Vertical tail surfaces
 

In reports of flight testing and development over the years from the First World War to the present day, there is a continuous thread of aircraft requiring enlargement of the vertical tail surfaces following initial prototype testing. Aircraft continue to sprout dorsal/ventral fins etc to this day and yet I haven't heard or read of any that have needed a reduction in vertical surface area. Why the reluctance to design them large enough in the first place? OK, I can see that structural weight is a consideration, but it must be more difficult to redesign the structure to accommodate subsequent enlargements.

You raise an interesting point which I will leave others to properly address.

I guess some of the reason has been later versions of aircraft often had bigger donks fitted.

Also transonic conditions were not easy to calculate or tunnel test in the mid 20th Century and often severely reduced directional stability when push came to shove.

The production Victor had a reduced size fin compared with the first prototype.

Calculation of the area of the fin and rudder and its effectiveness as a stabilser is notoriously difficult, even with todays' advanced numeric methods. The difficult arises primarily because of the difficulty of predicting boundary layer behaviour. The boundary layer is already thick by the time the airflow reaches the tail area, and its thickness varies according to angle of attack. The wake flow over the fin area is therefore relatively sluggish, and is dependent to varying degrees on the downwash from the wing (in general low wings result in more efficient fins than high winged aeroplanes). Another variable that is difficult to predict in the case of prop driven aircraft is the helical wash from the prop.

These influences are extremely difficult to calculate when trying to predict flight behaviour across a wide flight regime and even more so if the aircraft is required to be spin-certified. The real behaviour of the aircraft can only be determined in flight testing, when deficiencies become apparent.

Designers always tend to err on the small side when designing fin area as generally a smaller fin will be lighter and cause less drag. Most of the time, the intelligent guesswork is good enough, but as you note, sometimes it is not, resulting in the various retrofitted strakes that you see on some designs.

The B-52 is another example of the production aircraft having a smaller fin than the prototype.

Though I completely agree with the observation, I can also say that some aircraft have fin changes (addition of dorsal fin, in particular) to change the behaviour of the rudder. I understand that in some cases (Cessna 180 to 185 for example) adding area to the vertical tail would not have helped, as much as making what is already there work with greater effectivness.

Another fairly obvious reason, is the addition of floats to a landplane, where the additional keel effect forward of the C of G results in a need to balance the keel effect by adding a ventral fin, finlets.

Pilot DAR

But with the Mk2 Victor, the engine power was almost doubled. We did our instrument ratings in the aircraft, the assymetric approach was always flown with two engines at idle as one without power wasn't really assymetric due to the engines being so close to the centreline. On the assymetyric go around, VMCA was a real issue. If you used full power at approach speed, you would lose control.

Am I right in thinking that if one wishes to be trendy - i.e, stealthy - AND have a lot of fin area, canted twin fins are the popular answer.

I've not heard yet what happens to the stealth if the aircraft banks so as to present a vertical surface in profile, discounting radar absorbent materials etc.