Marutaka project Black Widow pt 2

[Eric Robson]

continuation of previous post ,                                                                                                                                                                                                            Just after take off on the 3rd flight it started turning to the left as it was climbing, I tried correcting it and then it started to go into a spin  I throttled back but it was too late and to low for recovery. the all up weight is just under 12lb the span is 72" I am sorry I don't know the wing loading.

Edited April 1, 2022 by Eric Robson
added some text.

[Chris Walby]

Thanks Eric, I'll work out the wing loading, but just looking at the numbers supplies it seems ok. The videos show plenty of power although if you limit or cut power on one motor its will generate significant problems. I am a bit of a Mossie nut and quite a few full size were lost on take off if one engine faltered as pilots powered up the remaining engine to compensate. I'll see if I can find the link from the pilot talk about flying the rebuilt NZ Mossie and the biggest concerns with flying it. Stall is about 85knots, but its min air speed on one engine is about 115knots due to drag/lack of prop wash lift etc. If you have height and airspeed then that gives you the time to reconfigure controls to keep going and stay in control, but on take off you have none of those! The full size pilot said his course of action would be to shut the other engine down and land ahead using whatever hedges/fields were there. Then phone the owner and let them know the situation! 

 

Seen a lot of twins spin in with a tip stall, drop wing, but not just pancake which is leading me to thinking mine was a result of reduced power on the port side and C of G still rearward.

 

Thanks for your response and all the best

 

[Eric Robson]

I have just had a PM regarding the Black Widow and revisited this thread. It has had many flights since the rebuild and the problem was a duff battery.

It uses 2 4s batteries one in each nacelle and on  the first flight after rebuild it was fine tracked straight on take off and flew with very little trim adjustment.

The next set of batteries it veered to the left and as the wind was light and straight down the strip I pulled it up and brought it back to take off point, checked the wheels for binding, all free. I then had someone hold the plane and run the motors up one started slowing up quite a bit so I removed the batteries and one was still 90% while the other had dropped to 60%. I replaced the low battery and gave it a good run on the ground, no problems. since then it has been fine the only problem is getting decent piano wire for the undercarriage legs which have to be straightened at regular intervals, Most of the piano wire nowadays can be cut with a junior hacksaw whereas it had to be cut with a grinder.  

Edited December 10, 2022 by Eric Robson

[Mark Turner 12]

Could you run a pair of wires so that you could in future connect two batteries in parallel so you create effectively one large battery which would always allow equal power for each motor even if one battery was not quite up to scratch?

[Eric Robson]

 Hi Mark,

had I known it was the battery at the rebuild I would have done that but now it would be a major task,  

[Chris Walby]

10 hours ago, Mark Turner 12 said:

Could you run a pair of wires so that you could in future connect two batteries in parallel so you create effectively one large battery which would always allow equal power for each motor even if one battery was not quite up to scratch?

I don't think that will work. A recent experience with my Grumpy Tiger Cub might help to explain.  Been flying my GTC with the original rather tied Nanotec (4S3300) lipos + my GNB (4S4000), lipos that give it more noticeable top end + the GNB are a tad lighter, but same physical size.

 

Few weeks ago did all the usual pre-flight checks (Nanotec displaying 98% charge) and proceeded to taxi and take off. Once up I gave it full power and one ESC went into LVC (limp mode, 0 to about 50% power) so circled and landed with no drama (Peter Miller did say it would fly on one motor no bother and he was right!). Disconnected the lipo and checked it over on the bench and all seemed ok, but this time did a full power check on the deck and one ESC did the same (LVC). I am not one for picking things apart at the field so just put it to one side and flew another model.

 

A couple of days later I twigged what had happened. As the GTC has one battery and two ESC's in the nacelles  supplied with nice big cables the battery voltage sags under load. Now here is the twist to the issue and explains what happened, as the load increased so the battery voltage sagged. But once the battery voltage sagged to the point of the first ESC to go in to limp mode the limp/reduced power, so the voltage would recover (less total power drawn from the battery). This would mean that the other ESC would not go into limp mode. Changed the battery and problem went away (checked lipo and one cell was under par).

 

So for Eric's balance lead to work its going to have to be a very large cable and IMO even that would not work as any volt drop would cause one ESC to go into LVC before the other. At worst the balance lead would mask a poorly lipo and then load the other accordingly with the result that you think you have say 8 mins flight time but the reality is you have only 4 min. 

 

[Eric Robson]

You are right Chris , a low cell in a battery would pull the others down, I have had a recent case with a Parkzone SE 5. 

I have 3 Overlander 3 s 2200 all bought at the same time and treat in the same manner. They have had a lot of use but 2 weeks ago I was flying the Se 5 when it started making a sound like a duck, it also slowed up so I brought it down thinking something was catching all seemed clear, I held it and gave WOT it was fine, the battery was checked 85% remaining so it took off again and on climbing the sound came back, this time it was loosing control on power so I got it down ASP, the battery was still showing 0ver 60% so the conclusions were the esc or motor was at fault. At home I tried another battery and it seemed OK so all of the batteries were charged and I marked the suspect one, next day I used a different make of battery and all went well after 5 flights I put the suspect battery in and despite being fully charged it lasted 2 mins. before the loss of power showed,  I had tried to recover the battery by cycling but to no avail it is now scrapped (properly)