Register Forms FAQ Wikiposts Calendar Search Today's Posts Mark Forums Read

 Tech Log The very best in practical technical discussion on the web

 23rd Oct 2006, 01:32 #1 (permalink) Thread Starter   Join Date: May 2002 Location: still in bed Posts: 178 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.
 23rd Oct 2006, 01:56 #2 (permalink) Join Date: Mar 2003 Location: Canada Age: 74 Posts: 25 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
 23rd Oct 2006, 02:17 #3 (permalink) Join Date: Jul 2006 Location: USA Posts: 445 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.
 23rd Oct 2006, 02:21 #4 (permalink) Join Date: Jul 2006 Location: USA Posts: 445 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.
 23rd Oct 2006, 05:04 #5 (permalink) Thread Starter   Join Date: May 2002 Location: still in bed Posts: 178 Thanks, at 200 psi your aqualplaning speed is 481 kn? wich is the gross value of the gravity of the contaminant? (water)
 23rd Oct 2006, 06:41 #6 (permalink) Join Date: Jul 2006 Location: USA Posts: 445 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. PantLoad
 23rd Oct 2006, 07:36 #7 (permalink) Join Date: Jan 2006 Location: EU Age: 36 Posts: 366 Oxford teaches this formula which you mentioned above: V (speed kt) = 9 (rootsquare (tyre pressure / Specific Gravity) it's easy and it works =)
 23rd Oct 2006, 08:22 #8 (permalink) Join Date: Dec 2002 Location: UK Posts: 1,569 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).
 27th Aug 2010, 12:43 #9 (permalink) Probationary PPRuNer   Join Date: Aug 2010 Location: france Posts: 1 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
 27th Aug 2010, 13:42 #10 (permalink) Join Date: Oct 2009 Location: UK Posts: 1,272 Google found this on the front page: Et Voila: http://ntrs.nasa.gov/archive/nasa/ca...1964000612.pdf
27th Aug 2010, 14:11   #11 (permalink)

Join Date: May 2008
Location: Euroland
Age: 46
Posts: 147
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.

 29th Aug 2010, 11:05 #12 (permalink) Join Date: Nov 2002 Location: Heart of Europe Posts: 190 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.
 30th Aug 2010, 11:11 #13 (permalink) Join Date: Aug 2005 Location: Europe Posts: 97 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
 30th Aug 2010, 18:13 #14 (permalink) Join Date: Aug 2005 Location: Europe Posts: 97 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.
 27th Sep 2010, 19:17 #15 (permalink) Join Date: Dec 2008 Location: Netherlands Posts: 96 Hydroplaning more info
 27th Sep 2010, 21:46 #16 (permalink) Join Date: Aug 1998 Location: Ex-pat Aussie in the UK Posts: 3,641 The NASA doc above is the definitive source, as it is based on original research. The important things to take from it: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. 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.
 27th Sep 2010, 22:00 #17 (permalink) Join Date: Jan 2006 Location: Between a rock and a hard place Posts: 820 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.
 28th Sep 2010, 01:55 #18 (permalink) Join Date: Aug 2007 Location: Between East and West Poles. Posts: 69 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.!!!
 28th Sep 2010, 04:05 #19 (permalink) Join Date: Oct 2005 Location: 45 yards from a tropical beach Posts: 1,104 Aquaplaning continues It is also important to know that, once started, aquaplaning can continue well below the speed of onset.