Flying the Bearhawk - Position Error and Stall speed.

Position Error

Position error can be caused by a number of things and normally occurs due to airflow effects around the static ports, and airflow meeting the pitot at high angles. This affects all aircraft types.

Before doing any slow flight close to the ground, it's a very good idea to calculate the amount of position error between IAS and TAS due to airflow entering the pitot tube at a very high angle of attack. For STOL aircraft when flying at high AOA, it's common to see very low IAS for a given TAS.

Position error varies between aircraft so it's particularly important to establish the correlation before using approach speeds recommended by others.

Many of the modern EFIS screens will show a TAS that is derived from the IAS, so there can be a corresponding error in the TAS at high AOA such that the low TAS appears to confirm a low IAS.

The GPS groundspeed however is derived independently and provides a good means for comparison. This is the method used by the National Test Pilots School in the link below.

This Kitplane Article explains everything in greater detail and has a link to a spreadsheet used by the National Test Pilots School. The advantage of the spreadsheet is that it utilizes GPS and is a simple test to perform.

Stall Speed

My Bearhawk has a stall speed of 38 KIAS (37 KTAS) at 2200lbs, 14" CG, F3 and F4, power on. It is normal to see a lower stall speed with power on compared to power off, partly because of the "blown flap" effect, and partly because the additional airflow over the elevators gives greater elevator authority.

Power off, it stalls 7 knots higher. With power off, the elevator authority is reduced and it can be difficult to raise the nose high enough to get a full stall. 

This is particularly noticeable when performing stalls at a mid to forward CG with power off where it may appear that the Bearhawk has a very docile stall, when what is happening is that the elevator is losing authority prior to the stall (as the airspeed reduces), and this causes the nose attitude to reduce slowly while giving the impression of a very docile stall. However when properly stalled the control stick makes a marked "jolt" forward, commensurate with the stall occurring. The aircraft pitch attitude decreases simultaneously. A mild wing drop can be induced.

The addition of vortex generators reduces the power off full flap stall speed by 4 kts, but they had no effect on the power on full flap stall speed. They do however improve the low speed handling further.

Overall, the Bearhawk exhibits gentle and predictable stall characteristics.