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[Guide] How to check for prop wear.
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fans1cafe718
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Deucalion Posted at 4-30 14:27
Here is my first test inside the house. I set the drone on the floor, started motors, lifted off, hovered for a bit, yawed CW for a couple revs, hovered, yawed CCW for a couple revs, hovered, landed, picked the drone back up. Notice that even though I am yawing CW or CCW, there is very little to see in the RPMs. It looks like I am hovering the whole time. Which is why I added the Gyro to make sure.

[view_image]

Very nice!

...so, rear-left engine heavy/high rpm's...no wind...no mouvement...hover?

Props not a cause?

...your rear-left engine identified by color blue...confirm?


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120ccpm
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Occams Razor Posted at 4-30 14:14
That doesn't make sense.  The aircraft's attitude will change whenever the props rotate at different speeds.

Did you mean attitude or altitude?  If you meant altitude then it will not change.  Think of it this way, when the rear propellers have a higher rotation speed than the front props then the AC will tilt forward.  When the front end of the AC is pitched downward, some of the thrust created from the propellers will be converted into forward thrust which moves the aircraft forward and some thrust is downward thrust which will maintain the altitude.  The FC knows the exact combination of front and rear RPMs to keep the AC at the right pitch to achieve a forward speed and maintain a certain altitude.   When flying backwards, the same concepts apply and the front motors will spin at a higher rate than the rear props to achieve an upward pitch of the front end of the AC.

Mmmm... nope, in steady forward flight, the front/rear ratio is going to be pretty much the same as during a hover, otherwise the AC would flip. A bit more power on the rear to induce the tilt, of course, but then back to hovering ratio.
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fans1cafe718 Posted at 4-30 14:31
Very nice!

...so, rear-left engine heavy/high rpm's...no wind...no mouvement...hover?

Yes, RearLeft Blue. No wind, in house, original props. I am doing some outside, but there is a lot of wind.
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fans1cafe718
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Deucalion Posted at 4-30 14:57
Yes, RearLeft Blue. No wind, in house, original props. I am doing some outside, but there is a lot of wind.

...hmmm...rear-left heavy...in favorable conditions...

...geeez...
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m80116
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For sheer aerodynamics alone the upper part suffers more resistance moving forward thus demands more uplift from the propellers / rpm from the motors.

How dearly might one miss hallmark007 saying something that shoots so high in the sky that would settle the argument immediately among everyone's dismay.
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fans1cafe718
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m80116 Posted at 4-30 15:31
For sheer aerodynamics alone the upper part suffers more resistance moving forward thus demands more uplift from the propellers / rpm from the motors.

How dearly might one miss hallmark007 saying something that shoots so high in the sky that would settle the argument immediately among everyone's dismay.

...for the masses, in simple words....?

...you talking about angle of attack?

...or, shape of wing?
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Zbip57 Posted at 4-30 11:53
"rear motors will show higher RPMs than front motors, because there's a bit more weight on the rear of the AC."

Re-did the measurements and, long story short, you are correct, there is bit more weight on the front motors. Sorry for the confusion.

It's a pain to measure the weight on the motors because the body gets in the way but at end I got 131g front and 118g rear (give or take 1g).
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Deucalion
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I really didn't want to change props because those never gave me a bit of trouble. But in the name of science.

All new props, same flight pattern. Start motors, liftoff, hover, yaw CW, hover, yaw CCW, hover, land. It is obviously a crapshoot what shape a prop is when you open the bag. I will probably change that one front one with one of the others and see if I can get them to match.

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m80116
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Put one hand out of your car window while you move, now put the other hand behind the first one. You can even try swapping hand positions. Very easy concept.
Let's hope some deeply intelligent mind won't chime in and say: are you suggesting I drive with my knees!?
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fans1cafe718
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m80116 Posted at 4-30 15:43
Put one hand out of your car window while you drive, now put the other hand behind the first one. You can even try swapping hand positions. Very easy concept.
Let's hope some deeply intelligent mind won't chime in and say: are you suggesting I drive with my knees!?

...lol...is like pi***ng in the wind?

...funny...

...but, not recommended in flight...

