It should be fairly easy to get some qualified answers from experienced pilots that still fly these planes today.
15meter/sec is actually fast but I'd say it's half of what I'd expect to cause such a strong rotation. At 5m/s a plane shouldn't turn like that at all I feel.
Here's a little calculator to translate the values:
http://www.unitarium.com/speed
I'd just ask the pilots of vintage planes though, anyone has got some contacts? It's the easiest and most reliable way to fix this. Eventually it can even be tested at lower wind speeds.