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Old 23rd Oct 2006, 01:32   #1 (permalink)
 
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Aquaplaning formula

somewhere I have got the formula that aquaplaning speed = 9 *rootsquare of Tire pressure.

Tire pressure in PSI or different? can some kind ofyou give me an example?
thank you.
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Old 23rd Oct 2006, 01:56   #2 (permalink)
 
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From my early years I seem to remember the formula as 9x the square root of the tire pressure in PSI and the speed in miles per hour.
Hope this helps
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Old 23rd Oct 2006, 02:17   #3 (permalink)
 
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Aquaplaning/Hydroplaning

1. Take your tire pressure in kg / sq cm
2. Divide this by the specific gravity of the contaminant
3. Take the square root of this value
4. Mulitply this by 34
5. The result is the speed of aquaplaning in kts.


Source: Airbus "Getting to Grips with Aircraft Performance" Section 5.5.2.4, Page 82.


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Old 23rd Oct 2006, 02:21   #4 (permalink)
 
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Addendum

What we're talking about is "Dynamic Hydroplaning". The above formula does not apply to the other two types (Viscous and Reverted Rubber Hydroplaning).

Some may argue that the specific gravity factor considers Viscous Hydroplaning...but, I believe it does not.

Others...please comment.


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Old 23rd Oct 2006, 05:04   #5 (permalink)
 
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Thanks,
at 200 psi your aqualplaning speed is 481 kn?

wich is the gross value of the gravity of the contaminant? (water)
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Old 23rd Oct 2006, 06:41   #6 (permalink)
 
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Not quite...

First, the specific gravity of water is around 1. It varies, slightly, based on temperature and degree of contamination (how pure it is).

Second, did you take the square root the tire pressure? Note, the formula I cited is based upon tire pressure expressed in Kg/cm*2 (not pounds per square inch).

I've read several places that the principle of 9 X the square root of the tire inflation pressure (in lbs/in*2) has been re-evaluated. Some engineers consider a more accurate formula as 7 X the square root of the tire inflation pressure (in lbs/in*2).

Then, there's the theory (I've read this somewhere, but I honestly can't remember where,) that the formula is incorrect if, upon touchdown, there is no initial wheel spin-up, i.e. a real greaser of a landing on a runway that is contaminated with water after a heavy rain storm. In this case, you may never realize any braking action.

Interesting stuff.

In any event, it's important to note that the coefficient of friction (mu) is very low with standing water. In fact, I've seen figures published where the mu with standing water is not much higher than that of wet ice.

To some degree, tire wear plays a role, too. How much tread is on the tire has some bearing...but not as much as you might think.

Years ago, Professional Pilot magazine had a really good series of articles on braking, hydroplaning, coefficient of friction, etc. You may be able to retrieve these somehow via the internet.

In ProPilot treatise, the author discussed how a pilot in a typical runway over-run accident, many times, experiences all three types of hydroplaning. Initially, the pilot is faced with dynamic hydroplaning. As the landing roll continues, reverted rubber hydroplaning comes into play. Finally, as the aircraft skids through the touchdown zone of the other end of the runway, viscous hydroplaning is experienced.

In countries that are lesser developed (a nice way to put it), the runways are not crowned, not grooved, and not regularly pressure-sprayed (to remove rubber and other contaminants). Attempting to land during or immediately after a heavy rain puts a pilot into the 'test pilot' category. Whether or not he'll get stopped...well, all bets are off.

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Old 23rd Oct 2006, 07:36   #7 (permalink)
 
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Oxford teaches this formula which you mentioned above:

V (speed kt) = 9 (rootsquare (tyre pressure / Specific Gravity)

it's easy and it works =)
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Old 23rd Oct 2006, 08:22   #8 (permalink)
 
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The subject has been covered in Flight Testing. The thread provides links to references which associate SG with aquaplaning speed and also indicates that a simple formula might not apply to all aircraft.
In particular modern aircraft tires have lower hydroplaning speeds than those predicted by the well-known and commonly accepted equation because of the differences in the footprint dimensions of the newer tires as compared to the older bias-ply tires.
The diagram in the linked presentation indicates that the hydroplaning boundary ranges from 9*SQR x tire pressure to 6*SQR x tire pressure (psi).
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Old 27th Aug 2010, 12:43   #9 (permalink)


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Formula aquaplaning

Hi,

I would like to understand...How do you find this formula? And can you demonstrate this formula??
I'm french and I'm doing research on this topic...Can you help me???

