Hi all,

I recently helped a friend build a 17" quad that wont take off the ground.
When battery is plugged in we get the normal regular beeps and then when armed the motors start
and all is well however even under full thrust the motors accelerate but will not lift off the ground.
We first assumed that one battery did'nt supply enough current to the largish motors so we connected 
2 in parralel but no difference.
I get the immpression that rotational speed of the motors are somehow lacking!
We then assumed that iNav might be the problem (we both are new to iNav) so we flashed the FC to 
betaflight and tried the custom rates, aggressive and default.. no difference.

We now suspect that the motors are too small for the weight... don't know.

I will start with a list of the major components used:

Frame HSKRC LX800 17 inch.    https://th.aliexpress.com/item/32994376918.html
Motors EMAX 12N14P.     https://th.aliexpress.com/item/32484476504.html  (they wont take 6s)
Propeller 17 inch Carbon Fiber.
ESC Aikon 55 Amp
FC Matek F722 HD.
Receiver and Camera DJI Air Unit.

Batteries x 2 paralled 4S 5000mAh. 
 
Total all up weight 5.6 Kg (2 batteries included)

Probably will add GPS and LED's

The Motor Specification is as follows:

600 KV, Length 23.4 mm, Diameter, 41.5 mm, Cells Range 3 - 4S, Maximum thrust 1.78 Kg.  

Maximum total motor thrust 7.12 Kg is this enough?? 

The frame will happily accomodate up to 19" props.

Can someone advise us what to do next,
If the motors are the problem...
What motors should he be looking at??

Most freestyle or race quads have a thrust to weight ratio of somewhere between 8:1 and 12:1. You have a thrust to weight ratio of 1.3:1, if we're being kind. If you ever manage to take off then you probably won't be able to manoeuvre.

As to motor size, I honestly don't know. Even X-class quads draw the line at 13" props (IIRC). If you want something to throw 17" props then you'll probably have to look into industrial or filming drones, and most of them use more than 4 motors for redundancy. For example: the DJI Matrice throws 21" props using 6010 motors, but has 6 of them.

Wow... that was an expensive mistake.
Thanx Banelle
How much thrust per motor would my friend need to get a ratio of 10:1?
Is there a formula/website?
Would this be correct.
10 x 5.6=56 ÷ 4 = 14kg thrust per motor?

Hello Titanv!
Firstly, I'd PM V-22, he is extremely helpful with design. [Rate him if helps you ]

It seems that your props are simply to large.
Also 1780 grams thrust each is way to little for a 17 inch, but smaller props may help out also... (less drag or the motor/prop = more rpm/thrust.)

Your equation is correct, although a 5/1, Thrust to weight ratio is more than plenty for a quad of this size...

And lastly, try ecalc.

Tried 15' props and got it off the ground but thats all...
Tried eCalc but its a little beyond me...
My friend has decided to upgrade the motors to suit 17' so I will contact V-22 for advice.
Thanx all

Check out the prop/Kv wizard on eCalc to start. With your quad size/AUW, you'll need much larger motors in that Kv range and a much larger ESC to power it. 

What are you trying to accomplish exactly? 

   

Hy V-22
My friend has built a 4in, 3x 5in, 3x 7in, 2x 10in and now he wants a 17in...
No particular reason...
He intends to put a GPS and xfire to hopefully fly it 2 to 3kms distance.
I know I'm asking alot and probably stepping over boundaries but can you select motors and ESC required.
I have tried eCalc and its complicated.
My friend basically selects parts and I do the building and initial test flight for him.
As you can tell.. selecting parts for a 17in didn't work out that well.
Between the 2 of us we generally are able to get a build working.
Both of us are novices and in dire need of professional advice.

3110 seems wayyy too small for a 17in quad pretty sure that is something you would use for a heavy 7in or an 8in