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-   -   Purpose and operation of rudder pedal shakers on the Harrier (https://www.pprune.org/military-aviation/339323-purpose-operation-rudder-pedal-shakers-harrier.html)

stilton 15th Aug 2008 22:37

Purpose and operation of rudder pedal shakers on the Harrier
Can anyone explain how this system works ?

CirrusF 16th Aug 2008 07:30

Google can:In the slow speed flight regime, the airflow over the aircraft can becomeless predictable. For instance, variations in the wind caused by gusts orground obstructions can change the airflow over the aircraft relative tothe aircraft's body and relative to the flight path of the aircraft overthe ground. Also, the aircraft can rotate around its yaw axis due tocontrol inputs so that the body of the aircraft is no longer aligned withthe flight path of the aircraft. Misalignment of airflow over an aircraftrelative to the aircraft's body is known as the angle of sideslip.Excessive angle of sideslip can create serious hazards in the slow speedflight regime. For instance, in the Harrier, if more than a 10°angle of sideslip exists, the aircraft can enter into an uncommanded roll,which can be unrecoverable. To prevent this hazard, the Harrier uses aweathervane-type probe extending into the airflow in front of the cockpitwhich measures angle of sideslip. The weathervane-type probe aligns withthe relative wind, allowing the aircraft to determine angle of sideslip bymeasuring the difference between the rotational position of theweathervane-type probe and the normal alignment of the body of theaircraft with the airflow. In the Harrier, if the angle of sideslipbecomes excessive, a warning is provided to the pilot by a pedal shakerwhich shakes the rudder pedals. The pilot can also visually monitor theangle of sideslip by observing the amount of angular offset of theweathervane probe. However, the Harrier's weathervane probe is fixed inplace even when the Harrier is in normal wing borne flight. This increasesthe drag of the Harrier at higher speeds.

stilton 16th Aug 2008 07:33

Understood, but, which pedal do you push if one is shaking ?

Green Flash 16th Aug 2008 09:00

If the pedals are shaking violently, then pushing any of them may be pointless. The operation of a black and yellow handle, however, might allow you to return safely to earth in a somewhat exciting manner and probably not the way you expected to when you took off!:}

bonajet 16th Aug 2008 09:50

From rather old memories, you apply rudder to the vibrating one. The rolling problem of the Harrier with sideslip was worst around 30 to 100 kts but was, obviously, exacerbated by AoA. One of John Farley’s specialities was flying down the runway whilst rotating the aircraft around the yaw axis - keeping the AoA near zero - without mishap. Very impressive to watch.

CirrusF 16th Aug 2008 11:31

[QUOTE]However, the Harrier's weathervane probe is fixed inplace even when the Harrier is in normal wing borne flight. This increasesthe drag of the Harrier at higher speeds.[QUOTE]Is the "weathervane probe" literally a small weathervane? Presumably with a repeater in the cockpit? And why don't they use an ultrasonic probe which would have no drag at higher speeds? For the uninitiated, ultrasonic weather vanes are more accurate and reliable than electromechanical vanes, and no more expensive - I have one on my boat after the old mechanical one blew off in a gale!

Barn Doors 16th Aug 2008 14:17

Can confirm that the you do indeed kick down on the pedal that is shaking. It is designed to prevent excessive build-up of side-slip which can bite you seriously in the Harrier if coupled with high power and moderate AoA in the 30-120kt flight regime; i.e. decelerating into / accelerating out of the hover. Google Intake Momentum Drag. In the Harrier II the pedal shaker is triggered at 0.06g (lateral acceleration). Believe John Farley can explain in great detail why this particular number was chosen but this 'g' equates to around 1/2 aileron input to maintain wings level with the gear down (zero-scarf nozzle) at typical approach speeds, giving you a margin to correct any potential problems. Side-slip + AoA + High Power (intake air momentum) = departure and it's killed a fair few unwary chaps in the early days before it was really understood. Nowadays the modern Harrier is fitted with a Stability Augmentation System to 'smooth' down the ride....

Hope this helps...

NoHoverstop 16th Aug 2008 14:35

Is the "weathervane probe" literally a small weathervane?

Presumably with a repeater in the cockpit?
No. It's right in front of the pilot. It has beta-lights* on it so it can be seen in the dark.

And why don't they use an ultrasonic probe which would have no drag at higher speeds? For the uninitiated, ultrasonic weather vanes are more accurate and reliable than electromechanical vanes, and no more expensive - I have one on my boat after the old mechanical one blew off in a gale!
Not sure such things existed in the mid 60s, but even if they did they'd have been a poor choice because it would have entailed a significant installation. The windvane uses no power, has no wiring, weighs bog-all, has negligible drag and is dirt cheap.

*tritium plus some sort of fluorescent coating

Two's in 16th Aug 2008 17:47

ultrasonic weather vanes are more accurate and reliable than electromechanical vanes
All sophisticated helo's have a 5'' piece of red wool on the front for the same thing - low failure rate unless the knot comes loose...

ARINC 16th Aug 2008 17:54

I must admit when doing the functional check for the RPS I never really understood what it was for until now !! Incidently we did not check the HUD symbology during the functional.

JF has it all here in his reprinted lecture...

