Balanc­ing Act

Model Airplane News - - FLIGHT TEST -

The pre­dom­i­nant fac­tor that de­ter­mines how an air­craft of the same de­sign will per­form and han­dle is the cen­ter of grav­ity (CG). With­out get­ting into aero­dy­nam­ics, the ef­fects of CG are as fol­lows:

For­ward CG

• In­creased lon­gi­tu­di­nal (pitch) sta­bil­ity

• In­creased stall speed

• De­creased cruise speed

Aft CG

• De­creased lon­gi­tu­di­nal (pitch) sta­bil­ity

• De­creased stall speed

• In­creased cruise speed

• Poor stall/spin re­cov­ery

As you may have guessed, train­ers and many scale mod­els ben­e­fit from a for­ward CG. Aer­o­batic pi­lots tend to fa­vor an aft CG due to the in­creased ma­neu­ver­abil­ity. Of course, if taken too far, the plane can be­come so un­sta­ble as to be un­con­trol­lable or un­re­cov­er­able from a stall/spin. No doubt, ex­pe­ri­enced pi­lots have their own method of de­ter­min­ing the sweet spot for their par­tic­u­lar fly­ing style. Pi­lots just get­ting into aer­o­bat­ics may not know where their plane falls in the CG range. The method that I (and many oth­ers) use to get a quick check on the CG is sim­ple (as­sum­ing you can fly in­verted for a short dis­tance): At 3/4 throt­tle, pull into a slight climb and roll in­verted, and see what the plane does. If it im­me­di­ately starts de­scend­ing, it is on the nose­heavy side. If it con­tin­ues on the same path or it climbs, it is on the tail-heavy side. Most pi­lots feel the most com­fort­able when hold­ing a slight amount of for­ward-el­e­va­tor pres­sure when in­verted.

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