An Eleccy WOT4XL - with a twist!

[Geoff S]

I put the 'arming' plug in one of the motor connectors on my DB Gypsy (Cirrus) Moth after much thought (and a bit of discussion here) because to put it in the motor connection would have necessitated extending the battery wires quite a bit. It works a treat with no problems at all and it allows me to connect the battery safely and check the controls before arming the motor.

On a couple of occasions I've forgotten to connect the arming link and wondered why the motor only stuttered so it works as it should. The run attempt was very brief (2 seconds tops) so I can't see that it's likely to cause any damage.

I'm not a huge fan of arming plugs when the battery connection itself is as easy to access as an arming plug might be and I have a motor disable switch (with a spoken warning) programmed into my transmitters.

I like the neat battery mounting. Velcro holds batteries very firmly ... but often so firmly they seem to make every effort to become a permanent feature

Geoff

[Biggles' Elder Brother - Moderator]

Posted by Geoff Sleath on 25/09/2018 18:06:27:

I like the neat battery mounting. Velcro holds batteries very firmly ... but often so firmly they seem to make every effort to become a permanent feature

Geoff

Don't I know it! Especially if you can only get your fingers into the hatchway, not your whole hand so you can't really get a good grip on it! That is why, on the bottom I've used non-slip matting rather than Velcro, it stops the battery sliding very effectively while the Velcro strap holds it down against the mat basically. Its an arrangement I've used in a number of models - so far sucessfully! (Fingers crossed!)

BEB

[Biggles' Elder Brother - Moderator]

Today has been all about the catches on the hatch. A bit of a fiddly job of course, because you have to get everything aligned correcty so the hatch will sit flat and flush with its surroundings.

I used two cocktail sticks - cut off to about 6mm length protruding - at the front and a single sprung catch at the rear.

The balsa in this area is quite soft so I decided to line the locating holes with brass tube that was epoxied in:

A rebated channel was cut in the rear to accept the sprung catch:

Again the catch was epoxied in and then covered over with a bit of scrap balsa. I coated the moving parts of the catch with vaseline before hand to stop any glue that "strayed" from completely gumming up the mechanism! And again a brass tube was let into the balsa to receive the pin.

So here's the final product with the hatch in place:

I need to iron the black covering back into place! Its lifting at the edges due to some "rugged" handling! That exposes "woody bits" which makes the hatch look a poor fit - I can assure you its not, look at this close up:

So why have I fixed a washer to the catch lever? Because the hatch is such a snug fit I can't get it off with just the smooth lever - my fingers slip off it! I need someting to get my fingernails under!

Right, just the aileron servos to do. They were on soak test today - passed fine - so onward and upward!

BEB

[Devcon1]

Boy are you quick at this malarky BEB. I wish I could progress so quickly.

I will copy the arming solution.

[Biggles' Elder Brother - Moderator]

Hi, well she's 'finished',..

I had limited time today - just an hour or so in the morning. So it was a quick "weight-in" and check the balance.

What's the final weight? It's coming in at 4.19Kg with a battery on board. That compares with 4.8Kg original. We've saved 610g (about 1lb 6oz). The majority of that of course being the lead that has been removed. This new weight rings the wing loading down to 23 oz/sqft, which bodes well for general agility.

What about the balance, are we there? Well, no, but we are very close. The recommended CoG position is between 100 and 105mm behind the leading edge of the wing at the root, given the conservative nature of most manufacturer's recommendations in this area I will go for 105mm. We are at around 111mm or approximately 6mm to far back. That's close, TBH I'm almost tempted to fly it there as my experience with the Ripmax WOT4 was that, at the recommended CoG I felt the plane was nose heavy and I actual have my CoG on that model perhaps 5mm back from that recommendation, where i think it feels much better and more responsive. But I'm also a coward! So I'll stick at 105mm for the XL - at least for the first few flights.

Before I rush to add some lead to the nose - albeit a modest amount - I do have two tricks up my sleeve here that might make the shortfall even smaller:

1. When I weighed the model I was very keen that, as far as possible, it should a fair like-for-like comparison. So I used the same battery - the one that came with the model. But that is only a 4500mAh job and I have designed the conversion of this model for a 5000mAh - which is slightly, but significantly, heavier.

2. By using a slightly thicker bit of foam on the battery tray back stop I have got room to just move the battery a very little further up the tray which will of course also move it forward, not by much admittedly, but along with the heavier battery, every little bit helps and we only need a relatively little bit!

I don't believe that with these two ruses we will get away with no lead, but will minimise it. I'm not expecting to need more than a very few ounces.

Now I'm away this weekend, so it will be Monday before I can report back on all this - see you then; unless you happen to be at the Don Valley Fly-in, in which case I'll you there!

