on the area

(AC) within the peripheral jet (Section 4.7) though, of course, there was some

ccntribution from the jet momentum flux (mj). Typically, at an h/t- value of

3.57, * = j.16 and 1.10 with stability Jets closed and open respectively. c c

Throughout the investigated height range, good agreement was achieved with the

StantoI1-Jones exponential theory' for the cushion pressure producedby a sunple

peripheral Jet of total head HJ (Fig.B):-

5 -2X H = l-e IThere x = t/h(l +cosU) . J

Some reduction in lift was observed if the craft was tilted from eero incidence.

In general, the pitching moments become increasingly nose-drum as the

incidence was increased (Figs.l2(c), $3(c), 14(c)) thus demonstrating inherent

stability though the static margin was reduced as the hover height increased.

The few measurements made with the stability jets closed indicated that they had

negligible effect on the pitch stability.

&.4 Intake modification

The original air intake (Fig.Mb)) had been designed tc give maximum effi-

chancy in hover. Unfortunately this resulted in an installation with relatively

sharp intake lips whose performance in a cross-flowwas rather suspect. Atuft

survey was therefcre r&e end this revealed signifYcant flcm separations in the

intakes even at quite low mainstresm speeds. Ccnsequently further intakes were

built with the more generous lip radii cf 5.4% and 10.8% of the det threat

diemster (Fig.&(b)). These gave appreciable lift gains in the presence of main-

stresm flew (Fig.g(a)) although they had nc measurable effect at hcver. Further

benefits were achieved by f&ring the cabin lines into the rear intake and extend- ing the front centre part of the forward intake to the bows of the craft as detailed

inFigs. and It(b).

By increasing the intake efficiency, these improvements gave a greater intake

mass flow-rate and hence sn increase in momentum drag but, as the associated

lift gains were proportionately greater, there was a s&&t reduction in the

drag/lift ratio (Fig.S(b)). Only small changes in the moment characteristics

were induced by these modifications (Fig.S(c)).

The effects of forward speed and incidence (Sections 4.2, 4.5, 4.6, l+7)

were investigated in detail with this optinised intake design installed but

the remainder of the tests were made early in the programme when the sharp lipped

intakes were fitted and lack of' tlrne prevented repeat testing after the msnu-

facture of the improved intakes.

J+5 Effect cf mainstream speed at zero incidence

The combined effects of lrainstresm speed and hover height at zero incidence

are illustrated in Figs.10 and. 11 with and without the stability jets represented.

No simple relationship could be established for the variation of the lift-values

with height and speed but this was hardly surprising in WEW of the effects of the various parameters on the several contributions to lift (see Appendix).

Although integration of the cushion pressure distIibutia (Section l+.7) indicated

some slight reduction U-I the cushion lift as the mainstream speed was initially

increased from rest, the aerodynsmic characteristics external, to the cushion were

sufficient to prcduce continuously increasing overall 1st throughout the tested

speed range.

The drag was also male up of several components each uf which was influenced

by various parameters and thus again complex chenges were observed as the height

and speed were altered. At 1~ mainstream speed the dra$lift ratio (Figs.lO(b)

and II(b)) increased virtually linearly with the speed parameter

i-

% F and nearly all the drag could be attributed to the loss in horizontsl 0 0

momentumat the stake. The reduction zn the rate of grodh of the drag/lift

9

ratio at high speeds was mainly due to the rapid rise in aerodpamic lift

mentioned above though there was also some reduction an the growth rate of the

intake momentum drag once the mainstream speed becsme comparable with the

mtslce velocity as flow separations from the intake l!.ps then reduced the

fan efficiency and mass flow rate (Section 4.2). A very significant result was

the substantial drag rise as the ground clearance was increased thus indlcatlng

that operation of ACVs at excessive height would xncur penalties in propulsive

power as well as in lifting poser.

The lift and. drag changes were accompanied by a forward movement of the

effective centre of pressure as the mainstream speed was raised frcsn zero

(Figs.lO(c) and II(c)) until a critical speed was attained sfter whxh there was

a fairly sudden rearward movement of the centre of prossure back tcwards the centre

of area. This cratical condition oocured when the mainstream dynamics pressure

was comparable with the cushion pressure as under this condition the front air

curtain broke dcwn with a oonsequentlal rearward movement of the centre of

cushion lift (Section 4.7). Unsuccessful attempts were made to delay this

flow breakdown by fitting a spooler immediately behind the front jet aligned

firstly tsngentlal to the jet and later perpendicular to it.

Large nose-up moments mxst be expected from a desrgn incorporating upper

surface intakes as thear associated. momentum drag must effectively act hagh up

on the craft. From simple sink flow considerations, Whitley and Bissel12 pre-

dicted that the momentum drag should act half a d&meter above the xntake lips,

a suggestion used in estimating the momentum drag contrrbutions on Fig.lO(c) but

this could not account for even half of the indicated moment. Other experimental

work at the R.A.E. on llftmg-fan models 3 has also indicated that momentum drag

effectively acts far higher than suggested by simple sink theories and consequently

the lntske flow phenomenon needs further consideration.

