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May 04, 2018, 07:54 AM
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tekochip's Avatar
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Hobby King Phoenix 2000 Stall Problem


On a few occasions my Hobby King Phoenix 2000 has suddenly pitched nose down and hurdled straight for terra firma. This has always happened while in a stabilized glide (no power) and seemingly, out of nowhere. Iím lazily gliding along on a full battery and suddenly the nose pitches down. I believe the initial upset is caused by strong thermals, but I have never seen the Phoenix pitch nose up, one wing high, or anything other than nose down like a lawn dart. Most flights are long and pleasant, and then comes the Jart flight.

Once on the way down I wait a few seconds for the airspeed to build up and then attempt to recover. Sometimes the craft recovers on its own and levels out, but other times the nose down attitude continues and shows no sign of responding to control input. On one occasion I had pitched nose down on purpose to burn some altitude with the flaperons deployed and it seemed that the craft has a terrible time recovering from a nose down attitude. As if once the nose is down, there is just no way to pull it back up again.

She has crashed three times like a lawn dart, straight out of the sky for a couple hundred feet, and then buried 4Ē in the mud. To her credit, the damage is quite minimal and a few drops of glue later, Iím back in the sky. Iíve simulated radio failure, and the servos never twitch during loss of signal. CG where it belongs, solid hinges, 4 good servos. On one plummet I did drop the flaperons and that seemed to slow the fall, so I donít think itís loss of signal. It always seems to be in response to a strong thermal.

So what do you think, is there something keeping the Phoenix 2000 from recovering from nose down?
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May 04, 2018, 10:10 AM
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Originally Posted by tekochip
So what do you think, is there something keeping the Phoenix 2000 from recovering from nose down?
Your problem is not why it is not recovering. Your problem is why it is pitching nose down in the first place!

You need to do a front to back check of every aspect of your model - linkages, servos, RX aerials, all servo connectors etc. Do it slowly and very carefully, disconnecting all servos wires and reconnecting them firmly to check nothing is wobbly. Test the battery under load and check it has the capacity and current delivery ability needed. Put all the servos on a servo tester and let them run for a few hours to ensure there is no issue there. If your TX has RSSI/signal strength telemetry get it set up so it's logged to the TX for analysis afterwards. Finally do a thorough range test with the model on the floor and TX in range check mode - walk around it 360 degrees and check it passes the test in all orientations. RSSI telemetry is very handy here.

If you do all those things you should identify the problem. If you don't, unfortunately you are at the point of replacing things - I would start with the RX and elevator servo.

Most likely causes - sticky elevator linkage, dodgy elevator servo (dirty pot or similar), damaged elevator or servo horns, damaged RX aerial, poor receiver install (aerials blanked by carbon).
May 04, 2018, 12:45 PM
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I've got about 10 hours of normal flight in her, and the elevator servo was replaced after one of the crashes. The nose down seems to occur in the presence of strong thermals, as if that's the way the bird responds to turbulence.
May 05, 2018, 12:02 AM
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Even if a thermal somehow made the model point straight down, if it's trimmed right it ought to pull out even if you don't do anything. My guess is that stall speed for a model like this ought to be between 20 and 30 feet per second. That means that it ought to be back up to flying speed in a second or so.

I'm not sure what would cause a sudden dive like this unless the c.g. is too far back. However, if the inability to pull up isn't a radio problem, then maybe something is flexing. For instance, if the pushrod isn't well supported enough, it might buckle under compression. An overly flexible tail or wing might do it too, though I suspect that would only be caused by damage. Or maybe you've got a missing tooth on a gear in the elevator servo. Try moving the servo through its full range while putting light finger pressure on it. Doesn't take much pressure, so don't overdo it.

Suggest you start with checks that are easy to do. If you find the problem that way, you may not have to go through the time consuming ones. If this happens, just make sure that the issue you find fully accounts for the problems you've been having. If not, keep going.
May 05, 2018, 06:14 AM
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I think I know what's causing the stall. I live in the very rural plains of Illinois, and this only happens occasionally on gusty days that might spawn dust devils. While gliding very close to stall speed a strong gust comes from behind and stalls the aircraft. Now a brick, I have trouble righting the aircraft in the strong tailwind.
May 06, 2018, 10:27 PM
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If that's what it is, the solution is to fly faster and be prepared to dive a bit to get airspeed back. Seems like it would have to be an awfully nasty dust devil to prevent ANY response to the controls, once you've dived for a couple of seconds.

I once wanted to land a model, but a dust devil kept picking it up. On the 3rd time around, I got too impatient and ended up dorking it much too hard, requiring repairs.

I've seen dust devils take things up violently, haven't seen sink that's quite as intense. Maybe you're encountering micro downbursts, if they exist?
May 07, 2018, 06:05 AM
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I have had a servo that was working fine on the ground, but in the air it did not have enough power.
My Phoenix is in the mail, but if the pushrods are running in a tube, make sure the tube is fixed as it should be. Read somewhere that on a Bix3 you supposedly would lose close to 50% of the power if not, as the rod would flex along the tube.
May 08, 2018, 08:37 PM
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Povlhp I think that's the problem. Once the bird gets nose down the little HS55 doesn't have enough torque to give me any elevator. On one occasion I intentionally was diving and the Phoenix seemed slow to respond to elevator.


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