# Forces on an aircraft - thrust, lift, drag, weight

I'm extremely sceptical about the wikipedia page on aircraft flight mechanics. When describing 'straight climbing flight', it says:

lift is unable to alter the aircraft's potential energy or kinetic energy

This seems like nonsense to me, given that 'lift acts perpendicular to the vector representing the velocity of the aircraft relative to the atmosphere'. In other words, I think there is always a vertical component of lift (except for the conditions where angle of attack for non-cambered aerofoil is 0 or 180 degrees). The reason I want to know this, is it also claims that

in straight climbing flight, lift is less than weight

I also can't believe that this is always the case, since an aerofoil can generate lift when it has a zero angle of attack if it is cambered. So if the aeroplane is going fast enough, not only can the lift can exceed the weight, but just the vertical component of lift can exceed the weight.

I'd appreciate criticisms of my understanding as well as the wikipedia page. (I wonder if it's an incorrect interpretation of the source, 'Aerodynamics' by Clancy).

Thanks

Rather than try to debug a Wikipedia page, I suggest two things:

1. Take your time and read this delightful on-line book. Also, why not get a copy of Stick and Rudder? It's been a classic for 70 years.

2. Take an introductory flight lesson. It's a lot of fun and totally safe. They let you take off and fly around, and then you will understand all the basics. Don't worry about landing - they will do it for you.

How airplanes work is not a mystery. They've been around almost as long as automobiles, and they are just as well understood.

If we define lift to be perpendicular to the aircraft's velocity, then it is true that the lift will not change the aircraft's energy. The change in energy due to a force is that force multiplied by the distance over which the force acts. But if, by definition the aircraft is not moving in the direction of lift, then that distance is zero, and the change in energy is zero.

The text you have quoted is confusing; I wouldn't read too much into it. Like always in physics, the key to success is to carefully define what your terms (vectors) mean, and apply those definitions consistently.

• Lift doesn't have to exceed the weight for an airplane to fly. If it did, the airplane would be in a kind of circular motion, going into an Immelmann turn. The majority of the time, the lift and the weight balance each other out so that the airplane is in straight-and-level, unaccelerated flight.
– Greg
Commented Jun 17, 2013 at 4:51
• Yes, I agree with you. Commented Jun 17, 2013 at 13:46

The article defines lift as a force vector perpendicular to the aircraft's path through the air, not the force vector needed to balance gravity. Similarly, thrust is considered a force vector parallel to aircraft's path. If you imagine an aircraft pitching up from level to vertical (and its flight path inclining accordingly), as its pitch increases, the lift and thrust vectors rotate along with it. The thrust vector begins to acquire a "true" vertical component, which adds to the "true" vertical component of the lift vector. Since the sum can't change for non-accelerating flight, the lift vector must be reducing its "true" vertical component by exactly that amount as it rotates.