4.6 Effect of incrdence

At low mainstream speeds, the overall lift was reduced by angular deflec-

tions of the model in either the positive or negata.ve sense (Figs.l2(a), Ij(a),

14(a), and 15(a)). However, at speeds where the mainstream dynamic head had

become comparable with cushion pressure, the influence of external aerodynasncs

gave some laf't increase at positive attitude and appreciable gains xere available

at higher speeds.

Throughout the investigated range of hover heaght the craft was statically

stable(@) ) negative at low mainstream speeds (Flgs.l2(c), 13(c), 14(c)) but

IO

the pitch stiffness was reduced by increases in either hover height or main-

stream speed (Fig.16) leading to almost neutral stability in the vicinity of

the critical speed (q. = Po ). Fortunately, further increase in speed led to 0

some recovery m stability. Thus, extra pitch controls (Section 4.10) would

be required for safe acceleration through the critical speed range.

Forward propulsion would be available from negative tilt (Figs.l2(b), 13(b),

l&(b), 15(b)) but the resulting speed would be llrmted by the sngle at which the

bows touched the ground. Higher speeds would require separate propulsive system

(Section 4.8). At high speed the optimum perfonnence would no longer be achieved

at negative incidence as the powerful effect of the positive slope of the lift/

incidence ourve on the drag/lift ratio would imply a large positive inclination

for the most economic operation.

4.7 Pressers distribution under the model

The static pressure distribution on the lower surface of the model was

investigated over ranges of hover height, mainstream speed end incidence; values

along the centreline being illustrated in Figs.1 7 end 18 for the modelwith

stability jet nozzles closed and open respectively. Increase of hover height

gave the anticipated reluoticn in cushion pressure and, as already m&zoned

(Section 4.3), integration of the pressure measurements gave good agreement

with the Stanton-Jones exponential theory' (Fig.8). However, with stability

P Jets open, the $ - values were somewhat greater thsn predicted by the theory

J though, of course, the absolute values of cushion pressure were reduced by

cpening the stability Jets (compare Figs.l-/(a) and 18(a)).

Increase of mainstream speed from zero initially caused scene reduction

in cushion pressure towards the rear of the cushion (Fig.lT(a)) but when the

mainstream dynamic head exceeded cushion pressure the resulting breakdown of the

front curtain led to a rapid reduction in pressure in the front part of the cushion

which was accompanied by a pressure rise in the rear part of the cushion. Inte-

gration of the overall pressure distribution implied that there was an initial

slight loss in cushion lift associated with a forward movement of the centre

of pressure but above the critical speed the breakdown of the front curtaln

led to a substantial rearward movement of the centre of cushion lift.

Angular deflections of the hovering model led to some pressure increase

under the downward going part of the craft but there was an even greater pressure

11

loss under the upward moving part of the craft SO that there was an over&l

lift loss (Figs.l8(a), (c) end (d)). Increase an mainstream speed at positive

incxlence resulted in en initial reduction an the cushion Pressure but, at

higher speeds, there was an ancrease in pressure originating at the front of the

cushion though rapidly extending rearwards as the speed was further increased

(Fig.l8(c)). At negative lncadence, the mainstream interference effects pro-

duced pressure losses at both the front and back of the cushion (Fkg.l8(d))

and thus, although there was a lift loss, the centre of pressure movement was

less rapid then on the undeflected model.

4.8 Directional and lateral stability

Yawing moments, sideforces and rolling moments have been plotted against

sideslip angle in Fig.19 for a cruising speed Just below the critlcal speed.

The negative values of yawing mcment at eero sideslip angle arose frcm the pre-

cessional moment Produced on the mcdel by the rotation of both fens in the sams

direction; full-scale they rotate in opposite directions so this effect would not

be expected. ?4ith fins installed, the model was directionally stable over the

range +lZ" of sideslip though it was completely unstable in the absence of fins.

As might be expected from their high position, the fans increased the negative

value of ev as well as doubling the value of Yv.

The effect of variation of mainstream speed was only investigated on the

model without fins (Fig.20). Although the results show linearity at

J-- 90

7 - values below unity, some non-1inearat.y was present at higher speeds.

0

Reductions in the values

sistent with the effects

sideforce direction:-

lr9 Prooulsion motors

of-2 ad-2 dp with increase in

of the intake momentum drag component

A CY (intake) ,X $ sin P . 0

speed mere con-

acting in the

In general, the model propellers were driven at 6860 rpm which was the

dynamic scale speed for the proposed full-scale installation though a few check

tests were also made at 8C$ of this speed. Blade pitch could be manually

adJusted and a range of blade tip angle (# . t=p

) between 7.5o and 16.75'was

12

tested so that the effect of blade ooarseness could be investigated through the

speed range. The resulting thrust cc-efficient ( KT = thrust per PrOPs ller has

> v.

p n2 a4

been plotted against the advance factor J = z in Fig.2l(a) where the implied ( >

full-scale values of thrust and mainstream speed are also indicated. Measurements

were also made on a Weston analyser of the input power to the electric motors;

the measured values (which make no allowance for motor efficiency) were non-

dimensionalised with respect to the power consumption with the blade tip set at

11.5' at zero maznstresm speed (Fig.Zl(b)).

