Auto take off settings for catapult- please help

hi guysI'm launching a x8 off a strong catapult and am having trouble with the throttle delay and MINACC parameters. I have done about 10 launches in auto mode without the prop on and have been monitoring when the motor spools up. It is usually way too late and the x8 has already glided down to the ground. Do I need to change MINACC down to 2/3 and the. Play around with throttle delay or keep MINACC at 5 and disable the throttle delay (Tkf_thrt_delay)completely. I do not have the depth on the catapult trolley to start the motor prior to launch, I want the throttle to kick in just as it is clear of the catapult. Can anyone help me with some ideas or a good starting point as it just seems to be getting worse and worseThanks in advance.X8 5kg mtow710kv scorpion20000mah14x8 folding prop

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  • Here are some settings I've found to work ok for full auto takeoffs on heavy x8s

    Mode AUTO (obvious, but sometimes overlooked)

    TKOFF_THR_MINSPD 0
    TKOFF_THR_MINACC 5
    TKOFF_THR_DELAY 5
    TKOFF_TDRAG_ELEV 0 (not used)
    TKOFF_TDRAG_SPD1 0 (not used)
    TKOFF_ROTATE_SPD 0
    TKOFF_THR_SLEW 0
    TKOFF_FLAP_PCNT 0

    also, C of G should be exactly on the "dimples" on the wings. And there's some good info here, but I would use a lower value TKOFFTHR_MINACC of about 5 as above.

    http://plane.ardupilot.com/wiki/automatic-takeoff/#catapult_launching

    Hope that helps.

  • Hi Patrick,

    I second the idea of a slightly lower pitch prop, it's like having a lower gear to climb a hill. You may take a hit on cruise speed but it shouldn't be much.

    Dwgsparky is right, a lower angle launch will give you more acceleration, but at the penalty of being closer to the ground. A higher angle will give more clearance but the catapult needs enough pull to deal with this. 20 degrees is good compromise.

    here's a heavy x8 with a short catpult https://www.youtube.com/watch?v=wHVMGpLo104

    Cheers,. 

    • Moderator

      Hi Icarus

      The launch angle can be quite shallow 20 degrees is a bit steep as you are very close to the stall, most catapults are between 10-15 degrees depending on the aircraft, If the aircraft is above flying speed when it leaves the catapult then flying close to the ground does not matter and in some cases it can actually help by using the ground effect to support the aircraft while the speed builds up further. 

      The X8 in the video is being launched on the our ASM300 catapult from  Air-Vision-Air  

      This is being operated in the mountains of Russia and the X8 is heavily loaded at approx 6.5Kg.

      most of the wing is glass fibre skinned to prevent damage on landing. 

      Home
      UAV\'s for photos and mapping
  • 3702179757?profile=original

    This is my setup. Works spot on. The bungee on the ground tended to pull the X8 down. Put it up on a hinged pole. I have the throttle delay at about 1 second but could have it at 3 seconds if i wanted it to. I havnt flown it for a while, i would have to fire it up and check over the settings.

    https://storage.ning.com/topology/rest/1.0/file/get/3702179757?profile=original
  • Moderator
    Not trying to sell you a catapult. Your X8 is overweight and to make it fly you need all the airspeed you can find. Even if you time this correctly you will still never get to flying speed before it touches the ground, you don't have enough thrust to get airborne. You need the prop to run .
    • Hi DWG 

      Sorry, I should have worded that differently, was mot meaning to act as though you were trying to sell one of your catapults. I managed some tests today (15 knot winds though) - With MINACC at 5, Thr_Delay at 8 (8/10ths of a second) - it successfully cleared the catapult and launched although it lost some altitude and was very close to the ground, still successful in getting up though. The weather got even worse so had to call it a day. I will try to reduce the delay to 7 and then 6 and so on to see if we can  get a slightly quicker start. I also need to try some different propeller combinations. TOW was 4.2kgs so I need to also add incremental weight to get it up to 5kgs. I will keep you posted with the results.

      Also, does anyone have some code that will average out amp consumption for the duration of a flight? It would be handy to average out from start to finish (Particularly in very windy conditions) to get an idea of overall efficiency

      Thanks 

      Patrick 

       

      • hi Patrick

        you asked 

        Also, does anyone have some code that will average out amp consumption for the duration of a flight? It would be handy to average out from start to finish (Particularly in very windy conditions) to get an idea of overall efficiency

        The easiest way is to graph the part of a TLOG and look at the average that is calculated - it is visible in the legend at the top of the graph.   That way you can do different parts of a flight at different speeds and compare the efficiency.

        Here is a screen grab of a very variable flight... it was a composite X8 doing a mapping lawnmower  mission flying at 14 or 15 metres per second... you can see the amp draw averaged "659" and maxed out at about "6000". Divide by 100 to get amps... so average 6.59 amps with 60 amps on takeoff and on sharp turns at one end in the wind.

        cheers

        p

        3702736218?profile=original

        mark1 current draw.JPG

      • Moderator

        No problem, I will look at my settings tomorrow. 

        To improve your launch you can also reduce the pitch on your prop. to allow the motor to accelerate faster. The pitch of the catapult is also important, reduce the angle for a faster launch, The easy way for the amps calculation is to take the mAh for the total flight divide by the hours.  ie 6000mah /0.5 hrs flight = 12amps average consumption. The MP can display the mah used on the HUD and this calc can be done anytime in the flight   ie 2000mah after 15 mins = 2000/0.25 = 8amps. 

        Have fun

        • The launcher is sitting at 11.5 Degree pitch

          Thanks for the explanation, I was wondering if there is a script that could do this automatically and give an average reading every few minutes, somthing that takes everything into consideration and averages it out.

          This particular run saw amp draw as low as 3.9amps (Very strong headwind) - I will see how we go with some changes and will update

          • Use an airspeed detection of around 4m/s. Disable all others. No throttle delay. Launch in Auto Mode.

            Works a treat, never had a failed launch over 18 mnths. Remember prop safety though don't arm the pixhawk till just before release. 

            See four launches here...

            https://youtu.be/Lmdekiv3OSQ

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