Ramp gear loads
 

There may be some confusion about what the nose wheel (and indeed the main gear) needs to cope with on a ramp takeoff.

It is not so much the high value of the load as the length of time the modest load is present.

On a VL at the max sink rate the vertical loads that the leg has to accept are obvious and designed in from the off.

What is different on a ramp is that the significantly lower load than that above is applied for a serious length of time. This means that the lower load can still close the oleo. If this happens shock loading will occur which is not acceptable. At high rates of closure of the oleo (heavy VL say) the damper does not have time to do other than act as a simple locked strut. (Imagine holding a car shock absorber in your hand – if you try and close it quickly it will appear rigid. If you apply a small load throughout the TV prog you are watching you can close (bottom) the thing no sweat).

With the Harrier the gear had never been designed for a ramp of course but we soon realised that it was not high loads but the time the loads were applied that affected how close we got to closure and shock loading. In the end luck was on our side and a small change in the SHAR nose damper characteristic was all that was needed. Funnily enough this was to avoid shock loading after leaving the ramp when the leg suddeny extended from nearly shut to free. To ease this effect the last few feet of the ramp have a reducing angle.

Out of interest the backup plan B was to pop a disposable, crushable collar round the leg to avoid bottoming shocks.

Happy days.

Engines

Thank you for your considered reply and apologies for the rant like nature of my post, I just get a little frustrated sometimes. I should ban myself from posting on PPRuNe within an hour of reading!

Backwards,

Absolutely no apologies required - free forums and all that. Offence never taken.

I think your advice on delaying posting is an excellent idea - I could do with following it as well.

Best Regards

Engines

One of the advantages of going back to the B is the fact that UK ramp and weapons requirements were always in the design spec.

There is a 12 degree ramp centre field at Pax River for testing, although it is a bit overgrown having been not required for the past 18 months. No launch's yet but I'm sure that a video will be available on the internet when it does happen. The Aussies may also be interested for their Canberra class.

During the 72 STOs and VLs conducted on the USS Wasp by the F-35B they did not all use the entire length of the deck, far from it in fact. The SHAR needed pretty much the full length of the deck to get up and no-one thought that was a problem.

On Ark in 2010 we had a USMC AV8B do a vertical takeoff from 1 spot to hop ashore to Cherry Point, due to a nose leg oleo issue which would have caused the problems JF details above. It was lightly fueled and clean under the wing but proved the power of the 408/107 Pegasus. Just because you have a ramp you don't have to use it! :}

As an interested bystander I've never quite understood the purpose of ramps for takeoffs. It would seem to be an (inevitably less than 100% efficient) way of obtaining vertical speed at the cost of forward speed.

Given that the thing will fly regardless if it's going forward quickly enough, why is this tradeoff worthwhile? Is it some sort of special voodoo relevant only to aircraft with some sort of vertical lift system like Harrier or F35B?

How much would it increase MTOW, becuase my back of a fag packet calculation says its proportional to thrust or empty weight, all things been equal on the deck length say 600ft). So I came up with a figure of 4600lb extra.
Adding the very long deck, my calculation says the F-35b could operate at close to the MTOW with quite a bit of a margin. If this is then case the SRVL really is crucial. Is the F-35b going to used its maximum on the UK carriers?

Its ballistics, the same principle was used to fire cannons the longest distance, so its very old voodoo. Plus the more convenient aspects such as pointing towards then sky not sea.

BTW, has the f-35B got a name yet, say Strike Harrier?

Phil R

"Given that the thing will fly regardless if it's going forward quickly enough"

Unfortunately this is not true, or you would be sort of correct.

Harriers need wingborne flight to take-off when heavy.




If you try to throw a stone as far as you can, do you throw it level, or up at an angle?


Imagine if during the entire time of flight of that stone it could accelerate.


The harrier needs to get to a certain speed to fly using it's wings.

It takes a certain distance to accelerate to that speed.

That speed, and thus distance changes with weight. ie you need a longer acceleration distance if you are heavier.

If you just shot off the end of the deck, your maximum acceleration distance to get to flying speed is like throwing the stone level.

If you fit a ramp, then you have given yourself a lot further to accelerate before things get wet.

Essentially, if you use a ramp you can get airborne without actually having enough speed to fly. You are just hoping to accelerate enough before you fall back to the water.

Tourist,

Nice explanation of the way the ramp works - it took me some years to 'get it' like you have. For Sea Harrier, the ski-jump essentially gave the aircraft around three quarters of a mile 'runway in the sky'. It's the nearest thing I have ever come across to being 'something for nothing'. And it was another Royal Navy invention. (We're quite good at this lark.)

The subtlety of ski-jump with a STOVL aircraft is that it can vector its thrust through its CG so that on leaving the ramp, while not capable of wing borne flight, it can use some of its thrust to support itself while still accelerating. The other 'not so subtle' part is that a STOVL aircraft has a control system that can work well below flying speed. Taken together, these allow the STOVL aircraft to maximise the benefit of the ramp (ski-jump) by scheduling thrust vector with airspeed to fly away safely. Ramp launches are also an extremely easy task for the pilot to carry out. And as I said previously, pointing at the sea, not the sky, is a simply massive safety win.

Ski-jump will be used on QE2 class to reduce the amount of deck used for launch and ease handling aircraft for launches. What will be interesting is if they stick with using JBDs for launch.

