In ongoing efforts to learn all the capabilities of the P4P, I was practicing flying in ATTI Mode.  During my first serious effort in doing so, I was practicing around a baseball field at about 8 to 10 feet high.  Knowing that when in ATTI you can fly faster and stick sensitivity is increased, but I also found out the hard way about something else I should have already known.  Flying at faster speeds in a copter requires an increased pitch or tilt to translate the thrust to more speed in the horizontal direction, in doing so you experience a measured loss of lift because the lift vector has changed from more up to diagonal.  Which means you will require more thrust/lift to compensate for the loss.

Unlike, yet similar to fixed wing aircraft there is a basic balance of lift, thrust, drag, and weight to achieve stable flight.  As an aircraft flies a constant speed it maintains that balance and therefore level/sustained flight (see diagram #1).  In the case of a copter to maintain a hover both thrust and lift have the same vector, up, while weight remains the same and drag is experienced at the props spinning (for the sake of simplicity we'll suspend explaining & differentiating parasite vs induced drag).  As the copter achieves the proper balance, it will hover (see diagram #2).  Now if the copter is going to move horizontally the rotors/props must be canted to change the thrust vector to achieve this action (see diagram #3).  It is also important to note that in order for the copter to increase speed it will require an increased angle of tilt along with increased prop speed.

What I learned the hard way that day was that loss of lift thing.  In the course of me flying, I did not notice the insidious drop in altitude until it was too late.  Before I could react the bird hit the dirt at a shallow angle and cartwheeled, thankfully and luckily, I only lost a prop on that mistake.  However, after the dust settled and I inspected the drone and operationally tested it I got to thinking, who else may not realize these nuances of flying in ATTI; thus the reason for this thread.  Truth in advertising, I did have some help puzzling this out from some of those more experienced folk.  As one put it, helicopters are always in a controlled fall, something for us to keep in mind.

So, besides my points on the subject, I would like to start a conversation with others out there that might have questions about flying in this mode or to hear from the more experienced among us who have done a lot of flying in ATTI in hopes that they can share their knowledge, mistakes, and lessons learned with us all.

Note: for all you Aeronautical Engineers out there and armchair ones too, please, keep in mind that not everyone here has that level of understanding.  While I am happy to entertain such a level of discussion, please try to keep it at a level that everyone will understand, or take the extra step to explain the more technical aspects you may bring up, this way all can follow the discussion.  Let’s talk flying in ATTI Mode…

Actually this is a great topic. I built my own drone, 450 size, and hoon it around in Atti mode which I love. It really does improve your drone skills.
One thing more I will add to the mix is that the effect of yawing and banking can also decrease height because the drone has to change the power to the relevant motors to achieve this.

I quite like Atti mode although i do not use it as much as i should, i find that the movements feel quite smooth compared to the other modes

Aloha Genghis,

     May I say that is an excellent start to an interesting discussion.  Mahalo!  

     Happy to hear you survived the Phantom gymnastics, BTW it is to be included in the next Olympics (well drone olympics anyway).

Aloha and Drone On!

One of the problems we have here is speed of Flight Controller reaction and compensation.

The Multi-Rotor Flight Controller is designed to maintain an attitude of the MR and has no interest in height or horizontal movement unless instructed to by your sticks or onboard data flow as in barometer / GPS etc. via the IMU.

So Genghis experience is one of Flight Controller failure to maintain height regardless of stick command. It is also correct as many will know - ALL flying machines are actually 'falling' due to gravity not only rotor machines. The fall is either compensated by increased rotor rpm causing increased lift - or increased forward speed of a fixed wing causing increased lift.

Now take the MR - if you are flying at or near max speed - the amount of reserve rpm left to maintain height as the 'tilt due to speed request' increases is not there. So the MR gives you the speed, but cannot maintain height as you are maxed out on the motors ....

