Gross Weight Testing

Today I continued adding ballast to the Bearhawk to test how it performs at heavier weights.  My ballast is 80 pound bags of concrete, so I started with 160 pounds in the back seat.  The weight wasn’t really noticable during a few stalls and some phugoid tests.  Since I was flying with close to half tanks on that flight, this weight gain was not much above my solo full fuel weight.  On the next hop I added 160 pounds to the front right seat.  As with before, this weight didn’t seem to matter very much. 

For the next flight I added two more bags to the back seat.  By now, the pitch stability was still pretty good, though the phugoid definitely started to show a higher amplitude.  The takeoff roll was noticably longer (perhaps 700 feet or so) and the uphill back-taxi required much more engine power.  The latter indicator might have also been related to the somewhat soggy runway.  For the first time, I was able to land tailwheel-first.  The landing flare was much easier.

For my final configuration, I added 105 pounds to the baggage area and another 80 pounds to the front right seat.  To recap, that’s 240 pounds in the front right seat, 320 pounds of concrete (plus another 20 pounds of fly-away-kit), plus 105 in the baggage area.  I was at about 1/3 fuel, which would mean that when I flew up to HKY to top up the tanks, that would put me about 8 pounds short of the max gross 2500.  I should mention that with each additional weight increase, I also measured the main landing gear tread.  As the weight increases, the gear spreads out.  Bob advises that the center-to-center distance of the main landing gear tires must not exceed 72″ at max gross, an fortunately mine did not.  

That heavy configuration had my CG at 21.88 inches by my calculations.  Bob specifies a limit of 22.5 inches, but I didn’t really care for flying it at 21.88.  If I encountered a little updraft, I found myself working the elevator control back and forth to maintain level flight.  If I trimmed for level flight at around 80 knots, bumped the stick aft about 2″, then let go, the resulting pitch up was 1.6g all the way up to around 25 degrees.  While I thought all of this through over lunch at HKY, I decided not to test any further aft.  Instead I made a note that 21.5 will be my operational limit.  To get there for the next flight, I moved the 80 pound bag from the baggage area to the back seat, and left the 25 pounds of kitty litter in the back.  Then I topped off the tanks and took off.  As before, the most noticable difference was the longer takeoff roll and climb rate.  

I had intended to fly around the pattern a few times, but all of this extra weight made it apparent that my tailwheel tire was a little bit low on air.  On the second landing (which was not one to be especially proud of anyway), the tailwheel started to shimmy as I slowed through about 25 knots.  I attributed the shimmy to the low tire pressure and departed for home.  On the way I tried to validate my calculated high-altitude performance with a climb to 10,000 feet.  I had to level off several times along the way due to high CHT and/or oil temperature.  This tells me that I need to improve the effectiveness of my cooling, since the OAT was right around freezing.  I have made a few adjustments and am going to try again on another day.  

In summary, I would say that the fully-loaded Bearhawk, particularly at a fully-aft CG, is a whole different animal than an almost-empty one.  This is the case with every other airplane that I’ve flown in both conditions, so I’m not surprised.  The good news is that from the family-hauling standpoint, I won’t be that heavy or that far aft.  My concrete family is at least 300 pounds heavier than my regular family, at least as of now.