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-   -   Harrier transition to and from hover (https://www.pprune.org/military-aviation/414038-harrier-transition-hover.html)

tartare 2nd May 2010 21:55

Harrier transition to and from hover
 
Question for any Harrier drivers out there.
Was asked by number one son about Harrier hovering... and realised I didn't know the answer myself.
When transitioning to the hover, are nozzles rotated to vertical and then airspeed left to wash off, while engine rpm is gradually increased to counter the effect of the unloading of the wings?
Similarly, when transitioning out of hover, increase power and rotate nozzles slowly back as wings develop lift?
Obviously it's not like a helicopter - you couldn't just slam the nozzles back and apply full power - it'd drop out of the sky...
How are you taught to fly the transition to and from hover?

LateArmLive 2nd May 2010 22:50


When transitioning to the hover, are nozzles rotated to vertical and then airspeed left to wash off, while engine rpm is gradually increased to counter the effect of the unloading of the wings?
Yes, that's right. Nozzles are usually rotated in 2 chunks - 40 degrees selected downwind, then hover-stop (approx 80 deg as the jet sits 8 or 9 deg nose up in the landing attitude) at the deceleration point which is about half a mile from the desired landing spot. The pilot will be flying 10 alpha until the hover-stop is selected; he will then keep the jet level with increasing power as the wing lift gradually reduces. Below about 45kts the aircraft will be pretty much wholly jet-borne.

Fore and aft speed can be controlled in the deceleration and hover by pitch, although nozzle movements are generally used if more than 5 deg NU/ND is required.


Similarly, when transitioning out of hover, increase power and rotate nozzles slowly back as wings develop lift?
Exactly that. From the hover, add some power, pitch up a couple of degrees then nozzle out. The rate of nozzle movement depends on how much excess power is available.

Getting in and out of the hover is the trickiest part of VSTOL ops. In a steady hover the jet is pretty well balanced and flies like a sluggish helicopter. The transition can be tricky as all the variables are constantly changing. Keeping the aircraft in balance is of paramount importance, as Intake Momentum Drag can flip a jet over in fairly short order. I'll let a QFI explain that one........;)

Hope this helps

tartare 2nd May 2010 23:36

Tks for that!
R/e water injection - I had heard about this - I assume water is sprayed into the compressor to increase the effective volume of air moving through the engine and also cool the engine ( I understand the Pegasus can only be run at full hover thrust for around a minute at any one time?)
Also, I note there's a yaw vane in front of the cockpit.
I assume this is to ensure that hover transitions take place into the wind to prevent the jet weathercocking - rolling over - seem to remember some film of a spectacular GR3 ejection at low level when the jet yawed unexpectedly..

Trim Stab 3rd May 2010 09:22

How is the vertical landing carried out? It looks rather crude from the outside, as if the pilot just chops the power at about 30ft. But presumably you try to make the drop as smooth as possible? Or is the seemingly brutal drop deliberate in order to lessen the chance of instability in ground effect, or FOD ingestion?

NigelOnDraft 3rd May 2010 09:37


I assume water is sprayed into the compressor to increase the effective volume of air moving through the engine and also cool the engine
Direct into the turnine area IIRC... Basically reduces JPT, allowing more fuel / thrust / RPM keeping within the (now higher) JPT limits.


I understand the Pegasus can only be run at full hover thrust for around a minute at any one time?)
There's a big table of limits... but the top rating is for 15s only :eek: (and flashing lights to warn you!)


