POF final recap -
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Explain the Axis of rotation
the line through the rotor head around which the blades rotate, at 90 degress to both the Plane of rotation and the tip path plane
Explain the Shaft Axis
The line consistent with the rotor shaft
Describe what Coning angle is
The angle between the blade and the plane of rotation. The further the Tip path plane and the plane of rotation are apart, the greater the coning angle
What are the 3 causes of dragging in a n articulated rotor system
- Changing blade radius of rotation
- Hookes joint effect
- periodic drag changes
What does changing blade radius of rotation mean
When rotor blades are allowed to flap there is a slight change is the radius of the path the blade C of G follows - this can also be referred to as Conservation of Angular momentum
Describe hookes joint effect
this is similar to dragging through changing radius or rotation but due to the movement of a blade to reposition itself relative to the other blades when a cyclic pitch change is applied.
When the blades are viewed from above they appear to be a equally spaced relative to the shaft axis.
Explain periodic Drag changes
the Blade meet continuously changing airflow as it rotates, the variation in relative airspeed of each blade will cause variations in the amount of drag on each blade which causes lead and lag. Period Drag changes causes produce less leading and dragging than the other 2 forces
Describe Phase Lag
this is when the blades flap to equality and alter the attitude the disc, The blades will however reach their highest and lowest points 90 degrees later than the point where it experiences the max and min increase and decrease of cyclic pitch change. Phase lag is always 90 degrees
Describe advance angle
this is a mechanically set angle between the attachment point of the pitch change link on the swash plate and the blade. This is unchangeable and depends on the helo type
What is translational Lift
This is the total rotor thrust gain resulting from forward speed - it reduces induced flow as the clean air blows over the front of the disc. This happens at around 12-15 knots of airspeed or headwind
What is a retreating blade stall
If Airflow reversal occurs then a retreating blade stall is possible, Airflow reversal causes a loss of lift at the root of the retreating blade. this causes the remainder of the blade to have a higher load on it requiring it to produce all of the thrust. This requires that portion of the blade to have a higher angle of attack ( to try and produce the additional lift required) as the AOA steadily increases to try and produce this additional lift requirement it will hit a point where the airflow breaks away from the blade causing it to stall. As it stalls it will cause the blade to flap down, this will actually increase the AOA further stalling it even more so. This starts at the tip and moves inboard = bad SHIT.
What is airflow reversal
This occurs when the speed of rotation of the retreating blade is high and low at the root, but the airflow from forward flight has an equal value for the whole length of the blade. when the airflow from forward flight is greater than the rotational velocity (at the root end) The airflow will be reversed of that portion of the blade causing a loss of lift.
What are the 3 effects of the flare
- Increase RRPM - due to reduction of teh inflow angle
- Thrust reversal - This is due to the TRT vector changes from tilting forwards to rearwards
- Increase in TRT - Due to the disc tilting rearwards while in forward flight introduces an airflow from underneath the disc, reducing inflow/ increasing AOA and TRT
Describe blade sailing
Its an extreme form of flapback that occur at LOW RRPM in gusty conditions.
The Low RRPM means that wind coming across teh disc will cause significant differences in the airflow velocity between eth advancing and retreating blades which amplify cyclic inputs and cause flap forward and flap back.
blade sailing becomes critical when wind speed is near or greater than the speed of teh retreating blade.
The use of the rotor brake can aid in reducing the chance of Blade Sailing in windy conditions
What are the two combinations of two forces that create the Total reaction
Lift and drag, and rotor drag and rotor thrust
What the three major types of 'powers' the Helos engines have to overcome
- Rotor profile drag - the form drag created by dragging the rotor through the air
- induced power - this is associated with TRT
- parasite power - created inflight due to disc tilt
in reference to the power graphs where is the Maximum ROC speed
Where the maximum power "in hand" exists - or the speed where there is the largest difference between power required and power available.
in reference to the power graphs where is the Maximum Range speed
See Graph on Page 50 of study guide - but in Rocks words "Tangent that Mo FO"
in reference to the power graphs where is the Maximum Angle climb when a helo cannot hover OGE due to power constraints
Ref Page 52 of the study guide, Downward tangent from Power available to the near side of the power required curve
What effect does adding weight to the helo have on Power required graph
It causes it to move UP and RIGHT
What effect does higher altitudes have on Power required graph
It causes it to move UP and RIGHT
During a constant rate of Climb is the total thrust required greater than the trust required in a stable OGE hover?
NO - the same power is required once a stable accent is established
During Auto rotation describe the sections of a blade and the forces acting on each section
The Tip or A section (at the Tip) has high speed which causes the lift and drag vectors to create a TR that is aiming rearwards - causing a dragging force
2/3s from the root of the blade or the B section has a slightly slower speed so its TR created is points near straight up cause nil drag or driving force
1/4 - 1/2 from the root or C has an even slower speed which creates an AOA that results in the TR titling Forward which creates a driving force on that blade
At very close to the root the airflow does not create any force
Describe the Avoid curve
It is an envelope of height/airspeed combination to be avoided as a safe Engine Off landing cannot be assured
in forward flight during an auto rotation what happens to the driving section of the entire disc
It deforms and moves towards the left hand side of the disc, this is due to the inflow angle increasing on the retreating side of the disc and the inflow angle decreases on the advancing side. Handling characteristics is virtually teh same as powered flight
At which speed is translational lift the highest
When a Helo is in a stable climb or a hover is the tail rotor thrust the same?
Yes, Power required is the same therefore anti torque force is the same
Define what a loss of tail rotor effectiveness is
an Uncommanded rapid yaw to the right
With regards to Loss of tail rotor effectiveness Wind from which direction is worse - Left or Right
wind/airflow from the Left creates higher chances and more opportunities for Loss of tail rotor effectiveness to take place. this is also true if you are in a right hand turn, as this creates a relative airflow coming from the left hand side
Talk us through the required control inputs to take a helo from a OGE hover in a constant rate climb.
As collective is pulled up, power is increased and the pitch of the rotor blades is increased. This creates more rotor thrust and torques the A/C to the right, left pedal is required to maintain a heading as you accelerate upwards. once the desired climb rate is established the collective can be lowered to maintain a steady hover, peddle can be adjusted accordingly
Describe a vortex ring state
in a hover, if the collective is slightly lowered there is now a ROD flow that is opposing induced flow - at the root the induced flow is quickly overcome and the angle of attack will increase producing greater thrust. At the tips the vortices redirect the flow outside teh rotor disc then draw it inwards from above teh disc which adds to the induced flow. this increases the local inflow angle and reduces angel of attack - which means the AOA is reduced at the tip and increased at the root.....blah blah more to go
SOMEONE TAKE OVER AND PARAPHRASE THIS SHIT!!!
what are the conditions that are suited to vortex ring state
- Little or no horizontal airspeed
- a rate of decent
- induced flow/powered flight
Look at a bunch of power graphs
What would you like to do?
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