Obelix Posted July 9, 2012 Share Posted July 9, 2012 All good stuff but I've a question or three. How is "onset speed" defined? What units of measurement in the formulae are used? Psi or metric equivalent, mph or kph? (I can't work out if it matters!) By what you write, am I right in thinking that tyres pumped up beyond the recommended pressures would aquaplane at higher (onset) speed than those with just the recommended pressures? Oh, and what sort of training/job function involves this technical stuff? I'm intrigued. Or is it late and I should get off to bed and stop being dim? Apologies I should have said. The pressure figures are in PSI and the speed is in KTS. It's important to note they are rough rules - it's not exact but the figures are reasonably good and supported by a lot of real world testing. Higher pressures increase the speed of onset of aquaplaning but they have other deleterious effects like generally poor grip, much increased tyre wear etc. It's important to know this sort of thing for aircraft - if you are sideslipping onto a runway for example you want the tyres to grab and guide you otherwise the wind will push you off the side - if you have wet conditions though and you land above aquaplane speed then you end up falling off the side as the tyres wont grab. Also you can end up being unable to brake before the end of the runway if stall speed is above aquaplane speed - Qantas Flight 1 that slid off the runway in Bangkok was a victim of this as I recall. - Edit: it was in 1999 if you go looking for it and as I recall the aircraft finally stopped on a golf course making for a rather unconventional hazard on the way to the green. No fatalities apart from the flight crews pride. Link to comment Share on other sites More sharing options...
Rupert_Baehr Posted July 9, 2012 Share Posted July 9, 2012 All good stuff but I've a question or three. How is "onset speed" defined? What units of measurement in the formulae are used? Psi or metric equivalent, mph or kph? (I can't work out if it matters!) By what you write, am I right in thinking that tyres pumped up beyond the recommended pressures would aquaplane at higher (onset) speed than those with just the recommended pressures? Oh, and what sort of training/job function involves this technical stuff? I'm intrigued. Or is it late and I should get off to bed and stop being dim? Back to my post: I learned about aquaplaning when I was learning to fly. (part of the training for my Airline Transport Pilot's licence) From my original post: Onset speed is in Knots (Nautical miles per hour.) I knot is 6080 ft per second. 1 mph is 5740 ft per second. 45-54 Knots is a 'guestimate' It's a (tiny) bit higher than the mph figure ... but how accurately can you guarantee you tyre pressure? Mathematically, that's irrelevant, too. "Don't go faster than 45 mph on any road where there is 2mm or more of standing water and you are unlikely to aquaplane." It's good advice. backed by sound physics. You are correct in assuming that the greater the tyre pressure the higher the aquaplane onset speed ... but do the sums! Tyre Pressure (psi) 25 Square root of tyre pressure 5 Onset Speed. (kts) 45 Tyre Pressure (psi) 36 Square root of tyre pressure 6 Onset Speed. (kts) 54 I suggest you substitute mph for knots (a tiny bit of safety 'for the wife and kids' If the tyre pressure was 60psi (cracking on a bit for car tyres!) Tyre Pressure (psi) 100 Square root of tyre pressure 7.75 Onset Speed. (kts) 69 70mph is the motorway and de-restricted dual carriageway speed. how many people stick to it? How many cars are running around on tyers inflated to 60 psi? In the real world, the onset speed is above 'domestic speed'(30 mph) but below the National speed limit. If you know what's coming, you can learn how to deal with it. (nowadays, I prefer to avoid aquaplaning - but there are techniques which can help you to get around it.) I've aquaplaned more times than I've eaten hot breakfasts (I tend not to eat breakfast) I landed at about 130 knots and 2mm of water (on the runway) is hardly uncommon in England, is it? The problem is (IMO) that not only are students not taught how to recover from aquaplaning, it appears that they aren't even taught that it exists! Link to comment Share on other sites More sharing options...
Obelix Posted July 9, 2012 Share Posted July 9, 2012 I knot is 6080 ft per second. 1 mph is 5740 ft per second. *cough* are you sure about those figures? Link to comment Share on other sites More sharing options...
