60” homemade wing- 6-8s mentally fast setup- LOTS OF ADVICE PLEASE :)

[Billy_boy_2010]

60" homemade wing- 6s mentally fast setup required

Hi Guys

This is the first time I have written anything in the "members hanger" so please go easy on me :D


The below is a 60" wing which I bought used on ebay. The guy I bought it from was a real RC nut and built it with his own hot wire cutter. The back has various measurements written in felt tip pen so he could get the proportions right and he obviously knew his stuff.


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Anyway- I dived right on in there and mounted a big 185g 850kv motor in there which made about 750W with a 12" prop. However, it required two fairly heavy 4S batteries in the front, and an extra couple of smaller batteries and weight in the front to balance. Essentially- it was too heavy and flew poorly without power. Also- I don't think the low kv motor really suited its character either- taking a while to spin up.

After a while I removed the motor and fitted a much lighter smaller motor I had lying around- this one was 750kv and was about 300W. This allowed me to shave off perhaps 300g of weight- which certainly helped the flying characteristics- but it still needed a fairly big prop to generate any power and it cruised at about 60% throttle from the weedy little motor. It was installed with an EzOSD and, 1km out, the EzOSD disappreared (with video signal fine) and 5 seconds later the plane seemed to drop a tip and possibly stall *badly*. I was way out of my league and, she circled and crashed her way to the ground from perhaps 100m high. With hindsight this may have been a stall, a failed BEC losing my control of my elevons- I don't know. Anyway- I was flying over fields as I always do and, despite a very helpful farmer and hours of walking up and down fields- she was gone. 1 month later, after a month of very harsh November weather, the farmer calls me so I turn up with a bottle of wine and theres the plane. Most of the items had faded in the elements- but everything, except the ESC and BEC (which I haven't tried) still works fine. I cant risk them on a decent plane now- but the £90 control Rx is used in another expendable plane and seems fine.

Anyway- on its return I tried to beef it up with a bigger prop one day and gave up after about 5 failed launches. Turns out just wacking an oversized prop on a motor doesn't automatically give more power. My guess was the motor wasn't even managing 750kv with the big motor so thrust and power was reduced.

So, after losing interest in this for a while here I am with an idea.

I have a skyfun- which is fast and fun but quite heavy and loud! And a TBS cap- which is much slower and quitter. I also have another 60"- which is a far superior machine- with a full autopilot and a much more powerful motor. So- what to do with this big wing?

I have decided to try and bridge the gap between the "small and fast" plane and "big and slow/efficient" wing. I want to make this 60" beast very fast. I have never run anything above a 4S setup before- so im aiming for 6-8S. IME the concern with very big planes is they are so heavy- and flying a heavy plane is much more intimidating. So, despite all this- I want to keep the plane light. Im happy to sacrifice flight duration for this. My plan is to have a mentally fast and loud big wing to zoom around in for 5-10 mins- or, on the occasion that im feeling more relaxed- fly it fairly high and then glide around high up semi efficiently- enjoying its lightish weight. I know from the original setup that a flying weight of no more than two 4S 3000 batteries is my weight limit.

I have also cut into the back of the plane with the intension of bringing the motor (S) much further forwards. This should mean I can hit CG more easily without loading the thing up with batteries.


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(she still looks a bit messy with all the tape and wires on her. I was hoping to avoid relaminating her, but with all the sparring I will need to add I may as well relaminate her anyway so will rip it all off and the laminate with it).

Any ideas on how best i can fill the holes? Some simple expanding foam perhaps? I will be mounting the single battery in the very middle of the plane to improve handling a bit.

Sparring

This is going to need loads- considering the original two were made from plywood! A 4-5mm square one as the main spar should take the brunt of the flex- with an A frame style ones in each wind nearer the front made from 3-4mm fibreglass to give me at least some RF friendliness.

6s vs 8s

Why would you ever go 7s? It seems to me to be the ignored battery combo, much like 5S?

I would love an 8S setup- my charger can handle it too. But all the 8S motors seem to be huge heavy things with very low KV.

ESCs seem to have a 6S limit too. And so do some of the UBECs that I plan to use, and the step down voltage components that I will use to power the video system. So, sadly, 6S seems sensible.

In this plane last I used 2* 313g batteries- so I want to keep the 6S battery to around 600g which should allow for a bit of extra sparring.

ESC

On decent planes I don't mind spending £ on escs- but as this is just a bit of fun I would go cheap! Both the HK redbrick (which I have never used but have never trusted in truth) and the Turnigy Plush 60-100A ESCs have a limit of 6S. However, the HobbyWing Platinum range can handle up to 12S (!) and the 100A version is around £40 which makes it a bit of a steal IMO. I have used their flyfun range exclusively for ages and really rate them.

Motor

I want this wing to have the some beserk flying characteristics of a 3000kv motor on a 3s Setup. (which seems to be the high speed combo that the skyfun and funjet guys fly with). That means I need a 1500-2000kv motor. Fairly normal for a largish plane running 3-4s- but pretty quick for a 6S battery setup.

