Aerodynamic balancing.docx

Aerodynamic balancing The ways and means of reducing the magnitudes of Chαt and Chδe are called aerodynamic balancing. The methods for aerodynamic balancing are:

1. Set back hinge, 2. orn balance and !. "nternal balance. Set back hinge or over hang balance "n this case, the hinge line is shifted behind the leading edge of the control #see u$$er $art of %ig.&.&'. (s the hinge line shifts, the area of the control surface ahead of the hinge line increases and from the $ressure distribution in %ig.!.! it is e)ident that *Chαt *and *Chδe * would decrease. The o)er hang is characteri+ed by cb  cf  .%igure &.& also shows ty$ical e-$erimental data on )ariations of Chα and Chδ with cb  cf. "t may be added that the changes in Chα and Chδ also de$end on #a' ga$  between nose of the control surface and the main surface, #b' nose sha$e and #c' trailing edge angle #%ig.&.a and b'.

Horn balance "n this method of aerodynamic balancing, a $art of the control surface near the ti$, is ahead of the hinge line #%ig.&./a and b'. There are two ty$es of horn balances 0  shielded and unshielded #%ig &./a'. The following $arameter is used to describe the effect of horn balance on Chα and Chδ.

arameter#( arameter#(reaof reaof horn'3#meanc horn'3#meanchordo hordoff horn'#(rea horn'#(reaof of control'3#me control'3#meancho anchordof  rdof  control' %igure &./b shows the areas of the horn and control surface. %igure &./b also shows the changes 4Chα and 4Chδ due to horn as com$ared to a control surface without horn. orn balance is some times used on hori+ontal and )ertical tails of low s$eed air$lanes #see %ig.&./c'.

Internal balance or internal seal "n this case, the $ortion of the control surface ahead of the hinge line, $ro5ects in the ga$ between the u$$er and lower surfaces of the stabili+er. The u$$er and lower surfaces of the $ro5ected $ortion are )ented to the u$$er and lower surface  $ressures res$ecti)ely at a chosen chord wise $osition #u$$er $art of %ig.&.6'. ( seal at the leading edge of the $ro5ecting $ortion ensures that the $ressures on the two sides of the $ro5ection do not e7uali+e. %igure &.6 also shows the changes 4Chα 4C hα and and 4C 4Chδ hδ due due to inte intern rnal al seal seal bala balanc nce. e. This This meth method od of aero aerody dyna nami micc  balancing is com$le- but is reliable. "t is used on large large air$lanes to reduce Chα and Chδ.

Remark: Tab Tab is also used for aerodynamic balancing. Tabs – Introductory remark The The meth method odss of aero aerody dyna nami micc bala balanc ncin ing g desc descri ribe bed d earl earlie ierr are are sens sensit iti) i)ee to fabrication defects and surface cur)ature. ence, tabs are used for finer ad5ustment to make the hinge moment +ero. Tabs are also used for other $ur$oses. ( brief  descri$tion of different ty$es of tabs is gi)en in the following subsections. Trim tab "t is used to trim the stick or bring Ch to +ero by tab deflection. (fter the desired ele)ator deflection #δe' is achie)ed, the tab is deflected in a direction o$$osite to that of the ele)ator so that the hinge moment becomes +ero. Since the tab is located far from the hinge line, a small amount of tab deflection is ade7uate to bring Che to +ero #%igs.2.1&a and b'. (s the lift due to the tab is in a direction o$$osite to that of  the ele)at ele)ator or,, a slight slight ad5ust ad5ustmen mentt in ele)at ele)ator or deflec deflectio tion n would would be needed needed after  after  a$$lication of tab. Though the $ilot subse7uently does not ha)e to hold the stick all the time, the initial effort to mo)e the control is not reduced when this tab is used.

Link balance Tab "n this case the tab is linked to the main control surface. (s the main surface mo)es u$ the tab deflects in the o$$osite direction in a certain $ro$ortion #%ig.&.11'. This way the tab reduces the hinge moment and hence it is called 89alance tab.

Servo tab "n this case the $ilot does not mo)e the main surface which is free to rotate about the hinge. "nstead the $ilot mo)es only the tab as a result of which the $ressure distribution is altered on the main control surface and it attains a floating angle such that Ch is +ero .The action of the tab is like a ser)o action and hence it is called 8Ser)o tab. This ty$e of tab is used on the control surfaces of large air$lanes. Power boosted and ower oerated controls and !ly"by"wire

The control force increases in $ro$ortion to the cube of the linear dimension of the air$lane and to the s7uare of the flight )elocity. Conse7uently, a low )alue of Chδ is re7uired, to restrict the control forces within human limits. "t may be as low as ;.;;;2. This is not achie)able consistently due to sensiti)e de$endence of Chδ on

uncertainty in fabrication. The alternati)e systems for o$eration of controls are as follows. #a' ydraulic $ower boosted systems in which the effort of the $ilot is boosted by a hydraulic system. #b' ower o$erated systems in which the mo)ements of the $ilot alter settings of  electrical electronic systems which in turn cause the mo)ement of the controls. This led to fly
%lyhen they mo)e in the same direction, they $ro)ide $itch control and when they mo)e in different directions they $ro)ide control in roll.

%– tail "n some some olde olderr air$ air$la lane ness the the func functi tion onss of hori+ hori+on onta tall and and )ert )ertic ical al tail tailss were were combined in a ?
, where @A is the free free stream @ach @ach number. number. Thus, Thus, C=α and in turn the tail effecti)eness decreases significantly at high @ach numbers. ence some military air$lanes ha)e two moderate si+ed )ertical tails instead of one large tail. %or e-am$le see @"B<26@. This has a higher )alue as com$ared to the configuration with single )ertical tail located in the $lane of symmetry.