It really has very little to do with aerodynamics in fact ... its pure lift based on rotor rpm and FC ability to increase to compensate. If you push left stick up (vertical command) - you will immedaitely detect a lowering of forward speed to allow FC to gain power spare for the command....

Now I know someone is going to argue. Fine - its what we are here for - to discuss and put forward observations / possibilities. My above is based on quite a number of years flying rotor craft .. multi and helicopters.  

Nigel

I fly a fair amount in ATTI mode with my P3P when filming near ground, over fields for example (5-12 meters AGL), and haven't experienced Ghenhis's problem. Since this is done at modest speeds for good video, I guess it would agree with Nigel's comment.

Excellent Subject.  Is ATTI mode used for Indoor Flying or where GPS is probably sketchy?

Wingshooter Posted at 2018-2-6 02:32
Excellent Subject.  Is ATTI mode used for Indoor Flying or where GPS is probably sketchy?

I'm sure many will have different opinions, but because DJI quads are stable with GPS (P Mode) most don't even consider that ATTI Mode is just as viable to use, albeit more difficult.  Before the enhancement of computing power and GPS, ATTI would have been the primary mode to use, and I'm betting many of the long timers here have that experience because of that era.
Yes ATTI would be necessary for indoor use, short of picking up enough satellite coverage the system will kick you over to that.  You can still get position stabilization from the VPS in this case, the GO 4 app calls it OPTI Mode.  However, when selecting ATTI the vision system is disabled and the altitude is established by the use of the barometric altimeter, which is not nearly as accurate.

You can also loose height when flying fast in sports mode, you don't want to use sports mode too close to the ground.

I suspect some of the problem comes from pressure changes caused by the speed which cause the barometer to read height incorrectly so the flight controller maintains height incorrectly, the height always corrects when you slow down.   

I wouldn't choose to use Atti indoors, the OPTI mode is much safer as long as you have enough light.

solentlife Posted at 2018-2-6 02:10
One of the problems we have here is speed of Flight Controller reaction and compensation.

The Multi-Rotor Flight Controller is designed to maintain an attitude of the MR and has no interest in height or horizontal movement unless instructed to by your sticks or onboard data flow as in barometer / GPS etc. via the IMU.

Nigel thank you, that explanation is helpful and useful.  
I would only clarify one point and dispute slightly another.  This has everything to do with aerodynamics; if your thrust available is exceeded then you will lose lift in order to maintain altitude in this case, which is what you were describing.  Albeit here you were noting the tilt part of the equation, meaning reducing the tilt will allow for more lift but reduce horizontal thrust (aka speed) again pointing out that balance between the four forces of flight.  This is why the quad is speed limited to the extent that at some point the rotors rpm cannot go any faster and the tilt cannot be increased anymore without losing lift altogether.  At this point you would be what we call flying on the edge of the envelope, which according to your information, is apparently what I was doing unwittingly pushing the craft to that edge where it could no longer maintain enough lift while meeting the speed demand i.e. thrust limited in that circumstance.  In some ways it is very similar to a high-speed stall, where high-speed stalls can be corrected instantaneously simply by backing off the lift demand.  In the quads case the uncommanded descent can be instantaneously corrected by backing off the speed demand.  I'm sure if we could get an aero expert to weigh in here they'd be able to graph this for us and I'm betting those graphs would likely look fairly similar to one another.
Yes fixed wing and copters are controlling their fall, with one significant difference, when the thrust is stopped for a quad it falls out of the sky while a fixed wing can glide (unless you are an F4) and choppers can attempt an auto rotate assuming they don't have rotor failure or seizure.

Thanks so mUch for this helpful and useful info

Genghis9 Posted at 2018-2-6 03:01
I'm sure many will have different opinions, but because DJI quads are stable with GPS (P Mode) most don't even consider that ATTI Mode is just as viable to use, albeit more difficult.  Before the enhancement of computing power and GPS, ATTI would have been the primary mode to use, and I'm betting many of the long timers here have that experience because of that era.
Yes ATTI would be necessary for indoor use, short of picking up enough satellite coverage the system will kick you over to that.  You can still get position stabilization from the VPS in this case, the GO 4 app calls it OPTI Mode.  However, when selecting ATTI the vision system is disabled and the altitude is established by the use of the barometric altimeter, which is not nearly as accurate.

