Longitudinal Control (SS&C 7)

Question 1

What three items in the stability equation could be used to create a trim moment to balance a change in lift coefficient?

Answer 1

1. Wing moment about the a.c.
2. c.g. position (xa/c)
3. Angle of attack of the horizontal tail (HT)

Question 2

For an aircraft that has a horizontal tail, the effectiveness of what parameter is limited in its ability to create a trimming moment in response to a change in equilibrium lift coefficient?

Answer 2

The moment about the aerodynamic center.

CMac

Question 3

The magnitude of Cm_ac is dependent on what three items?

Answer 3

1. Geometric Twist
2. Aerodynamic twist
3. The trailing edge flap (if installed)

Question 4

On a flying wing, what is the major trim control device?

Answer 4

The flaps

Question 5

As c.g. moves forward, equilibrium CL is _____.

Answer 5

Reduced

Question 6

As c.g. moves aft, equilibrium CL ____.

Answer 6

Increases

Question 7

What are the two basic types of horizontal control?

Answer 7

1. All moving surfaces (stabilators)

2. Combination of a horizontal stabilizer and an elevator flap.

Question 8

Deflecting the elevator increases the ____ of the horizontal tail as a whole.

Answer 8

Chamber

Question 9

A upward elevator deflection is considered _____. (Sign conventions)

Answer 9

Negative

Question 10

A downward elevator deflection is considered _____. (Sign conventions)

Answer 10

Positive

Question 11

What is the elevator control power?

Answer 11

The magnitude of the change in pitching moment coefficient per degree of elevator deflection.

Question 12

What is the definition of the elevator effectiveness parameter? (Def, not Eq. )

Answer 12

The rate of change of the tail angle of attack with respect to elevator deflection.

Question 13

True or False

A downward deflection of the elevator changes the angle of attack at zero lift by shifting it left.

Answer 13

True

Question 14

True or False

A downward deflection of the elevator changes the CL_stall value by shifting it down.

Answer 14

False. It shits upward.

Question 15

How does elevator deflection change the pitching moment curve?

Answer 15

It only shifts it up and down along the Cm axis. The slope does not change.

Question 16

The elevator angle required to trim to the desired CL is proportional to _____, and inversely proportional to ____.

Answer 16

dCM/dCL
CM_delta_e

de = de0 + ((dCM/dCL) / Cm_de)*CL

Question 17

The slope of the elevator deflection vs CL_trim is dependent on _____.

Answer 17

The c.g. location with respect to the N0

Printed Handout: Pg. 21

Question 18

When the c.g. is at the neutral point, the slope of the deltea_e vs CL_trim will be _____.

Answer 18

Zero

Question 19

The most forward c.g. location in free flight is determined by ____

Answer 19

The elevators’ ability to trim that aircraft at all lift coefficients up to CL_max.

Question 20

If the c.g. moves forward, ____ elevator is required for trim.

Answer 20

More

Question 21

If the c.g. moves backward, _____ elevator is required for trim.

Answer 21

Less

Question 22

The extent of the aircrafts c.g. range depends almost entirely on the _____.

Answer 22

Elevator power derivative

Question 23

The magnitude of the elevator power depends on what four terms?

Answer 23

1. Slope of the tail lift curve (a_t)
2. Tail Area (S_t)
3. Tail length from the c.g. (l_t)
4. Elevator effectiveness (tau)

Printed Handout: Pg 25

Question 24

Being in ground effect ____ the tail angle of attack. This requires more _____ to keep the aircraft in trim.

Answer 24

reduces
Negative (up) elevator deflection

Question 25

True or False

The reduced downwash experienced in ground effect increases the slope of the wing and tail lift curves.

Answer 25

True

Question 26

What should be assumed if nothing else is known about the downwash angle?

Answer 26

Assume the downwash angle in ground effect is half that of the free flight value.