I am doing some prop test and need a motor capable of 25K plus rpm. Wondering if it should be and in-runner or out-runner type. I have both 4S and 5S packs for power, likely won't be pulling more than 40 amps and only for short 15 sec run time (just enough to measure thrust and amps). What brand and size should I look at?

Best,    DennisT

John,
This is for smaller props 6" diameter and less (1/2A size).

Best,    DennisT

Just to kind of give you a baseline, i tested 8" props up to almost 18000 rpm and i was pulling some serious amps. (close to 90 amps) on a 75 amps speed control. I was doing this for 15-20 seconds on a castle edge and a 1400kv outrunner so i could log everything.. if you are just doing a prop test then you can oversize the motor and speed control and hitting 25k should be easy especially if you use a 6s battery. You can adjust the motor and battery once you have the numbers and you know how many amps the prop will pull at those rpm. Remember if it is a bench test your amperage will be higher. Make sure you have a prop capable of spinning that fast without it flying apart too.. I did smoke a few motors and speed controls in flight tests, but i was trying to get a flite steak sized plane to over 100mph for 2 minutes. I think i would have made it as I did get it to the low 90's a few times then kind of got distracted with other things and never went back to it.. There are  many factors to consider so ramp up gradually and watch your numbers. I feel it is doable and most likely has been done already.

John,
This is for smaller props 6" diameter and less (1/2A size).

Best,    DennisT

A Cobra  C-2820/8 Brushless Motor, Kv=1450 on a 5 cell battery should do the job.  Look at the prop chart.
https://innov8tivedesigns.com/cobra-c-2820-8-brushless-motor-kv-1450.html

Any good 60 amp ESC and I would use a servo tester to control the ESC.  such as

 https://www.ebay.com/itm/333879969440?hash=item4dbccc06a0:g:ji0AAOSwlYtgHXzI

John,
I order the servo controller, for the motor I looked at what Innovative had to offer, Since I will be testing less than 6" diameter props (likely less than 5.5") this motor looks like it would fit: Cobra CP-2210-2200 Champion Series FPV Race Wing Motor, Kv=2200. I could order two of them and use one for the test stand and use the other for a test ship. What do you think.

Best,    DennisT

Looking at the prop chart,  this is a good fit.  The fact that it runs on 4 cells is a +.  Also a 40A ESC should work.

J Rist