Ski jumps - why do they work?

[Cron]

I posted this on Questions and got a reply from Mark 1. Maybe Rotorheads can provide some more replies.

I'm sure it's obvious, but I can't see it. Why does a ski jump offer an advantage to Harrier performance? Thanks in advance for information.

[The Nr Fairy]

IIRC, the ski jump principally improves performance by allowing a greater take off weight because the aircraft is launched upwards by the ski jump rather than having to attain the requisite flying speed by its own efforts, so you can either carry more, or alternatively have a smaller carrier do the same job as a larger one without a jump. It also reduces the risk exposure by giving the pilot more time to react in case of engine failure.

[Thomas coupling]

Sometimes, I really wonder about the grip some so called professionals have on aerodynamics and aviation.

How, in the name of faith do you expect a 10 ton Harrier to jump over 3, sometimes 4 carriers, if there is no ramp before hand??????

I ask you:D

[Heliport]

I'm hoping John Farley will see this thread and respond.
Subject to anything John says -

The ski jump improves Harrier take-off performance by pushing it upwards at the moment of launch and allows it to carry a greater payload. I've always assumed less fuel is used in a ski-jump than a vertical t/o, but I don't know if that's true or a significant saving.

I think (not sure) a Harrier can take off with full fuel and full armaments load using a 12 degree ski jump.

The first two Invincible Class carriers were built with 7 degree slopes. Ark Royal was built with a 12 degree slope -

http://www.clash-of-steel.co.uk/gallery/pages/med/m_ARflight-f.jpg

[Flying Lawyer]

I think the ski jump also improved handling, safety and ship pitch motion limits compared with short take-offs on a flat deck ship because the aircraft isn't rolling along the pitching runway for as long.

Health Warning: I'm only a PPL and I'm no engineer. :D

[John Farley]

Cron and chaps:

There several ways to get your head round why the ski-jump improves a Harrier STO performance without getting into the maths. But some may not be familiar with the basics of the VTO and STO so forgive me for tidying those away first.

A Harrier will only VTO if its weight is less than some 96 percent of the thrust available. (Not 100% as you need a tad to spare to accelerate the thing upwards as well as carrying the weight)

So, at max AUW you will typically be short of three or four tons of lift (actual amount depends on the mark of motor and airframe) This calls for an STO where you need to go fast enough on the surface for the wing to carry the excess weight plus some margin to stop it staggering, then when you bang the nozzles down the thing will get airborne on a mix of engine and wing lift.

Now imagine doing this from a flat deck. Life is better than from a runway as the deck height above the water is free and so you can delete the margin needed to stop the runway stagger. You shoot off into the air regardless and can also rotate the aircraft to the optimum AOA – which you cannot do ON the runway because of the bicycle main gear being well aft of the CG.

But all of this ship advantage vanishes (and then some) in rough water. If the ship is pitching you may get to the end of the deck when the bow is down and finish up diving towards the sea. Pulling out from this dive requires the ability to pull g, which means a considerable margin of lift is needed. So, once you are using all the deck to accelerate to as high an 'end' speed as possible you have no option but to reduce STO weight. And this weight reduction can be very large so quickly negating the benefit of the STO in the first place. So you become a fair weather only toy.

Now stick a ramp on the end where the exit angle is greater than any conceivable bow down tendency and you make every launch start with a climb courtesy of the ramp and the gear. End of ship motion problem Now consider a ramp angle much greater than that needed for the above and you really start to win. Your question is Why?

The secret is the ramp gives you TIME in the air even if you have zero lift.(I trust you can accept that even your bike or car would fly for quite a few moments before it gradually fell back towards the sea if you shot up a ramp at seventy or so kts)

If your bike or car is now a Harrier - where the engine can carry say 70 percent of the weight - then even without ANY wing lift you will only sink back towards the sea at 30% of gravity. If you arrange that the wing lift actually carries half of that 30% lift deficit then gravity only sucks you down at 15% of its normal rate. Which means you curve upwards for many seconds before you get level, let alone sink. You use this TIME to accelerate the wing to a higher airspeed (through the proper use of nozzle angle) to get more aerodynamic lift and so fly happily away. Even though you started at a weight that could NEVER have flown off a flat deck in a calm sea – because then you would not have had enough acceleration time before you got to the water.

