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Author Topic:   hyroplaning speed
cadet
unregistered

Posts: 544
From: Australia
Registered: Dec 98

posted 19 March 1999 02:19            

hello,

does anyone still remembers the formulae for calculating hydroplaning speed ? i believe it has something to do with tyre pressure. no?

thanks

 

pterodactyl
Experienced PPRuner

Posts: 379
From: australia
Registered: Sep 98

posted 19 March 1999 04:22               


Hydroplaning Speed Knots = 9 * Sqrt Tyre Pressure (PSI)

Example 100Psi Tyre Press

Hydro Speed = 9 * sqrt 100 = 9*10 = 90 Kts

This is for Hydroplaning INITIATING SPEED. Many years back it was suggested that once hydroplaning started it did not cease until speed reduced below 7.2 * sqrt Tyre Pressure
in that case 7.2*10 = 72kts. This aspect seems to have been either forgotten or disproved. Any opinions,recollections?

 

Slimbitz
Experienced PPRuner

Posts: 14
From: UK
Registered: Nov 98

posted 19 March 1999 10:32            

Seems you have it correct as 9*sq root tyre pressure in psi.

No technical evaluation, but I am aware of the hydroplaning speed for my car, and it seems that the steering goes looser at about that speed. Also continues for about 10mph below onset. Probably confirms your numbers.

 

Windshear
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Posts: 544
From: Australia
Registered: Dec 98

posted 19 March 1999 10:46               

Dactyl ... you're info is correct, however that 9^P applies to a tyre rolling from a dry surface to a flooded surface.
If you land and touch down on surface water deeper than the tyre grooves, or with smooth tyres on any wet surface, then it's 7.7^P tyre pressure for hydro-planing speed
ie 100psi 7.7* = 77kts.
I guess this is a good lesson about anti-skid systems being operative as well.

 

pterodactyl
Experienced PPRuner

Posts: 379
From: australia
Registered: Sep 98

posted 19 March 1999 11:13               

Thanks Windshear. I couldn't remember if it was 7.2 or 7.7. Didn't want to branch out into viscous or reverted rubber hydroplaning.
Flooded runway is more likely to be encountered.

 

Pom Pax
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Posts: 93
From: Kalgoorlie, W.A. , Australia
Registered: Oct 98

posted 19 March 1999 13:47   

cadet
If you wish to delve further into the subject than a simple formula I suggest you try the competition dept of one of the major tyre makers.
Other critical factors are tyre width, no of grooves and weight plus the water deepth thread deep ratio already mentioned. And remember as the tyre progressively fails to clear the water in front of it a "bow" wave developes increasing the apparent water deepth.

 

Checkboard
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Posts: 698
From: Perth, WA, Australia
Registered: Aug 98

posted 19 March 1999 15:55   

I heard that, due to the factors mentioned by POMPAX, that these formulae are really only usefull in technical evaluations, under controlled conditions, and are so innaccurate under practical conditions as to be useless.

Yes the tyre will hydropane - but I wouldn't bother using these formulae in real life.

 

Ignition Override
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Posts: 502
From: Memphis, TN, USA
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posted 21 March 1999 06:34   

Does anyone out there operate into any airports where the airport info. or ATIS gives out water depth on runway? Do any companies have criteria for allowed water depth (other than i.e. one-half inch max takeoff and 1 inch max landing) on takeoff/landing except not during or immed. after a major rainshower? Obviously not accepting tailwind/downhill landing!

 

pterodactyl
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Posts: 379
From: australia
Registered: Sep 98

posted 21 March 1999 07:26  

Before runway grooving became common manual guidance was given by some operators for various water depths and some effort was made to give depth info. Establishing water depth is an impossible task really if you expect results within one quarter inch because of runway slope both longitudinal and transverse as well as the tide effect of a crosswind pushing water to one side.

Some broad guide lines are given by some operators with regard to the appearance of the surface.For example surface darkened by moisture = damp,Some glossy patches = Wet,shiny surface with no dark patches = Flooded. All very well on the ground,not so good on late final approach especially at night. Fortunately better tyre design and runway grooving and drainage has reduced the frequency of encounters a little but regardless of all that if there is sufficient water present aquaplaning will occur.

[This message has been edited by pterodactyl (edited 21 March 1999).]

 

Astroboy
Experienced PPRuner

Posts: 47
From: Darwin, Northern Territory, AUSTRALIA
Registered: Jul 98

posted 29 April 1999 12:53   

The rule(s) of thumb that will score you maximum points in an Aussie ATPL exam are

9 x (sqrt Tyre Pressure) for a ROLLING wheel encountering a depth of water, as would be the case in an aircraft accelerating to TO.

7 x (sqrt Tyre Pressure) for a STATIC wheel encountering a depth of water, as would be the case for an aircraft touching down on landing.

