Consider British Airways as an example. Their website [provides][1] a list of car seats requirements and one of them is:

> must be designed to be secured by means of a normal aircraft single lap strap ...

Therefore, it looks like you need to buy a car seat which meets the requirements or to check rules of your airline (if it is not British Airways). 

**(UPD: DON'T DO THIS (see below why))** However, returning to British Airways, [the different section of the same page][2] states:

> If your infant is less than two years old and you haven't reserved a
> carrycot position, your infant can sit on your lap using an extension
> seat belt. The cabin crew will provide this for you and show you how
> to use the seat belt.

It looks like this additional seat belt is measure against, as you said, "a kid turning into a projectile". 

**UPD:** there are numerous concerns regarding safety of such belts (e.g. a [Flyertalk post][3]). According to the [Federal Aviation Administration][4] (the national aviation authority of the United States):

> ... the safest place for your child on an airplane is in a
> government-approved child safety restraint system (CRS) or device, not
> on your lap ...

So it looks like you need to buy a special seat and (according to FFA): 

> make sure your CRS is government approved and has "This restraint is
> certified for use in motor vehicles and aircraft" printed on it.


  [1]: http://www.britishairways.com/en-gb/information/family-travel/getting-ready-to-fly#carseats
  [2]: http://www.britishairways.com/en-gb/information/family-travel/getting-ready-to-fly#freeseatwithinfant
  [3]: http://www.flyertalk.com/forum/12537464-post2.html
  [4]: http://www.faa.gov/passengers/fly_children/