In the past it used to be very hard to fly gas powered remote control airplanes. However these days, they come 90% assembeled, and are generally easy to fly! http://www.raidentech.com/arfnigasai.html
Some gas powered remote control airplanes are suitable for six or seven year old children, however, many rc airplanes are advisable for ages eight and up.
I had a difficult time finding this information. This link gives several different gas powered planes for sale. The specifications for each plane are listed. Hopefully this helps.http://www.nextag.com/remote-control-gas-planes/stores-html?nxtg=38230a28050c-593B61A4F52745FD
Electric remote control airplanes are difficult to build if you do not have step by step directions or at least some previous hobby building experience. The more practice, the more efficient you will become at building.
Battery powered remote control cars are more efficient and economical.
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There are many retailers where one can purchase a gas-powered remote control car. One can purchase a gas-powered remote control car online at Amazon, eBay, and XenonProject.
The average entry level remote control car will cost about $75.
It is common for remote control planes to crash. I would start off with cheap one until you become familiar with how to handle them.
For someone looking to purchase a gas powered remote control car, websites such as Amazon, eBay, Nitro RCX (which claims the lowest prices),and Xenon Project.
A great site for buying gas powered remote control cars is hobby-estore.com. You can put together a gas powered rc car with your ideas there. The cheapest one I found was about $390.
Gas Powered RC Boats are exactly what they sound like. They are remote control boats that are powered by gas. Check them out here! gas-powered-rc.buycheapr.com
Gas operated remote control cars are faster, but require much more maintenance and care.