Idea: Series hybrid in airplane using auto engine and avoiding the pitfalls of auto conversions

I have been thinking this back and forth now quite some time. This idea is quite simple, the purpose is to fix the most critical problem with auto conversions, achieve better aerodynamics, propeller placement and mass and inertia distribution.

Auto conversions most often fail, no surprise, because of the reduction gear or belt. The core engine is not the root cause in the problems and many problems with the reduction belt or gear system can not be seen beforehand because the dynamics of the vibrations of the engine, propeller and their inertia forces affecting each other is a bit more complicated than one could think at first – it is not that simple to make these parts to last for hundreds or thousands of hours.

So we came up (with Kate, we usually talk with Kate about these things and we kind of invent these things together, I usually happen to be the one who writes them down – and it is usually so that Kate is the opponent into which I test my idea’s feasibility before I write it here) with the idea of having a auto engine, possibly a diesel engine, running at constant power, most likely exactly at the optimum point of the engine, always. Then all the power variation would come from the electric motors which would drive the propellers. The idea is that the diesel engine only runs a generator.

The downside of this idea is the additional weight from the generator, batteries, motor controllers, electric motors and the props (depending how many electric motors are used, it is also possible to use just one if that is preferred). However, there are two several things possibly good about this:

– First the diesel engine burns less fuel, resulting smaller fuel tanks.
– Secondly the gearbox system is saved. The gearbox system can be very heavy duty in a high power aircraft engine and they still have tendency to fail. Possibly something like 40-50 kg is saved straight away.
– Thirdly the aerodynamic advantage – optimal aerodynamic shape without using long extension shafts and couplings to deal with the dynamics of the rotating shaft connected to a non-optimally rotating propeller and the power pulses of the diesel engine. Now there is the chance to put the engine anywhere in the airframe where it best fits and propeller drive don’t need to be considered at all.

Then there is the redundancy thing. Brushless DC electric motors usually never fail, but the prop can still fail in bad circumstances. Therefore having two independent props for the one diesel engine could be advantageous. Same thing with the batteries – if the diesel engine fails, the batteries could be sized such that the aircraft can fly without the diesel engine for example for 30 minutes in level flight. That might be enough in most cases to get safely on the ground, except on middle of an ocean. The most likely place for the engine to fail is the takeoff. This takeoff stress would never happen with this engine configuration – the engine would be run always at optimum and safe power, never on takeoff power. The extra power for the takeoff can be easily taken from the batteries if they have proper capacity and the electric motors are powerful enough. On takeoff the batteries at full power are not discharging that quickly, because the diesel engine is recharging the batteries at the same time. The takeoff power can be rarely used for longer than 5 minutes on an aircraft equipped with Lycoming engine either, so having a limited period of time for the full power is not that big problem.

Generator and electric motor can have very high efficiency, and the gap to a efficiency of a reduction belt system is not that great. Best electric motors (though heavy ones) are around 98% efficient.

On descent the diesel engine could be shut down providing there was enough battery capacity. The motors could actually regenerate also batteries when the pilot wants to decelerate the plane.

Maintenance cost would be like a single engine aircraft, but the reliability geared towards a twin. Of course there is the one little fine print: the battery pack is expensive and it has an expiration time and date, unfortunately. But nothing is perfect and without compromises.

Any comments about this idea? This surely would not be a racer as the power to weight ratio would be rather poor, but anyhow I am thinking, providing it would be efficient enough to climb adequately, this would be a quite economical thing to fly and also easy conversion-wise, almost stock auto engine would be okay, no reduction gear and prop installation and an assembly that takes the push or pulling loads, would be needed. Also waiting on the airport would not waste any energy, since props can be completely stopped when the plane does not need to move. For example Lycoming IO-360 consumes about the same amount of gasoline per hour when waiting on IFR clearance on the ground than our Toyota Prius car on highway. Consuming zero amount of fuel when still on the ground, but still being ready, would save some liters.

And answer to the question, why diesel and not gasoline when gasoline engines can be run very lean and quite great specific fuel consumption values can be achieved in optimal conditions – it is quite simple: availability of the 100LL/Avgas seems to be becoming poor. There has been three 100LL operators in Finland, but two of them decided to discontinue this year. There is only one left. When that only one decides that it is not profitable enough, there is no 100LL available for anybody and the whole country’s fleet of Lycoming and Continental based planes are grounded. The Jet-A1 is not going anywhere, so engine that can burn the jet fuel would be a safe bet. Jet engine, turboprop, or turbofan are out of the question because those are not available in meaningful sizes and power classes – there is not a small turbofan that would have high pressure ratio and bypass ratio available, nobody manufactures such a thing. And it is unlikely anybody will in the future because this personal flying all is a very niche market unfortunately until it changes for better (if it ever does).

