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# 80mph on 175 punter boards. possible? - Page 2

Quote:
 Originally Posted by Phil Pugliese well, maybe an African Swallow.
'ow did you know that: (Your Highness).
Quote:
 Originally Posted by Wear The Fox Hat Looking at Ghosts figures, they are claiming it took him 8 seconds to accelerate from 35 to 65mph, and then 8 seconds to decelerate to 25mph. I guess it's possible, but highly unlikely.
Why is it so strange for someone to take 8 seconds to accelerate 30mph, or to decelerate 40mph? I don't understand at all, surely the important thing is the grade of the slope not the amount of time to top speed - that's more for motorized vehicles, no?

I was a World Cup speed-skiing event a few weeks ago and they went from 0 to 170 khm (over 100 mph) in about 8 seconds then came to a complete stop a few seconds later.
Quote:
 Originally Posted by Bode Klammer That log shows an aceleration from 31 mph to 52 mph in one second. That's about 0.6 g's, which require straightlining a 40 degree slope, WITH NO FRICTION. Averaging the accelerations over other intervals results in accelerations around 0.25 g's, which might occur at lower speeds, but probably not at higher speeds because of wind resistance. For example, in ordinary skydiving, freefall terminal velocity is about 120 mph. At 60 mph, the air drag would be one quarter of that. That means you need 0.25 g's just to overcome air resistance. I can't say that the above log is impossible, but it implies that you were straightling some seriously steep terrain. BK
Actually, it's worse than that -- going from 31 to 52 mph in a second is 30.8 ft/s^2 of acceleration, or 0.96g -- almost a full g. To get that, you'd basically need to drop an object vertically down with no resistance of any sort. I just find it impossible to believe that a skier could attain close to 1g of acceleration when you take friction and wind resistance into account on a sloped surface.

I see a couple possible sources of error (both in the data and in the follow on arguments presented here, including mine above). First, the Garmin specs indicate that the unit samples data once per second. If you are experiencing such rapid accelerations, then we need a much faster sampling rate -- at minimum 10 per second. With a data rate of one sample per second, this unit is not appropriate for instantaneous speed at all, but would clearly be geared for near steady-state speeds where the acceleration in 1 second is low. I bet that if you did a 0-60mph run in a very fast vehicle (with say a 4 sec time) or a motorcylcle, the GPS would show some very sloppy results just because it can't update fast enough.

Second, the position error of GPS is generally in meters. I did a google search and found that the RMS error of most Garmin units is about 5 meters or less (note that this is different from the accuracy of GPS). In a 1 second frame, that would say that velocity uncertainty could be as much as 5 m/s or 10.3 mph. So there is a lot of potential uncertainty in the measurements.

I do a lot of work with instruments, and my gut feeling is that handheld GPS units with a 1 sample/sec data rate and a 10mph uncertainty are not the best tools to measure a body with rapidly changing speed. Now, if you could reach a sustained speed while skiing and hold it for about 20 seconds, then I think we could get more dependable data from GPS.

And by the way, I am not necessarily disputing the speed claims, as I have seen amateurs clock in at 48mph on NASTAR courses with radar. I am just saying we need more convincing data than what the consumer GPS devices can give. Heck, if you can keep a sustained speed for 5 seconds, I think a stopwatch could give us highly accurate numbers, far better than GPS. Using satellites up in space to triangulate the position/velocity of a skier on the ground is the classic "using a sledgehammer to swat a fly" scenario.
Just where is the antenna for the GPS located while all this speed is going on?
Not exactly sure, but in the nav systems of the aircraft I have flown, measuring speed via Doppler shift requires radar transmissions. That's why we generally didn't use it (because we didn't want to be emitting) on "special" missions. I don't think the handhelds everyone is referring to have a transmitter that can measure Doppler shift.
Quote:
 Originally Posted by newfydog We need physicsman to step in and point out that drag is porportional to the square of the speed...that is, the drag of your suit at 80 is nearly twice as big as it is at 60.
You seem to be implying that the suit should make a large difference in speed. This isn't the case, for the same reason you imply it should be. I could be taking your post the wrong way though...

Without illegal (except for speedskiing) aerodynamic enhancements, you won't be able to change drag that much with a suit is my guess. You are correct that reducing drag will improve top speed more than increasing the pitch...

