The Ground Cushion - A FJ Question

[ProfessionalStudent]

http://www.glumbert.com/media/flylow

This was posted a wee while ago, but I was wondering would some of these shots have been "in ground effect"? I've heard FJ mates talking about riding the ground cushion and I've even heard them say that if you're already on/in it, it's actually really difficult to break through it and spank in?

Any thoughts? Or utter horse? Anyone stupid enough to try?

I know that ground effect and ground cushion is familiar to us RW mates and I've experienced it on landing in FW, but never at 300+ kts.

And obviously any anecdotes would be from "a friend of a friend".

[mbga9pgf]

http://www.glumbert.com/media/flylow
This was posted a wee while ago, but I was wondering would some of these shots have been "in ground effect"? I've heard FJ mates talking about riding the ground cushion and I've even heard them say that if you're already on/in it, it's actually really difficult to break through it and spank in?
Any thoughts? Or utter horse? Anyone stupid enough to try?
I know that ground effect and ground cushion is familiar to us RW mates and I've experienced it on landing in FW, but never at 300+ kts.
And obviously any anecdotes would be from "a friend of a friend".



Some Gen HEre

http://en.wikipedia.org/wiki/Ground_effect

Mentions Wing tip vorticies playing an effect, which would suggest as speed increases, induced drag decreases (decreased AOA), therefore wingtip vortex strength decreases, THEREFORE ground effect decreases. Dont know whether you would want to prove this at 420 KIAS within a 1/4 wingspan of the ground!!!!

Someone may know in the tech forum.

[GPMG]

I thought that the ground cushion on a helicopter was lost once you moved into transitional lift.

Surely a fast mover is going way beyond the speed where ground cushioning could be felt?

[BEagle]

'Ground cushion' is perhaps not the same as the 'ground effect' being discussed here?

I was always under the impression that 'ground effect' existed below a height of 0.8 x wingspan.

A 'friend of a friend' :hmm: once used to fly HM's tin triangles at such heights as < 0.8b (about 80 ft) and 300 KIAS, it was much easier than flying it at 50 ft but didn't leave quite such a good rooster tail behind the aircraft over the sea. Or so I'm told.....:E

[BOAC]

Having flown 'lower than normal' a while back in my military time, I can assure you that there is a noticeable push-force below a 'certain height' presumably caused by the effect of the positive pressure zone under the aircraft. It is well-known. Whether it occurs at < 0.8x w/span I know not, not having my tape measure with me at the time.:)

Thinks - that would have been < 17.6 ft...............:eek:

[27mm]

Maybe this topic should be directed to a tp, but I recall from the F4 that standard landing technique was to drive it all the way down to the runway - no flare being required, due to ground effect providing a "cushion". Apparently this is also the case with Typhoon.

[stillin1]

"I recall from the F4 that standard landing technique was to drive it all the way down to the runway - no flare being required, due to ground effect providing a "cushion".
Not in the couple o thousand hours I did in em:\ If there was a cusion it was a very small and not plumped-up one! T'was the ruddy huge "gear" that saved the visits to the dentist, since it was designed to plonk the jet into a carrier's cable:ok:
and - flaring is for poofs:E

[brain fade]

Ground effect is related to the wing vortexes being constrained by the presence of the ground. This limits induced drag giving a bit extra lift when you need it most. Not speed related except in as much as more speed = more lift.

Or not, as the case may be.:}

[BEagle]

Lift dependent drag is proportional to the inverse square of TAS, whereas zero lift drag increases with the square of TAS.

So the reduction in vortex magnitude is certainly 'speed related', I would venture to suggest.

Regarding the F4 landing technique, I agree with how Rob Prest described it in his book F4 Phantom:

'100 feet ... 50 feet ... a slight tension now ... the neccessity to fine down the movements, hold it steady in the groove ... the earth flattens and expands, but life is still graceful, relaxed as my eyes tell my brain to tell my hands (don't ask me how) that the time is right to make a little backwards movement to check our rate of descent slightly (the purist never crashes the Phantom onto the ground as the QFIs taught us to do at OCU Conversion) and at the moment critique, draw back the throttles and settle the main wheels ... feather soft and belying our 170 mph onwards race ... onto the damp runway ... easy.'

[MightyGem]

I thought that the ground cushion on a helicopter was lost once you moved into translational lift.

Yes it is, but this is about Ground Effect.

[Cyclone733]

According to one of my old instructors, the Buccaneer could be flown very low over the sea. In his words "38ft and trim nose down", I gather this is to do with Wing in Ground Effect, similar to the Ekronoplane.

Website on WIG http://www.se-technology.com/wig/index.php

[BEagle]

44ft wingspan, 0.8 x 44 = 36, so that would seem correct.

Bucc was rock steady at low level. As Bruce C described it "You don't look for things to chase, you look for things to ram!"

[Yeller_Gait]

A legend in its own lunchtime, it was designed to rapidly move assault troops across the (flat & calm) Caspian Sea.

www.youtube.com/watch?v=q6HQSNERadQ

Y_G

Edited to add:

Around 250-300 mph if I remember correctly.

[Kitbag]

Fantastic machine, guess operating in a harsh environment (high speed @ literally sea level) must have been quite punishing. Still reminds me of the things Jerry Anderson used to come up with each week on Thunderbirds and Captain Scarlet. (Sorry, showing my age)

[Avtur]

I remember a very competant Captain of mine pushing the stick forward on Norman Nimrod during a go-around over runway 27 at Gib at 30 feet saying" look it won't go down no matter how hard I push (fnar fnar)"... Not sure how much forward pressure he was actually pushing on the control column, but he managed to impress me as my brown system was in the process of failing.

[Roland Pulfrew]

I remember a very competant Captain of mine pushing the stick forward on Norman Nimrod during a go-around over runway 27 at Gib at 30 feet saying" look it won't go down no matter how hard I push (fnar fnar)"... Not sure how much forward pressure he was actually pushing on the control column, but he managed to impress me as my brown system was in the process of failing.

Ah yes the Mighty Hunter. WIGE doesn't just apply to FJ, I think all aircraft bar perhaps the slowest will do this. I certainly remember 30-ish feet in the Mighty Hunter and not being able to get lower. I am not sure but IIRC it also a function of speed as well as wing span. Certainly at any reasonable speed you could feel the 'cushion' of air under the wing and it was very difficult to get any lower.

[tacr2man]

WW2 Mosquito seemed to utilise this effect quite often, good example on "the Mosquito Story" video or would it be good concentration?

[oldbilbo]

it was designed to rapidly move assault troops across the (flat & calm) Caspian Sea.

Er, I believe the intention was to move them rapidly across the Baltic, into Denmark and Schleswig-Holstein. There was this little story going around about how they and their assault infantry were earmarked for the taking of NATO airfields. Oh, and chemical weapons....:eek:

AFAIK ;)

[Jaguar Pilot]

Ground effect is caused by a reduction in wing downwash angle. This means that the total aerodynamic reaction vector "tilts" slightly forward.
Since the rearward component of this total reaction is induced drag, then the latter decreases. A very low aspect ratio wing, such as that of the "Caspian Sea Monster" results in a large tip vortex - the more intense the vortex, the greater the downwash, and thus the greater is ground effect.
In Aircraft Performance, an aircraft is "considered" to be in ground affect when the height above ground is equal to or less than half wingspan, but this has to be taken with a pinch of salt, (unless you are sitting the ATPL exams), since there are so many variables.