What is a good angle to depitch the ikara broad blade to? I need to take care not to break the spar.
The optimum pitch of the propeller for an indoor endurance rubber powered airplane depends on many factors.
Indoor flyers usually talk about pitch-to-diameter (P/D) ratios for propellers. For Wright Stuff airplanes a P/D ratio of about 1.8 will yield excellent results.
If you keep the width of the rubber motor constant, increasing the pitch will cause it to spin at a slower rate and use up the turns more slowly. Decreasing the pitch will cause the propeller to spin faster and use up the turns quicker.
Re-pitching an Ikara prop is a difficult process, often leading to broken prop spars and unbalanced pitch.
It is far easier to vary the width of the rubber than to try to opimize your flight by re-pitching an Ikara propeller.
If your plane lands with too many unused turns, increase the width of the rubber.
If your plane lands dead stick, i.e. the turns are all used while the plane is still in the air, decrease the width of the rubber.
But changing rubber widths is not easy to do unless you have a stripper or a friend that will do the stripping. Proper use of a rubber stripper is an acquired skill, and of course, depends on the accurate and repeatable use of a thickness gauge, which itself is a tricky business because it is difficult to precisely measure the dimension of a soft deformable material like Tan Super Sport rubber.
Some suppliers and kits will give you a range of rubber widths you can try. But for Wright Stuff the longest duration in the air can usually only be achieved with the ability to vary rubber widths down to thousandths of an inch. For example going from .095 inches to .097 inches in the width of the rubber could produce an extra fifteen seconds in the air.
The best Wright Stuff planes have props that are balanced. They don't shudder and vibrate the tail and waste energy. Balance is achieved on several levels. The weight of each blade should be equal. If one blade spins down to the bottom when there is no motor attached and no wind, you know it is heavier than the other blade. People then trim the blade. This is a bad idea. Now the blades may have unequal area leading to assymetric thrust. Blades may have unequal pitch due to manufacturing tolerances. Checking the pitch of each blade requires a pitch gauge. Again if the pitch is unequal there will be an imbalance in the thrust and resulting vibration in flight. Then there is the problem of wobble in the prop bearing.
Experts can detect and fix all these problems in the propereller and its bearing but the fixes are complex and delicate. They can provide the winning edge. For example the weight of the blades can be equalized by adhering a suitable length strip of 1/2" wide 3M Scotch tape on the rear surface of the underweight blade.
My advice to novices is to switch Ikara props to one that exhibits the least vibration and then optimize the rubber width for that prop.
Good luck to all at your upcoming regional and state competitions.