Small Hybrid Electric Aircraft

Discussion in 'Electric Propulsion' started by litespeed, Aug 14, 2019.

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  1. Aug 14, 2019 #1

    litespeed

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    litespeed

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    Following on from discussion on aircraft suitable for electric power, and the need for a big enough battery that is light enough.


    If we think of it purely in the terms of The ability to carry X amount of energy that can be converted into usable propulsion during a flight. This opens up the field in terms of design. Any Source is now open be it gasoline- petrol, lpg, hydrogen, solar, biofuels and of course batteries.

    Ideally using a combination of 2 or more sources of energy provides a greater potential in energy output, increased redundancy and longer range. So that means hybrid systems which for us right now tends to be gas running a alternator to make power, then a electric driving the prop and batteries as well. It can get complex and expensive real quick and weigh quite a bit.


    For a homebuilt aircraft it may actually be doable and not cost a arm and leg. The trick will be doing it in a small enough package at the lightest possible weight. The greater the size aircraft, higher power needs, means we should only try for single seat designs. So sub 300kg for the rest of world and part 103 for the USA.

    If we look at what they are doing in drones whether a glider shape or helicopter or even now the large multirotor rigs they all tend to run a hybrid system for max range and max payload. Range can jump quite a bit into hours not minutes. Or for some days of range.

    So we should follow the leaders and just adapt the ideas.

    It can be as simple as a motor glider with a small hybrid stashed behind the seat and electric setup turning the prop. The magic comes in getting the right combination to suit your airframe.

    If your allowed 4 gallons in a part 103 then a 4 hour range could be done if you only need a small amount of power for sustained flight- so low drag ideally. With the greater 300kg limit comes much greater potential as fuel carrying can be relatively huge. But also does the power for most designs.

    We really just need to think of a systems approach in designing. Even better if we design the airframe to suit the power system. Even if that means we just select the best airframe to suit the abilities of a chosen power system.

    It also means we should look at the complete system weight of a current beloved aircraft design. So that means anything at all in the aircraft related to propulsion. So motor, fuel lines, tanks, weight of full tanks, engine mount, cooling fairings, cowl, prop, any electrics related to it. It adds up to a lot of weight esp on some designs.

    It becomes a large mass fraction of the total weight of the aircraft. Often half or more. This is the weight we really should be considering when we look for a solution in powering air chosen design. So if you X aircraft has a total figure of 300kg and all that stuff weighs in at 120kg then that 120kg is the design goal.
    The figures are made up and just to illustrate a point*

    The modern small UAV and model aircraft stuff is far ahead of the game in packaged power that is cheap and light.

    If we have a airframe that needs 4.6kw to stay in powered level flight, I have found a hybrid for the job, forget the price- thats a rip but nice.

    A bit heavy in some ways but nice. Dual engines for redundancy plus battery.
    https://www.aliexpress.com/item/330...9.0&pvid=1ab275b0-9f3b-4111-8f4d-1c0e87e3acda

    Or 10kw which is much better. And still would weigh almost nothing. 3.175 kg actually for 10kw .
    https://www.aliexpress.com/item/330...8.0&pvid=b740c193-1fdc-464c-8f7d-a700d060e69c

    Its all a matter of scale of course, and a twin 10kw hybrid could be as little as 7 kg or less. So you can have longer range from combining the gasolines superior energy density to the electric systems instant power for takeoff and boosted power.

    Using the electric systems full potential for boost power means we can do max safe drain on batteries and still get a extra 10kw at the prop when required from the gas engine.

    For longer range cruise we run the gas motor as needed for charging/powering the prop.

    Also we get redundancy of engine and battery provided power.

    This could mean a great deal more range on a much smaller package, the battery can be much smaller and thus cheaper and have a lower max current draw need- so less energy to have to cool off as a bonus. The tank and hybrid can be low weight as well.

    Now we only need a suitable engine that is efficient at 10kw but can punch 30 kw if needed. These tend to be light, cheap and reliable. Same with the controllers.


    Now as its a hybrid we add solar film cells on the wings and tail - the printable type from previous discussions.
    That can give 3kw dependant on design.