DT Ralge Posted July 9, 2012 Share Posted July 9, 2012 Back to my post: I learned about aquaplaning when I was learning to fly. (part of the training for my Airline Transport Pilot's licence) From my original post: Onset speed is in Knots (Nautical miles per hour.) I knot is 6080 ft per second. 1 mph is 5740 ft per second. 45-54 Knots is a 'guestimate' It's a (tiny) bit higher than the mph figure ... but how accurately can you guarantee you tyre pressure? Mathematically, that's irrelevant, too. "Don't go faster than 45 mph on any road where there is 2mm or more of standing water and you are unlikely to aquaplane." It's good advice. backed by sound physics. You are correct in assuming that the greater the tyre pressure the higher the aquaplane onset speed ... but do the sums! Tyre Pressure (psi) 25 Square root of tyre pressure 5 Onset Speed. (kts) 45 Tyre Pressure (psi) 36 Square root of tyre pressure 6 Onset Speed. (kts) 54 I suggest you substitute mph for knots (a tiny bit of safety 'for the wife and kids' If the tyre pressure was 60psi (cracking on a bit for car tyres!) Tyre Pressure (psi) 100 Square root of tyre pressure 7.75 Onset Speed. (kts) 69 70mph is the motorway and de-restricted dual carriageway speed. how many people stick to it? How many cars are running around on tyers inflated to 60 psi? In the real world, the onset speed is above 'domestic speed'(30 mph) but below the National speed limit. If you know what's coming, you can learn how to deal with it. (nowadays, I prefer to avoid aquaplaning - but there are techniques which can help you to get around it.) I've aquaplaned more times than I've eaten hot breakfasts (I tend not to eat breakfast) I landed at about 130 knots and 2mm of water (on the runway) is hardly uncommon in England, is it? The problem is (IMO) that not only are students not taught how to recover from aquaplaning, it appears that they aren't even taught that it exists! Thanks for the explanation(s) - very informative, I ought to hang around with a few more pilots, it seems, to know about such things! As for drivers not knowing too much about it, well I deal with such lack of knowledge every day. I shouldn't complain since it keeps me in work. Far more mundane than the "high-end maths" of the formulae is the truth that drivers don't know that water dispersal in tyres drops off like a stone off a cliff below 3mm tread (for starters YouTube and Vicki Butler-Henderson 3mm in wet or http://www.rospa.com/roadsafety/adviceandinformation/vehiclesafety/tyresafety/tread-depth.aspx) - they neglect their tyres - they don't check pressures regularly enough - they don't measure their tread and only have a vague idea that their tyres are legal - they think tread gives grip - if they know 1.6 mm is the legal limit, they will drive the tyres down close to it (and lease car deals stipulate 2mm before changing) but assume "legal" is "safe" - more than a few buy part-worn tyres for economy. These tyres do not come with a health warning, I guess. - the speed choices that they make with these well-worn tyres are exactly the same whatever the weather - a few will also drive through standing water on cruise control despite the fact that they know to ease off and keep a steady hold on the steering in the hope of a dry landing (still heading the way you want to go) the other side of the puddle. - they admit on here to "flooring it" through standing water (but, mind you, only when nobody is around) And for every one of those on here that is not similarly ignorant, there are plenty others ahead of you or behind you that are and they are driving on shallow tread and low pressure. (let's hope each corner is equally worn and inflated) Link to comment Share on other sites More sharing options...
Rupert_Baehr Posted July 9, 2012 Share Posted July 9, 2012 *cough* are you sure about those figures? LMAO - In a hurry -as usual. Try 'per hour' (I was being lazy anyway. - mea culpa. Link to comment Share on other sites More sharing options...
DT Ralge Posted July 9, 2012 Share Posted July 9, 2012 There is one more bit of maths that you can help me with, Rupert (or anyone else). I have heard said that a tyre is designed/meant to ship some 75 litres of water per revolution. A quite impressive feature based, no doubt, on having a decent amount of tread (how much?) But nobody has ever been able to tell me: - at what speed - with how much standing water Is the "standard" always 2mm as seems to be the case with the calculations around the onset speed (above)? OR am I over-thinking this and 75-litres is purely the volume of tread on an average new tyre? Link to comment Share on other sites More sharing options...
DT Ralge Posted July 9, 2012 Share Posted July 9, 2012 LMAO - In a hurry -as usual. Try 'per hour' (I was being lazy anyway. - mea culpa. And aren't there 5280 feet in a mile? Link to comment Share on other sites More sharing options...
Cyclone Posted July 9, 2012 Share Posted July 9, 2012 So having looked into this formula it is quoted and requoted all over the place, and every time it specifies that this is about planes. The shape and profile of a plane tyre is significantly different to that of a car tyre, maybe the pilots here can explain more. I think that trying to apply this formula directly to car tyres is probably very misleading. My physics isn't up to the challenge anymore of calculating this, but in a car I suspect that the width of the tyre and the tread pattern make a large difference to the aquaplaning speed. If the tread pattern only made a small difference then nobody would worry about 1.5mm, 3mm or 6mm of tread depth (or racing slicks). I think we've all driven through water at >45 mph and not aquaplaned before, which would suggest that the formula is giving a misleading result for a 'normal' psi car tyre. Link to comment Share on other sites More sharing options...
Obelix Posted July 9, 2012 Share Posted July 9, 2012 So having looked into this formula it is quoted and requoted all over the place, and every time it specifies that this is about planes. The shape and profile of a plane tyre is significantly different to that of a car tyre, maybe the pilots here can explain more. I think that trying to apply this formula directly to car tyres is probably very misleading. My physics isn't up to the challenge anymore of calculating this, but in a car I suspect that the width of the tyre and the tread pattern make a large difference to the aquaplaning speed. If the tread pattern only made a small difference then nobody would worry about 1.5mm, 3mm or 6mm of tread depth (or racing slicks). I think we've all driven through water at >45 mph and not aquaplaned before, which would suggest that the formula is giving a misleading result for a 'normal' psi car tyre. You are dealing with 2 different things. Tread depth and so forth affects how much water you can disperse and hence affects the *depth* of water that you need to induce aquaplaning. However, once you have sufficient water to start aquaplaning (be that a mm for a slick or a couple of centimetres for a good wet weather tyre) it doesnt matter about these things, and the speed and tyre pressure are what count. Link to comment Share on other sites More sharing options...
Cyclone Posted July 9, 2012 Share Posted July 9, 2012 Given the nature of what aquaplaning is, the width of the tyre and weight of the vehicle must be relevant as well. Link to comment Share on other sites More sharing options...
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