JF, thanks for that excellent explanation of nose leg loads. I was involved with early analyses at Fort Worth and yes, it was leg closure that was the issue, not leg loads. Ramp profile design is actually quite a complex area, with some interesting variance in ideas on opposing sides of the pond. The USN have never put ramps on LHD class ships due to USMC insistence that the entire deck be left clear for mass helo launches.

Best Regards as ever

Engines

All variants are Lightning II; correct nomenclature would be F-35A Lightning II, F-35B Lightning II, F-35C Lightning II, F-35I Lightning II, and CF-35 Lightning II.



Maybe it's not too late to go back to the Boeing X-32 :p

Bah. That's what the manufacturer and the US DoD call it. Colonial.

Let's have a proper British name that has an association with an outstanding pilot's aircraft.

Has to be..... Sea Fury. Any other calls?

Engines

Yank names are better, in my not so humble opinion.

Can't really call it by the PPrune endorsed name of 'Dave' anymore, given camoron's defence disinterest.

Cool Fanned Luke?

Incidentally Engines, did you get your sea and sky a bit muddled in your earlier post or am I as befuddled as usual?

Engines

My pitch for the best British name would be the Buccaneer II

HM Forces' name for JSF?

Jump Jet Fanny and her Rolls-Royce/Pratt & Witney Tw@t?

...I'll get my coat...:}

Er.... Lightning?

Between avoiding "bottoming shocks" (if you can't stay off an RN ship, don't fall for the "golden rivet" line) and "it was leg closure that was the issue" this thread seems to be under the influence of Round the Horne scriptwriters...

Back to topic. Ski jump is physically simple but you can get hung up in the idea of "where does the force to move the aircraft up come from?" when in fact what you have done is changed the vector (presumably, though, the jet would be moving a little faster coming off the end of a flat deck, given the same distance, weight and thrust).

And Engines is right: the USMC does mass trooplift and giving up deck space is taboo.

:hmm:
Which is why I think a British name might be in order.

I think it will be the death knell for the cat and traps - which are bonkers when you think about it.

That basically gives me my answer, thanks.

'Scuse my venturing into this forum, as I have nothing to offer other than enthusiasm - flying off these carriers is something I'd very much like to be doing in a few years time.

Does F-35B have the ability to vector thrust using just the jet nozzle? Obviously the Harrier could 'prop' itself up by using x° of nozzle with minimal aerodynamic drawbacks; however I can't imagine that having two big flappy doors open on top of the jet are going to be a huge help when you want to gain as much forward speed as possible before your otherwise-inevitable swimming lesson?

re post 849 ,Peter we, Excuse me for the late response but I didn't think this thread would go so quickly,

As for a link, I will look it up but I remember reading about this issue on a Dutch engineering forum where they quoted from this article, I believe;
JSF ski jump tests due in 2011 - Jane's Defence Weekly
which stated that;
The question was why it didn't happen yet, seeing that testing is supposed to go rather smoothly and someone who is in a position to know stated that possible Gear problems when used with the Ramp was the reason for the delay.

Also the 13° stems from the Navantia designed carriers both used by the RAN and the Armada Española, 2 likely future F35B clients (certainly the Spanish).


Also as an aside, I don't know if there is any trueth in it (only read about it in a keypublishing-forum thread years ago) but supposedly the real reason the MARINES didn't opt for the Ski-jump was political iso operational.
It did have something to do with the effect it could have on the perceived necessity for large CATOBAR carriers for the NAVY and the argument of deck polution was more used as a rather convenient excuse, which sounds logical as it wouldn't be too hard to imagine a small extension at the bow functioning as a ski-jump without intruding on the available helicopter spaces.

Even more logical if you realize that the newest America class will get even more F35B's on a regular base vs AV8B's on the older ships adding to the importance of fixed wing ops on an otherwise MARINE landing ship (and V22's which could also benefit from it, I would guess).
Any thrueth to this, anybody knows more about the real reason(s) ?




Last remark/question, wasn't the ,nearly completed, Graf zeppelin carrier supposed to have 2 slighty upward sloping CAT's to help improve launch performance, sort of the first (albeit modest) ski-jumpish aided performance tool ?

Crowsnest
 

So no mention of Crowsnest yet. (Hammond -> New Airborne Carrier based early warning) Guess this is baggers for Merlin??

Also a bit obvious, but I assume forumites do realise QEC are replacing Ocean LPH and Ark? So when SDSR said 12 F35 (was C now B) it was not because of aircraft buy budget alone, but because of an expected mixed deck. (F35, Junglie Merlin, AS Merlin, Wildcat, and now I guess Crowsnest ????? ) Think this position creates a broad number of assumptions that influence comment here!

1) USN no longer looks at this as lightening their carrier strike load - hence really not sure they care as much as they did
2) Lots of sea time now. Can't see them leaving Pompey without 12 shiny F35s aboard
3) Ignoring how to land and take off, these 12 aircraft need to do a lot. CAP, CAS, Maritime strike ........ Think the training load will be a bit higher then land based deep strike. As I assume and hope 12 will be in the minority of F35's, I think too busy to go to sea won't fly, so to speak! Only stopper would be another Afghan. Unless the current state is extended I really can't see anyone wanting to get involved anytime soon in a repeat anywhere.
4) Following on from 3), anything we do get involved in will much more likely to be in the littoral areas, launched from QEC
5) Agree to the concern about putting these in harms way, but how else do you do the LPH role? BTW US strike groups use 3 escorts and an SSN. Think that can be done. The gap is AEW (Crowsnest) and # of F35s available. Ooops :-)