There are many other advantages to the ski-jump in terms of handling and safety but your question was about performance.

Hope that helps.

John

[Cron]

It does. Thanks all.

[Smoketoomuch]

John, a further question if I may. US Marine Harriers [a different beast I know] use the flat decked ships, they 'appear' to go to full flaps as they go off the end. Is this an artefact of nozzle position or something else? Or are my eyes deceiving me? Timing looks like it's critical.
And, only semi-seriously, [thinking of energies, PE and KE] would the ramp on RN ships be equally effective if it was at the back of the ship to give a boost to the take-off roll?

Maybe one at each end? :)

[Hilico]

Oooh. Oooh. Oooh. Can I just say - an unexpected dose of John Farley and I didn't even have to wait for next month's Flyer. What an early prezzie. Oooh.

Oooh.

[Tinstaafl]

For extra time in the air why not use even steeper ski jumps? 45deg comes to mind from my modicum of artillery experience.

[NickLappos]

John Farley's explanation is perfect, but let me try a slightly different tack that I think compliments John's:

The concept is energy management, almost identical to the helo concept of hovering in ground effect to accelerate while attempting a barrier takeoff. While trundeling down the deck, the Harrier can devote almost all its thrust to accelerating the beast to some appreciable forward speed. If it were resting on its plume of air, literally nothing would be left to accelerate it. The ramp lip points upward so some of the ground speed (deckspeed) is converted to climb that allows quite some lofting of the aircraft. This lofting buys time to let the aircraft transition to forward flight, and negates the problem of down deck angle in a pitching sea, as John so well puts it.

Helos could use exactly the same technique, if they were so vastly overloaded. Wisely, we have not been so burdened, yet.

Look for this jump technique for V-22, as its rolling takeoff weight, as compared to hover weight, creates a compelling argument for a rolling takeoff.

[Head Turner]

Nick - I am way ahead of you here!

Way back in the early days of AAC we were equipped with the ever powerful Skeeter which would stay on the ground at max power if the temperature was anyway near warm.

We 'discovered' the ski-jump method of aerial flight which provided enough 'up' to allow the little beast to reach translational lift speed (12 knots) and from there the Skeeter would fly really well.
The Skeeter has wheels and we were used to planning our take-off where there was and adequate area for a rolling run which culminated in a ramp at about the 12 knot ground speed position. And it worked very well

Limited power was all the power we had on a warm day. We learnt techniques to take-off and land that would totally confuse the present breed of rough operators. We had to refine our senses to identify the very smallest whiffs of lift.

This experience has helped me take-off in a 206B/B20 engine at 11,000 feet at 30 degrees C from a village football pitch. Here we didn't use a ski-jump because the skids wouldn't like it.

[John Farley]

Smoke too much

Your observations are correct. With the Harrier II wing (GR7, AV-8B etc) the flaps are so large they could not be put down beyond about 30 deg when the nozzles are aft (as in the STO ground roll) without being blown off. So their selection is armed by the pilot during TO checks after which they travel automatically with the nozzles.
As for a down ramp at the start of the deck roll, I agree that would have to help performance. I think its downside (sorry) is the further reduction in space usable by marching bands. You could also use a large hamster wheel as well, then after N revs to build up speed FLYCO (or any of the old guys who used to fire the catapults) could open a flap and let you loose.

Tinstaafl

Too right. 45 deg IS the optimum angle if you are only considering performance (those early brown jobs knew a thing or two). But we stopped our trials at 20 deg because the law of diminishing returns was rearing its ugly head plus a whole host of adverse ship related issues were appearing on the scene. 15 deg is the practical man’s choice as it provides real benefits to the Harrier with negligible disadvantages to the ship.

The ski-jump has been described as the 'runway in the sky' as it just gives you time to accelerate to flying speed. The Russians use it for flying Su-27 and MiG-29s from ships rather than bother with catapults. But not all aircraft can benefit from a sk-jump. You need a high thrust weight ratio to make the most of the time provided and you need to have aerodynamics that provide adequate control of attitude at well below normal flying speed.
Like the Russians above.

