THE HOBBISTAR 60 advanced trainer from Hobbico was finished in last month's article, including all the modifications. The firewall was reinforced, as were the nose gear, servo tray, and wing mounts. Separate aileron servos were added for more exact control and flaperon use. Bolting on the wing made for a cleaner and more precise airframe, relegating rubber bands back to the slingshot factory.

    The modifications required a few extra work hours, so now is the time to find out if it was all worthwhile. But before we trek out to the field, it would be best to get everything into proper balance. That does not mean becoming "one with the universe," although that would probably make for better flights. It means making sure the airplane is in balance and that all control surfaces and servos are properly set.
 
    The original plan was to photograph the Hobbistar 60 during the balancing and control-surface-adjusting phases, but conflicting publishing deadlines had the model flying several months before this article was written.

    Sport Aviator (www.masportaviator.com) published a "Test Pilot" report using this aircraft. The usual preflight-adjustment photos were not taken during the initial setup rush for that article.

    I thought I could take the pictures now, but during yesterday's video phase of the test flights I planted the Hobbistar 60 in the corn. Never try an Outside Loop at only 30 feet altitude with the wing flaps deployed—not even for a video. The law of gravity is strictly enforced at our flying field, and picture-taking provides no protection.

    The Hobbistar is a strong airplane and it did survive, but the wing sustained damage that cannot be repaired in time for this article's publication. To learn how to fix a damaged wing, go to Sport Aviator's "Flight-Tech" section. Every tragedy does have a purpose, so the damaged Hobbistar might help other pilots restore their "rekitted" airplanes.

    Before my thumbs turned dumb, all the flight photos and flight tests were performed. The flight part of this series survived. For the preflight checks I'll use another excellent advanced trainer: Hangar 9's Arrow.  

Click on photo to view large image with caption

Balancing Two Times: The instructions for almost all model kits, including RTF and ARF aircraft, detail setting the front-to-back CG. Since the airplane may not survive its first flight if the CG is too far from its proper setting, airplane manufacturers consider setting a proper CG to be important. That is understandable.

    However, there is another CG: the model's lateral balance, which must also be set for any aircraft to fly well. Few manufacturers include setting the lateral balance in their instructions. Although an airplane will survive flights with a poor lateral balance, the pilot will not enjoy the experience.

    I briefly covered setting the front-to-back CG in the second article in this series, about building an RTF. Since this CG is critical for a safe first flight, a more detailed discussion is necessary.

    Why is this CG so crucial? From a pilot's standpoint the aircraft's front-to-back balance determines the airplane's "stability" (colloquial—not engineering—terms are used here) and the effectiveness of all control surfaces—especially the elevator.

    From the aircraft's standpoint this CG determines the balances between thrust vectors, lifting forces, and the control surfaces' force moment arms, among other ratios. If the CG is too far toward the rear, the airplane will be unable to maintain level flight and may become less roll stable. The elevator, if it even has enough authority to control the airplane's pitch, will be extremely sensitive and small inputs will have big pitch results.

    The excessive pitching movements could cause parts of the wing, notably the outer sections, to stall (lose lift). This condition results in wild rolling movements and sometimes even in snap rolls, where the airplane violently rolls and pitches around the CG. The ailerons also provide big results from small inputs.

    The usual result of flying with an excessively rearward CG is neither a satisfying nor happy experience.

    If the CG is too far forward, the elevator might not have enough authority to raise the "heavy" nose during slow flight and landings. Roll rate and rudder effectiveness diminish, and the airplane becomes "sluggish" and boring to fly. Maximum airspeed also drops. The airplane will survive flying with a heavy nose, but the pilot might find installing a new nose gear after every nose-first landing tiresome.

    As with everything else mechanical, an airplane's CG is a compromise that designers agonize over, and they sometimes hold their noses while specifying it. Many designers set a CG range so the pilot can determine the aircraft's CG according to experience level. For many 40- to 60-size sport models, the usual range is 3/16-1/4 inch on either side of the specified CG point. Most new RC fliers would be best served by setting their airplanes' CGs at the most forward points and then working backward as experience levels increase.

    Look in the instruction manual to find the proper CG setting for your airplane. The correct point for the Arrow's CG is cited as 31/8 inches back from the wing's LE.

    Specifying the CG as the distance tailward from the wing's LE is the most common method. Some advanced competition aircraft quote the CG in a fuselage format, but training and sport aircraft seldom do.

    For high-wing airplanes use a felt-tip pen to mark this distance on the wing's bottom, just outside the fuselage. For low-wing aircraft mark the CG on the top of the wing, again just outside the fuselage. You can remove these marks later with alcohol.

Page 1 • 2  • 3 • 4