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SSSETOWTF
17th Apr 2003, 23:02
Does anyone out there have any facts/figures for the amount of wing lift you could expect from the Harrier's wing whilst you're in the hover?

The reason I ask:

When flying the GR7 from Illustrious, the boat was usually turned into wind for landing. I've always thought (is it pyschological?) that a stiff 20-30 knots over the wing makes the jet hover at a slightly lower power setting than I'd expect.

Using the very conservative figure that the wing is producing 1000lb lift at 50kts (the number in the books for RVLs), and assuming the lift coefficient stays the same, surely by just factoring for the difference in dynamic pressure, you should get around 200lb at 20kts. Can you make that assumption?

I'm now flying an AV8B II+ with the USMC and they insist on turning the boat around and running downwind for landings. According to the USMC software, you lose about 400lb performance if the apparant wind over the wing is greater than 15kts.

I've asked around what their reasons are. No one seems exactly sure - have heard 'laminar flow breaks down below 50kts', 'stronger wind means the SAAHS is going to be working the reaction controls more' and 'stronger wind blows out the LIDS cushion'.

Have no idea what they're talking about with regard to laminar flow breaking down - certainly don't remember that rule from university.

I can see how in a gusty wind the SAAHS would work harder - but a steady wind (like that produced from a boat steaming along at 30kts) should be fine shouldn't it? And besides it'd have to be working really hard to use up the 400lb their software says it does + whatever lift effect the wing's producing.

As for the LIDS effect - I was always under the impression it only really cut in around 15-20'. While on a windy day you do get a bit of 'suck down' I've never thought of it as being so bad that I'd e.g. come into the hover on a 20kt day at a land base 400lb below my VL figures.

Operationally, it would really help if the boat didn't have to keep turning into wind for launch and downwind for recoveries - especially when someone calls up for recovery low on gas (eg bingo-ing out of Iraq...) and there's a launch cycle in progress. Basically I'm just trying to clear my name for the huge faff I caused the other day by being able to tell them they didn't need to turn the boat around.:uhoh:

Hilico
18th Apr 2003, 00:48
Now please bear in mind I know ABSOLUTELY NOTHING about flying a Harrier and last flew as PIC in a C150 25 years ago, BUT:

I was talking to an ex-SHAR pilot the other day and having a bit of a laugh how these Wings Channel documentaries always say that landing back on the carrier is the difficult bit. You just stop and land, surely?

He said that the SHAR was least stable between 30 and 120kt.

Now, doesn't that mean that if the carrier has turned into wind, you're always going to be in the unsweet spot? Going downwind, you'd have a much lower airspeed, but from this information, more stability.

I'll get my coat.

John Farley
21st Apr 2003, 01:04
The main issue with regard to the Harrier and wing lift at low speed is the angle of attack of the wing. In a zero aispeed hover this is minus 90 deg as the engine efflux is inducing a not inconsiderable downwards flow of air all over the wing. Thus producing a negatve lift. The extent of the downwards flow over the wing will depend on the thrust in use and any slight airspeed. How this average wing AOA varies with speed also depends on the flap angle and the nozzle angle

The next thing that is likely to affect your hover preformance is the temp of the air entering the engine. A 20,000 lb class jet engine will lose about 100 lb of thrust per 1 deg C rise of intake air temperature. This may not sound much but it means 10 deg costs you 1000lb which starts to become real money. So recirculation of exhaust gases from either the far field or the near field can be of major concern and affect how you choose to operate.

LIDS effects peak at about 1400 lb up when standing on the wheels and become very small above about 8ft main wheel ground clearance. The LIDS are very beneficial on VTO because the (average) force of 700lb is aplied for several seconds during a slow go. Thanks to Mr Newton´s laws of motion this really does improve the VTO perf. A VL is quite different as the 700lbs is not applied for anything like as long because good jetmanship requires (for various reasons) a very positive rate of descent down through the ground effect layer.

I mention these non contentious issues just to show how the application of simple common sense can lead a young lad astray. The rest I will leave to the current experts because it seems to me that after a time Frank Sinatra (or any other old b***er) should refuse to climb back up on the stage and sing another song – however tempting the invitation might seem.

