Harrier Hovering Autorotation

[NickLappos]

How the other half lives.....

http://www.dailymotion.com/popular-week/video/xvfl7_crashavion

[MightyGem]

Yes, this happened a couple of years ago, at a display at the seaside town of Lowestoft in the UK. The pilot moved the nozzle positon lever instead of the throttle, and then didn't have time to correct his mistake. :{

[Ascend Charlie]

And i bet the kid in the funny yellow hat is wishing he had kept watching.:*

[Staticdroop]

Ooops, bet that required many repetions to the chaps in the bar:\

[Barshifter]

Im fairly sure the rumour at the time was that he survived the ejection only to break his leg when he parachuted onto the fusalage of the stricken Harrier in the water.

Regards

Barshifter

[Graviman]

Interesting demonstration of advantages of low disk loading, over the need to maintain engine power with high disk loading.

Mart

[scooter boy]

Great that the pilot got out but... but what a balls up!

The MOD have insurance though - it is called the inland revenue.

Isn't it sad that our taxes funded this terrible waste of money?



SB

[Flying Lawyer]

Isn't it sad that our taxes funded this terrible waste of money?Not IMHO.
It's a pity the accident happened but even top pilots such as those creamed off to fly the Harrier make mistakes - just like the rest of us in our various occupations.

Fortunately, the pilots also survived the incidents below. Nothing to do with hovering or rotary flight, but amazing pictures nonetheless.


http://img.photobucket.com/albums/v146/FlyingLawyer/Tbird_ejection.jpg




http://img.photobucket.com/albums/v146/FlyingLawyer/Lightning_crash.jpg




FL

[topendtorque]

Fortunately, the pilots also survived the incidents below. Nothing to do with hovering or rotary flight, but amazing pictures nonetheless.


The first (finger trouble at an airshow - bit low) I would say would qualify as the worlds fastest ride to a desk job,

but the second,

I cannot remember any details if I ever heard them??

[MightyGem]

From the Pprune archives, via google:

The place was Hatfield, 13/9/62, and the pilot was George Aird, a civilian test pilot with Hawker Siddeley Dynamics. The Lightning became uncontrollable after an engine bay fire had weakened a tailplane actuator, and the pilot was forced to eject from low level. Unfortunately he landed on some greenhouses, breaking both legs in the process!

The pic was taken by a bloke covering a ploughing contest for Farmer's Weekly

The tractor was a Fordson Major, and the plough was made by Aveling Barford, and in those days the driver would have been paid £5.10.6d per week for a 50 hour stint, Mild beer was 7-1/2 old pence per pint and larger had still to be thought of in the UK, a portion of fish and chips was 9d and only the American forces had Anoraks, Oh what glorious days we have missed!

[Bertie Thruster]

Mighty; I got a few of those glorious years! (and bought my first pint of mild at the "Hoste Arms" in Burnham Market circa 1965, only 'slightly' underage!)

The only ejections I ever witnessed were from a F18 over Hampshire in 1980. The subsequent crash was spectacular, the aircraft narrowly missing the Officers married quarters and primary school at Middle Wallop.

[Graviman]

The place was Hatfield, 13/9/62, and the pilot was George Aird, a civilian test pilot with Hawker Siddeley Dynamics. The Lightning became uncontrollable after an engine bay fire had weakened a tailplane actuator, and the pilot was forced to eject from low level. Unfortunately he landed on some greenhouses, breaking both legs in the process!


He must had made a full recovery, since my old man later knew George Aird as the Hawker Diddeley chief test pilot. Very helpful guy apparently, particularly since the purpose of the engineering tests was not always well explained. :uhoh:

Mart

[as355f1]

Yes, this happened a couple of years ago, at a display at the seaside town of Lowestoft in the UK. The pilot moved the nozzle positon lever instead of the throttle, and then didn't have time to correct his mistake. :{
just a thought, but would moving the throttle not have the same outcome i was of the understanding they needed full power to hover ?

[treadigraph]

Bertie, this was the F-18 that had displayed at Farnborough the previous week... anyone know tech reasons why said pilot felt obliged to eject? Call it vulgar curiosity if you will...

