After watching wheels take off, literally and figuratively, I’ve noticed some areas of strength and weakness with some potential for improvement. None of this is new, it’s just shared observations that hopefully designers will take into account when devising their next generation of coolness.
One of the most serious cart incidents I’ve read about involved a machine with no bars going up the sides. Lots of carts are this way and I never gave it any thought. We should. The quick version is that a bad launch resulted in wing material winding into the prop, pulling lines along with it. One line draped around the pilots neck and was yanked by the prop, causing a line burn across his neck. Needless to say, that could have cut him at a very bad place. Bars that go forward, all the way to your feet, and above your chin, would reduce the chances of this happening.
The most common malady for launching on wheels is flipping over, usually from an oscillation with the wing. Of course the best cure is to never accelerate unless the wing is overhead, tracking straight and under control. Thankfully, skill building with a cart is relatively benign since you stay on the ground and at slow enough speed not to lift off.
Inflate, get the wing tracking straight, then practice turning by first turning the wing then following it around with the cart. Lead it. To go straight, start the wing coming out of the bank before reaching your desired heading. Mastering this skill will go a long way towards avoiding rollover accidents.
1. Use 4 wheels instead of 3. The trade off is that quads are heavier and have more rolling friction since two front tires must negotiate possibly uneven terrain. Also, consider cart-style steering (like a car) since uneven terrain doesn’t make the wheels turn like it does on the single pivot system. Unfortunately, nobody seems to build a quad with cart steering.
2. Wider Wheelbase. This is most important where the most weight is—the back wheels. On long carts, where the weight is more in the center, then the importance of a wide wheelbase transfers equally to the front wheels.
3. Long frame to prevent turtling. If you’re trying to launch a cart in much wind, and/or you’re aggressive during initial inflation by backing the cart up a fair amount, the initial inflation can tip the cart back on its frame. Having the rear wheels well after reduces this likelihood dramatically.
4. Low wing pivot point. Usually the wing’s pivot point is the base of carabiner–that’s where the wing and cart pivots around. Being lower makes the cart more tip resistant both left/right and fore/aft.
5. Keep the cage mounted line holders, if used, fairly low on the cage. During the inflation lunge, there’s less pull-down on the cage rim and decreases turtle tendency.
6. Lower Center of Gravity (CG). Mounting the engine low and having the pilot sit close to the ground are the two primary methods. The pilot should be pilot high enough and/or padded sufficiently for decent crash protection (covered later).
7. Balanced Center of Gravity (CG) fore/aft. To prevent turtling the aft wheels must be far enough back.
8. Crash protection can include structure and padding. Roll bars or a strong enough cage should prevent the pilot’s head from being rolled over. Stirrups or other means should keep the from extending outward or upward and padding on the bars (pool noodles or pipe insulation work) can reduce damage from colliding body parts.
9. A Suspension system could be employed, at least on the main weight-bearing wheels, to reduce the consequence of a hard landing.
10. Wide enough riser hook-ins to reduce the amount of left-right swinging and torque effects. They should not be wider than what the wing was certified for.
11. Big wheels to resist digging into soft sand and handle bumps better. They also add some amount of floatation although, if over-water flying is anticipated, an auto-inflating system (like Agama) should be used since the tires would tend to float the craft upside down.
There’s also the issue of protection after an upset. How well does the cart protect its occupant during an accident. Since flipping is so common, it’s one area that must be explored. Some common accident types are hand and foot injuries when pilots extend limbs in an automatic response to tipping over. I’ve done it myself, thankfully on a light enough machine, and slow enough speed, that it caused no harm. But here are some areas where possible improvements remain.
If you know of other considerations that could make for a better cart-based machine, please send it to firstname.lastname@example.org.
1. The pilot’s head should ideally not hit the ground during a rollover. Many cart machines use just lap belts instead of harnesses and it’s possible this may be less than ideal although I know of no statistics suggesting it. Consider the results of coming down after a wing collapse or severe oscillation and you can see how design elements become important. If the machine has bars near the pilot’s head, maybe they should be padded.
2. Can the pilot’s feet easily come off the steering pegs? It could be best if the feet were sufficiently captive so they don’t come away from the frame in into the “twisting zone.”
3. Consider using some form of A-Assist or Speedbar Puller to make inflation easy without having to hold the A’s.
Below you’ll find some of these features illustrated but it’s not perfect. In a rollover the pilot’s head is slightly above the cage and his chin is below the bar level.
4. Wheel brakes on the back. On cars and motorcycles the front brakes do more due to forward CG shift but, on our craft, rear wheels bear way more weight and steering is already limited.
5. Have the motor tilted up enough to keep the prop blast ABOVE the wing while idling for launch. If the wing inflates just sitting there it makes control harder and may pull you back.