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

NoHoverstop 5th May 2010 18:03

try teaching a computer all of that
 
Well of course with our HS 1174 we 'ad it tough...

Firstly, 'the book' says helpful things like "minimise sideslip when the ASI registers" (i.e. at all speeds above 'hover'). So as a boffin trying to turn that into something a computer can understand it helped to know the science behind an appropriate, formal, interpretation of 'minimise' and what sorts of min IAS the ASI would actually stir into life at (bearing in mind that the air data computer used real gears and cams to do its sums). Some helpful chaps had left a nice folder (marked "John Farley's Rolling Moments") of plots in an acquired filing cabinet and and summarised the worst cases into 1/2 roll-stick boundaries as functions of sideslip and incidence for different speeds from 30 kts upwards. We also put in a 'don't be silly' limit on the rudder, to limit the amount of 'you're only making it worse' rudder (and hence yaw puffer) that could be demanded by our whizzy flexible experimental flight control system in the worst bit of the transition region. Of course, it was allowed to do some of that in the name of flexibility; no point in paying good money for a high-authority FCS only to cut out massive chunks of that capability with over-cautious lines on graphs coded into software.

Testing it all was of course great fun. Having learned the lesson from the initial phase over Bedfordshire (debrief: "no, we don't know what sideslip you got to, the sensors were off-scale, why didn't you stop sooner?") we switched to the skies of Wiltshire; obviously one doesn't want to do this sort of testing near one's own house. Once sorted with a 'complete' clearance, it was then a case of sticking to one's guns when the control law designers complained that the software limits prevented their wonderful stuff from freely expressing itself. Well, mostly anyway. Again, going back to earlier experiences in Bedfordshire skies, it was apparent that having got safe limits to let us begin to see what we could really get computers to do for the next generation, we would then learn where it was useful to relax those limits. Sometimes this required the safety pilot to take the boffin for a hover and say "look, your limits are here...., but actually you can get away with THIS...". Discussions were kept short, mindful that over-prolonged debate would be terminated, permanently, by lack of fuel (so discussions in the hover over how much nozzling-out to allow were very brief indeed). All straightforward really. Then we had to extend the FCS clearance for take-offs and rolling landings. And ski-jumps (the aircraft isn't even properly flying when it leaves the ramp), and real shipboard ops etc etc. So really we were very fortunate that JF and his colleagues had done such a good job in understanding (and explaining) the nature of V/STOL and designing such a good jet in the first place :ok:

noprobs 5th May 2010 20:01

To get back to the original question, it is important not to try to descend too slowly when near the ground. One major problem is hot air reingestion, which can lead to an engine surge. There is also some handling instability as all that thrust (which is a must) comes back up and hits the airframe. As the mainwheel touches down and starts to upset the weight/thrust balance, you then need to get rid of the thrust quickly, which jet engine fuel control systems generally don't cope with too well. The Pegasus therefore, even with the current twin, dual-channel digital system, needs a rapid-acting mechanical fuel flow relief system to make it happen.

IMD is a whole other area that has been discussed at length on this forum, but JF's diagram does help to simplify the root cause. The resultant roll is the real killer. If by skill or luck you avoid it, the outcome may be acceptable. I have seen Harriers doing inadvertent 360 degree (or in one case 450 degree) yawing turns during a decell and then land safely. Incidentally, IMD can also occur in pitch, as other jet pilots have found out to their cost, specifically with over-enthusiastic take-offs. As to JF's backwards formation lead, as a Harrier pilot, I found it a little less amazing than his continuous yawing flight along the line of the runway. One thing to beware of in rearward flight is the reversed action of the tailplane (elevator). While the reaction controls continue to work in the natural sense, a high reversing speed can lead to the canard-like tailplane acting such that rearward control column movement (relative to the pilot) produces a nose-down pitching moment.

Rudder pedal shakers and yaw stabs do help to keep you out of trouble, but few mechanical devices are trouble-free. When yaw stabs were still a novelty, I well remember taking off from a narrow strip that had very tall jungle close to each side of it. Shortly after lift-off, the system decided that it needed to demand maximum yaw, giving me a very impressive view of the trees before I booted the rudder back the other way!

But at least life in that cramped cockpit was never boring.

jimgriff 6th May 2010 08:02

Double Zero- Ballistic canopy spreaders are a pyrotechnic device where the base of the canopy is "shot" out in all directions (360*) by small "bullets" attatched to the shroud lines /canopy base by a "gun" which is timed to fire when the chute has reached full stretch but not yet full of air.
Normal parachutes use airflow to "inflate" the chute but sometimes this can prove too slow when there is insufficient airflow going into the chute to inflate it as happens with very slow speed ejections.
The balistic spreader "forces" the mouth of the chute open to catch any availible air in the chute at low speed.
Can be a complicated proceedure and yet another "widget" to worry about when you need a system to work flawlessly 100% of the time.

Helmut Mann 4th Mar 2014 13:02

Hi All,


My apologies for digging up an old thread.


I am beginning a scratch build scale AV8B and I have a question which research on the web hasn't answered for me. If anyone can assist I'd be grateful:


I understand there are puffer jets on the wingtips of the Harrier for roll control during the hover. My guess is there are also jets on the nose and/or tail for pitch. Is this correct?


Is there also a puffer jet or pair of jets for yaw?

