Lesson 2: Aerodynamic Forces
Section 2 - More on Thrust and Drag
THRUST
Thrust is the force that moves an object, such as an aircraft, along a specific path. It is the force that overcomes the resistance of an object moving through air. In an aircraft, thrust comes from propellers or jet engines. The force of thrust on an aircraft has a direct bearing on its speed.
The term “thrust to weight ratio” refers to the ratio between the thrust exerted and the weight of the aircraft. For example, in an F-16, which has 25,000 pounds of afterburner thrust, the thrust to weight ratio is 6.2 to 1. Thrust is needed to overcome the aerodynamic drag. It is the force that pushes an aircraft through the air.
When an aircraft is in level flight at cruising speed, the horizontal and vertical forces cancel each other. The thrust equals the drag, and both are represented by the dimension of force (F). Thrust can be generated by propellers driven by rubber bands to rockets. Try to place a small object in a rubber band, pulling it taunt towards you. Release the rubber band and watch the object propel. You are watching the action of thrust.
Force is equal to the rate of change of momentum with time. Momentum can be represented in accordance with Newton’s law of force in terms of rate of change of momentum or mu (mass times velocity).
To review, Newton’s law of force is force equals mass, m times acceleration and can be written as:
F = ma
In expanded terms, the equation can be written as:
F = ma = m(du/dt) = d(mu)/dt
Where mu is the product of the constant mass and the velocity or the momentum. The force is then equal to the change of momentum with time.
All systems of propulsion result in momentum to the oncoming air. A propeller speeds up the air ahead of it.
In a wind tunnel with a propeller, the air in the test section is blown against the propeller. The propeller is driven by a motor that imparts kinetic energy to the air, which speeds up the incoming air. The resulting increase in momentum produces the force of the thrust. This happens when an airplane is in flight since the airplane’s speed in still air is identical to the speed in the wind tunnel.
A jet turbine burns liquid fuel, and the hot gas of the burnt fuel increases the volume of air and accelerates the flow leaving the turbine. The force produced here is resulting from a change of momentum that increases both mass and velocity.
A rocket, on the other hand, carries its own supply of oxygen needed for combustion of its fuel. It is the only power plant that can function without air or where the oxygen in the air (outer space). A rocket spews out a high-speed jet of hot gases, producing the force of thrust.
Once thrust is induced by any of these means, Newton’s Third Law takes effect. Recall, action equals reaction. In other words, a force equal to that produced by the rate of change of momentum, but opposite in direction, pushes an aircraft forward through the air. Newton’s Second and Third Laws often served as reference in the advancement of aircraft and rocket propulsion in the twentieth century. This can be reflected from balloons and the Wright Flyer to the Apollo Flights putting men on the moon and the Space Shuttle of today.
In our previous section, we began a discussion of drag which is the force experienced in the direction opposite to that of an airplane’s motion. Drag resists the forward motion of an object meaning a force is applied against that object as it tries to move. This can be seen in a car with less surface area pointing in the direction it is moving that will have less drag affecting its performance. Drag or aerodynamic resistance is common in our daily lives.
We experience this force fighting us when we swim, stick a hand out the window of a moving car, ride our bicycle against the wind, or ski downhill.
Any object moving through a fluid will eventually be brought to rest by the drag unless it counters this force.
Issac Newton equated the force exerted by a fluid on a body with the change of momentum in the fluid as being due to the presence of the body. This force is drag.
REYNOLDS NUMBER
Reynolds Number is an important term to introduce here as the drag coefficient of an object depends on its shape and Reynolds number.
Reynolds number is represented in the following equation:
Re = ulp/µ = ul/v
Where Re designates the Reynolds number, a dimensionless quantity.
By using the Reynolds number, which is the ratio of inertia to viscous forces, when we are testing a model, we can change any of the air properties, or wind speed and mode length, to duplicate the Reynolds number of the full-size flying object. If we use a 1:10-scale model, all that is necessary to do in the wind tunnel to obtain complete similarity is to blow the wind in the test section at ten times the flight speed of the airplane.
We can then measure drag and lift forces and determine the distribution of pressure. When we obtain results, we can apply these directly to the full-size aircraft.
An example of a Reynolds number for a moving object is an automobile at sea level of a given length is a linear function of the speed, or twice the speed, twice the Reynolds number.
With a known flight speed and length, the Reynolds number can be computed for the many things that fly through the air.
