What is a Constant Speed Propeller

8/17/2019 What is a Constant Speed Propeller

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What Is A Constant Speed Propeller? Part 1: The Basics

by CARLVALERI on OCTOBER 11, 2011

A listener to The Stuck icA!ast recentl" asked ho# aconstant speed propeller #orks$ Be%ore #e ans#er this&uestion #e 'ust understand #h" airplanes ha!e constant

speed propellers and the (asicso% ho# a propeller #orks$

Why The Constant SpeedProp?

The propeller con!ertsrotational po#er %ro' theen)ine into thrust$ Therotational po#er is calledtor&ue$ To (etter understandtor&ue* i'a)ine "our t#istin) a

lid o%% a can o% peanut (utter$ The 'uscles in "our handand ar' con!erted into po#er to turn the peanut (uttercap is called tor&ue$

So'e cans o% peanut (utter are !er" hard to open so #eeither need to use 'ore po#er (" e+ertin) oursel!es orhandin) the ,ar o% peanut (utter to so'eone #ho isstron)er$ Another #a" #e can )et the can open is (" usin)our stren)th 'ore e%%icientl" (" usin) a tool that con!erts

the po#er in our hands and ar's into )reater tor&ue$ -ou'a" ha!e seen such de!ices at a kitchen store or on lateni)ht TV sellin) %or ./$//$ I kno# #hat "our thinkin)$ When a' I )oin) to sa" 0But #ait theres 'ore23 4o# that


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#e ha!e our ,ar o% peanut (utter open lets see ho# thisrelates to our propeller$

Si'ilar to the tool helpin) us open the ,ar o% peanut (utter

the constant speed prop is a de!ice #hich allo#s us toe%%icientl" con!ert po#er %ro' the en)ine into thrust$ Tounderstand ho# the constant speed prop #orks lets %irstre!ie# ho# a %i+ed pitched propeller #orks$

Propeller Basics

Be%ore po#ered %li)ht* pilots #ere %l"in) )liders and so'e

o% the 'ost success%ul #ere %lo#n (" the Wri)ht Brothers$The challen)e in keepin) these earl" air 'achines alo%t #asin ho# the" could 'o!e the plane throu)h the air under itso#n po#er thus producin) enou)h air speed o!er the #in)s to sta" alo%t$

an" earl" atte'pts at po#ered %li)ht %ailed (ut aco''on the'e a'on)st the earliest %l"in) 'achines #as a

de!ice called a propeller$ This de!ice had t#o s'all #in)sattached to a rotatin) hu( on the %ront o% the %l"in)'achine rotatin) !erticall"* 'o!in) air ,ust as a %an 'o!esair$ A %an 'ust (e attached to a (ase or it #ill %all %or#ardsince the air (ein) pulled throu)h the %an produces a%or#ard %orce called thrust$

The Wri)ht Brothers #here success%ul in placin) an en)ineon an aircra%t and con!ertin) the po#er to turn thepropeller and produce enou)h thrust to cause the aircra%tto take o%% under its o#n po#er$ We ha!e co'e a lon) #a"since those earl" da"s (ut the (asics o% propeller operationare the sa'e$


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A propeller (lade is desi)ned si'ilar to a #in) (ut it 'ust #ork #hile rotatin) a(out a center hu($ Si'ilar to a #in)the propellers 0cord3 is an i'a)inar" line dra#n %ro' thecenter o% the leadin) ed)e to the rear o% the propeller$ The0(lade an)le3 is a 'easure'ent o% the distance (et#eenthe propeller cord and the plane o% rotation* #hich is %i+edin one position on 'ost airplanes$ When #e speak a(out

propellers thou)h #e dont talk a(out the (lade an)le #etalk a(out the pitch o% the propeller$

Propeller 0pitch3 is the distance in inches #hich thepropeller #ould scre# throu)h the air in one rotation$ When chan)in) the (lade an)le #e chan)e the pitch o% thepropeller and use the ter' controlla(le pitch propeller$There%ore #hen #e chan)e the (lade an)le #e are

chan)in) the distance the propeller #ould scre# throu)hthe air and are controllin) the pitch o% the propeller$There%ore #e call the' controlla(le pitch propellers$ 4ote:i% "ou are seekin) "our %li)ht instructor ratin) it is a )oodidea to understand the di%%erent ter's pitch and (ladean)le$


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Why Use A Constant Speed Propeller

ost sin)le en)ine aircra%t are e&uipped #ith a %i+ed pitchpropeller #hich is e%%icient at a speci%ic %or#ard speed$

4or'all" the propeller desi)n o% a %i+ed pitch propeller is aco'pro'ise o% e%%icienc" (et#een cruise and cli'($ Thereason a propeller is 'ost e%%icient at a speci%ic speedthrou)h the air is due to 0propeller slip3$

Propeller slip is the di%%erence (et#een the theoreticalpitch* )eo'etric pitch* and the actual pitch* e%%ecti!e pitch*

o% the propeller$ An e%%icient propeller is one #hich has theleast a'ount o% slippa)e and there%ore con!erts 'oreen)ine horsepo#er into thrust$ A constant speed propellerkeeps the propeller pitch ad,usted %or 'a+i'u' e%%icienc"%or 'ost conditions o% %li)ht$

