Firstly I'm not a Luscombe expert, what I do have is some expertise on the art and science of airworthiness - which I teach at a couple of universities.
Anyhow, the definitive limitations data on the 8E are in
FAA Type Certificate Data Sheet No. A-694, however this doesn't show either a stall or a manoeuvre speed.
I confess that I got the manoeuvre speed from an
unapproved checklist for an 8A online, however if we look at this again.
- Your stall speed is 48mph, we'll assume (as is generally the case for an aircraft of this type) that there's little difference between CAS and IAS.
- It'll have been certified in the normal category to an early version of FAR-23. This will have used a maximum g value of 3.8
- The manoeuvre speed is therefore defined as the stall speed times the square root of the g limit. So that's 48 * (3.8^0.5) = 93.5 mph - okay, a little higher than I said (and this is at MAUW).
- As you say, at lower weights, the manoeuvre speed reduces. However for present purpose this doesn't matter. The reason for this is that Va at MAUW is used for determining the strength requirements for the fin and rudder (amongst other things).
- The requirement then and now is that the "limit" (maximum load expected in service) equates to the worse of full instantaneous rudder deflection at Va or 1/3 deflection at Vne. You can assume that the load goes with the square of airspeed.
- So, if you apply full rudder at 100mph, the fin and rudder will see loads of about (100/93.5)^2 = 1.14 times the limit load. In other words, it's seeing 14% above what it was designed to take.
- It is likely that a 1.5 safety factor was applied between limit and ultimate (although some companies use 1.4 I think this is unlikely for the Luscombe). So, 1.14/1.5 = 0.76, you are actually 24% under the load at which the structure was designed to fail when it was new.
But realistically, as you've said, the aeroplane is old and couldn't possibly be expected to be as strong as it was at manufacture - due to work hardening, internal corrosion, the odd small dent and so-on. So, to be honest by routinely sideslipping above Va you are taking the fin every time to within a small (but uncertain) margin of the failure load.
The result was probably a rapidly growing fatigue crack at some point which, I'd guess, is the likely reason for your fin failure. (And from what I can see, appears consistent with your most recently posted photo).
G