Collective Pitch Quad support

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Chris,
I’m not sure that I understand your comment here. Are you saying that if, and its a big if, a CH-47 suffered a dual engine flameout that they would not have enough rotor energy to allow for a successful entry into autorotation (i.e. rotor speed bleeds off too quickly to give the pilot enough time to get the collective down). Or are you saying that they could get into autorotation but it wouldn’t be controllable. Or that they could get into autorotation and set up a glide profile however couldn’t successfully land it because rotor speed would bleed off too quickly on the final collective pull at touchdown.

I agree with Brad that the Chinook can autorotate like any other helicopter. Now with the collective pitch quad, it all depends on whether the rotors are interconnected when the motor stops. If they aren’t then I think all bets are off. If they are interconnected then I think it could get into the autorotational profile but I would agree that the entry in the event of sudden engine failure and success of pulling it off at the bottom would rely on the rotor inertia.

just my 2cents

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With a CH-47 there is no such thing as a regular autorotation. Basically, if you survive a single-enge out it was considered successful. The helicopter is destroyed or badly damaged on contact with the earth in a both engines out auto. While both engines out are extremely rare, there is only one known instance where the crew survived a CH-47 crash with both engines out.

The far more common fatal failure in the CH-47 is de-sync where the pinion coupler or shaft to the front rotor breaks. The helicopter breaks apart in flight when a de-sync happens. Out of 12 fatal crashes due to mechanical failures with CH-47’s, three were caused by de-sync - the most famous one in Germany in 1976. Any other helicopter can easily recover from drivetrain failure - a CH-47 can’t.

http://www.chinook-helicopter.com/history/aircraft/C_Models/74-22292/74-22292.html

In Afghanistan the Russian Mi-26 Halo was used to rescue two of them that crashed on the sides of mountains because the US Army’s CH-53 couldn’t lift them. Both CH-47 crashes were single-engine attempted landings, one at 8,500 feet, the other at 10,200 feet. The one was decided too badly damaged to salvage. So the Halo picked the repairable one off the mountain and first brought it to Kabul, then later to Bagram Air Base.

Six months later another CH-47 crashed due to an engine fire north of Bagram at 3,900 feet. The Russian Mi-26 came in and picked that one up too. Then later picked them both up and delivered them to a ship to be transported back to Fort Campbell, Kentucky for repairs. The Russian company, Sportsflite, that operated the Mi-26 Halo charged the US $900,000 to rescue those two CH-47’s - $300K apiece to pick them off the mountains, and another $300K to pick them up at Bagram and deliver them to the ship.

This is a video of one the CH-47’s being brought into the base - notice they had to fire up the Apaches and get them out of the way so they didn’t get blown across the desert.

The CH-47 does NOT have an exceptionally good service history.

1 Like
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No doubt if you have a sync shaft failure, autorotation is not possible. Density altitude is also factor in doing autorotations for any helicopter. I’ll have to chat with some of our army students about this. It is interesting with the current fleet of military helicopters. Most are dual engine so pilots with dual engine aircraft never practice dual engine autos. Mostly they practice single engine out emergencies under favorable conditions. And some communities don’t even practice single engine out recoveries to touchdown. So they are doing it for the first time during the emergency. My point is that we rely on the reliability of the systems and pilots have less experience so when it comes time that they have to do it in an emergency, the odds of them doing it successfully are not as good.

We have many students that come to the school with very little auto experience. During the course, they will learn about autorotational assessment testing and height velocity diagram development testing. They will conduct the flight exercises in an OH-58C as it is the most forgiving aircraft when it comes to autorotations.
Thanks for your insight.
Regards,
Bill

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Yes, autorotation is not practiced by CH-47 pilots even in the sim because they are mechanically unable to perform an autorotation. They are designed for engine redundancy instead.

