23

www.balloonlife.com

03.2001

InBillBird’sarticle,High Wind Tech-
niques
,elsewhereinthisissue,hetalks
aboutFalse Heavy. As far back as 1987 an
issue ofBalloon Life contained anarticle
titledFalse Drop (Hangar Flying),the
termpromptedletterstotheeditordis-
agreeingwiththeterm“FalseDrop”or
“FalseHeavy.”TheHangar Flying ar-
ticledealtwithapilot’sexperienceof
tryingtolandhisballoonwitha strong
lowlevelwind shear thatsomehowseemed
to“suck”himintothegroundasheen-
tered it.
What we are going to look at here are
situations thatcan cause aballoon to “drop”
caused by low level wind shears. It should
be notedthat these situations canhappen
atanyaltitude,butthattheeffect,of
hittingtheground, are most pronounced
near the surface.
Theelementsorsituationsthatwe
are going to look at are; False Lift, dimin-
ishedcapacityofenvelopefrombeing
“struck”byaforce,lossofheatdueto
conduction,low leveljetstreampulling
air out ofthe balloon, and air flowoverthe
lower portionof ballooncreatingdown-
ward lift.

False Lift
First thing that we want to state is that
there is nothing false about lift. Lift is lift
whetheritisgeneratedbythepressure
differential between the air inside the bal-
loonandthe air outside or by the flow of
air over the topof the balloon.
In your training youwere told about
theeffectsof air flowing over the top of
theenvelope. While the balloonis static

on the ground the shape of the top forces
theflow ofair tocompressoverthetop
creating a lowpressure area (seefigure 1).
Thislowpressureareacreatesliftthe
same wayanairplane wing does.
As the balloon takes off and acceler-
ates to the speedof the air massthis flow
of air over the top diminishes. Thusany
lift createdby it is no longer available. If
youwereatequilibriumbeforetakeoff
and havenot changed the density ofthe air
insideyourballoonyouwillnothave
sufficientlifttostayinthe air,because
you have lostpartof your lift.
Let’s take a look at why this happens.
While you are static onthegroundthere
are two lift forces at work. The first is the
heating of the air inside the envelope. The
second is the flow of air over the top. The
lift from each of these two elements com-
bines tocreatethelift necessarytobe at

equilibrium.Witha littlemore heatyou
are able torise.
What you have been taught is that the
air flow over the top is false lift because it
wasnotcreatedbyapplyingheattothe
envelope. Thisis correcttoa point. The
lift is real and aslong asthe speedof the
airflowandtheballoonremainsuffi-
ciently different the lift will continue to be
generated.Thisiswheretheproblem
arises, it’s like going to a Chinese restau-
rantfor dinner, the foodisgreat andfills
youupbutanhourlateryourhungry
again. The same thing happens here only
sooner. As you accelerate you lose the air
flow lift and must add more heat to main-
tainthesamerateofascent(thesame
holdstrue at equilibrium or in a constant
rateofdescent).Thereisnothingfalse
about this lift, it is real, it just doesn’t stay
withyouandyou needtoreplaceitwith

by Tom Hamilton

Nothing Is False Revisited

The air around a balloon can create upward and
downward lift as well as take away the lift you
generate with heat.

IMAGE nothing010301.gif

Figure1

www.balloonlife.com

03.2001

24

somethingelse like heat.
Nowlet’s take alook at what happens
witha low level jet stream (see figure 2).
As you are flying along and the top of the
balloon is in a faster movinglayer of air.
You are at equilibrium withtwo compo-
nents of lift, heat and air flow. You would
liketolandsoyouallow the balloonto
cool or vent to begin a descent. As the top
oftheballoonmovesoutofthefaster
movingairyoulooseoneofyourlift
components. This combined with the de-
scentyouinitiatedacceleratesyourde-
scent. Youarenow movingmuchfaster
than planned.You apply heat but the close
proximitytothegrounddoesn’tprovide
sufficienttimetorecoverandpow,you
plantone.Backintotheairyougo,a–
swing’n.
Your totallyembarrassedandwon-
derwhatiswrongwithyour pilotskills
today?Ifyouarenotawareofthelow
levelwindshear youwillprobablyjust
passitoffassloppyflying.Andthis
doesn’t have to happen closeto theground,
itcan just as easilyhappen at 3,000 feet,
onlyyou recover before hitting the dirt.