The implementation possibilities have challenges; namely no such electric motor available (would require custom motors possibly), etc. And the weight also causes penalty for the efficiency and speed of the plane. But the power to weight ratio will be with this arrangement a lot better than on a pure electric aircraft. And pure electric aircraft is feasible, why an electric aircraft with a generator and a fueltank added would not be.

And by the way, even if it is first of April at the time of writing this, this blog post is not an April fool.

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  1. Karoliina and Kate,I've always thought this was a great idea. There are a couple more reasons as well.1) You can put the diesel engine where you want for CG purposes. There are a lot of good reasons for wanting to put the engine in the nose, even if you would really rather have the propellers somewhere else. You would have to have some kind of active cooling of the engine because you don't have that hundred-horsepower fan blowing on it all the time — but that also allows better control over engine operating temperature. Overcooling a Lycoming is very easy to do.2) You can get the benefits of a constant-speed prop, without the complexity. Internal combustion engines generate full power only at a specific RPM. With a fixed-pitch prop at zero airspeed, the gas engine can't turn the prop fast enough to generate full power, so we have built constant-speed props to allow the engine to always run at that one speed. With an electric motor, we would size the prop so that at full power at cruising speed the prop tips would be at .8 mach, but with the flat torque curve of an electric motor, we'd be able to generate full power (at a lower RPM, true) at zero and climb airspeeds. The prop would turn faster as the plane accelerates, but that's by design.3) Aviation engines (as opposed to auto engines) are designed to generate power at low RPMs to keep the prop tips subsonic. Because of this, the engines have much larger displacement for the same power. We'd be able to get significantly lighter powerplants for the same horsepower running them at high RPMs.4) Running the generator and engines at relatively high voltage (say, 220 volts) will allow modest cable gauges. You won't have cables anywhere nearly as thick as battery cables going from the generator/battery to the electric motor(s)Finally, I think there's a case to be made for a battery-less system, at least at first. You get so many benefits from separating the diesel and electric systems, and the battery is a huge dead weight for 95% of the time flying. The battery required for 30 minutes level flight would be at least 100 kg with current technologies, and high-power batteries have extraordinary failure modes.I'll do a spreadsheet over the next few days to see just how much weight penalty you would get for a modern 100 hp airplane.

  2. It is possible that 30 minutes of battery capacity might be overkill. But some battery capacity would be needed to do what I was thinking about: have poor power to weight ratio in the diesel engine but have great takeoff power from lightweight electric motors, e.g. 50-100% exceeding the performance of the diesel on takeoff. Yes, the high voltage is a good idea, it does not need to be limited to 220 volts, it could be as well 400 volts. Yes, I had just the idea that the diesel would be run at high constant rpm. Sounds that you have a interesting excel sheet for the weight estimations, would you mind sending me a copy of it?Kate has the additional idea what she's been thinking for over an year, and this could be also used with this and it would be even simpler with this configuration: electric turbo compound: turbo used to turn a generator -> more power to the electric motor. This would be quite tricky to implement, and would require custom heavily modified turbo and that requires speciality that it must be outsourced, we do not have that kind of expertise and skill and it comes with risk of being overpriced for the benefit. I think the engine placement question is not so straightforward that foremost nose is the best (or at least that same could not be achieved with a more novel placement): diesel engines are water cooled and airplane cowlings are always designed for aircooled engines, except some planes, of which a quite good example in my opinion is P51 Mustang. Placing the engine elsewhere than the nose does not change the mustang scoop cooling. Unless someone explains and proves why the mustang scoop actually does not work.

  3. Well, a very brief research seems to show that the problem of this kind of a consept would most likely be the poor efficiency of currently available alternators. A 100 kW diesel delivering approximately 70 kW for the electric motors doesn' make much sense.

  4. Safety in a crash might suffer, but this would afford you the ability to have the engine/genset aft and the pilot forward of the wing. Visibility and entrance/egress would be improved in a low wing.Also, Sonex has a flying all electric 2 seater. I believe they developed their own motor.

