How to build a launcher for a RC plane

Sometimes you need a runway for taking off but there is not any around or just your aircraft is too heavy for being launched by hand. in any case, what you need is an initial speed in your aircraft for taking off and this can be achieved by the simple solution of a launcher. Is this really a simple system? fortunately the answer is yes! The working principle is simple, it does not take more than five minutes to set up the launcher, and it can be easily transported by only one person.

Before going to a more detailed explanation I have summarized the building steps in the following video.

I will show you how to design a launcher for an airplane from 1Kg up to 4Kg, with a wingspan from 1m up to 3m. However, you can always scale up the launcher since the working principle is the same.

The materials can be found in a regular Do It Yourself store, the list is after the image:

catapult_letters

  • 1x Elastic Rope 3 / 5 meters (I will show you later how to choose it).
  • 1x Nylon Rope 2 meters.
  • 6x Metal rings, two of them have to be chained.
  • Self-driving screws and a couple of L-Shape metal pieces.
  • 2x Big tent pegs.
  • 2x Metal plates.
  • (A) 2x Elbow PVC 25.0 x 6.0 mm.
  • (B) 2x Elbow PVC 32.0 x 5.0 mm.
  • (C) 4x T-shape PVC 32.0 x 5.0 mm.
  • (D) 2x Tube PVC 32.0 x 3.0 x 1570.0 mm.
  • (E) 2x Tube PVC 29.0 x 1.9 x 1000.0 mm.
  • (F) 2x Tube PVC 32.0 x 3.0 x 555 mm.
  • (G) 2x Tube PVC 32.0 x 3.0 x 270 mm.
  • (H) 8x Tube PVC 32.0 x 2.0 x 100 mm.
  • (I) 4x Tube PVC 25.0 x 1.9 x 90mm (inside A and D in the above image).

Working principle: When you stretch out the elastic rope, you are adding potential energy to it. This energy will be transferred to the airplane as kinetic energy, so the airplane will gain velocity. The airplane needs velocity (airspeed) with respect to the air (remember that the velocity is a relative quantity) in order to to climb up. This is why you should always face the launcher against the wind. Depending on the angle of the wings with respect to the airspeed (angle of attack) the wings will generate a different force lifting up the plane.  You can adjust the initial angle of attack by modifying the height of the the T-shape tubes. The optimal angle of attack depends on many factors, for instance, for my low wing fighter I set it to about 15 degrees. The initial angle of attack can be calculated easily from the height of the launcher and Pythagoras theorem.

Configuring the elastic rope: How much do you need to stretch out your elastic rope? That depends on how tight or soft is your rope. The potential energy of your rope depends on two factors, how much you stretch out the rope (x), and the nature of the rope (k).  In fact, the expression for the potential energy is E_p = \frac{1}{2}kx^2. Whereas x is easy to estimate, k usually  is not. However, it is easy to compute it following this ten minutes lecture by Professor Walter Levin. Just add some known weights to an edge of your rope and compute its elongation.

Please, pay attention that this equation for the potential energy only works when the rope is in its linear behavior. Usually the smaller is x with respect to the total length at rest of the rope, the more you can rely on the formula for the potential energy. This is why I choose a six meters elastic rope length, because I usually stretch it out about 1 meter, i.e. about a 16% more with respect to the resting length of the elastic rope.

Once you know how to compute the potential energy, you have to estimate how much velocity you need to transfer to your plane. You can roughly estimate that the 80% of the potential energy will be transferred as kinetic energy E_c = \frac{1}{2}mv^2 to your plane. Therefore, to know how much I need to stretch out the rope we need to equal both expressions for the energy.

0.8 k x^2 = m v^2 \implies x = \sqrt{\frac{mv^2}{0.8k}},

where m is the mass of your airplane. You have already computed k, you should know m, and the velocity v depends on your plane. For my fighter I set a velocity v of 50km/h (equivalently  to 13.3m/s). The k of my rope is about 150N/m, therefore I need to stretch out my rope for

\sqrt{\frac{1.5\cdot 13.3^2}{0.8\cdot 150}} \simeq 1.5m

If you have questions, just comment in the post and I will try to help you.

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