In the hover, there was little change in thrust with blade setting, suggesting

that the blades stalled at a $t&, value of 9'. At fine pitch settings the thrust

decayed rapidly as the nmlnstream speed was increased but this decay could be

delayed by coarsening the blade angle though more power would then be required

from the motors. Operation of the rear lifting fan gave large reductions in the apparent thrust at low mainstream speed (approximdtely 257&loss at hover) as

some of the slipstreamwas drawn into the rear intake.

At zero incidence the propellers gave some benefit to the lifting system

(Fig.22) presumably through the slipstream augmenting the inlet total pressure

but at negative incidence this was counterbalanced by the downward component

of thrust. In consequence of their high position on the craft, the propeller

thrust produced a useful nose-down moment increment and analysis of the results

at zero incidence (Fig.23) showed that the simple relaticnship:-

AM (propellers) = 0.85 AD (propellers) lb ft

applied throughout the incidence range. This showed excellent agreement with the fact that the propeller axes were 0.85 ft above the moment centre.

The lateral and directional stability o" L the craft with propellers and fins

fitted was only investigated at one speed (E = 0.87) and the results show

that the directional stability was now limited to +5' of sideslip angle (Fig.24).

Consequently larger fins would be required to give the same handling characteris-

tics as those experienced on the basic craft. An engine failure was simulated

by rotating the starboard propeller whilst holding the port propeller stationary.

Originally, directional control on the GC.2 had been obtained by deflec-

tion of vanes in the peripheral jet but this was insufficient at the higher

speeds available with propulsive units imta~ed. Extra control was therefore

provided by cutting rudders in the fins (Flg.2). Curves of yawing moment,

sideforceandrolllng moment have been plotted against sideslip angle for a range

of ridder angle in Flg.25. At low sideslip angles the directional stability Was

unaffected by sldesllp angle but fm stall gave an antisysmetrlc set of yawing

moment curves at higher sideslip angles.

4.20 Tailplane

As previously mentioned (Section 4.5), the nose-up moment increased rapidly

as the mainstream speed was increased snd some form of trim control was thus

desirable. One possible solutzon was to carry a tailplane between the fins though

this would need to be mounted high up to allow for the opration or rudders.

Measurements were made of the lift, drag and pitching moment of one such

installation (Fig.2) with the tailplane at various settings between 0 and 25O

and, in order to obtain the tallplane ccntributions, measurements were also made

with the tailplane removed. The tallplane contribution to pitching mcment has

been plotted against tailplane angle (Fig.26) for a range of mamstreem speed.

From these curves, the mean downwash angle at the tallplane was determined as that

tailplane setting required to give zero oontributicn to pitching moment; values

so deduced have been plotted against mainstream speed in F'ig.27. As expected

from consideration of the effects of the flow into the rear Intake, the downwash

angle decreased rapidly at fxrst with increase in mainstream speed but only at a

very slow rate once the ma~stream speed exceeded about a half the intake velocity.

Operation of the propulsion motors caused some Lncrease in the mean downwash

angle at low mainstream speed but had neglaglble effect at p r

90 - values above 1.

CO

Overall pitching moments are plotted against speed parameter in Fig.28 with

the tallplane set to give maximum nose-down moment contribution on the model both

with and without the external propulsion units; curves are also shown for the model

without a tailplane. At d--

2% - values above 0.7 insufficient thrust was being pO

0

produced to balance the drag (Section &.y) so an attempt was made to assess true

performance by estimating the moment contribution produced by the additional pro-

peller thrust required tc give zero net drag; m Section 4-9 it was demcnstrated that propeller thrust acted along the propeller axes. The estinated overall

moment on the model with drag trinmed in this msnner is given In Fig.29 where It

can be seen that the tallplane would be inadequate below the oritl0a.l speed. Some

improvements may be pcssible by fP33ng the tailplane cn low bocms behind the craft

so that the favourable lift arm could be increased whilst eliminating the adverse

drag srm.

4.11 Effect of sealing front peripheral jet

An alternative method considered for trim control was the variation of

flow through the front part of the peripheral nozzle. A simple test was made at

an h/t-value of 1.73 with the extreme ccndition represented by complete closure

of the front nozzle between the stability jets; i.e. just over a tenth of the

air n*rtain was removed. The results have been compared in Flg.30 with those

for a fully open nozzle.

In the hover, a lift centre movement of 1.1% of the craft length was obtained

at zero incidence though tilting the craft reduced the available trim control

which also deteriorated with increase in mainstream speed. Naturally some

benefit would be available from a more forward position of the centre of gravity.

As was only tc be expected, at the higher speeds complete closure of the front

nozzle had negligible effect as even with the front nozzle open the air-curtain

was blown back and adhered to the under-surface if the mainstream dynamic head

exceeded the cushion pressure.

Throughout the speed range, closing the front nozzle had little effect

on 1st end drag.