[Chesty Morgan]

as355f1, not necessarily. It depends on weight. The MAUW exceeds the maximum thrust from the engine even with water meth injection, I think. It does have to be fairly light to hover. John Farley would be a good chap to ask for specifics, he frequents this place from time to time.

[John Farley]

The first steps towards hovering using jet engines for direct lift were all taken by the RAE Farnborough. In 1951 they instigated a model rig test of a jet reaction attitude control system. The success of this led to them contracting Rolls-Royce to build a man carrying hovering rig called the Flying Bedstead which flew in 1954. This in turn led to a contract on Shorts to produce the SC1 aircraft with four separate lift engines plus one for propulsion to investigate the transition manoeuvre. A double transition (accelerating onto the wings and back again) was achieved with the SC1 in 1960. Later that same year Hawkers started to use the P1127 to look at the notion of jet lift (initially PV) using a single engine with swivelling nozzles. This was a less complex way promising greater reliability when compared to the SC1 but the penalty was it relied on the pilot to correctly operate two controls with the left hand (throttle and nozzle lever)

Clearly if you accidentally close the throttle or raise the nozzles when in the hover the thing will fall on the ground. People make mistakes. Whether they recover from them depends on many factors. I accidentally closed the HP cock (so shutting down the engine) at the start of a decelerating transition on one occasion. The dying whine woke me up so I relight the donk and gave myself a talking to.

Even though today they are operated by six nations and have been in service with the RAF since April 1969 there is still much rubbish written about them by people who don’t do their homework – or in some cases even think at all before they post something. Sorry, rant over. I need my breakfast.

[vaqueroaero]

The scoop on the F18 ejection can be found right here:

http://www.ejectionsite.com/thunderbird6.htm

[MightyGem]

That'll be an F 16 then. ;)

[smarthawke]

I stand to be corrected, but I thought the Harrier accident was a result of the pilot's flying suit cuff catching the nozzle lever as he went to move the throttle and took the nozzles 'up'.

[topendtorque]

Now all we need to know is ~ After Capt Strickland rocketed towards his boss's office at 225 knots, did he get a desk job or was he given another rocket to end up as a super salesman for AcesII?:{

[Graviman]

Even though today they are operated by six nations and have been in service with the RAF since April 1969 there is still much rubbish written about them by people who don’t do their homework – or in some cases even think at all before they post something. Sorry, rant over. I need my breakfast.

Regarding diskloading, John, my post was reference a previous discussion with Nick Lappos who neatly showed that even with some sort of emergency power (batteries were considered) low diskloading in a rotorcraft has inherent disadvantages during engine failure. Apologies if your breakfast was spoiled - post now clarified. :ok:

I read your post with interest, and as an engineer would like to understand more about the problems associated with this mode of S/VTOL. This is a very good resource:

http://www.harrier.org.uk/history/history_farley.htm

Mart

[Graviman]

The linked site in the last post is pretty informative. Just one question, although i have not yet finished my homework:

How would hovering a Harrier compare to hovering a helicopter? From the site i get the impression that the pilot is balancing the aircraft, much the way a glider pilot balances on the main wheel using many tiny stick inputs. In comparison an R22 seems to be more a question of limiting the frequency of cyclic movements.

Is this about right?

Mart

[NickLappos]

In case someone wonder why we still have Helicopters when there can such things as Harriers:


http://webpages.charter.net/nlappos/p1.jpg


http://webpages.charter.net/nlappos/p2.jpg

[B Fraser]

The first steps towards hovering using jet engines for direct lift were all taken by the RAE Farnborough. In 1951 they instigated a model rig test of a jet reaction attitude control system. The success of this led to them contracting Rolls-Royce to build a man carrying hovering rig called the Flying Bedstead which flew in 1954. This in turn led to a contract on Shorts to produce the SC1 aircraft with four separate lift engines plus one for propulsion to investigate the transition manoeuvre. A double transition (accelerating onto the wings and back again) was achieved with the SC1 in 1960. Later that same year Hawkers started to use the P1127 to look at the notion of jet lift (initially PV) using a single engine with swivelling nozzles.

