I had this knife...

...and I threw it over to my enemy for HIM to use.

What does it take to forge a good throwing knife ? Obviously the blade must be lighter, but what exactly makes a regular dagger NOT a good throwing blade, and what makes a throwing blade not a good all-purpose knife ? Is "aerodynamism" (translating from french here, not sure its even an English word) a concern while shaping that kind of a blade ? What is involved in balancing the weight of the blade versus its handle ?
'Paul-Etienne'

Some of the nicest (and most realistic) sequences involving knife and axe fighting are seen in Last of the Mohicans. (If you have not seen this film - I'd suggest renting it. Its actually worth owning, the historic detailing is very exacting. Its also beautifully shot.)

First thing that is almost always shown incorrectly in modern film. Any thrown weapon spins as it travels through the air. This means that the DISTANCE to the target is some multiple of the spin rate. You want the weapon point to be forward when it gets to the target. In theory you could give the weapon an extra flip as it leaves your hands to adjust for this. In actual fact, anyone that I've ever seen throw something and hit the target correctly actually measures off the range (as a multiple of the spin rate). Hope this makes sense.

Every weapon has a spin rate. People who throw regularly have specific things they throw - for which they have determined the spin rate / range combination. I doubt ANYONE could grab a random weapon and throw it instantly and correctly to hit a moving target. (So much for film heroes)

The impact of throwing puts incredible strain on the weapon. This particularly for knives. The blade is spinning as it moves forward. If it hits a hard target, the point sinks in a small distance (imagine a wooden door). Then the rest of the mass is still vectoring forward and rotating. The end result is that the blade is more likely to shear off at the tip. For this reason Specially designed throwing knives are usually much thicker through the spine or back (double / single edge) and most commonly have fairly wide angles at the point.

Although ANY weapon can in theory be thrown, specially designed throwing knives tend to have the weight balanced forward. Measure the centre point based on length - then expect the front to be heavier than the handle end. This makes the blade hit harder on impact. That's why you see the leaf shape so common to double edged throwing knives. Also that kind of wide bowie pattern for single edged. Lessening the handle weight also tends to reduce the impact stress described above. In any case, you want the point to be located along the long axis of the knife (at the centre line).

(As a side note, the balance on any knife is related to intended use and personal preference. A knife with a lot of weight in the handle tends to make a fast and whippy feel to the blade. This is often a preference for knife FIGHTING - depending on the style employed. A general purpose camp knife often will have the weight forward - to allow for more impact when doing heavy chopping. Generally kitchen knives will have the weight to the hand - again allowing for easier control of the edge and point. This marks the difference between chopping and slicing actions.)

You mention aerodynamics. There is no reason to change the design of the blade so that it flies better. Wind resistance does not effect the travel over the short distances involved The changes all come from the functional aspects described above.

For axes, the same rotational shock mentioned above is applied to the handle - right where it attaches to the head. This is why the Norse style throwing axes have the angle of the handle offset slightly (at a diagonal to the line of the head / edge). This offset reduces the strain on the handle at impact. The specific style goes back to the Franks - who's name actually is derived from their word for these small specifically throwing styled axes.

Now an editorial note. Why would you WANT to throw a knife? Truth is that the damage you can do to a target with a knife that is thrown is extremely limited. If you are such a great shot that you can measure your distance and then hit an extremely small area on the body (likely while its moving too)...
If you could hit: an eye or the throat - you might be able to stop a person. IF you hit them in the rib cage (most likely) then there is about a 70% chance of hitting a rib. You would have to hit one of the gaps - and then have the blade at the same angle as the gap (extremely unlikely, as the direction of spin is usually more up and down than side to side).
If you hit someone in the gut (the other likely hit area) You will not KILL them. Odds are good they will go into shock - which has the result they will scream like a pig - or decide to kill YOU. A body in shock may not feel secondary damage - at all.
If you hit someone any place else - you will put them into shock and generally piss them off. You will not kill them, just slow them down some. You have also just given them a nice new weapon to use.

Axes are a different matter. A throwing axe will weight something from 500 gm to maybe 1.5 kilo (tops - that last is pretty heavy and takes two hands to throw). This is anything from twice to six times the delivered energy of that knife. Truth is that even the BACK end of an axe hitting you in the head may just knock you cold. Mind you, a head shot is a tricky one to try for.

I should note that other than those fairly common and specifically designed light throwing axes, there are no samples of knives designed for throwing from the Viking Age. Knives from the period are based on the SEAX (sax) profile, single edge with straight line, often slight forward balance to the blade.

(Note to my readers: As you have seen, I am just finishing my heavy period of Festival shows. This is a much older reply to an e-mail, polished off a wee bit to provide some content over these weeks. I intend to be back to my normal activities pretty soon!)