The main problem is rotor inertia with multi-rotor helicopters. And the collective pitch quadcopter (the proper name for it) also suffers from this. A basic understanding of autorotation must be had - the rotor is driven by cyclic pitch during autorotation and collective is used to control rotor speed. All helicopters have what is called “best autorotation speed” and achieving and maintaining that speed is critical to successful autorotation landing. It is not increase in collective pitch that builds rotor inertia prior to landing. It is cyclic in the flare that speeds the rotor up, and then collective is gradually increased to maintain lift, using the stored energy in the main rotor.

CP quadcopters are mechanically incapable of autorotation because they lack cyclic pitch. Simply trying it with collective pitch will result in a crash because you can’t keep the rotors in sync. Stall one rotor and it’s all over except for picking up the pieces. Since you are using the collective in a negative pitch range to drive the rotors, you are using your rotor speed for stability control and there is no stability control in negative pitch with inverse induced flow thru a rotor - it requires cyclic for that. The rotor will just flutter and rock wildly side to side.

So I think it’s a basic misunderstanding of how autorotation works. Full size helicopters do not even have negative pitch (except for the Lynx).

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@ChrisOlson, what does “de-sync” mean here, how you use it? The rotors are out of sync? Out of mesh in other words? In that case they would clash, and I’m sure it would fall out of the sky in that state. Or, does it mean something else? I assume that whatever it is you are referring to leads to catastrophic failure of the rotors or airframe, and that’s a long way from simply having the capability of pitching the rotor(s) for auto-rotation.

I’m not disputing that both engines out on a CH-47 would be a quick descent to the crash site - but even your comment that your friends say it would be stable all the way to the crash site means that it is still under control. I also wasn’t implying that a Chinook would auto-rotate to a safe landing “as well as” any other helicopter, especially a light one like the Bell 47/H-13, which can auto-rotate to land and still has enough rotor energy to take off again. I was saying that it “can” auto-rotate due to differential collective. My saying “does auto-rotation like any other helicopter” wasn’t implying “just as well as any other”, it was saying “is capable of” meaning mechanically, physically able to pitch the rotors into the airstream using differential collective. Actual performance of the machine and pilot is a whole other story that I wasn’t attempting to get into. I also was acknowledging the fact that a CH-47 wouldn’t do it “as well as” by saying “just a good bit heavier, wink”. So please don’t take offense to my suggestion, here.

In fact - I didn’t just say this out of the blue. Out of curiosity about this, I had asked someone about it who also has lots of helicopter experience and time in the CH-47, when we were comparing it (being one of the only tandem rotors in the world) to these heli-quads and talking about capabilities of lifting, stability, flight characteristics, etc. I specifically asked about auto-rotation in the CH-47, and he said “sure it can” by differential collective. So that’s when the light bulb went on for me about what these quads can potentially do using collective pitch rotors instead of fixed pitch propellers.

I already new that the normal fixed-pitch quadcopter was highly inefficient. I knew that the first time I saw one, when I said “Yeah, cool, but I bet it won’t fly very long though”, because I understood the limitations of that. We already invented the most efficient lifting platform - it’s called a helicopter, hahaha! So, when I was thinking about using helicopter rotors instead of props, I started searching for that - and here I am, thanks to all of you guys. I am interested in developing collective pitch quads for high-endurance lifting missions (Chinook style), not for 3D aerobatics, or racing competitions. So my interest here is in stability and hover efficiency, not inverted flight dynamics, and that’s where my curiosity about the Chinook came from.

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@ChrisOlson - So my post was late to this explanation - that’s what I thought you meant by de-sync.

We won’t have that problem on heli-quads, because the rotors don’t mesh - they are separated enough to have clearance from each other. And by putting a one-way bearing on each rotor, you can eliminate the engine/drivetrain failure problem, assuming you are using one motor for all.

