I have a e-flite park 400 out runner that I want to put it on a stryker. Thing is a 10 4.7 prop will not fit between the rudders, so what prop? Im thinking i can incerease the pich because Im decreaseing the diamiter. But how much pich? Ive cut down a 11 7 to a 6 7 will it work? Ive ran it up and it dosnt seem to have alot of power. Im not trying to bulid a rocket, just want to get good perfromance with good run time. thanks!

I've built a couple lite Strykers. You might try a 8060. Or, I've cut down a 9070 to fit.
It depends on the motors KV and your batt voltage. As long as you don't over amp the motor or battery, a smaller prop is fine. Butch

Generally, you swop an inch of diameter for an inch of pitch - but that's only a crude rule of thumb. Also you can't go past a 'square' prop (6x6 or 7x7) without losing a lot of efficency (unless it's a really fast plane, and that will still have problems launching)

Thrust is (crudely) calculated by the volume of air 'pumped' by the prop, i.e. it's swept area, pitch and rpm.

A 10" prop has a swept area of about 78 square inches
A 6" prop has a swept area of about 28 square inches - about a third.

You've increased the pitch by about 50%, which is nowhere near enough to compensate for losing that much prop area.

I'd cut the prop down as little as possible, and add the same number of inches of pitch - but it's usually worth buying a prop in each size around the calculated size and just trying them.

The easiest way to speed the motor up is higher battery voltage - if you're using a 2s battery, try a 3 or 4s.