A good subject, and cautionary tale.

You also have auto hold in Sport mode. Flying indoors in P mode, as you say it would use the optical sensors in the base to hold position if no GPS/GLONASS available. Unless not enough light, then it will automatically default to ATTI mode.

My thought would be that some ATTI experience is desirable should the system default to ATTI, flying under bridges or close to tall buildings perhaps, or thick tree canopies above, where satellite navigation may become unavailable.

This is a bone of contention for some in the Mavic camp as ATTI mode cannot be manually selected on the Mavics to allow practice.

Nigel_ Posted at 2018-2-6 03:31
You can also loose height when flying fast in sports mode, you don't want to use sports mode too close to the ground.

I suspect some of the problem comes from pressure changes caused by the speed which cause the barometer to read height incorrectly so the flight controller maintains height incorrectly, the height always corrects when you slow down.   

Agreed ref OPTI and agreed ref Sport Mode in the case of lift vs speed (thrust) the same ideas and principles will apply.
You know I thought the same thing about the baro readings, sine we don't know where the port is located on the bird, I don't anyway, it is likely that high rates of speed could effect those barometric readings causing a descent or maybe even a climb.
Reference Sport Mode and your thorough explanation earlier, I have a question.  How is it that you can go even faster in Sport Mode than in ATTI Mode?  In other words if you can reach a speed limited circumstance that you described in ATTI but then turn around and get even more performance in Sport what changed?  I'm guessing prop rpm, in Sport Mode the prop speed is increased over what is available in ATTI, correct?

Edit: After looking at the manual I see that the max tilt angle is increased from 35 degrees in ATTI to 42 degrees in Sport which I would think accouts for some difference in speed capability but increases lift problems

Hu G ... my comment about not being aerodynamic was in keeping with your request to simplify.

Of course all balancing acts of thrust / lift / drag etc. are all part of aerodynamics - but MR's differ significantly as you point out - they are basically a brick with multi rotors to lift them up.

Onto a point by another - yes like many others - I grew up without fancy flight controllers on my models ... I flew heli's and various when even gyros were not fitted. They were heavy mechanical items that we just could not suffer the weight of.

I have a number of MR's still that I fly indoors and outside when calm ... they are great fun as you have to FLY them ... unlike the P3 / P4 which flies itself. Sorry but the P's are a long way of from most other MR's in that fact.

As to indoor ... don't know about P4's .... but my P3P if I fire her up indoors - automatically drops into VPS mode ... P-Opti .... I just leave switch in P-GPS position and it decides for itself.

Nigel

Aardvark Posted at 2018-2-6 03:42
A good subject, and cautionary tale.

You also have auto hold in Sport mode. Flying indoors in P mode, as you say it would use the optical sensors in the base to hold position if no GPS/GLONASS available. Unless not enough light, then it will automatically default to ATTI mode.

Aardvark what is your reference for "auto hold" in Sport Mode.  I see it written on the RC under the S1 switch but it encompasses all P, S, & A modes so I'm not sure what that is supposed to mean.  I don't see any ref to it in Sport Mode either.  Are you by chance referring to baro and or GPS?  Just curious...

solentlife Posted at 2018-2-6 03:52
Hu G ... my comment about not being aerodynamic was in keeping with your request to simplify.

Of course all balancing acts of thrust / lift / drag etc. are all part of aerodynamics - but MR's differ significantly as you point out - they are basically a brick with multi rotors to lift them up.

My apologies, I misunderstood the point you were making in the manner you were doing it.  Now I get it and much appreciated.

Yes, I agree, it is best and recommended that indoor flight ops be done in OPTI

So, I take it you would suggest Phantom pilots practice using ATTI, yes?

