Aerodynamics I A study guide on aerodynamics for the Piper Archer Introduction The purpose of this pilot briefing is to discuss the basic aerodynamics of the Piper Archer. Please use the following references: - Pilot’s Handbook of Aeronautical Knowledge - Flight Theory for Pilots
A study guide on aerodynamics for the Piper Archer
- Flight Theory for Pilots
In other words, an airplane takes a “bite of air” with the propeller, throwing air back behind over the aircraft. This is the action. The airplane reacts to the propulsion of air, by moving forward.
Consider when you blow air over the top of a piece of paper. The paper rises, displaying the effects of lift.
The rise of the piece of paper is due to the low pressure that was created between the air stream, and the original paper position.
The velocity of the air increased above the paper, thus the pressure decreased, causing lift.
Air going over a wing. Notice the shape of the wing creates a Venturi. The low pressure develops on top of the airfoil.
Notice how the wing is thickest at the middle. It then thins out at the trailing edge. This creates curvature to the wing.
The curve of the wing means that the molecules of air traveling on top of the airfoil have a faster velocity than the molecules of air traveling underneath. According to the Bernoulli, this difference in velocity is what contributes to the pressure differential above (LOW) and below (HIGH) the wing.
Chord Line – The exact line from the leading edge to the trailing edge of the wing.
The next slides will explore the weight & balance of the Piper Archer
Prior to every flight, the weight and balance is calculated for that particular day.
It is known that weight x arm = moment.
Weight = the actual weight of the object/person in pounds.
Arm = “the distance from a datum, to the applied force”
Moment = the product of the weight multiplied by the arm
For the Piper Archer, the envelope that the C.G. must remain within is only 11 inches.
When C.G. shift is calculated for weight loss during flight (due to burning fuel, decreasing the weight in the fuel tanks), it actually is only shifting a few inches.
It is uncommon for the C.G. to reach the forward or aft limits of the envelope. But, it is essential to check it each flight.
Well, perhaps you might, but you sure won’t make it off the ground!
For level flight (top picture) the CG is before the center of pressure
With a small angle of attack (middle picture) the center of pressure is slightly before the CG
With a large angle of attack (bottom picture) the center of pressure is ahead of the CG
When the angle of attack increases, the center of pressure moves forward
When the angle of attack decreases, the center of pressure moves rearward
Resultant force is the average between the force of lift and the force of drag.
Tying it all together, it would make sense that the center of pressure would move during flight.
This is because throughout the flight, your lift varies (you climb, descend, level off) and your drag varies (fly with/without flaps).
Remember, when you climb and descend, you are increasing, or decreasing your angle of attack.
When the forces of lift and drag are constantly changing, the center of pressure is constantly changing.
Always land beyond an aircraft generating significant wingtip vortices.
Rotate prior to their rotation point. ALWAYS give yourself plenty of time to avoid them. Remember to sidestep upwind.
Problem: Have you ever seen a Piper Archer out climb a 727? Probably Not.
So, what good will it do to rotate prior to their rotation point if you can’t remain high above their climb out path? You will eventually fly through them. Time will solve this problem so that the vorticies can dissipate.
Positive static stability means that initially, the aircraft will return to its original position. After being disturbed, it wants to go back.
Neutral static stability means that initially, the aircraft will remain in a new position after being disturbed.
Negative static stability means that initially, the aircraft will continue away from its original state, after being disturbed.
Dynamic stability is the aircraft’s response over a period of time.
Maneuverability, controllability, and stability are each unique design characteristics, don’t mistake them for the same thing!
When you enter ground effect, the following phenomena occur:
“On entering ground effect:
1. Induced drag is decreased
2. Nose-down pitching moments occur
3.The airspeed indicator reads low
Upon leaving ground effect:
1. Induced drag is increased
2. Nose-up pitching moments occur
3The airspeed will read higher (correctly)”
Page 72 Flight Theory for Pilots
According to the diagram, in ground effect, less thrust is required to maintain any given velocity, compared with the thrust required out of ground effect. Because less thrust is required, also displays the correlation as to why you can maintain level flight at a slower airspeed.
“Therefore, the wing will require a lower angle of attack in ground effect to produce the same lift coefficient or, if a constant angle of attack is maintained, an increase in lift coefficient will result” (PHAK 3-7).
Adverse Yaw – You change the camber (shape) of the wing with the ailerons when executing a turn. The upward wing has more lift than the lower wing. In adverse yaw, the aircraft tends to slip towards the upward wing due to the differential of lift.
In a turn, an increase in lift results in an increase in drag. The more drag on the upward wing causes a shift/twist around the vertical axis resulting in an uncontrolled turn.
Adverse Yaw is one explanation for why a rudder is essential.
Stall speeds are based upon 1G, wings level, unaccelerated flight.