Lecture 12 Flashcards
What are tandem rotor helicopters?
Aircraft that have two main rotors that rotate in opposite directions
Why do the two rotors on a tandem rotor helicopter rotate in opposite directions?
This counter rotation eliminates torque, and eliminates the need for a tail rotor
What are the benefits of having two rotors on a helicopter?
Twice as much wing area
Larger CG range
Eliminates the horsepower needed to drive a tail rotor
The tail rotor on the _____ provides as much as _____ of thrust and requires as much as _____ to run.*
CH-53
3500 pounds
1500 HP
On the _____ the rotors are connected by a _____, so that they are _____ and do not _____ with each other while running.*
Boeing CH-47
drive shaft
synchronized
collide
What does the collective stick do?*
Where is it located?*
Increases or decreases pitch of all main rotor blades at once or collectively
Pull up = aircraft goes up
Push down = aircraft goes down
On left side of pilot
What does the cyclic stick do?*
Where is it located?*
Increases the pitch on the blades at one point in their rotating cycle
Push forward = helicopter goes forward
Pull backward = helicopter goes backward
Push left = helicopter moves left
Push right = helicopter moves left right
Located in front of pilot
Why can tandem rotor helicopters move backward and single rotor helicopters can not?
No longer worried about hitting the tail rotor with the main rotor
No longer worried about hitting the tail rotor on the ground
What do the anti-torque pedals do?*
Where are they located?*
Controls direction helicopter faces
Single rotor: changes the pitch on the tail rotor blades
Tandem rotor: tilts main rotors in opposite directions
Push on the left pedal = helicopter yaws left
Push on the right pedal = helicopter yaws right
Located at pilot’s feet
What is feathering?*
Increasing and decreasing the pitch of the rotor blade
The blade _____ travel _____ in the _____ amount of time than the blade _____ causing an _____ of lift along the span of the blade.
tips
further
same
roots
imbalance
What are the retreating blade and advancing blade?
Retreating blade: the rotor blade moving backward
Advancing blade: the rotor blade moving forward
Where are the most and least effective areas in producing lift on the main rotors?
The outer 25% of the blade is very effective at making lift
The inner most area is least effective at making lift
Airspeed _____ across the blade.
increases
Which blade tip has the highest airspeed in forward flight?
Advancing blade
How is lift equalized along the span of a helicopter blade?*
What is the term for this?*
A twist that lowers the angle of incidence between the blade root and the blade tip
High angle at root, low angle at tip
Wash out
What is dissymmetry of lift?*
In a forward flight, the speed of the air over the rotor blades is different on the advancing blade and the retreating blade
Causing a greater lifting force on the right side (advancing blade) of the helicopter than the left side (retreating blade)
Does dissymmetry of lift cause the helicopter to roll? If not, what happens? How?
No, the helicopter pitches up due to gyroscopic precession
What is gyroscopic precession?*
On a spinning object the resulting effect happens 90 degrees than the input force
What is the solution to dissymmetry of lift?*
Install flapping hinges
How do flapping hinges fix dissymmetry of lift?*
A flapping hinge lets the blade flap up when there is more lift, and down when there is less lift
Flapping causes the lift to equalize on each side
How is lift equalized using flapping hinges?*
Flapping changes the relative wind approaching the blades, thus changing the AOA, and therefore changing the Coefficient of Lift
As the Advancing Blade rises, the Relative Wind seems to come from slightly above so the AOA is smaller
The changing AOA temporarily decreases lift on the advancing side to offset the higher velocity
As the Retreating Blade falls, the Relative Wind seems to come from slightly below so the AOA is larger
The changing AOA temporarily increases lift on the retreating side to offset the higher velocity.
What is the Coriolis effect or the “ice skater effect”?*
Blade flaps up: center of gravity shifts towards the center of the aircraft (ice skater pulls arms in) so the blade wants to try to speed up
Blade flaps down: center of gravity shifts away from the center of the aircraft (ice skater puts arms out) so the blade wants to try to slow down
What problem stopped helicopter development for years?
Coriolis effect
What is the result of the Coriolis effect?
The advancing blade wants to go faster
The retreating blade wants to go slower
Causes a very nasty rotor vibration and a pitching up tendency
What is the solution to the Coriolis effect?*
Lead-lag hinges
What do lead-lag hinges do?
Allows the blades to speed up and slow down if they want to
The blades lead ahead on the advancing blade, and lag behind on the retreating blade
What new problem did lead-lag hinges create?*
Massive vibration caused by lead-lag movement
Who was Igor Sikorsky?
One of many early helicopter developers
Found the solution to lead-lag vibration
How did Sikorsky eventually figure out how to fix lead-lag vibration?
Noticed excessive wear at the limit stops for the lead-lag hinges on the rotor blades
Centrifugal force was not great enough to make the blades lead and lag slowly
Instead they had be leading and lagging violently
A dampener was installed to slow the leading and lagging, and the helicopter flew perfectly**
What is the solution to lead-lag vibration?*
Lead-lag dampeners
What is a fully articulated rotor head?*
Where is this design most common?
A rotor with flapping hinges and lead-lag hinges
Most common on helicopters with more than two rotor blades
What is a semi-rigid rotor system?*
Where is this design most common?
A rotor with a lead-lag hinge and no flapping hinge
Helicopters with two blades
How does a semi-rigid rotor system work?
A central pivot to allow the rotor to teeter at the center
Allows the advancing blade to rise and the retreating blade to fall
The relative wind change produces the desired AOA change on each side
How does the Coriolis effect affect a semi-rigid rotor system?
Since the blades are rigidly connected one must go down as the other goes up
The Center of Gravity of both blades appear to move toward the center at the same time
Basically eliminates the Coriolis effect
Why are semi-rigid rotors designed the way they are?
So the rotors hang below the pivot point
Center of Gravity does not shift
What is a rigid rotor system?*
There are no hinges in the rotor blades
How does a rigid rotor system handle flapping and lead-lag?
Elastomeric bearings and composite flexures
List advantages of a rigid rotor system
Improved control response
Improvements in vibration control
No risk of ground-resonance
More expensive
How does the Starflex system accommodate flapping?
A flexible composite resin arm that bends up and down
Two strong composite resin plates hold the arm and withstand the centrifugal forces of the rotor spinning, keeping the blade attached
How does the Starflex system accommodate lead-lag?
Polymer fitting allows the whole blade and plate assembly to flex
Polymer fitting also acts as the lead lag dampener
What type of helicopters does the Starflex rotor head work best on?
Light and medium lift helicopters
What are the advantages of the Starflex system?
No hinges that require lubrication
No need for an additional lead lag dampener
No hydraulic fluid to leak
All of the parts are exposed for easy inspection
How is gyroscopic precession fixed?*
The pitch input to the main rotor is automatically put in 90 degrees before you need it
Example:
To move forward, push cyclic stick forward
Increase in pitch happens on the left
Gyroscopic precession causes this to be felt at the rear of the aircraft
Moving it forward