Anywho:
-Cd times frontal area is a linear relationship.
-The relationship between the component force pulling the skier down the hill and the pitch of the hill is not linear. As in, on a 45 degree pitch that component will be 70.7 percent of its maximum.
-Assuming you aren't wearing stupidly baggy clothing, the suit is going to change your Cd, not your frontal area. Given that the human form hooked to skis with boots in a rather limited tuck position isn't exactly "pretty" aerodynamically, the Cd will always be quite high. Changing the Cd by say, 33 percent with the suit would be generous.

A 33 percent change in the Cd does not equal a 33 percent change in top speed. Since the drag increases exponentially with speed, the 33 percent change in drag will have a significantly smaller than 33 percent effect on the top speed.

If, in fact, the drag force quadruples as the velocity doubles (which is a really lousy and likely wrong assumption to make without a wind tunnel or CFD) then a skier with 2/3rds the drag will have this relationship:

Fd=Cd(V^2)A

Fd=(2/3)Cd(V^2)A

Lets define the component force down the 45 degree hill for my roughly 100 kilo self:

Fs=mg(sin 45)
Fs=100 X 9.8 X .707
Fs=693N

Now, lets add some snow friction and call the leftover our max drag force for equilibrium:

Fs(.95)=Fd

Fd=658N

And lets solve for the speeds of the suitless and the suited by plugging into the first two eqn's:

A=.75m^2
Cd=.5

658N=.5(.75m^2)(V^2)
658N=.375(V^2)
1754N=V^2
V=41.8m/s=150km/h (man I love metric units)

658N=((2/3).5)(.75m^2)(V^2)
658N=.25(V^2)
2632N=V^2
V=51.3m/s=184km/h

18% difference, if we reduce drag by a third.

I totally guessed on the area and Cd, if I did it again I'd guess the actual area smaller and the Cd larger.

### drag

Take a look at the size of the speed or dive brakes on a fighter or a sailplane.

They are quite small. On many, you can't even exceed the manuvering speed with the nose pointed at the ground.
Quote:
 Originally Posted by Yuki Just where is the antenna for the GPS located while all this speed is going on?
Let's not get personal: .

Yep, the data is limited. Chord length and not arc length speeds are recorded, so recorded speeds are slower than actual speeds if your in a turn, but not if your straightlining. There is only a 1 second refresh rate. Average speed over the 88 feet, not maximum speed at the end of it, nor minimum speed at the beginning of it. Positon error may cancel or be cummulative. Vertical speed gained going down a short steep section is not accounted for until it is converted into horizontal speed at the transition to a flatter surface. What's worse, what felt like the fastest run of the day (straight down elevator shaft) suffered from a lost signal (Murphy's law), only to be picked up after I had slow down to play in the moguls where it joins up with avalanche.

It is like swatting flies with a sledgehammer, but I don't want to be looking at my watch while skiing along at 60mph. Put the GPS in your pocket and forget about it. It ain't perfect, but I think it's pretty good. It matches up with my experience of radar tested runs, bike runs, and car runs. If you get three data points in a row that say over 60, your probably doing about 60.

RE vertical speed converted to horizontal speed. Ever catch 10 feet of air and land on an inclined surface? I hope you are able to keep up to your skis. How about ski down a cliff and have a transition from near-vertical to near 50 at the bottom of it? Feel any extra pressure on those legs? I almost herniated myself more than once (back in the days when I was into that sort of thing).

BTW. Interesting point re blue versus black runs. Many runs on the double blacks that day were slower than this run on the black, because I had to start the other steep sections at a very slow pace, and had to avooid other skiers, but started the black section of memory lane already cruizing, with nobody in sight.
Quote:
 Originally Posted by skier219 In a 1 second frame, that would say that velocity uncertainty could be as much as 5 m/s or 10.3 mph. So there is a lot of potential uncertainty in the measurements. .
Good post, but what you show is that the maximum error is in the 10mph range, and that's if you only spend a brief second at your max speed. If you watch a gps you'll see it refining its position with each reading. If you get four or five sample points accuracy becomes much better. I've used them in geological surveying for years and they just keep getting better. Since the government turned off the dithering factor a few years ago many handhelds will give the same location as a professional survey model if they have a few readings to zoom in on.
Put one on the dash of your car...you won't see it jump around anything near 10 mph once you reach a steady speed. I would think anyone with the slope and balls to hold their max speed in a straight line for five seconds would get a pretty good number.

