Hybrid Power Could Help Drone Delivery Take Off

Troy Mestler April 30, 2015 at 7:24am

@ Rob:

I agree with you that a traditional gas helicopter will have longer flight times than a series hybrid multi rotor, and yes, Top Flight's statement is incorrect. But I think there are some other benefits of the series hybrid approach that are overlooked (taken from my post here):

"

a series hybrid multirotor is actually much simpler. no reciprocating parts (other than the engine, which a heli would also have), gears or tail rotor to deal with.
redundancy/boost power with a battery, which leads to increased safety.
this is related to complexity, but we built this series hybrid from scratch in a couple of months with no prior experience with multirotors or series hybrids. we would not have been able to do the same with a helicopter. and so, we expect the production version of this system to be much cheaper.
less vibration. because of electric, non-mechanical power transmission, you can soft couple the gas engine to the frame of the quadcopter. (i.e., you can isolate the vibrations with tuned rubber mounts.)
the ease of control of the multirotor is preserved.
we can shroud the propellers for safety. not sure if you can do this with a heli, can you?

I agree you take an efficiency penalty hit when you convert gas to electricity as well as additional weight for electric motors, but the ease of using electricity vs. dealing with a spinning mechanical shaft, in my opinion, more than makes up for it. if you wanted to hook a sensor payload up to it or add another motor/propeller combo, there would be little additional cost."

As with all aircraft, the series hybrid multirotor (SHMR) approach is a compromise. The tradeoffs are (1) much more flight time than battery multi rotor, but less than a gas helicopter. (2) more complexity than an electric multi rotor with introduction of an IC engine, but much less complexity than a gasoline helicopter.

With regard to (1). Yes, helicopters are more efficient and can therefore fly longer than an SHMR due to larger disk area, but by how much? The company I work for, SkyFront, was able to create a prototype of an SHMR that can fly for 2 hours (see video below). Cheap commercial off the shelf parts were used. With the prototype that is currently being developed with better parts and engines, we have good reason to believe that it will be able to fly for 3.5 to 4 hours with a 1 kg payload. I know that helicopters could likely fly longer, but how much longer? 5 hours? Then the question becomes, will the 20% increase in flight time justify the additional complexity/cost of a traditional helicopter?

With regard to (2). If you look at why electric multi rotors have been so successful, it is because they are incredibly simple. If you believe this will continue to be a driving force for aircraft choice in the future, then, in my opinion, a series hybrid multi rotor makes perfect sense over a traditional gas helicopter for many types of applications.

But I could be wrong here, and you surely have more experience with multis/helis than I do, and so I would love to hear what you have to say about these points.

Best Regards,

Troy

In reply to Rob's post:

To me, the argument for hybrid can only go like this:

1. We must use multirotor because of some external factor. (belief that it's safer, or because multirotors are all the rage currently and thus attract funding easily).

2. How do we power it? Batteries, fuel cell, or hybrid?

Without first declaring #1, there's no point thinking about #2.

Acorn March 30, 2015 at 4:02pm

Gary,
Could the + or - 10 to 20% be leveled off with say a solar charged supercapacitor, thereby extending the capacity of the main power supply (lipo batt)?

Gary McCray March 25, 2015 at 12:05pm

Hi JB,

Not making sweeping claims against all forms of hybrid in any case, just this particular application of any sort of gas engine - generator - electric drive hybrid in this case.

Simply less (a lot) efficient than straight fuel - mechanical drive for multicopters under any operational circumstances.

Could you make one fly, sure, but it's performance envelope and endurance would pale in the light of an equivalent straight fuel engined variable pitch multicopter.

The simple fact is that for any gas - electric hybrid to have an advantage it needs more than one power requirement and the further apart those power requirements are, the more it favors a possible hybrid solution.

In a multicopter you just don't have that, you really have a single continuous power requirement with a very small deviation around it.

A single (non-complex - non-hybrid) solution is much better suited to that.

Diesel electric trains are great hybrids, high efficiency constant speed engines with battery backup and requirements to accelerate huge and varying loads to a variety of speeds, hold or vary those speeds up and down inclines and then recover some of the energy when braking is necessary.

Pretty much the opposite of a multicopter - generally one power requirement: hover + or - 10 or 20 percent all the time when flying.

Best,

Gary

Wipo March 25, 2015 at 11:52am

@ Rob:

For reliability, look at starting up a gas engine in cold weather. I don't think that you can deny that starting a gas engine in winter or summer is the same. In winter it will take more effort to fire up a gas engine. And it will need a longer warm up time to be sure it does not cut off at low power.

Even my 70cc scooter needs specific carb tuning for every season of the year. If I don't change carb (air/fuel intake/mixture) the engine does not run smoothly/cut off/overheat. And that’s the maintenance issue I refer to.

As far as I know these issues do not occur with electric motors.

Another factor for choosing this hybrid application could be sound. As from my experience a direct drive electric motor is quieter than a direct drive gas engine. NOISE is a real pain in the ass. In Belgium we even have RC clubs where it is forbidden by law to fly with gas engines because the RC airfield is close to a residential area.

JB March 25, 2015 at 9:39am

Rob

Mazda are still doing it recently with the Skyactive and a turbo, but just not as well as the Prius with hybrid. The Prius is doing it with just electronically controlled valve timing without any form of forced induction which is typically needed for a driveable Atkinson cycle, because the hybrid can take up peak requests. So turbo out and hybrid in...plus all the other hybrid benefits in a car.

