Hi Tonny, greetings from the UK.
First of all the -9A. I think you will have some difficulty fitting a larger tyre to the -9A because it has to fit inside the "U" shaped nosefork. It certainly needs it though for safety. I had a -9A
(Picture 3rd down here.) and put about 45 hrs on it before I sold it, partly because it was so unsuited to soft ground. The flip over issue appears to be associated with bumps and holes. I never got involved with that but there is another related problem. The wheel is very small and has about 275 lbs on it before the take off role starts. Because of the long gear legs, on soft ground this weight increases significantly because of the forward moment. I worked out on the back of an envelope that it was probably over 350lbs as you put the power in. Once you have enough air flow over the elevator of course the nosewheel lifts and flight is pretty well instantaneous....or at least, levitation into ground effect. It did have the effect of doubling the (short) ground role though.
Landing can be a bit frightening too! Once you loose elevator authority the wheel comes down and you stop in a hurry. Its very hard on the gear.
The -4 will have a much easier time on my strip. The typical flying weight is less (and the gross) and by having a 20% wider tire the loading on the ground per square inch much reduced. Also when you start your takeoff role you have a positive angle of attack so you will have some lift from the wing and the prop is pointing upwards! On the -9A with the tip forward of those long legs it appeared sometimes to be thinking about Australia.
Bottom line the drag is caused by the tires sinking in. Thats why the tires on the S'cub are so large. It even handles snow half way up the wheels quite well. The issue with tail draggers is to keep the stick on the rear stop as you touch down until you are down to a very slow walking speed if it is soft. With snow I try and touch down with power on so there is plenty of airflow over the backend. I have never heard of a -9 flipping over.
So good luck with your -9A. My suggestion is keep it on hard runways or firm grass. I am not sure what you have mostly in Belgium. I have been to Namur and Achen...one of each.
The downside of putting bigger tires on is I will undoubtedly loose a little off the top speed.
Cheers!
PS When I started there was no RV-9 option, only the -9A. They said at the time there never would be a -9A. If I was building it again it would only be a -9.
