9 x The Square Root Of Tyre Pressure

[The Devil's Advocate]

Could somebody please explain this one to me? It's something I came across recently, but sure don't remember doing it in my ATPLs.
Am I correct in saying that it means, 9 times the square root of your tyre pressure will give you your tyre limiting speed?

Anybody:confused:

[TopBunk]

DA

From recollection, 9x sqr pressure in psi gives you the aquaplaning speed in kts.

So if pressure is 225psi, 9 x sqr 225 = 9 x 15 = 135 kts aquaplaning speed.

HTH

[Milt]

Calculate the approximate aquaplaning speed for your car and store the number in the recall drive in your brain.

It may save my life someday if you are coming the other way!!

[Old Smokey]

It's a fairly good approximation, but don't forget that the result is in Miles per Hour, not Knots.

Like all fairly good approximations, it does not consider other variables such as

(1) Water dispersal capability of the tyre/tires, and the surface of the runway (e.g. grooved / not grooved),

(2) Your tire/tyre profile (Tall skinny tires/tyres are better),

(3) The friction coefficient of your tire/tyres (wear and tear) and of the runway.

It's a guide-line 'ball park' figure, not an empyric value and must be treated with caution as such. The actual aquaplaning speed may be considerably lower or higher.

Regards,

Old Smokey

[The Devil's Advocate]

Thanks for the replies guys, cleared that up for me. Interesting to note its mph Smokey, i'll keep that in mind. So if thats the calculation for aquaplaning speed, is there an easy calculation for tir limiting speed or are you just going by manafacturers published numbers?

[BOAC]

DA - no way to 'calculate' - it should be part of a/c limitations in your ops manual or else is so high as to be irrelevant.

[The Devil's Advocate]

Thanks BOAC :ok: just wondered after having a tire blow out recently. How's winter with AEU, they keeping you busy?

D.A.

[enicalyth]

http://i28.photobucket.com/albums/c220/enicalyth/Aquaplaning.jpg

Presumably aquaplaning speed is higher for fuel rather than water because it squishes out of the way easier?

[Alex Whittingham]

Actually I think the formula is derived empirically, and the value is in knots not mph. From an old typewritten NASA technical memorandum (NASA-TM-85652):

The contact pressure between tire tread and pavement establishes the escape velocity of bulk water drainage from beneath the tire footprint. High pressure tires can expel surface water more readily from the footprint than low pressure tires. When the aircraft ground speed equals or exceeds the escape velocity of water drainage from the footprint, choked water flow occurs. The tire has now reached a state of total dynamic hydroplaning. Test results .... indicate that the critical aircraft groundspeeds required for this total hydroplaning condition to occur on a flooded (runway water depth is greater than tire tread groove depth) pavements with an unbraked tire are approximately:

Spin-down (rotating tire) speed, knots = 9 sqrt Infl. pressure, psi
Spin-up ( nonrotating tire) speed, knots = 7.7 sqrt Infl. pressure, psi

......... It is important that pilots be aware that the lower hydroplaning spin-up speed, rather than the high hydroplaning spin-down speed, represents the actual tire situation for touchdown on flooded runways.

[Gary Lager]

enicalyth - please accept my apologies and ignore me if you were just being flippant, but the correction for specific gravity is more useful when considering contaminents such as snow or slush, which both have SGs below 1.0. (Dry snow ~ 0.35, slush from 0.5-0.8 I think)

I do agree with you in essence though - a fluid with a lower SG is less able to sustain the required (shear?) force due to lower density.

But if you find yourself landing a jet aircraft on a runway flooded with fuel, I suggest you're shortly going problems other than hydroplaning to worry about! WOOF!

[enicalyth]

Agreed contaminants normally means slush etc. Woof is not the worry either. The worry really is what is the specific gravity under the wheel? Once pressure is applied ice for example reverts locally to water (under a skater's blades for example). But what happens under the wheel when an oily, sooty mix is present? What s.g. refers then? Getting the s.g. wrong gives wildly erroneous answers. The fuel s.g. was an example and I once had a near-incident with worn but nonetheless legal tyres on a notorious runway. Any layman would have called the surface contaminated but the management response was to query the specialised meaning of "contaminant" which evoked to them slush and because slush was not present binned my complaint as ludicrous. I have since read the Airbus TOSTA document with more than usual care!

[woof]

But if you find yourself landing a jet aircraft on a runway flooded with fuel, I suggest you're shortly going problems other than hydroplaning to worry about! WOOF!

I wasn't even there.

Sorry, back to the subject

[Gary Lager]

Oops forgot std noise for enormous explosion is 'woomph' - expect he'll be along in a minute now.

enicalyth - point taken.

[RatherBeFlying]

The absolute worst surface is ice with air above freezing which invariably is coated with a film of water -- it can take considerable time to simply get back on one's feet.

Next down on the list is compacted slush at temperatures just below freezing. It has a firm grip on the surface and contains a significant amount of water which makes its presence known when pressure is applied -- say from an tire.

While hydroplaning may come into play on these surfaces, the underlying contaminant may well remain between tire and pavement.