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Still under control is a relative term. A CH-47 falls out of the sky at 3,000 ft/min with one engine operating and the operating engine is destroyed by increasing power well beyond 100% torque to break the fall prior to the crash. With both engines out the impact force is not survivable. There was one skilled pilot that managed to land one with both engines out and the crew survived. But the machine was scattered in a 250 yard long trail of wreckage on impact. The pilot said thankfully he had level ground to crash it on. If there would’ve been a ditch or similar it would’ve been all over.

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It doesn’t make any difference. De-sync with any multi-rotor helicopter is catastrophic. The blades on a CH-47 don’t hit each other in de-sync failure - it’s aerodynamic forces that tear the machine apart because it becomes unstable. You can’t run one set of blades at 5 degrees of pitch and another at 8 degrees. This is what CP quadcopters do for stability, and it is the reason they vibrate like a paint shaker.

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Brad, this is test that was done by Boeing and US Army engineers to evaluate de-sync failures in the CH47. It was done at Aberdeen Proving Ground (Maryland). The rotors were deliberately put out of sync to study the effects on the airframe by inducing an imbalance in the rotor system. The driveshaft to the front rotor was cut so they assumed differential speed.

The video is incorrectly labeled something about ground resonance, and this test back in the day had nothing to do with ground resonance. It was a vibration test to try to determine why the airframe breaks apart when the rotors become unsynchronized.

This is a rear view of the test - the engines were shut down as soon as the shaft was cut and the helicopter tore itself apart in spool-down.

Someplace online there was a whitepaper on this test - I don’t remember if it was published by Boeing or the US Army, but I can’t find the link to that whitepaper right now. If I remember correctly, this helicopter had airframe damage from in-flight failure and emergency landing, so it was used in this test.

In full-size helicopters we use a DynaTrac (just one tool) to track the blades to eliminate vibration issues. It used to be done with sticks on the ground, holding a piece of cardboard near the tip path plane of the rotor and making marks on the cardboard to see what the rotor track was. These days it is done in flight with electronics and we’ve found that perfectly tracking blades don’t always yield the lowest vibration in helicopter rotor systems. With the electronic trackers like the DynaTrac we can get them so smooth a glass of water sitting on the panel in flight doesn’t even have ripples in it.

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@bnsgeyer - thanks for the reply. I have a few responses, to clarify what I have learned.

In this case, no I didn’t try to read the radio’s output during calibration - I was just trying to get it to work. But I know the radio puts out 988-2012 microseconds as it’s range, and I did the manual calibration with TX stick full-up, then full-down, so that’s what the ESC was getting. Since then I have manually adjusted the servo out put range, and found that the motor runs at 1137 minimum idle, and maxes out at around 1800 - the high one is hard to tell after the fact through mission planner. I don’t think this will work anyhow, because if I set the servo output to the minimum ESC value, then when the interlock switch is set low, it goes to this value and the motor runs. I am going to stick with the 1000-2000 range for now, just for sanity. At least I have the motor running now - thanks! Maybe soon I can go on to experimenting with the throttle curve option…

As for this, I don’t think that is exactly right - at least it doesn’t match what I am seeing. I was wrong when I said I expected 1600, for the same basic reason, but actually there is different explanation. I had read somewhere in the docs that this RSC setpoint was a percentage of the servo range, and I was erroneously using 80% of 2000 as my expected result, when it should have been 80% of the range. With the servo range set at 1000-2000, 80% is 800 and the result is 1800 - exactly what I got. And I have confirmed by experimenting that it is percentage, not PWM + bottom setting, now that I have a running motor. The description in the “Full Parameter List” in mission planner says that the units are PWM, that the available setting range is 0-1000, and the note says “PWM passed to the external motor governor when external governor is enabled”. But, it actually works as a percentage of the range. It just is coincidence that 1000-2000 is a 1000 range, so the setpoint value comes out as setpoint+1000. But if you change the range to 500 (1250-1750), then what’s output is 1650, which is 1250+400, 400 being 80% of 500. I think the units and notes explanation needs to be updated in the software, so that it is correct.

