Return To Home Speed

charlie999 · Jul 20, 2015

Is there a set speed for the P3 to RTH at? Does it use full throttle or a little less?

I've used RTH a couple of times to test but they have only been short so haven't had time to work out if it used full throttle so to speak.

I remember using the RTH feature on my P2 once when I hadn't noticed the battery running so low so initiated the RTH but it was fighting a head wind on the way back and seemed to be standing still. Panicking about the battery I took control and got it back much faster myself by giving it full throttle.

nhoover · Jul 20, 2015

It's similar to the P2 - not full speed and no setting.

rmfa · Jul 20, 2015

nhoover said:
It's similar to the P2 - not full speed and no setting.

You can hold 'forward' on the right stick to make it go faster. You have complete control of the bird while its in RTH mode.

ImJim · Jul 20, 2015

rmfa said:
You can hold 'forward' on the right stick to make it go faster. You have complete control of the bird while its in RTH mode.

Didn't even know that. Thanks! Imagine that the bird is auto-landing (RTH mode) but.. Not exactly where you want. If you turn right, will it continue to go down while you correct its position or will this action interrupt it? I know there's a button to interrupt RTH, but you know..

bobmyers · Jul 20, 2015

ImJim said:
Didn't even know that. Thanks! Imagine that the bird is auto-landing (RTH mode) but.. Not exactly where you want. If you turn right, will it continue to go down while you correct its position or will this action interrupt it? I know there's a button to interrupt RTH, but you know..

Yes, but speeding it up will also drain the battery faster

ImJim · Jul 20, 2015

bobmyers said:
Yes, but speeding it up will also drain the battery faster

Yes what? Yes it will interrupt it or yes it will keep the RTH?

Meta4 · Jul 20, 2015

bobmyers said:
Yes, but speeding it up will also drain the battery faster

Perhaps .. but not much.
The Phantom goes faster by tilting more - not by having the motors rev harder, which is what makes the Phantom climb higher.
It's better to get your Phantom home earlier by going faster than to have it run out of battery because RTH couldn't punch a headwind.

ScottyT · Jul 21, 2015

Meta4 said:
The Phantom goes faster by tilting more - not by having the motors rev harder, which is what makes the Phantom climb higher.

I don't believe that is correct. Quads tilt for horizontal momentum, sure - but they need to rev harder to gain speed. And they still need to offset the constant 10m/s of gravity at the same time so more power is needed than opposed to just hovering in one position.

taroh · Jul 21, 2015

Here's instant calculation:

(edit: sorry, correction below!!)

tilt[deg]: power[hovering=1]: y accel: accel/power ratio
0 : 1 : 0 : 0
5 : 1.00382 : 0.0871559 : 0.0868243
10 : 1.01543 : 0.173649 : 0.17101
20 : 1.06418 : 0.342021 : 0.321394
30 : 1.1547 : 0.500001 : 0.433013
45 : 1.41422 : 0.707108 : 0.5
60 : 2.00001 : 0.866027 : 0.433011
89 : 57.3106 : 0.999848 : 0.0174461

Actually "y accel" is horizontal vector of thrust, almost proportional to terminal speed of traveling - drag increases if P3 leans, thus max effective angle will be < 45 deg on real P3. Anyway Phantom is limited to lean <= 30 deg (I remember), then higher speed is more effective by power to travel constant distance.

ScottyT · Jul 21, 2015

taroh said:
30 : 1.1547 : 0.500001 : 0.433013

Interesting. Where'd you get those calculations from?

At 30 degrees tilt, you'd get 0.5m/s/s acceleration with just 15% more power from the standard hover? So after 60 seconds it will have covered 900 meters and be at a speed of 30m/s?

RedHotPoker · Jul 21, 2015

ImJim said:
Yes what? Yes it will interrupt it or yes it will keep the RTH?

Yes, you can steer it to a better landing spot during RTH function..

RedHotPoker

taroh · Jul 21, 2015

ScottyT said:
Interesting. Where'd you get those calculations from?

At 30 degrees tilt, you'd get 0.5m/s/s acceleration with just 15% more power from the standard hover? So after 60 seconds it will have covered 900 meters and be at a speed of 30m/s?

Sorry, that's only factor where hovering as 1... but it's almost 10 (9.8) m/sec^2, in case of 30 deg it's 5m/sec^2. And donot forget drag by air... accelertion (force) decides the terminal speed.

taroh · Jul 21, 2015

Sorry again for correction... I'm so bad at physics.

Of course, increasing angle of attack, need more power but speed increases (till 90 deg).
Here's correction, with final speed, (final speed / power) by approximated model of drag by P3 body.
description and assumption:
aoa: angle of attack, pow: relative power P3 needs (hovering = 1.0), yacc: horizontal acceleration (gravity = 9.8m/sec^2 = 1), vterm: terminal velocity for horizontal moving. The P3 is modeled as thin paper (cross-section volume increases in proportion of sin(aoa)), no vertical lift/drag concerned. P3 max speed be 16m/sec at 30 degree.

aoa pow yacc vterm vterm/pow
0 1.000 0.000 0.000 0.000
1 1.000 0.017 2.590 2.589
5 1.004 0.087 5.808 5.786
10 1.015 0.176 8.294 8.168
15 1.035 0.268 10.323 9.971
20 1.064 0.364 12.198 11.463
25 1.103 0.466 14.059 12.742
30 1.155 0.577 16.003 13.859
40 1.305 0.839 20.513 15.714
45 1.414 1.000 23.308 16.482
60 2.000 1.732 36.480 18.240
89 57.299 57.290 1122.970 19.599

Anyway velocity/power ratio is better as it leans. Max speed (16m/sec at 30 deg) and half speed (8.3m/sec at 10 deg), battery life differs only 15%, efficiency differs 1.5 times. Result: better max speed RTH.

I calculated by Perl script. Again, I'm not so good in physics... anyone give me advice.