# Oh, Now They Tell Me

A while back I bought a radio-controlled helicopter to fly around the apartment — it isn’t something designed to withstand much more than the gentlest of breezes — and broke it in almost record time. A harsh learning curve. I strayed into enemy airspace smashed into the lights above the dining-area table and snapped one of the rotor spokes. Oh, well. I suppose it’s fixable, but I haven’t gotten around to it yet. Add it to the list.

Now arXiv tells me why tiny helicopters are so hard to fly

[M]oments of inertia drop in proportion to the fifth power of vehicle size. This gives small helicopters quicker response times, making them more agile. But the real killer is that the main rotor tip speed in a small helicopter is the about the same as it is for a large helicopter. So the ratio of the rotor moments to the moments of inertia can become huge and unmanageable.

So it’s all because of scaling. Curse you, scaling laws! A disproportionately large curse!

One thing I noticed, though, in my brief career as an indoor chopper pilot, is that there is a “ceiling effect” in play, which I presume as an analogue to the ground effect. When the ‘copter gets within a blade length or so of the ceiling, it has more lift and slams into the ceiling. You have to reduce power a lot in order to get it to disengage, and once that happens you have to react quickly. Once the helicopter drops more than the blade length, which only takes a fraction of a second, it’s essentially in freefall because the lift has been reduced so drastically. Full power often isn’t enough to arrest the drop before you hard-land on the floor, or open pizza box on the table.

One way I’ve seen the ground effect explained (without using a bunch of aeronautical engineering terms that I don’t really know) is that when the air meets a hard surface it has nowhere to go but to the side, and that’s harder to do compared to free space, so the pressure increases, which means more lift. (Looking at Bernoulli, you’re trading flow energy for pressure energy.) Similarly, I assume, when you are at the ceiling, it’s harder for the air to get in and replace the air above the rotor, which gives a pressure drop, likewise increasing lift.

Works for me, unlike the current state of my helicopter.

## 0 thoughts on “Oh, Now They Tell Me”

1. Ha! I have two of those systems (one at home, one at work) but not enough hands to shoot down something I’m simultaneously flying. Also not enough USB ports to attach a webcam to the launcher (IIRC both like a direct feed rather than a hub) to do targeting.

2. Couldn’t you attach very large fan blades to the rotor shaft of the helicopter? It would allow the rotors to move much more slowly, and presumably solve some of your problems.

3. No excuse, you’re a physicist…you should be able to do the dimensional analysis. 😉

In any case, the moment of force generated by a disturbance or control deflection should scale with the fourth power of the size, so the response scales inversly with the *first* power of the size of the vehicle. Still tricky, when you consider the disparity in size over even a hobbyist’s R/C helicopter.

If you’re willing to try again, I recommend the following helicopter from Fry’s. It’s a fair bit bigger than the ‘Air Hogs’ class of toy helicopter and has all 4 degrees of control, so it’s a good one to practice with. Pretty resilient as well (you should still buy some spare blades). As an added bonus, you may get the chance to tinker with it to make the controls work the right way…some of them are configured with the left and right sticks swapped (I guess European models fly this way)

Oh yeah, and the battery charger may overheat and burn out (it’s Chinese, what can you expect). Then you’ll have to get a ‘real’ hobbyist battery charger if it does (about \$30). Gosh, I feel like I’m trying to sell a ‘Happy Fun Ball’. :p But it flies nicely.