Did I get myself into Vortex Ring State?

[JulesTEO]

Hello everyone,

I'm not sure how to address this topic, since I would love to gather some true insight from this incident rather than starting a speculation brainstorm.

Also I'm really not looking for advice; it is clear that this incident was self-induced, and as long as I don't repeat the same maneuvers the H will be just fine.

So, long story short: I was trying to snap a cool image of my H tilted, (I did get it):

But, in the process I believe I got myself into a Vortex Ring State, resulting in a minor crash with my H.

• The scenario was as follows:

----
Trying to get a snapshot of a fully tilted Typhoon H, I began to (very agressively) roll back and forth at maximum rate. (imagine tilting the aircraft as it is shown in the picture above, going side to side while remaining almost in the same position)

After going, back and forth, 6 times in quick succession, the aircraft lost lift completely and it quickly sinked in a wobbly fashion.

Since I was just very close to ground the aircraft made ground contact, but quickly recovered.

I was able to regain altitude with only minor scratches on the camera.
----

So, for those of you with knowledge on aerodynamics, my question is: Did I get myself into Vortex Ring State by tilting the H back and forth so quickly? Or was it something else?

I have everything recorded, but I don't want to upload the video just now without first setting some context around it.

Greetings!

 

[Steve Carr]

VRS would be my first guess. The back and forth movement could have set up an area of turbulence under the H.

 

[Dr Delta]

VRS would be my first guess. The back and forth movement could have set up and area of turbulence under the H.

Yup. Multirotors generate quite some propwash.
Turbulences tend to stay noticable much longer then one would expect.

 

[PatR]

My theory has the aircraft spending more time in an extreme bank where prop positioning will not generate the lift necessary to off set the weight.

I have done similar and observed a fairly rapid altitude loss after several rapid oscillations. The aircraft quickly recovered and stabilized altitude after returning to level flight. In your photo, look closely at the angle the props are oriented. If you drew a straight line through the center of each propeller perpendicular to the blade length it would demonstrate the lift vector of the prop. As flight requires a balance of forces, lift over weight, thrust over drag to be successful we could view the lack of vertical thrust, which for a multirotor is lift. Too little lift is being generated to keep the weight aloft.

 

[FlushVision]

Hello everyone,

I'm not sure how to address this topic, since I would love to gather some true insight from this incident rather than starting a speculation brainstorm.

Also I'm really not looking for advice; it is clear that this incident was self-induced, and as long as I don't repeat the same maneuvers the H will be just fine.

So, long story short: I was trying to snap a cool image of my H tilted, (I did get it):

But, in the process I believe I got myself into a Vortex Ring State, resulting in a minor crash with my H.

• The scenario was as follows:

----
Trying to get a snapshot of a fully tilted Typhoon H, I began to (very agressively) roll back and forth at maximum rate. (imagine tilting the aircraft as it is shown in the picture above, going side to side while remaining almost in the same position)

After going, back and forth, 6 times in quick succession, the aircraft lost lift completely and it quickly sinked in a wobbly fashion.

Since I was just very close to ground the aircraft made ground contact, but quickly recovered.

I was able to regain altitude with only minor scratches on the camera.
----

So, for those of you with knowledge on aerodynamics, my question is: Did I get myself into Vortex Ring State by tilting the H back and forth so quickly? Or was it something else?

I have everything recorded, but I don't want to upload the video just now without first setting some context around it.

Greetings!

These days with modern drones it is quite difficult to get into VRS, particularly if it has six props. VRS was something to be very aware of on earlier quads. Descent speed limitations imposed on drones by the firmware has largely gone towards making VRS almost a thing of the past. However, it has not been eliminated completely and some thought should still be exercised to ensure it is avoided: Never descend straight down with full left stick...always come down in a diagonal fashion flying into the wind.

Your statement: ' the aircraft lost lift completely and it quickly sinked in a wobbly fashion' tells me that this was VRS initially induced by making the aircraft go back and forth, 6 times in quick succession causing the aircraft to lose lift. The start of the descent wasn't VRS but VRS was quickly achieved once the aircraft had begun it's descent. The aircraft had been flown beyond it's capability to remain airborne.

 

[PatR]

Also to consider is that load factor on an aircraft increases with angle of bank. Multirotors are a bit more complex in this area than fixed wing as they bank for every directional change due to opposing prop RPM increases and reductions. A 60* angle of bank doubles aircraft weight and thrust must be added to compensate. In the photos the aircraft is at a roughly 45* bank angle, increasing aircraft weight by ~1.3 times its weight in level flight. To obtain that photo the aircraft had to be spending a minimum time of at least twice what it spent in a level state due to opposing bank oscillations. Probably closer to three times longer. As the Typhoon H does not have an abundance of thrust reserves it could not compensate for increased loading.

 

[JulesTEO]

Thanks for the feedback.

Honestly I wasnt even familiar with Vortrex Ring State prior to this, mostly because as @FlushVision said, the H makes it almost impossible to achieve. I guess every flight is a new lesson.

I will keep this incident in mind if (for any reason) I have to recreate these maneouvers in the future.

As I mentioned, I didnt want to share the video without some context. Now it has been set, here's the video of the incident:

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Greetings!

 

[CraigCam]

Check your telemetry and you will see that your power dropped during those 6 fast tilts. I use that maneuver indoors to fly down charged batteries to prep for charger storage. I’ve observed a battery at 14.8 drop to LVC of 14.3 on the ST meter during my stick stirring. So it can be a double whammy of wash and power loss and if you’d been higher up and just let go of sticks, it would have stabilized. Good to learn how far you can push your skills.

