Bungee Launching an EDF Jet

I know, your first thought is add some retractable gear and voila. Well there are a couple of problems with that. The first problem is the added weight and the second is steering the aircraft while it's accelerating. Enter the EJF bungee Launcher - 10 years in the making - looks like this



Bungee launching is particularly suited to FAST jets with low weight and narrow airfoil sections like the HET series electric jets because it takes them safely past stall speed with a repeatable trajectory. The guide rails make sure there is no interference in roll and the pitch angle is positive.

Question : Bungee or Hand Launch?
How do I reliably get my electric jet in the air since it doesn't have any landing gear to speak of? To prepare for my excursion in this area I used two of my smaller electric jets: the FVK HE-162 using a Mini Fan and a SU-27 Pusher Prop Jet. The most popular methods of launching these kinds of planes are Hand Launching and Bungee Launching (e.g. Surgical tubing. or Hi Start) .

The Hand Launch Horrors
For those of you who haven't experienced this you might think hand launching is the most cost effective and reliable way, and most have a mental picture of gracefully tossing the plane from their hands with a nice gentle altitude gaining departure, and all is happy and fine. Wakeup!!! We're not on the edge of a slope with a 15 to 20 mph constant head wind on our launches! We need airspeed and we need it now! The few people that do try the carefree method will become either very accomplished re-builders or join the 20 second club of pilots. My second tip of the column is: Just don't even think about it.

Yes, all of us who have been doing the one man band method will argue that "nobody else can either fly it or hand launch it as good as myself so that's why I do it". I, too, have been in the same situation and on occasion biff it. The only advice I can give here is you need two people. The pilot and the launcher. Oh, did I forget to mention you need a specific launcher type person? OK, you need to find someone that you trust and who knows how to run reasonably fast and can throw about two pounds in the air at the correct release angle into the wind and not squeeze the servos out of it while do so. Umm, I didn't say it would be easy. Ever think about what would happen if the motor stopped working right after launch or your friends changes his wrist angle? That's right - CRUNCH! Its much better to get the plane airborne with enough speed for safe flight even if the motor isn't working at all. For that we need to rely on a bungee.

Bungee Launching
Before jumping into my adventure I collected as many bungee launching methods as possible and then came up with what I use and enjoy. What I'm going to describe will not be new developments in bungee launching but the specific details of the method I now use to reliably launch my electric jets wherever I go. You will be able to teach most anyone how to do it. As you might gather I suggest two people: the pilot and launcher. The stress factor is much less when you're able to concentrate on the aircraft instead of the gymnastics of launching as well as flying.

One of the mysteries to bungee launching is where does the tow hook mount? For this question I experimented with two different locations. On the HE-162 1" from the tip of the nose and for the SU-27 it was placed 4" in front of the C.G. Both spots worked very well. One of the reasons you don't mount it on or behind the C.G. is that it would go nose up and look like you were launching a kite and probably rip the wings off at the very minimum. Remember, you're trying to get past the stall speed and attain level flight- not launch the aircraft to some desired altitude. For the tow hook I wanted to try something a little different so that I could easily retrofit it on any of my existing aircraft. For the HE-162 I used a single medium sized du-bro nylon control horn with the normal either side sandwich mounting method and added some epoxy. On the SU-27 I used the du-bro nylon right/left control horns and CA glued them together and used the normal sandwich mounting method. Both are strong and launch great.

The bungee is comprised of 15' of surgical tubing and 10' of towline, three steel split rings and a steel anchor stake. I found that pacing off 45' to 55' of stretch was fine in my tests. The approximate weight limit is around 45 ounces. I'm sure there are other distances that work however I didn't test them all. It also works out better if you don't have so much stretch on the bungee that the launcher has trouble standing with it. The following technique is very straight forward and has three complete steps.

The Reliable Launch
1. The pilot should retrieve the bungee and verify that the anchor stake is properly set and all split ring connections are sound before each flight. After a couple of times it becomes a natural habit when walking back to the launch area verifying the bungee along the way. The launcher should carry the aircraft like the Bob-E shown here (not a scale jet but much faster than most scale craft in the air) and radio with the switches off and follow the pilot to the launching area. As you reach the launching area it should be clear why the pilot does these steps with the bungee since anything to do with the flight process should be the pilot's responsibility such as direction into the wind and launch placement.

2. The launcher sets the aircraft and radio down and then the pilot hands over the bungee to the launcher holding onto the split ring. The pilot now turns on the radio and then the aircraft and does the pre-flight surface and power tests. The pilot then hands the aircraft to the launcher, it should be the opposite hand to the one holding the split ring. The pilot should move behind the launcher about six feet and to either side 3 feet for visibility.

3. The pilot then verifies just the elevator or ailerons/rudder, applies a small amount of up elevator and announces 'Ready' the launcher then applies the split ring to the towhook and stands up straight with the aircraft over head and then acknowledges with 'Set' and waits for the final command from the pilot which is 'Launch' the launcher then releases the aircraft with a slight pushing forward motion and then stays still or kneels down. The pilot then applies throttle and makes a level accent followed by a slow turn away from the flight line area.

And there you go, I hope you can see that two people cooperating make a nice stress free and predictable launch every time. I'm not saying that potential incidents can't happen like when the launcher has that sudden itch that must be scratched and you almost have a pre-mature launch condition (hopefully with the receiver on). Or, the pilot bumps the throttle startling the launcher. I must say that one of the most interesting launches is my SU-27 with it's pusher prop. This is where the trust factor comes into play between the pilot and launcher (similar to the holder and kicker in football). Since this method instructs that the aircraft is always clear of the launcher before throttle is applied the Puller, Pusher and EDF are all accommodated. The acceleration effects of the bungee launch are well over the power output of the throttle for the first few seconds anyway.

This leads me to the new EJF Bungee system. Check out the video on this post.

I would also like to say that during the creation of this column no electric jets were hurt or killed. :)