Stick-Free Long. St&Cnt (SS&C-8) Flashcards
The pressure distribution over the surface of a plane flap is governed by what two parameters?
- Angle of attack
2. Deflection of the flap (or elevator)
The asymmetric pressure distribution causes the flap to ______.
Float up
The floating tendency is due to what derivative?
C_h_alpha
As the flap floats upward, the pressure distribution changes along the chord. What stability derivative determines this phenomenon?
C_h_delta
C_h_delta provides a _____ force with respect to float.
Restorative
C_h_alpha causes a _____ force with respect to float.
Divergent
True or False
On most control surfaces, it is important to get the floating tendency as close to zero as possible.
True
For the pilot to control the aircraft without undue force, ____ must have a shallow (low) slope, especially for high-speed airplanes.
C_h_delta
Perkins and Hage, Pg. 273
Why is lifting line theory not successful in predicting hinge moment effects?
Because it doesn’t take into account viscous (boundary layer) phenomena.
True or False
Generally on most control surfaces, it is important to get the floating tendency to as near 0 as possible.
True
The slope of ____ must be as close as possible to zero to try and reduce the floating tendency.
C_h_alpha
What is the major technique used in aerodynamic balancing?
The set-back hinge.
If the setback hinge is placed too far aft with respect to the pressure distribution the control is referred to as ______
Overbalanced
What are the three methods for the prediction of hinge moments?
- Semi-analytical/empirical analysis
- Wind tunnel tests
- CFD
Why are wind tunnel tests, in order to study hinge moments, often difficult and ineffective?
Because dynamic similarity is almost impossible to achieve and requires some form of artificial transitting of the local flow to approximate a solution.
What are the three primary geometric factors that can affect the hinge moment?
- Nose shape
- Nose gap
- Hinge axis location
What are the three most common design options that can affect the hinge moment?
- Horn balance
- Sealed control surface
- Beveled trailing edge
What is the benefit of a horn balance?
Outer aerodynamic surface moments balance control surface moments.
What is the benefit of a sealed control surface?
This eliminates air bleed from the lower to the upper surface.
What is the benefit of a beveled trailing edge?
It controls the aft separation of flow, increasing the effectiveness.
What is a trim tab and what is its primary purpose?
An ancillary surface attached to the trailing edge of the control surface. It is used to create moments about the control hinge axis to trim or balance the flap.
True or False
The trim tab can be used as a primary control device.
True
True or False
The deflection of the trim tab determines the equilibrium elevator angle.
True
True or False
The trim tab allows one to set a controlled deflection without a constant input force from the pilot.
True