I have always been a supporter of flying in ALL modes - you never know when its needed.

I have had both my P3S and P3P drop into ATTI mode without warning - first you know is drift ... and the pop-up along top of screen !! and if you are not practiced in it - it can spell disaster.

Like I say - I have many hours flying far less 'stabilised' Multi's ... and so its a heart jerker when it happens - but I can handle it.

Nigel

Genghis9 Posted at 2018-2-6 03:58
Aardvark what is your reference for "auto hold" in Sport Mode.  I see it written on the RC under the S1 switch but it encompasses all P, S, & A modes so I'm not sure what that is supposed to mean.  I don't see any ref to it in Sport Mode either.  Are you by chance referring to baro and or GPS?  Just curious...

On controller, as you say auto hold 'On'-P-S and 'Off'-A.

If you're in sport mode the aircraft uses GPS/GLONASS to hold position.

When in ATTI mode GPS/GLONASS is still active (so the aircraft knows where it is and can RTH if needed) but it is not used to hold the aircrafts position. Sports mode is P mode on steroids without the Obstacle Avoidance, and additional thrust being available.

Genghis9 Posted at 2018-2-6 03:44
Agreed ref OPTI and agreed ref Sport Mode in the case of lift vs speed (thrust) the same ideas and principles will apply.
You know I thought the same thing about the baro readings, sine we don't know where the port is located on the bird, I don't anyway, it is likely that high rates of speed could effect those barometric readings causing a descent or maybe even a climb.
Reference Sport Mode and your thorough explanation earlier, I have a question.  How is it that you can go even faster in Sport Mode than in ATTI Mode?  In other words if you can reach a speed limited circumstance that you described in ATTI but then turn around and get even more performance in Sport what changed?  I'm guessing prop rpm, in Sport Mode the prop speed is increased over what is available in ATTI, correct?

The fact that it can tilt further in sport mode than atti suggests that atti is not limited by the amount of lift it can generate, an inaccurate barometer caused by the airflow around the drone seems a more likely cause of the issue in atti mode.  Also I note that after dropping a bit of altitude in sport mode it then maintains the incorrect altitude so clearly can generate enough lift.

I would expect pressure around the aircraft to drop with speed due to the aerodynamics, which would result in a higher height reading than the real height.   A little puzzling that the P3 doesn't seem to suffer, but it does have a lower top speed and the effect will increase rapidly with increasing speed, not linearly.  It is not an easy effect for the firmware to compensate for since it will also depend on wind speed and wind speed is unknown.

Genghis, many thanks for starting this important topic. It has proved challenging for me, but a necessary evil to be learnt. I needed to use it for my Flight Test to get my PfCO, but more importantly I need it to know that I can fly when GPS is out. The main thing I have noticed, as I do not fly fast with my Phantoms, is that the 'braking' distance is much increased., essentially none at all, and reverse thrust is needed to bring the aircraft to a a hover. An important point when flying more quickly as you can with ATTI mode. I find that I have been helped a lot in flying in ATTI mode by flying a racing drone. Currently only on simulator, until the weather improves. Freerider is, well, free, with one scene. If you want the other scenes it is $5. Plugging my remote in from my racer lets me have the feeling of flying with it very well.
I would also add this question to others for consideration. If you are to fly each mission as safely as possible, is flying in ATTI mode fulfilling that premise? I agree that at times it is essential, and the skills needed to fly in ATTI mode are important, thus I have included a small paragraph in my Ops Manual to clarify that all missions be undertaken in GPS mode unless emergencies dictate otherwise.
One final note of caution, I never land with ATTI mode. Far too many variables, especially if there is a wind.

OK ...

First - I apologise to 'N' but I discount the barometer effects as described ... as its not a sensor that sits out in the 'wind' so to speak ! Any pressure change will be outside the shell and transient ... and out some way from the sensor which is located in the central body.

