Have I Built A Brick?!

[Ad's]

I think I've built a brick, or got something wrong anyway.

over the winter i thought i would have a go at building my first own design. i thought i'll keep it small so if it dont work i have not lost much but secretly i really hoped it would fly, anyway i built it and put a slow fly type motor up the front with an orange prop (9x7 i think?). A 5 amp speed controller with BEC, connected up to a 8.4v Nicd. The model it self is a high wing trainer ish style nice and simple, here's the spec.

wing span-820mm(32.28in), wing chord-125mm(4.92in) AUW-445g(15 3/4 oz)

Trouble is it likes the ground too much

I think if i worked it out right it has a wing loading of 42g/sq.dm(13.5 oz sq.ft) But i dont understand what this all means? how does this all relate to my model, have i built a brick or is it just not got enough ooomph? if anyone can help steer me in the right direction i would much appreciate it as i would like my plane to ROG if poss

[Alan Cantwell]

first thing i saw was 8.4ni-cad, thats a HEAVY thing, now, i dont know anything about electric propulsion, but this seems a tad heavy for this type of model, are the incidences correct? what section is the wing?

[Frank Skilbeck]

the wing loading is fine, but based on the battery and speed contoller rating you can't have more than 40w of power, which is why it won't lift off. You should be aiming for at least 80w per lb and over 100w per lb preferably. You'd need a motor/prop that pulls around 15 amps on a 8.4v Nicd (a lipo 2s or 3s would be much better).

[Tony H]

I reckon you are very low on your power for the weight of the plane try an 11.1v 3s Lipo.

Here is a electric plane calculator which will help you get the power to weight and wing area right for what you want it to do. I usually like to have around 120W/lb, but I do like power.

http://flbeagle.rchomepage.com/

Edited By Tony H on 19/03/2012 23:38:37

[Alan Cantwell]

told you i knew nowt about lecky,

[Tony H]

Looking at the Calculator, you are way too low on power. What Kv Motor are you using?

[Tony H]

is the wing tappered, or straight??

[Tony H]

If my Calculations are correct, you will need roughly a 1500MAH 3s Lipo, an 12-15amp ESC , an 1200Kv Brushless outrunner motor with a 9x7 prop. This should give you the required power and duration around 8min flight time and 111W/lb

[PatMc]

Tony, what you're suggesting would undoubtedly have enough power to fly the model but IMO given the model's small size it would be a bit of a rocket, difficult to see, difficult to handle & prone to stalling. Reducing the AUW would be my first priority.

Ad's, what are the break down weights of the airframe, battery & motor ? Is the motor brushed a type ? What capacity are the nicads ? What sort of performance are you wanting from the model ?

[Keith Simmons]

Any photos? That will help us with your query.

[Frank Skilbeck]

Pat, what Tony has recommended is probably lighter than the Nicd set up. Thats the beauty of Lipo's and brushless motors they are not only more powerful but lighter as well, talk about having your cake and eating it.

I also have a 800mm wingspan Corsair which weighs 14 ozs and flies on 70w. so I don't think the plane is too heavy and if you were looking to lighten it then one of the first options would be to change to Lipo batteries.

[Ad's]

Hi all, sorry i couldnt reply sooner, must admit I am pleasantly surprised at how many want to help, well here goes with what i know (i'll accept the laughs as i'm sure this will create a few!).

My wing is a constant chord and i drew it by eye to the shape off a clarke y section

the prop is a JP Slowfly Prop 9x7 (9070) and the motor is a GWS A, brushed, which was matched to it on the chart on the back of the packet, the Nicd is 8.4v ( 1.2v 150mAh for each cell, 7 cells in total)

Like I say I'll expect it'll take longer for the replies as people will need to get off the floor nursing their ribs from laughing. I know that lipo's would make life easier but as i struggle to get to grips with the basics of all thing sparky, i didnt want to go down that rought just yet

[Daithi O Buitigh]

Consider switching to Lipos as the equivalent of adding a drop of nitro to the glow fuel - a lot more oomph from tne prop spinning gubbins

[Frank Skilbeck]

I had a look on the web and i guess that you are using the brushed motor with a 7:1 gearbox, which according to their datasheet gives you about 2800 rpm on a 9 x7 prop, which is less than 12watts. You'd need to do some very serious weight saving to get that to work.

A small outrunner brushless on a 800mah 2s lipo would quite easily give 80W which would suit this model perfectly, something like a 1700kv motor, 8 x 5 prop, 15A ESC and a 2s 800 mah lipo.

[Ad's]

oh, that'll explain why it wont work how i hoped. so the only way to get it to fly is to start again with the whole power set up then, i guess i'll have to take your good advice and try an all new brushless/ lipo system. i feel like i'm biting off more than i can chew in this area. i thought building the model would be the difficult bit as you are going by your own design but i think i was wrong, the electrics bit is,to me anyway, by far the most difficult bit of the lot ! I guess if i buy what you have advised it'll be ok, question is what on earth could i use the existing set up for? sounds to me that it would just about power a paper aeroplane, even then i'm not so sure! would be nice to be able to use it on something though, shame to waste it.

