Racecar Engineering – September 2019

80 http://www.racecar-engineering.com SEPTEMBER 2019

lateralgripathigherdownloads,butthiseffect

is notquantified,orindeedcertain.It is quite

likelythattheextralateralgripgeneratedby

increaseddownloadis relativelysmall– thismay

providepartoftheexplanationtowhy‘unstable’

LSRcarsdescribedabovedonotappearto

bedifficulttodrive,asthedestabilisingforces

involvedcouldberelativelysmall.

TheperformanceofThrustSSC(which

becamethefirstlandvehicletoofficiallybreak

thesoundbarrierin1997)wasactuallyseverely

impairedbyspraydragcausedbyanunderbody

shockwavepenetratingtheporousdesertand

kickinguploosesand,whichimpingedonthe

rearstructureandsuspension.Bloodhoundhas

beendesignedtominimisethisproblem.

Wheel-driven

It is worthnotingherethatwheel-drivenLand

SpeedRecordvehiclessufferanadditional

complication,inthatthedrivenwheelsare

providingsome(possiblyall)oftheirgripto

traction,leavingnothingforlateralstability.This

wasnota majorconcernforDieselmaxasit was

four-wheeldrive.Toprovidefurtherconfidence,

thepowertotherearwheelswaslimitedto

makesurethatrearwheelRPMdidnotexceed

frontwheelRPM,toavoidthepossibilityof

spinninguptherearwheelsandproducing

anoversteermoment.Clearlythisis nota

solutionfortwo-wheeldriverecordvehicles,

whichwillalwayshavetofacestability

challengesathighpowersettings.

Ofcourse,gustsensitivityandyawstability

mustalsobeexplored.Whentravellingat

supersonicvelocitiesanycrosswindgustwould

beexperiencedasbeingsharp-edged.Too

muchstabilitywouldjerkthecarhead-to-

windsocanbejustasproblematicastoolittle

stability.Inconsideringtheyawstaticmargin

above,it is thereforenotenoughforit simply

tobepositive.It alsoneedstobecarefully

bounded,whichleadstotheabovetargetbeing

setattwotothreepercent.

Theproblemis thatyawstaticmargin

is difficulttoassess,withoutextensive(and

expensive)computermodelling.Worse,it is

almostimpossibletomeasureobjectively

oncethevehiclehasbeenbuiltandis running.

However,experiencewithBloodhounditself

suggestsa testapproachthatmaybeofuse.

Runwaysuccess

DuringtherunwaytestingofBloodhoundin

2017 thecarwastestedatanairportandata

timeoftheyearwheregustycrosswindswere

thenorm.Inaddition,theinitialcartestingwas

conductedwithoutthetail-finfitted.Without

thefin,theyawstaticmarginwouldclearlybe

negative.Thiswasdeemedacceptable,dueto

thelowspeeds(under200mph)andhigh-grip

rubbertyresbeingusedontherunway.Once

thefinwasfitted,thestaticmarginbecame

positive.Thedifferencecouldclearlybeseenin

frame-by-framepost-runanalysisofin-cockpit

video.Withoutthefin,thecrosswindgusts

wereblowingthenosedownwind,followed

rapidlybydrivercorrectionsonthesteering

wheel.Oncethefinwasfitted,gustswould

causethenosetomoveintothewind,with

thedriver’scorrectionsnowintheopposite

direction,downwind,tocorrect.

Thisuseofin-cockpitvideoprovidesa

cheapwaytoassesstheyawstaticmargin

throughoutthespeedrange(whichis

otherwiseactuallyprettymuchimpossibleto

measure),basedonthewaythecarhandles

asthecrosswindvaries.Fortestrunswithout

a crosswind,theeffectcanbesimulatedby

puttinga small‘kick’intothesteering,tosee

howthecarresponds.Weplantousethis

technique,whichis similartomethodsfor

measuringaircraftstability,duringthehigh-

speedtestingofBloodhound.

If crosswindeffectsarea concern,thenone

obviousprecautionis tooperateattimesofthe

daywhencalmconditionsmaybeexpected–

suchasearlyinthemorning.Ona longtrack,

localobserverswillalsobeneededtowarnof

localweatherconditionsalongthetrack(in

additiontoensuringthatthetrackremains

clearofpeopleandanimals).However,limiting

theyawstaticmargin(andthuslimitingthe

gustresponse)remainshighlydesirable,asit

increasestherangeofconditionsinwhichthe

recordcarcansafelyberun.

Dynamicstability

Whilestaticstabilityis a measureofwhether

a vehiclewillreturntoa neutralposition,

dynamicstabilityis themeasureofhowit will

doso.If it settlesstraightbacktoequilibrium

it is saidtobedynamicallystable,if it oscillates

abouttheneutralpointit hasneutraldynamic

stability,andif it suffersdivergentoscillations

it is dynamicallyunstable.

Thetwopossiblemodesofdynamicpitch

instabilityareheavingandporpoising. The

namesareself-explanatory.Normalsuspension

TECHNOLOGY – LSR AERO STABILITY

As the record car accelerates towards the 600mph mark

the desert actually fluidises at the wheel contact points

The arid ground of Hakseen Pan in South Africa. At lower speeds the solid wheels will sink 7mm to 8mm into this surface