A bit of a head scratcher.
|Biggles' Elder Brother - Moderator||08/02/2018 10:23:12|
15748 forum posts
I had a bit of a weird experience last weekend and as I'm struggling to explain it I thought I'd share it with you wise lot and see if anyone can come with a credible explanation.
I have a ARTF WOT4 which I'm using as a winter hack. Its relatively new but established, the Tx timer tells me its done 217mins, so I reckon that's about 20 flights or so, which matches what I would have guessed at.
First flight on Sunday afternoon. I do all the pre-flights, all seems well. Line up for take off and open the taps. She accelerates briskly away as usual, I ease back on the stick - nothing. No rotation. I pull back on the stick, still nothing. She's behaving like an RC car and showing no interest at all in becoming airborne! I have other controls, but no response to elevator.
At this point I make a mistake - I'm so wrapped up in "why the hell won't this rotate?" that I don't notice the runway end rapidly approaching! She goes "smack" straight into some very dense rough. Oh dear.
Quick examination shows the damage is not too bad. The undercarriage plate has pulled out - but its a clean break. The prop is broken. One of the mounting blocks for the cowl has come loose. I got off pretty lighly.
So I put it in the car and we go home.
I always leave repairs a couple of days - I find I have a lot more motivation if I let the dust settle as it were! So yesterday I get it out on the bench. Two objectives; do the repairs of course, but equally important try to figure out what happened.
Now I full expect to find that for some reason the elevator has stopped working - that's the obvious explanation - either the servo has packed in.or the gears stripped, or the linkage come loose etc. For whatever reason I expected it to be likely I would have no elevator.
Visually the elevator looked fine - everything was in place. I connected up a small external ESC to just power the servos and check them out. Everything was fine. Smooth, full range, operation. All the rates are fine too.
I note that the battery tray has broken loose, but surely that is most likely to have happened in the impact at the end of the runway? But suppose it didn't? Suppose it happened during the take off run - unlikely but possible. Could the battery have shifted forwards, moving the CoG so far forwards I couldn't rotate?
Two things argue against this; firstly its a tail dragger so the battery would have had to slip "uphill" - why should it do that? Secondly, there was only about 1-1.5cms of space in front of the battery, if it did slide forward then that is far as it can go and I doubt that is enough to shift the CoG far enough to give me that big a problem.
For all its faults the shifting battery theory is best I have been able to come up with so far. So thinking caps on chaps - anyone got any other suggestions for the cause?
|Simon Feather||08/02/2018 10:26:01|
237 forum posts
local gravity well?
|Simon Feather||08/02/2018 10:37:02|
237 forum posts
... seriously though, I can't see that if the battery moved only 1.5cm it would have a significant effect. Presumably your pre-flights included all the usual control surface movement checks (all surfaces moving the RIGHT way).
We have a simulator in our club hut, and last week the normal transmitter wasn't available, so they tried it on a different make; the net effect was that the ailerons and elevator were both reversed. That was entertaining - but one thing I noticed, trying to take off a taildragger using the normal "hold full up in then let it out gradually" wasn't working - as you say, I had a racing car, not an aeroplane. Once I realised and stopped relying on muscle memory, I did for fun do a takeoff, circuit and landing with aileron and elevator reversed, but it took quite a lot of concentration.
So, it *could* be a reversed elevator perhaps?
|J D 8||08/02/2018 11:04:25|
1320 forum posts
How cold was it when you went flying BEB ? And what surface?And was it wet?
|Colin Carpenter||08/02/2018 11:24:08|
|577 forum posts|
Soaking wet , boggy grass runway ? I've seen this with my Sebarts ! Colin
|Ron Gray||08/02/2018 11:32:04|
|1553 forum posts|
I had a similar experience the other day, notbwith a Wotty but with my Easy Street. I had replaced the esc with a new one which was very slightly heavier than the old one. I had positioned it slightly further back on the fus to compensate for the extra weight. On roll out it didn’t want to lift off but with full up it did unstick, just. It then proceeded to fly about a foot off the ground until I switched to high rates to give more elevator control. One hairy circuit and landing later I decided that the cog was obviously wrong so back at home I moved the esc further back by about 20mm. Took it back up the field and it was back to normal flying. In my case, the new esc weighs about ¼ oz more than the old one and that makes a significant difference to the handling, so it could well be the battery position.
|Martyn K||08/02/2018 11:45:30|
5005 forum posts
I had an interesting and exact opposite problem with my Taranis BEB.
The model (a Meteor 40) has been very reliable and I have had many flights with it.
Just before I last flew it, I spotted that I had a duplicate (different) model in my Tx list. To be safe I deleted it. I then prepped the Meteor and did the usual pre-flight checks and all appeared OK.
Started up, full throttle check, waggled the stick and everything moved in the right direction.
Started the take off run and the model leapt into the sky - way over elevated. This was confirmed by the next couple of minutes of flying. Rather concerned that something had moved, I then killed the throttle using the programmed throttle kill switch - and the model went into a no elevator dive and got badly damaged on the arrival.
It appears that somehow I had managed to get a mix of throttle and elevator. It appears that hitting the throttle kill switch had put an override in on throttle and also elevator. Unfortunately, the model was too badly damaged to prove this theory at the time and I haven't been able to replicate it since. However, I have since wiped the transmitter and copied all the model back into memory and it seems ok again.
The only thing I can think of is that deleting that model from memory caused some form of corruption.
|2810 forum posts|
Intermittent fault on elevator servo/connectors? Change it out just in case?