Something like this could work, which kicks out around 800W on a 4S battery- even better if I had two :D Total flying weight of motors 240g

http://www.hobbyking.co.uk/hobbyking/store/__15169__NTM_Prop_Drive_Series_35_36A_1800Kv_875w.html

If I went for a single bigger motor (347g) designed for 6S, it will probably end up weighing the same as the twin setup will need two motor mounts

http://www.hobbyking.co.uk/hobbyking/store/__22035__NTM_Prop_Drive_50_50_580KV_2000W.html

Its capeable of 2000W- but at 600kv- it will have the same sort of personality as a 1200kv motor on 3S- which is a bit tame for my tastes!


Can I run a 4S motor on a 6S, or a 6s on 8S setup and just expect lower battery life? If I prop it down and perhaps push the wattage just a bit- will it do any harm to the motor in the short-medium term? How does the motor know what voltage its running anyway?!

What are the pros and cons of a twin motor pusher setup?

Pros:

Cancel torque with counter rotating
Easier to source two smaller higher KV motors than one bigger more powerful one
Safety if ESC fries

Cons:

Heavier?
Louder?


Anyway- sorry for the waffle and lack of pics. I would be really grateful for responses to the following questions.


Can I run two motors/escs off a single battery?

What are the pros and cons of a twin vs single pusher setup?

Can I run a motor on a higher than rated battery setup, provided I keep the total wattage produced to near its 4S limits? They say that heat kills a motor- and with a lower current going in a motor (the power made by the higher voltage)- the potential for heat must stay the same.


Please let me know your thoughts- I would love to get a hobbyking order in this week.

[FPVSteve]

lol, I have nothing to say except that I need to up my game tape-wise to reclaim the title of "The Gaffer".

[Billy_boy_2010]

Lol. None of it is structural- just added and bodged each time I added another component. The rebuild will have everything seated within the foam before lamination :)

[skyscraper]

My experience of trying to go quick is here. (Hopefully to be continued sometime...)

I (EDIT: eventually)used a 2150 kV motor and a 6S setup, but the plane was 1 only meter span. ( I was working from F5D which use 5S and 2500 or so kV) I think with a bigger plane you wouldnt have much luck with such a high kV motor. You would just get no power. As the prop diameter gets bigger so you can have a coarser pitch, so you can go to a lower kV motor.
E.g on a F5D you typically use a 5 x 5 prop
Say a F5D is 40" span. Scale it up to 60" and your prop would be 7.5" x7.5". Simplifying greatly and imagining your prop is corkscrewing through a  solid medium, you can see that you only need a motor of 2/3 kV  ( e.g 1400 kV) to get same speed on bigger plane. N.B Simplifying almost to absurdity here but hope you get the idea. Higer speed = less power.

What I found is that going fast things get expensive. You need to experiment to the get right combination of motor prop ESC and lipos. ESC needs to have a good range of timings. High discharge rate Lipos, F5D typically dissipate around 1kW, so you need to look at 2kW I would say. The experimenting starts to get expensive since you find that you need quality gear. Cheap gear doesnt tend to meet the required specs.  Cheap motors dont have good enough magnets and the magnetic circuit decides how much power can be produced. Put more power in when magnets are saturated and it just gets hot real quick! EDIT: this is why it is best to stick to manufacturers recommended voltage range)

Also need a strong vibration free airframe to go quick.. Dont want wings folding on you  :laugh:

All that said  I hope to get back to my speedster at some point. It was a real buzz to fly even if so brief!

Anyway will watch with interest.


regards
And

[Billy_boy_2010]

Very interesting indeed thanks very much.

After sleeping on it overnight- I have several 3000 4S Zippy lipos which really arent doing much. They are quite big and heavy considering their capacity (for comparison the 4000 4s Compact ones are almost the exact same size) so im now thinking i could use them in series to make a 8S setup.

This would allow a single heavy motor like this

http://www.hobbyking.co.uk/hobbyking/store/uh_viewItem.asp?idProduct=31156

A whopping 450g! and two batteries the plane would be pretty heavy. I would scale the prop down as much as possible and rely on a higher pitch for speed. But my crappy 4S batteries would likely only cope with 10C which would give me 30A :( I assume that two batteries in series double the voltage but keep the C rating the same- 2* 3000 4S = 1* 3000 8S. Whilst they claim a C rating of 20C- i suspect 10C is closer to the mark.

You're right. It's getting expensive. This was supposed to be a bit of expendable fun.



Ok sod that. Next thought is how quick can i make it with a single 3000 4S battery. Will use a 4S 2000+ KV motor and aim for 100MPH. If i run a single battery the setup will be super light and allow for very low stall speeds so i can cane it around rugby fields like i do my TBS cab but with a wing nearly double the size. She should cruise quite efficiently if shes light but I can nail the throttle and get out of trouble pretty easily too!

So- im back to this 1800Kv motor again

http://www.hobbyking.co.uk/hobbyking/store/__15169__NTM_Prop_Drive_Series_35_36A_1800Kv_875w.html

which I can prop to kick out perhaps 800-1000W and use a high pitch prop for top speed. I have an old 60A plush esc lying around too which will hopefully be sufficient (1000/15V = 65A).

The only other option is i run two of these motors, get 1600W instead of 800W, and probably run two batteries in parallel to handle the current load. Motors are 130g, mount takes that to 200 say. Second battery is 300g and esc and wires lets call it an extra 600g for double the power but higher stall speed.

Opinions please :)