For the background to this thread, and the model and it's flight videos, please see:
http://www.rcindia.org/electric-planes/another-spad/msg319668/#msg319668

Manu and were having an interesting discussion and thought we'll share it here.

It started when he wanted to know if he should set the cruise speed in the autopilot at 20% over the stall speed, which he had calculated as 8m/s. I said to set it at 2-3 times the stall speed, say 20m/s.

I estimated from the model size and stall speed its weight as about 1100gms. Actual was 1070gms.

I had a feeling that with the DYS 3536 1000kv motor and 9x7 prop on 4s that the model was overpowered and at the same time the power system was overloaded.

So I asked him to check the current draw at full throttle.

He said 44.1 amps, and 13.28v on a fully charged battery at 16.6v.
He also said max power is way less according to the data sheet

Of course. Putting over 600 watts into a 480w motor!

The next question was: Lipo capacity and rating? 2200mah and 30c?
Ans: It's a Li ion 3500mah 3c

Aah! The penny drops.
Current drawn at over 4 times its max discharge rate!
And 600w in a 480w motor.

So the power system was in fact overloaded as I suspected.

In the next post I'll talk about my feeling that the model was over powered...

Discussion part 2

There's a long established thumb rule that a (non aerobatic) model needs only 50w/Lb for good take off and climb performance.

Or say 125w for a 1070gm model.

The stall speed of 8m/s is a fair estimate by Manu, but could be higher as the CL Max of a coro wing may be well below 1. Let's assume a cruise speed of 20m/s and top speed of 25m/s.

Now a 1000kv motor will do 15000rpm on 15v without load.
Let's assume 80% efficiency under load, or say 12000rpm
Or 200 rev/sec.

Now airspeed of 25m/s is 1000 inches/sec
At 200 rev/sec, this needs a prop pitch of 5".
Or 6", assuming prop efficiency of 80%.

At 25m/s, the thrust from 125watts will be 5 Newtons, or say 500gms.
Or a thrust to power ratio of ~47%

Is this enough? The answer is YES.
A 900kg Cessna 172 has thrust/weight ratio of less than 25%.
(Out of curiosity one can check the p/w ratio of an airliner)

Now the thrust depends not only on the pitch, but also the diameter of the prop.
So what dia to use?
The dia that the motor can turn at the required rpm.

Here's another Rule of Thumb.
A 9x6 prop @10-12k rpm will produce about 4gms thrust per watt.
So with a 125w 1000kv motor, we can reasonably expect 500gms thrust.

In the next post, we'll look at the performance potential of one or two motors.

The basics have to be revised 😁

The max current that can be drawn from the lithium ion cells is
3.5 (3500mah converted into Ah) * 3c ( the discharge rate of the cell or the battery) which is 3.5*3 = 10.5 amps  


The dys 3536 1000kv motor draws 44.1 Amps at full throttle
and by power = voltage * current  , (13.28*44.1 )  = 585W

Now I have a doubt here, how come a motor rated for 480 W on a 10" prop on a 4s battery draw more current than it is supposed to, according to the data sheet.  The ideal current draw on a 4s must be 480w/16.8 = 28A. Is the data sheet incorrect or practical conditions differ so much from the test conditions or is my reading incorrect D.

Now coming to smaller motors , say a 2212 1000kv which is rated 173W on a 10inch prop on a 3s.

The current draw would be  173W/12.6v = 13.7 amps

Now, for a decent performance ( say 10 mins of flight time and not so trainer like airspeed).
Which motor is more desirable. The one which has a higher power rating (flying the model at low throttle) or the one which has lower power rating (flying the model at more percentage of throttle).

Even though I started reading about aeromodelling back in 2011, I still have so many doubts. I really appreciate Iyer Sir's patience that he still attends to my questions.

Finally got around to testing a few motors.

First was a 2822 1200kv. It was from Riccs, but identical to an EMax CF282#.
On a 3s lipo it spun a 9x4.7 SF at 7200rpm drawing 9.5amps /120 watts, producing 475gms thrust.
A bit marginal for a 1070gm model, especially due to low pitch speed of about 14-15m/s.

Next was a Turnigy D2830 1000kv.
On 3s it produced 525gms thrust, at 11.8v, 7.6a, 89w. Forgot to note down the rpm.

Then tried it with a 3300mah 4s at 50% charge (15v)
It produced 720gms thrust, spinning the 8x4 at 12100rpm drawing 9.6amps at 14.5v, 140w.
Adequate power for the model, with a pitch speed of 20m/s.
Remember this was with A half charged lipo.
With a full charge, there would be plenty of extra power.
At under 10amps draw, it's hardly loading the battery at 3C.

Now Manu's motor weighs 102gms. The 2822 is 42gms, and the 2830 is 59gms.

I was curious to see how a much smaller (20gm) high quality motor would compare.

So the T Motor AS2304 1800kv was removed from the 3d foamy and mounted on the test rig.
It produced 650gms thrust on a T Motor 8x4.4 prop drawing 178w, 15.5a at about 11v.
I could not measure the rpm as my optical tachometers don't work in artificial light.
But the data chart of the motor says:
665gms thrust at 10.85v drawing 14.88amps, 161watts, turning a GWS 8x4 at 11082rpm.
The motor is rated for max 168w. It drew a little more than that probably the prop 8x4.4.

I expect the rpm would have been about 11000 or so. Giving a pitch speed of about 20m/s.

The conclusion is that a much smaller motor would have flown the model much more efficiently (ie, at far lower current draw)
The T Motor 2304 could fly it, but would need to run full blast. So too small for this model.

But the Turnigy 2830 1000kv with an 8x4 on 4s would do just fine.

Pic of test rig attached.

Well, I am buying a new motor then. 😀.. Thankyou Sir. One more doubt, where should I use a high power motor like the dys 3536

Changed the motor as per Iyer Sir's instructions, bought a dys 2836 1120kv motor. Flew it Today continuously for 20mins on a full charged 1850mah 4s lipo

Really happy. 😃