Limited Flying Qualities Evaluation Of The American Falcon

Jim Payne

Originally published March 1994

American Falcon Sailplane

Introduction
Test Item Description
Test Objective
Test Methods
Exterior Inspection
Cockpit Evaluation
Takeoff
Tow
Approach to Stall Investigation
Stalls
Longitudinal Static Stability Investigation
Maneuvering Flight Stability
Steady Heading Sideslips
Roll Performance
Dynamics Investigation
Thermaling
Landing Pattern
Landing
Recommendations
Overall Impressions

Introduction

This report presents the results of a limited flying qualities evaluation of the American Falcon that I conducted on January 08, 1994. I flew two flights totalling 1.3 hours at California City Airport, California.

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Test Item Description

The test sailplane was an American Falcon, serial number 001. It was assembled by Advanced Soaring Concepts from their prototype kit. The Falcon was a 15-meter class glider with removable wingtips. The sailplane had a t-tail with fixed horizontal stabilizer and elevator, full span trailing edge flaps and ailerons, single retractable main wheel, dive brakes on the upper wing surface, nose hook, and forward hinged canopy. The flap control had positions of +15, +10, +5, 0, and -5 degrees deflection. The empty weight was 582 pounds. The test was flown at 790 pounds with the center of gravity (CG) at 11.5 inches aft of the datum. Allowable CG range was 8 to 13 inches. The design load factor limit was -3 to +6 g with a safety factor of two. Serial number 001 had a placard limit of 125 knots. Subsequent kits have a limit of 145 knots.

The tested Falcon was externally representative of current kits. Improvements in some of the internal details are being incorporated into kits.

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Test Objective

The test objective was to evaluate the flying qualities of the American Falcon with respect to its suitability as a cross country sailplane.

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Test Methods

To gather data I flew some representative mission events such as thermaling and flew some specialized flight test techniques. All data were hand recorded. Forces were estimated.

All airspeeds were indicated because I did no pitot static tests to determine the instrument or installation errors.

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Exterior Inspection

This prototype appeared to have solid construction. The various parts fit well. The flaps and ailerons were sealed. The elevator and rudder were not sealed. The wings did not have internal seals.

The controls did not have automatic hookups. (An automatic elevator hookup has been designed and incorporated into subsequent Falcon kits.) A large access panel over the right side center fuselage offered easy access for hooking up the ailerons, flaps, and spoilers. The hookups consisted of bolts with self locking nuts. This was positive and simple but not as expeditious as other systems.

The aft fuselage had a shelf suitable for storing survival equipment. A builder planning to add an oxygen system should engineer the bottle mounting before bonding the fuselage halves.

The total pressure source for the airspeed indicator was a tube mounted about half way up the vertical fin. The static pressure source was ports on both sides of the tail boom. A total energy probe was mounted on the vertical fin.

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Cockpit Evaluation

The seat pan was of molded composite construction and was comfortable without a cushion. The advertised cockpit size was for a 6-foot 4-inch 260 pound pilot with parachute. The installed seatback positioned my 6-foot frame too far forward when I wore a full length Security parachute. With a Strong backpack parachute I just fit. Without a parachute I fit the cockpit with room to spare. Tall builders that fly with a parachute will probably need to mount the seatback further aft.

The rudder pedals had a fixed base with ground adjustable foot pegs. Builders with big feet will want to reposition the nose vent inlet tube for more left foot toe space. The gear lever, spoiler lever, and flap lever were all of rugged construction. The gear and flaps had positive locking detents. The motion of the gear lever was opposite the direction of motion of other similar sailplanes.

The canopy was forward hinged and was held up by a locking pin actuated by a knob in the cockpit. When locked open, the canopy was difficult to reach when strapped in the cockpit. The latches were abeam the pilot's shoulder and could be easily overlooked. To jettison the canopy, the pilot pulls a knob that releases the front hinge attachment.

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Takeoff

I flew two takeoffs behind a Cessna 182 towplane. The wind was a right quartering headwind of less than five knots. The Falcon was pitch sensitive at the test CG due to light stick forces. The lack of a trim system added to the pitch sensitivity. At the test CG, the pitch sensitivity was a potential problem for a low time pilot. Further testing at a more forward CG may show decreased pitch sensitivity.

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Tow

With a nose hook, the tow was easy to fly. Towing at 65 knots indicated airspeed (KIAS) required about three pounds of forward stick pressure. The roll force gradient was greater than the pitch control force gradient. With +5 degrees of flap, it was easy to see over the nose.

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Approach to Stall Investigation

I flew 1-g approach to stall investigations with flaps at +15, +5, and -5 degrees. I also flew a 1.4-g approach to stall investigation with flaps at +5 degrees. During each investigation, I checked the control power available in the pitch, roll, and yaw axes as I slowed to stall speed. During each test, control power was better than or equivalent to other high performance sailplanes.

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Stalls

I did Phase A stalls (coordinated stalls with no aggravated control inputs) with flaps at +15, +5, and -5 degrees. I also flew a 1.4-g Phase A stall. All stalls were entered with a airspeed bleed rate of about 1 knot per second. During each stall, the Falcon buffeted noticeably beginning at 3-to-4 KIAS above stall speed and the airspeed indicator oscillated ±1 KIAS. The 1-g stall speed at flaps +15 was 32-to-33 KIAS. At flaps +5 the stall speed was 34-to-35 KIAS and at flaps -5 it was 38-to-39 KIAS. The Falcon was controllable up to the stall. At the stall, it exhibited a g-break (a nose down pitch typical of straight wing aircraft) and recovered immediately with forward stick.