  5. This kind of concept would require custom generator and motors, that is the price of this idea. What is available on market may not be good enough, or more specificly, have proper set of specs for this purpose.Yuneec (a maker of electric aircraft) is building their custom motors. This may eventually change as electric cars will become more popular (as this requires one or two powerful but relatively lightweight but efficient brushless DC motors and controllers), but the situation at the moment is not so great for buying off the shelf parts.Why I got this idea for single engine diesel-generator instead of two (for redundancy) was that the diesel has poor power to weight ratio. Having two diesels would mean two times the poor power to weight ratio, meaning twice the overhead and penalty. That would also require more electric motors to produce the additional needed power and the concept gets out of hand very quick. That is why I did not propose two diesel-generators.Unfortunately the engine placement is not that ideal in terms of fireproofing and crashproofing than having the engines in wings. There are pros and cons for every concept.

  6. Check out the hybrid buses they build in Lahti – they weigh the same as their ordinary models.Series hybrids with supercapacitors instead of batteries.The very heavy diesel engine is downsized.Modern car engines do all kind of tricks to produce torque at low speeds etc. A hybrid engine wouldn't need all this complexity with the valvetrain etc…I don't know the life issues with supercapacitors, but I'd imagine that they could be close to forever? They have lousy energy per mass but it doesn't matter that much if it's only for a short while.Say, a takeoff incident. Two minutes of full power or four minutes at half power?Pipistrel had some hand wound motors for self-launching electric sailplanes. Very high power to weight ratio.

  7. This is an interesting idéa that I've been analysing for some time. A hybrid powertrain will probably need a different airframe compared to air-cooled aviation engines, but I think this is an advantage. Free placement of the engine(s) for best COG envelope and the propeller for aerodynamic efficiency. A series hybrid also gives us the ability to design a cross connected dual powerplant – dual prop configuration for redundancy.Why not look into hydrostatic drive as an alternative to electric transmission of power. It would likely be much lighter and cheaper while retaining the advantages of different (and variable) RPMs at powerplant and propeller.Engine efficiency at cruise wuld be enhanced with a single smaller engine powering at best BSFC, and a second (similar) powerpack used for take-off/climb and cruise redundancy.

  8. Karoliina,Im a student in a aviation training institution in Malaysia. Right now im doing a research on whether or not a Campro (an auto engine: http://en.wikipedia.org/wiki/Campro_engine) can be used in Eagle 150b. Can I ask you a favour? I need to know what data do i need, and what i need to do other than comparing both engine performance (Campro and Eagle 150b engine).Im sory if what Im asking is not related to your topic.

  9. Nice ideas but I would go to full electric: double motors and double batteries. One can have so much more batteries instead of having a diesel engine and generator and all additional weight that comes with them, that it must be more efficient to replace that complexity with batteries only. And how much space you save!The only point you loose is the possibility to easy cabin heating with combustion engines. On the other hand the electric motors make also a small amount of heat but is that enough to heat a cabin?I would go full electric .. it is so easy and simple. And the time is on the side of the electric systems also: battery technology will continue to evolve.

  10. Have you seen: http://www.terrafugia.com/aircraft.html ?Spendy, and don't know for sure, dut think the engine is a Rotax.