Diminished Capacity
Thesecondwayinwhichawind
shear can increase your rate of descent is
by diminishing the capacity of your enve-
lope. Our topic ofdiscussion here is about
gettingpulled down, but againthis situa-
tioncan happengoingup just as easily.
Flyingalongyouchoosealanding
spot and begin to descend. Below is a low
levelwindshearwiththeairnearthe
surface movingmuchfasterthanthe air
mass you are in. As you enter the lower air
massitislikegettinghitwithasucker
punch(figure 3). Itknocks inthe side of

theenvelopedecreasingthe capacityof
theenvelope andpushingthe air out the
mouth. The bigger the differencebetween
the winds of the two airmasses the greater
the effect.
Nowyou no longer havethe lift capa-
bility that you had a few seconds ago. You
beginheatingbut againthereisn’tsuffi-
cienttimebefore yousmackthe ground
and recoilback into the air, a–swing’n.
Thiscanalsohappenwhengoing
from a faster moving air mass to a slower
movingairmass.Thistimeinsteadof
getting“hit”inthebacktheenvelopeis
“hit” inthefront,bythewallofslower
moving air. The effect is the same. Again
thiscanhappen at any altitude.
A 1991article,Landing Flight Path
Through Low Level Shears
,byFrank
Hines,expanded on this topic.Hines com-
paredthe effectsbetweenlightlyloaded
and heavily loadedballoons.
If a balloon descends abruptly from a
30 mphwind into a 15mphwind, it will

experience an effective abrupt increase in
wind across its surface from no wind to a
15 mph wind.This will remove thebound-
arylayeronitssurfaceandgreatlyin-
creaseheatlostbyconduction(seedis-
cussion below), while causing distortion
inthe form of a “dish.” Thisconditionis
very dangerous to the low level flight of a
fullyloadedballoon. Itshouldbe noted
that as the envelope lowers into the slower
windandbeginstodistortandslow-up,
theeffectivewindspeedoverthetop
beginstoincrease.Airmovingover the
top of an envelope produces false lift and
there could be no worse time for this to be
happening.
A “dish”usuallydoesn’taffectthe
flight pathof a lightlyloadedballoon as
muchbecausenointernalliftingheatis
dumpedbutisonlyredistributedinside
thedistorted envelope. Heatlossisonly
by conduction and won’t be too large but
must be replaced quickly. Since a lightly
loadedenvelope hasitsheatuphighin-

IMAGE nothing010302.gif

Flying with the top part of
the envelope in a faster
moving layer of air adds a
second component of lift.
This is caused by the ven-
turi effect over the top of
the balloons.

Figure2

IMAGE nothing010303.gif

Figure 3

25

www.balloonlife.com

03.2001

sideandthefabricbelow theequatoris
loose and non-taught, momentary closing
of the throat happens easily.
To understandwhata “dish” can do
tolift,weneedtounderstandhowthe
“heat line”fluctuates under different load-
ingconditions.Manycommercialbal-
loonpilotstaketheirambienttempera-
ture, maximum altitude desired, and maxi-
mumsustainedinternaltemperaturede-
siredandgototheirballoon’sperfor-
mance curves and interpolate their maxi-
mum lift. They subtract the fueled system
weightandtheirweightandthenadd
passengerstogettoallowedmaximum
gross weight. This is done to increase the
profit margin ofthe flight. This is legal but
not wise. Responsiveness has been turned
intocash.Thisresultsintheenvelope
reaching its maximum sustained internal
temperature andneedingmostof its vol-
ume filledwith less dense hotair togen-
eratethelifttogetofftheground.A
lighter-than-airballoon rises consequently
asteelshipcanfloatthoughit’smade
from heavy materials. The ship floats be-
causeitweighslessthanthevolume of

water it displaces. A balloon “floats” in
the air because the hot air inside it weighs
less than the volume of air it displaces.
The bottom of the heated air is down close
to its mouth. We all experience this in
summer if we put two passengers in an
AX-7 and hit a shear. You can feel the
envelope “belch” warm air into your face

and you know you have to add some heat.
Noticethatanormaltwo-secondsingle
burnonaheavilyloadedballoononly
adds a small percentage of heat compared
toitstotalneededhotairvolume.The
sameburn in a lightly loaded balloon adds
a muchlarger percentageof liftingheat
comparedto itstotal needed hotair vol-
ume.Thisisasimplewaytovisualize
responsiveness. See figure 4.
If a heavily loadedenvelope experi-
encesamajor“dish”,importantlifting
heat can be squeezed out and heat lost by
conduction.Hineswrites,“Thislifting
heat dumping will be at the throat but also
could be at the cap seal on parachute tops.
My experience has been that the dumping
is quick and momentary.” This condition
isextremely serious if youare on a final
approach to landing because you will not
have enough altitude to add enoughheat.
Rememberthatnotonlymusttheheat
loss be replaced to make the balloon neu-
trally buoyant, but more must be added to
stop any downward momentum. As stated
earlier,alightlyloadedenvelopeonly
displacesitsinteriorheatasshownin
figure 5, but will still lose heat by conduc-
tion.Itwillregainitsshapeandthena
reasonable amount of heat will need to be
added. When you feel agood shearin your
face, always check the throat opening and
add some heat immediately. (Hines found
thatheatlossbyconductionwascon-
firmed by a balloon with a scoop tethered
insome wind. The scoop pressurizes the
internalvolume slightlybut the burn ca-
dence increases greatly compared to free
flight. This is to replace heat lost onlyby