While I don't recommend it, it is sometimes good to look at the continuous reading rather than the max speed record. I've about killed myself on my bike doing that.
Quote:
 Originally Posted by bjohansson Not exactly sure, but in the nav systems of the aircraft I have flown, measuring speed via Doppler shift requires radar transmissions. That's why we generally didn't use it (because we didn't want to be emitting) on "special" missions. I don't think the handhelds everyone is referring to have a transmitter that can measure Doppler shift.
If you have precise position and time data, you do not need rather imprecise radar speed data. This is why car mags etc. all use GPS groundspeed now. Granted, they don't use GPS units in a coat pocket...

In fact, very fancy GPS units often use doppler shift info from the signals of the satellites to increase accuracy of speed and position data. Some very fancy hardware calculates winds aloft using doppler info from the satellite carrier frequencies.
Because of the design of GPS, you don't need to transmit anything to measure doppler shifts.

I don't know if the handheld units make use of any doppler positioning/speed data at the moment, but I'd be surprised if they didn't now or soon. One of the benefits is that this data is usually available even when there aren't enough satellites in quality view to make a standard position fix...read cell phones/handheld GPS in urban environs etc.
Does a speed suit make a difference. Yes! I don't think people could ski at 154 mph without a speed suit. 80 mph is another story however. A skier doing 80 without a suit would likely hit 85 or maybe 90 with one.
Quote:
 Originally Posted by Yuki Take a look at the size of the speed or dive brakes on a fighter or a sailplane. They are quite small. On many, you can't even exceed the manuvering speed with the nose pointed at the ground.
Cd of a cleaned up aircraft: .005-.06 (the latter if its like, a Navion. A 172 Cdminimum is like .021)

Skiers/cars/etc. we are talking about Cd's an order of magnitude bigger.

Put some relatively small panels in the right places and you can really screw that up quickly. Lots of dudes don't like putting the speed brakes out on 737s because the whole damn plane starts shaking pretty noticeably and it unsettles the self loading freight.

### aero factors

The FIS (no rant please) ... even limited the type of stitching on the suits for the speed events.

skiingman ... I only know of one Navion in the area .... I'll be perfectly willing to watch you test it "dirty" .... as long as I'm on the ground. I'll help you cut it loose from the weeds and saplings though.
Quote:
 Originally Posted by Ghost Edit: If there were NO FRICTION you could convert potential energy to kinetic energy and end up with a speed of the square root of (2*9.8 m/s * hieght).
Um, not sure about your maths there, I would have thought that you could only achieve that if you were travelling vertically, with no horizontal movement - i.e. freefall, cause otherwise your acceleration will be based on the angle of the slope - at 45 degrees, it's about 0.7g (6.9ms^-2)
Quote:
 Originally Posted by newfydog What makes skiing so special?
Locale and aspect.
Quote:
 Originally Posted by Phil Pugliese well, maybe an African Swallow.
Quote:
 Originally Posted by Yuki The FIS (no rant please) ... even limited the type of stitching on the suits for the speed events.
Yes...the Speedwyre suits supposedly gave benefits of up to 30%. Saw one of those old press releases when looking for the Cd of a skier earlier.

Man it pissed off the Europeans....
Quote:
 skiingman ... I only know of one Navion in the area .... I'll be perfectly willing to watch you test it "dirty" .... as long as I'm on the ground. I'll help you cut it loose from the weeds and saplings though.

I've got this buddy that is graduating from Embry Riddle and is a pretty newly minted CFI. He was telling me over a beer at Christmas that I should go get a PPL this summer, training with him. But the local flight school flies 152s, he's gotta be 240lbs, and I'm 210-220lbs. Haha, add in some good and hot weather and forty years of getting beat on by students, and we'd be lucky to climb at a couple hundred feet a minute....no thanks.
Quote:
 Originally Posted by L7 Locale and aspect.
Compared to downhill mountainbiking the same slope? I have a well calibrated cyclometer on the bike and a wrist strap gps. The max speed function doesn't stop working when I bike down a ski slope.
On really winding single tracks the distance comes in about 5% shorter, because it thinks I'm cutting those corners (of course, that would give a lower speed, not a high error.)