The above correspond very closely to hydroplaning experienced in our Metros. During the wet season, a single CB cell may drop an inch or two of water on a runway in minutes. Makes for fun xwind landings!

 

Skycop
Experienced PPRuner

Posts: 277
From: UK
Registered: Jan 99

posted 29 April 1999 22:57  

RAF teaching was not to attempt to "grease it on" if landing on a soaked runway. This helps ensure good initial contact between tyre and terra firma to get a good tyre rotation. (That's my excuse and I'm sticking to it).

 

pterodactyl
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Posts: 379
From: australia
Registered: Sep 98

posted 30 April 1999 12:10   

Yes they do say that don't they.
However the real reason for not greasing it on is to avoid floating past usable runway and initiating all retarding devices in a timely manner as soon as possible.If sufficient water depth is present aquaplaning will occur regardless of the weight on the tyres. This is because the tyres "spread"(at a given tyre pressure more square inches in contact to support weight).The onset of aquaplaning is dependent on tyre pressure not weight.The early use of retardation methods,engines to idle,spoilers up,max flap setting for conditions,reverse as appropriate helps to reduce speed as soon as possible to a speed where tyre adhesion to the surface is achieved and THEN those extra square inches of tyre contact due to weight will help braking effect.

Upon touchdown when aquaplaning is present there is not the usual degree of nose down pitch associated with mainwheel adhesion to the surface coupled with braking effect present on a dry runway and positive nosewheel contact with minimum delay is important especially with crosswind present.

When braking action is apparent the nose down moment increases transferring more weight to the nosewheels and if large forward stick is maintained the negative lift normally present from the tailplane at touchdown is reduced resulting in even more weight transferred to the nosewheels from the mainwheels.

On the Convair 340/440 Metropolitan it was recommended to use back stick, increasing down load on the tail and hence total apparent weight, to counter this tendency(not to raise the nose) as brakes became effective and keep as much weight on the mainwheels as possible during the latter part of the landing roll.This aircraft had a pronounced nose down attitude with nosewheel on the ground.
Quite the reverse on a DC4 where forward stick was needed because of the nose up attitude and wing lift.
GET IT ON THE GROUND AND STOP IT!

[This message has been edited by pterodactyl (edited 30 April 1999).]

 

Skycop
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Posts: 277
From: UK
Registered: Jan 99

posted 01 May 1999 22:52   

Pterodactyl, I see where you are coming from but by your argument it would seem that resting a hammer on a nail would result in it being driven home just as well as hitting it. I don't think we have the complete answer as yet.

 

pterodactyl
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Posts: 379
From: australia
Registered: Sep 98

posted 02 May 1999 03:53    

Very well Skycop. Certainly will get those spoilers up quickly especially as wheel spin up may not occur.
But when the undercarriage recoils back in shock AND there is enough water present you are back where I am. The need, by all means possible to reduce speed as much and as rapidly as possible to assure braking action is paramount. That assumes that you really want to do this in the first place ... I guess we are caught out, rather be somewhere else, wouldn't you?

 

Checkboard
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Posts: 698
From: Perth, WA, Australia
Registered: Aug 98

posted 02 May 1999 09:30    

Resting your hand on the water will allow it to slip through, slamming your hand on the water won't!

 

Skycop
Experienced PPRuner

Posts: 277
From: UK
Registered: Jan 99

posted 02 May 1999 21:36    

Ptero, I agree with you but if your u/c oleos are working correctly there shouldn't be much recoil and by then the wheels should hopefully be a spinnin' so Root (9xP) should be the order of the day. The best way to avoid all this worry is to come to the hover and land on skids Happiness is a big chopper!

 

IBTheseus
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posted 03 May 1999 03:02   

Pterodactyle
Didn't Boeing produce a film when experimenting some years ago with aqua planning showing that it did not matter if you made a positive touch down, the wheels would still plane above the water surface untill the 7?* Square root of the tire pressure? So yes it is very important to touch down accurately to give the tires a brakes every opportunity to do their stuff

 

pterodactyl
Experienced PPRuner

Posts: 379
From: australia
Registered: Sep 98

posted 05 May 1999 10:38    

I B Theseus
Yes I do recall a video on the subject but not sure if it was Boeing or Douglas.Yes you are right, regardless of the weight on the wheels, if enough water is present and tyre speed above 9*Sqrt Tyre Pressure aquaplaning occurs and wheel spin up is negligible. Wheel spin up may not be sufficient to activate spoiler deployment. My point was that it is important to get it on the ground firmly and on target without wasting runway to deploy spoilers(oleo squat swithces) and all other aerodynamic retarding devices as soon as possible as they are the only means of retardation until below aquaplaning speed when tyre adhesion enables braking effects.
THEN maximum weight on the wheels gives the most square inches of tyre adhesion and most available braking.

[This message has been edited by pterodactyl (edited 05 May 1999).]