All three machines are to be found in The Science Museum in South Kensington. The P1127 is in the aviation gallery and the other two can be found adjacent to the Apollo X capsule on the ground floor. The aircraft are rather poorly displayed which is a shame.

Whoever flew the bedstead must have been wearing their "extra brave" trousers.

[John Farley]

Graviman

If you have any specific questions that you feel are not answered in my lecture on the site you quoted then send me a PM and I will do my best to help.

I see you dabble with the R22. Take my word for it if you can hover one of those then (from a stick and rudder point of view) you would find the Harrier a doddle to hold steady in a hover.

Why? Because as you know when you move any helicopter control it is necessary to make an adjustment in another. EG Lift the collective to gain a bit of height and the rotor tries to slow down so you open the throttle to compensate which then requires a different rudder input to match the new torque. When you open the throttle in the hover in a Harrier it just goes up. Nothing else happens. Similarly use of any of the reaction controls does not make the RPM sag as waving the cyclic can in a chopper. There are some aspects of the Harrier which might seem harder than flying an R22 but hovering is NOT one of them.

Indeed I have been known to suggest to overconfident fixed wing pilots who think they are little aces (regarding their stick and rudder skills) that they go and try to hover an R22 – they soon find they are not top of the heap after all.

Nick

I am quite sure you have your tongue in your cheek and fully understand what you are posting about. The trouble is many other people will not and could go away with the wrong idea.

You have correctly suggested that the means used to support a Harrier in the hover (jet lift) is less efficient than a wing (Blackhawk rotor) at producing the required force. The problem with such a simple (even patronising) comparison is that the Harrier is not intended to do a job in the hover (airshows excepted!) but merely to spend 30 secs in getting from flying on its wings to arriving vertically on a small bit of surface – with all the advantages that flow from that with regard to operating site flexibility.

As to efficiency your readers might like to know that a Harrier uses less fuel in doing a VTO and accelerating to cruise speed on heading towards the target than if it takes off conventionally from a runway. The only exception to this is when the runway is pointing along the track you need and the aircraft is towed from dispersal to the runway before start up.

For the general readers I should perhaps finish by saying that a heavier than air device needs to generate the force needed to hover by grabbing a mass of air and accelerating it downwards such that the product of mass X velocity equals the weight of the hovering device. A Harrier grabs a (relatively) small mass of air and gives it a pretty big velocity a Blackhawk rotor grabs a (relatively) huge mass of air and so only needs to give it a relatively small velocity to get the same end result

The lower the velocity used the greater the efficiency of the system.

In that context the prop rotor on a tilt rotor machine grabs rather less air and so has to give it a higher velocity which is going in the less efficient direction.

The Boeing offering for the JSF competition used Harrier direct lift principles (very simple from the mechanical complexity standpoint but poor efficiency) the Lockheed used a combination of jet lift and fan lift (more efficient but more complicated) In the end given the reliability of modern complex systems (better design, better materials, better lubricants, better etc etc) the efficiency advantage of the Lockheed system won the day. The better efficiency due to the lower V used by the fan part of the system enabled the same basic powerplant (as used by Boeing and Lockheed) to hover an aeroplane that was over 20% heavier. A bit more than a marginal advantage.

[NickLappos]

John,

Physics is not tongue in cheek. The Harrier is a STOL-hovering machine, anyone who thinks it is not does not read the brochures. And if airshows were the only place a Harrier needed to hover, the HMS Invincible would have many Harrier-shaped dents in its sides. If it did not hover to land on small places, its awful payload would make it a non-starter in the aerospace world. I cannot tell you how often people ask "Why do we have helicopters when jets can take off straight up?"

All direct thrust machines are similar, even when one's tongue is not in one's cheek! The rotor on the Harrier is inside its fuselage, as part of its lift fan, and the high disk loading of that fan make it necessary that a Harrier use 1,000 horse power to lift one pound.

My slides explain this inexorable law where power is consumed by high disk loading so that it becomes easier to answer that question, "Why do we have helicopters when jets can take off straight up?"

The answer is, "Because only Governments can afford the fuel bill and poor payload performance of a Harrier!" (Now THAT is a tongue in cheek statement!)

