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New C.G. location, FP101 Aircraft

Over the past few months, we have received an occasional report from builders who have just completed and test flown FP101 Aircraft, that the aircraft required an extraordinarily long take-off run to become airborne and that climb performance was very poor. In addition, one aircraft has been reported to stall at 5 mph below its maximum cruise speed.

These incidents, it should be noted, have been very few in number and not peculiar to any one part of the country. The vast majority of FP101 aircraft completed and flying to date, fly very well indeed and meet or exceed published take-off and climb figures.

We at Fisher Flying Products, Inc., however, are primarily concerned with the quality and safety of the aircraft we produce, and even one report of abnormal flight characteristics from one of our customers is cause enough for us to search and test until we establish why that particular aircraft does what it does. Attempts to duplicate the above described flight characteristics at the factory level were unsuccessful until a recent incident involving a factory test FP101 shed light on the problem.

The aircraft involved is a standard FP101 except that it is slightly heavy, (approx. 280 lbs. because of a very heavy paint job and a 9 lb. seat), and is equipped with optional brakes and electric start. The aircraft had been balanced using the prescribed CG balance point at the aft gear leg attach fitting. This aircraft performed normally during approx. 10 hours of test flights until the horizontal stabilizer was removed and replaced with a test horizontal stabilizer, 1 1/2 pounds lighter than the original.

The next attempt to fly this aircraft resulted in a take-off run over 3 times the normal distance, and when finally airborne, a climb rate of near 0 ft. per minute!! Further calculations revealed that the CG of this particular aircraft exceeded the forward limits of the CG envelop. by a considerable amount, and an extreme nose heavy condition, existed. It also became apparent, that most FP101 aircraft, when balanced using the prescribed CG point, were flying in the forward portion of the CG envelope, and minor variations in pilot weight, seat position, fuel load, etc. could easily result in a nose heavy condition.

The airfoil used on the FP101, as installed on this aircraft, has a design center of lift at 30% M.A.C. The following procedure should be followed to insure that the CG balance point used conforms to the center of lift.
1. From the leading edge of the wing, measure aft 14 1/4 and mark on bottom of wing this location.
2. Raise the tail of the aircraft until the bottom of the wing at the root is horizontal as determined by a spirit level.
3. Using a plumb bob, plumb downward from the 14 1/4 mark and mark the bottom of the fuselage where the line crosses the bottom longeron.

4. This is the new CG Balance point and in most instances will fall approx. 3/4 1 behind the rear landind gear attach point.

5. Balance aircraft on the new point with with all flight equipment installed, pilot on board in the flight position and fuel tank empty. The aircraft should balance level under these conditions, nose down if the pilot leans forward, and nose up if the pilot doubles his legs and leans back.