Calculating top speed on ice...

[milisavljevic]

Nice to see the art of putting pen to paper and doing it long hand is not lost.

Viv...does this mean I can ship all of my notes and calculations off to you, for safe-keeping? Hmm?

I have bankers boxes full of sketches, notes and calculations. After the BushHorrors, I try to avoid
carrying too many notes when I travel, and now I have boxes stranded on both sides of the border.

M.: If you can't do it on paper, at least the outline and proofs, then you shouldn't be doing it at all.

Cheers!
M.

[Johansson]

Thanks for the formula Irvine, I´ll do some calculating this evening and see what I can find out. The remaining question is how I can estimate the air resistance and how it changes with speed since it will be the main limiting factor.

I know the risks well so this is no suicide attempt, with my motorcycle riding gear along with back protection I should be able to walk away from a fall. The good thing about a flat ice track is that you won´t start to roll and tumble as you do on gravel or hard packed snow, you just slide along it (getting skinned alive of course without the proper safety gear) until you stop. As it is now even 20km/h will be noted as a record for jet propelled sleds so there is no reason for me to go faster than what I feel is safe, nevertheless it is interesting to find out how fast it could go.

Since you can die at that speed I don't see why your not using the formula I sent which is a piece of piss for anyone with a standard 2 dollar op shop calculator. One or two other formula come to mind and the problem is very simple to work out. 15 minutes and those equations will tell you everything you want to know. In case you forgot falling on hard ice at 200kmh will skin you alive.

Useful equations
F=MxA
s= ut + 1/2at^2
V^2 = U^2 +2as
t= (v-u)/a

Example of frictional force calculation:

A 15kg body on a horizontal surface is subjected to a force of 125N to the right. If the co-efficient of friction between the two surfaces is 0.6 we want to know A- the frictional force, and B the acceleration of the body.
Solution : Fw=mg = 15x9.8 = 147N
first, calulate the weight force as above and, from this obtain the normal reaction Newtons.
Hence, n = 147N
Using Ff= uFn
Ff= 0.6 x 147 = 88.2 N
The resulting force acting to the right Fr is Fr=125-88.2 =36.8 N.This is the unbalanced or resultant force causing the change in motion and using newtons second law of motion: a= F/m = 36.8/15= 2.45m/s^2
Piece of cake, now substitute and do it Johansson and save your skin.

[milisavljevic]

Hejsan Anders!

I will stop pestering you with many questions, but I still need a few more pieces of data from you.
May I assume that your kick will be or can be as long as the turbo-kick? And just how long is that?

The kick frame weighs in at 10kg, the fairings 2-3kg...perhaps 5kg for the fuel system
and enough fuel to get the engine started and run the track.

Got it! That's a minimum of 17-18 kg, before we get to the pulsejet and its mount.
And of course, I need to know if your 75 kg includes your helmet, flamesuit and kit.

A friend of mine wants to build a jet powered ice yatch (spelling?) to race with next year.

Ice yacht! Very cool! I will wait with great patience for future developments.

Cheers,
M.

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[milisavljevic]

.

As it is now even 20km/h will be noted as a record for jet propelled sleds so
there is no reason for me to go faster than what I feel is safe, nevertheless
it is interesting to find out how fast it could go.

Well said.

M.

[milisavljevic]

The remaining question is how I can estimate the air resistance and
how it changes with speed, since it will be the main limiting factor.

Spot on, Anders.

I decided to run the preliminary calculations in my head, ie., no paper, no pencil, no slide rule, just
the "noodle". Once I had worked through Anders problem from two (2) different directions, I used a
calculator (non-graphing, thank you!) to verify my "noodle-numbers". I am pleased to report a result:

Unless our Mr. Johansson goes out of his way to squelch any aerodynamic "slipperiness" in his kick,
the final determinate of Anders' top speed over 2000 meters will be when he backs off the throttle.

Let's call this speed the "brown trousers limit".

Anders: I don't think 200 kph is unrealistic, but I will run detailed calculations once I get all the data.
If the kick hits 200 kph, Thunderchine's output will approximate 20 kW (requires careful modeling).

Cheers,
M.

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[Irvine.J]

It had nothing to do with one up-man ship and everything to do with giving Johanssen numbers which he asked for in the question posted at the top of the thread.

Calculating top speed on ice... Was the question, It's unfortunate you saw it that way, Johanson is a talented and intellegent person who I have nothing but respect for, and as far as the threads gone I'm the only one who has thrown proper equations at him which is what he asked for so he knows exactly what to expect.

Good time to say that I'm leaving the forum now too.

Johansson I've given you the answer to your question. In regards to drag force through air friction, the total frictional force can be applied by the previous set of equations once you know the
two elements below

Drag force: Start with your co-efficient.
D=1/2CρAv2
C Drag Co-efficient - expect to be at least as high as 1.
ρ is the air density
A Cross-sectional area of the body
v is velocity.

Then your drag force Fd= (CDpAv2)/2
You'll note the change as you increase your velocity value. Remember to apply the new values to your force friction (Ff) equations later also to determine the total force you have accelerating your sled with each new value. (Graphing software would now be helpful)
Look at the hyperphysics for more info.

Hope I've been of some assistance Johansson.

[Johansson]

Hey Irvine, I´m thankful for your help with this since my math skills aren´t what they once were. There are most likely different ways to get an answer to this question but the laws of physics cannot be bent so as far as I can tell your equations should give me an estimation of what to expect. Based on experience and testing there are more accurate ways of finding this out using computer simulations and such but that assumes that you´ve got access to that software.

