hh-60g asymmetric roll rates capt jake hobson 8 june 2014
TRANSCRIPT
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HH-60G Asymmetric Roll Rates
Capt Jake Hobson8 June 2014
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• The data presented in this briefing is drawn from the technical report: Roll Rate Data Collection and Analysis on the HH-60G Helicopter, published by the 413th FTS July 2009.
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The Problem
• HH-60G experiences difficulty in rolling out of right hand turns
• In steep loaded right hands turn we experience a 50% reduction in available peak roll rate when trying to roll wings level
• 40°/sec when rolling out of a left hand turn vs 20°/sec when rolling out of right hand turn
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Why does this happen?
• Mixing unit design– Limited left cyclic throw
• Aerodynamics – G-Loading (symmetric effects) – Coning Roll
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Limited Left Cyclic Throw
• At high collective settings we can’t push cyclic left as far as normal!
• With collective full up, left stick throw reduced by 1.7 inches (30% of left authority) when compared to collective full down.
• With full cyclic throw we would gain a 47% increase in available blade pitch on aft blade when collective full up.
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Limited Left Cyclic Throw
• The Army experienced exacerbated right rolling tendencies on a Wide Chord Blade project on their Lima model helicopters.
• They were able to include mixing unit fixes which increases left cyclic throw, and increased left roll rate.
WCB with anhedral tip on UH-60M
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G loading
• Increased G load increases the “weight” of the Aircraft.
• Airfoil control authority remains the same regardless of G loading.
• Results in reduced roll rate at high G.• Affects left and right roll the same.
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Coning Roll
Normal unloaded forward flight
Little difference in inflow angle between forward and aft blades
Loaded forward flight
Large difference in inflow angle between forward and aft blades
• Recall that as load on the aircraft increases the blades will flap up.
• This can be due to increased collective, weight and/or G-loading.
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Coning Roll
Normal unloaded forward flight
Little difference in inflow angle between forward and aft blades
Loaded forward flight
Large difference in inflow angle between forward and aft blades
• Note that in a loaded condition the forward blade will experience a higher AOA since inflow approaches the forward blade from below.
• The opposite occurs on the aft blade, resulting in a decreased inflow angle and reduced AOA.
• We end up with a lift asymmetry.• Due to gyroscopic precession this lift asymmetry results in
a right roll.
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Coning Roll
• In order to counteract the coning roll pilots must apply left stick.
• In a right hand turn this necessary left stick migration results in only 20% lateral stick envelope remaining.
• This is all we have to roll out of a right turn.
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Recovery
Turn Type Collective Recovery
Peak Roll Rate (deg/sec)
Average Altitude loss
Left/Loaded Fixed 40 76’
Right/Loaded Fixed 20 43’
Left/Loaded Reduction 50 80’
Right/Loaded Reduction 40 227’
The data in the chart above is taken from an aircraft attempting a 60° level turn at 120KIAS
• Collective reduction has been shown to reduce the effects of coning roll.
• AHC principles suggest we reduce collective to increase roll rate when terminating right hand turns.
• However collective reduction can results in significant altitude loss.
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Recovery
Turn Type Collective Recovery
Peak Roll Rate (deg/sec)
Average Altitude loss
Left/Loaded Fixed 40 76’
Right/Loaded Fixed 20 43’
Left/Loaded Reduction 50 80’
Right/Loaded Reduction 40 227’
The data in the chart above is taken from an aircraft attempting a 60° level turn at 120KIAS
• Recovery technique used will be situational and based pilots good judgment.
• Note: Torque effects not discussed in test report and should be considered
• Also, not discussed in report is potential of nose down pitch reducing g-load and its effects on roll rate (obvious low level dangers)
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Misconceptions• Initially US Army cited Retreating Blade Stall as a
factor contributing to the roll asymmetry. Test’s conclude this is not a factor.
• Tail Rotor Thrust erroneously cited as an key factor in the roll asymmetry. – See slide notes for deeper discussion/justification
• Using sideslip for recovery shown to be ineffective – Resulted in unanticipated
pitch excursions and airspeed loss
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Conclusion
• The Problem• Why it happens
- Mixing unit- Coning Roll
• Recovery Considerations• Misconceptions