Hybrid Power Could Help Drone Delivery Take Off

A prototype hybrid quadcopter can fly several times farther than conventional battery-powered ones.

A new hybrid gas-electric aircraft could make drone delivery more practical. Developed by a startup called Top Flight Technologies, the six-rotor drone can fly for more than two-and-a-half hours—or 160 kilometers—carrying a payload weighing nine kilograms.

The aircraft’s range is many times that of any quadcopter on the market—the most popular type of drone for its maneuverability. Almost all quadcopters run on batteries, and can fly for only about 40 minutes between charges with a payload weighing just a couple of kilograms. The new drone can also fly more than twice as far as a radio-controlled, gasoline-powered helicopter of similar size.

Top Flight’s technology is distinct from what’s being developed by Google. The Google aircraft takes off vertically but then reorients itself and flies like an airplane. But the company isn’t saying much about how far the prototype drones can travel or how much they can carry. And demonstrations of the aircraft in Australia involved delivering relatively lightweight packages, such as dog treats, vaccines, and a first-aid kit.

The efficiency of Top Flight’s drone is made possible by using batteries to supplement the gasoline engine. Power can come from the batteries, from a gasoline generator, or from both at once. And because the gasoline engine doesn’t have to supply all of the power, it’s possible to use a much smaller and more efficient one. Unlike a hybrid car, however, Top Flight’s drone doesn’t capture energy from braking.

Top Flight isn’t the first to try hybrid technology with drones. The U.S. Army and Air Force, together with an Oregon-based company called Northwest UAV, have experimented with hybrid airplanes. A representative of Northwest UAV says the company is also working on a hybrid system for use with multi-rotor craft such as quadcopters.

Top Flight was founded and is advised by researchers from Draper Laboratories and MIT. It has selected a relatively simple type of hybrid engine, known as a series hybrid, in which there is no mechanical connection between the gasoline engine and the rotors. The engine serves only as a generator that charges the battery or supplies electricity to the electric motors. Long Phan, cofounder and CEO of Top Flight, says “future vehicles will fly well over three hours—we already have the new engine to do it.” The company is also developing object-avoidance technology and other safety features—which likely will be required for drones, at least in the United States (see “FAA’s Caution Not the Only Obstacle for Drone Delivery”). Phan says the company hopes to start selling its hybrid drones by the end of the year.

Full article here Hybrid power for drones 

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Comment by Gary McCray on March 22, 2015 at 4:47pm

In several places on Blogs and Discussions here I have written negative thoughts about this method of powering a multicopter in particular.

However, my negative comments only have really related to off the shelf equipment being used to accomplish it.

The problem is one of efficiency and weight, currently available engines and generators simply weigh too much and are too inefficient to provide a reasonable solution.

You need a platform specific light weight high performance engine and ultrahigh efficiency brushless generator to make this technology feasible.

Engine energy conversion efficiency is unlikely to exceed 25% and typical heavy generators are lucky to get much over 70 percent mechanical to electric conversion and you lose another 40 plus percent converting the electricity back in to thrust.

Even with the extremely high energy density of gasoline, you are really stretching it.

An engine driven variable pitch Quadcopter would be much more efficient and consume considerably less fuel than this sort of hybrid even if it were totally optimized.


T3
Comment by Jared S on March 22, 2015 at 7:09pm

Yep, and their power required numbers to lift their design payload are not realistic.

Comment by JB on March 22, 2015 at 8:47pm

If you compare the energy densities between fuel and lithium batteries then it is possible and even very likely.

18650 cell is 200Wh/kg and petrol 10,000Wh/kg. So even at a low 10% conversion rate from fuel to prop, so only 1000Wh/kg left available at the prop, that's still 5x better than lithium. The weight and space saved by replacing the batteries can be used for the drivetrain instead. Or put differently 200g of fuel instead of a kg in batteries would allow 800g of hybrid energy conversion gear etc to give the same range. So 3x better sounds about right for what is available as mini hybrid power trains now. Most brushless motors can be used as generators and there's even ESC's with regen to regulate them to DC.