Second 'Embay' notes a characteristic of ATTI mode - the lack of braking. This is due to not having GPS positional info causing a spot determination to stick to. Basically when in P-GPS mode - you let go of sticks - the position at that point is then the position the FC will determine to hold. In ATTI mode - the position is not used to hover or maintain location - therefore the AC will carry on moving in the direction of last travel, but slowing due to drag. You need to give it a bit of back stick to stop it.

For precision flying - GPS mode is preferred for DJI style gear ... but ATTI mode is better for racing / aerobatics etc.

Nigel

solentlife Posted at 2018-2-6 05:33
OK ...

First - I apologise to 'N' but I discount the barometer effects as described ... as its not a sensor that sits out in the 'wind' so to speak ! Any pressure change will be outside the shell and transient ... and out some way from the sensor which is located in the central body.

The sensor is inside the shell, the shell has some high pressure on the front, which because of the 45 degree angle is actually the top where there are no ventilation holes, and low pressure behind, which because of the 45 degree angle is actually the bottom where all the ventilation holes are located, thus the inside of the aircraft is effectively in a low pressure area.  The low pressure will continue as long as the aircraft is pushing through the air and will increase with speed.

Great info - thanks

Nigel_ Posted at 2018-2-6 06:20
The sensor is inside the shell, the shell has some high pressure on the front, which because of the 45 degree angle is actually the top where there are no ventilation holes, and low pressure behind, which because of the 45 degree angle is actually the bottom where all the ventilation holes are located, thus the inside of the aircraft is effectively in a low pressure area.  The low pressure will continue as long as the aircraft is pushing through the air and will increase with speed.

Sorry but the pressure differential and the effects are far less than imagined or eluded to here.

Nigel

To be honest, I learned how to fly a drone on a $30 CX-20 in atti mode, and that's what I recommend to everyone: buy a cheapo Chinese drone and learn how to fly it without any assistance. A single crash on your experimenting on the Phantom can cost more than the entire cost of a cheap drone, which you can score between $20-50 depending on features and sizes. And, because you don't care about it, you try more adventurous things and learn more in the process. And, if it dies, you're only out a couple dozen dollars. If you kill your P4...

solentlife Posted at 2018-2-6 07:58
Sorry but the pressure differential and the effects are far less than imagined or eluded to here.

Nigel

Not sure how much the pressure would drop behind the Phantom, but on the front side, the pressure increase at top speed in sports mode should be about 0.05psi which is equivalent to -28.13 meters change in altitude.  Seems to me that there is potential for a noticeable error if the barometer isn't completely shielded from pressure changes due to speed...

Genghis9 Posted at 2018-2-6 03:44
Agreed ref OPTI and agreed ref Sport Mode in the case of lift vs speed (thrust) the same ideas and principles will apply.
You know I thought the same thing about the baro readings, sine we don't know where the port is located on the bird, I don't anyway, it is likely that high rates of speed could effect those barometric readings causing a descent or maybe even a climb.
Reference Sport Mode and your thorough explanation earlier, I have a question.  How is it that you can go even faster in Sport Mode than in ATTI Mode?  In other words if you can reach a speed limited circumstance that you described in ATTI but then turn around and get even more performance in Sport what changed?  I'm guessing prop rpm, in Sport Mode the prop speed is increased over what is available in ATTI, correct?

IMHO getting this technical about flying in Atti mode can end up causing more confusion , yes for some how exactly the aircraft behaves within every movement may be interesting to know but when actually flying a lot of this stuck in your head will do you no good.
I’m not rubbishing the technical format of aircraft movements with every stick movement it has its place.
But for me learning to fly Atti mode is more about hand eye coordination knowledge of the wind and a good recovery plan.
First is to learn how to control and get a good feeling for how Aircraft responds to stick movements. This should be done in open area at altitude of at least 30/40 ft well clear of all obstacles. First you should learn how to control the position of the aircraft , ( try learning to hold it in a space of 3/4 ft ) moving on try to learn to yaw Aircraft around until it’s battery is facing you (in phantoms case ) , you will then be able to use controls in a manner that most suits most people ie left is left and right is right.
Aircraft at 50ft distance and 40ft altitude try first to land aircraft safely, when your comfortable with this then move on to trying to bring it home and land close to you.
It is very important to learn how to land safely and to always have a couple of safe options to land apart from homepoint , this is what we need Atti for.
Using Atti for just fun is a great way of getting used to Atti mode but I would always recommend flying at altitude that is well clear of all obstacles, and don’t be afraid at any time to switch back to P mode.