5 COIELUDING RJiNAEG

These tests have shown that serious trim and stability problems require

solution before air-cushion-veh=Gles oan operate at relatively high speeds. The nose-up moment contribution associated with the intake momentum drag predominates

over trim considerations at low mainstream speed, but when the mainstream dynamic

pressure beoome comparable with the cushion pressure the front air curtain breaks

down leading to a rearward movement of the centre of lift whilst simultaneously

the pitch stiffness is reduced to sero. There is some recovery at higher speeds.

Forward propulsion can conveniently be obtained by the installation of pro-

pellers mounted above the craft; their thrust moment then tends to counteract the

intake momentum drag moment. However, care should be taken in positioning propel-

lers so that their high energy slipstream is not swallowed by the intakes as such

an eventually would lead to increased intake momentum drag.

Operation of craft at excessive grcund clearance would net cnly mcur a

high lift power penalty but greater propulsive power would also be required.

Analysis of the results on thi 8 compcsite model has proved difficult

because the effects cf varying mainstream speed cn the aercdynemics of the

upper and icwer surfaces are interconnected; 1.e. intake efficiency changes

with speed and this leads to changes in internal mass flow rate end. cushion

pressure. Consequently a full understanding cf the problems wculd require

three models:-

15

(4 an intake model for the upper surface aer~os;

(b) an efflux mdel to study the air cushion; end,

(4 a composite model to check the interference effects.

16

Appendix

ANALYSIS OF LIFI! AND DRAGFORCES

The lack of uniformity uf flw through the nozzles precluded any rigorous

analysis of the results but an attempt has been made to give a crude estimate of

the various components of lift and drag.

In order to investigate the impcrtance of the lift external to the dr-

curtain, the integrated cushion lift (PC SC) and vertical components of jet moments (mJ sin 0) were subtracted frcm the overall balance measurements. Curves

of theseaduoed values plotted against mainstream Qnsmics head (Eg.31) were

parallel indicating thatatall speeds and heights there was a ccnstant CL-value

of 0.28 (based on overall plan area) attributable to the external aerodynamics.

The slight stagger of the curves may have been produced by the entrainment of

air into the standing vortice external to the curtain. Thus the ntsjor components

of lift could be expressed in the form:-

LIFT = PcSc+0.28qoS+Emjsin0

Simple consideration of the drag results shcaed that, after remcval aF the

intake momentum drag, derived fraw measured mass flow rate (Section l+..Z), the

re maining drag varied direct&jr with both hover height and nninstresm dynamic

head. Thus drag could be expressed:-

DRAG = iA + B (h/t)jq, + pVo VIAL .

The first twotefinsrepresented the profile drag external to the air-curtain and

a termwhich, beiag dependent on "he cross-sectional area of the passage under

the craft, could be regarded as 'curtain drag'. Transposition of these terms

into more appropriate forms gave:-

Prdile drag = o.03qo s

'Air-ourtain'drag = 0.667~ hbc

Intake momentum drag = pVo Vi Ai

The impbed profiLe drag component was only just over half the value established

for the bssic craft without fans (Section 4.1). Such an improvement couldbe

attributed to the elimzd.ion of skin frictian on the lower surface of the craft

and also to B.L.C. effects on the stern of the craft induced by the air curtain.

Appendix

A ccmpas~~son of measured drag with empirical vdlues derived from this analysis

is given in Fig.52 where the importance of the intake momentum drag component is clearly demonstrated.

18

Table 1

Mdel details

Plan area

Cushion area (area within Peripheral Jet)

Intake area (at throat)

Peripheral Jet exit area

S

SC

*i

*.i Stability Jet exit area Overall length of craft

Overall width a? craft

Length of cushion

Width of cushion

Thickness of peripheral nozzle

Inclination of peripheral jet to horizontal

Propeller diameter Tailplane area

Tailplane chord

'"$~da$nt to C.G.)

A-

ds

b

c

bO t

e" D

P sT

CT &T

33.05 sq ft

8.95 sq ft

0.94 sq ft 0.70 sq ft

0.51 sq ft

5.00 ft

3.00 ft

4.17 53

2.33 ft

0.667 in 30" inwards

0.995 ft 0.923 sq ft

0.w ft

2.04 ft

583

SYAIBOLS Geonetric

S planarea

sc cushicn area

Ai intake throat area

5 exit area of peripheral nczzle

*s exit area of a stability Jet

8 length of cr.&%

0 length of cushion (between fore and aft nozzles)

b width of craft

h height of peripheral nozzle exit above ground

t nozzle thickness

e inclmation of peripheral jet to horizontal

n fan speed rpm

Pressures and velocitzes

pO

. . H. 4 90 Vo V. I v

V::

"J

qj s,

cushion pressure

total head at exit of peripheral nozzle

total head at exit of stability Jet nozzles

mainstream dynsmc hcad

mainstream speed

3.ntake velocity

exit velocity from peripheral nozzle

exit velocity from nozzles cf stablllty jets

exit Iucmentum flux

peripheral Jet dynamic head

stability Jet dynsmic head

Forces andmoments

L lift

D drag

Y sideforce

In pitching moment

n yming incment

e rolling mcmcnt

20

%=$ cD =-+

c* = y

so=

SYMBOLS (conta)

ce = e cbsb

Prawller characteristics

DP propeller diameter N propeller speed (revs pr second)

v J advance factor &

P $ tiP blade angle (tip section)

thrust KT thrust coeffxient =

Q NL "$

P power input

po ~CWCX input at V. = 0 with $tip = 11~5'

21

2

J&. Author

1 Stanton-Jones

Title, etc

Some design problems of Hovercraft.