John Farley's Lecture

During the early 1970's a package was developed to help the pilot deal with the issue of sideslip during mid transition where intake momentum drag makes the aircraft directionally unstable. An autostabiliser working through the yaw reaction controls was produced to reduce this instability. Additionally, an indication was provided in the head up display (HUD) of a safe limit of sideforce, allowing the pilot to limit any lateral out of trim that might otherwise result in a loss of lateral control. Finally, in case the pilot was distracted from noticing the HUD warning, rudder pedal shakers were fitted which operated as the HUD sideforce limit was reached. The system shook the pedal that the pilot needed to use to reduce the sideforce, not only reminding him to use his feet, but also indicating which foot.

stilton 16th Aug 2008 19:14

Truly fascinating, thanks for the replys.

There is a certain logic there, in aircraft with stick shakers you push a little to recover !

John Farley 16th Aug 2008 21:38

Stamp out vibration

stilton 17th Aug 2008 02:51

Thanks for the expert opinion John, would activation of the pedal shakers be fairly common or a rare, considerered to be extremely hazardous phenomena ?

John Farley 17th Aug 2008 11:03

The anti-VSTOL sideslip package was introduced to address the very high rolling moment with sideslip that the 201sq ft metal wing aircraft exhibited. The trigger was set at 0.06 lateral g because in those conditions if combined with 12 ADD at 100-120 kt you needed half aileron to keep the wings level.

There is no simple answer to your question because the seriousness of the situation depended not just on the 0.06 lateral g being reached but how far and how fast you exceeded that lateral g as well as the ADD and airspeed at the time. However I don’t believe there were any well known dramas after the package which included the HUD display and artificial directional stability reached all service aircraft.

The whole topic now only applies to the remaining metal wing aircraft (the sole one in the UK is the VAAC plus the Indian and Thai Navy fleets).

The 230sq ft plastic wing aircraft has a greatly reduced rolling moment due to sideslip and indeed until aileron droop was added (to increase STO performance in certain circumstances) then said rolling moment was effectively zero at 12 ADD. When I looked at it on my first flight with the plastic wing back in 1979 I had full rudder (gasp if you are a metal wing man) 120kt and 12 ADD yet had to let go of the stick to see if I was holding any aileron.

noprobs 17th Aug 2008 15:41

Just to clarify some points of confusion that may have crept in here, the critical formula in the transitional speed range with the metal wing involved sideslip, angle of attack and airspeed. If you could keep any one of these to zero, the other 2 would not bite; this was the key to John Farley's amazing flights along a line in the critical speed range while yawing continuously - he kept the AoA at zero. In normal use, it was more useful to keep sideslip, as betrayed by lateral G, to a low value. Hence the RPS, backed up by the HUD sideslip ball, and the vane if the electrical stuff didn't work. Many a student Harrier pilot will have heard the instructor's cries of "Vane, vane, VANE!" during a dodgy transition (or a low speed fight).

Intake momentum drag is another interesting subject covered at length here some time ago. It relates to high power settings and will indeed produce unwanted pitch or yaw, but not roll.

Operation of the RPS was very common, and not a matter for extreme reaction. On a gusty day, you could get both pedals going in turn so you just had to do your best to mean out the sideslip while reducing AoA and IAS.

By the way, while the aircraft vane was a very simple device, the only place it became complicated was in the simulator. With no real airflow to drive it, it had to be motorised to comply with the simulated flight conditions.

Double Zero 17th Aug 2008 21:33


I had the honour of working with J.F, though I'm a ground or 'sitting there' if airborne, type.

I take it from your post you know what you're on about, but I've seen film attributed to intake Momentum Drag, quickly developing into a very high rate of roll, and an unhappy ending ( beyond seat envelope / reaction time of the pilot ) - so even if a secondary effect of Intake Momentum drag, it certainly always seemed to me that the catastrophic roll was the bit to avoid !

I remember J.F. describing the shaker system - which he as you say was instrumental in developing, flying right on the edge repeatedly - as " a rather nice relaxing calf massage " !

noprobs 19th Aug 2008 10:24


I'm not doubting the significance of intake momentum drag, which you quite rightly point out can cause yaw leading to sideslip which might lead to uncontrollable roll, depending on the IAS and AoA. The film to which you refer does indeed show such a sequence of events. On the sound track, you will hear the (American) advice from the ground to accelerate away, but phrased something like "Stow the theta-J lever". IMD in yaw is particularly noticeable in the Harrier as it makes it directionally unstable at low speed - it always wants to point out of wind. In a 360 degree spot turn, little rudder is needed for the first half, but more has to be applied to turn back into wind.

I once saw another pilot (co-incidentally also American) survive a similar incident to the one in the film. He was decelerating with a strong crosswind when he let too much sideslip develop, and he took a while to catch up with the rudder action while trying to accelerate away. This resulted in a rapid yaw through 450 degrees followed by an acceleration to wing-borne flight 90 degrees off his original track. Not only did this scare that pilot, and also quite a few of us spectators, but not least the pilot of another Harrier decelerating in the same original direction, trying to cope with the crosswind, who suddenly got a windscreen full of jet flying across fairly close straight in front of him!

I seem to remember a filmed example of IMD in pitch with an F4. I think that it was a new aircraft on a delivery flight (from St Louis) that just carried on rotating up until the crew ejected.

By the way, thinking of St Louis, I always found it a bit odd when confronted by the huge sign over one of the freeways there proclaiming "St Louis - Home of the Harrier".

Returning to the original subject, there was a fixed relationship between the head-up sideslip ball and the shakers. I think that the point at which the central vertical line became tangential to the ball equated to the 0.06 lateral G at which the appropriate shaker operated.

John Farley 19th Aug 2008 14:47

Nearly - 0.06 g was with the ball tangential to the inside of either vertical line rather than the centre marker.

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