    So now we have a extra 3kw for power so lower fuel burning needs.

    Run the little devil hybrid on biofuel and its a green machine.

    A well designed prop that folds with engine would be a easy fit to a glider or other designs. A Facet Opal comes to mind but even a wind bag draggy machine could still benefit from a hybrid.

    I am sure it is possible to roll your own mini hybrid system at home without a massive cost. If we only need battery boost at takeoff of 20kw of total max power 30kw and then run the 10kw max on gas and or solar cells.

    So we have
    Small cheap battery and more reliable
    smaller cheaper gas engine and super low weight
    smaller fuel tank
    Smaller cheaper controllers
    Smaller propulsion engine or engines

    Everything is considerably cheaper than previously and now at the sweet spot for our sport.

    Because everything is now compact, its easily distributed around the airframe it can open up design a bit.

    You could use your battery if needed in flight as it would be getting a charge from the hybrid/solar as needed. So at times you could be silent running on electric only- still have noise though.

    Now I would not expect a long engine life on some of these but the idea has merit for some airframes. And they are very easy to re ring/ change a cylinder.

    Or completely replaced with another you have well run in etc.


    These are simply just examples but a much higher output version could be done.

    The idea of a inline version that has the gas engine connected to the prop engine can also be done. Reduces systems but concentrates weight to a single point.

    Any suitable gas engine could be done like a rotax 40hp or 100hp two stoke or four stroke if light enough. They do it in UAVs and they do it in cars. BMW has a small hybrid car which uses a motorbike engine.

     
    Last edited: Aug 14, 2019
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  2. Aug 14, 2019 #2

    litespeed

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  3. Aug 14, 2019 #3

    litespeed

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    Here is a video about NASA doing similar stuff.

    Notice the Halbach array engine on the twin. Cool.
     
  4. Aug 15, 2019 #4

    pictsidhe

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    Yes, possible, but you need to do a lot of maths. I'm not sure you'll like the results. Start with the airframe and iterate until you get a low enough power requirement.

    Series hybrid is a lot of extra weight, If your engine needs to be running anyway, stick a prop and a motor-generator on it.
     
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  5. Aug 15, 2019 #5

    tspear

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    I started a thread on series hybrid a few years ago. I talked to enough engineers, and even paid a little for some research. It only "pencils" out if you believe you will be fully electric at some point in the future and want to work out the kinks in the engine, batteries, and and control systems.

    Tim
     
  6. Aug 15, 2019 #6

    litespeed

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    Yes,

    Lots of pitfalls and it will suit some designs much better than others. As stated it all comes down to numbers, some designs and mission profiles it will be a winner and some it is just a complicated way of getting power.

    As I alluded to above in my very long winded post- some designs will really benefit and a clean sheet design can allow for the distributed power system to be a advantage.

    It does not have to be a serial hybrid, they have advantages and disadvantages.

    I do not propose hybrids to be a replacement for any petrol powered aircraft but for special cases where it makes sense. Especially if high electrical loads are needed. But some trade studies might find suitable candidates like a Lazair and some of the other small part 103 planes - as long as it is not too draggy.

    The part 103 rules might be a good start due to the weird treatment of battery weight and low fuel allowance.

    Also it gets around the need in some cases for quiet operations on takeoff, and then flight can continue with the gas power when away from complaining neighbours.

    Yes, it will suit those who wish to go full electric later on when batteries improve or later convert to potentially hydrogen fuel cell.

    A converted glider rather than going a pop up engine or a jet as some do, would be a good candidate. Engine on nose and stashed inside is the small generator and some boost batteries.

    There is merit in the idea as seen by the growth of its use in UAVs esp the medium size ones. But it will be like anything in aviation- a trade off.

    If we are looking at clean sheet designs then everything is up for grabs.

    Naturally the simplest is just a prop on a gas engine, but this is about hybrid systems that might use mutiple energy sources including solar to give power. Then it starts to make some sense.

    A good example would be Sunseeker, a small hybrid on board would vastly increase range. You still have batteries just not as many. 20kg of hybrid and fuel would give a much greater range than 20 kg of battery.

