Hi John
"So I was thinking, what are the limiting factors on airplane speed during a race?"
Usually the pilot and the pitman:))
Seriously though, it could be one of many different variables. It all depends on how many of them are affecting the airspeed. The main culprits are, and this is in no particular order, Engine, leadout position, Prop size, fuel tank (positioning and venting), and to a certain amount, balance point. I left out the lines since everyone has to race on the same size lines anyway.
Since you already have a ball park figure for airspeed, optimizing your airplane/engine combination for maximum speed can be done with an afternoons flying, providing all the previously mentioned variables are close to optimum or can be changed easily.
First up is the balance point. For racing, around 12-15% is ideal, it gives enough response without being twitchy, and it helps keep the model a bit more stable in flight and during landings.
Second; In order for the model to track the circle properly for maximum speed, the leadouts must be positioned correctly. I have use of a computer program that can determine if they are. I would need to know: Speed of the airplane in MPH. Line length, and diameter. Half of the wings span. Weight of the model. If you email me, I can send you the program (it's PC based) along with some basic instructions and you can get the location spot on.
Third; are you getting the most from your engine? If your local rules state that the engine be stock, then there's really not much that can be done other than fine tuning. If your rules allow, remove the head shim. Since it is a sport engine, the LA 15 engine has woefully low compression in stock form. A boost in venturi size would also be beneficial if the rules allow.
Fourth, and one of the direct questions you asked about...Props. The 7x5 APC is pretty good, but probably not the best fit for the low compression and small venturi of the LA 15 engine when used for racing. Try and get some APC 6.5x5, or 6.5x5.5 props. This series has worked very well for us on .15's for sport Goodyear and Clown racing. I would start with the 6.5x5. This will allow the engine to wind up more, and should bring you a speed increase, as well as increased acceleration off the ground.
You may not have experienced any speed increase with the LA 15 over your Fox 15 due to both of them making similar power at the rpms you were running with the 7x5-7x6 props. The LA 15 certainly has more potential than the Fox, and I'm certain would favour the increased rpm's of the smaller diameter props more than the Fox.
I know this is more info than you asked for, but it is all related when it comes to trying to increase an airplanes speed.
I hope you find this helpful
best regards, Les Akre