If the model has flown OK up to now, then I'd say that excludes mechanical set-up, CG, incidences etc. If all other channels are OK, then radio seems OK. Unlikely to be the TX as I guess that you fly other models with no similar problem. I'd go for the elevator servo/wiring/ something jamming it intermittently. Had something similar on a mate's model with a dodgy rudder servo.
Edited By Cuban8 on 08/02/2018 12:10:56
|Martyn K||08/02/2018 12:08:46|
5005 forum posts
The connections were all OK. The servo tray epoxied to the g/f fus side had become dislodged but everything was still plugged in. I originally thought it may some form of cross coupling with the servo cables but it seems OK when I tested it at home.
|Don Fry||08/02/2018 12:29:10|
4150 forum posts
BEB, I take it the tail lifted, and it was running on the main wheels. Also, when you roll to inverted, is the forward stick; a hint, or a push. If a push, it might be a CG problem with a "normal forward" CG compounded by the battery moved forward. Running on its wheels, tail up, the battery would not be moving uphill.
But it's either CG, or a temporary elevator servo problem.
Peace of mind says change the servo, glue the tray back. Offer dues and homage to Gravity, and try again.
|Concorde Speedbird||08/02/2018 13:06:21|
2720 forum posts
I've noticed with my Taranis that if you don't set rates on all positions of a three position switch, then one position cuts off all control from that channel. Not suggesting this is the case, but it is an important point with open source Tx's.
So you had sufficient airspeed (presumably) but insufficient angle of attack to fly. Wot 4s have slightly semi-symmetrical wing sections which suggests there may have been a slight bit of down elevator/pitch down. My Wot 4s have always got off the ground incredibly quickly, further providing evidence of this.
Ground conditions- grass length, dampness- make a huge difference I have noticed, as others have mentioned.
Is the thrustline correct?
I am not sure just the C of G moving slightly forwards could create enough stability in pitch to remove all the pitch authority, presuming you run quite high rates like I do on Foss aeroplanes, and I would have thought a nose over would occur is the C of G was this far forwards.
Could you see if the elevator was responding during the take off roll? I often glance it when I take off.
|The Wright Stuff||08/02/2018 13:15:30|
1381 forum posts
BEB, in the spirit of brainstorming, just to add another possibility into the mix:
Is it possible that the aircraft wasn't going as fast as you thought it was, due (possibly) to some illusion, trick of the light, etc? If the grass was wet and/or there was a lack of amps from the battery due to the cold, then it could have never achieved flying speed.
The elevator remains a possibility, but I think I join the others in thinking the shift of battery position unlikely to be the sole cause. If the battery moved 1.5 cm, then the CoG probably only moved a few mm. You could of course check it by putting the battery all the way forward and balancing it, but I would have thought it would make little practical difference for a plane of this type, i.e. within the normal range of operation.
Whatever the outcome, let us know the conclusion - we're all on tenterhooks!!!
21 forum posts
|Possibly a very cold battery, not delivering enough power.as you apply up elevator the tail goes down, drag goes up, model doesn't.|
1468 forum posts
Servo tray loose
|2810 forum posts|
Having made a living before I retired of repairing mechanical/electronic industrial equipment, I know to my cost that very often, one over-thinks matters and delves too deep, sometimes making the situation even more unclear and difficult to resolve quickly. One of my old colleagues, would steadfastly refuse to look at drawings/wiring diagrams/manuals until he'd gone over a fault using eyes only for some time, and more often or not, he'd find a broken wire, concealed smashed switch, bit of bubble gum covering a sensor (bored operator), usually something simple but manifesting itself as a much more complex fault.
Could be a weird one, but I'd be very surprised if BEB's problem model has something super complex - mind you, that doesn't mean it'll be easy to find.
|Richard Ashworth||08/02/2018 13:49:56|
|88 forum posts|
Are you sure that the wing bolts were fully fastened?
I had a similar "racing car' experience with a foamy Wot4 (first flight of day) where I had part tightened the wing bolt (only a third) to check the ailerons were OK with the wing "on" as one connector had been difficult to connect, all worked ok but forgot to fully tighten the bolt.
Racing car, full elevator, 6 ft up then full power dive into terra firma. New fuz.
Only worked it out 6 hrs later sitting at home trying to work out what happened and remembered a change to the usual pre flight routine.
Less likely with two bolts on the ARTF Wot4 but they normally lift off nose up in feet.
|1395 forum posts|
This is not strictly true, you do not need to set rates on all positions, however if not set up the line order correctly it can default to no rates.
You need to read the "Getting Started" section of this documentation. It is explained fully there.
|1395 forum posts|
You can go back and look at your previous copies of the model files, use the simulator to see exactly what happened.
You do use the Companion to do regular backups of your model files every time you change everything.....
|Mike Blandford||08/02/2018 15:27:48|
544 forum posts
I'll ask a silly question - You were trying to take off into wind, not downwind?
|Martyn K||08/02/2018 15:46:03|
5005 forum posts
Yes - I have done that and also checked it using Companion. There was definitely no obvious programmed mix between Throttle > Elevator - but there was one - or something very similar in flight. It could have been a receiver fault but I wouldn't have thought that would have been the case. There definitely wasn't a mix between Elevator > Throttle or I would have noted the change in engine note when it was on the ground.
Yes - everything backed up regularly as well - and version controlled
Edited By Martyn K on 08/02/2018 15:46:51
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