Takeoff

5urin) takeo%% #e need the 'ost po#er and thrust$Re'e'(er #e are not 'o!in) throu)h the air !er" %astand a lo# (lade an)le does a %e# thin)s$ 6irst* it keeps the An)le o% Attack s'all and e%%icient #ith respect to therelati!e #ind$ Second the en)ine turns at a hi)her RPand there%ore con!erts the 'ost %uel into heat ener)" %or a


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)i!en ti'e$ Third* the lo# pitch causes a s'aller 'ass o%air to (e handled (" the propeller (ut the hi)h RP andlo# speed causes 'a+i'u' thrust$

Climb

As #e li%to%% and cli'(* the speed o% the airplane increasesand auto'aticall" chan)es to a hi)her an)le* a)ain )reaterpitch$ The chan)e in the (lade an)le increases the airhandled in one re!olution$ The ne# (lade an)le #ill keepthe RP constant durin) the initial cli'($

A%ter the initial cli'( and at a sa%e altitude the pilot #illselect a po#er settin) to initiate the re'ainder o% the cli'(to altitude$ This is done (" %irst reducin) the po#er ("decreasin) the 'ani%old pressure$


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4e+t #e 'ust increase the (lade an)le and lo#er the RP$ A )ood rule o% thu'( is to 'ake sure the 'ani%oldpressure is less than or e&ual to the propeller RP (ut o%course "ou #ill re%erence "our en)ine operatin) 'anual tore%erence the proper settin)$

As "ou cli'( to altitude in a nor'all" aspirated* non tur(o

char)ed en)ine* "ou #ill need to constantl" increase "our'ani%old pressure as "ou cli'( since the air is less denseas "ou cli'($ I% "ou cli'( to hi)her altitudes the throttle #ill e!entuall" need to (e increased to 'a+i'u'$ A%ter thethrottle is at 'a+i'u' po#er settin) there is not 'uch "ou can do* e+cept (u" a tur(o char)ed en)ine$

Cruise

Le!elin) o%% at cruise altitude let the airplane accelerate tothe desired airspeed$ Since "ou #ill no lon)er need thee+cess po#er to accelerate* reduce the po#er$ Reduce ("%irst reducin) the 'ani%old pressure and then increase the (lade an)le to decrease the rp'$


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A%ter e!er"thin) has sta(ili7ed in cruise and (e%ore "ouco'plete "our cruise checklist this #ould (e a )ood ti'e to !eri%" the proper po#er settin) and lean the en)ine$5urin) "our pre%li)ht "ou esti'ated a speci%ic true

airspeed* TAS* and a po#er and %uel (urn$ ost airplanesha!e a &uick re%erence chart a!aila(le to !eri%" the cruisesettin)$ I% "ou dont ha!e one I think it is a )ood idea to'ake one and la'inate it %or &uick re%erence$

Descent

5urin) a descent "ou 'ust reduce the po#er and point the

nose o% the plane do#n#ard$ This #ill nor'all" increasethe airspeed #hich the propeller #ill auto'aticall" ad,ust%or and increase the (lade an)le$

In the ne+t part in the series on ho# a constant speedpropeller #orks #e #ill (e talkin) a(out the 'echanics o%ho# the propeller #orks$ But %or no# kno# that there are'echanical stops that pre!ent the (lade an)le to (e

increased (e"ond a 'a+i'u' settin) #hich #ould occurdurin) hi)h !elocit" descent$ There%ore e!en #ith thepo#er set at a 'ini'u' "ou #ill e!entuall" see anincrease in rp' as "ou increase the airspeed$

As "ou slo# %or landin) the pitch o% the (lade #ill decreaseuntil reachin) a 'echanical stop$ Be%ore "ou land "ou #illneed to place the propeller control in the lo# pitch andhi)h rp' settin) (ecause "ou 'i)ht need the po#er %or a)o around$ I% #e ne!er did )o arounds "ou could lea!e theprop control alone until a%ter landin)* (ut #e all kno# thisis not a )ood idea (ecause #e all plan to )o around one!er" landin)$ Ri)ht?


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With that said as "ou (eco'e 'ore e+perienced in %l"in) aconstant speed prop "ou #ill notice that "ou #ill (e a(le toset the RP durin) cruise and ha!e the &uiet sound o% alo#er rp' settin) all the #a" do#n to touch do#n$ As "ou

slo# the aircra%t the an)le o% the propeller (lade decreasesuntil it reaches its stop$ To add to passen)er co'%ort andto (e a s'oother pilot "ou should )et to kno# #hatairspeed the propeller #ill contact the 'echanical stop #hile the throttle is at a lo# po#er settin)$ I% "ou ad!ancethe prop le!er to 'a+ RP "ou #ill not hear an" chan)e inRP #hich 'eans "ou are at the 'echanical stop$ 5o thisand "ou and "our passen)ers #ill appreciate the lack o%noise created (" the chan)e %ro' a ha' %isted pilot$

4o# that "ou kno# ho# a constant speed propeller #orks%ro' an operators perspecti!e lets see ho# the 'echanicalde!ice o% 'o!in) the propeller #orks$ We #ill look at (othconstant speed propellers and %ull %eatherin) propellersused on t#in en)ines$ 5o "ou kno# #h" #e #ould #ant a%ull %eatherin) propeller on a t#in and not on a sin)le

en)ine aircra%t? Sta" tuned %or Part 8 The echanics o%The Constant Speed Propeller$

Sa%e 6l"in)2

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What is a Constant Speed Propeller