Suckdown and the F-35B

[LowObservable]

F-35 Production Costs, Line Changes Drop: Lockheed (http://defense.aol.com/2011/11/01/f-35-production-costs-line-changes-dropping-steeply/)

The company has also gathered "a lot of data," Lawson said, that will enable engineers to come up with ways to mitigate a phenomenon called "suck down," in which turbulence under the F-35B as it lands vertically can create a vacuum that pulls the plane down too rapidly -- a potential danger, especially for a pilot returning from a mission with unexpended bombs or missiles.

Interesting - although not clearly described (it's more of an induced downdraft). This has never been disclosed as a development issue before, although it is a standard issue with jet lift (that's why the Boeing JSF wing was so high off the ground).

"That will enable engineers to mitigate"? "Especially for a pilot returning from a mission with unexpended bombs or missiles"?

It sounds like this is something that they're still working on, and that affects bring-back. Wonder if some of this was behind the UK's change of mind?

Note too that they VL with the inner bay doors open - presumably an attempt to "mitigate" by trapping the rising "fountain" between the front and rear nozzles, as the Harrier LIDS do.

[Shackman]

Sounds more like 'recirculation' - ask a rotary mate.

[rotorfossil]

The vacuum bit doesn't sound right; surely the problem as in all vertical lift vehicles, helicopters include, is one of re-circulation, particularly in zero to light wind conditions.

[jamesdevice]

presumably something akin to the Bernoulli / Venturi effect?
The rapid downdraught dragging adjacent air along so creating a pressure drop?

Sorry my fluid dynamics isn't good enough to explain more

[LateArmLive]

Shackman Sounds more like 'recirculation' - ask a rotary mate.


Yes, I'm sure you're correct. The Lockheed Martin design must not know what they're talking about:ugh:

[jamesdevice]

not even a new phenomenon
see http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910014795_1991014795.pdf from 1991

[John Farley]

Chaps

There are two things mixed up here No 1 the technicalities of suckdown (negative ground cushion or negative ground effect in more technical terms) and No2 what was printed.

Taking the easy one first the issue of suckdown is very straightforward to understand if you take a moment to consider things.

It is a totally aerodynamic problem and nothing to do with re-circulation.

When a SINGLE downwards vertical jet hits the ground it has nowhere to go and radiates horizontally in all directions. Should some of this very high velocity air be in a position to pass outwards below a wing it will generate a reduced pressure under that wing (thanks to Mr B) and so a suckdown effect happens.

If there are four nozzles - as in a Harrier - some of each nozzle sheet can actually meet its opposite number from another nozzle under the centre point. These opposing flows then have nowhere to go but up – the so-called fountain effect - producing an upward force locally under the fuselage. As LowObservable remarked the Harrier LIDS (lift improvement devices) were conceived to maximise this beneficial fountain effect.

However whether you are talking about the positive or negative aspects of these flows you have to be very close to the ground for them to apply. In the case of the Harrier this is below about 8ft wheel height. (NB I delight in using the present tense because they still fly with five nations).

The suckdown force (I am sticking with that term because it is so descriptive) works like any other in that F=Ma (thanks to Mr N). So it produces a downwards acceleration. NB NOT a velocity but an acceleration.

An acceleration being a rate of change of velocity with time we at last come to the kernel of the issue. This suckdown needs TIME to actually change the vertical velocity of the aircraft. The acceleration is quite small actually – Harrier of order 0.05G – so providing you do not hang about on the way down you just won’t be able to measure the increase in vertical velocity. That is why it is good Harrier VL airmanship to keep a positive rate of descent on as you let down from a hover and not try for a pansy feather like VL.

In the case of the F-35B (of which I know nothing but we can still apply the fundamental principles above) the jet velocities are clearly higher so I would expect a slightly greater downward acceleration but even if it was double the Harrier (which I doubt) it is still small and time is still our saviour.

So much for the factual background.

Now turning to the article by Mr Whittle I feel that the word vacuum (which is not sensible in physics terms) came from his pen and not Mr Lawson’s mouth and was probably Whittle’s attempt to help his story along once he got to a keyboard.

For those posters who suspect it is all about re-circulation I would add that the design of the B’s two poster layout quite brilliantly deals with this issue as there is more thrust from the fan than the rear nozzle and so the ‘cold’ efflux from the fan splashing rearwards stops the hot rear efflux from penetrating forwards towards the intake end of the business.

[airborne_artist]

Very clearly put JF, and completely clear even to an aviation numpty like me.. You should write a book :ok:

[Just This Once...]

http://www.rafjever.org/Booksandfilms/ViewfromthehoverFinalcover.jpg

Yes, go on John, write another book as soon as you can.:)