Discussions

Catapult launch systems.

Posted by Subashish Sasmal on March 5, 2010 at 5:38am
Hi guys,
   I've a question: its just that most of the UAVs which are catapult launched are pusher configurations. Is there any particular reason for it?
Besides, I've been wondering that going for catapult launch means great forces on the underbelly. What structural changes are necessary to secure the plane against that? I mean does reinforcing the underbelly have great changes on the GTOW?

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Replies

  • Joe March 5, 2010 at 10:10am
    The degree of reinforcement is going to be a function of the size of your plane and the length of the catapult track. To reach the same airspeed on a shorter catapult will require greater acceleration. Likewise the greater the mass of the aircraft the more force will be required. This relationship is simply F=ma.

    For example:
    Assume that you know your minimum airspeed is 30 mph (13.4 m/s), you reach that speed in 1 second, and your mass is 2 lbs (0.9072 Kg). The acceleration will be 13.4 m/s^2 (a=deltaV/time). Then you will need to apply 13.4*0.9072 = 12.156 Newtons (2.7 lbs) of force. This seems low until you consider that the track for this example would be 6.7 m (21 ft) long.

    It is also easy enough to start with a certain track length, mass, and air-speed and calculate the forces. This all assumes that the acceleration is smooth and constant over the length of the track.
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  • Developer
    Mark Colwell March 5, 2010 at 9:12am
    It took me awhile to figure out how to launch my Stryker_II. Full force level hand launch, at 15 - 20 degrees.. Still no prop to hand contact, I have a bungee I could use to get to 300'. I used it when I flew a Airtronics Eclipse 2m electric glider with 10 C NiCads, but that's what Lipos & brushless motor is for. You could use a meat cutting glove on launch hand, and take it off when up and steady.
  • Developer
    Doug Weibel March 5, 2010 at 8:21am
    Reinforcements do not need to be extensive....

    Another reason for catapults is that with some there is a severe danger of having your hand travel through the prop. When it is spinning at 20,000+ rpm that it a problem ;)
  • Tim McMahan March 5, 2010 at 6:16am
    I am new to rc's but have flown tractor planes, pusher planes, and "prop in slot". pushers in particular have no air moving over the control surfaces until the plane is up to speed. This making launching them tricky at best sometimes. Particularly on the speed planes. My flight buddy Jon is big into speed, check out this video of his new Blackbird. Pay close attention to the launch. The plane has NO control until its up to probably 30mph. He finally got a successful launch by letting it roll completely over while building speed, then recovering before it hit the ground. Scary to watch, but it worked on about the 3rd or 4th try. We were both like "we need a catapult.
    As for structural changes to the airframe? I can't help, we haven't gotten that far yet.
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