I.A.S. Report N&l-45, A.R.C. 23752 1961

&I the nature of aerofoil characteristics with a

sink located in the upper surface including compari- son of theory with some fan-in-mmg experiments.

8th Anglo-Amerwan Aeronautical Conference,

September 1961

Wnd tunnel experiments on a lifting-Jet in a

bluff body with and without wags.

A.R.C. C.P. 859 July 1964

2 '&it&y-Bissel

3 W.J.G. Trebble

FIG. 2 SCHEMATIC VIEW OF MODEL NOTE:- PORT NACELLE AND PROPELLER NOT SHOWN

I , I .

60-O in

in MODEL SCALE 0 2 4 6 8 10 12 14 16 18 20

t1 I I I I f I I: I 01 23456789’0

ft FULL SCALE

FIG. 3 GA OF MODEL WITH HEBA FANS

AND MODIFIED INTAKES

LIP RADIUS

PULLEY

FIG.4 a DETAILS OF HEBA FANS AND INTAKES

.

II.0 In, 1451n - I-1 Il... --an

Lvl ,n PlOtEL SCALE A

0 2 6 I I 3

0 I ft F:LLSCAl.E3

4

FINAL MODIFICATION TO FRONT INTAKE OTHER INTAKE DESIGNS HAD CONSTANT SECTIONS SHOWN BELOW :-

AiyAt5,n&fn

ORIGINAL LIP MEDIUM LIP RADIUS LARGE LIP RADIUS ITC HOOEL SCALE

0 I 2 3 4 I

I

0 , 2 ft FULL SCALE

FIG 4 b DETAILS OF INTAKE LIP SHAPES FOR HEBA CONFIGURATION

@5 @5 CL CL 0 0 195 195

z “s.:: z “s.:: A A 792 792 0.4 0.4 . . 60 60

i =HOVER HEIGHT AT i =HOVER HEIGHT AT CENTRE OF MODEL CENTRE OF MODEL 0.3 0.3

0.2 0.2

0.1 0.1

a LIFT AT ZERO SIDESLIP

1

-0.4 -0-3 -0.2 -0.1 I

b PITCHING MOMENT AT ZERO SIDESLIP

FIG.5 a a b AERODYNAMIC CHARACTERISTICS OF BASIC MODEL INTAKES AND EXITS SEALED ; NO FINS

.

-0.4 -0.4 -03 -03 -02 -02 -0 -0 I I 0 0 0 0 I I 0.2 0.2 0.3 0.3 0,4 0,4 05 05 CL CL 06 06

C DRAG AT ZERO SIDESLIP C DRAG AT ZERO SIDESLIP

. d DRAG AT ZERO INCIDENCE

e LIFT AT ZERO INCIDENCE

f PITCHING MOMENT AT ZERO INCIDElkE

FIG. 5 c-f AERODYNAMIC CHARACTERISTICS OF BASIC MODEL INTAKES AND EXITS SEALED i NO FINS

-0 Of -0 Of

-0.10 -0.10

-0 I5 -0 I5 h SIDEFORCE , 4T ZERO INCIDENCE

i ROLLING MOMENT AT ZERO INCIDENCE I

FIG.5 g-i AERODYNAMIC CHARACTERISTICS OF BASIC MODEL INTAKES AND EXITS SEALED

0 0 0

v I.22 0.71

+ 3.28 1. 16

A 4.62 I.38

a TOTAL HEAD

b JET VELOCITY

FIG.6 a & b TOTAL HEAD AND VELOCITY OF PERIPHERAL JET AT NOZZLE

% = 3.57; “=O; Pco=2-45 Ib/sq ft

9000

‘i - 8000

7 000

1 i 6 Anti \\ I I ---

t\

I\\ - 5000 \\

- 4000

- 30001

- 2000

- 1000

\

\

\\

t\

I

It RAN& OF

CRAFT OPLRATlNG

WEIGHTS I--

l

I I I I 0 O-5 I-0 I.5 - 2.0

GROUND CLEARANCE kt FULL SCALE)

FIG. 7 EFFECT OF HOVER HEIGHT

0.2 0.2 0 STABILITY JETS CLOSED 0 STABILITY JETS CLOSED X STABILITY JETS OPEN X STABILITY JETS OPEN

0 I *o 2.0 3.0 , 4.0 5-O 6.0

FIG. 0 EFFECT OF HOVER HEIGHT ON CUSHION PRESSURE

Q LIFT

I’ 3

I”/L

0~2

0.1

0

0.06

YLC

Cl.05

0.03

o-02

0.01

DYNRMIC = CUSHION

I I C PITCHING MOMENT

I I

33-

0

I-

%,

2.3

2c

I.5

I 0

0.5

0

OL

D/L

Oi

C’ 1

03

0.2

0.1

C

I3 I 95 35.6 3.74

0 3.57 S-4 2.45 X 495 17.9 I.66 P 7*92 IO.5 1.02

b I.5 2.0 a LIFT

I INTAKE MOMENTUM

- DRAG

I I I 1 I.0 I.5 2.0

b DRAG

FIG. IOa (L b EFFECT OF HEIGHT AND MAINSTREAM SPEED ‘

AT ZERO INCIDENCE STABILITY JETS OPEN

.

