GETTING AIRBORNE
There are three common methods to an EDF Take Off. They include:
ROG ( Rise off Ground )
Hand Launching
Bungee Launch
We will look at each method in detail below.
ROG – Rise off Ground
Much like a full scale jet, a ROG take off is one where a long stretch of run way is used to build speed and then leave the ground. If you are coming from prop planes, the technique used is slightly different along with the climb out. Be sure to double check that your Center of Gravity is correct and run over all pre-flight checks.
First be certain there is a long bit of runway where you can safely get your plane up to speed. EDF jets take a much longer distance than a similar weight prop plane. Once the plane is placed at one end of the runway facing in to the wind, it is time to advance the throttle. In order to get up to speed, you must commit to WOT.(wide open throttle) Once the plane reaches a high enough speed pull back on the elevator until airborne. Depending on the Angle of Attack,(Angle main wings makes with runway) use of the elevator to break the surface may be necessary. Maintain WOT until you are at a safe altitude before begining the first turn. With no propwash to keep air flow over the control surfaces, increased airspeed is necessary to compensate. Once up to a safe altitude 70% throttle can be used to maintain a cruising speed.
If you run out of runway throttle back all the way, retrieve the plane, and check that the gear survived. If all looks good repeat the process. If it is near impossible to get airborne, it may be best to double check your angle of attack. Negative Angle of Attack will make it exceptionally difficult to rotate the plane off the runway.
Hand Launching
Hand Launching is generally limited to a plane constructed out of foam. Wing Loading should be less than 20oz/sq ft. A higher wing loading will launch easier with higher power. Under powered EDF jets will be more difficult to launch. Be sure to double check that your Center of Gravity is correct and run over all pre-flight checks. An incorrect Center of Gravity for a first launch may be your last.
Before hand launching it is necessary to find the positioning of how you want to hold the plane and your tossing motion. Practicing without releasing the plane may help. Generally, holding the plane just behind the center of gravity is ideal. Many prefer to have the plane right side up with an overhand style toss. However, there are a couple different variations that may work better. An under hand toss holding the top of the plane so that it is right side up may feel better. Some planes even have an inverted preferred hand launch. Select the variation that you are most comfortable with.
To hand launch face the wind. A friend may be best if you are not comfortable launching alone. Aim the airplane at a 25 degree climb angle. It is often best to use Wide Open Throttle during a hand launch. Unlike prop planes, torque roll is not as significant. With the plane aimed for 25 degree climb out and throttle is wide open, firmly toss the plane forward. A firm launch in to the wind is required to get the plane up to speed. Maintain full throttle and transition from throwing the plane to grabbing the control sticks as quickly as possible. EDF jets do not have prop wash passing over the control surfaces. The plane after launch will not yet be up to a comfortable flying speed and may dip back down. Carefully bring the plane up to a safe altitude before making the first turn and trimming out.
Bungee Launching
Bungee Launching is the toughest method for getting an EDF in to the air. Bungee type launching is best for heavier planes such as composite airframes that do not have landing gear. It is also the method most prone to a crash, if not done correctly. This includes both the setup of a bungee launch and the execution.
What is Needed?
In order to Bungee Launch an airplane, there are a few items needed. In addition to the plane itself, a launch ramp, tow rope with hook is required.
Selection of Components
The Launch ramp must be long enough for the plane to grab a split second of height and speed before exiting the ramp. This length should be built at a minimum of 1.25 times the length of the airplane. The angle of glide, off of the Launch ramp is typically 15-25 degrees.
The tow hook should be placed in your airplane at the recommended location. Be certain it is in the correct location with the correct amount of reinforcing. There will be a significant amount of load on this hook. The hook must also disengage once the plane is in the air at speed. A typical location is just in front of the center of gravity.
The tow line must be capable of providing the necessary amount of tension required for takeoff. The length of tow line will fall between the lengths of 15-50 feet. Lighter, smaller airplanes will require a smaller length of tow rope while larger planes will need more length.
The tow rope tension is a critical part of the bungee launch system. It is common to allow for 5 times the weight of the plane in bungee tension. A 1 kg plane would require 5kg of bungee tension while sitting on the launch ramp.
Releasing the Bungee and Flying Out
The technique for launching an airplane from a bungee is just as important as setting the system up. Once your plane is setup and waiting to be launched on the ramp under tension and pointed directly in to the wind, there is only one seconds from the time of release to the time the bungee releases. Be certain to launch in to the wind as not doing so can result in a quick crash.
When ready and just before hitting the release, check over all control surfaces. You may rev up the motor and leave it at a slow RPM in order to not effect the launch from the bungee. With a properly selected launch system you do not want thrust from the motor effecting the launch. The motor being turned on at a low speed will provide confidence that there will be power once the tow hook releases.
With everything set hit the bungee release. As the plane pulls out follow its every move with the control surfaces to keep it flying in a consistant straight path. Once the tow hook releases apply full throttle and climb out to a safe height.
Things that can go wrong – Bungee Launch
If your setup is not properly selected or you are not facing in to the wind, there is a higher chance of crashing during the launch. It is important to pay close attention to these fine details.
Be certain to check over the strength of your tow hook mounted on the plane and that it is in the correct location. It’s also a good idea to re-check the Center of Gravity of your plane. If either of these are incorrect, the plane may not track straight during the launch.
If the tow hook does not release from the tow rope, the plane will be hurtled towards the ground with great force.
A Bungee Launch system will release your plane with no thrust from the motor at 50km/h and up. With the proper care and caution, you will be able to make launch after launch safely.