None of this above takes away from the fantastic aeronautical achievement of the Harrier, a direct product of the lighter, more powerful turbine engine, and the moxy of a group of excellent British engineers who did what the vast American aerospace machine could not do. Like the tilt rotor, neither configuration would be possible if recips were the only game in town. As turbine engines get more efficient and lighter, we will find a larger and larger niche for direct lift aircraft (Like F-35 is doing). But as engines get lighter and more efficient, the helicopter will carry more payload yet, so it will benefit even more.

The disk loading power gap will never close.

[MightyGem]

the HMS Invincible
Very, very minor point Nick, but unlike in "the USS Nimitz", Royal Navy ships are simply "HMS Invincible" as in:
And if airshows were the only place a Harrier needed to hover, HMS Invincible would have many Harrier-shaped dents in its sides

[NickLappos]

MightyGem,

Thanks! As a died in the wool Horatio Hornblower and Jack Aubrey fan, I should have known better!

[MightyGem]

That's OK,I prefer Alexander Kent's (http://www.bolithomaritimeproductions.com/Alexander%20Kent%20Novels/default%20-%20AKNovelsUS.html)Richard Bolitho books myself.

[Geoffersincornwall]

My heating engineer is a Richard Bolitho - then, with a handle like mine you might have guessed! His type of 'gas engineering' does not involve a 'turbine' however, otherwise it would cost as much to maintain my gas boiler as it does to maintain a Harrier. I bet the crabs wish they could manage on a £150 a year!


Cool thread!


G

:ok:

[John Farley]

For those who are interested in correct Harrier information please read (slowly) on.

If you divide the current service max STO weight by the aircraft empty weight the number you get is 1.93 to 1.98 depending on the individual aircraft mod state. Work is ongoing to increase this into the range 2.06 to 2.1

For any fast jet to have a payload equal to its empty weight is very good (as this is the sort of fraction airliner designers aim at) but when it is a fast jet that also contains a bunch of kit to enable it to offer the operating site flexibility that stems from a VL then it becomes quite remarkable.

I do appreciate that when I delivered the early aircraft to the RAF in 1969 things were not so good as today, but even then I used to fill it with fuel, do a VTO and fly for 75 mins to a German base. The payload fraction on STO in those days was closer to 1.7.

Since then (38 years ago) things have moved steadily on with four major engine upgrades to the UK fleet. Indeed the latest engine to go into UK Harriers has 3000lb more thrust at ISA +35 (yes that is 50deg C) than the engine it replaced.

Despite these facts people are still writing here about ‘its awful payload’.

The Harrier’s extreme hovering inefficiency in terms of power used does not matter when the total time it is needed to be used during a landing is less than a minute. The total weight of fuel used in this process is between 100 and 200lb depending on the time the pilot chooses to spend flying below wingborne speeds (partially jetborne) before coming to the hover

The Harriers job (that is to say the military role it was designed to carry out) is to attack targets. It does not have a job to do in the hover except at airshows when various manoeuvres are used to entertain the crowd (plus demonstrate to those in the business the considerable margins of handling and control that exist beyond those needed to carry out a routine VL at the end of a sortie). At such times efficiency is of no concern.

It is true that the Harrier could fire some of its armament options from the hover and might in very limited circumstances improve aiming accuracy by so doing. However unless the target was a hospital or unarmed women and children queuing for soup I think most pilots would prefer to be flying a high speed during an attack. It is also true that the Harrier is the only fast jet in service anywhere where the pilot can retreat while facing the enemy, but I do not believe this has been a factor in any procurement decision to date.

Why do we need helicopters when we have fast jets that can hover?
Because we have the need to do jobs in the hover that often involve considerable time and so must use a much more efficient system to provide the necessary lift. Such work in the hover also requires the most benign possible conditions under the hovering device and jet lift necessarily involves the opposite with high exhaust gas velocities and temperatures.

[John Farley]

http://img.photobucket.com/albums/v145/johnfarley/AirtoAir082.jpg

[slowrotor]

Those are very interesting points you have made, particularly the 200lb of fuel required to hover and touchdown. I always thought the Harrier was the most inefficient fixed wing. The VTOL capability does not seem to be much of a penalty.
Thanks for your insights.