[the wizard of auz]

Your all wrong........................ the real reason for the ski jump is so the blokes and blokettes have something to ride their skate boards on when they have a bit of free time. shame to waste all that flat deck space. :}

[Red Wine]

Don't be silly.

Harriers don't have brakes, and something has to stop them running off the flat end at the front of the boat!

[Tinstaafl]

G'day John.

What sort of ship & aircraft adverse issues are there with the ski jump? And how do they change with increasing slope?

[John Farley]

Hi Tinstaafl

The only adverse ship issues are you have to build the thing. Up to 20 deg nobody saw the extra top hamper as significant. It does reduce the amount of flat deck for helo ops but the matelots love the extra storage space underneath it – warships never have enough – so they really use it to the full.

As to the aeroplane it needs no mods to cope up to 20 deg. Not even for gear fatigue as the gear was designed to cope with RAF rough ground spec useage and the RN do none of that.

May sound all too good to be true but that is really how it worked out. A total win situation.

Launch safety is hugely increased because you will fly for secs even if the donk seizes or the nozzles jam at the end so you will always have stacks of time to eject. With a flat deck launch, escially if the bow is down a tad, it is doubtful if you would make it before you got to the water. After all you don't launch expecting a big drama so it would tkae a second or so to realise what had happened. And that is about all the time you would have.

The handling is also superb. Talk about low workload. You get delivered into clear air at the right nose up attitude and appropriate AOA – all the stuff you normally have to fix for yourself. Normally the Harrier has a big trim change leaving ground effect from a runway STO -3 T/P as you leave the ground to +8 (out of 10) in free air!

When I started telling the boffins – especially the govt ones - how low the workload was off the ski-jump they clearly thought I had a company hat on and was overstating the case to have Invincible fitted with a jump after she was launched. So I did a hands off launch with an instrumented aeroplane for 35 secs after leaving the end. After staring at the staight line traces and the kine-theodolite external films (which also showed the tailplane not moving against a painted scale on the side of the aeroplane) they shut up and told their masters the RN needed the mod to the ship.

See you Saturday

[Tinstaafl]

Ta John,

Not often aviation gets something for near-as-dammit nothing... What max. 'g' does the ramp cause? 1.5?


As for Saturday: I wish! I'm working that night so no bashing for me... :sad: Hope everyone else has a great time though.

[John Farley]

Tough T!

Another time then. The big g (up to 3) tends to come with the smaller ramp angles because once the angle starts to increase beyond 10 you get so many ballistic seconds even at max weight you can afford to start from very close to the ramp and so only have 50 kts or so when you hit it.

Initially we could see the gear being bottomed at high weights and high speeds and had ideas of changing the damping of the oleo (rigid would be ideal) to stop any risk of shock loading at the end of its travel. But it turned out not to be necessary. Another win win win thing.

(I'm sure its obvious but normally gear is designed for the landing impact which can be a very big force applied for a very short time which gives high closure rate that is resisted by the dampers. Gears often break on a heavy landing long before the oleo has bottomed for this reason. Change the time of applied force to a second or so going round the ramp and even a much reduced force can bottom the damper)

[BRL]

The man in the pub says the ramp was designed by a 'housewife' from somewhere like Kent.

I don't believe him as he was drunk.

Anyone know any different?.

[John Farley]

The short answer is yes.

The longer version is that in the early 1970’s Royal Navy engineer Lt Cdr Doug Taylor, wrote a paper on ways of launching heavier and higher performance aeroplanes from cheaper and smaller ships.

The ski-jump for the Harrier was one of several ideas included in his paper.

Doug won an award £20,000 (or thereabouts) for his efforts.

[yakker]

The idea stemmed from Lt. Cdr. D. R. Taylor RN in 1973

Sorry just realised JF beat me to it

[Onan the Clumsy]

I thought it was because someone accidentally ran a carrier into the dock at full speed and bent the deck up a few degrees. The 'new design' was the most plausible excuse he could come up with in the short amount of time available to him.

Fortunately for him, the rest, unlike his career, is history.

:}