Lift
What is lift ?
- Lift is that part of the force generated by moving air over a surface perpendicular to the moving air.
- Lift is in fact part of the Total Reaction, it can be said lift does not appear on its own, only as part of the total reaction.
The other component of the Total Reaction is Drag.
- There can be no lift without Drag, but there can however be Drag without lift.
- Lift can be generated by cambering the surface of an object or inclining that object with respect to the the oncoming airflow..
What causes lift ?
There are a number of theories as to what causes lift these are summarised below, individually these theories can not completely describe the lift force but combined they produce answers which are pretty close to reality.
- Bernoulli - A pressure differential between the upper and lower surfaces of an object causes that object to move in the direction of the lower pressure.
- Newton - An action and reaction as the air is deflected by the object as it passes through the air.
- Coanda Effect - Airflow rotation due to viscosity.
Mathematically
We can calculate the lift force by using the following equation
L=CL ½ ρ V2 S
where
- L
- is Lift Force. [N]
- CL
- is Coefficient of Lift. [dimensionless]
- ½
- is the constant number a half. [dimensionless]
- ρ
- air density. [kg m-3]
- V2
- is the relative velocity of the air squared. [m2 s-2]
- S
- is the surface area of the object the air is travelling over.[m2]
What this means
Increasing any one of the factors involved in lift creation will increase the total lift itself.
As helicopter pilots the factors we can control directly are
- CL with the Cyclic and Collective altering the geometric pitch angles, and as a result angle of attack.
- V2 is analagous to Rotor RPM (in a hover, in translating flight V2 also includes a factor due to the motion of the helicopter) so we can control this as a function of Engine RPM with the throttle.
Indirectly we can control ρ by flying at higher or lower density altitudes. Higher density altitudes mean that the air density is reduced so the total lift force will be reduced if all other factors are still equal.
The term ½ρV2 is known as dynamic pressure.
Web references
Explanation Of Lift
Physical Description of Lift