Thanks for the help and anymore ideas are welcome, now where's that tissue paper

[Tony H]

My setup would be lighter and probably give you more power than you need as your AUW weight was with the heavier Nicds included, you could get away with a 2s (around 1000mah) lipo setup and a 10x6 prop, be careful props can make huge differences in amps and watts and that includes the same size props using different manufacturers and can overload your ESC so you will need a watt meter to test the setup and a range of props.

You could use your existing setup but it would have to be in a large wing area, slow flying aircraft like a light weight powered glider. You need power and speed to get a heavy small wing area plane to fly but you need very little for a large wing area aircraft.

In other words you would need to build a powered glider around 50-62" wing span to be able to get some use out of you old less efficient and heavier power setup. Good Luck! or move with the times and go all out Lipo and brushless, trust me you won't regret it!

Edited By Tony H on 20/03/2012 22:34:18

Edited By Tony H on 20/03/2012 22:34:48

[Tony H]

Second thoughts a 10x6 may be too big for your plane.

You could try a 1000MAH 2s Lipo with an 8x6 prop, 12-15amp ESC, and a 1600kv brushless motor. Given your AUW wil be reduced with this setup I would estimate you would get around 90-98W/lb which would be sufficent for slow to meduim sports flying.

I personally would go with a hotter faster setup but I like speed and power!

[PatMc]

I had an MPX Lupo parkflier that I used to fly indoors & occasionally outside. It used the same GWS motor/gb with a 270mAH 7 cell nicad. I think the prop was 8" or 9" & the model weighed about 10oz. The Lupo had to be flown at less than half throttle or it hit the rafters in the sports hall. In an effort to slow the model down by reducing the weight I changed the motor to one of these with a 10A ESC, 7" prop & 2s 700 lipo which knocked about 2oz off the AUW. It did fly a little slower & at even lower throttle. A pair of lipos was more than enough for an evenings flying time.

The GWS motor it wasn't worth messing about with so got binned.

Actually the 150mAH nicads & GWS motor are about the same weight as this 1220KV motor with a 2s 1000mAH lipo

IMO the overall weight is still too high for a pleasant flier at the size this model is. Although Frank's Corsair is about the same span it has greater area & I reckon it's wing loading to be about 10oz/sq ft. For the model in question I'd have aimed for a max of 8 -10oz/sq ft but since it's already built there's little to lose by trying a it with cheap brushless & lipos. As already said the power will have increased dramatically.

Good luck, hope my pessimism is proved wrong.

[Peter Miller]

Put a .15 ic engine in it and it will go like stink!

[Olly P]

and actually stink.....

[Simon Chaddock]

Ads

I too knew nothing about brushless and LiPo but started with a RTF plane that used a brushed motor NiMH set up that I knew would fly.

It was not too much of a step to replace the 9.6V NIMH with a 3s LiPo (11.1V) pack that weighed just a little less but had significantly more capacity. Result - a significant improvement.

When the brushed motor gave up I replaced it with a brushless that was recommended for the same diam & pitch prop with a 3s LiPo. Result - an even bigger improvement.

Only then did I feel confident enough to start designing my own but to start with as just a series of 'improvements' on the original design.

Try to move forward just one step at a time, preferably from something that you know works. That way nothing gets too complicated.

[Ad's]

Thanks for all the help guys,sounds like I've messed up in the design stage of my little project as it was meant to be a little light plane that i could sneak off to the field in the summer evenings, a little plane for flying in a small place. didnt think it would have to fly fast to stay up and i could poodle about the sky forgetting about the days work just gone, instead i've made something that sounds like the only way to make it fly is to go the speed of sound! blimey how wrong could i have been? oh well i'll try as suggested and up grade all its gubbin's and fingers crossed ay. As for the poodle about model, back to the drawing board. I suppose thats all part of the fun of it, shame i dont understand all this electrickery stuff though as it would make my best guesses a bit more in the ball park, ho hum

[jerm]

if you were going to put in an ic engine ,your going to have to strengthen model. if you built it for electric i would stick with it, you will get there eventually and learn lots along the way

[Ad's]

I've just been looking on the BRC Hobbies website and they have a combo motor set that i think might be right for my model its an emax cf2812 1500kv brushless outrunner with a 20A ESC, there is a 2s 7.4v 1300Mah lipo too, rated at 20c that i think will be ok, I might have to do some mods to make it all fit in the model though, does this all sound ok?? Also i was wondering if Im using a lipo for the juice now how does that affect the RX, would it all work fine. The ESC has a BEC i think, or would i need some sort of regulator for it as the volts are different to the NiMh cells that im used to?

[Tony H]

That sounds about right for your plane with a 9x7 prop. The ESC has a 5v 2A BEC which is fairly standard, so you don't need to worry about the voltage to the RX as the RX lead will go through the BEC and to you RX at 5v. I'd estimate around 95-105w/lb with that setup.

Don't worry about having too much power you don't have to use it all, but it's nice to have to get you off the ground quickly and out of sticky situations.

 

Edited By Tony H on 25/03/2012 21:11:20

Edited By Tony H on 25/03/2012 21:13:17