I thought I had plenty of reserve to return...and the forced landing warning was inducing panic

[albatross911]

I think it's an entrance to an underwater tunnel?

It is. Out on a bridge then into a tunnel and out to another bridge, I think.

 

[Grizzo]

The other day I was flying out to the sea going to a man-made island about 3.5km away, which has been on the local news lately. I flew with sports mode on and reached it with 78% battery left which means the distance used up just above 20%. I spent about 5 minutes filming there thinking of returning before reaching 50%, which should give me double the battery required to return in case of emergency. At about 58% it gave me a warning that I only have enough battery to return home. I thought for a few seconds and took the advice to start returning with sports mode on. I realized that I must be against wind as the battery kept dropping quicker than the way over, and I descended to about 20m over the water hoping to catch weaker wind...50%...40%...30%...20% and I was still 1km away...15% and 600m away, knowing that it will start forced landing at 10% I ascend to 35m to give me more time to pick a landing spot just in case...10% and 400m to go and reached land I kept it just above some rocks on the coast in case it drop out from the sky, but the forced landing warning is so rapid that my heart was pounding and palms sweating...

Finally it reached home point with 5% left. After turning off everything my hands were shaking...I have lost a Phantom 2 into the ocean before and been flying the Mavic for over 200 flights so I won't consider myself a novice, but that forced landing warning sound was really making me nervous. Also my girl friend heard it and kept asking me what's happening which further induced the stress. In less experienced hands a pilot can easily panic under the pressure and make the wrong decision.

Can DJI make the warning to sound just a few seconds and then go silent? I'm sure we can hear the warning when it goes off and attend to it. Keep it going for the entire duration from 10% onward really does not help the pilot handle it calmly.

Also when we go long distance always leave more reserve than you estimate to be enough. I looked at the log and on the way out the speed under full throttle in sports mode was 60km/h while on the way back was 45-50km/h. Another flight and another lesson for me.

60km/h on the way there and 45-50km/h on the way back doesn’t sound like the wind was all that strong compared to how fast the battery went down on the return trip.

How old is the battery and it’s general performance during other flights? Maybe worth checking.

Glad you got it back.

 

[albatross911]

60km/h on the way there and 45-50km/h on the way back doesn’t sound like the wind was all that strong compared to how fast the battery went down on the return trip.

How old is the battery and it’s general performance during other flights? Maybe worth checking.

Glad you got it back.

I'll check its health later and update.

A quick edit on Google Photos of this flight.

 

[sar104]

It does increase the power to the motors. The motors are always generating lift and power is used in the process. If the aircraft is pitched more horizontally it need to generate more lift and use more power in doing so. Drones to not generate lift like an airplane wing does.

Do you have a reference for this? I agree with the statement that more pitch requires more motor thrust to maintain altitude, but the need for a higher motor power limit in sport mode implies that in P-GPS mode the limitation is motor thrust rather than pitch. My understanding is that P-GPS imposes a pitch limitation, not a motor thrust/power limitation, and that sport mode simply increases the pitch limitation but still within the same motor performance envelope.

 

[Kilrah]

My understanding is that P-GPS imposes a pitch limitation, not a motor thrust/power limitation, and that sport mode simply increases the pitch limitation but still within the same motor performance envelope.

That's correct, but he didn't say either that allowed motor power was changed, just that the higher pitch results in higher motor power.

 

[genesimmons]

what direction were u flying from, the pics u posted dont look 3.5 km from the land in the back of the pic, thats a very cool place to shoot, hope to see more pics/vids from this location,

 

[Koelkop]

Just to add my experience from flying long distance.

I have a mavicpro platinum, I found that flying at between 42 and 45 to give me the best distance. Furthest I have flown was 14.4km 7.2km out and back. Landed at 12% battery.

I just switch to sport mode without going full throttle. What I like about sport mode, you can switch to the little radar and see how much power you are using. It gives you a nice indicator of how hard you are pushing it.

 

[roadfly]

HK Macau Bridge!!! where were you when you flew the drone?

