Toothless Midnight Fury

[Duane]

I suggested that kiddo's car next year be a Blue Angels jet, but he decided he'd rather have a dragon. Then his 1st grade teacher gave the assignment of doing some project with a parent, and writing up a how-to article about doing that project. And we saw the great movie 'How to Train a Dragon'. So it was obvious which dragon we needed to build now. Here is Toothless:





No weights yet, and no room for them in the back end. Total weight is 2.4 oz, with 70% of the weight on the front axles: backwards of what it should be for performance. Have already broken a rear tail fin twice; too bad it wasn't the right side like in the movie.

Wings are adjustable, so they can be kept within 2.75" when necessary, or spread out for looks. Air drag should be small.

Matte black spray paint looked too dull and dark compared to the movie posters. So we rubbed graphite power over the whole body, which gave it just the right amount of grayness and sheen.

[FatSebastian]

Nice! What are the wings made from? Will the car race head first, or tail first?

[Darin McGrew]

Very nice design! Thanks for sharing!

[Duane]

What are the wings made from? Will the car race head first, or tail first?

The wings are cut out from a thin stiff sheet of plastic, from a box that once held Christmas ornaments. Cut into a batwing profile, then creased at the shoulder and elbow joints, and attached with double-sided sticky tape. Thick construction paper would work okay too, but could pick up additional creases during play.

This car will likely race only at home. Racing tail first would nicely fix the weight distribution problem, but the delicate tail would need reinforcement against crash stops. The wing tips might flutter and cause air drag, when going backwards. Also, Midnight Furies are fine with acrobatic maneuvers but are indignant about being asked to fly backwards.

[FatSebastian]

Thanks for the added info, Duane.

Also, Midnight Furies are fine with acrobatic maneuvers but are indignant about being asked to fly backwards.

Excellent point! Nevertheless, if you can ever coax it down the track backward (and/or forward), I would be curious to know what behavior you might observe with the wings.

[Duane]

if you can ever coax it down the track backward (and/or forward), I would be curious to know what behavior you might observe with the wings.

Yes, me too -- in particular, whether decorative wings (or other thin decorative artwork edge-on to the airflow) causes much drag penalty. I'm planning to use this technique for wings, twin stabilizer tails, and cockpit profile for a Blue Angel F/A-18 Hornet for next winter's racing. From a front view, it will have nearly the same low frontal cross section drag as a plain fast slab pinecar. If it works well, that would open up the possibility of cars that are both very fast and very imaginatively styled, without use of child-unfriendly tools.

Air drag from these decorations have three parts:
friction (a small effect here), and
form drag caused by surface-layer separation and rear vortices (practically nill for edge-on sheets), and
form drag caused by the artwork twisting or flopping in the wind like a flag, vane, or stalled wing.

Using stiffer sheets helps with the twisting issue and with survival from rollovers etc. But paper works great for 'first drafts'.

Our plastic test track w/o timer isn't adequate for measuring these small effects by timer or by face-offs. Anybody have a test track in south San Francisco Bay area?

[Duane]

Our plastic test track w/o timer isn't adequate for measuring these small effects by timer or by face-offs. Anybody have a test track in south San Francisco Bay area?

Another possibility: When Toothless & kiddo return home from school, take Toothless on a 40mph training flight, using our car's sunroof as a crude wind tunnel. The wing will probably pull off, giving us a qualitative before and after comparison of its contribution to total drag.

[FatSebastian]

Yes, me too -- in particular, whether decorative wings (or other thin decorative artwork edge-on to the airflow) causes much drag penalty.

Besides a drag penalty, I would be curious if large protrusions complicate or aid one's ability to align the steering down-track. That is to say, what if the large decoration is not quite "edge-on" to the direction of travel, such that differential pressure (lift) causes a delicately balanced rail-rider to experience unwanted stability issues? Conversely, could fins forward of the CoM help dampen or correct unwanted oscillations?

take Toothless on a 40mph training flight

On a 4' high track, aren't we're looking at top speeds closer to, say, 11 mph?

[Duane]

take Toothless on a 40mph training flight

On a 4' high track, aren't we're looking at top speeds closer to, say, 11 mph?

Sure, but when driving at 11mph, I don't think I could judge (just from the forces on my hand) a difference between some small body drag w/o wings, and a 20% larger drag w/wings. Maybe with those drag forces amplified 16x, I could tell. I was thinking of mounting Toothless on a stick, like a marshmellow, and holding the stick out through the opened sunroof or window.

A proper car-borne windtunnel would involve somehow attaching Toothless to a tether, wrapped around a pulley, with some calibrated spring tension on the anchored end. And some way to keep Toothless hanging straight in the wind. Hmmm.

[Duane]

Conversely, could fins forward of the CoM help dampen or correct unwanted oscillations?

In my limited experience, the problems of steering, fishtailing, and losing energy on the guide rail seem to be more acute on the flats when the car is decelerating, than when accelerating down the ramp. So maybe a rail-riding technique based on sideways force from a vane could be as or more effective than trying to manage this from subtle wheel/axle tweaks?

[FatSebastian]

I was thinking of mounting Toothless on a stick, like a marshmellow, and holding the stick out through the opened sunroof or window.

Please post pictures of that!

So maybe a rail-riding technique based on sideways force from a vane could be as or more effective than trying to manage this from subtle wheel/axle tweaks?

The speeds are relatively low, and the change in angle of attack would be relatively small - one might need a fairly large vane? Sounds like a science fair project waiting to happen...

[sporty]

K, Busting a bubble perhaps.

Many flying animals, their wings have a material that not only allows for the air to go through it just slightl, but also conforms and changes shape as they fly, to capture more wind or reduce the lift or drag.

The material would likely have to be lighter and more giving, but it may also do what most wings are intended to do, capture the wind for lift, so may indeed slow you down.

But still neat and fun stuffy to play with and perfect for the family to tinker.


Sporty