    That is when it starts to work well.
     
  7. Aug 15, 2019 #7

    litespeed

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    Range extension in electric aircraft is a area where this is suited.

    We all seem quite rightly worried about range anxiety, just as we use to with electric cars.

    A small 10kw hybrid weighs approx. 10lbs installed with tank. 10lbs of fuel would provide a considerable range boost when needed. Call it 7 litres for that weight and you have approx 90 mins or more of range on a low drag aircraft.

    [​IMG]
    This is a 10kw unit and weighs 3.175kg only.
    That is 90 mins on a package as small as a cubic foot plus a tank hidden of 7 litres capacity.

    If you can find such a tiny space on a electric aircraft and substitute that 20lbs of battery for the hybrid system and fuel. You are good to go.

    When we look at the expense of a glider with a launch power system and a wish to cruise aka motorglider it can be a winner. People are putting Jet engines on them and it works but costs a bomb and drinks like a sailor for the power output. This is a far more efficient way of getting the power needed. And would be considerably cheaper to install, run and service.

    It does not have to cost anywhere near as much as the ones I showed above- you could DIY a system for under $2000.
    20lbs does not take up much available weight or size. But it does provide a great deal of extra energy to augment a electric aircraft.


    We often look at things to make our aircraft go further or be safer through redundancy or even parachutes when power is lost from battery exhaustion. A hybrid package can be seen in this catergory of nice things to have when needed.

    It does not have to be a big heavy lump of engine and alternator. That is old school thinking.

    This is 2019 tech and very compact and light- that is what makes it work. Just like old lead batteries and brushed motors made no sense. But modern alternatives do.
     
    Last edited: Aug 15, 2019
  8. Aug 15, 2019 #8

    litespeed

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    Yes in some cases the best route will be a small serial hybrid, with a prop on the front. The gas engine only engages when needed for additional power and charging.

    Some suit serial hybrid and others would benefit from distributed hybrid.
     
  9. Aug 15, 2019 #9

    litespeed

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    Here is a video and they discuss use in UAV and in manned aircraft in the future.

    Note: They estimate that a small 3.75 kg hybrid including fuel provides the same range as 17 kg of battery using Lipo.

    That makes a big difference.
     
  10. Aug 15, 2019 #10

    BBerson

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    That unit might work for a FAR103 with minimal batteries. Bolt the prop to the engine. It has electric boost for takeoff. Cruise is with engine only, no external electric motors. The advantage is the electric start and the electric starter is also a boost motor for takeoff. Only a small amount of generator power would be tapped for recharge of the 20 pound battery in flight
     
  11. Aug 15, 2019 #11

    BJC

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    I have an interest in pure electric powered aircraft, but I don’t expect to see one that matches my wants in the next few years.

    What is it about a small hybrid electric aircraft that you find attractive? (Question directed to litespeed and anyone else who finds it attractive.)


    BJC
     
  12. Aug 15, 2019 #12

    Hot Wings

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    Range/flight time.

    It only makes sense if you are dealing with something where a reduced or regulated top speed is acceptable. For a clean part 103 it's an elegant way to keep good climb performance, and not have to lug around a large heavy engine that has to be throttled way back (further reducing efficiency) just to stay under the speed limit. Same theory applies to LSA, just bigger parts.

    Motorgliders are another niche where a hybrid may be useful. I ran the numbers through my spread sheet just last night for the AV-36. If you want to cruise in motor mode faster than about 65 mph it makes no sense. On the other hand if you are willing to cruise at best L/D and want to be able to self launch the numbers start to turn around. Being able to distribute the various hardware also helps with weight and balance on a flying wing.
     
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  13. Aug 15, 2019 #13

    Hephaestus

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    [​IMG]
    It does open up some "different" possibilities for homebuilts. 2 small electric EDFs like the EADS concept...

    Weight savings could be significant, some of the small industrial engines could be easily adapted - build an appropriate generator set. It's a point design, very specific operating parameters very consistent loads and operating environment... Offers options for weight savings if you do it right.

