The Tikrit University theoretical analysis looks dodgy to me. I don’t think they have correctly modelled the increase in pneumatic tyre contact area with increasing load.
I know the NASA TN D-2056 is 50 years older, but with due respect for an ancient civilisation, I don’t think Iraq has regained its lead over the USA where technology is concerned. NASA’s TN includes theory backed by British and US experimental data. See what you think of it.
It does precisely predict minimum hydroplaning speed for my Maule, subject, as noted above to changing the constant for radial tyres. The 20 knots is measured in practice with two GPS (non-WAAS in the Garmin VIRB, and a GTN650 in the cockpit).
I’m reluctant to comment about the river Derwent accident, not least because the pilot is not here to defend himself. People who are less squeamish may call him a liar, but I won’t.
Your analysis of taildragger water-skiing is correct. With aft c of g it’s in stable equilibrium. The problem with a nose wheel is that while hydroplaning speed is independent of vertical load, drag is not. So the system looks unstable to me – which is why I wondered if anyone has tried it and proved me wrong. Failing that, is anyone willing to offer a C172 to be sacrificed in the interests of science?
The majority of the ditchings indeed seem to be successful in that people get out safely, and then it becomes a matter of surviving the water.
But we all know the saying : ‘lies, damn lies and statistics’. Under stress one may not be proficient enough to make a greaser and/or one may face horrible waves.
I did an egress course years ago. Very useful. Especially being put into the water in a simulated cabin, buckled-up and inverted or semi-inverted. To add to the experience also once blindfolded, not only simulating night conditions but also a few feet below the surface in murky waters.
You are in relatively warm swimming pool water, you know what’s going to happen, have people around you to assist if things go wrong, but still, quite a bit of adrenalin. Extrapolate that to a real world situation.. One of the main lessons was to fight the intuitive reaction to unbuckle right away when your are submerged and upside down. If you do that you may start floating in the cabin, and no way to find ever the lever to open the door. So, first firmly grip the door handle or some other clear reference point with your left hand and then unbuckle..
Bottom line: anyone can save him/herself and his pax after a greaser into the still water, floating and open up the ideal canopy (yep, the Diamond 
) step on the wing and start thinking about which aircraft to buy next. Now, what if…
This made me to conclude the following:
-don’t just take a life vest but actually put in on before flight. Well, in a SEP anyway. I am always amazed to see when SEP pilots take vests with them on board but don’t put them on, or worse, leave them in the bag compartment. If I fly with someone in a SEP to cross water and that person is not putting on a vest, I ask him/her to do so. 9 out of 10 times it turns out to be a struggle to put in on correctly just standing on the apron. Point made..
-Rafts. An interesting trade off between a bigger, fully fledged one with everything in and on it, or a light one which you can actually deploy in a worst case. So, I ditched (pun intended) my 18 kg one for a 4 kg one that I can quickly grab and take with me on the way out, even in a tedious scenario.
The chances of ditching in a DA42 are pretty remote as long as you don’t exhaust your fuel. But you never know. Electrical fire?
So in the twin I don’t put on a vest. But I do have a that light raft nearby.
edited: don’t worry. I am a happy and optimist kind of guy 
I chose the university of Tikrit paper simply because it was the first one that came to hand that seemed to address the question. There seem to be a lot of papers out there using similar but much more complex models – e.g. trying to model the behaviour of water as it is displaced by different tread patterns or against various different road surfaces. They didn’t seem directly relevant, and they made my head hurt. Most of the older articles that I suspect might have been from more august institutions were simply hard to come by. I don’t have ready access to an academic library any more. I miss it.
I would agree with you that I would tend to distrust modelling of this sort of thing, and put more faith in a good honest experiment. But does your experiment really confirm the prediction made using Horne and Dreher’s formula, given that you predicted an aquaplaning speed 50% higher than one you have demonstrated?
The NASA paper talks about films of water trapped between tyre and rigid paved surfaces and only deals with water thicknesses up to about half an inch, so I don’t see the direct relevance to landing on deep water where you’re trapping the water between the tyre and more water, the water being deformable. Presumably you have another source for the depth of water not making a difference above an inch?
Regarding one point I made: The reason the tyre pressure is more important than the vehicle weight on a road, is because to a first approximation, doubling the vehicle weight will cause the tyre to deform in order to double the contact area with the rigid road surface. Something analogous will doubtless happen with a tyre on water, but it seems to me that unless the tyre is soft, the size of the contact patch will be increased by deforming the water rather than deforming the tyre.