[Graviman]

Graviman

If you have any specific questions that you feel are not answered in my lecture on the site you quoted then send me a PM and I will do my best to help.

I see you dabble with the R22. Take my word for it if you can hover one of those then (from a stick and rudder point of view) you would find the Harrier a doddle to hold steady in a hover.
...


John,

It is fair to comment that i dabble with the R22, since i can't afford to play! Once again i am grateful that PPRuNe allows me to discuss aircraft design with test pilots of your calibre.

Having read your lecture and some other resources, i do have a query regarding the stability of the Harrier in takeoff/landing hover. In particular how longitudinal and lateral dihedral remain effective in hover. This is something that puzzles me after the comment about the stability augmentation system being considered unnecessary for good pilot control. After the front landing gear adverse yaw problem i imagine a large vertical stabiliser helps, but the anhedral wing required for such a large amount of wing sweepback must counter lateral dihedral.

This really comes from my realisation that all helicopters have built in static stability, due to the rotor flapback (it becomes reduced and cross coupled in rigid rotors - thanks Nick). In theory if the cyclic was frictioned into position a teetering helicopter would remain still - i have no intention of trying this! The pilot is really there to provide dynamic stability, to damp down movements for the intended flight direction. Larger machines have SAS as part of AFCS to provide much the same service.

Which stability augmentation systems has the Harrier settled on? I will order the appropriate paper from I.Mech.E or SAE to understand in more detail, so just want an initial guide.

Thanks in advance.

Mart

[John Farley]

Mart

The easiest way to get your head round what is happening with an un-autostabilised hovering Harrier at speeds of less than (say) 30 kt is that no significant aerodynamic forces are produced by the wings, tailpane and fin.

This means it is just a hunk of stuff supported by the engine thrust and your piloting task is to keep the aircraft flat (or to be more precise in the same attitude it has on the ground before a VTO) so that the engine thrust stays vertical.

Clearly if you let the roll or pitch attitude change the thrust will develop a component in some direction that is other than vertical and off the aircraft will wander.

I say un-autostabilised because when the first RAF and USMC guys were converted there were no two seaters and no simulator. We chose to convert them autostabs OFF so that there was no chance of them experiencing an autostab hard over failure case. Back in the 60s electromechanical things were not as reliable as they are these days and we saw the slightly twitchier handling with stabs off as very much the lesser of the two evils.

At such low speeds (and without aerodynamics) the aircraft is neutrally stable in roll and pitch. So it does not particularly want to move in those axes however if it is disturbed in the least there is no damping to slow down the motion and certainly no stiffness (stability) to correct the motion. Only the pilot can correct the motion and so restore the attitude by moving the stick instinctively just as if flying normally. This moves the ailerons and tailplane which having no airflow over them achieves nothing. However the aerodynamic surfaces are mechanically linked to the shutters of the appropriate reaction controls so the effect to the pilot is that the ailerons and tailplane work as normal despite there being no airspeed.

Now for two big ‘buts’. Firstly the story in yaw is not so simple. Because the intake momentum drag acts ahead of the CG and increases with RPM it has a big influence in the hover. If the nose is not kept into wind (even 10kts) then the aircraft will yaw and try and fly tail to wind. So in yaw the hovering aircraft is not neutrally stable it is actually UNstable. This requires continual footwork – not terribly difficult to do but you MUST control the heading at all times. You can CHOOSE to have it 90deg to the wind if you want, but you will HAVE to hold it there with your feet.

The second ‘but’ is that although people may not realise it when they fly normally in a fixed wing aircraft they are using ‘rate’ controls. In other words say you put on a bit of aileron, the aircraft starts to accelerate in roll but if you hold the control where you first put it the normal aerodynamic roll damping comes into effect and stops the aircraft continuing to ACCELERATE in roll leaving you with a steady RATE of roll. There being no aerodynamic damping in the hover the reaction controls are pure acceleration controls. In the 1950s there were two schools of though about this. Pilots would over-control if faced with acceleration (vice rate) controls and finish up oscillating. Or they would not!

Hawkers thought pilots would be able to cope so they went ahead on that basis. Others decided that it would be necessary to produce artificial damping and stability to effectively leave the pilot with a rate control system.