D-06

o-04 I

a 1.95 0 3.57

O-03 ___ * 4.95 A 7.92

I I, I ,/-

l=MiE6ii ASSUMING THIS ACTS HALF DIAMETER

ABOVE INTAKE LIPS (%.=357)

o-01

0 l-5 2.0 25

c PITCHING MOMENT

FIG. IOc EFFECT OF HEIGHT AND MAINSTREAM SPEED AT ZERO INCIDENCE

STABILITY JETS OPEN

0.G 2 L

0.!

0.‘

0-i

0.:

0.

(

ICI 15

a LIFT

1 k .

b DRAG

‘ FIG. llaab EFFECT OF HEIGHT AND MAINSTREAM

SPEED AT ZERO INCIDENCE NO STABILITY JETS

O.IC

m I

L-C

0. OS

o*oe

O-07

o.oc

0.05

.

o-04

.

O-02

002 :USHION ‘RESSURE I I

I-- 1.5

‘%&

2.0

c PITCHING MOMENT

5

FIG. Ilc EFFECT OF HEIGHT AND MAINSTREAM SPEED AT ZERO INCIDENCE

NO STABILITY JETS

05 I.0

Cl LIFT (Lo= 35.6 lb, f&=3.74 lb/sq ft)

03

0 L

02

0.1

0

- 0.1

b DRAG

FIG. 12 asb EFFECT OF INCIDENCE AT h/, = 2.25 STABILITY JETS OPEN

-I

c PITCHING MOMENT

FIG 12~ EFFECT OF INCIDENCE AT h/t =2.25 STABILITY JETS OPEN

20

I.5

I.0

0 05 I.0 I.5 2.0 25

a LIFT (Lo=254 tb, Pco=2.45 lb /rq tt)

b DRAG

FIG.13 a rb EFFECT OF INCIDENCE AT h/t =3-57

STABILITY JETS OPEN

5

*

.

,

-10 -8 -6 -2

-r I

B

I I I I

c PITCHING MOMENT

FlG.13~ EFFECT OF INCIDENCE AT h/t =3*57

STABILITY JETS OPEN

3.5

‘/L,

3.0

2.5

2.0

I.5

I.0

05

0

05

D/L

0.4

0.3

0.2

0-I

0

-0.1

0.5 I.0 I.5 20 - 25

a LIFT (Lo= 17.9 lb, P&= I.66 lb/q ft)

T

b DRAG

FIG. 14 a rb EFFECT OF INCIDENCE AT h/t = 4.95 STABILITY JETS OPEN

-t \, WI.253 .

0.573 -002. . -0.04

-006

c PITCHING MOMENT

FlG.14~ EFFECT OF INCIDENCE AT h/t =4-95 STABILITY JETS OPEN

6.5

k c 60

5s

50

4:

4c

3.5

3c

2!

2-c

I.= w

I.0

0.:

,

,

j-

)-

i-

)-

j-

0.5 I.0 I.5

a LIFT (Lo= IO-5 lb, Pco=I-02 lb/q ft)

FIG 15a EFFECT OF INCIDENCE AT h/t = 8-10 STABILITY JETS OPEN

.

o-3 0

b DRAG

FIG.15 b EFFECT OF INCIDENCE AT h/t = 8.10 STABILITY JETS OPEN

0.004

d (W dcl,

0.002

C

ii % i-z-

o*oor

iTi E -0.004 lL i cn

s -O*OOE

Y p -0*008

2 Lr t; -0-010

-0~012

FIG. 16 EFFECT OF MAINSTREAM SPEED ON

PITCH STABILITY STABILITY JETS OPEN

0‘

.O-

)

,o -

o-

0

a h&3.57;d=0 “4

I I 1

o-2 0.4 0.6

b h/t = 4.95; d=O

FIG.17 a 8 b PRESSURE DISTRIBUTION ALONG CENTRE -LINE OF

UNDER-SURFACE OF CRAFT WITHOUT STABILITY JETS

-

0 02 04 06 O.8 a/{ I-0

a h/i= 3.57; d= 0

b h/t = a~io;&=o

FIG. 18 a 6 b PRESSURE DISTRIBUTION ALONG CENTRE - LINE

OF UNDER - SURFACE OF CRAFT WITH STABILITY JETS

P C LOCAL

(lb/se ft>

I 0 02 0.4 0.6 0.8 +e I.0

1 C

3-o

P ’ LOCAL

(Ib/s$+t)

0

.