Horne and Dreher’s paper talks about aquaplaning starting when the dynamic pressure in the water film between the normal contact area of the tyre and the road surface is sufficient to overcome the weight of the aircraft. For a pneumatic tyre against a rigid surface this criterion will be met when the pressure in the water film is equal to the pressure in the tyre. But if the tyre is significantly more rigid than the column of water beneath it, the contact area will be much bigger than the tyre requires, and its size will become independent of the tyre pressure.
Barnes Wallis and team have shown that it is possible for a very dense, almost totally rigid tyre to support its weight on water: http://www2.eng.cam.ac.uk/~hemh/dambusters/ricochet_johnson2.pdf
I’d be willing to wager your aircraft would still aquaplane at similar speeds, rather than sinking like a stone, if you could pump the tyres to 1000psi.
This one turned out OK.All be it close to shore. Not sure why he did not take the big runway, but nice execution of the ditching
http://www.kitv.com/story/31123525/cessna-lands-in-water-off-lagoon-dr-pilot-ok
kwlf wrote:
I would agree with you that I would tend to distrust modelling of this sort of thing, and put more faith in a good honest experiment. But does your experiment really confirm the prediction made using Horne and Dreher’s formula, given that you predicted an aquaplaning speed 50% higher than one you have demonstrated?
Yes and no…
Yes, the Horne formula is for old bias-belted tyres, while Alaskan Bushwheels are modern radials. According to the theory and experimental data in NLR-TP-2001-242 (see link in earlier post) this entails a change of constant from 9 to 6.4. Using the latter figure and 10 psi inflation pressure, the Horne formula predicts hydroplaning above about 20 knots.
But no, I’m not saying we can touch down at 21 knots and be sure of hydroplaning! It’s nice to have a factor of safety, just as when flying a conventional approach. I may be accused of being over-cautious, but I like a safety factor of at least 2 (so 40 kts) when touching down for a full stop. For a practice touch and go, it doesn’t hurt to touch even faster.
On the subject of water depth, it may help to bear in mind that water is effectively incompressible, and then to visualise the aircraft tyre as stationary, supported by a stream of water. When viewed from that frame of reference, it is easy to see why the depth of the stream, over an inch or so, has little effect on the upward force on the tyre.
For 1,000 psi Horne’s formula predicts hydroplaning at 280 knots, so yes, it’s fair to say that an empirical formula for pneumatic tyre hydroplaning doesn’t work well for rigid tyres or boats.
BeechBaby wrote:
This one turned out OK.All be it close to shore. Not sure why he did not take the big runway, but nice execution of the ditchinghttp://www.kitv.com/story/31123525/cessna-lands-in-water-off-lagoon-dr-pilot-o
I think he was afraid of the landing and handling fees so he took the cheaper way out and did a “water Landing”.
Looks like an insurance job to me…;-)
My thoughts exactly.
aart wrote:
I am always amazed to see when SEP pilots take vests with them on board but don’t put them on
When I was training for the PPL in 1992, an angry renter came to the office at the school/FBO. After returning the rented plane, he recieved a considerable bill for repacking the vests after he and his passenger had unpacked them in order to wear them for the short over-water flight. In Denmark many of the non-local flight are more or less over-water flights, and with prevailing cloud bases, parts of the VFR trips will often be beyond glide range to land unless you are very conservative about the weather.
The renter had to pay, of course, because the airplane was not airworthy for IFR without packed vests, because the package is the proof that the vest is approved and legal.
I then reckoned someone else had done the risk analysis and immediately took note: SOP is that vests are left in their plastic wrapping until immediately before you drown. When I started thinking about it a couple of years later, I bought a vest intended to be carried from the beginning of the flight.
National rules (Denmark is not Part-NCO’d yet) states that whenever single-engine airplanes cannot reach land without the engine – or fly IFR over any kind of surface – there must be an approved vest on board for every seat in the aircraft.
So in order for the Dakota to pass the annual, there has to be 4 approved and packed vests in it. In order to actually survive a ditching, however, we have also bought 4 vests to carry for over-water flights. So we are keeping 8 vests serviceable :-) Not many do that.
And myself, would I ever fly beyond gliding range of land without putting on a vest? Actually I do that frequently, either because of in-flight change of routing, or because the actual airplane only has the packed vests. It might be about having learned to fly in an archipelago.
With Part-NCO, survival equipment will no longer have to be approved, IFR will not matter, and vests will only be required for the actual number of persons on board. That will help, at least if Danish “CAA” recognizes EU-law, which they have been slow and reluctant to do so far. It could actually, perhaps, theoretically, lead to more GA pax using a vest.
For reference, in Australia – the most highly regulated (and enforced) aviation environment on the planet – in a SEP out of gliding range of land it is mandatory that all occupants actually wear a life jacket when below 2000ft…with of course a predefined fixed penalty regime and strict liability….