Its like everything we do in life ... always better to practice BEFORE its needed ...

Its like testing the stall of a fixed wing machine ........ take it up 3 mistakes high .... keep pulling back on sticks till she mushes and falls off ... you then can recognise it ... you have seen how to recover ... and you hopefully do not need that plastic bag for the bits when it happens later.

Forget all the **** about pressures ... lift ... all the bits and bobs. Basically you need to know what to do with the sticks to get her under control and safely home.

Nigel

hallmark007 Posted at 2018-2-6 08:27
IMHO getting this technical about flying in Atti mode can end up causing more confusion , yes for some how exactly the aircraft behaves within every movement may be interesting to know but when actually flying a lot of this stuck in your head will do you no good.
I’m not rubbishing the technical format of aircraft movements with every stick movement it has its place.
But for me learning to fly Atti mode is more about hand eye coordination knowledge of the wind and a good recovery plan.

hallmark thank you for your input, this is the reason for the discussion and your input is valued for sure.
I'm sure everyone here is intelligent enough to decide for themselves what information is useful for them and what is not.  I don't think too much knowledge is bad, but I agree with you some don't really prefer more knowledge just more skill.  I believe that is the reason we have some folks with PhDs and others who are Master Carpenters both equally valuable as human beings, obviously for different reasons.
Fly Safe

Aardvark Posted at 2018-2-6 04:50
On controller, as you say auto hold 'On'-P-S and 'Off'-A.

If you're in sport mode the aircraft uses GPS/GLONASS to hold position.

Did not know this...good to know
Thanks

djiuser_iGJqCoTNTQSa Posted at 2018-2-6 08:06
To be honest, I learned how to fly a drone on a $30 CX-20 in atti mode, and that's what I recommend to everyone: buy a cheapo Chinese drone and learn how to fly it without any assistance. A single crash on your experimenting on the Phantom can cost more than the entire cost of a cheap drone, which you can score between $20-50 depending on features and sizes. And, because you don't care about it, you try more adventurous things and learn more in the process. And, if it dies, you're only out a couple dozen dollars. If you kill your P4...

Sage advice, thanks

Nigel_ Posted at 2018-2-6 08:22
Not sure how much the pressure would drop behind the Phantom, but on the front side, the pressure increase at top speed in sports mode should be about 0.05psi which is equivalent to -28.13 meters change in altitude.  Seems to me that there is potential for a noticeable error if the barometer isn't completely shielded from pressure changes due to speed...

I really do think there is something to this, but having no technical knowledge of the Phantom design I can only take in both points from both Nigels...
In an aircraft, pressure altimeter readings are compensated for the effect of temperature, but in this case, I doubt that that factor equates to any significant measure and does not explain the problem discussed.
This makes for an interesting discussion, I give it that, but unless a DJI design engineer comes on and explains this system in detail or an aero engineer with Phantom knowledge can do the same, I'm not sure we can get to the bottom of this theory one way or the other.

I was a reviewer for a number of Multi Rotor machines ... mostly micro to smallish size. Indoor mostly but some capable of use outdoors ... along with mini helicopters.

Believe me - all of them were 100x harder to fly than any DJI product. But they taught you how to ... if you didn't learn - they were ornaments !

A guy earlier recc'd a cheap 'toy drone' ........ yes exactly.

Nigel

Late at night, and up early in the morning.