Please
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Old 27th Aug 2010, 13:42   #10 (permalink)
 
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Google found this on the front page: Et Voila:

http://ntrs.nasa.gov/archive/nasa/ca...1964000612.pdf
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Old 27th Aug 2010, 13:49   #11 (permalink)
 
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What's all this "rootsquare" business?

Don't you mean "square root"?
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Old 27th Aug 2010, 14:11   #12 (permalink)
 
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Quote:
Then, there's the theory (I've read this somewhere, but I honestly can't remember where,) that the formula is incorrect if, upon touchdown, there is no initial wheel spin-up, i.e. a real greaser of a landing on a runway that is contaminated with water after a heavy rain storm. In this case, you may never realize any braking action.
Which is why we have learned that you should make a positive landing (i.e. not a greaser) when there is a possibility for aqua planing.
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Old 29th Aug 2010, 11:05   #13 (permalink)
 
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rudderrudderrat

Thanks for that doc. I've been looking for that one for a while. Always good to simply go to the source of something for insight.
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Old 30th Aug 2010, 11:11   #14 (permalink)
 
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Take-off hydroplaning vs. Landing hydroplaning

The 2001 report NLR-TP-2001-216 Safety aspects of aircraft performance on wet and contaminated runways gives additional and updated information related to the NASA/Horne's equation. Have a look at chapter 2.3 Hydroplaning in this report and you will see that there are differences related to "spin-up", "spin-down" and radial tyres.

Table 1 in the report shows Typical take-off, landing and hydroplaning speeds of commercial jet and turboprop aircraft


www.nlr.nl/smartsite.dws?id=4381
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Old 30th Aug 2010, 18:13   #15 (permalink)
 
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Aquaplaning - Hydroplaning - radial tyres

In the May 2010 NLR/EUROCONTROL report by G.W.H van Es, - A study of runway excursions from an European Perspective - following message is found:

Modern aircraft tires like radial tires can have lower aquaplaning speeds than the older cross-ply tire designs. This fact is not very well known to the pilot community.

www.nlr-atsi.nl/id~14919.pdf

Last edited by tribo; 30th Aug 2010 at 18:47.
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Old 27th Sep 2010, 19:17   #16 (permalink)
 
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Hydroplaning more info

also see; www.skybrary.aero/bookshelf/books/1147.pdf
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Old 27th Sep 2010, 21:46   #17 (permalink)
 
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The NASA doc above is the definitive source, as it is based on original research.

The important things to take from it:
  1. The formula only applies if the tyre can be supported by the fluid - that is that the depth of the water above the surface of the runway is greater than the tyre tread depth.
  2. The speed required to support the weight of the aircraft is (in almost every normal case) greater than the touch down speed of that aircraft.

... so the pilot "clued up" on aquaplaning understands that:
  • it is worth noting the tread depth on the walk-around.
  • That you don't need to worry about "aquaplaning" if the runway isn't flooded
  • that regardless of the calculated speed, you always need to worry about "slippery" conditions if it is wet (or contaminated with rubber, or oil or slush etc.) and,
  • that, if asked by a pedant, it is only the main wheel pressure you need to calculate - as the nose wheel doesn't (in almost every case) have any braking, and at the speeds in question, steering authority is still with the rudder.
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Old 27th Sep 2010, 22:00   #18 (permalink)
 
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For what it's worth...

JAA Learning Objectives states, for dynamic aquaplaning:

Non-rotating tyre: 7,7 * sqrt PSI
Rotating tyre: 9 * sqrt PSI

... the reference, if I recall correctly, is the NASA document linked to above.
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Old 28th Sep 2010, 01:55   #19 (permalink)
 
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Another Aquaplaning Formula

The formula for aquaplaning speed in Knots is 9 x square root of tyre pressure in PSI.

The formula for aquaplaning speed in Miles per Hour is 10.4 x square root of tyre pressure in PSI. Forget where I read this---was many years ago.

Now think about driving a Volkswagon Beetle where I believe the front tyre pressure is 16 PSI.

So using the last formula we arrive at an aquaplaning speed of 41.6 MPH.
Not a good idea to speed in the wet then.!!!
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Old 28th Sep 2010, 04:05   #20 (permalink)
 
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Aquaplaning continues

It is also important to know that, once started, aquaplaning can continue well below the speed of onset.
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