Canberra UAV Demonstrates Helicopter Flight with Downstream FBL Controller

I wanted to bring attention to the work that Tridge and the rest of the CUAV team have done to demonstrate the possibility of flying a Helicopter with a standard FBL controller handling the rate control duties.  They have been testing gas powered helicopter for next round of the Outback Challenge which will require a long-range VTOL aircraft.  Helicopters are a natural target for this mission of course.

Several of the CUAV team are experienced RC helicopter pilots, but not as familiar with installing a Pixhawk on a helicopter which can be difficult, especially in the case of gas engine helis.  As such, they were more comfortable having a normal Flybarless Controller handling rate control. In this case, the Skookum Robotics 540.   Tridge has made changes to the code which allow for a pure control pass-through from the RC Rx, through the Pixhawk, and straight to the FBL controller. This pass-through occurs in Acro mode, whereby the Pixhawk running ArduCopter has no effect on the flight control, so even if the Pixhawk should have a major AHRS/EKF failure, the helicopter is still controllable. 

Of course, the Pixhawk is capable of controlling a Helicopter without any FBL system, and this is the most common arrangement.  But the possibility of running an FBL controller in series with the Pixhawk helps lower the barrier to entry for many existing RC Helicopter pilots.  And also offers helicopters similar failsafe-function to airplanes, where they can survive the loss of the autopilot.

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Comment by JB on March 10, 2016 at 9:22pm

Hey Rob buddy

"There's nothing scary about gas.  Gasoline is safer than LiPo batteries."

Sorry but that statement is factually wrong! ;-)

Fuel and then Dynamite

https://youtu.be/83BinHEbvvQ?t=217

Lipo explosion after impact (being wacked by a stick!):

https://youtu.be/gz3hCqjk4yc?t=50

A 18650 shorted explosion after a few minutes (release of internal cell pressure build up)

https://www.youtube.com/watch?v=ZTzEHsJVZhA

I know which one I'd rather be next to... ;-)

Without wanting to detract from the efforts and subject of this post I'd like to say I don't agree with the perceived risk of fuel in comparison to batteries.

Your statements purporting the position that the likelihood of a lipo failure is higher than gas would need to be evidenced a bit further. Asking ad-hoc at a airfield might not yield accurate results. Gas is now typically less common than lipos on airfields, so one would expect the likelihood of lipo fire to be higher lipos overall. However, the likelihood of occurrence has no impact of the actual risk consequences, if comparing the risk consequences of two energy storage types directly, which is what is required to assess the individual risks of the different platforms.

Typically any type of fire is safe if it can be contained. Fire fighters therefore always seek containment first, then extinguish the fire.

Reasons why fuel is more dangerous than electric propulsion (at least in my opinion!)

  1. Due to the poor conversion ratio of combustion engines some 4-5 more fuel energy needs to be carried - the volume of fuel might be less than a comparable battery, but the amount of energy contained is significantly higher (Lipo up to 2.7MJ/L fuel a massive 44MJ/L) - also the reason for fuels better range of course, but most of that energy is wasted in conversion!
  2. Fuel is a liquid that can easily disperse upon impact or even in air from a leak (fuel line rattling loose etc) - it can douse people and property with fuel. A significantly higher risk to create bushfires, especially considering that most lipo fires are due to charging and happen in the direct care of users ie they can put the fire out. I'd consider the secondary fire risk and the resulting damage injury of "explosion" to be considerably higher with fuel
  3. Fuel is classified as a "explosive " as a vapour (hence "knocking" in internal combustion engines instead of deflagration) given the right conditions and a source of ignition - which are plentiful on impact of a heli - they say a table spoon of petrol contains the same energy as a stick of dynamite (velocity of detonation is higher in dynamite though)
  4. A lipo fire has lower energy and typically a slower detonation rate, to the point that unless there is a pressure buildup in the lipo cell that leads to explosive rupture the battery burns over a considerable time with a smaller flame instead of all at once in a large fireball like fuel (As they say a half empty fuel can is more dangerous than a full one)
  5. A lipo fire can be further mitigated if there a multiple cells (18650's etc) in various locations/protected from eachother from impact etc. Fuel is harder to manage/mitigate.
  6. Faulty Lipos are more likely to fail whilst charging on the ground which should always be done under supervision. The risk on the ground with a user intervention is orders of magnitude lower than whilst airbourne and no firefighter in range
  7. Generally a engine needs more maintenance and through the complexity of parts is less reliable and subject to fatigue and wear - this increases the risk of it failing in flight and coming down where it can't be readily extinguished 
  8. Overall a fuel system has many more critical points of failure than a electrical one, where the primary concern is the failure of the battery. ESC and motor fires are a order of magnitude less common. Fuel lines, carburetors, exhaust systems (Backfire), fuel tanks etc are all possible ignition/failure points, that can possibly result in unrecoverable in-flight failure with uncontrolled ground impact, that an electric platform with separate control battery could still recover from. A plane can glide, a heli autorotate etc. Neither fuel or battery aircraft fly well whilst on fire, but the severity of the battery fire will likely be less, so it's more likely to be able to recover. ;-)