I I I I I J

0.2

d h,to:3.5,; .“1”_ 4.50 Oa8 +t? ‘*O

FIG. IS c sd PRESSURE DISTRIBUTION ALONG CENTRE-LINE OF UNDER-SURFACE OF CRAFT WITH STABILITY JETS

30

PC LOCAL

b/ sq ft)

20

IO

o-4 0.6 O-8 “le 10

e h /,=a-10 ; q,=o

30

P %JCAL

(Iblstft)

20

f h/t=e.lO; c$,= l-22 Ib/sq ft 5 me= I.13

FIG.18 er f PRESSURE DISTRIBUTION ALONG CENTRE-LINE OF UNDER-SURFACE OF CRAFT WITH STABILITY JETS

.

.

0 r 02 04 06 08 O/P ’ 0

g h/+3.10,q~ = 2.09 Ib/sq ft; q = 1.48 co

40

P ‘LOCAL

Gbjsq ft)

30

-I 0

h h/t= 8.10;,~~=3-28 lb /sq ft i j-=1 86 c,

FIG.18 g ah PRESSURE DISTRIBUTION ALONG CENTRE -LINE

OF UNDER-SURFACE OF CRAFT WITH STABILITY JETS

YAWING MOMENT

I I I I

b SIDEFORCE

c ROLLING MOMENT

?5

I

r

5

FIG. 19 a- c YAWING MOMENT, SIDEFORCE AND ROLLING MOMENT

AT,/%, = 0.87; h/t = 3.57; w -0 ORIGINAL INTAKES (rL = 0 018 d,)

.

.

a YAWING MOMENT

I I IcyI I I I l-?-Ed

b SIDEFORCE

c ROLLING MOMENT

FIG 200-c EFFECT OF LIFTING SYSTEM ON LATERAL STABILITY

h/t ‘3.57; 0~ ~0 i NO FINS ORIGINAL INTAKES (rL= 0.018 d,)

a THRUST INCREMENTS

r I I I I I I

‘/p, i I I i I I ,

15

P. 15 POWEk REQUIRE0 h&=0 WITH d=llS”

0 01 I

02 03 04 05 0.6 ,

b POWER REQUIREMENTS

1

FIG.21 a &b EFFECT OF BLADE ANGLE AND MAlNSTREAtvl SPEED j ON THRUST COEFFICIENT AND POWER REQUIRMENTS

h/t =3-S? ; oc = 0 ORIGINAL INTAKES CL =O 018 dr)

PROPELLERS

NO PROPELLERS

WITH PROPELLERS

n =6860 rpm

FIG.22 EFFECT OF PROPELLERS ON LIFT AND DRAG

. h/t = 3.57; Pco= 2.8 lb/se ft

ORIGINAL INTAKES (rL=O.Ol8di)

E C

>

PROP PROP tPm tPm

) 0 6860 ) 0 6860 : + 5490 : + 5490

CENTRE OF PROPELLERS IS 0.85

2, 2.0

Am Am PROPS PROPS

(lb ft) (lb ft)

I

‘A80vEcGr I’0

5.c > -4.0 -

0

/ ? 0

Q

3 -i 3 -I

x

7 0

0 /

‘x

f

X

\ \

-T-- LlFtlNCj FANS INOPERATIVE

-‘/

>

/

X

- -1-O

- -2.0

- -3.0

- -4.c

- -5.0

0 x

/

/x

I

FIG.23 EFFECT OF PROPELLER THRUST

ON PITCHING MOMENT

d=O; h4 = 3 * 57

ORIGINAL INTAKES (rL= O*Ol8di)

1 2 ADPROPS

(lb)

r

a YAWING MOMENT

.

FE- 031 -25 -20 - Lb

nom -.

. . I -10

1

1

- -- b St DEFORCE

--I 004

‘\ 5

0.02

Ykk

\

- z.

-6

+

-002

-7 -0.04

?! 5

c ROLLING MOMENT

FIG. 24 a-c EFFECT OF STOPPING ONE PROPULSIVE MOTOR . ON DIRECTIONAL AND LATERAL STABILITY

= 0.87, hp3.57; a =0, UNDEFLECTED RUDDER; k,=0*044

ORIGINAL INTAKES (rL= 0.018 di)

Cl YAWING MOMENT

c ROLLING MOMENT

25 a-c EFFECT OF RUDDER DEFLECTION IN PRESENCE OF SLIPSTREAM, k,=O-044

ORIGLNAL INTAKES (rL= O.OIEidi)

I-0

0

-I*0

-2.0

FIG. 26 TAILPLANE CONTRIBUTION TO PITCHING MOMENT NO NACELLES, HEBA FANS, h/t = 3-57, d=O, NO STABILITY JETS

2s

\ “\ f- WITH NACELLES

20 WITH PROPELLERS

E0

IS

S/ NO PROPELLER i i !

