CIRRUS POWER LEVER

Flying - - FRONT PAGE - By Rob Mark

There’s some­thing to be said for sim­plic­ity when teach­ing peo­ple to fly or merely check­ing out in a new air­craft. Sim­plic­ity was Cirrus’ mo­ti­va­tion be­hind this month’s fea­ture, the com­bined throt­tle and pro­pel­ler con­trol that’s stan­dard on both the Cirrus SR20 and SR22.

In com­plex air­craft, a throt­tle con­trols en­gine ac­cel­er­a­tion, while a sep­a­rate blue pro­pel­ler lever ad­justs the pitch of the pro­pel­ler through a pro­pel­ler gov­er­nor. To­gether, they cre­ate a given power set­ting, and that’s how, in the Cirrus, the old throt­tle con­trol came to be known as a power lever.

On their first glance into the cock­pit of a Cirrus, some pi­lots might think the sin­gle black leather­wrapped han­dle on the throt­tle quad­rant is con­nected to a tur­bine en­gine be­cause there’s no blue pro­pel­ler con­trol knob nearby. The mix­ture con­trol next to the en­gine con­trol is the only real give­away that un­der the hood sits ei­ther a 215 hp Ly­coming or a 315 hp Con­ti­nen­tal re­cip­ro­cat­ing power plant.

Op­er­at­ing an air­craft equipped with a vari­ablepitch pro­pel­ler, also known as a con­stant-speed prop, re­quires train­ing in or­der not to mis­man­age the use of the throt­tle and pro­pel­ler con­trols. Com­bin­ing the throt­tle and the pro­pel­ler con­trol into a sin­gle power lever not only elim­i­nates the need for a sep­a­rate pro­pel­ler con­trol, but it also re­duces pi­lot work­load while help­ing pro­tect the en­gine from dam­age that might be caused by an in­ad­e­quately trained pi­lot. Some pi­lots have ex­pressed

dis­dain for the Cirrus com­bined sys­tem, be­liev­ing it of­fers no con­trol over en­gine ro­ta­tions per minute. The pro­pel­ler is ac­tu­ally con­trol­lable, just over a nar­rower rpm range.

On Cirrus air­craft, the power lever is con­nected to a me­chan­i­cal cam also at­tached to a pair of ca­bles run­ning from the throt­tle quad­rant and out the fire­wall. One ca­ble is con­nected to the en­gine throt­tle, the other to the pro­pel­ler gov­er­nor. By us­ing a ma­chined slot in the cam, the sys­tem me­chan­i­cally moves the pro­pel­ler con­trol as the throt­tle con­trol is moved fore and aft. The cams are slightly dif­fer­ent for the var­i­ous mod­els be­cause travel dis­tance, re­quired rpm changes and other fac­tors vary. Only the SR22 Turbo does not have a ca­ble-con­trolled gov­er­nor, mean­ing the pro­pel­ler is fixed at 2,500 rpm.

The pro­pel­ler gov­er­nor senses en­gine speed us­ing fly­weights and the throt­tle set­ting through a ca­ble con­nected to the power con­trol lever. Mov­ing the power lever for­ward causes the gov­er­nor to me­ter less high-pres­sure oil to the pro­pel­ler hub, al­low­ing cen­trifu­gal force act­ing on the blades to flat­ten the pro­pel­ler pitch for higher-rpm oper­a­tion. Re­duc­ing the power-lever de­mand causes the gov­er­nor to me­ter more high-pres­sure oil to the pro­pel­ler hub, forc­ing the blades to an in­creased pitch or re­duced-rpm po­si­tion. Dur­ing cruise flight, the gov­er­nor again ad­justs the pro­pel­ler to main­tain an rpm set­ting. Any change in air­speed or load on the pro­pel­ler re­sults in a change in pro­pel­ler pitch.

While power set­tings vary some, de­pend­ing upon the Cirrus model, full throt­tle on take­off should pro­duce about 29 inches of man­i­fold pres­sure while spin­ning the pro­pel­ler at 2,700 rpm. As the pi­lot re­tards the throt­tle and de­pend­ing upon the po­si­tion of the cam, the prop be­gins to au­to­mat­i­cally slow, but will not drop be­low 2,500 at high­power set­tings. The be­hav­ior of the prop gov­er­nor at re­duced power is, how­ever, ex­actly the same as with a con­ven­tional prop con­trol set at 2,500. Once man­i­fold pres­sure drops low enough that the pro­pel­ler can’t main­tain 2,500, the rpm will, of course, be­gin to de­cline. On the Cirrus, as power is in­creased from idle to max­i­mum, the pi­lot will feel a slight bump through the han­dle as the power lever moves past the three-quar­ter point. This con­firms the pro­pel­ler has be­gun to move above the 2,500 rpm stop.

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