...better pee in your nomex suit
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m80116 Posted at 4-30 15:31
For sheer aerodynamics alone the upper part suffers more resistance moving forward thus demands more uplift from the propellers / rpm from the motors.

How dearly might one miss hallmark007 saying something that shoots so high in the sky that would settle the argument immediately among everyone's dismay.

I don't follow, either. What "upper part" are you referring to?
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fans1cafe718
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120ccpm Posted at 4-30 15:54
I don't follow, either. What "upper part" are you referring to?

...he must be reffering to upper part of wing.

It's more curved...thus giving lift.

...it's simple physics.
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fans1cafe718 Posted at 4-30 15:58
...he must be reffering to upper part of wing.

It's more curved...thus giving lift.

Actually, the lift comes from the bottom of the wing. The air across the top is going faster (because of the curvature) which means less pressure, thus more pressure on the bottom.
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Deucalion Posted at 4-30 16:10
Actually, the lift comes from the bottom of the wing. The air across the top is going faster (because of the curvature) which means less pressure, thus more pressure on the bottom.

...yes...agree...lol...am getting old

...lower on top = more lift from bottom.
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Deucalion Posted at 4-30 16:10
Actually, the lift comes from the bottom of the wing. The air across the top is going faster (because of the curvature) which means less pressure, thus more pressure on the bottom.

Correct Bernoulli’s principle, 10 out of 10... lol.
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ABeardedItalian
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I never thought I would have these kind of results today, what a fun little project this has been.

To remind everyone this is what the Original props look like:

Now here's the original props reinstalled:


The RPM difference between the Rear Left and Rear Right has grown again but resembles close to the original graph but a little tighter together.

Now this is where things get interesting, I decided to swap the Original Rear props to the front motors and the Original Front props to the Rear. Immediately something was wrong, on take off I heard the props clipping the body. I thought i was chopping grass at first but then I realized what was happening. I started the camera and did some bounce hovers and you can hear the props striking the body, also the drone has a totally different sound.



Here's the graph from this flight:


I went back inside and replaced the front props (Original Rears) with the replacement props I had on the Rear earlier. So now the Rear Props are my original Front props and the Front props have been replaced entirely. I went back out and took off, immediately it sounded better and repeating the same flight actions from before but the props never strike the body.




Here's the graph:


Look at how nice and even that graph is while hovering.

Here's the same forward/back/up/down test with the above setup, I think this is the best looking graph I've produced yet.


It looks a lot similar to the one I posted previously when I only changed the Rear Props:


Lets overlap the two:


So my mind has started to race since experiencing the prop strikes, have all those posts about prop strike been an early sign of prop fatigue? With the increased rpm/work from the rear motors do the props flatten/deform quicker then the fronts? With the "Fatigued" props on the front motors I immediately had striking issues. This would indicate that the Rear props are indeed fatiguing and flattening out, are my Rear props inherently weaker from the deformation/flattening out under takeoff stress which was shown in the video as it also happens when landing. The right front also showed signs of increased RPM over the other motors and makes me question if this was the Prop that was striking the body.

I would love to know if someone who is experiencing prop strikes has their issue resolved simply by replacing the props. No more of this discussion about the props being to close to the body, it explains why only some have strikes and the rest fly fine.

We don't need weights to flatten the props, this is happening on it's own from natural use. Now I have questions, do props exposed to higher temps experience quicker deformation? Does the props being bent in the case cause the crystalline structure of the plastic to break down causing accelerated fatiguing? Are the props just to thin to sustain form for longer then X flights? So many new questions I have.

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Deucalion
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Ok, I changed the front right prop and got a pretty good set. The gyros look higher because we didn't have such much movement when we set it on the floor and picked it up. So the scale is different.

GoodProps.png




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fans1cafe718
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ABeardedItalian Posted at 4-30 16:32
I never thought I would have these kind of results today, what a fun little project this has been.

To remind everyone this is what the Original props look like:

...very exp Irish dude Hallmark has made very harsch tests on props...and the thing still flies...

I think its software..this is my 2 canadian cents opininon on this.

