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Battery and drone speed questions

Skiiingwonderz

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Hi everyone,

Let me start out by saying I’m a total drone noob. I’ve had my MM for about a month and done about 30 flights. I’ve read the owner’s manual a few times, read posts on the forums and FB groups and watched a few dozen YouTube videos.

Today I was flying over a river near my house. I was trying to get over the river and to the ocean. The UAV Forecast app showed a light wind of 8 MPH sustained and 10 MPH gusts. It looked like it would be a tail/cross wind heading out.

I made it out 15,500’ in S mode with 71% battery remaining before losing the picture. I let it RTH until about 12,000’ and then took over manually in S mode. I noticed the speed was lower than it was on the way out which made sense since I was flying into a bit of a head wind home. I still thought I’d make it back easily.

At about 7400’ out with 34% battery left, I started to really lose speed from 21-23 MPH all the way down to 5-10 MPH. Around this same time I briefly lost the video picture again. I’m assuming I lost speed because the controller lost communication with the drone? If I was in RTH instead of flying manually, I take it transmission/signal issues wouldn’t have slowed the plane down?

The drone then took its time getting back to speed and managed to only fly at 10-15 MPH until 3700’ out when the speed nose dived down to 6 MPH with 12% battery left. I had to find a grassy area away from the river to land it. I was able to locate it without issue and it seems fine even though it hit a couple branches on the way down.

Is it normal to lose that much speed in S mode when you hit 12% battery remaining? Would that have happened if I was in P mode? The reason I ask is because watching videos online like DD’s, he seemed like he was able to maintain speed until near 0%. So I’m wondering why mine lost speed? Could it have been wind gusts?

Thanks in advance.
 
Last edited:
It was most definitely wind.
I would always advise a small test run into and against the wind before doing any sort of distance flying like that.
Also in my experience it is not wise to use S mode flying into the wind as the tilt/power-consumption ratio provides diminishing returns
 
It was most definitely wind.
I would always advise a small test run into and against the wind before doing any sort of distance flying like that.
Also in my experience it is not wise to use S mode flying into the wind as the tilt/power-consumption ratio provides diminishing returns

Can't argue with the first part. As to not using S mode to fight a head wind, that advice will be wrong much of the time. There are too many variables to make a blanket statement like that.

Pilot's learn early on that for an out and return flight, a tailwind component, unless it shifts around become a tailwind for the trip home (which rarely happens for me it seems) is almost always a net negative to speed and efficiency. A steady crosswind is bad in both directions - no way to win.

If considering efficiency, speed made good over the ground vs fuel consumption is foremost. Flying with a tail wind, one wants to use the most efficient speed and consider the tailwind as a gift. Generally higher is better due to wind speed gradient increasing with altitude. Conversely, flying into a headwind, speed made good over the ground is usually more important than efficiency. Using S mode and flying as fast as possible* can be pivotal in making distance towards home. While doing so, it would be smart to descend if that increases ground speed. One might also back off the throttle while watching the affect on ground speed. Reduce throttle, but only if doing so does not make ground speed suffer too badly.

*The variables, head wind speed, ground speed, distance remaining, and fuel remaining, are all variables that one would ideally be able to graph and come up with the best speed to fly "on the fly". That probably ain't gonna happen. Best you can do is watch the parameters and adjust the only things you have control over, altitude, throttle, airspeed, and ideas. The old pilot's saying: "Never run out of airspeed, altitude, and ideas all at the same time".

The Mini had an max airspeed just under 30 mph (we'll assume 30).
If the headwind is 25 mph, max level will net 5 mph ground speed.
Reduce throttle a little bit to 27.5 mph. You will now take twice as long to get home. Your efficiency had better double to make up for the drop in speed . . . trust me, it won't.
 
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Interesting. Thanks for the tips.

So if you’re flying out in Sport mode with a tail wind and your max speed is 30 MPH, are you using more battery than you have to by being full throttle? If you let off the throttle a little and are still able to hit that same 30 MPH, are you then using less battery? Or does the software do that automatically to conserve battery?

Has anyone hacked a MM yet to make it fly faster than 30 MPH?
 
Interesting. Thanks for the tips.

So if you’re flying out in Sport mode with a tail wind and your max speed is 30 MPH, are you using more battery than you have to by being full throttle? If you let off the throttle a little and are still able to hit that same 30 MPH, are you then using less battery? Or does the software do that automatically to conserve battery?

Has anyone hacked a MM yet to make it fly faster than 30 MPH?

Aircraft are dumb, without instruments, aerodynamically, they don't have a clue about ground speed - imagine a huge box of air moving along over the surface of the earth with a small airplane flying in the box. If the box is blowing along at 30 mph and the airplane if flying inside the box at 30 mph, the airplane's pitot tube airspeed indicator will show 30 mph even though it's moving along at 60. Plane turns 180 and heads back. It's still doing 30 mph but over the ground it's doing 0 mph.

So, to answer your question is, if you let off the throttle and airspeed remains the same (unlikely by the way, unless you start a descent you will usually slow down) but use less battery power. Then it starts to get complicated rapidly. There are two types of drag the aircraft must deal with, induced drag, which decreases with speed, and parasitic drag, which increases with speed. Propellers provide most of the lift, and their efficiency decreases with speed (higher rpm and airspeed with a rotorcraft), as the moving blades (airfoils) create drag like the wing on a fixed wing aircraft. I'm a simple pilot, not an engineer, so that about tops my description without pushing my pay grade :c).
 
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