An aviation engineer, and someone who works with ROV's - I could answer the questions - but I need to trawl through all the replies first.

First thing first though - the P4 has a finite amount of power.

If you max out the lift and then tilt forward - you lift rate will decrease.

Robbing Peter to pay Paul.

We have similar with the workclass ROV's.  The flying much like the drones.  We even have this written into the ROV's software - if one type of hyd pump is fitted - then we can adjust which thrusters have priority - laterals or vertical.

My tuppence worth at the moment.  Basic - but that's what we need at this point.


Just be glad we not getting into the coriolis effect on helicopter blades, and why lateral dampers works so well at stabilising conventional layout helis.

Whether flying as a hobby, or for work, in my humble opinion learning ALL aspects of your bird is a requirement to be safe.

Safe flight......if you are in a neighborhood shooting real estate, or in a field or construction area doing mapping,  being able to flip the switch to take control of your aircraft is a requirement.  You MUST know what to expect from your aircraft, and practice is what it takes.   I was flying a Drone Deploy practice map using 3 batteries at our local field...and once a battery was at 30% I switched to ATTI and flew back using the screen as well as visual,  1000' to 1500' returns.  It's different....and practice is needed to be comfy with it.

It's great to understand the forward motion at speed, and loss of altitude, and the reasons WHY....it helps to make sense of it all!  But practice the flight, and master your aircraft.

Let's be safe out there!

Randy

Rigger73 Posted at 2018-2-6 14:05
Late at night, and up early in the morning.

An aviation engineer, and someone who works with ROV's - I could answer the questions - but I need to trawl through all the replies first.

Thanks!
Look forward to your comments...

rwynant V1 Posted at 2018-2-6 15:23
Whether flying as a hobby, or for work, in my humble opinion learning ALL aspects of your bird is a requirement to be safe.

Safe flight......if you are in a neighborhood shooting real estate, or in a field or construction area doing mapping,  being able to flip the switch to take control of your aircraft is a requirement.  You MUST know what to expect from your aircraft, and practice is what it takes.   I was flying a Drone Deploy practice map using 3 batteries at our local field...and once a battery was at 30% I switched to ATTI and flew back using the screen as well as visual,  1000' to 1500' returns.  It's different....and practice is needed to be comfy with it.

Good words

One characteristic of multis that I haven't seen mentioned, but is actually vital when flying in atti or manual mode, where max power is typically available to all the motors. As you approach full power the control responses on a multicopter degrade significantly. If the flight controller permits absolutely full power (as many do in manual mode and sorta do in atti mode), the aircraft becomes very difficult to control in all axes. This isn't a bug - rather, it's generally what you get when you have 4/6 fans going completely full blast. The only option the flight controller has for correction or maneuvering is to slow certain fans down; since the fans are maxed out, it can't speed them up in the usual way any more to induce a maneuver. Thus the very limited control at or near full power.

The main symptom of this (besides the throttle stick up against the stop, that is) is negative stability oscillations with poor damping (meaning oscillations, even if they start small, tend to get worse and worse rather than diminish) - yaw may be very very sensitive and once started, the a/c may continue to turn even when you release the stick. Same thing with attitude oscillations, which can become violent with very bad results when near the ground.

DJI controllers in GPS mode are very good about making sure that they maintain an ample reserve - at full stick, the motors are nowhere near maxed out so there's plenty of reserve power left to maintain good control. In Atti mode, this is less so. In manual mode (not available on the P4P or mavic pro, but it is on other older controllers like the Naza or Wookong), the aircraft becomes almost dangerous at full stick, especially near the ground.

Finally, atti mode in the P4P, for example, still has the level attitude "autopiloting" enabled - meaning it naturally returns to level and stick inputs have to be held in to maintain forward, rearward, etc. flight. And it still will attempt to maintain altitude at least to a significant degree. So it won't fly quite like a helicopter or a multi in manual mode. As an example of the difference: Both sticks full forward on a P4P in Sport mode and the a/c will climb as hard as it can at forward speed as fast as possible. In a copter like my F550/Naza in manual mode, the same inputs will yield an outside loop if you're high up enough, or pile into the ground if you're too low .