 

[RayOZ]

Do you have a reference for this? I agree with the statement that more pitch requires more motor thrust to maintain altitude, but the need for a higher motor power limit in sport mode implies that in P-GPS mode the limitation is motor thrust rather than pitch. My understanding is that P-GPS imposes a pitch limitation, not a motor thrust/power limitation, and that sport mode simply increases the pitch limitation but still within the same motor performance envelope.

Isn't it just simple vector calculation of the speed, vertical component (lift) and horizontal component (thrust)? The drone aims to maintain altitude. When pitch is increased, the less lift with the same motor power, so the motor power needs to be increase to maintain altitude, which in turn increase the trust component (horizontal speed) as well.

 

[tcope]

I agree with the statement that more pitch requires more motor thrust to maintain altitude

and that sport mode simply increases the pitch limitation but still within the same motor performance envelope.

More motor thrust is generated y increasing the speed of the motors. If you agree with this then you agree with my statement. The second part of your quote appears to contradict your first statement.

In order to maintain the current altitude with a higher degree of pitch, the Mavic needs to generate more upward thrust. This is done by increasing the speed of the motors. Lacking this and you'd be generating power from nothing, which is not possible.

In order to generate forward momentum the Mavic pitches forward. All things being equal it converts some of the motor/prop energy that is maintaining it's altitude into forward momentum. By converting some of that upward thrust power to power used to move itself horizontally, it's going to lose altitude (as mentioned, you can't divert some lifting power to move an object forward and still maintain altitude). So how does the Mavic maintain the same altitude? It needs to create more power. As it gains forward speed, it can slightly decrease it's pitch (which is creating the forward momentum) but it still needs to pitch, which means more power is required to maintain current altitude. It may need to increase its pitch though, such as if more wind resistance is encountered. But still, any pitch (to generate horizontal movement) is going to require more power. If this could be done by pitch alone (and no additional power), you'd be creating energy (movement) out of nothing.

Proof would be E=mc2 and Newtons Laws of Mechanics.

 

[albatross911]

what direction were u flying from, the pics u posted dont look 3.5 km from the land in the back of the pic, thats a very cool place to shoot, hope to see more pics/vids from this location,

I did post my flight log and a video earlier, you are welcome to check out the links for details.

 

[albatross911]

HK Macau Bridge!!! where were you when you flew the drone?

From Tai O, check my flight log for details.

 

[albatross911]

This is going into more technical in a direction that I did not expect but it's good discussion anyway. For me I never intended to range test my Mavic for this flight nor any flight in the future. If I couldn't reach the island by 30% battery I would have given up and returned home. There are just too much variables for stretching the range limit and the results could be catastrophic, at least for my wallet.

I was really hoping to say that the forced landing warning is really heart wrenching and would suggest DJI to change it to silent after maybe 10 seconds. It's driving me into panic mode and attract unnecessary attention around. So far no one seems to pick up on this issue. Maybe not many has gone down to 10% and experienced it first hand?

 

[sar104]

More motor thrust is generated y increasing the speed of the motors. If you agree with this then you agree with my statement. The second part of your quote appears to contradict your first statement.

In order to maintain the current altitude with a higher degree of pitch, the Mavic needs to generate more upward thrust. This is done by increasing the speed of the motors. Lacking this and you'd be generating power from nothing, which is not possible.

In order to generate forward momentum the Mavic pitches forward. All things being equal it converts some of the motor/prop energy that is maintaining it's altitude into forward momentum. By converting some of that upward thrust power to power used to move itself horizontally, it's going to lose altitude (as mentioned, you can't divert some lifting power to move an object forward and still maintain altitude). So how does the Mavic maintain the same altitude? It needs to create more power. As it gains forward speed, it can slightly decrease it's pitch (which is creating the forward momentum) but it still needs to pitch, which means more power is required to maintain current altitude. It may need to increase its pitch though, such as if more wind resistance is encountered. But still, any pitch (to generate horizontal movement) is going to require more power. If this could be done by pitch alone (and no additional power), you'd be creating energy (movement) out of nothing.

Proof would be E=mc2 and Newtons Laws of Mechanics.

