I remember that R/C dive test for C.G. Unfortunately, it doesn’t inform as to whether the C.G. or the longitudinal dihedral, or both, want adjustment; as either or both may produce the same observed results. Also, as I know that you know, the aeroplane must be trimmed for hands-off level flight at a specific power setting before commencing the test dive. For a number of reasons, I don’t think that this test works for full-sized aircraft.
With regard to the Spitfire’s ability, or lack of it, to easily do forward or side slips, etc., no less a Spit expert than the then recently-appointed RNVR Lieutenant- Commander, FAA, Jeffrey Quill, in his 24 February 1944 report to Fifth Sea Lord, Rear-Admiral D.W. Boyd, who was investigating reports of accidents and pilot’s problems flying mostly Seafire L Mk IIC’s (RN adaptation of the Spitfire Mk Vc) onto RN aircraft carriers Attacker, Battler, Stalker, Hunter, and Unicorn, during Operation Avalanche (the Allied invasion of Salerno, Italy, 9 September 1943) in which 42 of 106 aircraft were written off due to deck- landing accidents; wrote in part:
“(1) Pilots had to be trained to employ a curved approach to the deck as the crabbed approach was acceptable only for skilled and experienced pilots:”
“Wings of the Navy, Flying Allied Carrier Aircraft of World War Two” p.133
Capt. Eric Brown
Apparently, the real-world Spit doesn’t like to be slipped very far any more than our A2A Spit does. We may have gotten it right after all. Of course, we never stop tweaking and fussing and… (sigh).
The question of sideslip in these prop fighters has come up many times during my association with them.
The Mustang for example was noted during testing by North American to have issues with sustained side slip due to lack of aileron effectiveness to maintain the slip angle. This was true for the 51 and ended up being so noted in the Dash 1.
What wasn't noted however was that with the power and airspeed back as is the situation in the pattern for example, the Mustang can be slipped effectively by "scrubbing off" altitude cross controlled. I've done this many times myself. In the 51, with the barn doors hanging off the training edges as they do, you don't really need all that much slip in a curving close in approach, but you can certainly perform a rudder lagging aileron leading descending base to final turn if you want to scrub off some excess altitude on final.
All in all, I would totally agree with North American on the SUSTAINED sideslip notation and I would only recommend cross controlling these fighters in the pattern to pilots having more than the average association with their handling qualities.
Both the Spitfire and the Mustang have more than adequate flap available to control the approach profile without having to slip them, but it's notable to make the fine point distinction between sustained sideslip and a TEMPORARY normal slip condition being available if required.
This subject has come up during my career often enough that I mention it as part of my familiarization discussions with pilots flying the real world aircraft.