Also, I note there's a yaw vane in front of the cockpit.
I assume this is to ensure that hover transitions take place into the wind to prevent the jet weathercocking - rolling over
Yes - not necessarily "into wind", you just keep it straight with rudder so the relative wind is fore/aft. If you don't the rudder pedals start shaking, the QFI in the back/on the radio shouts "Vane", and if you ignore all that :{ Critical period is 30K-120K - again IIRC. Below 30K no probs :ok:


How is the vertical landing carried out? It looks rather crude from the outside, as if the pilot just chops the power at about 30ft.
Not really "chop" the power, but you do not "cushion" the landing - just keep the RoD on until impact :ooh: then idle straight away, and nozzles aft. It has been known at this point for people to do this in the wrong order (no names ;) ) which is great fun for any observers...:D


Or is the seemingly brutal drop deliberate in order to lessen the chance of instability in ground effect, or FOD ingestion?
Spot on :D

NoD

BOAC 3rd May 2010 11:50

To amplify NoD's coverage:

My QFI used to shout 'Vane ****'

This thread is bound to send a call to John F who will no doubt remind you all that the
Intake Momentum Drag mentioned by LAL is effectively a function of a mathematical product of engine power x yaw angle x angle of attack, and that by keeping ANY one of those at zero, it is avoided (engine power - yes, that was a problem....:))

The ejection Tartare mentions is most probably the unfortunate US Marine who fatally forgot 'the rule'.

VinRouge 3rd May 2010 14:51

To answer all these questions and more, I strongly reccommend JF's book "A view from the hover". Simple laymans explainations of lots of phenomena.

"intake momentum drag" sounds hideous but is relatively simple to understand. A jet engines thrust is actually a NET value, a summation of all the forces in the engine. The front fan creates significant drag, as the airflow entering the fan has to slow down considerably. This creates a force ahead of the centre of gravity, which, if the aircraft is allowed to yaw, will create an unstable situation whereby the force creates more yaw, which generates more force.

Of course, this yaw at slow speeds is combatted in the harrier using jets of bleed air, if the yaw force exceeds the reaction jet force, you are outside of restoring control to the Aircraft.

Thats how I understand it anyways!

BOAC 3rd May 2010 16:27

'IMD' occurs on ALL jet engined aircraft with any angle of sideslip. It is the lack of directional stability from the fin at low IAS that causes VSTOL a/c to have this problem, and of course the Harrier experiences this 'force' much further forward on the fuselage than other a/c.

BrakingStop 3rd May 2010 16:48

IMD
 
Intake Momentum Drag happens to all jet aircraft. If an ac is experiencing sideslip then the air being drawn into the intakes changes direction from the original relative airflow in order for it to pass directly into the intakes. This change of direction is a change of momentum and exerts a yawing moment on the side of the fuselage forward of the intakes. This yawing moment is IMD. At normal speeds (>120 kts) the tail has enough aerodynamic stability to overcome this yawing moment and so most normal ac don't observe any IMD. At slower speeds (30 - 120 kts) IMD can be a huge factor because the tail has little aerodynamic stability at these speeds. Once the yawing starts the pilot, if they catch it, is unlikely to have enough rudder authority (either aerodynamic or reaction control power) to stop the yaw from building and the ac will depart. IMD is avoided in the Harrier by keeping the vane straight (using rudder) and hence sideslip at zero. Alternatively, keep the AOA at zero, as has already been mentioned.

Standing-by to be corrected by JF.

BS

Trim Stab 3rd May 2010 17:11


Critical period is 30K-120K
How does that effect carrier landings? Does the carrier have to sail at a speed and heading to keep the over-deck wind within certain limits?

Madbob 4th May 2010 09:01

Tatare

NOD should also have mentioned FUEL, or more importantly the lack of it when down to hover weight.

On cool/cold day it is less of a problem but a hot one a VL might not even be possible but whatever the situation the trick is to keep the decel period to a minimum, establish over the pad, get into a stable descent and land without running out of fuel first!

Flashing lights certainly concentrate the mind.....

MB

lifts a gift 4th May 2010 09:07

IMD
 
IMD, well explained by BS is not in itself a problem. It just makes the jet go sideways. The problem is that swept wings producing lift do not like to go sideways. The into wind wing loses sweep and Cl increases. The downwind wing has its effective sweep increased (and is shielded somewhat by the fuselage) and therefore loses lift. Result mega, rapid, uncontrollabe rolling moment. Not good during VSTOL.

