We have recently presented a paper on the multi-rotor aircrane operations that we do now fairly regularly. So we want to take the opportunity to share some of the material here.
The paper is available for download at the end.
Motivation / typical launch methods:
We need a method to safely launch unconventional aircraft for flight testing. Especially on a first flight of a new design, we want a method that gives the pilot an opportunity to learn how to fly this new aircraft. Other reasons include the difficulty of launching designs with low pitch stability from bumpy grass runways, and the potential weight savings of removing the landing gear.
Widely used launch methods for UAV are:
- Runway – this requires one, which is not always given. Further, a grass runway can be too bumpy for low pitch stability aircraft. Also, a runway requires a full landing gear, which adds weight and complexity over a simple landing skid.
- Catapult – This puts huge loads onto the airframe to accelerate to flight speed in a very short distance. A catapult is also typically very bulky and quite dangerous to operate. For a first flight, the catapult launch is very risky, since a crash will occur almost immediately if the pilot does not have full control right after launch.
- Hand launch – This is limited to low wing loadings and overall airframe weights. A bad throw will almost inevitably result in a crash.
- Car launch – This requires lost of space and, obviously, a car. This is not really practical for low budget university research.
Ideally, we want to lift the test aircraft high up into the air and release it for a safe launch. This is the aircrane technique, which we discovered by a news article on the droptests of the Dream Chaser orbital vehicle a few years ago.
Our approach:
Inspired by NASA’s technique of performing glide tests by lifting the test aircraft with a helicopter to the target altitude, we have developed a similar method based on a multi rotor air crane. Below are two videos and some photos that show how it works.
Lessons learned:
- make the cable long
- attach cable at crane CG
- test aircraft (load) needs to be in a defined attitude for a predictable launch
Future work:
These techniques are not perfected yet. Further work is required to:
- Reduce the twisting of the tether and the load by adding a swivel into the cable
- Reduce drag for the forward speed launches by applying thrust on the load
- Improve stability of the launch cradle on the test aircraft by improving on the design of the frame
