im putting a 75ax in my pulse 60 what prop should i use with this set up?

I would use a 14 x 7 or a 15 x 7 both with in the range of the manufacturers specs.
If you feel that the 15 x 7 is to much for this engine, I guess that you can drop to a 15 x 6 and check the revs.

here's a chart I found on the net for prop sizing; this should help you out. If the "starting prop" doesn't have enought "bite" to the air, move to the "alternative props". According to this chart, a 15" prop is reserved for .90 or bigger engines, not .75's...... do a google on Sizing Model Airplane Propellers and you'll get some good results to help you out


Prop Chart For Two - Stroke Engines
Alternate Propellors Starting Prop Engine Size
5.25x4, 5.5x4, 6x3.5, 6x4, 7x3 6x3 .049
7x3,7x4.5,7x5 7x4 .09
8x5,8x6,9x4 8x4 .15
8x5,8x6,9x5 9x4 .19 - .25
9x7,9.5x6,10x5 9x6 .20 - .30
9x7,10x5,11x4 10x6 .35 - .36
9x8, 11x5 10x6 .40
10x6,11x5,11x6,12x4 10x7 .45
10x8,11x7,12x4,12x5 11x6 .50
11x7.5, 11x7.75, 11x8,12x6 11x7 .60 - .61
11x8,12x8,13x6,14x4 12x6 .70
12x8,14x4,14x5 13x6 .78 - .80
13x8,15x6,16x5 14x6 .90 - .91
15x8,18x5 16x6 1.08
16x10,18x5,18x6 16x8 1.20
18x8,20x6 18x6 1.50
18x10,20x6,20x8,22x6 18x8 1.80
18x10,20x6,20x10,22x6 20x8 2.00

...

a friend of mine has a 14x7 on a big stik 60 w/an os 75ax and it is a great combo

Look at the mfg's recommendations and get a couple of those that suit your intended use of the airplane. Then go compare them and pick what works best. And then pick up a couple of other brands that're the same or close to the same specs, and compare them. It's amazing how different performance can be on props with the same diameter/pitch numbers printed on them.

Most beginners think there is a perfect prop and expect to swing by the LHS and pick up ONE and live happily ever after. It don't really work that way. And they believe that the more experienced modelers would know which is the magic prop for their model.

My workhorse airplane's OS75AX has a couple of favorites. It does well with a Zinger 13x7, but lots better with an Evolution 14x6 in spite of the fact it turns that 14x6 about 800rpm less on the ground than the 13x7. It likes a couple of 14" Zingers and TFs but not as much as the Evo. And it performs with a MasterAirscrew 13x8x3 really good. That supposedly "less efficient" MA is at least a tie for best prop on that engine on that airplane.

What prop should you use? Whichever one your engine on your plane in your hands likes best. Don't stop at the first one you buy based on whatever you're going to base the purchase on. Test a couple to find the diameter/pitch range and then test a couple of different brands. And expect to find that the pitch printed on different brands doesn't always pull like the same pitch printed on the other brands. You'd be surprised how few magic mfg's there are.

I have an APC 14-7 on my Goldberg Skylark 70 with the OS .75 AX engine. It flys just fine, suites my needs. I've really never needed to experiment because it just worked and I left it that way.. if it ain't broke, why fix it!! All the recommended props are fine.

The chart put up by spacemonkey is a tad light for this engine, which performs more like a 90 than a 75. Prop it accordingly.

But, as mentioned, it would never hurt to have a few various sizes around and try them out. See what one(s) get you off the ground the best, do good vertical maneuvers, and slows the plane down appropriately for landing. There will be one that stands out for your particular application. Buy a couple of them, then enjoy your flying!

CGr.

This is a quote from RCKen who did a review of the OS.75AX. Notice the props that OS recommends.

"OS recommends the following props for this engine, 14x6, 14,x8, and
15x7. You might get away with dropping that down to a 13x8 prop, but I
wouldn't go any smaller of a prop than that."

I know it may sound too small but I ran a 13x6 on a .75-AX in Super Sportster 60 and the plane was a rocket.

I would start with a 13-6 APC. The engine will have a much broader needle and the power will give unlimited vertical.