The rolling moment depends on a) IAS b)alpha c) sideslip. Any one of these zero - no probs (no lift). Either b or c large and the other two small big probs.

Given that one must accel/decel through the 30-90 kt band where the jet is directionally unstable, and that to lift a war load wing lift is needed at these speeds b and or c must be minimised.

In my day in the GR1-3 8 to 10 units of alpha was reasonable 12 a bit gamey.

Sideslip is minimised by 1) Yaw autostabs 2)Rudder pedal shakers 3) HUD sideforce symbology and 4) THE VANE

Early signs of things going wrong are a lateral stick displacement to keep wings level. This is quite attention grabbing and the recovery actions are simultaneously 1. Rudder to get the vane straight. 2.Full pwr through the limiters to increase pwr of the 'puffers' and inc vertical lift thereby reducing AoA. 3. Stick fwd to reduce AoA. 4.Change trousers.

Hope this is of use. For all a/c but particularly the Bona-Jet

Lifts a Gift. But thrusts a MUST

Spon Clayton 4th May 2010 11:02

Assymetric lift is the problem, as described above. Below about 30 KIAS there is no lift and assymetric zero is zero, thus no uncontrollable rolling moment. Zero alpha achieves the same effect-hence it is in the recovery drill.

Pontius 4th May 2010 14:29


How does that effect carrier landings? Does the carrier have to sail at a speed and heading to keep the over-deck wind within certain limits?
Not a snag mate 'cos you're pointing into relative wind, so you don't get any of the nastiness described above. For instance, if the boat is sailing at 20kts into a 20kt wind then you'd have 40kts over the pad (I think the fisheads called it a deck). Now, this would take you into that dreaded band of IAS but it didn't matter because the wind would be aligned with the runway, your vane would be straight and your feet would be shaker-free.

Sometimes the cod faces enjoyed doing things which didn't involve the airfield being driven into wind and it could make things more interesting than your every day arrival. For instance, if you've got a tailwind on the boat from 45 degrees off the right side, then you'd come into the hover on relative bearing of 135, off the boat's front and vertically land facing that direction (and then have fun with the yellow shirts trying to park you).

Finally, IF it was all going for a can of worms and the boat HAD to face a particular direction AND you had to land from straight up the back of it, then the fisheads had a little spin-wheel where they could adjust their speed so the relative wind was within limits.

The real fun was when you were accelling and decelling during a display, with a 'challenging' on or off crowd wind. You obviously wanted to keep everything looking bona, so had to keep pointing down the crowd line but with the vane at 90 degrees and, if you hadn't already turned it off, the shakers going ten to the dozen. Quickest accells and decells I ever did (to get through the 'nasty' 30-120 regime).

Doesn't happen on a Boeing :)

Double Zero 4th May 2010 14:46

On an unpleasant note for a minute, isn't 30 - 120kts pretty much the same regime as the AV-8A Stencel seat is regarded as extremely dodgy ?

You know, the Stencel which Art Nalls was forced to fit to Sea Harrier XZ439 after Martin Baker, in an inspired move, refused support ?! :ugh:

John Farley 5th May 2010 10:43

A quote from page 80

The other problem stemmed from the intakes and meant that, if left to its own devices, a P1127 (or early Harriers) flying slower than about 100 kt wanted to go tail first. The pilot literally had to use his feet to keep the aircraft pointing into the airflow. This was directly analogous with the need for the pilot of a tail-dragger aeroplane to use his feet to stop it swinging and ground looping when landing, especially in a crosswind.

The reason for this was that the aerodynamic stabilising effects of the P1127 fin were no different from any other aircraft so faded away as one got slower. Unfortunately there was a destabilising force that increased as flying speed reduced and so defeated the residual efforts of the fin. This force was called intake momentum drag. It exists on all jet engine intakes and gets greater as rpm are increased. Thus whenever you were flying slowly and necessarily using jet lift not wing lift, up went your rpm and up went the intake momentum drag.

