Effect of CG on Stall
Discussion
The Bearhawk has a very wide CG envelope compared to many light aircraft, and therefore exhibits it's own particular characteristics when operating at either end of that envelope. Those Bearhawks that are fitted with an IO540 engine may have an empty CG close to, or even forward of, the forward CG limit (10.5"), due to the addtional 150lbs weight of the engine.
With the CG forward of 15", during a power off stall entry the elevator may lose authority resulting in a very gentle lowering of the nose. This can be mistaken for a stall, however the main wing may not actually be fully stalled. By performing a power on stall entry at the same forward CG, the additional airflow over the elevator will increase elevator responsiveness enough to induce a full stall.
With the CG aft of approximately 16" there will be sufficient pitch authority even at low speeds with power at idle, to enter a normal stall.
The take-away point is that if the aircraft exhibits very stable docile stall characteristics at a forward CG, realize that the main wing may not be fully stalled - as defined by an aft movement of the Center of Pressure and immediate pitch down - although it can still develop a very high rate of descent, and that when flying with the CG further aft the stall characteristics may be very quite different with a more abrupt classic stall.
Air Exercise
Note: This may only be possible if the specific aircraft has an empty CG forward of 14" (typically with a larger engine installation) and may not be possible if the empty CG is already aft of 14" (typical with an IO360).
In addition to practicing normal stall entry and recovery, perform the following exercise. Load the aircraft to a CG forward of 14" and configure for a power OFF stall.
As the aircraft approaches the stall and when the pitch attitude decreases (nose drops), hold the control stick fully back. Observe the aircraft descending in a stable condition. Then gradually apply power and observe the nose of the aircraft pitch up as the elevator regains authority from the resulting airflow over it. Continue to add power and observe the main stall occur and the nose pitch abruptly down as the Center of Pressure moves aft on the wing. On many Bearhawks a small "kick" can be felt through the control stick at the point of aerodynamic stall.
Observe that the first pitch down was due to the elevator losing authority, and the second pitch down was a full aerodynmic stall of the main wing. Observe that the stall characterisitcs of the Bearhawk may vary significantly depending on the CG position.
Forward CG handling differences - Loss of elevator authority
Discussion
The most noticeable difference with a forward CG is the potential to lose elevator effectiveness on approach when at low speed and idle power. Particularly on Bearhawks with a heavier engine (and hence more forward empty CG), a loss of elevator authority can be mistaken for a docile stall. The risk is that if flying an approach at a low airspeed, if the power is reduced to idle the aircraft may pitch down, perhaps significantly, and can develop a high rate of descent.
Caution - Engine Failure
In the event of an engine failure when at low speed, there may not be sufficient airflow over the elevator to prevent the aircraft from developing a high rate of descent. At low altitudes, on approach or during take-off, there may not be sufficient altitude to recover the airspeed.
It is recommended to fly at or above an airspeed that ensures sufficient elevator authority in the event of power being reduced to idle, or the engine stopping.
Air Exercise
At a safe altitude, and with a forward CG, set up an approach configuration and reduce the power to idle while reducing the airspeed to below 50 KTAS. Observe the reduction in elevator authority and the nose lowering. Recover the elevator authority by adding power.
Aft CG handling differences
Discussion
The pitch control becomes increasingly sensitive at aft CG, particularly when aft of 21". With the CG in this range the stick forces become lower and this can be observed through the need to trim decreasing.
The rudder also becomes more sensitive as the CG moves aft. This can be noticeable with ground handling during the landing roll as the CG is further aft of the main wheel, and closer to the rudder.
Air Exercise
During an all up weight check, load to an aft CG. Observe the differences in handling at this part of the CG envelope.
Adverse Yaw
Discussion
The Bearhawk generally exhibits more adverse yaw than most modern light aircraft.
Therefore it is essential to practice keeping the ball centered by coordinating rudder pedal input with roll.
Air Exercise
Take both feet off the rudder pedals and put them on the floor. Commencing from level flight, roll the aircraft up to 45 degrees AOB both left and right. Observe the initial movement of the nose of the aircraft in the opposite direction to the roll.
Coordinated Flight
Discussion
The Bearhawk has a very powerful rudder making it very effective at low speeds. The large rudder surface area, and in some instances a large cowling area also combine to make it fairly sensitive in yaw at cruise speeds.
Like all aircraft the Bearhawk can exhibit fuel system issues if not flown with the ball centered. These include transfer of fuel between tanks (with the fuel selector in the BOTH position), fuel sight gauge inaccuracies, and in some cases engine stoppage.
Air Exercise
Demonstrate the method of resting the weight of the feet on the pedals to increase yaw stability. Allow the candidate time to become familiar and to develop muscle memory by just flying the aircraft and practicing coordination. Consider an exercise where the candidate can follow a winding river to practice coordinating roll and yaw in the turns.
Fuel Sight Gauges
Discussion
The fuel sight gauges can only be read accurately when the ball is perfectly centered. If the ball is not centered, then the fuel will move to the same side of the tank that the ball is out to. The sight gauges will then indicate more fuel in one tank, and less fuel in the other tank.
Air Exercise
In level flight with the ball accurately centered and fuel approximately half tanks, place a finger on the sight gauge at the position of the fuel level. Have the candidate do the same on their side.
Next, with both tank quantities now marked, ease pressure on the left rudder pedal to move the ball deliberately out of balance towards the right.
Observe the fuel level on the right sight gauge appear to decrease, and the fuel level on the left appear to increase. Discuss the fact that the tank quantities have not changed, only the position of the fuel in the respective tanks has changed.
Discuss what the outcome might be if rather low on fuel, and one tank was then selected (thinking it contained more fuel), then the ball moved to the opposite side (potential un-porting and possible engine failure).