Given the limited nature of this test, further stall testing with different CGs, higher airspeed bleed rates, and aggravated control inputs is warranted.

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Longitudinal Static Stability Investigation

I flew the Falcon from stall speed to 122 KIAS. Because of a lack of pitch trim device the normal trim checks and flight test techniques were not done. Zero stick force at the test CG was at 45 KIAS (trim speed). Estimated breakout and friction forces were less than one pound force. Qualitatively, increasing airspeed required forward stick deflection and forward stick force. Reducing airspeed required an aft stick deflection and aft stick force. Estimated pitch stick forces were less than five pounds force at all airspeeds tested.

The lack of a pitch trim system increases pilot workload. A pitch trim system should be added.

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Maneuvering Flight Stability

To test maneuvering flight stability, I flew elevated load factor turns up to 2-g. Increasing load factor required aft stick deflection and aft stick force. Reducing stick deflection and stick force decreased the load factor. The stick forces were light.

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Steady Heading Sideslips

To test static lateral-directional stability, I flew sideslips up to full rudder deflection in both directions. Opposite aileron was required to maintain straight flight, indicating positive stability. All forces were light. Increasing rudder deflection and force caused increasing sideslip. When released the rudder returned toward neutral.

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Roll Performance

To test the roll performance at 50 KIAS and flaps +5, I made coordinated full deflection rolls from estimated 45-to-45 degrees of bank angle. The estimated time required was 4 seconds. The Falcon had enough rudder control power to easily keep the yaw string straight.

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Dynamics Investigation

Classic aircraft, that is aircraft without feedback systems in the flight controls, have five dynamic stability modes. In pitch, they have short period and phugoid (long period) modes. The short period is the most important pitch mode to the pilot because it is the mode that most impacts pitch maneuvering. The phugoid (long period) has such a slow oscillation that most pilots do not even know it exists. Because of the long period, pilots can tolerate low phugoid damping.

Lateral-directionally classic airplanes have Dutch roll, spiral, and roll modes. Dutch roll is a coupling of roll and yaw oscillations that cause the nose to wander. The frequency is high. Pilots like high damping. Spiral is the response to a bank angle disturbance. If positive, an airplane will roll back toward wings level. If the time for the bank angle to double is over about 12 seconds, pilots often do not notice an unstable spiral. The roll mode is the dynamic response to an aileron input.

The short period mode was tested using pitch control doublets (one forward and aft control input) to induce a pitch oscillation. The natural frequency was high and the damping was greater than 0.6.

The phugoid mode was tested by accelerating above trim speed and releasing the controls. Because of minor atmospheric disturbances, the test damping data were inconclusive. The Falcon at the test condition had either a lightly damped or neutral phugoid damping. The phugoid period with flaps +5 was 19 seconds. The period was long enough that the phugoid would probably never be noticed by most pilots.

The Dutch roll lateral-directional mode was tested using a yaw doublet (one right and left rudder cycle). The period and damping were high.

The spiral lateral-directional mode was tested by rolling into an estimated 20 degree bank angle and releasing the controls. At flaps +5 degrees the Falcon increased its bank angle to an estimated 40 degrees in about 5 seconds. This is a lower lateral stability than other similar sailplanes. Many pilots would likely find the higher workload to be fatiguing on long cross countries. (The test Falcon's wing spar was built with 50 percent more carbon than engineering indicated was required, making it the stiffest wing on any composite sailplane I have flown. This coupled with the low dihedral was the likely reason for the low spiral stability.)

The dynamic roll response to aileron inputs was like that of similar sailplanes.

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Thermaling

I made several turns in areas of reduced to zero sink. The airspeed and bank angle were easy to control although the Falcon required more opposite aileron than similar sailplanes. It was easy to fly coordinated.

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Landing Pattern

I flew the landing patterns with flaps +15 and spoilers as required. The flaps by themselves did not cause enough drag for a steep approach. Flaps and spoilers provided a suitably steep final approach angle. The mechanization of the spoiler handle resulted in high forces when intermediate spoiler was selected. This made it difficult to accurately hold the spoilers at a midway setting.

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Landing

Landings were similar to those in other high performance gliders. Except for making too large of a spoiler movement near the ground that resulted in a slight, but easily controlled, balloon, I noted no problems. However, the light stick forces could lead to pitch sensitivity.

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Recommendations

I recommend the following changes to the Falcon:

  1. Increase dihedral effect so lateral stability is increased. The easiest way to accomplish this is to add winglets (the factory is making molds). Reduced wing stiffness would also add dihedral.
  2. Add a pitch trim system. A pitch trim system that incorporates a spring would reduce pitch sensitivity by increasing pitch control forces and would reduce pilot workload. (A trim system is being designed.)
  3. Design a different spoiler lever layout. A different spoiler lever that increased spoiler mechanical advantage and increased spoiler handle travel would make modulation of spoiler position easier.
  4. Increase maximum gross weight to 525 kilograms (1,157 pounds) and add a water ballast system with capacity to reach maximum weight. To be competitive in 15-meter racing class the Falcon needs to be able to fly at the same wing loading as other sailplanes.
  5. If tuft testing indicates flow separation at the wing roots, add blended wing root fillets. This will improve the climb performance during circling flight at the possible cost of less stall warning.

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Overall Impressions

Representing excellent value for money, the Falcon had solid construction and was suitable for cross country soaring, likely offering builders many hours of safe, fun soaring. While I did no glide performance testing nor comparison flying, a well built Falcon with competition sealing and a water ballast system could potentially be competitive in 15-meter class.

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Revised -- 2 March 1997