Once Hawkers got going it became clear that pilots could adapt to acceleration controls provided a few criteria were met. The control runs must be as low friction as possible with no backlash at all and the forces available from the puffer should be able to produce an acceleration of approx 2 'pie' radians per sec squared. Anything less ‘punchy’ and you tended to get out of phase with any oscillation and struggled to damp it out.

This was especially so in roll where there was very little inertia (if you think about it inertia resists acceleration and so acts as a quasi damping term) Therefore the greater pitch (and yaw) inertia was a help to pilots.

I have not answered some of your points about anhedral etc directly because it is not a factor in the hover only in transition which is another story and this post is too long already.

Hope that helps a bit.

John

[NickLappos]

Mart,
John is absolutely correct, the hovering task in a vectored thrust is relatively simple one as compared to a helicopter, mostly because the nasty cross couplings and rotor responses to changes in velocity and direction (the bulk of the problem we face) are simply not there.

OTOH, the concept of hovering and the need to control acceleration/attitude while making a double integration to achieve position is still there, so that some training is necessary, but the Harrier "feels" like it has SAS even when unaugmented.

The XF-35 has a great deal of augmentation so that it is a position=hold control while hovering. Look at the videos of it hovering to see the rapid, automatic nozzel movements commanded by its stability systems. see the clip "X-35B Landing" on this page and look closely at how hard the rear nozzel is working:

http://www.jsf.mil/gallery/gal_video.htm

[Graviman]

John,

This information is spot on - i can see why you got your C.Eng! ;)

I understand what you mean about footwork, by comparison of hovering an R22 downwind. By intake momentum drag you are referring to the intake air inertia leading to a continued state of differential intake flow on both sides? So the effect increase with RPM is due to higher flow rates. Did Hawkers consider an internal rudder designed into the intake plenum, to redistribute intake flow with pedal input?

I had not appreciated the point about acceleration control of roll and pitch - definately more challenging than a roll rate control glider. Helis suffer a brief accel transient, but keeping inputs below about 2Hz the delay does not induce PIO. I can understand the need to keep pilot lag well below 180 degrees, and 2Pi rad/s^2 allows a high freq response. Comanche reportedly achieved 0.9 rad/s so i imagine it's initial accel was also pretty good. [Edit: typo over accel and rate]

Do you have any rough details about the pitch/roll rate control system eventually selected? I have i mind a project, although likely some years off, so would like to gain an understanding of design practicalities of control systems.

Regarding the anhedral, i imagine some of the Hawkers dynamicists figured on the possibility of dutch roll with such a large amount of sweep back on a high wing. Gliders gently spiral diverge from a heading. Not flown for a while, but there is a nice little RC heli sitting on it's pad patiently waiting the pilot.

----

Nick,

Looks like we posted at the same time!

OTOH, the concept of hovering and the need to control acceleration/attitude while making a double integration to achieve position is still there, so that some training is necessary, but the Harrier "feels" like it has SAS even when unaugmented.

Interesting. Since the response is very linear, as you say unlike a heli, i can see that this would become second nature after training. The XF-35 nozzel definately works hard to keep that aircraft stable. Then again since the pilot objective is keeping a good horizon level, i imagine it would become second nature.

Mart

[John Farley]

Mart

Intake momentum drag is rather simpler than you perhaps think. It is a simple drag force that acts at the intake and is caused by the high velocity air that is travelling from nose to tail having to suddenly loose some of this longitudinal momentum as it starts to move radially in the engine. The snag from a Harrier pilot’s point of view is that this drag force acts parallel to the relative airflow and so not necessarily along the aircraft long axis. With any side slip there arises a component of the IMD acting across the long axis and since the intakes are ahead of the CG this tries to yaw you tail to wind.

Yes, the anhedral is there to reduce the excessive rolling moment due to sideslip that would otherwise exist thanks to contributions from sweepback, the fin and rudder and the high wing combined with a mid CG.

John

[Graviman]

Thank you again, John. This clears up my misunderstanding about IMD nicely. So as RPM goes up drag goes up.

Being an engineer i tend to find the complicated solution first!

Mart