IMAGE nothing010304.gif
IMAGE nothing010305.gif

HeavilyLoadedEnvelope

LightlyLoadedEnvelope

Figure4

Effective volume of
one burn is a smaller
percentage oftotal
volume of heat.
Therefore balloon is
less responsive

Effective volume of
one burn is a larger
per centage of total
volume ofheat.
Therefore balloon is
more responsive

Bottom ofheat line in a
fully loaded balloon

Bottom of heat line
in a lightly loaded
balloon

Figure5

Conduction
heat loss

Heavily
Loaded

Major heat
loss through
exhaliation

Heat line

Heat line

Lightly
Loaded

Conduction
heat loss

Lowered
heat line

www.balloonlife.com

03.2001

26

conduction. See figure 6.)
Heatlosscanchangetheslopeof
your approachandunknowinglymake it
steeper.Seefigure7.Thestrongerthe
shear,thegreaterthechangeinslope.
Knowledge of this andof the importance
toaddheat quicklycouldpreventanex-
cessivelyhard landingor an accident.
Some signs of shears to watch for are
anymovementof crownlines,handling
lines,throatropes, or skirt; windin your
face;or evenbasket movement.It isim-
portant to realize that as the balloon low-
ersintotheslowermovingair,distorts,
and starts slowing, the effect of heat loss
canbe maskedbyfalselift.Evenif the
shear is a mild one, false lift will momen-
tarilybethere.Whentheballoonslows
enough and gets out of the faster moving
air, the hidden false lift and dishing disap-
pear and downyou golackingheat.

Conductive HeatLoss
To understand what is or what could
happen to the envelope, we need to look at
what the envelope experiences in terms of
“effective wind” over its surface that re-
sultsinheatloss.Secondly,weneedto
understandwhat a “dish” in the envelope
could doto its lift, discussedabove. The
above two effects of descending through
ashearcould happen together, in theworst
situation, or oneof them alone. Severity
dependsonloadingconditions,fabric
porosity, speed of descent, and last but not

least, strength of wind shear. Technically
the mechanisms of heat loss are called
heat loss by conduction and heat loss by
volume dumping, respectfully. Heat loss
by conduction is actual h eat passing
through a media that in our case is the thin
envelope fabric and the fabric coating.
Due to the complexities of heat loss in a
porous envelope, our discussion will be
limited to relatively new envelopes with
negligible porosity. The amount of heat
loss by conduction, therefore, is largely
dependent on envelope surface area, the
difference of inside temperature and am-
bient temperature, and the speed of the
wind blowing across the envelope’s sur-
face. Since we are looking at wind shear
effects to a landing balloon, then wind

speedis our dynamic culpritandsurface
area andtemperaturedifferencescanbe
consideredconstantandignoredfor this
topic.
Tostart, we needtolookatwhatis
happening to the heat inside the envelope
during normal level flight. A balloon fly-
ingatlevelflightexperiencesnonetair
flowacross its surfaceand thereforeexpe-
riencesminimumheatloss.Whatheat
lossitexperiencesis duetoconduction.
The envelope has a thin boundary layer of
warmerthanambientaironitssurface
that actsas a blanket and helps keep heat
inside.Thisisanaturalphenomenaof
physics. Human skin alsohas a warm air
boundary layer on it when it is in a colder
environment. In both cases, the layer can