  11. I too am interested in a hybrid design even at a small weight penalty. My particular interest was a low cost multi-engine trainer I posted this on a yahoo forum and also in a letter to Kitplanes before I saw your writeup. Since then I saw the Sonex eflite project that has a nice 45kW electric motor that might work for a multiengine hybrid.Below a letter i wrote to KitPlanes when their editorial said that electric engine airplanes are not ready for use. (edited to fit comment character limit ) [in the article, two 87 pound electric engines produced 500 HP; used in Killacycle a drag race all electric motorcycle ]batteries are rated 1000 cycles from the battery maker's website, http://www.a123syst ems.com/ at nominal conditions/A CALL FOR AN ELECTRIC HYBRID MULTI-ENGINE TRAINERThis is a proposal for an electric powered multi-engine trainer aircraft, with a gasoline engine / battery hybrid power source. The energy density of gasoline makes it the preferred source of power. Twenty gallons of gasoline is about 120 pounds and it can provide 660 kW-h of energy. A hybrid aircraft can have advantages in terms of lower operation cost, improved reliability and reduced maintenance costs over traditional multi-engine trainers, to offset the weight penalty.- ReliabilityMulti-engine aircraft have twice the probability of an engine failure in flight compared to a single engine aircraft, but they may be have better chances of safely landing at an airfield. The loss of either the battery or the gasoline engine during takeoff or flight will depend on ability of the control system to switch to a single source, and the ability of the remaining system to supply sufficient power for the aircraft to land safely. – Overhaul costsFor traditional gasoline engines, the overhaul costs of two engines is often much more than the cost of a single engine of the same total power. – Proposed design parametersThe electric motors would be capable of 75 hp (56 kW) each for take-off. The total power of 150 hp is consistent with 3 and 4 seat aircraft. The hope is the weight penalty associated with the overall hybrid design will leave the designer with a solid two seat trainer.- Ducted FanIf we elect to keep the cruise speed to less than 100 mph then a ducted fan could be used for each engine. A ducted fan provides good thrust at higher rpm with a smaller overall propeller diameter. The higher rpm is good for electric motor weight efficiency and keeps the tips from going transonic. The ducts give improved thrust for take-off, when compared to a free propeller. The small diameter of the ducted fan means that the two engines can be kept closer to the centerline of the airplane, reducing yaw during engine out simulations. – Gasoline EngineThe gasoline engine probably wants to be at least 100 to 120 hp to charge the battery quickly for touch and go type training. A low weight and high rpm engine would be ideal compared to the low rpm typical of aircraft engines. In the 100 to 120 hp range typical light weigh engines would be the Rotax 912ULS or 914, a Subaru RAM, or Hirth 3701. – BatteriesThe battery needs to be able to supply enough power to clear obstacles during take off with a loss of the gasoline engine and then a few minutes at lower power to return to the airport. Lithium ion as used on KillaCyle and Nissan. From A123 website, 100 pounds of batteries gets you 4.5 kW-h of power at 139 kW continuous, which covers the 112 kW (150 hp) engine power requirement. Producing a continuous 112 kW would deplete the 4.5 kW-h rating in 7.5 minutes, without charging by the gasoline engine, but a lower power setting would still allow longer cruise and safe landings.So Karoliina and Kate, how can we get started on a multiengine hybird plane project?Norm

  12. I too am interested in a hybrid design even at a small weight penalty. My particular interest was a low cost multi-engine trainer I posted this on a yahoo forum and also in a letter to Kitplanes before I saw your writeup. Since then I saw the Sonex eflite project that has a nice 45kW electric motor that might work for a multiengine hybrid.Below a letter i wrote to KitPlanes when their editorial said that electric engine airplanes are not ready for use. (edited to fit comment character limit ) [in the article, two 87 pound electric engines produced 500 HP; used in Killacycle a drag race all electric motorcycle ]batteries are rated 1000 cycles from the battery maker's website, http://www.a123syst ems.com/ at nominal conditions/A CALL FOR AN ELECTRIC HYBRID MULTI-ENGINE TRAINERThis is a proposal for an electric powered multi-engine trainer aircraft, with a gasoline engine / battery hybrid power source. The energy density of gasoline makes it the preferred source of power. Twenty gallons of gasoline is about 120 pounds and it can provide 660 kW-h of energy. A hybrid aircraft can have advantages in terms of lower operation cost, improved reliability and reduced maintenance costs over traditional multi-engine trainers, to offset the weight penalty.- ReliabilityMulti-engine aircraft have twice the probability of an engine failure in flight compared to a single engine aircraft, but they may be have better chances of safely landing at an airfield. The loss of either the battery or the gasoline engine during takeoff or flight will depend on ability of the control system to switch to a single source, and the ability of the remaining system to supply sufficient power for the aircraft to land safely. – Overhaul costsFor traditional gasoline engines, the overhaul costs of two engines is often much more than the cost of a single engine of the same total power. – Proposed design parametersThe electric motors would be capable of 75 hp (56 kW) each for take-off. The total power of 150 hp is consistent with 3 and 4 seat aircraft. The hope is the weight penalty associated with the overall hybrid design will leave the designer with a solid two seat trainer.- Ducted FanIf we elect to keep the cruise speed to less than 100 mph then a ducted fan could be used for each engine. A ducted fan provides good thrust at higher rpm with a smaller overall propeller diameter. The higher rpm is good for electric motor weight efficiency and keeps the tips from going transonic. The ducts give improved thrust for take-off, when compared to a free propeller. The small diameter of the ducted fan means that the two engines can be kept closer to the centerline of the airplane, reducing yaw during engine out simulations. – Gasoline EngineThe gasoline engine probably wants to be at least 100 to 120 hp to charge the battery quickly for touch and go type training. A low weight and high rpm engine would be ideal compared to the low rpm typical of aircraft engines. In the 100 to 120 hp range typical light weigh engines would be the Rotax 912ULS or 914, a Subaru RAM, or Hirth 3701. – BatteriesThe battery needs to be able to supply enough power to clear obstacles during take off with a loss of the gasoline engine and then a few minutes at lower power to return to the airport. Lithium ion as used on KillaCyle and Nissan. From A123 website, 100 pounds of batteries gets you 4.5 kW-h of power at 139 kW continuous, which covers the 112 kW (150 hp) engine power requirement. Producing a continuous 112 kW would deplete the 4.5 kW-h rating in 7.5 minutes, without charging by the gasoline engine, but a lower power setting would still allow longer cruise and safe landings.So Karoliina and Kate, how can we get started on a multiengine hybird plane project?Norm

