My Soon to be Solar airplane had its first flight today without cells.
It was a good news /bad news flight....
Good news... It has plenty of power and the pitch trim stability
seemed to be good.
Bad news...
It has a spiral stability problem.....
It was a bumpy strong thermal day and it got upset and entered
what looked like a classic case of the death spiral....
With full opposite rudder the spiral just got steeper until it hit the ground.
With such a slow aircraft this all happened in slow motion.....
It was in the CSRC long grass so the only damage was one wing tip.
It is easily repairable.
Now to speculate on why this happened.
It is a Rudder Elevator bird with no Ailerons....
1)I believe that I do not have enough Dihedral by a factor of two.
It was recommended that I have at least 10 degrees.....
I read this as ten degrees total angle at the wing root, 5 degrees on both
sides.
The general consensus on the flying field is that 10 degrees refers to both
wings,
and the total angle at the center section should be 20 degrees.
Should it be 10 total or 10 each side?
2)The vertical stabilizer fails the "that looks about right" test by
looking to be too large. Since I have a large moment of inertia I
was targeting the vertical tail volume of 0.025 to 0.030
The calculated Vv of the plane as flown is 0.030
21" span * 10" cord on 44" arm aft of a 14" cord wing with 178 in span.
(The 10" cord includes 4" of rudder and 6" of stab)
Vv = A_vert * arm_vert / A_wing * span
Vv= ((21*10)*44)/(14*148*148) =0.030
Is this too much v stab?
With the fairly long AR stab, full rudder seems like it would induce a
rolling moment, the wrong way.
The way it is made it would be trivial to reduce the span by 2.5" to 18.5"
Vv=0.0265 or to
16" Vv=0.0229 Should I do this?
Thanks in advance for any help....
Paul
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