I guess I had assumed the parameter file Tridge provided was for the 3.6.0 version, since that’s what’s mentioned on the page in the docs, so if it was for an older version, then I simply didn’t know that - but it makes sense now. As soon as I know this is working (by flying), I will post the new parameter file. I am also using a new Pixhawk 4 with the PM07 power management board, where Tridge had used a previous Pixhawk, so there may be some quirks there as well. I am also trying to get the battery monitor set up. I killed my first battery because I wasn’t monitoring the voltage - duh!!

Anyhow, thanks for your help Bill!
Brad

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@ChrisOlson - Wow! Thanks for these videos!

It’s apparent to me now that what we are seeing here is the harmonic that is produced in the airframe by the rotors operating at different speeds. So it appears that this de-syncing does more than just allow rotor interference - it’s allows harmonic oscillation to occur where any difference in forces between the rotors is amplified to catastrophic failure. Very interesting stuff! I will definitely look for the paper on it now. Drives home the idea that keeping them turning at relatively equal speeds is pretty dang important…

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What do you mean by “mechanically unable”
Also CH-47 does have longitudinal cyclic which is mixed with DCP for pitch control and trimmed forward flight.

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@kd0aij @bnsgeyer @tridge

Here is a photo of the setup

PART_1528569397378
PART_1528569397378.jpg1920×1080 163 KB

I got the pixhawk off of hobbyking, gps is neo m8n, taranis x9d transmitter. And it’s an old Stingray frame.

The firmware is Heli 3.5.7, installed through the default menus on Mission planner

Below I use Mavproxy, because I had tried qgc, mission planner, and apm planner, with the same issue, mavproxy seemed like what @kd0aij was using.

Below is the param file after setting all params to default using ‘param set sysid_sw_mrev 0’, and rebooting the pixhawk.

default.param (21.5 KB)

then I run ‘param load WLToys_V383_HeliQuad.param’, copy and pasted from https://github.com/ArduPilot/ardupilot/blob/master/Tools/Frame_params/WLToys_V383_HeliQuad.param

Here is a mavproxy printout
mavproxy copy.txt (2.8 KB)

then I ran param save filename.param, to get the below, as you can see frame_class has successfully changed to 13, along with many other important changes

after_load_heliquad_param.param (21.5 KB)

then I reboot the pixhawk, and run param save again., As you can see the frame_class has reverted to 6

after_reboot.param (21.5 KB)

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also here is the result of running param diff between after loading the tridge’s params, and rebooting the board:

MAV> param diff after_load_heliquad_param.param
STABILIZE> Loaded 789 parameters from after_load_heliquad_param.param
FENCE_TOTAL 0.0000
FRAME_CLASS 13.0000 6.0000
GND_ABS_PRESS 100219.6094
GND_TEMP 0.0000
INS_GYR2OFFS_X 0.0049 0.0065
INS_GYR2OFFS_Y 0.0126 0.0124
INS_GYR2OFFS_Z -0.0045 -0.0031
INS_GYROFFS_X 0.0116 0.0135
INS_GYROFFS_Y 0.0647 0.0678
INS_GYROFFS_Z -0.0064 -0.0059
SERVO1_MAX 2050.0000 2000.0000
SERVO1_MIN 950.0000 1000.0000
SERVO2_MAX 2050.0000 2000.0000
SERVO2_MIN 950.0000 1000.0000
SERVO3_MAX 2050.0000 2000.0000
SERVO3_MIN 950.0000 1000.0000
STAT_BOOTCNT 1.0000 2.0000
STAT_RUNTIME 690.0000 1290.0000
SYSID_SW_MREV 120.0000

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Bill, they can’t produce enough rotor inertia to perform a successful landing without power. They basically stall and fall out of the sky. Like I said, there is one pilot known to have done it with both engines out from 1500 AGL in Afghanistan, but it happened behind enemy lines, the wreck was quite impressive and the fact that the crew survived it meant it was considered successful. The crew was picked up by a UH-60 and they abandoned what was left of the CH-47. IIRC that incident involved both engines catching on fire.

