HH-60G Asymmetric Roll Rates

Capt Jake Hobson8 June 2014

• 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.

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

Why does this happen?

• Mixing unit design– Limited left cyclic throw

• Aerodynamics – G-Loading (symmetric effects) – Coning Roll

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.

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

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.

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.

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.

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.

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.

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)

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

Conclusion

• The Problem• Why it happens

- Mixing unit- Coning Roll

• Recovery Considerations• Misconceptions