BEB

[Ron Gray]

Well done BEB, you’ve done a good job on her! I too have found that the ‘book’ cog on WOT4s is far too forward, on my ‘standard’ one that I’ve just assembled, I have ended up moving the battery further and further back, now it is about 40mm further back from where it was with the cog at thev’correct’ location, I’ve also added 20g of tail weight to get it to fly as I want it to! Mind you I’ve also had to change the wing incidence by raising the LE by 2mm! What I’m trying to say is that based on what you’ve said, I would fly it as it is without adding any additional nose weight.

[Biggles' Elder Brother - Moderator]

Thanks Ron, yes you're probably right, but I have decided to retest the balance with the battery I'll actually fly it with and see where that takes us - always a wise move!

So, the battery that came with it is a slightly puffed Dynamic, 6s 4500mAh. The ones I will use are Zippy and Turnigy, both 6s and 5000mAh - and so should be slightly heavier, which will of course be an advantage. Weighing them the "scores on the doors" are:

Dynamic - 573g

Zippy - 750g

Turnigy - 815g

I am rather surprised that the Zippy/Turnigy combination are so much heavier - I had expected a weight gain in line with their capacity advantage - so 10% or so, say 60g. In any event it's much more.

I took the Zippy and tried a balance. I found that it will balance - mid recommended range - with a little lead to assist it.

Her she is on the stands:

And full view:

The balance position is very slightly tail down, but I'm not worried about that; the position is mid range and we have already said that the recommendations tend to be a bit conservative. So I fly her in that state.

Just how much lead is that?

60g (just over 2oz)!! So we took out 1.3pounds (22oz or so) and put back 2oz. Not at all bad, happy with that.

I reweighed her and she comes out at 4.35Kg, so missed the 4Kg target, but again a good result compared with the 4.8Kg we started with.

I haven't managed to measure the max power yet - its raining on and off here which is how I got permission to check the balance in the house! But I can't run her up there! Hopefully tomorrow. The power is of course unchanged from how it was before (same cell count, same prop etc.) and we know it flew like that so it should not be problem - but I am curious to know the power to weight ratio we have at the reduced weight. I'll try tomorrow.

BEB

[Bob Cotsford]

As for the difference in battery weights does the Dynamic have a lower C rating and correspondingly thinner leads? I've found a big difference in both size and weight of batteries of the same capacity depending on the C rating. Higher C seems to mean fatter cells with an accompanying weight increase though I'm surprised to see as much as a 40% increase for a 10% increase in capacity. With my 3S 2200s it's a couple of clicks of elevator trim difference, with 6S 3700 25C and 35C it does become more noticable.

[Biggles' Elder Brother - Moderator]

Hi Bob: yes I was somewhat surprised at how big the weight gain was too! The Dynamic and the Zippy are both 30C, the Turnigy is marked 30-40C, which might explain why it is the heaviest of all?

OK, ground power test finally done, and the result is,..... 1492W on full throttle. A very satisfying result indeed. That works out at 155W/lb - excellent, loads of power! Big verticals and loops here I come!

The current for that power was 73A, it's an 80A ESC so not a lot of headroom there, but as I'm not likely to be holding those power levels for extended periods I'm not too bothered.

One interesting point did emerge. On any model half throttle always delivers less than half power, the system isn't linear. But I find you can usually cruise OK on half throttle and help the endurance along. But on this set up half throttle was less than 400W! Now that is very low. It will be interesting to see what she flys like under those conditions - I am always surprised just how little power you need to just fly around once you are already "up there", but that does seem marginal. I may have to consider programming in a throttle curve - we'll see.

Just waiting for a reasonable day now to re-maiden her - hey-ho!

BEB

[Nigel R]

If you do a timed full power run, how do the three lipos stack up?

I've noticed some lipos seem to be very optimistically rated, for capacity and C rating.

If you look at capacity vs weight, lipos are quite consistent, given a particular C rating (as you'd expect with a relatively mature technology). And vice versa for C rating vs weight. But sometimes you can see the odd stand out, sometimes in very cheap lipos, a rating which is "too good to be true".

The worst case I noticed was some ZOP lipos (ebay!) which were rated at 4500mAh and 45C, yet were only about 80% of the weight of a similarly priced Zippy lipo rated at 4500mAh and only 35C. So, over 25% more C, but 20% less weight on the same capacity? And I'm not sure I entirely believe the Zippy ratings in the first place!

Another good lipo metric is cost per capacity, which demonstrates just how good value some of the small 3S 2200 size lipos are.

 

By the way, good work on the rebuild/reinstall - looks a proper job now. 

Edited By Nigel R on 03/10/2018 10:27:04

[Steve_Wright]

Bit of a resurection but I am just about to do the same on my XL that has been sat in the garage for years unflown. I am thinking on building out the motor mount into a battery tray arrangement I think, not a huge fan of long standoffs, they just look wrong.