This has been discussed elsewhere before and there are varying types of hybrid. In the case of a quad I'd use a Prius type hybrid with variable pitch props. That reduces the size of the engine, generator, motor and battery required to a minimum as they can all be dimensioned only for peak power for short 1 minute or so bursts, and it de -sensitizes the risk of the drivetrain by introducing two independent propulsion units for redundancy, without any additional weight. Plus this way most torque is delivered directly to the props from the engine itself, the motor/generator (in one) is only there for peak performance and doesn't reduce efficiency by having to convert to electrical, to charge the batteries or run the motors first.

BTW AC generators are typically over 90%, the ICE component rarely exceeds 50% however on large scale units. There are some interesting alternative designs slowly coming through like liquid pistons design etc. and other rotary engines that have the low weight to power ratio's required for doing this. They could push it beyond 10x current battery range. If you use Gary's pusher prop too, it could be even better! ;-)


T3
Comment by Jared S on March 22, 2015 at 8:56pm
Yes, JB, gas has a higher energy density, that is why gas aircraft are more efficient. The question they need to answer is: if I have a gas aircraft, why would I want to supplement it with electric and make it hybrid? There are certainly compelling reasons but efficiency and longer flight times are not it.
Comment by JB on March 22, 2015 at 10:47pm

Hey Jared

I would say that depends entirely on your aircraft configuration and what type of performance and flight modes you require.

In the case of Top Flight above they need the hybrid ICE configuration so that they can operate independent motors on each arm (6x) and have only one ICE engine onboard for propulsion. In their configuration with conventional multirotor layout with electric motors, they would seem they are simply replacing the batteries with a ICE powered generator so that they can make use of the better energy density of the fuel. Provided you keep the generation gear and fuel weight below the weight of a comparable lithium battery you end up with more energy available for extra range. With their configuration without variable pitch props, that is nearly the only way to have fuel based propulsion onboard.

At first glance this seems to be enough of a reason to do a hybrid design to increase range, however underneath these assumptions lie some factors that can't be optimized with a direct driven, fuel only propulsion system. Given that every aircraft has alternating flight states where different levels of propulsion energy are required, each element in the drivetrain and propulsion system end up having varying efficiencies at different states that do not always align. Typically a higher than "base load" output ICE is required to main flight performance at a safe level.

Simply put; with a hybrid drivetrain it is possible to have the instant required peak output for control and short temporary maneuvers, making the aircraft nimble and responsive, and at the same time reduce the energy production of the ICE to it's optimal efficiency band for continuous base load. That means that the ICE can run "base load" at one single tuned and optimized rpm level, for fuel consumption, vibration, torque load, combustion rate and thermodynamic and even generator electrical performance, with the absolute minimum ramp rate availability. This means the engine starts and is left running at a "constant optimized rate" regardless of the output demanded by the drivetrain, which leads to significant fuel cost savings.  

The generator can then also be dimensioned smaller etc, reducing the total fuel to propulsion system size. The electric part of the hybrid takes up the peak, or required ramp rate demand for propulsion, in that it can instantaneously respond to drivetrain demand for extra energy, but only for a limited time using it's much smaller battery pack. Further to this, depending on configuration, even the electric motor's can benefit from being hybridised in that they can run at higher voltages etc as well. There's millions of Priuses on the road doing just that, there's even more benefits in a prius that I won't get into here (like no gearbox or clutch, but it can still decouple the ICE from the drivetrain, or no parasitic accessories etc), besides just the ability to reclaim energy through regenerative breaking, which is typically thought of as a hybrid benefit. 

Depending on the configuration used, and the flight performance required, the level of "hybrid" optimization techniques between the various flight states increases the overall efficiency of the system. If you would try to fly the same sized aircraft with the same sized baseload ICE, and without the peak energy available from the hybrid electric motors, the aircraft would be so under-powered that flight would be quite unstable, especially in a multirotor aircraft.

