A 13-6 is pretty lightweight for a .90 FS. My .70 OS turns an APC 13-7 at a comfortable 10,300 or so before I back it down a bit.

You should be using a 14-6 (or so) on that engine. Less than that and you're simply converting glow fuel into noise. The Spacewalker is not a fast plane under any circumstances, so a high pitch prop is performing with very poor efficiency. The flatter pitch (ie: 6") will allow for greater static thrust and I feel, a better delivery of useful power inside the speed range of the spacewalker.

Good luck!

Try a 14-6 or a 14-8. The heavy plane probably dosn't need a "speed" prop, Unless you already know its exceeding 80 mph with what you've got and it acts like it needs more speed...

Remember that when you change pitch, you should also change dia... The 13-6 is a decent prop for break-in of the .90 class 4-stroke, but its definitly not the best for flying. A 13-4 would be way too light of a prop, and you might over-rev the engine (very bad for 4-strokes... bends pushrods and valves.)

13 lbs is on the heavy end for most sport modelers using a Saito .90. We tend to want the performance of a high power aerobatic aircraft, and for that we ned a high power to weight ratio. You'll NEVER hover that plane with that engine. It won't have the ability to climb vertically forever either. (A .91 4-stroke MIGHT over a 11 lb plane... but not easilly.) You'll find that most people would put a 1.20 on instead of the .90 with that heavy of a plane. (and might as well have the "ballast" be a heavier, higher power engine...)

Uh.....forget about hovering a spacewalker.....

Your .91 Saito ought to fly 'er fine at 10+ pounds. 14-6 or so should be fine.

True... the configuration of the Spacewalker doesn't lend well to hovering (control surfaces are small compared to most 3D aircraft, long fuselage....)

The O.S. .91 FS CAN give unlimited vertical climb to a 10 lb aircraft though. (Takes my 10 lb Fokker Dr1 straight up forever [as far as I let it...] at 80% throttle)