There is no provision in the US Army training, or the manual, for autorotating a CH-47. Nor was it ever tested at Boeing for certification of the helicopter. It is considered by both the manufacturer and the pilots to be not practical.

So, again, they do NOT autorotate like any other helicopter. Any CH-47 pilot describes them as being “real stable all the way to the crash site”.

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Chris,
Alright thanks for the clarification. Some of your statements were broad and I was confused by other issues you brought up.
I would say that the CH-47 is not the only helicopter with low rotor inertia that make performing successful autorotations challenging. I would agree that if the drive train failure resulted in the synchronization shaft failure then there would be no hope for a successful autorotation. As one rotor could slow and stall before the other.

As for the CP quad, I believe autorotations would be possible if all rotors were interconnected in the unpowered state and the rotors contained sufficient inertia. I disagree that cyclic pitch is required for autorotation. There is nothing that I have read on theory of autorotation that requires cyclic blade pitch. Certainly forward velocity is good as it reduces the descent rate and allows for the flare so as to bleed speed, arrest descent rate while maintaining rotor speed. But whether pitch control is done through cyclic or DCP does not matter. The fact is that you are increasing AOA on the rotor which results in the arresting of descent rate and forward speed.

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Well, that kinda gets back to the fact that in theory there is no difference between theory and practice - in practice there is. You can try to drive the rotor with a lot of negative pitch. But it results in the helicopter falling at approximately the same speed as a rock. Pulling 15G on just rotor inertia to try to arrest the fall usually results in a CH-47 style crash :slight_smile:

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this is what I took issue with and cyclic blade pitch has nothing to do with what you are discussing here. Autorotation does rely on collective blade pitch and like you said controlling the rotor speed. If you put in a lot of negative pitch then your rotor speed will go up and so will the descent rate. if you hold rotor speed constant then the rate of descent depends on gross weight, density altitude, and forward speed. these are all performance related. you can attain the forward speed by using either DCP or longitudinal cyclic blade pitch. typically you will want to get to the bucket speed because that will give you the lowest rate of descent.
I totally agree that if you too heavy or don’t have the rotor inertia then it will be difficult to successfully complete an autorotation.

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But cyclic has everything to do with it. Refer to your basic helicopter training manuals. The rotor has three distinct regions - driven (near the blade tips), driving (center section which provides the lift), and stall (which is drag).

During autorotation the driven and driving region shifts to the advancing side of the rotor disc, the stall shifts to the retreating side. What drives those blade tips is difference in AoA as it is lower on the advancing blade (creating less drag and more power), higher on the retreating side (creating more drag). Critical to autorotation is direction of the induced flow in the rotor and clean air to make it work. The cyclic pitch drives and stabilizes the machine, the collective pitch controls rotor speed and descent rate.

You can not autorotate a multi-rotor helicopter because in vertical descent the fuselage blocks induced flow. The front rotor blocks the induced flow on the rear and it stalls. It is not a windmill being driven by paddles. It is a flying wing that rotates. In powered flight the induced flow comes from above and that is why the CH-47’s rear rotor is higher than the front one.

During the flare you use the kinetic energy provided by forward speed to reduce your descent. With zero airspeed you completely lose this form of stored energy. Look at the aerodynamics of the flare, which is the critical stage of autorotation. The induced flow suddenly changes along with the lift vector on the disc. What happens to the cyclic when you add up elevator? Keep in mind the collective management right here is critical - the blades remain collective-feathered in the flare to preserve stored kinetic energy in the rotor system - because you’re going to seriously need it in the final stage of touchdown.

When you add the up elevator with the cyclic the advancing blade (still being the primary driving force) shifts the lift vector of the disc to the rear and converts the forward motion to stored energy in the rotor system. This is where maximum rotor speed is achieved. In addition there is a severe inbalance in the symmetry of lift on the disc during the flare so it requires significant aileron towards the advancing blade and then correction back to level at touchdown.