...btw, God Bless them dear military Canucks on helo crash this week in Med sea (off subject...sorry)
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ABeardedItalian
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Deucalion Posted at 4-30 16:40
Ok, I changed the front right prop and got a pretty good set. The gyros look higher because we didn't have such much movement when we set it on the floor and picked it up. So the scale is different.

[view_image]

Looks good to me.

...very exp Irish dude Hallmark has made very harsch tests on props...and the thing still flies...


I wish we brought up this discussion sooner, when Hallmark was changing his front props to his rear we could of evaluated the rpm data to see what if any change was made. I would actually like to see one of Hallmarks dat files but from his posting it sounds like his mini is performing fine. If you watched my video the mini was still flying, perhaps if my props weren't so "fatigued" it wouldn't of caused striking and I would of proceeded to do a normal test instead of aborting.
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ABeardedItalian Posted at 4-30 17:10
Looks good to me.

...very exp Irish dude Hallmark has made very harsch tests on props...and the thing still flies...

I think that the props have a pretty healthy margin of error, which is why the uncommanded descents and speed errors were rare. Even my original props, which had the most variation, never had a speed error. And now I see why. After running it up 400 feet at max speed in sport mode, and in wind, the max RPM they got, for one moment was 15,000.

Except that we want to test things, I wish I hadn't changed them. Knowing what I know now, I will wait for the FW to give me a warning before changing props.

So what about the bad props that some users get? I guess that is a QA issue and some of them come from the factory too far out of spec. But now the FW will warn them to change them, and they should follow its advice.

Here is the original props going up to 400 feet in wind max speed...



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fans1cafe718
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"QA issue and some of them come from the factory"

...they will never admit....BU HAU!! (no way, in english!)
...hmmm...


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ABeardedItalian Posted at 4-30 17:10
Looks good to me.

...very exp Irish dude Hallmark has made very harsch tests on props...and the thing still flies...

My props are fine and I’ve had them on since November , I store them in combo case, I often just bring my drone stick it in my pocket I don’t try anything special to protect my props except to stop them flapping about and I examine for imperfections before flying as one should, I have also noticed that it now seems many here have “bad” props without warnings and since FW update I’ve seen no drop outs or indeed involuntary descending.

Almost 6 months they’ve been stored in combo bag with these protective covers on.

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Occams Razor
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120ccpm Posted at 4-30 14:33
Mmmm... nope, in steady forward flight, the front/rear ratio is going to be pretty much the same as during a hover, otherwise the AC would flip. A bit more power on the rear to induce the tilt, of course, but then back to hovering ratio.

Look at this graph between front and rear RPM differential against forward pitch and horizontal speed.  The rear motors need to achieve greater power than the front motor to sustain forward pitch.  If you don't believe the data then run the test yourself and look at the RPM's of the 4 motors.  I suggest sports mode at max speed so that the data will be easier to interpret.   Here's an easy experiment to verify what you are saying - push down on the front end of a hovering drone and by your understanding the drone should sustain forward movement.  

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fans1cafe718
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hallmark007 Posted at 4-30 17:43
My props are fine and I’ve had them on since November , I store them in combo case, I often just bring my drone stick it in my pocket I don’t try anything special to protect my props except to stop them flapping about and I examine for imperfections before flying as one should, I have also noticed that it now seems many here have “bad” props without warnings and since FW update I’ve seen no drop outs or indeed involuntary descending.

Almost 6 months they’ve been stored in combo bag with these protective covers on.

hey...your other birds are fine...?

Just curious...thx.
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fans1cafe718
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Occams Razor Posted at 4-30 17:46
Look at this graph between front and rear RPM differential against forward pitch and horizontal speed.  The rear motors need to achieve greater power than the front motor to sustain forward pitch.  If you don't believe the data then run the test yourself and look at the RPM's of the 4 motors.  I suggest sports mode at max speed so that the data will be easier to interpret.   Here's another experiment - push down on the front end of a hovering drone and by your explanation the drone will sustain forward movement.

[view_image]

...this is to be expected...you seem surprised?
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Deucalion
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Occams Razor Posted at 4-30 17:46
Look at this graph between front and rear RPM differential against forward pitch and horizontal speed.  The rear motors need to achieve greater power than the front motor to sustain forward pitch.  If you don't believe the data then run the test yourself and look at the RPM's of the 4 motors.  I suggest sports mode at max speed so that the data will be easier to interpret.   Here's another experiment - push down on the front end of a hovering drone and by your explanation the drone will sustain forward movement.

