I built a largish fixed-wing UAV (Hugin 2.6m, roughly 8 kg MTOW if I recall). It's definitely too large to hand launch, and it needs a surprisingly long takeoff run (especially on grass) to get airborne and clear of trees etc. This is fine when I have access to a private road or something that can be used as an airstrip, but when that's not available, things become difficult.
I'm toying with the idea of improvising a cheap catapult that could get it airborne in a much smaller space, either using a spring-loaded or gravity driven arm or even just a stretched bungee cord of the right length and thickness. Before I experiment too much, I thought I'd ask whether anyone else has come up with a good solution.
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BTW,here's a video I just found of a bungee launcher that seems to do a nice job with the Skywalker X8: https://www.youtube.com/watch?v=PqEESKzy_iQ
1) Yes, I refer to the skywalker X8 but they are usually loaded with heavy batteries and you wont hand launch these beasts. do a search for the injuries caused to people trying to do that. they are also far from flimsy.
2) yes and no, We do just that to make a long bungee and only use a small portion of the stretched length, we still have about 60-70% of pull at the top of the catapult but there is a hidden variable that you are missing and that a is the mass of the bungee cord itself, the longer the cord the greater the mass and you will need a lot of bungee to make that work. The greater the bungee mass the slower your launch. also the bungee power is non-linear, the last 20% of stretch gives about 40% of the tension. (despite what manufacturers will tell you.)
The torsion spring is more linear but now you have to stop the fast moving parts at the top or end of the launch ramp , that is a lot of energy and torsion springs dont like to be stopped in that way.
what you are describing is a bungee launcher, not a catapult which is self contained. Fasten a cord into the ground at 100 feet , fully stretch the elastic and attach to the plane and release with a guide ramp for the first part of the flight. That is not a catapult launcher. the mass of the bungee cord for a large aircraft will try to pull the plane into the ground. it is common to use a combination of bungee and nylon line to reduce this mass.
The biggest danger with that type of launcher. if the plane does not make a good launch then the plane goes into the grass and the bungee continues to pull until something breaks. usually the aircraft.!!!
good luck, I look forward to seeing your results.
David,
Thanks for the additional comments and info. A couple of quick comments/questions for now:
1) Is this the Skywalker X8 you're talking about? https://hobbyking.com/en_us/skywalker-x-8-fpv-uav-flying-wing-2120m... I've seen videos of this being hand-launched, and it seems like a pretty light and flimsy foam airframe to be launched with a catapult, so I'm wondering if there's another airframe with the same name.
2) Regarding the non-constant acceleration: That can be avoided by using a very long spring or bungee and allowing the active "pull" distance to be only a small fraction of its total length. In fact, one of my ideas was simply staking a very long (50+ ft) bungee cord out along the length of ground, stretching it to the point of applying the required force and then enough further to cover the relatively short distance required to get airborne. Not perfectly constant force (unless the bungee is inifinitely long), but pretty close. A similar effect could be achieved far more compactly with a torsion spring.
Hi Grant
We aim at 18-20m/s launch speed, slightly higher than your 15m/s as the aircraft are different, The 3G is based on initial acceleration not constant acceleration as that is impossible with a bungee or spring system and difficult to measuere in practice,(even witha pixhawk) , That's why I suggest an electric stepper motor method where that can be achieved. Depending how you configure the bungees you have much less power when they are relaxed at the top of the launcher.
I have not done the calculations and the 3G is simply based on our knowledge from our catapults, Your 10m track is 2.5 times as long as ours (3.75m) so I took the 8G-10G of our system and divided it by the new length, not very scientific but close enough for discussion.
you also need to allow for the slope of the launcher which will require energy, ours are a much higher angle then your 10m version.
I am not trying to deter you but there are many challenges with the system you want! have you checked out a bungee launcher or even a rooftop launcher for a car or van? both of these work will for the larger sizes of aircraft.
Our pneumatic version will also be an option later in the summer.
I'm not quite sure where the 3G comes from if my target airspeed is, say, 15 m/sec (about 30 knots). In that case, a 1G acceleration should accomplish that over about 10 meters. 3G over 10 meters would give three times the energy, or 24.3 m/sec (about 48 knots). So I guess it comes down to what one assumes for the minimum launch velocity...and it's possible I'm underestimating that speed for the MTOW I cited and the wing area, lift coeff., etc.
Also, I don't know yet what to assume offhand for the acceleration from the aircraft thrust itself, which reduces the energy contribution required from the catapult.
Again, thanks for your thoughts!
Dwgsparky said:
I see where you are going but even a 10 meter track would still need about a 3G acceleration, not exactly portable !
If you are going that way then I would seriously recommend using using a large stepper motor with a suitable gearbox and a lightweight continuous nylon chain drive over a head sprocket and a tail sprocket. you can then tailor the top speed and acceleration for your aircraft..
you can easily power this of a car battery for portability.
Good luck , have fun
Very helpful information; thank you very much.
I'll add that I had been thinking about a relatively long track so as to greatly reduce the required g-forces. As a starting point, I believe a 10-meter track (still greatly reduced compared to the normal ground run) would allow flying speed to be achieved with an average of only 1G acceleration (not even counting the motor contribution), which would seem to greatly simplify the engineering.
Hi Grant
We have been building and providing medium sized catapults at www.Air-Vision-Air.com for members and others for several years.
These are mainly for fixed wing and flying wing aircraft up to 6Kg ( Typical Skywalker X8) . our part number ASM300
This is not a sales pitch for you!.
To do this for your aircraft is not going to be simple or cheap, for a successful catapult launch you generally need at least 8-10G at launch depending on the length of the catapult. To achieve that you need to engineer your structure so it will withstand the forces at launch and to control the tension in the bungees before release. You also need to stop the carriage and cradle before it self destructs at the top of the catapult.
The part that will give a lot of problems is the ramp section joints, have fun with that.
For example, for an X8 to launch from an 11 foot catapult you need at least 8 x 3/8 good quality bungees with a combined tension of nearly 200 pounds.
Yes it can be done but expect a struggle with some parts of this.
For our part we are finalizing the development of our own design pneumatic catapult for the summer aimed at the larger 6Kg-15Kg aircraft. Details in a few weeks.
Good luck on your project.
David