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Stalls

Overall, the Bearhawk appears to exhibit gentle and predictable stall characteristics.


Stall Speed


My Bearhawk has a stall speed of 38 KIAS (37 KTAS) at 2200lbs, 14" CG, F3 and F4, power on, with Vortex Generators installed. It is normal to see a low stall speed with power on compared to power off, partly because of the "blown flap" effect. Power off, it stalls 3 knots higher.


I also now have Vortex Generators fitted. Thorough testing was also completed to establish airspeed position error using the system from ITPS.


Without Vortex Generators:



Power OFF

Power ON

Flaps up

52


F1

50


F2

49

42

F3

45

38

F4

45

38

The "stall speeds" listed above are thought to be artificially higher when performed with no power due to the elevevators losing pitch authority at forward CG. With power ON, the stall speed reduces significantly.

 

With Vortex Generators:


Power OFF

Power ON

Flaps up

46

46

F1

45

43

F2

43

41

F3

41

38

F4

41

38



Notes:

  • Having the VG's decreases stall speed and improves handling at low speeds.

  • Having power ON also decreases stall speed by a significant margin.

  • The above speeds were tested at 2500lbs TOW.

  • All stall speeds can be reduced further at lighter weights.

  • Care must be exercised when interpolating the above stall speeds into approach speeds.

  • The lowest stall speed I was able to observe at a very light weight was 37 kts - only one knot less than the 38 kts I observed at 2500lbs.

  • On aircraft with large position error it is very common to observe a much lower IAS at high AoA approaching the stall.


Forward CG


With a forward CG and power off, there is much less airflow over the elevators and hence elevator authority is reduced at low airspeed. It can then be difficult to raise the nose high enough to get a full stall.

Because of this, when performing stalls at a mid to forward CG with power off it may appear that the Bearhawk has a very docile stall, but what is actually 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. This effect (elevator stalling first) can be desirable on a STOL approach so long as there is an awareness of the resulting high descent rate due to the pitch attitude lowering.


Aft CG


With a CG aft of approximately 14" the elevator will usually retain authority at low speeds and a normal stall can be induced. With power off, it may be difficult to experience a wing-drop. However with power-on a wing-drop can normally be experienced.


Flaps


Deploying flap lowers the power OFF stall speed by several knots. With power ON there was little difference noticed as the stall speed is already reduced by the additional airflow. There was no difference in stall speed noted between F3 and F4.


Reflexed Flaps

This wasn't tested on my aircraft. However it should be noted that reflexing the flaps (sometimes done to try and increase cruise speed) may alter the stall characteristics particularly if the ailerons haven't been reflexed.

Vortex Generators


The addition of vortex generators on my aircraft 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.



Aileron Reflex


Many Bearhawks have both ailerons rigged with their trailing edges reflexed above the trailing edges of the flaps and wingtips. This greatly lightens the roll response, reduces adverse yaw, and adds washout. Reflexing the ailerons also reduces or delays the tendency for a wing-drop to occur.


Due to this effect, my Bearhawk (with reflexed ailerons) can be flown in a relatively controlled manner into a stall, with a high sink rate. It is demonstrated with full flap, power on, the inboard wing area stalled and the outboard wing area flying. Power is kept on to ensure enough elevator authority to maintain a high AOA as the center or pressure moves aft on the inboard wing.


If a Bearhawk has been rigged with no reflex in the ailerons, then the stall characteristics may differ from those Bearhawks with aileron reflex.


Position Error at the stall speed


IAS position error can be quite high at the stall AOA. This manifests as the airspeed indicator displaying a very low speed at the stall, and is usually because the relative airflow is not in line with the pitot tube and only a reduced amount of the airflow is able to be sampled (causing IAS to under-read).


With flaps extended there can also be an area of low air pressure behind the flaps around the static ports (which causes IAS to over-read). Due to the variation in installation on an amateur built aircraft, the position error also tends to vary. There is is separate aricle on Position Error and how to measure it HERE.



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