Flightline 1.2m Spitfire Odd Behaviour on Maiden. Bad Crash.

[Stuart Quinn-Harvie 1]

Hi Folks, I wonder if anyone can shed any light on the crash I had with this model this weekend just gone. 

This was the model's maiden. The aeroplane was range checked and CG checked, run with a 4 cell battery ( 2700 ) using an FRsky i6 TX and matching RX. The balance point was checked with a cg machine with the aircraft upside down on the cg, and the same battery installed. It balanced slightly nose down at the point indicated by the manual ( 73mm from LE root )  

 

Control throws were as described by the manual, and I dialed in 40 per cent expo ( that's standard for me ) 

 

On takeoff the aeroplane shot upwards, and after some stick thrashing settled down for about 10 seconds, then began to respond to inputs very late and very sluggishly. I landed at the fist opportunity thinking that the battery might have come loose and might be sliding around the battery compartment. ( it literally looked as though some weight was moving back and forth inside the fuselage )  Landing was rough but I just got it down in one piece. 

Battery ( and RX /airframe)  was checked and resecured despite looking ok and a very careful second flight attempted. On this occasion the takeoff was better but after that the model was simply uncontrollable, after turning right it executed two independent loops, turned ( again late ) very steeply then I am assuming that the failsafe operated as it went into a steep right hand spiral dive with the motor off and dug a hole in the field. 

 

HAs anyone come across this kind of loss of control before? The only thing I can think of is a faulty RX or ESC. The TX was and is fine, and is used regularly for my other models and has shown no sign of poor behaviour.  One odd thing was that on set up, I checked the retracts and on throwing the switch, the wheels went up ( good) and the motor went on full throttle ) not so good. Fortunately the model was restrained. The problem disappeared when I reassigned the retracts to a different switch. 

 

Finally, the model was not fitted with a gyro. 

I would appreciate any insight that anyone might be able to offer. 

 

 

Edited April 15, 2021 by Stuart Quinn-Harvie 1
To add that the crash was on maiden.

[MattyB]

What is the BEC rating on the ESC? Sound to me like an RC power supply issue is the most likely cause... 

 

PS - Do you mean it was a Flsky i6 TX rather than Frsky?

Edited April 15, 2021 by MattyB

[Stuart Quinn-Harvie 1]

It's the factory installed one - 60 amps. I have to admit that that is what I was thinking as well, it's rated for 2 to 4s and the battery was 4s. 

[Stuart Quinn-Harvie 1]

Ah yes you are absolutely right, the TX is a Fly SkyFS I6S. The RX was/is Fly Sky FS IA 10B. 

 

And I've just edited the post to indicate that this was a maiden flight. 

 

 

[Piers Bowlan]

Just a couple of thoughts... Presumably the model was range checked before the maiden flight but was it ranged checked with the motor running. I just wondered if a faulty BEC/ESC is not supplying the receiver properly causing it to brown out when the motor is producing high power? Also, could faulty retracts drawing high current when retracted cause a voltage drop to the receiver (brown out again). What is the make of the model, ESC, retracts?

[Jon H]

Much of what you describe sounds like the expo. 40% is shed loads and i had a similar experience with a friends hangar 9 spitfire which at the time was up at 60% expo. I would knock all the expo out and drop your lowest rate by 20% and try again. 

[Shaun Walsh]

Was the correct direction of expo applied? If it was positive rather than negative it would have made the model very twitchy especially if the pilot was used to negative expo. 

[kc]

First of all commiserations on wrecking a fairly expensive model.    The fact that the retract switch caused full throttle suggests some mix or switch asignment was already in the Tx memory and should be checked to see if any more unknown mixes are there.  (Presumably the switch may have been arranged to be a throttle cut )

Either the motor cut because of failsafe due to signal loss or perhaps it cut due to too much current  being drawn or low battery.   I would check to see if all the controls are free enough not to cause a servo to use excessive current draw.   Are the servos man enough for that plane anyway?

 

[MattyB]

2 hours ago, Stuart Quinn-Harvie 1 said:

It's the factory installed one - 60 amps. I have to admit that that is what I was thinking as well, it's rated for 2 to 4s and the battery was 4s. 

No, the BEC is not rated at 60A - that is the ESC’s max current that can be pulled through it to the motor. The BEC is the part that powers the receiver and servos, and in many ESCs they may only have a 3-4A rating. My reasoning is that if your retracts and servos move together they could easily exceed this, which could then cause the voltage to dip at the RX and (maybe) explain the control issues you saw.

[Andy Gates]

I assume that you did not retract the undercarriage on either flight?

If you did not, then the BEC in the ESC should be up to the job even if they are small digital servos.

 

How was the range check executed, was the machine rolled around in the air to check there were no dead orientations?

