im working on a tower uproar with a 40 thunder tiger pro. what size prop should i use?

This engine has very peculiar power & torque curves. There is a sharp increase in power output right at ~ 12,500 rpm (like "comming on the pipe") & it is important to prop it correctly to take advantage of that sharp jump. An APC 9-6 will quickly let it get up there & wind out to its happy place @ ~ 16,000. A 9.5-7 (if you can find one), or 10-6, or 10-7 also work very well. Don't be afraid to wind it out, this engine still makes huge power as high as 20,000 rpm!!!!. Conversely it can lugg really brutal props if it has to -- it is capable of spinning an APC 11-7 @ ~ 12,700 static --- but that is really giving away power potential. For good vertical, it will turn an APC 11-4 @ ~ 15,000+.

i talked to a 3d guy and he told me that longer props are better cuz they create more air flow over the control surfaces. he reccomended a 12 x 4. (apc) any thoughs.

Your buddy is correct that longer props give more thrust -- at the same power load. However, your TT Pro 40 will turn that 12-4 @~ 13,000 --- 3,000 rpm short of its power peak, & the 11-4 will spin @ ~15,000 rpm -- only 1000 short of the power peak. I think that you will find the 11-4 a better choice. Total thrust will be about equal for the two props, but the 11-4 will give you ~ 20% more hp, 20% more torque & 15% greater prop slipstream discharge velocity (= quicker engine response, more level speed & greater entry speed into vertical maneuvers).

Keep the prop as short as you can on the uproar. I went to a 3 blade on mine to give it another 1/2 inch of ground clearance.

well still being kinda a newb, longer props wont do my any good if i break them on every landing hehe. 11 x 4 is smaller. but can i get away with a 10"? and then it would be a 10 x 5? or what.

A 10-5 APC would be fine (~15,300 rpm). A 10-6 will also do the job (~14,900 rpm). MAS props will flatten out at the revs that your engine is going to spin.

If you are concerned by the high rpm, remember that the torque peak on this engine is @ 13,600 rpm & the power peak is @ 16,000, so you want your revs to fall somewhere in between those two points.

I agree, 10x5 MAS or APC would be good. 10x6 would probably be ok too.

britbrat, these stats you have given me are for the 40 pro not the 46 pro right. and if so i guess ill be ok with a 10x5 or 11x4 right?

The stats were for a TT 40 Pro. A 10-5 will get the job done, as will the 11-4. Be prepared -- it's really going to scream.

i bought a 10x6 apc, hope that will work. it was the only closest size that my lhs had.

After you work out the prop busting issue, do give the APC 12x4 a try. I use one on my uproar 40 with a OS 46AX. Great slow control and will hang(first hover for me) on prop nicely.