  13. I too am interested in a hybrid design even at a small weight penalty. I posted this on a yahoo forum and also in a letter to Kitplanes. Since then I saw the Sonex eflite project that has a nice 45kW electric motor that might work for a multiengine hybrid.Below a letter i wrote to KitPlanes when their editorial said that electric engine airplanes are not ready for use. (edited to fit comment character limit ) [in the article, two 87 pound electric engines produced 500 HP; used in Killacycle a drag race all electric motorcycle ]batteries are rated 1000 cycles from the battery maker's website, http://www.a123syst ems.com/ at nominal conditions/A CALL FOR AN ELECTRIC HYBRID MULTI-ENGINE TRAINERThis is a proposal for an electric powered multi-engine trainer aircraft, with a gasoline engine / battery hybrid power source. The energy density of gasoline makes it the preferred source of power. Twenty gallons of gasoline is about 120 pounds and it can provide 660 kW-h of energy. A hybrid aircraft can have advantages in terms of lower operation cost, improved reliability and reduced maintenance costs over traditional multi-engine trainers.- ReliabilityMulti-engine aircraft have twice the probability of an engine failure in flight compared to a single engine aircraft, but they may be have better chances of safely landing at an airfield. The loss of either the battery or the gasoline engine during takeoff or flight will depend on ability of the control system to switch to a single source, and the ability of the remaining system to supply sufficient power for the aircraft to land safely. – Overhaul costsFor traditional gasoline engines, the overhaul costs of two engines is often much more than the cost of a single engine of the same total power. – Proposed design parametersThe electric motors would be capable of 75 hp (56 kW) each for take-off. The total power of 150 hp is consistent with 3 and 4 seat aircraft. The weight penalty of the overall hybrid design will still leave a solid two seat trainer.- Ducted FanIf we elect to keep the cruise speed to less than 100 mph then a ducted fan could be used for each engine. A ducted fan provides good thrust at higher rpm with a smaller overall propeller diameter. The higher rpm is good for electric motor weight efficiency and keeps the tips from going transonic. The ducts give improved thrust for take-off, compared to a free propeller. The small diameter of the ducted fan means that the two engines can be kept closer to the centerline of the airplane, reducing yaw during engine out simulations. – Gasoline Engineshould be at least 100 to 120 hp to charge the battery quickly for touch and go training. A low weight and high rpm engine would be ideal compared to the low rpm typical of aircraft engines. typical light weigh engines would be the Rotax 912ULS or 914, a Subaru RAM, or Hirth 3701. – BatteriesThe battery needs to be able to supply enough power to clear obstacles during take off with a loss of the gasoline engine and then return to the airport. Lithium ion are used on KillaCyle and Nissan. From A123 website, 100 pounds of batteries gets you 4.5 kW-h of power at 139 kW continuous, which covers the 112 kW (150 hp) engine requirement. Producing a continuous 112 kW would deplete the 4.5 kW-h rating in 7.5 minutes, without charging by the gasoline engine, but a lower power setting would still allow longer cruise and safe landings.So Karoliina and Kate, how can we get started on a multiengine hybird plane project?Norm