To understand why this destabilised the aircraft directionally we need to look at the airflow round the aircraft when viewed from above. In Fig 3 I have tried to indicate that everything is fine when the aircraft is pointing directly into the airflow.

Fig 3

http://img.photobucket.com/albums/v1...rley/CH4F3.jpg

However, what happens if the aircraft starts travelling slightly sideways through the air because of a cross wind or a deliberate move by the pilot is shown in Fig 4.

Fig 4

http://img.photobucket.com/albums/v1...rley/CH4F4.jpg

With the total airflow - represented by the blue arrow - now coming at an angle to the nose, we must think about its two components as shown. The green being that part which is straight on the nose and the red that part which is blowing directly across the nose.

The red arrow is of course the troublemaker because its effect is felt at the intake which is well ahead of the centre of gravity and so opposes the fin. Should the pilot allow the aircraft to swap ends and fly tail first you might think it would just be embarrassing for him because in his debrief he will be told to try harder on the rudder. Sadly he is unlikely to make the debrief because, at speeds greater than about 70 kt as the aircraft goes seriously sideways, the leading wing will generate much more lift than the other and the aircraft will roll out of control, thanks to what is termed ‘rolling moment due to sideslip’. Such asymmetric lift can easily swamp the aerodynamic and reaction controls.

Clearly some exotic technology was called for to help the pilot keep the aircraft pointing into the airflow. In fact all that was needed was a simple wind vane as seen on any church steeple. It is mounted in front of the pilot and always shows him where the airflow is coming from.

Later on Harriers were fitted with an artificial directional stability autostab (mid 70s) that helped keep the vane in the middle at mid transition speeds.

Re the Stencil seat this had an outstanding performance at low speeds because it used an explosive canopy spreading device which inflated the canopy quicker than the airflow could. The changeover speed between slow and highspeed modes was about 210 to 230 kts. The bad news was that if you got the low speed mode (with explosive spreading) at the top of the changeover band it might (might) break your neck. So either punch out above 250 kts or below 180 kts for a gentler ride. Not hard to organise as even gliding at 210 kts you could pull the nose up before ejecting.

Double Zero 5th May 2010 14:23

Mr. Farley,

Please excuse me asking stupid questions; after all I usually do !

I take the hint that Art Nalls' seat may not be so dodgy after all, but can't help thinking Martin Baker scored a huge own goal for UK Ltd PR...

What's this 'canopy spreading' lark then, some sort of coaming level MDC ?

Thanks for the diagrams, most instructive for a pleb' like me ( who's only going to get near flying a Harrier via a simulator ).

Ta,

DZ

BOAC 5th May 2010 16:58

How re-assuring and unusual to find so many people agreeing on something on PPRune:)

00 - John refers to parachute canopy, not a/c. The MDC was, I think unchanged from the (one that didn't quite work for me) GR3

A 1(spit) Sqn friend told me how rapidly they learnt when coming alongside Ark to use only bank to manoeuvre over the deck, and not to be tempted to turn the bona-jet 90 degrees across the 40kt wind to hover-taxy onto the spot...............................

Old-Duffer 5th May 2010 17:19

The Accident.....
 
....... to which BOAC and tartare refer was a development aircraft being flown by a Major Rosberg and it happened at a Brit Aerospace airfield (Dunsfold - I think).

All the test flights were filmed at the time, hence the loss of the pilot was clear for all to see. A few years later, the RAF sought permission to use the film as part of a flight safety film they were putting together and BAe refused. However, they relented subsequently.

The film 'starred' the actor Richard O'Sullivan as a pilot and one of the scenes included a chap called Chris Humphrey, who was unfortunately killed before the film's release, demonstrating a Harrier to iirc the Swiss.

BOAC 5th May 2010 17:47

O-D - I don't think I saw the film with 'Huggy Bear' in it, but the 'raw' footage was certainly shown on the OCU in the late 70's as a firm reminder.


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