Geez, enough of this, I'm only posting this crap because its more interesting than working on taxes.
A very simmilar discusion occured on TGR a while back. Many persons were up in the upper sixties on fat boards in flat light. They used a radar gun that I am familar with, although radar guns are not perfect it is very difficult to consistently screw up. Not only is it possible, there are far more people out there than you think that could hit 70 in ideal conditions. A DH course is a complete sheet of bulletproof ice, I would love to see Bode on a trail with freshly groomed snow he could probably top 85.
Quote:
 Originally Posted by newfydog Compared to downhill mountainbiking the same slope? I have a well calibrated cyclometer on the bike and a wrist strap gps. The max speed function doesn't stop working when I bike down a ski slope. On really winding single tracks the distance comes in about 5% shorter, because it thinks I'm cutting those corners (of course, that would give a lower speed, not a high error.) Geez, enough of this, I'm only posting this crap because its more interesting than working on taxes.
Bottom line is you're not reporting some insanely fast speeds like an average ski speed in bumps of 100km/hr. Sure you hit a high speed on the bottom of a world cup downhill course that had likely been water injected. I don't think you reported as high a speed as some of these guys are talkiing.

Could it be reasonably accurate, probably yes on the right aspect in the right locale. Does that mean it's always accurate, no absolutely not again varying with aspect and locale and time of day (satellite positions). Numbers I've seen you post are on the fast side of believable given the terrain you've been on.

Others have posted ridiculously high numbers especially for the terrain that go beyond believable. I don't understand why someone could look at a whacky number and assume it must be true because someone else got a believable number in a totally different situation, the accuracy is totally situational. I'm not talking accuracy of the unit itself but the situation it is trying to operate in.

### Hold it

There are a lot of erroneous eqns and numbers thrown around here. For one, the Drag force is normally expressed as:

D = 0.5 * density * Cd * A * V^2

It's fairly easy to write up Newton's 2nd law for the situation of a skier on a hill, and it gives something like this:

mass * acceleration = gravity force - friction force - aero drag force

m*(dV/dt) = m*g*sinQ - k*m*g*cosQ - 0.5*r*Cd*A*V^2

Here, m is the skier mass, V is velocity tangential to the hill, t is time, g is gravitational acceleration, Q is the angle of the hill from horizontal, k is a friction coefficient, r is the air density, Cd is the drag coefficient, and A is the skier's frontal area. dV/dt is the derivative of velocity with respect to time, or the acceleration. The friction force is assumed to be independent of the velocity, which is true for most objects, but I consider this to be questionable in the case of skis since there is a complex interaction between the ski, snow, and the film of water in between. But the friction forces are small compared to the other forces so the assumption won't have a huge impact on our discussion here.

You can go ahead and solve this nonlinear ordinary differential equation with separation of variables, but there's a lot to be learned just by studying it. For instance, we can assume the skier has reached a top speed (which I think is the focus of this topic) and their acceleration has gone to zero. This would be analagous to a terminal velocity. Then, we get:

0 = m*g*sinQ - k*m*g*cosQ - 0.5*r*Cd*A*V^2

or

V = SQRT[ 2*m*g*(sinQ-k*cosQ)/(r*Cd*A) ]

I plugged in some numbers representative of myself, and I looked up some other quantities from handbooks:

m = 101 kg
g = 9.8 m/s^2
Q = 30 deg (assume a fairly steep slope)
k = 0.05 (from handbook, representative friction coefficient for skis on snow)
r = 0.986 kg/m^3 (32F at 7200 ft elevation)
Cd = 0.9 (from handbook, typical skier)
A = 0.84 m^2 (measurements of me)

If you stick all this into the equation and solve for V, the answer is:

V = 34.83 m/s = 114.3 ft/s = 77.9 mph

So in other words, if I had a long enough run on a 30 deg slope, I would top out at about 78mph max.