IMAGE nothing010306.gif

Figure 6

Heat loss

Wind

IMAGE nothing010307.gif

Figure 7

Faster Wind

Sheer Line

Slower Wind

Slope
change

Area where heatlossis
temporari ly masked by
false lift

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03.2001

28

IMAGE nothing010308.gif

Bernoulli’s
Principle

The pressure of a fluid (liquid or
gas)decreasesatpointswherethe
speedof the fluid increases. Known
as Bernoulli’s Principle, discovered
more than200 yearsagoby the sci-
entistBernoulli, this law ofphysics
helpsustounderstandwhyliftis
createdas air flows over and around
thesurfaceofaballoonenvelope.
WhatBernoulli discovered was that
within the same fluid, in this case air,
highspeedflow isassociatedwith
lowpressure,andconversely,low
speedflowwithhighpressure.The
principlewasfirstusedtoexplain
changes in the pressure of fluid flow-
ingwithinapipe whose cross-sec-
tional area varied (venturi tube). The
fluidmovesatalowspeedinthe
wide sectionofthe pipe,butasthe
pipe narrows it must contain thesame
amount of fluid. In this narrow sec-
tion,thefluid moves at highspeed,
producinglow pressure (the venturi
effect).
Withtheballoon,airisforced
aroundthecurvedsurfaceof abal-
loonlikethatofanairplanewing.
The airfoil ofa wingisdesignedto
increasethevelocityoftheairflow
aboveitscurvedsurface,thereby
decreasingpressure. The same phe-
nomenon takes placewith the curved
surface ofthe balloon. Asthe air is
compressed to go around the surface
it speeds up and pressure is decreased.
Thustheobject,balloon,hasliftin
the direction of the low pressure.

be removed by wind. This makes the skin
loseheatrapidlybyconductionandwe
feelcolder. Thisis whatthe term “wind
chill factor” means that the weatherman is
alwaystalkingabout. Wealso cover our
heated fuel tanks to add insulation to their
boundary layer. This is to prevent the heat
we addedfrom being lost byconduction.
Thermopaneglasspanelsuseboundary
layersonthe inside panelstomaximize
theirinsulation efficiency to keep thecold
outside andthe warmthinside.
Changingair massescreatesa rela-
tive wind over the surface of the balloon.
Thatwindremov esthe“insu lati ng”
boundarylayerofwarmairresultingin
heatlossby conduction.

DynamicLow Pressure
The next element that we will take a
look at is fast moving air across the mouth
of the balloon(figure 8).

theballoonandtheliftgeneratedhasa
downward component.
If we were to take a balloon and place
itin a windtunnel and blow smoke byit
we would notice that the airis compressed
andforcedallaroundtheballoon.We
alreadyknowwhathappenswhenthis
occurs at the top of theballoon, lift with an
upwardcomponent.Thesameelements
are taking place over the entire surface of
the balloon. What happensif this wind is
onlyover the bottomportionof the bal-
loon, below the equator (figure 9)? Lift is
beingcreatedbutthistimethereisa
downward component to the lift.
The lower half of the balloonenters
the faster moving airmass and the surface
of the balloon causesthe air flow to gen-
erate lift. This lift is tangent to the surface
of the balloon and because it is below the
equator, where the tangent line points in a
downward direction, the lifthas a down-
wardcomponent.Thisdownwardcom-
ponent of lift pulls the balloondown.
Again, the effect is going tobe more
severeonafullyloadedballoonthana
lightlyloadedone.Thereasonisskin
tension. Alightly loaded balloon will have
moreslackinthefabriconthelower
portion oftheballoon. On a heavily loaded
balloon the fabric below the equator will
have greater tensionproviding asurface
where a low pressure can develop.
These elements can allcontribute to
the balloonaccelerating downward from
an equilibrium state. The first, false lift, is
a lossof anupwardlift component. The
othersare basicallythe oppositeof false
liftandmaybeforlackof abetter term
have been described as false heavy.
In reality there is nothing false about
any of these situations. They are real and
create real flightdynamics. Whatisim-
portantistobeawareof them,theiref-
fects,andconsider what actionsare nec-
essary when you encounter each. In many
casesmorethanoneoftheseelements

IMAGE nothing010309.gif
IMAGE nothing010310.gif

willbe at work.
Be aware of the elements around you
and how they can affect the performance
of your balloon.
Whathappens if youare caught in a
low level jet? Keep calm. Waiting until a
low level jet raises or taking your time and
using a very gentle slope through a shear
to get below it before landing appear to be
safe considerations.Lookfor abigfield
andexpect ahighwindlanding. Staying
on the ground initially should be your first
and best consideration.

This createsa dynamic low pressure
which can cause the air to be sucked out of
theenvelope.Itislikethediminished
capacity that was discussed above. As the
air moves over the mouth of the balloon it
createsthe dynamiclow pressure which
pulls the static air inside the envelope out.
Againwith diminishedcapacitythe bal-
loonwill accelerate down.
Ofall the possibilities discussed here,
this point will have the least impact on the
liftingcapability of the balloon.

Downward Lift
Similar to false lift discussed earlier,
only now it is happening at another part of

Figure 8

Figure9

Asairflowsar oundthe
lower portionof the enve-
lopeli ftvector sar e
gfenerated tangent to the
entiresurfacein thesame
way that thye are gener -
atedoverthetopofthe
balloon.beacuseofthe
angletheseliftvectors
haveadownwardcom-
ponent.

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