I am US citizen. Do I need UK visa for short stay as voluntary research student. My finance will be cared by my college in us and my parents.
As you mentioned you are a US citizen, so you will get 6 months entry on arrival at any UK airport, they will simply stamp your US passport and enter a date and a code manually,
that's what they did to me 4 months ago,
i hope this will help you to plan your trip,
Enjoy your trip.
Source: https://travel.state.gov/content/passports/en/country/united-kingdom.html (although this says only 90 days but experience and all sorts of unofficial sources claim six months).
The visitor rules are at UK Immigration Rules. These should be examined prior to departure.
The UK Site's Check if you need a UK visa determines which nationalities require a visa.
As stated by Timatic, the database used by airlines:
Visa required, except for Nationals of USA for a maximum stay of 6 months.
So no, you do not need a visa