I am a frequent visitor to a swedish motorcycle forum where the "cealing is high" as we say, the discussions can get very heated sometimes but we all get along anyway because we all share the same hobby and can take a hard word or two with a shrug since it is most likely said to tease instead of hurt. "Med glimten i ögat" as we say here in Sweden. I really hope that you decide to keep contributing to this forum since you are one of the builders here that I follow with great interest.

M: I went down and took a measurement on the kicks runners today. They are 200cm long and relatively stiff since the kick is an older model, the new kicks are cheaply built with plain steel runners while the old ones are profiled to make them less flexible. I won´t try to lenghten them since the runners are tempered and long enough as it is.

My weight is 75kg, fully dressed for the run I will weigh aprox. 90kg with the flamesuit on.

Assume that I have the balls to keep the engine at full thottle during the entire run, later on I will have to solve the breaking issue. A small parachute perhaps?

[milisavljevic]

Hejsan Anders!

They are 200cm long and relatively stiff since the kick is an older model. [snip!]
I won't try to lengthen them since the runners are tempered and long enough.

Roger. That covers the basic weight and dimensions. Working out Cd (drag coefficient)
will be fun: a sketch --a sketch, not art-- of your proposed fairings will be most helpful.

No worries, no hurries on that sketch.

Later on I will have to solve the breaking issue. A small parachute perhaps?

Adding a braking chute would be a prudent thing to do.

A small set of winglets, in dihedral would be useful for lateral stability.
Nothing fancy, just something to think about...I can help with this, too.

Umm...what are you planning to do about steering? This IS a big deal if you plan on going =FAST.

[James: ] I really hope that you decide to keep contributing to this forum
since you are one of the builders here that I follow with great interest.

I agree. James is one of the very few people here who can design,
build and test the most excellent* pulsejets -and- fueling systems.

Peace to All,
M.

*Expression used by permission, Bill & Ted Enterprises, a division of the Keanu Reeves Charitable Foundation.

[Johansson]

A sketch of the fairing will have to wait a while, its shape will depend heavily on how much space the fuel system needs so that is probably one of the last things I will make on the kick.

A kick is steered by bending the runners, you stand on the runners and use the handlebar to steer. That is why the older kicks with profiled runners are better, they are harder to steer but much steadier at speed. You can see on the attached pics what this fantastic contraption looks like.

[larry cottrill]

Anders -

That looks like a WONDERFUL idea. Just make sure you keep your thrust line low!

A thing like that will just fly, unless there are sizeable hills or a lot of roughness or some such to contend with.

L Cottrill

[tufty]

Working out Cd (drag coefficient)
will be fun: a sketch --a sketch, not art-- of your proposed fairings will be most helpful.

I'd start with a lower and upper estimate of Cd and calculate twice. I'd probably take around 1.0 as a upper limit, a bit more than the average "bloke on a bicycle", and around 0.6 as a lower, I doubt if handmade fairings are going to be massively efficient (or, if they are, would the end result end up looking like a kick). That should be enough to give you a rough guesstimate of the range of top speed you should be looking at.

[Johansson]

I doubt if handmade fairings are going to be massively efficient (or, if they are, would the end result end up looking like a kick).

You´ve got a point there, I´d rather have a slightly less streamlined vehicle that looks like grandma´s old kick than something looking like the spaceship from hell but runs a couple of km´s faster. This project is after all more show than go even if I want it to go fast.

Larry: Its design practically screams out that it wants to go fast, I hope that I can prove it next year.

[milisavljevic]

Hello Simon!

I'd start with a lower and upper estimate of Cd and calculate twice.

I agree with this guy!

And with this one, too:

This suggests a less rigorous approach (...) focused on resolving the ranges
(possible values) for each variable, and their various interactions.

Kinda scary if we are thinking alike...

And this is good enough for Anders' kick. It is also an exercise for the pulsejet model,
specifically the Thunderchine model, where the internal standard (of precision) is 0.1%.
With the exception of certain measures and acoustics data, this "standard" is (wildly!)
impractical; however, I will refine the Cd estimates as much as I can before the event.

What I mean by "fun".

I obviously have no idea what "having fun" really means...

Cheers,
M.

[milisavljevic]

Thank you for sharing your photos, Anders. Really helps to see what you're doing!

I doubt if handmade fairings are going to be massively efficient
(or, if they are, would the end result end up looking like a kick).

I'd rather have a slightly less streamlined vehicle that looks like grandma's old kick.

While I agree that Anders is not going to get a lot of bang for his (fairings) buck,
limited fairings do have another purpose besides a direct reduction in drag: they
help maintain a symmetrical flow of air around the kick. This is not an issue at
modest speeds, but it is an issue if Anders wants to go faster than say, 100 kph.

Anders: What I am suggesting is for you to consider fairing off the more, umm...
"inelegant" parts of your kick to balance the flow of air around it, to enhance its
stability, while maintaining a visual impression of grandma's kick (side aspect).

Even grandma would wear a varm knä filt whenever grandpa squired her around!

Hope this helps,
M.

[PyroJoe]

Be extra careful if a braking chute is used. Many great runs, end in tragedy with a fussy chute or a ill-defined anchor point. The front will lift, or the back will lift , sometimes both. ha

If things are going sour, a deployed chute can often make a bad situation worse. Since the rider in many of these experimental vehicles are the heaviest part, it usually ends with the vehicle and the chute stopping quickly and the pilot continues at speed.

If things are really bad the chute can pull your trajectory upwards several meters/yards and leave gravity to reign in your destiny.

There is something impressionable about a vehicle disappearing from beneath you, leading to a full body glide to terra firma and a beloved ritual of extracting gravel lodged under the skin. Something often laughed about, but not forgotten.

Joe