Model Forum / Radio Controlled / Air Models / November 2003

That's pretty big tires for a 40 size isn't it?
I have a 40-size Zero with 2" tires. Maybe smaller would be better, but your
CG must have been pretty close to catastrophe anyway I wold think.
Maybe I'm wrong...  Lesson learned anyway

Stefan

Hmm...I'd have thought the wheels should be located *very* near the CG so
there wouldn't be a profound effect when changing from foam to rubber...

/daytripper

I would think that if you also replaced the nose wheel (which you didn't
mention), the difference wouldn't be that great since its well ahead of the
CG. Why would you fly a trainer that "was never slightly nose heavy".
Rearward CG's increase pitch sensitivity but you should still be able to
bring the plane back unless it snapped on take-off. If so, this is likely a
result of the rearward CG. Tail heavy planes are also difficult to flare on
landing. I'm curious since like other posters, I have difficulty believing
the effect of the tires. Why worry about foam tires with a trainer? You are
right though that bigger tires are preferable off grass but not essential if
you have sufficient power.

Heh heh heh. I throw away the rubber boat anchors and buy light foam wheels.

Sorry for your crash.

Bob

I agree that it's a good idea to recheck CG on changes.  I have a hard
time believing the tires did this however.  The mains are close to the CG,
certainly closer than the 8 inches you thought it was behind.  My best
guess is that your CG was way off to begin with.  How did it go in?

In any event, there's the old theory that a tailheavy plane flies once.
That's true to an extent, it depends on the skill level of the pilot
flying it.

I once flew a trainer for somebody else who hollered for help and his
plane was so far to the rear that once you pulled power the nose jumped
up, landing was done under power and much down elevator.  You could not
pull power or the plane would nose up and you couldn't push it down with
elevator, it took both.  The lesson I'm imparting to you is that if this
happens again remember throttle and down elevator, and also consider a
steep turn if necessary.  Don't stall it, I flew my Duraplane once with a
CG well back and stalled it, first time I ever seen a perfectly flat spin,
I mean level, you could put a bubble level on it.  It took about a hundred
feet to recover, but then I was doing it for fun since I was done using it
for the students that day and felt like a challenge.

By the way, it's funny in a way but the reason the CG on that guys plane
was so far to the rear wasn't his fault.  His fuel tank leaked big time
and during a climb the fuel sloshed to the rear, the bulkheads held it
there in normal flight.  I didn't figure it out until I picked up the
plane and noticed the fuel dripping out from the tail.  It was one of the
GP tanks famous for busting up and leaking back several years ago.