A cross between a helicopter and an airplane, this concept has been intriguing researchers for 50 years.
During late World War II Germany had a problem (among many others…). Their runways were being bombed almost constantly. The holes on the runways made it almost impossible for their fighters to take to the air.
The Germans dreamt of a machine that could take to the air without a runway but that could also fly around like an airplane. The German company Heinkel took on the challenge and designed two vertical take-off airplanes, but due to the shortages of the war the concepts went nowhere.
After the war, many others picked up on the idea and tried to develop it into a practical flying machine. They are still trying.
Coleoptera is the scientific name for beetles. I assume that the name was given to these aircraft because of their vertical take-off and landing capability.
I see two basic requirements for an aircraft to be called a coleopter. First, it must be capable of taking off vertically. Second, the means of propulsion must be located at the bottom of the aircraft, close to the ground.
Beyond these basic requirements, many variations have been built. Turbine-powered propellers are a popular choice in the larger military versions, I presume because of their superior power to weight ratios.
An almost given requirement is a set of two counter-rotating propellers. Without them, the aircraft would spin out of control on its vertical axis.
There are two schools of thought once they are airborne. One says that they will tilt horizontally and fly away like a ducted fan airplane. The other school of thought says that it will remain vertical and fly around like a helicopter with the rotor on the bottom.
Most coleopters have been designed as personal transportation vehicles. For the sake of simplicity, most of these stay in helicopter mode all the time. Their compactness and relative simplicity is very appealing to designers.
Having the rotor at the bottom of the machine in a pusher configuration actually helps to stabilize it. Having the rotor on top, like in a helicopter, is destabilizing. I know this sounds completely counter-intuitive.
When a flying vehicle is tilted away from the direction it is traveling, a pusher rotor blasts the air in a direction that tends to bring the vehicle back in line with the direction it is moving.
The same principle applies to a (traditional) tractor propeller in an airplane versus a pusher propeller. The pusher propeller helps to stabilize the airplane.
Note that this is not the only principle at play. A helicopter’s stability benefits greatly from having a low center of gravity. A coleopter does not have this advantage. But the biggest mass in a small coleopter will be the machine itself, which keeps the center of mass relatively low.
An early American pioneer was Stanley Hiller. He designed several coleopters. Many are now at the Hiller Aviation Museum in California, which I was lucky enough to visit a few years ago.
Most of the Hiller designs remained in helicopter mode all the time. An exception was the VXT-8, which was a high-performance version (never built).
Looking at the other Hiller designs, you may notice a lip at the top of their ducted fans. This was a key performance enhancement that was lost on most of the other designers.
If you have read my previous articles, you should know that this lip uses the Bernoulli principle along with the Coanda effect to generate lift. This is exactly the same way that a wing generates lift, which is very efficient. In fact, Hiller reported that a whopping 40% of the lift came from just this lip. This was an almost doubling of lifting capacity! Way to go, Stan.
Stability and Control
Imagine the counter-rotating propellers. Their high angular momentum acts like a gyroscope. If you have ever tried tilting a gyroscope, you know that it fights you every step of the way. That helps with stability, but makes control difficult.
Coleopters have exactly the same problem. Having precise control in one is very tricky. A lot of different schemes have been devised over the years for controlling them, but none have been entirely successful.
Taking off vertically is very expensive, energy-wise. If the coleopter remains in helicopter mode, then flying one is like a low efficiency helicopter. Switching to airplane mode in flight would result in a much more efficient cruise mode. On the other hand, transitioning from horizontal back to vertical flight can be very tricky.
A helicopter can go into autorotation mode and land safely even if it loses power while flying high up. A coleopter cannot do that. Modern coleopters use redundant engines for greater reliability. Instead of two engines powering two counter-rotating propellers, they might use a total of four engines.
This is a very intriguing design. It is very compact, permitting usage literally from anyone’s backyard.
On the surface, it looks very easy to build, inexpensive, and easy to control. Several of the one-person designs have relied on weight shifting by the operator to steer. This works well as long as the total weight is kept low.
The efficiency issues should not be ignored, but a design that transitions to horizontal flight should manage a decent range.
The real problem with the design is with control while in helicopter mode. It reminds me of a hovercraft, which has a very tough time with precise directional control.
From watching the videos I linked to below, nobody has solved this problem yet. It looks solvable to me, though. Modern computer and sensor technology can do amazing things, and I have a hunch that this design is ripe for a breakthrough. The right solution would be as safe and easy to control as a Segway scooter.