As to differences between RAF, RN or USMC ops, well in my experience the units have different cultures. And if they are all different ask yourself - can they all be right?

Genghis the Engineer
21st Apr 2003, 01:37
Speaking as a very much relative Harrier Ignoramus compared to the above exhalted company a question nonetheless comes to mind reading what's been said already. Is the problem not wing lift, so much as that the airflow in which the jet is operating in jetborne or semi-jetborne flight is perhaps affecting the efficiency of the engine, either through causing a certain amount of warm efflux to enter the intake, or by affecting intake and thus compressor efficiency?

G

Feck
21st Apr 2003, 21:21
It would appear from the Farley-meister’s comments that all of the factors involved with hovering into wind are beneficial (fancy that!).

Wind over the wing: more lift.
Less induced downwards flow over the wing: more lift.
Less re-circulated air entering the engine: more thrust.
Also, with a tailwind, you’re fighting the SAAHS which is trying to turn the jet onto a sensible heading (it wasn’t designed by the USMC).

As for the LIDS, so you get a bit of suck-down. As you know, having the LIDS wire-locked closed is not perceived to be a factor for VSTOL trips, probably because we don’t VTO all that much.

From the above I would conclude that the USMC have decided, quite rightly, that in a headwind you are a stick-stirring, throttle-pumping arse and this has been worth changing their SOPs for.

Or they don’t know their arses from their elbows and are throwing common-sense out of the window because of a computer program.

Hope the cruise is going well, although it sounds like you’re rocking the boat a little! Just remember they’ve been doing this stuff since the 70s, and along comes a limey with all his…good ideas. It’ll never catch on, y’know.

LOMCEVAK
23rd Apr 2003, 23:44
If the ship is steaming into wind, you have a forward airspeed. In the Harrier II, where do you get the thrust from that is needed to overcome the drag? Do you nozzle aft a couple of degrees or fly in a slightly lower nose attitude? Whatever you do, if the ship steams downwind you will have to do the opposite (or at least something different)! As JF says, the slight changes in AoA resulting from the required technique could be the cause. Has anyone on your ship any contacts in the Harrier flight test world at St Louis or Patuxent River? I am off to Pax next week and will ask around to try and find some hard data - I am intrigued by the illogical.

SSSETOWTF
24th Apr 2003, 01:01
Thanks everyone for the help so far.

John Farley, Frank Sinatra sang pretty well the last time I saw him on the TV - can I not tempt to you to say more? And of course the Brits are right!

Lomcevak - I've only ever tweaked the nozzles in the hover on a couple of occasions alongside Illustrious, the rest of the time I always accepted the slightly low nose attitude. There might be something in that. I'm afraid I don't know any of the chaps at Pax or St Louis - I'd love to hear what they have to say on the matter as presumably they were involved in writing the rules at some point.

Went bouncing round the circuit the other day with this all very much in mind and noticed another slightly odd phenomenon - don't know if it's related. Flying off Illustrious, with lots of wind I definitely remember having to lower the nose a good couple of degrees to prevent aft movement as you take the power off to descend onto the deck. The other day, with only a couple of knots of relative wind over the great USS Bonhomme Richard I was drifting forward in the descent. Now I'd be fully prepared to accept this could be plain old ham-fisted throttle-pumping. Or could that be related to all the other aerodynamic/ingestion etc things that are going on?

John Farley
24th Apr 2003, 01:30
When hovering into a headwind I always preferred to crack the lever forward a tad from the hover stop rather than lower the nose in order to achieve the bit of rearwards thrust. There are several advantages to this, firstly I think the aircraft looks naff in a slighly low nose attitude (and the USMC taught me that it is always better to die than look bad) secondly the view and so one`s judgement of height is unchanged, but most impoirtant of all, the gear much prefers a mainwheel first VL. In the days when there was a big nose down trim change as you entered ground effect (P1127 and Kestrel configs) various mates broke the nose leg by letting it hit first.