Depends on a couple of things like how light it is. I think the best thing would be is just try things out. I was looking at some basic plane motors and they are around 2,000 Rpm so id start around there.
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But what if u dont know the rpm how do u find it
In general a plane motor is chosen for the model size it has to power. bigger model bigger motor. Look at some commercial models to get some idea of wing span/plane type and motors used. A lot of suppliers offer advice on their web sites Search for Als hobbies for example.
when trying to answer a question you have to first look at the 10 other questions that come with it. As said before size and weight are a factor in deciding on the right motor. You must also take into account the efficiency of the prop you plan to use, the amount of lift that your wings can provide and at what speed, also what type of use you plan to get out of it. my suggestion would be to look into an R/C plane forum and ask there they will have better specs