Overall, from a risk perspective, helis's have there own set of other risks we have discussed elsewhere and as Chris pointed out (blade inertia etc)  but from a fuel risk perspective that applies to all forms of fuel powered aircraft including airplanes, helis or hybrid quads, I'd prefer to use an electric one if the range allows. Personally I've had a few close run ins with just "a cup of fuel", with friends giving themselves third degree burns and months in hospital, from which I've learnt to respect the raw power of fuel. I can't see that happening from my experience with lipo failures (I've had a few) I'm not scared as such, but fear is a good self-preservation technique! ;-) 

Regards

Comment by JB on March 10, 2016 at 9:32pm

One thing I forgot to mention: Use LiFePO4 batteries if there's a fire risk issue, they don't burn well at all and have similar performance for long range:

Comment by Hellcat74 on March 10, 2016 at 10:53pm

Great effort guys.

@Tridge, well how does the controller handle vibes coming from the gas motor? How do you monitor its performance?

Comment by Hellcat74 on March 10, 2016 at 10:54pm

...sorry...I meant "how well" does the controller handle vibes coming from the gas motor?

Comment by John Bond on March 10, 2016 at 11:39pm

You can always find people to tell you what is more dangerous.  It's the type of aircraft or power source they are least familiar with.  There's some truth to that because it is for them.  It doesn't always hold for others though.  Sure you can come up with some absolutes like the power density of gasoline is greater than any lipo of similar volume, but I'd say someone used to handling gasoline is less at risk than a person dealing with lipos for the first time.  Nothing within the realm of normal RC craft is particularly dangerous if you have a bit of experience with it.

Comment by Ultrafuge on March 11, 2016 at 12:41am

Hi,

The method we're using is SBUS chaining, so we use a FBL controller with SBUS input (a skookum 540), and use the SBUS output port of the Pixhawk. That means just 1 simple cable between the Pixhawk and the skookum, which keeps wiring nice and simple.

Is there any difference (except simple wiring) compared to using the RCout channels of the Pixhawk as input for the FBL unit (refresh rate, fail safe, flight performance, etc.)?

Comment by JB on March 11, 2016 at 12:49am

@ Guy

I can agree that other then fire risks are more expedient to deal with, also that "lipo" fires are more common because of extensive use. But not all batteries can burn, or explode like lipos. Like the LiFePo4 batteries I showed.

Accordingly I would not place fuel and batteries in general in the same category. Lifepo4 are significantly less risk and are a good match to compete for the range that typically uses petrol ie less than 1C discharge.

I know the statement was specific to lipo  and gasoline, but even there I doubt their potential fire risks are the same. 

