Ch5 Flashcards
What do you adjust on a bell 212 to prevent geometric imbalance?
Check answer
Drag braces
What is the purpose of the tab on the trailing edge of a blade
Tracking adjustment
What do you adjust on a semi-rigid rotor to reduce vibration
Check answer
Dampeners
What are the two factors of vibration we are concerned with?
Amplitude and frequency
What does BIS stand for and what is it?
-Blade Inspection System
-use of thin wires along the spar that will cause a steady light if broken by a crack
(Flashing light is good, steady=change 9V battery and check again, if still steady, replace blade)
What does BIM stand for and what is it?
- Blade Inspection Method
- fill spar (which is pretty much the blade) with nitrogen. Gauge will be yellow and white if no leak (means no crack in spar) and red and black if there is a loss in pressure (means crack in blade)
Check question;
What unit of measurement is used for imbalanced vibrations?
Inches Per Second (IPS)
What are the CPM ranges of the 3 types of vibration (found as a result of MR imbalance)?
- Low frequency= 0-500
- Med frequency=500-2000
- High frequency=2000 plus
If a pilot complains about a tingling in his/her feet, what is it’s cause?
High frequency vibrations in tail rotor
What are the methods used to determine track?
- stick method
- flag method
- light reflector method
- pre-track method (manufacturer does this)
- electronic or strobe method (most common)
What method is not used to determine track?
Tape method (used for balancing)
After alignment, when balancing, blades may only be swept ____ of alignment points
Aft (never want a diving blade)
After tracking you must always test…
Autorotation rpm (very important step)
What is the purpose of blade alignment?
To align CofP with pitch change axis
What is the Astar flex beam made of and what is an advantage of this?
- fibreglass
- not subject to fatigue
What is the difference between a fibreglass and composite blade?
Fibreglass has a fibreglass spar and composite has a metal spar
What is the MAJOR advantage of a fibreglass blade?
It’s life
What are the 2 most common types of rotor heads?
- semi-rigid
- fully articulated
What is soft in plane?
Bells version of fully articulated rotor head (same thing)
Explain retreating blade stall
The condition of a rotor blade when its operating at an angle of attack greater than the max. angle of lift.
-occurs at high forward speed to the retreating blade and all blades during “settling of power”
Why were symmetrical airfoils used on early helicopters?
Lower cost and ease of construction
Why was wood used originally for helicopters blades? What shortcomings were found in wooden MR blades?
- because of their lifetime (it’s use depends on its condition)
- a short coming was blades were a matched set (to replace one, had to replace both)
What factors affect autorotation rpm?
- gross weight of heli
- density alt.
What is a “2 per rev” vibration?
It’s a low frequency vibration caused by the rotor itself
A “teeter totter” rotor head is more formally referred to as?
Semi-rigid rotor head
Has seesaw action or flapping axis
After tracking adjustments what must be verified on each aircraft?
Check that all blades are traveling within the same tip path plane
What is the function of a lead/lag dampener?
Dampens the hunting action of the lead-lag hinge to prevent geometric imbalance
How many types of lead-lag dampeners are in use?
3
- hydraulic
- multiple disc
- elastomeric
A faulty lead/lag dampener may induce what dangerous condition?
A shuffle with a 1:1 beat of the rotor
What is the difference between vertical and lateral vibration?
Vertical vibration is normally associated with track, while lateral vibration is associated with an unbalanced condition
