Freedom 21 Rudder Design Notes Summary of Rudder Design/Performance Information
can “cut-of” “cut-of” up to around around 10% 10% o the the rudder rudder Rudder T/E Cut-o Eect One can trailing edge chord with no appreciable efect on drag but with a signicant increase in lit. For exaple! a "% trailing edge chord cut-of on an #$$ aerooil produced a 10% increase in lit and a higher stall angle or the oil. (Ref 1 – p223) &he &he best best '() ratio is gi*en b+ a cross Rudder Ti! S"a!e #Cross Section$ section tip shape consisting o “,nie blade” edge! next best is a siple suare cut tip worst a rounded tip. (Ref 2 - p 52) $lso the tip tip cross section has an in/uence in/uence on the tendenc+ tendenc+ o the rudder rudder to *ibrate *ibrate at high speed. speed. $ suare suare cut tip has a o relati*e *ibration aplitude 2$3 o 1.0! a 4" included angle tip section rudder has an 2$ o 0.45 and a 50 o included angle tip section rudder has an 2 2$ o 6ero. 7oor tip shapes can ha*e 2$ *alues as high as 5.8. (Ref 3 - p128) suare cut cut tip prole prole wor,s well well Rudder %&ade Ti! Pro'&e #P&anform$ $ siple suare as does a tip prole consisting o a shallow elliptical tip o*er the rst 9"% o the chord blending into a suare tip. Other “cle*er” tip end shapes tend to increase drag. (Ref 2 - p51 3 &he leading edge edge sweep angle should should be iniised iniised : preerabl+ preerabl+ no ore than 10o and the rudder blade taper ratio should be in range 0.4 to 0.;. (Ref 4 - p87) Rudder (rea
alanced rudder blades iniise tiller loads and a,e steering easier : best balance ound to be around 19%. Ref 5 - p80) (s!ect Ratio “#oral” aspect aspect ratio or or sailing boat boat rudders is ?.? to 5.". 5.". (Ref 5 – p77)
#$$ 4-digi 4-digitt sip siple le oil sectio sections ns are best best sinc since e the higher higher (erofoi& (erofoi& S"a!e S"a!e #$$ perorance lainar /ow sections are *er+ critical o anuacturing accurac+ and are not as “robus “robust” t” in peror perorance ance ters toleratio toleration n o non-opti non-optiu u surace surace roughness roughness ouling. $lso the leading edge prole prole and accurac+ o the rst rst 5"% o lainar /ow /ow blade sections are are critical. &he best copro coproise ise o highest highest lit angle! best lit coe@cient and iniu drag is gi*en b+ a oil with a thic,ness(chord ratio in the range A% to 1?%. One should consider consider A% as the iniu iniu thic,ness(chord thic,ness(chord ratio ratio or a rudder rudder.. &he 4-dig 4-digit it #$$ section sections s also also gi*e gi*e a ore ore gradu gradual al stall stall characteristic. e 43 References
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1. 2. 3. 4. 5.
CarchaD $! Aero-Hydrodynamics of Saiin!! $dlard oles! 'ondon 1A88! =<># 0??A-1185"-;. Eutelle 7! "#e $esi!n of Siin! %ac#&s ! Cacillan! 'ondon 1A84! =<># 0-5555??;81. 'arsson ' liasson ! Principes of %ac#& $esi!n ! $dlard oles! 'ondon ?000! =<># 0-91;5-"181-4. 2acanti )! 'ee Rdder $esi!n ! 7roessional >oatbuilder Caga6ine! Gune(Gul+ ?00" Eerr )! oatbuilder Caga6ine! )ec(Gan ?00;
Com!arison of t"e Freedom 21 Rudder )it" Rudder Design Recommendations C"aracteristic
udder $rea
RecommendationFreedom Rudder
8% to 10% o &'7 1?.A% #oral Fin Heel3 15.?% IH &ande Heel3 1".0% IH &win 'it Heel3
udder >alance $spect atio &aper atio
19% optiu ?.? to 5."
?.5
0.4 to 0.;
0.9
'eading dge
10 degrees ax.
Ciniise 2$
19%
10 degrees <uare cut 2$ 1.03 : worst case 2$
2$ denotes “relati*e *ibration aplitude”3 $erooil
#$$ 000A or 0010
7roduction F?1 rudder section
N*TES
Fro the abo*e table we can conclude that the Freedo ?1 rudder is a relati*el+ large! highl+ tapered rudder! with excellent balance and an appropriate degree o leading edge sweepbac,. = intend to a,e + replaceent liting rudder or + IH twin lit ,eel F?1 to the standard F?1 planor to an #$$ 000A section with a 4" o degree ,nie-edge “*ee” cross section to the rudder tip and a "% trailing edge “cut-of”. = also intend to t a sall anti-ca*itation plate to the rudder to pre*ent entrainent o air at high speed which can e*identl+ reduce rudder lit and cause rudder *ibration to de*elop.
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= ha*e easured a nuber o F?1 rudder blades and the attached s,etch gi*es these diensions. 7erhaps these notes will be o interest to other F?1 owners planning to replace their rudder bladesJ
Kil >ishop )$L)$C :
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