 

[PatR]

I’ve done it with the gear down. The swings are a little more violent, but as mentioned, if higher it will self recover after letting go of the sticks before hitting the ground.

 

[KEITH KUHN]

VRS would be my first guess. The back and forth movement could have set up and area of turbulence under the H.

I like juuls he's like a surgeon when it comes to working on typhoon H
he's the kind of guy if I needed work on my typhoon h i could trust him to do a good job.
Keith Kuhn

 

[JulesTEO]

Check your telemetry and you will see that your power dropped during those 6 fast tilts. I use that maneuver indoors to fly down charged batteries to prep for charger storage. I’ve observed a battery at 14.8 drop to LVC of 14.3 on the ST meter during my stick stirring. So it can be a double whammy of wash and power loss and if you’d been higher up and just let go of sticks, it would have stabilized. Good to learn how far you can push your skills.

During the side to side motion voltage only dropped from 14.9v to 14.8v, but as the H began falling it went down to 14.5 while throttling to regain alt.

Just for fun I synched the video with the telemetry graps for Roll, Voltage and Altitude

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Greetings!

 

[Steve Carr]

Very nice setup Jules.

 

[FlushVision]

Having looked again at Jules video again I no longer think that the aircraft entered into VRS at any stage. The wobbly descent was simply a result of the aggressive backward and forwards motion rendering the aircraft without sufficient lift. If the aircraft had been higher up I believe it would have recovered so long as the pilot made no further control inputs before coming into contact with the ground. This was not a VRS event...IMHO.

 

[PatR]

Thank you.

Since the throttle was not at full the power available was limited, which in turn limited any possible recovery until after the descent was entered. As the system tried to save itself in an attempt to maintain the commanded altitude voltage dropped due to the increase in current to the motors. There were many aerodynamic forces in play and each contributed to the altitude loss.

IMO, multirotors never experience a VRX, but they do experience conditions where the operator reduces power to a level insufficient for the motors/propellers to generate enough for stability. As multirotors increase and decrease the speed of different motors simultaneously to maintain level flight, it is quite easy with some systems to enter a very fast descent and not maintain enough power to stabilize the aircraft. Those aircraft are in essence in a state of free fall with motors handicapped by insufficient power supply to perform effectively.

 

[AveryBadman96]

I'm a helo pilot and a drone pilot. VRS occurs during a straight decent to fast, without any movement in a forward, backward, or sideways movement. Decending too quickly into your propeller downward causes a sudden drop and loss of control. I doubt it was VRS as you were not decending, rather rapidly straining side to side, unless the drone did not compensate with thrust to maintain the elevation. To get out of VRS it is essential to increase throttle and pitch in any direction of forward, backward, or sideways. As a professional pilot I tend to always descend with motion forward or backward, rarely sideways unless no other option. That is why I always decend manually in angle mode in rabbit mode. Caution is that elevation is required to recover from VRS as the closer to the ground the higher the risk of VRS and less likely to recover. You cannot recover from VRS by simply increasing throttle, because as you remember you are dropping into your downwash making lift almost impossible.

 

[PatR]

As VRS is generally attributed to helicopters with a single rotor disc I find it difficult for it to be applicable to multirotors with numerous opposing rotation propellers. Although all of them generate upwards thrust relative to the hub face their rotational down wash effectively counters each other.

 

[AveryBadman96]

True more rotors less likely, but still possible as lift occurs by pushing air down by the props. I have not experienced this with my Typhoon H 480 or 920plus, but I have with my DJI P4pro and other quadcopters. Yuneec has the decent at such a slow speed that the likelyhood of VRS is minimal. DJI and other quadcopters boast a faster decent speed, but with 4 props and faster decent increases chance of VRS. There are videos of crashes with drones, and there ones not running into trees or cars that automatically lose control are most likely due to VRS, at least some.

 

[PatR]

Just for discussion purposes, I have platforms where throttle control is completely different from consumer drones. Bottom stick = low idle RPM, mid stick = hover, anything above middle is climb, with rate dependent upon aircraft weight.

Previous experience has shown that reduction of throttle from a high hover would permit the aircraft to descend at a speed commensurate with throttle reduction, which could be quite fast. The more throttle was reduced the more “wobbly” the aircraft became. However, with enough reduction of throttle the aircraft would enter a momentary unstabilized free fall before rolling inverted into an uncontrolled descent. If allowed to continue a crash was assured. If enough altitude was present, resuming full throttle would cause the FC to right the aircraft and resume controlled flight.

Those experiments are why I don’t believe multirotors experience actual VRS. What I know happens is that not enough current is being provided to the motors to effectively stabilize the aircraft, with the “wobbles” caused by motors provided only enough current to attempt stabilization, but lacking adequate current the motors induce destabilization as they cannot generate enough thrust to compensate for the weight of the aircraft. As one corner drops it is provided current to lift it back up but a shortage of total available current causes opposing motors to end up in a current shunt, starting the cycle all over again. The FC ends up fighting itself in see saw trying to provide current it is being denied through throttle position.

That’s my story and I’m sticking to it

 

[JulesTEO]

Woah, seems this was way more complex than I initially thought heheh.

All the feedback is very much appreciated, Whatever caused the "unrequested descent surplus" I'm glad I got it on video.

After 2 years getting to know the the H I thought I already knew all of its limits..

Greetings!

 

[PatR]

Jules,

I always enjoy discussions related to VRS and multirotors. I also enjoy the educational opportunity when rotary wing pilots join in with their experience. I have yet to find the physics that would marry the two genres with VRS possibilities but I’m open to being convinced they could share the same condition.

There’s also the sharing of multirotor functionality that others can learn from our individual experiences. I’m not as serious as I may sound. Think debate, not argument