Erratic braking on P3A

[msinger]

BudWalker, I took another look at Luap's video and see what you're describing. It appears the OP was thinking the throttle was controlling the speed.

idahoflyer, are you following the findings above? You basically went full reverse with the wrong stick.

 

[BudWalker]

BudWalker, I took another look at Luap's video and see what you're describing. It appears the OP was thinking the throttle was controlling the speed.

idahoflyer, are you following the findings above? You basically went full reverse with the wrong stick.

I wasn't there, but, another possibility is that reverse pitch came too late. I.e., there just wasn't enough time to slow down and stop. I really should try it on my own P3 first before make that judgement.

 

[msinger]

BudWalker, the OP mentioned this:

"I released the forward throttle before it hit the tree. I actually went full reverse for the last 40 feet and it just kept coasting."

You can see that throttle action in the video above. We'll need to wait and see what he says, but it sounds like he's saying he attempted to correct the situation using the throttle.

 

[BudWalker]

What the video, and my plot, shows is that he pulled back on the elevator in the last 3 seconds before the impact. The throttle had already been pulled back over the last 10 seconds or so. I know what he said in the OP; maybe he mis-stated throttle for elevator. (@idahoflyer please forgive me for putting words in your mouth). I don't think he was confused about throttle vs elevator. It was just too late for the speed and distance.

 

[msinger]

I guess I might have misunderstood what the OP was saying. I thought he was indicating that his Phantom was flying back at full throttle uncontrollably. But, perhaps it's as you say -- he just didn't let off the throttle soon enough. We'll wait for him to report in

 

[BudWalker]

Perhaps you meant to say elevator instead of throttle? I.e., it was flying back at full elevator and he didn't let off the elevator soon enough.

 

[msinger]

Perhaps you meant to say elevator instead of throttle?

Correct. I was referring to the "forward throttle" -- as the OP described it.

 

[idahoflyer]

You guys are correct that I was imprecise with my original description - when I said coming back full throttle I really meant that it in a "full speed" or "pedal to the medal" sense - forgetting that throttle has a more specific meaning in this context. I should have said I was coming back full elevator (nose down, max forward speed). I didn't get my left and right sticks backwards - you see them both pulled down at the end because I was trying to lose altitude and forward speed at the same time. I clearly didn't back off on the elevator soon enough (since I hit a tree!) but that was because based on my past experience flying this thing I was expecting it to brake more aggressively. The braking just seemed sluggish to me - hence checking with all of you pros to see what typical stopping distance from full speed is like for others. I've flown my P3 back to me at full speed at the end of flights before and just felt like this stop was particularly sluggish compared with what I'm used to. Whether that was normal braking or not, I've learned my lesson and will be giving it more margin for error in the future.

 

[mdowney58]

Far from having any real knowledge of this issue, but wouldn't full down throttle actually slow down the stopping process with full down elevator (reverse pitch)?
In my feeble mind, I would think the A.C. would brake faster with full reverse (down elevator stick) and either let the bird's computer handle the throttle, or throttle up.

Sent from my SM-N910V using Tapatalk

 

[BudWalker]

Far from having any real knowledge of this issue, but wouldn't full down throttle actually slow down the stopping process with full down elevator (reverse pitch)?
In my feeble mind, I would think the A.C. would brake faster with full reverse (down elevator stick) and either let the bird's computer handle the throttle, or throttle up.

Sent from my SM-N910V using Tapatalk

That's a really good point. Suppose the elevator controls only the pitch angle. Then for a given elevator setting the reverse thrust is determined by the throttle setting. But, if the flight controller is smart enough it can vary the pitch angle to achieve the desired reverse thrust. I.e., for lower throttle settings the FC would increase the pitch angle. There has to be a limit though; the pitch angle can't get too large. If the throttle is at a min then it doesn't seem like there would be even enough propulsion to pitch up.

Maybe that's what happened to this flight. The throttle was pretty low and the pilot stated the braking seemed sluggish.

 

[idahoflyer]

The interaction between throttle and elevator inputs (dropping forward speed and altitude at the same time) makes sense, and seems like a solid potential explanation. I may test that theory out sometime when I don't have anything around for it to hit if it doesn't stop quickly enough.

Now I just need to figure out if my sonar still works after taking some damage. I've mostly just been flying with VPS off to eliminate the possibility of damaged sensors giving inappropriate input, though nothing weird has happened the couple of times I've turned it back on. I can't tell whether or not it's getting a reading on the height above the ground when I'm in a low hover - usually when the VPS kicks in the bottom right number on my display in the DJI GO app changes from distance from controller to height above the ground (I'm using a Samsung Galaxy S5 so I'm not sure if something with a bigger screen would show both at the same time) but I haven't seen it show the height above the ground again since the crash.

 

[BudWalker]

The interaction between throttle and elevator inputs (dropping forward speed and altitude at the same time) makes sense, and seems like a solid potential explanation. I may test that theory out sometime when I don't have anything around for it to hit if it doesn't stop quickly enough.

Now I just need to figure out if my sonar still works after taking some damage. I've mostly just been flying with VPS off to eliminate the possibility of damaged sensors giving inappropriate input, though nothing weird has happened the couple of times I've turned it back on. I can't tell whether or not it's getting a reading on the height above the ground when I'm in a low hover - usually when the VPS kicks in the bottom right number on my display in the DJI GO app changes from distance from controller to height above the ground (I'm using a Samsung Galaxy S5 so I'm not sure if something with a bigger screen would show both at the same time) but I haven't seen it show the height above the ground again since the crash.

The sonar is always producing data. If VPS is off then the sonar data isn't used by the flight controller. You can check the value of the sonar by using DatCon (get it here) to convert the .DAT file. The column name is vpsHeight. Take a look at the vpsHeight documentation here.

 

[idahoflyer]

Thanks @BudWalker I'll download DatCon and give it a look.

 

[BudWalker]

I was doing some test flights to look into this issue. Thought you might find this particular flight interesting

The P3 is moving at 16 M/s with max elevator and min throttle (i.e. full forward and full down). The elevator was released and went to neutral at 363.6. The speed then started decreasing until it reached a speed of 3 M/s (at 368) and then stopped decreasing. The P3 continued at 3 M/s until the throttle was released and went to neutral at 370.2. At this point the speed decreased to 0. I.e. the P3 coasted and fully stopped only when the throttle was increased.

This example is close to the behavior of the P3 in the incident reported by the pilot. The pilot stated that it seemed like the P3 coasted instead of coming to a full halt. In addition, the throttle was around -8000.

 

[idahoflyer]

Does the calculated velocity take vertical speed into account too or is it just lateral movement? If vertical speed is included then it could have stopped moving laterally but still show non-zero velocity due to the vertical speed.

 

[BudWalker]

Does the calculated velocity take vertical speed into account too or is it just lateral movement? If vertical speed is included then it could have stopped moving laterally but still show non-zero velocity due to the vertical speed.

When you're right you're right. I completely missed that. Vel is a derived field and equals sqrt(velN**2 + velE**2 + velD**2). I'll implement a velH = sqrt(velN**2 + velE**2) but I suspect that the P3 was falling at 3 M/s and that was the velocity.

 

[BudWalker]

How's this?

There is a new beta that has the velH field. It's version 2.2.7 and can be obtained by going here.

 

[idahoflyer]

Interesting that even with the vertical speed factored out the horizontal speed still stays above zero until you release he throttle. Less dramatic looking than before but still coasting a bit.