As for ignition sources there are plenty of electrical components with enough potential from the control batteries that need to be carried as well. The question is where it fails/ignites/explodes. Exhaust gas temperatures easily exceed 280C, but I'd say most ignition is inhibited by not being in the right fuel:air ratio. Unlike methane as a gas that is fleeting, any liquid gasoline creates ignitable vapour at the right ratios for ignition.

Regardless of the likelihood, the result of ignition with fuel is however significantly worse as the consequence of a few 100ml of fuel can be catastrophic (injury, death, property destruction). The same cannot be said for for the consequence of a lipo failing directly.

But before establishing a accurate risk matrix for comparison, a lot more data and experimentation would need to be done. I'm currently unaware of any testing to support the premise that gasoline is safer than lipo, and would welcome some evidence to support the idea.

-

@John

A persons perspective is not indicative of the facts! |-)

Accordingly, unless tested and experimentally shown there are still risks even if people know how to deal with them, if at all they can. A gasoline aircraft is typically used for long range work, meaning it is also "out of range" of the operator to readily put it out, should it burst into flames. That application increases the risk.


Developer
Comment by Andrew Tridgell on March 11, 2016 at 2:19am

@Ultrafuge, the only advantage of the SBUS chaining is wiring simplicity, which consequently also makes vibration isolation a bit easier. It is perfectly OK to chain the Pixhawk to the Skookum with separate wires for each input.

The key to it really is that if ACRO mode is properly configured then it is a true pass-thru to the Skookum. That means you can flick to ACRO at any time and be flying your heli just like you would without the Pixhawk. The same thing works with other flybarless controllers.

It is also trivial to setup the rate gains in ArduPilot, as you just set P, I and D to zero. Then set VFF to the gain corresponding to how sensitive the FBL is to stick inputs. A value of around 0.5 or so has worked well for us on all axes.

Comment by Nicolas CLOES on March 11, 2016 at 2:46am

Great news, exactly what I and another user asked about some time ago : http://ardupilot.com/forum/viewtopic.php?f=66&t=7521&sid=1f... . I'm really happy that this could be done :-) And by the way, big respect to the whole team for all the fantastic job done...

@Tridge, can you confirm the following mixing parameters ? 

  • Swash Type (H_SWASH_TYPE) = 1
  • Flybar Mode Selector (H_FLYBAR_MODE) = 1
  • Tail Type (H_TAIL_TYPE) = 1

By the way, what about the SBUS channel-function routing order? Is it the same than the routing on the PWM outputs?


Developer
Comment by Andrew Tridgell on March 11, 2016 at 3:21am

@Nicolas,

Exactly correct, you need:

  • H_SWASH_TYPE=1
  • H_FLYBAR_MODE=1
  • H_TAIL_TYPE=1

You should also set RATE_*_P, RATE_*_I and RATE_*_D all to zero. Then set

  • RATE_RLL_VFF=0.4
  • RATE_PIT_VFF=0.4
  • RATE_YAW_VFF=0.3

they are just a starting point. You can then use the PID logging to do fine tuning so that desired rate equals achieved rate for each axis. We've found the tuning of those VFF values is not very sensitive.

We also found we needed to lower ACCEL_Z_P a bit from the default or you can get collective oscillation in ALT_HOLD mode.. A value of 0.15 works well for our Trex700 gasser.

Before we did our first flights we graphed the RC inputs against servo outputs for the roll, pitch, yaw and collective channels to confirm they matched exactly in ACRO mode. They did.

We had one additional complication with the skookum 540, which always requires collective on channel 6. The ArduPilot code puts collective on channel 3. Some FBL controllers allow the input channel mapping to be changed (eg. BeastX allows this) but Skookum 540 doesn't. So we are currently using a small patch to swap channels 3 and 6 in the PX4 HAL. That makes the SBUS chaining work with no fuss. We could instead have had separate wires for each channel between the Pixhawk and the Skookum, but that would make the wiring messy.

What we really need is a H_SWASH_TYPE=2 which puts collective in channel 6 and throttle on channel 3.

Cheers, Tridge

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