A collective-only control system is not capable of this. It cannot operate the blades at the changing angle of attack in the cycle, required to convert the forward kinetic energy to stored rotational energy in the rotor as the induced flow changes in the flare.

Autorotation in a normal helicopter is a total non-event once the pilot learns how to do it. It is easily 5x safer than landing a fixed-wing with no power because helicopters can bring the differential speed between the machine and ground (where all crashes end up) to zero before touchdown.

But the fact remains that a multi-rotor system has too many rotors that are too small to store any significant inertia. None of them are operating at the ideal angles of attack to perform a successful autorotation because they’re all operating in different planes of rotation, don’t have the clean induced flow required, and don’t have the proper energy management system to convert motion to stored energy - they are using collective for everything, which does not work.

The helicopter is a time-proven design that has stood the test of time for over 70 years. Then the multi-rotor people came along in RC and suddenly it’s the best thing since sliced bread. Except it’s not. The multi-rotor lift system has all kinds of leaks in it. It doesn’t actually fly - it is proof of concept that if you strap enough horsepower to a battery you can make a battery become airborne. It is the most inefficient airborne device ever created by the human race. Whether you change the differential thrust by changing blade pitch on the corners, or changing speed, doesn’t change its aerodynamics. It not in any way shape or form related to a helicopter.

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What I want everyone to notice is the distinct nose-down attitude of the helicopter in autorotation. The collective is feathered at this point. That is cyclic being used and it, along with the change in induced flow, is what is driving the rotor. You cannot achieve this with a dual or quad rotor system without using your collective on the rear, which is going to stall those rear rotor(s).

Try to do a vertical autorotation makes it worse because the clean air required by the lifting (driving) region of the rotor blades is broken up by the airframe and you get the fall-like-a-rock syndrome. Take your helicopter up to altitude and chop the power and drop pitch and let it fall. Notice how it flutters in the wind like a leaf and doesn’t build rotor speed. Slam in down elevator and get it into a dive and now notice how it starts to fly again and builds rotor speed. For anybody that wants to experiment this can be done quite safely from 150 feet altitude with a RC helicopter to give you a good idea of what happens in the first 100 feet of altitude loss using just collective pitch Then get 'er into a dive and turn the power back and you can easily pull the nose up and fly out of it. If you try to turn the power on and recover from that vertical fall you will crash it - the rotor remains totally stalled until you get back into translational lift with forward airspeed.

Why does this happen? Look at any airfoil and the critical AoA where it stalls. Once you induce a fall, accelerating at 32 feet/sec^2 minus drag, you cannot achieve enough rotational speed even with power to get that wing flying again. You have to nose the helicopter into the earth to get that AoA below critical so it flies.

If you guys want to try all this - have at it. But experience here says it ain’t gonna work :grinning:

I crashed a 600 trying out the new “low tech inverted flight mode” that was thrown in for CP quadcopters with a helicopter. I got the fall like a rock syndrome and then when I tried to fly out of it I found somebody had reversed the damn controls in inverted flight. Even under full power I could not recover and she went in hard inverted. I was a little less than impressed with that piece of code that was thrown in for helicopters without understanding how helicopters work - and we have since disabled it for conventional heli’s. But a high speed vertical descent using just collective results in a phenomenon called “vortex ring state” - which is normally settling under power. But it settles even faster without power and gets you to the crash site quicker. Only full 3D-spec machines can do that with extreme headspeed to keep those blades flying. And they bark pretty good when they hit maximum AoA just on the edge of stall. But for the rest of us you can do loops and rolls without any negative collective pitch at all. A loop or immelmann is a 1G manuveur - for a regular helicopter. A CP quad can’t do it without negative pitch. Because your CP quad is not a helicopter and it don’t fly like one. :grinning: