Several years ago, I attempted to perfect the holy grail of dangerous flying machines by strapping rocket motors to helicopter blades and hoping that I didn’t get horrifically injured in the process.
What I came up with was a simple flying device inspired by the humble helicopter seed. Here's the video!
These seeds, known as samaras, need no engine to spin through the air, thanks to a process called autorotation.
These vehicles require very little stability augmentation, because of the inherent static and dynamic stability present in the configuration.
A monocopter’s stability is derived from the careful balance of gyroscopic and aerodynamic forces present in flight.
So my final design is a totally uncontrolled free flight helicopter that, once lit, just does what it wants basically
But to get something like this to work and not just crash into the ground and explode, took lots of iterations and quite a lot of work.
Incidentally, I've actually built an electric helicopter based off the samara seed before. You can check it out in this video below.
Version 1
The first version was totally different to the final version as it wasn’t a monocopter. I wanted to try something entirely different to what I’d built before.
It comprised of multiple lifting surfaces within a disk, similar to those pulls string spinner toy things you might have seen. On the outside were two of the rocket motors opposite each other.
I CAD all of this up in within about 20 minutes on fusion 360 and then printed the entire helicopter out in one go on one of my Ender 3 3D printers.
Now I only bothered designing this first helicopter with crude blades which are not ideal. This version would simply serve as a proof of concept.
My first problem was that it turns out it's quite difficult to light two motors at once especially when it’s a bit breezy.
I decided to try using electronic ignition to solve the problem which is how I've always previously ignited rocket powered vehicles but there remained a delay between ignition that affected how well the aircraft launched and maintained altitude.
The version was officially scrapped and I realised that I should build a lighter aircraft that only needs one motor lit with a fuse.
Version 2
For version 2, I went back to the helicopter seed.
Theoretically a rocket powered monocopter could be as simple as a single rotor blade with a counter weight opposite which, helpfully, could be the rocket motor itself.
For the rotor, I could spend a lot of time CAD and printing one, or I could simply use a propeller blade from an RC plane which is already the perfect shape to be used as a monocopter wing.
I slapped together a quick proof of concept with a scrap of thin plywood, hot glue and tape.
I suppose this doesn’t seem like the most sensible way to build something that will be flying around at some serious RPMs, but the individual parts of the monocopter don’t have too much mass and therefore shouldn’t exert too much force on the tape - also I’d be standing well back.
I've found over years of building lots of experimental things like this helicopter that it always pays off to do the minimum amount of work to build the simplest version of your concept to check if it works.
The first flight was a success and I could now iteratively experiment with it before going back to the drawing board with what I'd learned and to start on a version 3.
One issue I had found concerned the centre of rotation vs the centre of gravity.
Previously I used the centre of gravity as the fulcrum point but as the craft starts rotating you can see that the rotation point is actually not where this centre of gravity actually is.
The reason for this is due to lots of complex aerodynamic and gyroscopic forces, but it’s a bit of a problem for launching the helicopter as really the guide wire should be precisely at this rotation point which is a bit difficult to find.
Look at this scrubbing through the slowmo footage. The helicopter is super unstable when it comes off the guide wire. It sorts itself out though but isn't good. Could overcome inherent gyroscopic stability and crash.
So I needed to redesign the helicopter to put the hole for launching the helicopter where this natural rotation point is rather than at the centre of gravity.
Roughly seeing where it was from tracking the slow mo footage. I CAD up a proper arm off with lots of different holes to find the right centre of rotation for efficient launches.
Performance Testing
Two motor types would be tested, a small L1 type motor and a larger L2 type motor.
Several tests of the smaller motor showed successful launches and consistent flight times of 16.1s, 17.2s and 16.4s.
The first of the larger motor tests resulted in a failed launch and immediate crash but luckily, I had printed and assembled a spare.
After a successful ignition of the motor, the monocopter launched into the air, far higher than the previous tests and resulted in a flight time of 39s!
If you're interested in making this project yourself, the print files are available for purchase here. They include the rocket helicopter STL, a printable launch pad STL as well as a PDF of instructions.
Thanks for reading and if you enjoyed this article, there are plenty more to check out over here.