[view_image]

One issue here is that RPM doesn't tell you that one is generating more lift than another. The prop could just be less efficient. In my tests with original and new props, in one case the left rear was at a higher RPM and in another it was the right front. You would first need to establish a scale for each motor and then run the test.
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fans1cafe718 Posted at 4-30 17:47
hey...your other birds are fine...?

Just curious...thx.

Yeah they’re all good no problems.
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fans1cafe718
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"One issue here is that RPM doesn't tell you that one is generating more lift than another"

...porked attempt to fix "uncommanded descents" in latest firm.

...long shot but...lol...hope am wrong.
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Occams Razor
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Deucalion Posted at 4-30 17:53
One issue here is that RPM doesn't tell you that one is generating more lift than another. The prop could just be less efficient. In my tests with original and new props, in one case the left rear was at a higher RPM and in another it was the right front. You would first need to establish a scale for each motor and then run the test.

If the rear RPM's are not generating greater lift then you would not achieve forward pitch and horizontal speed.  Please look at all the data on the chart.
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Occams Razor
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fans1cafe718 Posted at 4-30 17:50
...this is to be expected...you seem surprised?

I am not surprised.  Some users believe that you can achieve forward pitch and horizontal speed with all 4 motors running at the same RPM's.  
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Occams Razor Posted at 4-30 18:09
I am not surprised.  Some users believe that you can achieve forward pitch and horizontal speed with all 4 motors running at the same RPM's.

...indeed...
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120ccpm Posted at 4-30 15:41
Re-did the measurements and, long story short, you are correct, there is bit more weight on the front motors. Sorry for the confusion.

It's a pain to measure the weight on the motors because the body gets in the way but at end I got 131g front and 118g rear (give or take 1g).

120ccpm wrote: "It's a pain to measure the weight on the motors because the body gets in  the way but at end I got 131g front and 118g rear (give or take 1g)."

Cool.  That works out almost exactly to what I estimated when balancing it on a string.

If the 249g were split evenly between the front and rear at 124.5g each, then the centre of gravity would be located exactly halfway between the front and back motors.  The distance between the front and back motors is 5".


131 / (131+118) = 0.5261
0.5261 x 5" = 2.6305"

Or 0.1305" (1/8") ahead of the 2.5" centre between props.

I really should have measured it when I had it hanging on the string, but I had eyeballed the Centre of Gravity as being 1/4" ahead of centre.  That's pretty close!


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Occams Razor Posted at 4-30 18:07
If the rear RPM's are not generating greater lift then you would not achieve forward pitch and horizontal speed.  Please look at all the data on the chart.

Not arguing for or against that, just point out what I found with using RPM numbers for "lift". But, since you asked.

Short Answer - No, even when the drone is flying forward, it is still hovering, meaning the vertical lift from all 4 props is the same. If it wasn't the same, it would flip over. All the drone is doing when it is flying forward is balancing at an angle forward. Since all the props are at an angle, they all have to raise their RPM to create the same vertical lift that they did when it was hovering over one spot. But since they are at an angle, some of the thrust pushes the drone forward.

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fans1cafe718
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Deucalion Posted at 4-30 18:33
Not arguing for or against that, just point out what I found with using RPM numbers for "lift". But, since you asked.

Short Answer - No, even when the drone is flying forward, it is still hovering, meaning the vertical lift from all 4 props is the same. If it wasn't the same, it would flip over. All the drone is doing when it is flying forward is balancing at an angle forward. Since all the props are at an angle, they all have to raise their RPM to create the same vertical lift that they did when it was hovering over one spot. But since they are at an angle, some of the thrust pushes the drone forward.

