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Test Flight: Power-On Stalls

By Jack Dueck, Chairman - EAA Canadian Council, EAA 337912

In the last column, we covered the test flight for power-off stalls. We are now going to expand the test-flight envelope of this aircraft to power-on stalls at maximum takeoff weight.

Again I refer the reader to that excellent reference FAA Advisory Circular (AC) 90-89A. Not only does this AC cover this particular maneuver and test, but it also has a great deal of information on preparing for the flight tests, conditions, and cautions to take, and in general how to prepare and carry out these tests progressively and safely.

The stalls will be wings level, straight ahead, and in this test we will load the aircraft to maximum takeoff weight ensuring the CG position is comfortably within the designer's recommended range.

Aircraft Weight and CG position

The spreadsheet below establishes the weight of the aircraft at 1,750 pounds (designer's recommended maximum takeoff weight for the RV-9A), with its CG at 81.7 inches. (The designer's CG range for this aircraft is pegged at 77.95 to 84.84 inches.)

CG Calculation, Van's RV-9A, C-FBJJ, Oct. 27, 2007

Item Weight (pounds) Datum (inches) Moment (inch-pounds)
 
Empty aircraft 1,145 78.4 89,768
 
Pilot and passenger 370 92.7 34,299
 
Fuel 216 76.8 16,588.8
 
Baggage 19 122 2,318
 
Total 1,750 81.7 142,973.8

MTOW = 1,750 pounds
CG range = 77.95 inches to 84.84 inches

Methodology

Since this test carries with it a degree of risk, the tests will be carried out at sufficient altitude to recover from a departure from controlled flight. We will use an initial altitude of 6,000 feet AGL, which at High River, Alberta, is 9,500 feet. Also, as we raise the nose of our aircraft in full power configuration, we can expect a substantial altitude gain before the aircraft stalls and falls off.

Our first stall test will be with flaps fully retracted, full power, wings level, and straight ahead. We will watch for a pre-stall buffet and an audio and visual stall warning, and we will record the airspeed at the point of the stall and the altitude lost before recovery to level flight.

We will repeat this same process with flaps positioned in their takeoff angle of 20 degrees and finally again with full flaps applied.

Test Results

October 27, 2007, dawned clear and cool, with beautiful smooth air for flight. OAT stood at 1C, and the pressure altitude read 3,300 feet at our High River Airport of 3,431 feet ASL.

RV-9A
The author's RV-9A - the test aircraft

Throughout these tests, the aircraft held no surprises and behaved in its normal benign manner. The test results are summarized below:

Stall Test Results

Condition IAS Pre-stall buffet Altitude lost
  (kph)   before recovery
 
Flaps up 33 (estimated) yes - pronounced approx. 100 feet
 
Takeoff flaps (20) 25 (estimated) yes - pronounced approx. 160 feet
 
Full flaps 0 yes - pronounced approx. 200 feet

The photograph below, taken just before the stall break on test No. 3 (power-on, full flaps), shows the airspeed at 0, and the angle-of-attack warning lights lit up like a Christmas tree! Notice the high horizontal miniature aircraft bar on the artificial horizon and the full fuel burn of 8.7 U.S. gallons/mile on the fuel-flow engine analyzer (i.e., showing full power applied on a 160-hp Lycoming O-320). Also notice that the aircraft has climbed to an altitude of 10,700 feet ASL as the nose is raised and the airspeed has bled off.

There was a definite tendency for the aircraft to roll to the left in the stalls with flaps deployed, but it was easily controlled with application of right rudder. Again, this may be an indication of a slight asymmetrical flap extension, or more likely caused by the increased engine torque reaction and propeller slipstream effect.

Test Aircraft Panel

Conclusion

These tests show that this aircraft stays benign in its departure from controlled flight under maximum takeoff weight and full power conditions throughout the range of flap deployment. Again, these tests were conducted with the CG near the centre of the designer's recommended range. To explore these series of tests to the edge of the operational envelope, one final series of tests should be undertaken; that with maximum takeoff weight, maximum aft CG location, and full power through the full range of flap deployment.

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