Was the motor running during the range check?

 

Things I would check in the order I would check them...

1 - model memory used - correct one and has that memory been used before and some of the previous settings left over

2 - rebind and get the retract / motor issue resolved

3 - check antennae connections in the Rx - make sure they are fully connected and protected with paint / nail varnish so they can't move with vibration

4 - antennae positioning - are they running alongside other wires, metal or carbon fibre servo pushrods or being shielded by something metallic / carbon fibre

5 - recheck all soldered battery / ESC / motor connections - could there be a dry or poor joint cutting power off intermittently?

5 - full range check with motor running at different speeds with the plane in different orientations.

 

Good luck, let us know how you get on

[Stuart Quinn-Harvie 1]

Thanks for the advice folks: the Expo thing sounds like a good lead. 

Andy - to answer your questions: 

 

Firstly, you're right, I left the undercart where it was ( down) on both flights. 

 

No, I didn't roll the machine to check orientation dead spots - thanks for clueing me in on that. 

 

1   Model memory was a fresh one. ( I have only had two models on the TX before this one on memory three. ) 

 

2 Retract and motor issue was resolved by reassigning retracts. 

 

3 Will do. 

 

4 It's tight in there, from memory they were oriented ok if not exactly at 90 degrees to each other. 

 

5 Will do. 

 

5 Will do. 

 

Thanks again folks, this has given me some stuff to chew on. 

Edited April 15, 2021 by Stuart Quinn-Harvie 1
additional info

[Stuart Quinn-Harvie 1]

Additional: 

The model IS fixable, but it will need a full respray after I have re built and checked it. I'll respray it when it behaves ? ( Or rather, when I can get it to behave. ) 

[Nigel R]

15 hours ago, Stuart Quinn-Harvie 1 said:

after that the model was simply uncontrollable, after turning right it executed two independent loops,

It did that by itself, or, you flew the loops?

[Nigel R]

That retract problem. 

 

I would suggest in future that kind of issue is a no go for flying. Everything works, as it should, or don't fly. 

[Jon H]

Although not related to the current issue I would also recommend retracting the gear asap when test flying a warbird. The drag of the gear and its effect on c/g can make a big difference to the handling. Also if its all going wrong and you need to put it down straight ahead in the field, damage will be minimised with the gear up. 

[Stuart Quinn-Harvie 1]

Nigel and Jon: Noted - thanks. 

Nigel: Independently - I wonder if this was a result of ( now that I have checked it ) the massive positive expo ( which, frankly it never occurred to me to think about  - I therefore feel suitably idiotic ) and some the model naturally being slightly nose heavy, thus me pullling a "tiny" bit of up in. 

[leccyflyer]

Putting in 40%  expo in the wrong sense would certainly explain the erratic flight behaviour that you describe, making the sticks super sensitive around their centres. I typically fly such models with 40% "soft" expo and they are good as gold, so there's nothing wrong with a nice dose of appropriate expo. It doesn't help that different manufacturers use a different convention for expo - Spectrum has +ve expo for a soft centre, whilst Futaba use -ve expo for a soft centre.

[Graham Davies 3]

Bad luck old boy.

 

I think others may echo this, but I would steer clear of expo for maidens. You need to know what's really going on at early stages, and the needs for expo are not the same on all models. If you have a model with very soft aileron response and you start with a soft expo, you may find you are putting in huge inputs to achieve small effects. This results in you being in the 'high geared' section of movement, which is the dead opposite of what you are trying to achieve with Expo.

 

One thing; you describe the initial crash as a spiral dive. It sounds like it has stalled. If this occurred whilst it was still 'looping', do you recall when the engine cut? It sounds like the model found itself at a high angle of attack, with up elevator applied. Even with power, this is a stall situation. The spiral may have been the result of any reduction in airspeed stalling one wing. Tip stall and then the inevitable.

 

I would also have been very concerned by the gear/ throttle interaction. Having just survived a near disastrous 'accidental mix' issue just this week, I would consider this a risk situation. Changing the switch may have solved that particular problem, but left another one somewhere else.

 

We are of course, all armchair experts after the event! Hopefully there's something in here that will help you when the glue has set.

 

Good luck, let us know how you get on

 

Graham

[Stuart Quinn-Harvie 1]

Thanks folks. it is looking more and more like the expo issue. I should have the model ready to test fly again by Monday. 

I will be blanking the memory and re setting up the whole model, anc being careful to make sure that there is no expo to start with, as well as checking all soldered joints, and adjusting my range check procedure. I really want to thank everyone for your advice - I shall be updating the thread as soon as I have some more info. 

 

Cheers again! 

[GrumpyGnome]

Good luck with the re-maiden!

 

GG