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Performance

This section focuses largely on real world performance of the IO540 powered Bearhawk, but with some reference to the IO360 powered aircraft. Read in conjunction with the article on Engine Selection.


Landing Distance

A realistic landing distance expectation when learning to fly the Bearhawk would be 150 - 200m (600 ft) landing roll at 2200lbs (1000 kgs). On my aircraft this equates to 2 POB, overnight gear and 3 hours fuel.


With practice it can be consistently landed with a landing roll of 80-120m (450ft) at light weights and sea level altitude. It is important to understand that most airstrips are still around 300m (600ft) length, leaving a safe margin for error. It's also important to realize that most pilots don't carry a full payload into backcountry airstrips because the increased weight also increases both the stall IAS - and therefore the landing roll - disproportionately (a heavier aircraft results in an exponentially longer landing roll). Many backcountry airstrips are several thousand feet above sea level. Landing at a higher density altitude can significantly increase the landing (and takeoff) roll.


As a general rule of thumb when operating into short airstrips, keep the aircraft light. I usually stay below 2200lbs - 2 POB, some gear, and 120 liters fuel (30 gallons).


Takeoff Distance

Takeoff distances are very similar to landing distances. With an IO540 powered Bearhawk they tend to be slightly shorter, with an impressive rate of climb once airborne. The same effects of weight and density altitude apply.


Take-off and Landing distances - effect of engine size


IO360 For the same payload and a lighter aircraft, the landing distance is shorter because the total weight and therefore the stall speed are both less. The takeoff distance is very similar to an IO540 powered Bearhawk when at light weights.

IO540 The IO540 powered Bearhawk has an advantage in the rate of climb and particularly when hauling heavier weights. This is more noticeable at altitude.


Cruise Speed - IO540 with 29" ABW's

Lean of Peak - 110 KIAS

Rich of Peak - 125 KIAS


A real world expectation is to cruise at approximately 110 KTAS at low altitudes, and 125 KTAS at higher altitudes, while consuming 38-40 LPH LOP. The cruise speed can be increased by 10 KTAS easily, but with a comensurate increase in fuel burn to around 55 LPH.


Smaller tire sizes will increase the cruise speed by up to 10 KTAS, so a cruise speed of 120 - 125 KTAS at lower altitudes is realistic.



Rate of Climb

The IO540 shines in rate of climb. Under normal ISA conditions with at medium weights it would climb at 1500 FT/MIN at lower altitudes, reducing to 1000 FT/MIN at higher altitudes for an IAS of around 80-90 KTS.


Stall Speed

My aircraft is now fitted with Vortex Generators. Thorough testing was completed to establish airspeed position error.


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.


Due to variation in the various installations of the pitot and static system, there is a corresponding variation between amateur built aircraft in the IAS stall speed. Therefore it is important to establish position error (between IAS and TAS) before comparing stall speeds or approach speeds between aircraft.


Comparing airspeeds between aircraft that have not been properly tested for position error can give the false impression that one aircraft has a significantly lower stall speed.


HERE is a link to further discussion on how to establish position error.




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