Agreed - as @Kilrah pointed out - I read more into your comment than you intended, since it did not actually imply that the motor speed limit changes in sport mode.

 

[sar104]

Isn't it just simple vector calculation of the speed, vertical component (lift) and horizontal component (thrust)? The drone aims to maintain altitude. When pitch is increased, the less lift with the same motor power, so the motor power needs to be increase to maintain altitude, which in turn increase the trust component (horizontal speed) as well.

Yes - the motor thrust (force) vector F, which aligns approximately with the aircraft z-axis, decomposed into vertical and horizontal components at a pitch θ, solves for equilibrium in constant-velocity flight. Ignoring body aerodynamic lift, which is not large for a quadcopter:

F cosθ = Mg​

F sinθ = -D = -ku²​

where M is the mass of the aircraft and D is the horizontal drag, which goes roughly with the square of the airspeed, u. Eliminating θ ( since cos²θ + sin²θ = 1):

F = √(M²g² + k²u⁴)
​

Power, P, is approximately related to thrust by P² ∝ F³.

That rather simplified approach gives the following dimensionless relationship between airspeed, motor thrust, power and pitch angle:

 

[tcope]

Agreed - as @Kilrah pointed out - I read more into your comment than you intended, since it did not actually imply that the motor speed limit changes in sport mode.

Oh. No, the rev limit on the motors does not change,

 

[RayOZ]

Yes - the motor thrust (force) vector F, which aligns approximately with the aircraft z-axis, decomposed into vertical and horizontal components at a pitch θ, solves for equilibrium in constant-velocity flight. Ignoring body aerodynamic lift, which is not large for a quadcopter:

F cosθ = Mg​

F sinθ = -D = -ku²​

where M is the mass of the aircraft and D is the horizontal drag, which goes roughly with the square of the airspeed, u. Eliminating θ ( since cos²θ + sin²θ = 1):

F = √(M²g² + k²u⁴)
​

Power, P, is approximately related to thrust by P² ∝ F³.

That rather simplified approach gives the following dimensionless relationship between airspeed, motor thrust, power and pitch angle:

View attachment 35983

I didn't expect you to throw in the mathematical formulae and graphs.
So... whats the theoretical optimum speed for max distance?

 

[sar104]

I didn't expect you to throw in the mathematical formulae and graphs.
So... whats the theoretical optimum speed for max distance?

Interesting question, and a solution does fall out from this calculation:



The important issue is the sensitivity of the solution to the exact form of the drag equation and the error in the zero aerodynamic lift assumption. That needs a sensitivity analysis. I'll do that later.

 

[MGHimages]

Thank you Albatross for sharing this learning experience and all the data. As a relatively novice flyer, I appreciate the points to keep in mind when flying in risky areas where drone recovery is near impossible.

I recently lost my Mavic into the pacific while filming some sea cliffs in Hawaii. Due to intense sunlight washing out my usual display, I flew with my VR goggles. With camera pointed towards the cliffs, I flew sideways, in a slider type shot. I didn't notice that my path had me inching closer and closer to the rocks, and my spotter had wandered off. Instead of stopping to check clearance on my path, I kept filming this dramatic footage of waves crashing on the rocks. Next thing I know, the Mavic ran into an escarpment and tumbled into the ocean, where it rests to this day (since 2/21/18). Sure could have used some side OA!! I can give GPS coordinates for anyone in the Kona / Kailua area of Hawaii that may want to go snorkeling for a Mavic. I would have gone after it myself, but our flight home was leaving 2hrs later, and we couldn't miss it.

So, in spirit of this post, many lessons learned that day!

 

[sar104]

Interesting question, and a solution does fall out from this calculation:

View attachment 35991

The important issue is the sensitivity of the solution to the exact form of the drag equation and the error in the zero aerodynamic lift assumption. That needs a sensitivity analysis. I'll do that later.

It appears that the solution is not that sensitive to the form of the drag equation. In the calculation above, with drag proportional to the square of the airspeed, the maximum flight distance occurs at a pitch of 35°. Changing the exponent to 2.5 (a very significant change) only changes the pitch for maximum range by 3°, to 32°.