If you neglect friction between the skis and snow, you'd get about 81.5mph. So the friction effect is fairly small compared to the other stuff involved, but it has an impact. In the case of no friction (only aero drag), we see an interesting proportionality for V:

V ~ SQRT[ m/(r*Cd*A) ]

V is proportional to the square root of the mass m, and inversely proportional to the square root of r, Cd, and A. If a skier wants to speed up they can: put on weight, go to higher elevation or hope for a warmer day (less dense air), reduce their Cd with a suit and a well shaped tuck, or reduce their frontal area with a tuck (or a combination of all these). The effectiveness of all of these factors is diminished by the square root. An aside: the interesting aerodynamic aspect is that a basic tuck will reduce the frontal area, but a well shaped tuck can also reduce Cd.

So anyways, I do think it's possible for an average skier to go quite fast if they are on a steep enough trail with a long enough run, and if they can hang on long enough without doing a yard sale. Actually, at those speeds, it would be a mega yard sale.... We could go a little deeper and solver the ODE to get V as a function of time, and see how long it takes to reach the terminal velocity and how far the skier has to go. It may be that these speeds are impractical to attain. I'll save that for the next chapter!

Craig
Quote:
 Originally Posted by skier219 m*(dV/dt) = m*g*sinQ - k*m*g*cosQ - 0.5*r*Cd*A*V^2
Oh..... Yeah.... THAT! Of course! DOH!

I think he's swearing at us in Martian.

Quote:
 Originally Posted by skier219 D = 0.5 * density * Cd * A * V^2 m*(dV/dt) = m*g*sinQ - k*m*g*cosQ - 0.5*r*Cd*A*V^2 0 = m*g*sinQ - k*m*g*cosQ - 0.5*r*Cd*A*V^2 V = SQRT[ 2*m*g*(sinQ-k*cosQ)/(r*Cd*A) ] Craig
Ok. I take it back. Taxes are more interesting.

Actually Craig, a great post. Back to the original question...do I believe the 8omph? Well, Craig shows it is remotely possible, but I think no. 70 maybe. Anyone who could handle 80 would have done it on different skis.
Quote:
 Originally Posted by skier219 So in other words, if I had a long enough run on a 30 deg slope, I would top out at about 78mph max.
This is actually somewhat interesting, but I think we've forgotten the point of this thread. I don't think anyone (okay, maybe someone) is saying you can't ski that fast. The issue was being able to do it on a pair of "175cm punter boards".

Give me a 200cm+ straightish ski, and sure, but a pair of 175? I've tucked a short section of an easy blue grromer in southern PA in my 175s, and after about 5 seconds of tucking I had to stand up. Way too squirrley for comfort. And puting them on edge and letting them turn for stability would kill your speed.
Here's a chart showing the history of that hypothetical, mega yard sale inducing run down a 30 deg slope, obtained by integrating the ODE:

http://members.cox.net/harmony.hunter/skier.png

Starting from a standstill, it takes the skier 28 seconds to reach 0.5 miles, at which point the speed is approaching the max.

I have to agree with JohnH that this is possible but probably not practical. Then again, it would not surprise me that there are people out there crazy enough to do this, even on a set of recreational skis. Heck, I have been passed by gapers bent over their skis with their poles tucked and sticking straight up, going super fast. If they can do it, I bet a good skier with no fear could probably hang on and get close to the theoretical speeds.
Thanks for the explanation skier219, (shudders at the thought of solving Nonlinear diff eqns)..

....
Rahlves was "bagged" year for wearing a suit that did not pass the FIS permeability/air penetration test.
Quote:
 Originally Posted by JohnH The issue was being able to do it on a pair of "175cm punter boards". I've tucked a short section of an easy blue grromer in southern PA in my 175s, and after about 5 seconds of tucking I had to stand up. Way too squirrley for comfort. And puting them on edge and letting them turn for stability would kill your speed.
I saw a guy go 60 (at least) on a pair of 155 slalom skis (this was the year before the FIS limit of 165 went in, for those who wonder what he was doing on those). He ran a short training course and then straightlined Thunderbird at Mt Bachelor to the bottom. Ok, he was a Japanese National team member, but it got me thinking about what could be done on short skis.
Quote:
 Originally Posted by Yuki Rahlves was "bagged" year for wearing a suit that did not pass the FIS permeability/air penetration test.
Yeah, that is a very good rule indeed.

I've always been surprised by how much faster I stop if I fall wearing the suit than the regular DWR'ed pants and shell.
Nicely done skier219.
Looks roughly by eye like about 500 or 600 feet to 60 mph.
How about a 0-60 time (or better yet v:t graph)?
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