0’ I I / I o-5 I.0 1.5 m/P,, Co

FIG.27 MEAN DOWNWASH AT TAILPLANE AT ZERO INCIDENCE

h/t = 3.57, NO S TABILITY JETS ORIGINAL INTAKES (rL= O-018 di)

FIG.

0.06

0 04

0 02

0

WITH PROPELLERS

28 EFFECT OF TAILPLANE ON CENTRE OF PRESSURE

h/t= 3.57; STABILITY JETS CLOSED;ckO

0 06

m/c 0 04

0.02

0

- 0.02

SHADED AREA SHOWS

t

MOMENT AVAILABLE FROM

TAILPLANE DEFLECTION

--04 (-06<C,_<+O6)

FIG.29 ESTIMATED PITCHING MOMENT WITH PROPELLER THRUST ADJUSTED TO GIVE ZERO NET DRAG

h/t = 3 57, STABILITY JETS CLOSED,oC=O

ORIGINAL INTAKES r = 0.018d 6 3

i-5

L/ LO

I.0

0.5

0 0.5 I.0 l-5 '1

J

2.0

"4,

0

-0.1 1 I I

0.06 I O.OA

FIG. 30 EFFECT OF SEALING FRONT PERIPHERAL JET

ORIGINAL INTAKES (q-=0-018di); h/t = 1.73

CL= 0 280 BASED ON PLAN AREA (13.05 spft)

/ I I

IO 2.0 30 40 50 60

FIG.31 LIFT EXTERNAL TO AIR-CURTAIN

i 0 IO 20 30 40”o

(ft / 2 "Olx&A‘E) 70 80 90

FIG.32 DRAG ANALYSIS AT hh =3.57 AND FAN SPEED = 1288 r p m

STABILITY JETS CLOSED IQ =0 Prrnted in B~hplati for Her Majesty's Statmw> 3f:sce by the Royal Atrcraft Rstablmhnent, Fanborafh Dd 129528 X.3

‘

A.R.C. C.P. No. 983 533.6.013.67 :

December 1966 533.682

Trebble, H.J.G.

LOU-BPEED WIND-TLUNSL TESTS ON A 116TH SOUE MOLEL OF AN AIR-CU!XION-YEHICLS (BRITTFSI-NORMAN CUSHIONCRAFT C.C.2.)

Large nose-up moments occur a, lava-d speed, reaching a nrxxlnwm when the mainstream mc head is comparable wlth the cushion pressure. At

mver)

A&C. C.P. No. 983 December 1966

Trebble, W.J.G.

!i33.6.013.67 : 533.@2

LW4PEF.D WIND-‘DJNNBL ‘I%¶% ON A 1/6m SCALE NDtCt OF AN AIR-‘ZUS1ION-VEHICLG (BRI~-NGRI%N CUs1IONCNAFT C.C.2.)

mer)

A.R.C. C.P. No. 983 December 1966

5~3.6.013.67 : 533.682

higher speeds. the fmnt air curtain breaks dam with a reaWzi”t rear- ml-d mveB9nt or tha centre or CUshlM 1irt thu giving SC90 red”ct10” in the nose-up lKm?nt. Pitch stiffness Is reduced by a” Increase 1” either gmu”d cleamn~e or speed and 1s redlced to zero when the Wont cwtaln breaka dam; at higher speeds them 1s 801~ recolsry in stabil-

ICY. Attempts to improve pitch contml by Deans “I a tallplane or by thmttllng the fmnt jet proved 1”adeqIW.e.

higher speeds, the fmnt air curt&n breaks d-am with a resultant mar hl&+zher speeds. the front air curtain breaka dam with a res”ltant rear- m mveaent 0f the 0mt2P 0r CU.~~QI ilrt thts giving - mdl0ti0n rerd mmnt 0r tlr ~sntn 0r ~3k3h1m lift cm giving ~mramm 1” the nose-up II1I?nt* Pitch stirrness 1s r-+&cad by a” increase 1” in the noseup amant. Pitch stiffness 1s reduced by en lncreass 1” Cltbr mU.3 clearance or sped and is redlced to z.?m rhen the front either ground cle-s or speed and lsred”ced to wlro tin the Imnt curtal” breaks darn; at higher speeds there 1s SW mcowry In stabll- curtain breaks dam; at hlehsr speeds them 1s - reCovBry I” Stabll- 1r.Y. Attempts to f=rrove pitch contml by meows “I a cailplsns or by tty. ACttempts to lm,m=m pitch conU-01 by meana or a tallplane o,- by tlrottllng Lb? front jet pmved Inadequate. throttling the front jet p-wed lmdequate.

C.P. No. 983

Published by HER MAIESTY’S STATIONERY OFFICE

To be purchased from 49 High Holborn, London w.c 1 423 Oxford Street, London w.1 13~ Castle. Street, Edinburgh 2

109 St. Mary Street, Cardfl Brazennose Street, Manchester 2

50 Farfax Street. Bristol 1 258-259 Broad Street, Bumwham 1

7-l 1 Lmenhall Street, Belfast 2 or through any bookseller

,’

C.P. No. 983

S.O. CODE No. 23-9017-83