I lost the original question on this, but someone asked about the wisdom
of adding stiffness to the fuselage and whether that would make it more likely
to break. Good question, but the answer is not straightforward.
Generally, stiffness and strength go together, but they are actually
different things: A nylon rope might be strong, but not stiff while a
clay pot is stiff but not strong. Yes, if you make one element in a
complex system stiffer than the others it will absorb a disproportionate
amount of load and might be subject to failure. For instance, I added
unidirectional Kevlar "straps" to my aft fuselage and they are probably
stiffer than the glass fabric under them. They will absorb more of the
force exerted and yes, they might break first or possible de-bond from the
substrate during an overload condition. It would have been better to
design the whole system out of Kevlar, but I suspect the straps do increase
the overall strength and especially stiffness. Why do we want the
fuselage to be stiff, especially in torsion? Everything that has
stiffness and weight has a resonant frequency - the frequency at which it will
tend to vibrate. The tail will torsionally vibrate at some frequency and
this frequency is lower than it could be because of all the lead weights at
the tips. If that frequency is too low it could be excited by any number
of things, one being aerodynamic loads, possibly resulting in destructive
flutter. Since all three control surfaces are overbalanced it will not
likely be excited by a control surface itself. When I studied the shimmy
issue I looked for things that could have a resonant frequency in the
range of the observed shimmy. Three obvious ones came to mind.
There is the resonance created by the combination of nose strut/mount
stiffness and the gyroscopic precession forces of the wheel tire, a very
complex system. Another is the resonant frequency of the engine in its
isolators. The third is torsional resonant frequency of the tail, which
isn't has high as you might think. To demonstrate, take you fist and
pound on the tip of the horizontal stabilizer - I haven't measured it, but it
appears to be 10 Hz or less. That is in the same range as the engine.
What would happen if the excitation (the nose wheel turning side to
side) was the same frequency as one of the other major resonant frequencies?
It might trigger a catastrophic shimmy event. Why would a shimmy
trigger a torsional vibration of the tail? To start with the shimmy puts
a torsional load into the fuselage and besides inputs a lateral, or steering
load. The CG of the tail is above the neutral axis in torsion and
therefore when the tail goes side to side it generates a twist. The two
forces actually add together, increasing the effect. The resonant
frequency of the nose gear changes with speed, but what happens if the two
other large masses, the tail and the engine have the same resonant frequency?
When the resonant frequency of the nose gear matches the other two the
vibration could get severe. One basic goal of elastic design is to
separate the various resonances so no two can be excited at the same
time.
Skip all that - I assume the tail is strong enough (I think), but it
could always be stiffer and I like stiffer. The graceful curve of the
aft fuselage maybe impart slightly less drag and certainly looks good, but a
simple conical shape (Piper Cherokee) is probably more structurally
sound.
Gary