In essence, a hybrid system has the ability to de-couple load from supply, that increases the ability to optimize and to design a more efficient total system, by leveraging each components particular strengths and avoiding any individual weakness. Like in biology, "mono cultures" just don't have what it takes to survive! ;-)

Regards

JB

Comment by Rob_Lefebvre on March 23, 2015 at 5:56am

I'd like to see more specifics on this.  So far all I see is "Because Hybrid!"

Comment by Rob_Lefebvre on March 23, 2015 at 6:24am

Is there any evidence of the test flights, or this is all theoretical at this point?  Any data on AUW, fuel load, etc?  I find the claim that it can fly twice as far as a gas powered helicopter specious.  Which helicopter?  How much fuel did it have?  Is it using a drivetrain optimized for range as this one uses, or optimized for acrobatics?

It's like saying "I can run faster than a cheetah".  (a cheetah that has had at least one limb removed)

Not saying it's impossible, but, you can't just go throwing around claims like this without more details if you want to be taken seriously.  Both of their videos seem like more of an accomplishment in computer graphics, than mechanical engineering.  Where is the smoke from the 2-stroke engine? Or is it using a 4-stroke engine, good for efficiency, but bad for power/weight.  Perfectly stable on-board HD video from a machine with a shaking engine, and no camera mounted?  That's an interesting feat. 

I'm always very skeptical of flashy videos and tall claims.  There's a lot of malarkey in this space right now.

Comment by gustavo on March 23, 2015 at 6:32am
I do think that the reason is just how efficient in terms of converting the chemical energy to mechanical power you can be per unit of weight while keeping the multirotor flexibility .

So having a gas electrical generator , and electrical motors seams to me unbeatable with current known technology ...
Comment by JB on March 23, 2015 at 7:03am

Rob

Completely agree that there is a difference between somebody that makes claims in order to sell their product, and what that product can also deliver.

But the hybridization remarks I made, and the calculations I did some 18 months ago support the idea that flying RC hybrids will work, but so far I personally have only tested performance on paper. I have as yet not made a hybrid aircraft, but am progressing in that direction, hence also my posts on the other hybrid threads.

My first port of call is simply producing a lightweight ICE generator using a 2 stroke, to get some test data to see what rate of electricity requires how much fuel. I have the gear here setup already to do it, but i'm still looking for a ESC where I have better control over the regen/load than the ESC32 I'm using atm. I even have the planetary gear etc to sim a prius type hybrid, which also allows for rotor stop without a clutch, plus electric redundancy, but that will require some custom firmware to run, which is not my forte.

But as mentioned above, even if the total drivetrain efficiency from fuel to prop is only 10% the range should be in the order of 2-3x better (up to 5x) than battery in a multicopter. With some optimization, say getting it to around 20% efficient, will give you significant performance improvements over lithium, and make multi's fuel powered.

I think it's a very worthwhile en devour. 

Comment by Rob_Lefebvre on March 23, 2015 at 7:45am

JB, the "because hybrid" comment was not directed at you.  But rather this company.  And comments like gustavo's.  You can't just say "hybrid is more efficient because it's hybrid".

We know we can achieve 90% mechanical power transmission efficiency, with very little weight, and the technology is very mature and reliable.  That's a pretty high benchmark.

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.

If they hadn't made the claim that this can fly for longer than a gas powered helicopter (with no use-case or data to back up this claim) I'd be much more inclined to trust these guys are real.  But that just throws the entire thing into doubt in my mind, because:

The helicopter could use the exact same engine, thus basic BSFC would be the same.  Then, helicopter applies power through 90% efficient drivetrain.  Then helicopter has massive rotor disk area, which we know from momentum theory, benefits hovering efficiency.

To believe that a multirotor could fly longer, you'd need to come up with a hybrid system that approaches 90% efficiency, both in generator, and the motors.  You'd also need to make the hybrid system, and the multirotor frame, lighter than a helicopter of similar disk area.  Both of those are really hard to do.

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