That’s the aquaplaning formula for slick tyres, no?
You will be posting that famous multiple Harvard water-skiing video next 
Jacko wrote:
As IR holders may recall with fondness from their ground school , on calm water it’s not necessarily Vso but rather (speed in knots) = 9*(tyre pressure)^0.5 which determines when an aircraft becomes a displacement vessel or ground vehicle.
Actually, no. :-) And I took the “full” (pre-CBIR) tests. But this sounds more like something from ATPL theory?
Jacko wrote:
As IR holders may recall with fondness from their ground school
Another advert for European IR theory…..Very important to know the formula….I always feel unsure what to do on runways with standing water – but now know why – I couldn’t compute the aquaplaning speed. No wonder all those Australian and US pilots are sliding off runways…
Yes, Horne and Dreher’s formula, its derivation and its empirical verification are all in NASA TN D-2056. I’ve never found a copy on the net, but it has no copyright so I’d be happy to email or upload a copy on request.
On the other hand, the Dutch NLR-TP-2001-242 is here, and it gives a constant of 6.4 for radials which I have confirmed as accurate down to 10 psi tyre pressure.
Everyone loves those cool dudes in their mirror shades and T-6s, but please spare a shilling for this poor hard-working Scottish farmer washing his wheels between sheep fields:
I would have thought that a safe speed for water-skiing would be considerably different from the one given by the Horne and Drehn formula:
Start of the maneuver not at 1G
Lack of a solid surface beneath the film of water under the tyre
The water never, ever will be totally calm. If your tyres are 6", even 3" of swell or small ripples will be quite significant
In a taildragger, starting the maneuver with a low sink rate, you’re likely to be creating a fair amount of lift
Not saying that water-skiing is dangerous, but I don’t see that the formula is pertinent.
That said, the issue if you have a tailwheel aircraft, would it be a good idea to ditch by water-skiing then slowing until the gear sinks? Or might you be less likely to flip if you did a normal 3-point landing and plunked down fully stalled?
Pneumatic tyre hydroplaning speed is independent of vehicle weight or g. It is also largely independent of water depth over an inch or so. Trucks hydroplane at higher speeds than cars only because of higher tyre pressure.
According to Newton’s third law and Archimedes’ principle of levers, the system is not stable unless the centre of mass is well behind the main wheels (taildragger). I can’t see it working with nose wheel gear, as countless amphib skippers have demonstrated.
You are right about waves. Light ripple is OK, but any chop over abut 10" starts to feel hard on the main gear, even with 31" radial tyres.
Your last Q is a €64,000 one, and I would opine that the answer depends on stall ground-speed (function of wind and Vso) and minimum hydroplaning speed (function of tyre pressure) – and on sea state. For a Maule with 10 psi in radial tyres, Vso is about 40 kts (or maybe 35 in ground effect) whereas hydroplaning continues down to about 20 knots, so on a calm sea or lake it’s a no-brainier. Add a force 4 breeze with the associated ocean sea state, and the answer is probably to “plop and pray”.
One complication might be lack of elevator authority at slow speed with no engine. Not sure how that would affect the decision, and difficult to test on a poor farmer’s budget…
Is this what you mean Jacko?
Yes, that’s the idea. SuperCub handles differently, having crappy little flaps and proper ailerons , but the idea is to fetch up at the water’s edge all ship-shape at approximately 0.5 Vso. Same technique could be used for ditching, subject to the above considerations.
This is what it looks like from my right-hand tie-down ring. Note GPS ground speed in top left corner and pitch oscillation from about seconds 30 as we approach min hydroplaning speed – very handy, warns like a pre-stall buffet:
V. cool…
Jacko wrote:
Pneumatic tyre hydroplaning speed is independent of vehicle weight or g.
If that were true then why can’t you aquaplane a tricycle aircraft? In a tailwheel aircraft, increasing upthrust on the main gear increases the angle of attack; decreases the load on the tyres. In a tricycle aircraft, thrust on the main gear decreases the angle of attack, drag rotates the nosewheel down, the lift from the wing decreases. But if the aquaplaning speed is truly independent of downthrust, then the nosewheel should still aquaplane, no? There’s at least one high profile Jodel/Derwent case where it plopped.
This theoretical paper shows aquaplaning speed varying linearly with downforce: http://www.tj-es.com/index.php/tjes/article/viewFile/455/237
Oddly (to my mind) most of the studies in this area seem to be theoretical rather than practical.. Skim-reading a few papers I get the impression the phenomenon is surprisingly difficult to study practically.
That 20 knot aquaplaning speed is interesting – is that theoretical or empirical? I’ve admired lots of videos of Cubs taking off from sandbanks.