Rob_Lefebvre March 25, 2015 at 9:07am

I'm well aware of the Atkinson Cycle. I guess I missed your point because Atkinson cycle has no real connection to hybrid technology. It can be used on a standard car, in fact Mazda did so like 20 years ago.

JB March 25, 2015 at 8:56am

As well Rob. I linked to it a few posts ago here called the Atkinson Cycle: https://www.youtube.com/watch?v=hEfOxKmCstg

This is the system used in the Prius. As previously, I'm only mentioning it because all hybrids aren't simply "hybrids". Just to be clear though, I'm not saying that we do the atkinson cycle on a 4-stroke in RC scale model, but rather to point out that hybrids can leverage previously unavailable ICE optimizations that can lead to efficiency and performance enhancements. I don't think all of these "opportunities" have been fully investigated in hybrid aircraft yet.

In the case of a RC scale hybrid it depends on the application, but I don't think it's easily ruled out overall. I'm working on a spreadsheet to get a better overview in which type of airframe it makes most sense, to that end discussions like this are quite helpful, especially if we can take it up a notch and discuss some details.

The energy densities, ease of storage and refueling all add a certain "convenience" (wastes less time and effort) to liquid fuels that we have become accustomed to. Any system we develop to replace it should be better than we have now, not worse. With an ICE there's a lot of room for improvement, and I believe many of these have to do with de-coupling the drawbacks of one, to the benefit of another, in a hybrid arrangement. I've been a fan of hybrids since my teens when I first learned about diesel-electric trains, decades before they became cars!

BTW, as a matter of principle, I'm completely against the consumption of fossil fuel, despite driving a Prius due to the lack of a better option (beside my recumbent trike!), that doesn't produce more emissions elsewhere.... I'm a fan of biofuels in fact for transportation (over electric even), and often wonder why mankind abandoned Rudolf Diesels original fuel, vegetable oil, and Otto's ethanol, in favour of the black gold of the underworld! ;-)

(PS that is of course the sustainable and non-food related production of biofuels!)

Rob_Lefebvre March 25, 2015 at 7:44am

JB, how does the hybrid system help improve BSFC? I've never heard that claim. Is it just because it's constant speed?

Wipo: where is your reliability data? Is see the reliability claim trotted out quite often. But I've never seen data, and it disagrees with my own anecdotal evidence. I have helicopters that have flown much longer without failure than my multirotors. Multirotors are mechanically simply, but electrically complex. The can and do fail, particularly the ESCs. I know of a guy who flew a gas powered helicopter for 25 hours last summer, with no mechanical problems. And that was with a hobby engine with journal bearings and ABC piston, not an industrial or aerospace grade engine with ball bearings and piston rings.

Cold weather? What problems are you referring to? I've never had a serious problem running a gas engine, even small ones, in cold weather. In fact, I just ran my two stroke dirt bike at -2C yesterday. I've also started my gas helicopter below freezing too. With the proper fuel, it's no problem.

JB March 25, 2015 at 3:33am

Thanks Wipo.

You raise valid points on non-efficiency related benefits of hybrids whch all add to the value of the concept.

-

Gary buddy!

I have a nagging feeling you might have either missed some of my posts about hybrids or are mis-reading what I do or don't mean. My question two posts ago was: "what assumptions are you making on hybrid efficiency and performance, and ICE engine performance to come to that conclusion? Not all hybrid configurations are the same."

The Prius hybrid is a particular type of "combined" hybrid, which is both series and parallel, and it offers significant flexibility in it's design and operation. The top flight multicopter is known as a simple series hybrid, that does not function the same way, neither does it have the same efficiency or functionality. This is also not the only form of hybrid I'm advocating.

The Prius was also only used to demonstrate that using a hybrid drive train can improve the BSFC efficiency of the ICE itself. In particular in the Prius, this means the exact same ICE uses more fuel for the same kinetic output at the drive shaft without the hybrid attached, then it does with the hybrid attached. It improves the ICE efficiency, of the petrol engine itself, even without regen braking or the gearing optimization it provides to meet car operations.

So which "hybrid" are you referring to? Or are you making broad sweeping claims against every form without knowing their individual traits? I'd really like to understand your perspective on the subject, in a effort to learn more about the viability of applying hybrid technology to different RC scale air frames.

In order to have a technical discussion however, it would seem appropriate to understand more about the individual types of hybrids and their particular drawbacks or benefits for each application. To that end over the next few days I will endevour to seek out some more information on hybrids and will start a new blog post in order for us to discuss, in detail, which forms of aircraft might improve their performance with a hybrid.

I welcome you to join the conversation then so we can get to the bottom of what can be realistically expected or not.

BTW , even though I don't think you'll ever get a SNAP in a RC plane, I do believe you should at east use lifter tech to propel it instead of a prop to keep up with the times! ;-)

Regards

JB

Wipo March 25, 2015 at 2:15am

The reason why choosing this hybrid application and not a helicopter is maintenance.

Although more efficient, helicopters or multirotors with variable pitch props and/or direct drive gas engines will have a higher maintenance frequency and cost.

Multirotors with direct drive electric brushless motors and fixed pitch propellers have very little maintenance frequency and costs.

Because this is a hybrid solution the gas engine used to convert gas in to electricity is not a critical part of the multirotor. The battery will always have enough power left to make a safe emergency landing. Therefore the maintenance frequency an cost of the hybrid module itself can also be kept low.

Small scale gas engines used in direct drive applications are also more subject to (cold) weather conditions. Just call it reliability in general.

In my opinion reliability and maintenance would be the reason why this hybrid multirotor would be a good choice for this application.

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Hybrid Power Could Help Drone Delivery Take Off

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