LowObservable 4th Mar 2014 13:26

I recall reading a Raymond Baxter account of a sortie in G-VTOL that started with JF's famous/infamous takeoff, and JF's safety brief, which included a warning that Baxter would not hear EJECT more than once...

cornish-stormrider 4th Mar 2014 13:44

Harrier transition to and from hover
 
Kudos to digging up this thread
I never realised how much of a handful the harrier was, and why the jockeys regarded themselves as the creme de menthe,

A heartfelt RIP to all those it bit, and congrats to all who made it safely to the end of their time on the jet

Moreover I do declare this thread to be fascinating and such deserves the accolade of
"Science, bitch "

KG86 4th Mar 2014 14:52

Mention of the various fatals above reminds me of a rumour that was rife in RAFG in the 80s.

The German F104 had so many fatal accidents that it was nicknamed the 'widow maker'. But I was led to believe that the accident 'rate' for the Harrier GR1/3 was, percentage-wise, worse than the F104.

BOAC 4th Mar 2014 15:46

Helmut - you have been ignored!!

Yes, nose and tail for pitch and 2xtail for yaw.

thing 4th Mar 2014 15:53


But I was led to believe that the accident 'rate' for the Harrier GR1/3 was, percentage-wise, worse than the F104.
Percentage losses for all RAF Harriers was much higher than the F104, I seem to remember a figure of over a half were lost although I'm willing to be corrected on that.

Interestingly the Luftwaffe F104 percentage loss rate was around 30%, around the same as the Lightning and only slightly more than the RAF Phantom which panned out at around 28%.

Having said that the Luftwaffe had I think nearly 900 F104's so 30% of that is a lot of tin.

John Farley 4th Mar 2014 16:04

Helmut

Yep, there are puffers for all three axes.

http://img.photobucket.com/albums/v1...ps6a9e7390.jpg

The shutters are mechanically connected to the relevant aerodynamic surface so the effect is that the aircraft responds to stick and rudder in the hover as if it was flying conventioally. Magic. The front puffer is connected to the stick and opens if you pull back.

The air to the puffer system is turned off mecanically with nozzle angles less than 20 deg. So the pilot does not have to do anything - like remember to turn it on when slowing down.

kg86.

One can be led to believe all sorts of things in this life......

While here may I say that the Harrier has many unsung advantages. To name but three you can aim weapons very accurately in the hover (although best limited to targets such as hospitals or women and children queuing for soup) if the taget does fire back you can of course retreat while still facing the enemy (useful for some nations) but propbably best of all leading a display team by flying backwards means you can see just how well they are doing.

http://img.photobucket.com/albums/v1...psdc3d5e92.jpg

Wander00 4th Mar 2014 16:12

ISTR Gnat attrition was pretty high, and may have been at least part of the reason for re-introducing the Hunter at Valley, unfortunately about 3 or 4 courses after I had left.

dctyke 4th Mar 2014 17:01


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.

Anyone recall the title of this film, it had a very young air traffic controller (Mrs dctyke) in it.

bonajet 5th Mar 2014 03:35

You are correct "thing" - of the first 110 ordered up to XZ445 in 1979, I think 60 were lost, or 54%. By XZ999 at the end of 82 it was 67 out of 134 or 50%. The F104 has always got a bad press due to the large number in service.

Wholigan 5th Mar 2014 06:39

You may perhaps be interested in this discussion of the F104 accidents back in 2005.

http://www.pprune.org/military-aircr...accidents.html

bonajet 5th Mar 2014 07:53

Thanks RW, I hadn't read that thread. Lots of good stuff. The Aug 79 F104 crash at Yeovilton was, rumour has it, because the Display Control asked the pilot to complete his display quickly. He did.

Helmut Mann 5th Mar 2014 09:44

Thank you BOAC and John, that is what I hoped to see. Good work! It shouldn't be too hard from here..... ;)


Does anyone know what the two ventral strakes are for as shown in this image? Sorry I can't link to the image itself. Some sort of tech issue.


http://upload.wikimedia.org/wikipedi...ge_section.jpg

John Farley 5th Mar 2014 09:56

Helmut

As easy one. They are called LIDS (lift improvement devices) and are fitted to all Harrier II aircraft if the guns are not being carried. They give about 1400 lb more VTO lift capability by trapping the so called 'fountain' of exhaust air that bounces back off the ground and goes up towards the fuselage. If in your pic the gear had been down you would have also seen the cross dam that comes down in front of the stakes making a three sided box to contain the fountain. The LIDS work well to about 8ft wheel height by which time you have upwards momentum and can corry on with the lift and into the transition. The other postive effect once you have started to go up from the ground is that the temp of the air finding its way into the intakes reduces with corresponding increase in thrust (some 100lb per deg C).

Dan Gerous 5th Mar 2014 09:57

I'm sure someone will be along to answer it correctly Helmut, but as I had it explained to me once, it is an air dam. When the wheels are down, they, along with the strakes, make an enclosed area, which provides an air cushion, which helps with vertical landing. They either had to have these, or the gun packs fitted permanently.


Thanks to all who provided techy explanations in this thread, it was very interesting and easily understood.


(John beat me to it, and explains it better.)

Trim Stab 5th Mar 2014 09:59


Thanks RW, I hadn't read that thread. Lots of good stuff. The Aug 79 F104 crash at Yeovilton was, rumour has it, because the Display Control asked the pilot to complete his display quickly. He did.
Wasn't the Harrier crash at Kandahar also allegedly partly because the controller asked him to "expedite" on finals?

Fox3WheresMyBanana 5th Mar 2014 10:22


The shutters are mechanically connected to the relevant aerodynamic surface
But if one of those mechanical connections becomes disconnected, life in the transition gets quite interesting.....:ooh:


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