The Twin
Cylinder Cub (March,
2008)
The crash of the Taylor Craft was followed by the crash of the new
Cub. I was putting around real slow and it tip stalled and fell
40 feet and went nose in. Damage was serious but not
fatal, it took out the firewall, front deck, cowl, and forward
bulkhead. Amazingly the windows survived though the windscreen
was creased and crinkled. The wing had minor damage where a strut
pulled out half of one rib.
The Cub is now rebuilt but not yet flown. It took some patience
to piece together but it survived and looks
well. The windscreen has some slight blemishes remaining after
using a little heat to undo the crinkles, but will do. A new
cowl was needed and otherwise everything else was simply glued back
together.
Some effort has been given to setting the radio up to make the Cub a
better flying model. Checking the aileron throws showed 1/16 more
down than up. On a flat wing foil, a down aileron will have more
drag than the corresponding up aileron and on such a long wing as the
Cub has, the leverage forces are much greater and cause a yaw moment
along with the roll that produces a wallow. Two efforts can
reduce this. First, some rudder coordination will counter the
tail yaw and second some aileron differential will help balance the
drag between each wing thereby reducing the yaw force.
To
start, 50% differential has been dialed in meaning that the inboard
to the turn aileron goes up
twice as far as the outboard goes down. 35% rudder mix was
added to coordinate the turns and counter the yaw effect of the
turn. Finally,
per the suggestion of another flyer at the field who used to fly full
scale Cubs, to reduce tip stall tendency, some spoiler mix was given to
the
throttle. What this means is that as the throttle is reduced,
both ailerons trim upward to simulate washout. I've started with
1/4 inch at low throttle, which is fully neutralized by mid
throttle. The plane is ready for a nice day for testing.
I'm not sure however that this plane will achieve the nice qualities of
the Goldberg Anniversary Edition Cub above. The latter was built
with a generous amount of washout and is very forgiving and will stall
only forward. I do not know if this was a Goldberg design or was
a builder option, regardless it makes the plane a very nice fun plane
to play with.
Flying report
The aileron differential and rudder mix were exactly what the Cub
needed to stop the wallering. The aileron up trim did the job in
stopping the tip stalling at slow speeds. At half throttle,
ailerons begin moving towards a spoiler position with 1/4 inch total up
at low throttle. The rebuilt plane is flying well with these
trims.