View Full Version : Three forces, or four??

28th Apr 2002, 01:45
Kermode (Mechanics of Flight) and NASA (they know a bit about flight!!) both say an aircraft in flight - assume equilibrium, etc - is subject to FOUR forces (lift being one of them).

Many (including aerodynamicists) say THREE.

Would be interested to hear some constructive arguments and explanations...'cause I'm now totally confused.

28th Apr 2002, 01:49
Don't know how they get three;
1. Lift 2. Weight (mass x gravity) 3. Thrust 4. Drag

Ummmm that's 4.:)

Um when dealing only with the wing some may argue that drag is only a function of lift, hence 3 forces, but when dealing with the whole airplane or a real environment there are 4 generalized forces and many more if you start dissecting them. Exempli gratia drag is the sum of profile + induced + parasite + form + etcetera......

28th Apr 2002, 03:28
I'm hoping somebody will come along and explain more eloquently - and scientifically - than I can....but the vibe I get is that the "forces" are weight, thrust and RESULTANT.

The resultant can be broken down into two REACTIONS...lift and drag.

If this three force ideal is the correct one - I don't see why NASA (and many text books) call lift a "force"...which, by the way, I always have done.

28th Apr 2002, 04:24
As far as resultants you would have 3: one each in the XYZ direction, or if you prefer unit vectors in the ijk..... of course then the resultant could be described as one vector of length rho and direction theta. I do not know of pure aero guys describing the forces in flight as 3, I'm not pure aero, but in bar room chats and classroom lectures, never heard of it.

28th Apr 2002, 07:37
In some ways the three forces approach makes sense as it organizes them by the source of these forces:
Thrust - A side-effect of an infernal contraption that converts fuel to noise
Weight - Indeed mass x gravity, the origins of which is therefore in the matter used to create the airplane
Resultant - The force that originates from hurtling a suitably shaped contraption through a medium called 'air'

Then obviously you can dissect the resultant into the bits we want: Lift, and the bits we don't want: Drag.

They are all still forces! Just because you apply a bit of vector mathematics to combine or split one or the other does not change the nature of the thingy.

I wouldn't go so far as call one or the other approach the 'correct' one. Both have their merits, the four forces approach is still the one that is most easily absorbed by us dim-witted pilots, so it makes sense to keep on using it. The three forces approach is in my view only relevant for the purely theoretical types.

I could even put forward a two-force explanation: Mechanically induced force and Aerodynamically induced force! The first one would be the sum of Thrust and Weight, while the second one is the Resultant as described above. The end result is equilibrium again, so it must be right!!

It's just a different way of looking at things...