But that said, it's possible in the incident you had, this degradation in control might have been a factor. I know I've almost gotten into " P IO's" with my P4P pushing the elevator full forward in atti, though it's not nearly as bad as my F550 in manual mode (which is just plain scary to control at full stick).

Anyway, this is just a general precaution about really pushing the throttle stick up and up and into the stop in non-GPS modes. And really in general on multis, this is one of those things to avoid doing...

embayweather Posted at 2018-2-6 05:09
Genghis, many thanks for starting this important topic. It has proved challenging for me, but a necessary evil to be learnt. I needed to use it for my Flight Test to get my PfCO, but more importantly I need it to know that I can fly when GPS is out. The main thing I have noticed, as I do not fly fast with my Phantoms, is that the 'braking' distance is much increased., essentially none at all, and reverse thrust is needed to bring the aircraft to a a hover. An important point when flying more quickly as you can with ATTI mode. I find that I have been helped a lot in flying in ATTI mode by flying a racing drone. Currently only on simulator, until the weather improves. Freerider is, well, free, with one scene. If you want the other scenes it is $5. Plugging my remote in from my racer lets me have the feeling of flying with it very well.
I would also add this question to others for consideration. If you are to fly each mission as safely as possible, is flying in ATTI mode fulfilling that premise? I agree that at times it is essential, and the skills needed to fly in ATTI mode are important, thus I have included a small paragraph in my Ops Manual to clarify that all missions be undertaken in GPS mode unless emergencies dictate otherwise.
One final note of caution, I never land with ATTI mode. Far too many variables, especially if there is a wind.

On aircraft like the P4P, I wouldn't necessarily force myself to fly in atti mode all the time for safety purposes. It's sufficient for me to have done enough practice in atti mode to deal with a loss of GPS when on a mission. Otherwise, I just do all my flights with my P4P in GPS mode, even when it's fun flying. But I'll still try to do at least a battery a week in atti just to keep myself used to it.

As for landing in atti mode, it's only worth practicing that in low to moderate winds, and even then it's going to be dicy. The P4P is such a small aircraft and perched on top of a fairly unstable landing gear, if you have to put it down in atti mode in a 20+ mph wind it's just going to cost you a couple props probably no matter how good you are. Trying to practice it is going to be expensive too. So I'd just take the minuscule risk of GPS being out when you have to land on a 25mph wind day and just do all your landings in GPS mode. Unless you just have a pile of spare props, etc....

luciens Posted at 2018-2-6 20:55
One characteristic of multis that I haven't seen mentioned, but is actually vital when flying in atti or manual mode, where max power is typically available to all the motors. As you approach full power the control responses on a multicopter degrade significantly. If the flight controller permits absolutely full power (as many do in manual mode and sorta do in atti mode), the aircraft becomes very difficult to control in all axes. This isn't a bug - rather, it's generally what you get when you have 4/6 fans going completely full blast. The only option the flight controller has for correction or maneuvering is to slow certain fans down; since the fans are maxed out, it can't speed them up in the usual way any more to induce a maneuver. Thus the very limited control at or near full power.

The main symptom of this (besides the throttle stick up against the stop, that is) is negative stability oscillations with poor damping (meaning oscillations, even if they start small, tend to get worse and worse rather than diminish) - yaw may be very very sensitive and once started, the a/c may continue to turn even when you release the stick. Same thing with attitude oscillations, which can become violent with very bad results when near the ground.

Thanks for those insights
The thing I find interesting is the reduced control authority when at max.  It makes sense, but is this due to being thrust limited or power limited or is it both?
I think either way and based on inputs thus far, it seems consistent in saying that in ATTI you get increased performance capability while at the same time being placed closer to the operating edge of the craft's envelope.