Describe the power vs IAS graph?
What is the Lift equation and what does everything stand for in the equation?
- Lift = CL ½ ρ V2 S
- CL = Co-efficient of lift = Since the wing shape is fixed by the designer, any changes in CL must be due to changes in AoA.
- ρ = Air Density (Constant)
- V2 = Velocity or
- TASS = Surface area of wing (Constant)Therefore, what can we change: V2 and AoA (CL)
- Engine/s are not driving rotors
- Collective lowered to prevent RPMR decay
- The now descending aircraft has an airflow from beneath called ‘rate of descent airflow’
- This airflow acts on the blades in such a way as to turn the blades with sufficient speed IOT maintain controlled flight
Draw the basic vector diagram in auto?
What is washout?
- The manufactured twist in the blades designed to allow lift more evenly along the span-wise length of the blade.
Describe the forces on a blade in autorotation using the BVD?
How does change of pitch effect ROD and RPMR?
- AoA increases, TR tilts aft and stalled section moves further along blade.
- Driving region gets smaller.
- Drag increases
- Driven region moves inwards due to AoA pushing TR aft further.
- Aircraft has momentary decrease in ROD until RPMR decay commences due to not enough driving section along the blade. Unless blade is feathered again, rpmr decay will result in too high a ROD.
What Effect does High AUW & High DA have on RoD & RRPM?
- Initially a higher ROD due heavier a-c or less bernoullies as a result the Induced flow angles therefore greater. This increases Inflow angle which moves TR forward and makes for bigger driving section and therefore higher rpmr.
- Small amount of collective required to maintain RPMR which results in a higher pt for flare and less collective to play with at the bottom.
- OVERALL for those going to PNG:
- higher ROD, more collective maintaining RPMR, higher flare pt and less collective for the initial and cushion.
Describe Factor A in autorotation?
- To achieve forward flight from a vertical autorotation, the disk must be tilted forward.As the disk is tilted forward, the plane of rotation is also tilted.
- As the strength and direction of the rate of descent airflow has remained the same, this tilting causes the induced flow angle, AoA and rotor thrust to reduce.
- This reduction leads to an increase in RoD.
- Therefore Factor A increases the RoD in forward flight autorotation.
Describe Factor B in autorotation?
- As the helicopter moves into forward flight, the main rotor system is subjected to an additional airflow from the direction of movement.
- This airflow decreases the strength of the rate of descent airflow and decreases the induced flow angle further.
- The net result of Factor B is a decrease in AoA and TRT and an increase in RoD.
Describe Factor C in autorotation?
- As the helicopter is moving forward, the air that the main rotor system is encountering has not been subjected to the action of the rotor system to the same extent as in vertical descent.
- Therefore, the rate of descent airflow will effectively increase. This increases the induced flow angle, AoA and TRT of the main rotor system.
- The net effect of Factor C is to reduce autorotative RoD.
Describe Factors A, B and C together?
- IN SUMMARY:
- FACTOR A: Flat pitch, reduces AoA, Inflow angle, Increases ROD by decreasing TRT.
- FACTOR B: Forward component which reduces AoA and Inflow angle further increasing ROD by decreasing TRT.
- FACTOR C: Clean air from below increasing AoA and inflow angle which reduces ROD by increasing TRT.
What are the 3 things required for ground resonance and how do you prevent it?
- Fuselage vibrations
- Rotor vibes
- In contact with ground
- If at fly: lift asap
- If not at fly: shut down and stop the vibes asap
What factors effect ground effect?
- Height AGL
- Nature of ground
What is recirculation and what makes it worse?
- When air passing through a disc is recirculated back through again
- Hovering in close vicinity to an object will cause recirc. Eg, cliff, other aircraft...
How does an IGE hover effect the BVD?
- A divergent duct is created below 2/3 rotor diameter which results in a decrease in Induced Airflow (IAF).
- This reduced IAF increases the AoA which stands the thrust vector upward which results in a higher Total Reaction as well as a reduced drag.
- This further results in lowering of the collective to maintain height and therefore:
- Less power is required in IGE hover.
What is flapping to equality?
When a cyclic pitch change does not alter the magnitude of the thrust but simply the Disc Attitude.
What is phase lag and how is it overcome?
- When an action is felt 90 degrees in the direction of circulation from the initial force
- Overcome by advanced angle and control rigging
Describe Tail rotor drift and how is it corrected?
- The Torque required to overcome the torque created by the engines
- Can't have tail rotor on back and front
- 1 x tail rotor has to combat all torque
- Results in sideways movement
- Corrected by:
- Pilot inputs
- Control rigging
- Engine mounts
Describe Tail rotor roll and how is it corrected?
- A roll effect is created when the horizontal vector of the mast is at a different height to the horizontal vector from the tail rotor.
- Therefore creating a rolling effect around the centre of pressure.
What is flapback and when does it occur?
- As inflow across the disc increases with increasing IAS, the effect is it increases rotational airflow on advancing blade and decreases on retreating blade.
- Increased velocity on advancing blade = increased rotor thrust
- Blade flaps up at high point which is 12o'clock position
- Occurs about 10 - 15 kts
What is inflow roll?
- As air passes over the disc in forward flight it increases in volume creating higher induced flow at the rear and less at the front.
- An increase in induce flow decreases AoA and therefore rotor thrust.
- The result tips the blades sideways towards the advancing blade due to phase lag.
What is blade sailing and when does it occur?
- The slowing or accelerating blade is hit with gusty winds that increase the AoA on the advancing blade and opposite for the retreating blade which causes significant and damaging flapping of the blades.
- Occurs during start-up and shut-down of the blades.
What is translational lift?
- As the aircraft moves horizontally forward into clean air, the induced flow is reduced and therefore AoA increased and with it rotor thrust.
- Thus, less collective / power is required with forward airspeed.
- Usually around 20 kts
What is Rotor Profile Power?
Power required to drive the rotor at min pitch, operate the ancillary equip and tail rotor.
Describe airflow reversal?
- The retreating blade loses lift close to the blade root due to moving away from the advancing relative airflow.
- The faster the forward airspeed, the greater the airflow reversal.
- washout counteracts this effect slightly.
What causes retreating blade stall?
- Airflow reversal
- High forward airspeed
- High G manoeuvres
- Rough, abrupt or excessive control inputs
- Turbulent air
- High AUW/DA
Describe vortex ring?
- Recirculation from blade tip vortices causes the blade to stall from the outside in due to increased induced flow and decreased rotor thrust.
- As ROD increases the AoA of the blade root increases passed the stall angle and the root also loses lift.
- This combination results in an increased ROD despite collective being applied.
- Increased thrust increase root AoA and tip vortices making ROD increase.
- Power on
- IAS below 30kts
- ROD greater than 500fpm
- NB/ Can occur outside of these parameters
Signs and recovery technique of Vortex Ring?
- Increase ROD despite applying power
- Loss of control authority
- Recover is to find clean air whether that be to the side, ahead or even backwards.
- You must prevent tip vortices which will reduce ROD and further provide the root with and increased chance of gaining lift.