300km

Anatomy of a 200km ...crash

I've got a Cyclops EPO that I'm setting up with a view to attempting a 200km flight (lovely plane, I'm sure it's capable of it), but I've crashed it twice out of three outings, and I've got no idea what's going wrong.

The maiden flight in manual mode was a success (it came back in one piece), but the test pilot said it was pretty lively and had trouble trimming it.

The second flight was in manual mode too, it flew a circuit or two before it banked hard right and spiralled into the ground. The speed was about 90 km/hr so it wasn't a stall. The pilot said it just stopped responding to his input, we switched to Stabilize after he realized he'd lost it, but it made no difference.

3691104277?profile=original

The telemetry logs show a large right aileron input (green trace, see the white asterisk on the graph, high=right) to level it after a left turn, but it kept rolling right and didn't respond to the subsequent left input. The trace also shows some big right inputs, I don't know if that is panicked pilot input or a dodgy radio/connection. You can see the aileron trace go low when I switched to Stabilize, but it didn’t make any difference to the roll (red).

 

I repaired it and tested everything I could think of and found nothing unusual, so I flew again, this time in FBW A mode. It took off and flew very smoothly, but I didn't have much authority. Up elevator input barely made a difference, and it only turned very gently when I gave it right aileron and rudder. Then it never really levelled out. The right roll kept increasing (red trace below). I gave it some left aileron (I'm not sure that I gave it full left as indicated on the trace), which didn’t do much, and then it banked right and hit the deck.

3691104408?profile=original

 

In both cases it rolled to the right. In the first case it was like it kept rolling after a large right input, and didn’t react to the left input corrections. In the second case it flew the whole flight with too much right roll and didn’t react to the correction from the APM or from me. It feels to me like an aileron servo problem (failure, loose connection, brownout, jam, insufficient power).

Any insight or expert log analysis would be very much appreciated.

 

After two weeks of late nights building then repairing and analysing logs, I’m a bit burnt out and it seems hopeless rebuilding it if I haven’t found the problem.

 

Some of the tests I did:

  • Removed the power module, replaced JP4 and powered the whole thing from the UBEC. I monitored VCC while I moved and loaded the servos to see if the was any problem with the UBEC voltage/connection. It stayed at 5.0 V +/- 0.2V.
  • Moving servos through their full range, and jiggling all the power/servo connections to see if they would fail, loading the wing to simulate the load in flight.

 

Things I haven’t tested:

  • Anything at high power. Perhaps the ESC or motor caused interference. They both sit about 4 inches above the APM.
  • I don’t know if there is any telemetry record of the servo rail voltage if JP1 is removed?

 

Materials and specs:

  • Most gear used successfully in my Maja up until now.
  • DX8 radio, Orange DSM2 receiver with one satellite (range tested to 30m before flight)
  • APM 2.5, firmware 2.78, 3DRpower module (JP1 removed)
  • Turnigy 3A low noise UBEC powering servo rail
  • Turnigy D-Lux 30A ESC with red BEC wire disconnected
  • Hacker A30-52 motor
  • Vtail – mixing done in APM, RC is setup as T tail (according to instructions here)
  • Hobbyking telemetry module
  • Hitec HS-65HB servos x 6 (ailerons, flaps, v-tail)

 

Considerations/Complications

  • I have combined the power supply wiring for the flaps and aileron servos in each wing, and in the v-tail. Both get their power supply from the same red and one black wire, each have their own signal wire. Is one 26g wire enough to supply two HS-65HB 11g servos?
  • When I first connected everything I plugged the UBEC the wrong way around on the servo rail for 30 seconds or so. Could that have fried something? It "appeared" to work fine after I corrected that. Is there a way to check?
  • There is a large amplitude v-tail servo twitch that I get when I have the flaps switch in position 2 (50% flaps, no Elevator) if I have the elevator trim switch at 2 or 3 clicks of up elevator. Bizzare, it doesn’t happen at any other time. I still have normal manual control via the sticks; it goes away as soon as I move them more than a few mm.

Three log files attached from the three flights.

2014-03-09 10-43-33.tlog

2014-03-09 11-35-12.tlog

2014-03-13 07-27-49.tlog

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Replies

  • 300km

    IT FLIES!!!   I've solved my gremlin. 

    Thank you to everybody that offered help on this issue. 

    I started with a couple of hand launches without the motor to see how it glided, and it was perfect. Then I did one at 50% throttle just cruising a few metres above the runway in Manual mode and it was perfect. I didn't even have to touch the aileron stick.

    Then I flew it properly and it was effortless. Manual needed a little up elevator trim, but I didn't have to touch the aileron trim at all. I couldn't believe the difference.

    I didn't fly it fast though. I kept at 50-65% throttle. In auto mode I flew a few circuits and it handled well at all the speeds I tested from 11 up to 18ms. (40 to 65 km/hr)

    I probably had over an hour in the air, two battery packs. Cruising amperage is very low, as low as 2.5amps (on a 3S Lipo).

    So thanks again to everybody that helped.

    Now I've got a bit of tuning to do before I know if I'm ready for a 200km attempt.

    • If you didn't have to trim aileron from hand launch to flight speed it won't need any.  Only the elevator will change with speed.  Congrats!  Did you end up straightening the wing?

