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Going Fast on a Kite Buggy.

Apexx buggy at Ivanpah 2010
Apexx buggy at Ivanpah 2010
28th March '10.
I'm in a 777 somewhere above the Pacific, heading for the Mojhave desert at 1100km/hr to go buggying for a week (NABX 20th anniversary celebration of the development of the modern kite buggy). Yes, 1100km/hr, because there's a tailwind of more than 300km/hr at this moment.

If only this wind would stay with me to Ivanpah- then we'd be able to set a new buggy speed record with ease-and a little bit of terror I expect.
And this identifies the problem with kite buggy speed records; it's all about waiting around in some big flat open space for stupidly high winds then dragging off downwind with the smallest kite in the set. A drogue or windsock would serve as well.

A better test of buggy, kite and pilot would be the buggy speed/wind speed ratio- but wind speed and direction are too variable for this to be able to be measured reliably enough for fair comparisons.
Nevertheless, designing buggies and kites for aerodynamic efficiency- so as to mutliply the wind speed by the highest possible factor- enables a higher top speed in ordinary sorts of winds- so this is what we've set out to do.

Apexx buggy, posing at Ivanpah 2010.
Apexx buggy, posing at Ivanpah 2010
"We" is myself, Craig Hansen, Gavin, Steve Gurney and Matt Bedford.
All of us have contributed ideas along the way but mainly, Gavin (an aircraft engineer) has done the building, I'm the design consultant, Craig's paid the bills, Steve has worked with sponsors and freighting, and Matt has tried to keep us all realistic.

We're all to be crash test dummies but I expect that Craig will be fastest- 'cos he's just better at it than anyone else in the team.

But there's also another underlying relationship that makes this a rather open ended challenge. It's that heavier buggies are inherently faster- and there is no theoretical limit to this trend (except the strength of available materials). This is because aerodyanmic efficiency is determined by the lift to drag ratio of the entire kite/line/buggy system and a big contributor to the drag side of this ratio is the buggy and pilot. The size of kite that can be flown is set by the weight of the buggy and pilot (and by tire and surface characteristics- but more about this later), and while a kite's drag component is a fairly constant proportion of lift (about 20%) for kites of all sizes, buggy and pilot drag does not increase much as weight is added. Basically, total drag (of the kite/line/buggy system) increases with the square of dimension while weight (which determines how big the kite may be) increases with the cube. In dimensional analysis terms, every doubling of buggy/pilot weight (enabling a kite that's twice the size to be used) increases their contribution to drag by just 58.7%.

A buggy the size of an 18 wheeler, with appropriately scaled kite, would therefore be inherently more efficient (able to multiple the wind speed by a bigger factor) than any current buggy- so would be much faster (except downwind in a jetstream when the differences converge) .

Nevertheless, tempting as it was, we didn't go big this time, but stayed with a size that's transportable and useable with conventional kites and rigging.

Speed buggy at Ivanpah 2010.
Speed buggy at Ivanpah 2010
You're right, we're wimps.

The starting point, in November '09, was to decide on wheels. Is large diameter/narrow better than small diameter/wide? An easy choice you might think; but it's not. The more efficient a kite/buggy system is, the nearer to 'on the nose' the apparent wind (that is the direction of the wind as experienced by the buggy) can be, but it's never from straight ahead. In moderate winds, the apparent wind will be in the forward quarter at maximum speed, but in stronger winds, it will swing more to the side. When comparing the aero drag cost for different wheel options therefore, it's the side area of the wheels that matters most.

The question then, was not just to determine if larger diameter narrow tires are more efficient at resisting side force, but if they are, is it by enough to offset their greater aerodynamic drag (with apparent wind r a bit forward of side on).

Buggy tire force testing rig

Buggy tire force testing rig.
To help with this judgement, I built an instrumented towing rig to measure the rolling resistance cost of generating different amounts of side force at different speed for various tires and surfaces [see left photo].

In fact we haven't yet had comprehensive data from even the tire force rig on account of the load cells not yet getting themselves properly organised.

To do the job properly, I also needed a rig to measure the aero drag of each of these wheels at various speeds and angles, but time ran out and this hasn't been built yet.

But first impressions are that large diameter/narrow tires generate substantially more side force for less rolling resistance cost on compacted clay surfaces ( we are likely to run it mainly on dry lake beds)- and probably by enough to offset their extra aero drag (which we can roughly calculate), especially if their centres are as open as possible. As the starting point, we therefore chose tension spoke wheels (which were invented by Sir George Calley, the father of the aerplane by the way, did you know that?) rather than solid disc style.

The buggy we have built is just an interim guess- but from what we learn this week, backed by further tire testing, we should be able to progress towards an even better design.

Other aspects derived from the basic requirement to minimise aero drag- hence the ground hugging wheel struts and tray, supine seating position, and pilot fairing.
Not being sure about the aero drag/rolling resistance balance for the front wheel, we've made two options; fully fared small diameter, and projecting large diameter open spoked.

29th March '10
Arrived at Ivanpah 12.30am, and it's blowing like i've never experienced here before; 60 to 80 km/hr average, gusts reported to 125km/hr (not sure I believe this though).
Our team isn't ready for this; - kites not organised, no weight yet acquired (they're too expensive to airfreight) buggy not balanced, and none of us in flight mode yet.

But the Dutch team are ready:
Just before dawn, Arjen van der Tol on and an Apexx buggy using a 2.7m Peter Lynn Vapor (designed by Michel Dekker) hits 133.4km/hr- official, a new record! (previous mark 124km/hr, from Ivanpah '09).

The wind then dropped a lot, but in mid afternoon, with about 70km/hr true, Craig records 108km/hr, still accelerating, just as the lines broke.

Speed buggy filming for Discovery, Ivanpah 2010
Speed buggy filming for Discovery, Ivanpah 2010
30th March '10
Getting better and better. The speed potential is becoming obvious; Arjen (current record holder) did a run in our buggy this morning and hit 115km/h- in wind strength of less than half this,- and at the same time as a 'conventional' speed buggy recorded 104km/hr. We have a margin; I expect now, that when fully optimised, we will go 15% faster relative to the wind speed than has previously been achievable.

This suggests that with suitably strong wind, the buggy speed record will exceed 150km/hr sometime in the next few years.

But what are our chances during the remainder of this event?

Unfortunately, we've not only run out of wind, but are fast running out of kites: Every time a line breaks (often), the resultant shock load generally explodes the canopy.

But if even one survives and the wind hits 75km/hr again for just long enough, our chances are excellent.

And if not- then next time- and by then we'll have a more refined buggy, a year's more experience and stronger lines (currently 300kgm).

In the meantime; congratulations to Arjen, Appex and team; your skills, determination and preparation are a model for us as our quest continues.

Peter Lynn,
Ivanpah, Nevada, March 30 2010


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