    Don't know if any of you have a Honda eu2000 generator - super quiet, pretty light. If one could fly off it at cruise + a little battery power for the takeoff extra power + recharge during cruise. The noise reduction alone over a rotax 2/377 ...
     
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  14. Aug 15, 2019 #14

    litespeed

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    A variety of reasons led me to the Hybrid idea. In particular frustration that we are yet to see the benefits of electric flight in our sport whether that be Part 103, SSDR sub 300kg or even in LSA.

    I do admit to loving silent flight even if only for some of the flight profile.

    Also......

    The ability to have a redundant system
    Charging during flight
    Much lower battery costs and management issues
    Lower voltage systems are cheaper and safer
    Far lower weight of battery and complete system
    Ability to tailor a electric drive to your specific mission and airframe
    Overall efficiency
    The ability to upgrade system components without having to replace the lot as tech changes.
    We can get away from having a big heavy lump of engine in one spot. This opens up designs quite a bit and is a big safety issue- engines have a habit of hurting in a sudden stop.

    And cost- a new anything powered electric costs a arm and a leg.

    Also - because a lot say it cant be done.

    Additionally the ability to integrate solar into the system.

    And I like the idea.
     
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  15. Aug 15, 2019 #15

    BJC

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    Thanks guys, that helps me understand your motivation.


    BJC
     
  16. Aug 15, 2019 #16

    litespeed

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    The generator referenced above the EU2000 from Honda-

    It only puts out 2kw at start and 1.6kw constant, also it weighs 46 lbs. And it makes either 12v or 120 or 240v so not really what we want either.
     
  17. Aug 15, 2019 #17

    litespeed

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    BJC, what aircraft did you have in mind to be electric?
     
  18. Aug 15, 2019 #18

    Hephaestus

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    No it's optimized for the normal uses - not our specific uses. That said your brushless DC motor is pretty much an AC induction motor, 240vac in true sine wave into a VFD does make some sense. Your brushless ESC is pretty much converting back to AC at the end of the day.

    No not an ideal solution but at the same time it's a known quiet inverter generator for noise / power and reliability. As an example of what a power plant could be it's a good one - I don't think you'd be taking any existing generator and slapping it in an aircraft. I think we'd be looking at a few sizes up anyway.
     
  19. Aug 15, 2019 #19

    litespeed

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    Unless you can find a really light and compact four stroke with a added generator set then we should be looking at the model world for max hp/kg.

    The model guys have it sussed power and size wise. We do not need big torque to turn a prop- just to run the brushless on the end of the crank.

    If we want big power and compact then here is a candidate that just needs a dedicated brushless motor on the crank.

    And you get a 25kw charging system in a tiny package. It weighs 5.5kg add in mounts and brushless genset and approx 10kg.

    Yes, it will use more fuel than a four stroke, but that is far outweighed by the super low weight.

    https://www.aliexpress.com/item/323...9.0&pvid=d6d8a53d-efb8-4fc9-81d2-5e028e01ae8e

    Specifications:

    ² Exhaust Amount: 232CC

    ² Horse-power: 34.8HP

    ² Idling: 1200rpm/min.

    ² Pull: 56kg/1000m altitude

    ² Compression Ratio: 7.8:1

    ² Cylinder Diameter * Route: 46*35mm

    ² Recommended Propeller: 32*10, 32*12, 34*10, 34*8

    ² Spark Plug: DLA CM6 iraurita spark plug

    ² CDI Voltage: 6.6-8.4V, 2S LiPo

    ² Lubrication Ratio: 40:1~50:1 (normal flight), 92#93# gas,

    ² Weight: Main Engine 5080g; Exhaust Pipe 258g*2; Ignition 180g*2, Stand 22*4g
     
  20. Aug 15, 2019 #20

    Hephaestus

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    I'm betting the RC style ones wouldn't handle 25kw for hours. The generator and the motors would need way too regular overhauls...

    Whereas a Honda GX / briggs (or clone) with an appropriately sized generator of adequate size - likely could do it without breaking a sweat. And run longer term...
     

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