      • 300km

        Yes I straightened it, I twisted it by hand in the direction I wanted, then ironed it top and bottom and left it to cool. I had to do that twice and then it was perfect. That was last night, I checked it again and it was still perfect. Thanks for your help

  • 300km

    By way of an update, I haven't had much time to work on my plane lately. I've got it fully repaired so I could fly again, but I'm a bit nervous as I haven't found a smoking gun. There have been lots of good suggestions, but no smoking gun has been discovered. One crash each in a Manual and FBWA flight. The first one was quite aggressive flying (fast, steep turns) and the other was very gentle. In both cases the log shows the correct inputs and outputs were given to the ailerons and in both cases it rolled steeply to the right and hit the deck. Although the FBWA flight it pitched down and lost height BEFORE it rolled right.

    One thing I don't think I mentioned above was that the aileron trim was about 1/2 way to the left so there wouldn't have been as much authority to the left as to the right. I've adjusted the pushrods and brought the trim tab back to the centre.

    In addition I've:

    • Tested for interference by running it at full throttle on the bench and doing regular control surface movements.
    • Twisted the power cable between the battery and ESC, on the UBEC and the aileron/flap servo cables in the middle section of the wing.
    • Stiffened the wing with a T bar through the main carbon spar, and laminated the wing.
    • Planned to fly at a lower speed.
    • Increased roll and pitch P settings (that couldn't have been a factor in the first crash which was Manual mode)
    • Found the cause of the servo twitch (the very last bullet point in the original post). It was a DX8 firmware issue.
    • Setup the radio from scratch (previously I copied the settings from my Maja, and changed what I needed).

    I've got three keychain cameras that I'm going to mount to look at the aileron and tail control surfaces. That will tell me whether they are working properly in flight. I think I'll do a few hand launches and short glides first to see if there is anything funny to see before attempting a proper powered flight.

    I'm thinking about disconnecting the flap servo cables and just taping the flaps stationary, and switching to a T-tail in the interests of simplicity. I don't have any spare servos, or control horns on hand, so I'll have to order them in, and have the patience to wait.

    • If your ailerons are not equal in deflection after trimming something isn't straight.  You can only trim for one speed.  Below that speed the trim will be not enough, and above that speed the trim will be too much.  Also the wing with the up trimmed aileron will stall first.  Is it possible that the v tail is not mounted straight?  That will lead to a constant roll that most people use the ailerons to trim out.  Is it possible that the wing is warped?  Not in flight but sitting on the ground.  Back in the day when planes were mainly balsa you had to make sure the wing wasn't warped after it was covered.  The covering would shrink unevenly and twist the wing.   Depending on the airfoil cross section its an easy check.  Lay it on or next to a flat surface and check that it has equal distance at all points both on leading and trailing edges.  Is the angle of the tail set properly?  Is the angle of incidence correct between the wing/fuse and tail?  Do you have enough elevator throw and or surface area?  The last one I can think of is weight.  Is it possible the aircraft is overloaded?  Overloaded planes often fly like yours did.  The wing is partially stalled because the angle of attack has to be so high to overcome the extra weight it wasn't designed to carry.  It leads to a very fast forward speed, bad elevator authority, and dropping a wing when you put aileron input in.  That tip portion stalls first or is a bit more deeply stalled then the rest of the wing.  Produces less lift and that wing drops.  Banking compounds the issue because it requires more total lift (higher angle of attack) to keep the plane level.  

      • 300km

        Hi jg

        I placed my wings on blocks to give the servos and control horns some clearance and measured the distance from the middle of the leading edge to the block, and did the same for the trailing edge. Here are the results.

         

        left wing right wing
        Outside Middle Inside Inside Middle Outside
        Leading edge 9 10 9 11 11 7.5
        Trailing edge 5 6 4 2.5 6 6

        The left wing is within a few mm, but the right wing has a down turned leading edge at the outside relative to the rest of the wing, and the opposite wing. I don't know if it was like that before I applied the laminate. If it was then it would explain why I need to trim the ailerons to the left.

        Any suggestions how I could try to straighten it. I've put a 10mm stack of business cards under the leading edge at the outside, and placed a 5000mah battery on the trailing edge, and leading edge of the inside. I'll leave it for a few hours to see if it makes any difference. I've got a feeling I probably need to apply some fiber tape on the diagonal to keep it there.

        The weight definitely isn't the problem. It's a very light plane, and I have just the battery I need to balance C of G on the spar.

        • Well I can't say for sure that is a smoking gun, but I would say those measurements would be a smoldering fire at the very least.  That would cause a roll to the right and it would get worse as the speed increased.  Fixing it is easy.  You can either hold it in position and use an iron or heat gun and shrink the laminate to hold in in place.  Or like you have done hold it down and then shrink the laminate again.  You need to shrink on the opposite side of the twist to pull it back.  It is basically trial and error until you learn how to do it better.  I normally hold one tip down or wing half with something heavier, and twist the wing with my hand until its straight.  Then use the heat gun or iron to strategically shrink the covering to hold it in place.  If you have the sticky tape laminate, you may need to redo it.  Or you could try and "pull" the wing straight with a few well placed pieces of packing tape.  

        • Moderator

          A little judicious application of heat from a heat gun can help fix a bit of wing twist. I put a weight on and then blow a little heat and let the whole thing cool down. Check and re-do if needed.

      • 300km

        The ailerons weren't level after trimming, the right was about 1-2mm down, and the left 1-2mm up.

        The Vtail wasn't perfectly straight, the tip of the left side was about 2cm higher. That would give some unwanted yaw to the right when I give up elevator (but 2cm isn't much, I don't think it would have been a big effect, but it was an effect in the direction that it crashed in). I have fixed that now, but I'm thinking about switching to the T-tail because it seems much simpler.

        I'll have a look at your other suggestions tonight when I have some time. Thanks for your ideas. 

        • 100KM

          Did you have any Rudder-Aileron mixing? (APM or Tx)

This reply was deleted.

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