...still hovering in forward flight...?
...what are you saying/thinking?
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Occams Razor Posted at 4-30 17:46
Look at this graph between front and rear RPM differential against forward pitch and horizontal speed.  The rear motors need to achieve greater power than the front motor to sustain forward pitch.  If you don't believe the data then run the test yourself and look at the RPM's of the 4 motors.  I suggest sports mode at max speed so that the data will be easier to interpret.   Here's an easy experiment to verify what you are saying - push down on the front end of a hovering drone and by your understanding the drone should sustain forward movement.  

[view_image]

You say, "The rear motors need to achieve greater power than the front motor to sustain forward pitch."

Can you repeat that test and show the same graph with the drone going backwards at full speed in Sports-mode?
According to you, if you're right, the front motors should now need to constantly produce greater power than the rear to sustain a rearward pitch angle.

I'm saying the difference in speed is what causes the drone to change its pitch angle.  Once the desired pitch angle is attained the front and rear motor speeds should return to equal speeds to prevent further pitch rotation. For sure, once the pitch angle is no longer level in hover, both front and rear motors will need to increase speed (equally increase speed) to maintain altitude (height) since now the thrust angle is divided between providing vertical lift and forward thrust.  


If the drone could be pitched forward until its nose was pointing straight down, the props would be generating only forward thrust and no lift at all.  If the rear motors were still producing more thrust than the front motors, the drone would continue to pitch forward until upside-down.


All that aside, the whole reason we're in this thread is that evidently propeller deterioration has somehow resulted in the rear blades producing less lift than the front blades.  The flight controller compensates for that by increasing the rear motor speeds to the point where a motor overspeed warning is being generated.  So a comparison of motor speed plots isn't necessarily an accurate measure of the lift being produced, unless all four prop blades are replaced with equally fresh blades.

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fans1cafe718
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Zbip57 Posted at 4-30 18:58
You say, "The rear motors need to achieve greater power than the front motor to sustain forward pitch."

Can you repeat that test and show the same graph with the drone going backwards at full speed in Sports-mode?

The DJI Drones CANNOT MODIFY their props ANGLES of ATTACK in order to modify PITCH.

...so, only increase of rear/front RPM's will govern the flight of these machines.

Questions?
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fans1cafe718 Posted at 4-30 18:57
...still hovering in forward flight...?
...what are you saying/thinking?

Hovering as in the vertical lift at each motor is the same and the total vertical lift equals the weight of the drone. The FC is doing the exact same thing it does when it hovers, except it keeps the drone pitched forward at a specific angle, depending on how far the stick is being pressed. Going forward is just the side effect of it being at an angle.
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Deucalion Posted at 4-30 19:13
Hovering as in the vertical lift at each motor is the same and the total vertical lift equals the weight of the drone. The FC is doing the exact same thing it does when it hovers, except it keeps the drone pitched forward at a specific angle, depending on how far the stick is being pressed. Going forward is just the side effect of it being at an angle.

Hovering and forward flight is same dynamics?

No...you get extra lift as you fly forward....even more if into the wind.

....what are you saying?


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Deucalion
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I assume you mean the drone flying forward and at a constant altitude.

1. The drone is pitched forward, which means all 4 props are pitched forward. Since they are pitched forward, they must increase their thrust (RPM) so that the vertical component of their thrust equals the weight of the drone. Otherwise the drone would rise or fall. So that balance mechanism is in place (just like when it is hovering over one spot).

2. The drone must also keep the pitch constant. In other words the front and back rotors have to be pushing the same amount, otherwise the drone will tip forward or backward. Again, the same exact thing it does when it hovers over one spot, except then the pitch that it maintains is level.

3. The drone must also keep its roll balanced (zero if flying forward). Just like when hovering.

It is basically hovering pitched down at the front. All 4 props are generating more thrust to stay at the same altitude because at that angle only part of thrust is applied downward and the rest forward which is why it moves forward.
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Deucalion Posted at 4-30 19:31
I assume you mean the drone flying forward and at a constant altitude.

1. The drone is pitched forward, which means all 4 props are pitched forward. Since they are pitched forward, they must increase their thrust (RPM) so that the vertical component of their thrust equals the weight of the drone. Otherwise the drone would rise or fall. So that balance mechanism is in place (just like when it is hovering over one spot).

How can the props pitch forward in your equation..?

...think about it bud?

(hint...you cannot modify angle of attack)

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