  14. I too am interested in a hybrid design at a small weight penalty. My particular interest was a low cost multi-engine trainer I posted this on a yahoo forum and also in a letter to Kitplanes before I saw your writeup. Since then I saw the Sonex eflite project that has a nice 45kW electric motor that might work for a multiengine hybrid.Kitplanes' editorial said that electric engine airplanes are not ready for use. (edited to fit comment character limit ) in the article, two 87 pound electric engines produced 500 HP; used in Killacycle a drag race all electric motorcycle batteries are rated 1000 cycles from the battery maker's website, http://www.a123syst ems.com/A CALL FOR AN ELECTRIC HYBRID MULTI-ENGINE TRAINERThis is a proposal for an electric powered multi-engine trainer aircraft, with a gasoline engine / battery hybrid power source. The energy density of gasoline makes it the preferred source of power. Twenty gallons of gasoline is about 120 pounds and it can provide 660 kW-h of energy. A hybrid aircraft can have advantages in terms of lower operation cost, improved reliability and reduced maintenance costs over traditional multi-engine trainers, to offset the weight penalty.Reliability-Multi-engine aircraft have twice the probability of an engine failure in flight compared to a single engine aircraft, but they may be have better chances of safely landing at an airfield. The loss of either the battery or the gasoline engine during takeoff or flight will depend on ability of the control system to switch to a single source, and the ability of the remaining system to supply sufficient power for the aircraft to land safely. Proposed design – The electric motors would be capable of 75 hp (56 kW) each for take-off. This is consistent with 3 and 4 seat aircraft. The hope is the weight penalty associated with the overall hybrid design will leave the designer with a solid two seat trainer.Ducted Fan -for cruise speed to less than 100 mph a ducted fan could be used for each engine. A ducted fan provides good thrust at higher rpm with a smaller overall propeller diameter. The higher rpm is good for electric motor weight efficiency and keeps the tips from going transonic. The ducts give improved thrust for take-off, when compared to a free propeller. The small diameter of the ducted fan means that the two engines can be kept closer to the centerline of the airplane, reducing yaw during engine out simulations. Gasoline Engine -The gasoline engine probably wants to be at least 100 to 120 hp to charge the battery quickly for touch and go type training. A low weight and high rpm engine would be ideal compared to the low rpm typical of aircraft engines. In the 100 to 120 hp range typical light weigh engines would be the Rotax 912ULS or 914, a Subaru RAM, or Hirth 3701. Batteries-The battery needs to be able to supply enough power to clear obstacles during take off with a loss of the gasoline engine and then a few minutes at lower power to return to the airport. Lithium ion as used on KillaCyle and Nissan. From A123 website, 100 pounds of batteries gets you 4.5 kW-h of power at 139 kW continuous, which covers the 112 kW (150 hp) engine power requirement. Producing a continuous 112 kW would deplete the 4.5 kW-h rating in 7.5 minutes, without charging by the gasoline engine, but a lower power setting would still allow longer cruise and safe landings.How can we get started on a multiengine hybird plane project?Norm

  15. I too am interested in a hybrid design even at a small weight penalty. My particular interest was a low cost multi-engine trainer I posted this on a yahoo forum and also in a letter to Kitplanes before I saw your writeup. Since then I saw the Sonex eflite project that has a nice 45kW electric motor that might work for a multiengine hybrid.Below a letter i wrote to KitPlanes when their editorial said that electric engine airplanes are not ready for use. (edited to fit comment character limit ) in the article, two 87 pound electric engines produced 500 HP; used in Killacycle a drag race all electric motorcycle. Batteries are rated 1000 cycles from the battery maker's website, http://www.a123syst ems.com/ at nominal conditions/A CALL FOR AN ELECTRIC HYBRID MULTI-ENGINE TRAINERThis is a proposal for an electric powered multi-engine trainer aircraft, with a gasoline engine / battery hybrid power source. The energy density of gasoline makes it the preferred source of power. Twenty gallons of gasoline is about 120 pounds and it can provide 660 kW-h of energy. A hybrid aircraft can have advantages in terms of lower operation cost, improved reliability and reduced maintenance costs over traditional multi-engine trainers, to offset the weight penalty.Reliability – Multi-engine aircraft have twice the probability of an engine failure in flight compared to a single engine aircraft, but they may be have better chances of safely landing at an airfield. The loss of either the battery or the gasoline engine during takeoff or flight will depend on ability of the control system to switch to a single source. Proposed design parameters- The electric motors would be capable of 75 hp (56 kW) each for take-off. The total power of 150 hp is consistent with 3 and 4 seat aircraft. The hope is the weight penalty associated with the overall hybrid design will leave the designer with a solid two seat trainer.Ducted Fan- If we elect to keep the cruise speed to less than 100 mph then a ducted fan could be used for each engine. A ducted fan provides good thrust at higher rpm with a smaller overall propeller diameter. The higher rpm is good for electric motor weight efficiency and keeps the tips from going transonic. The ducts give improved thrust for take-off, when compared to a free propeller. The small diameter of the ducted fan means that the two engines can be kept closer to the centerline of the airplane, reducing yaw during engine out simulations. Gasoline Engine – The gasoline engine probably wants to be at least 100 to 120 hp to charge the battery quickly for touch and go type training. A low weight and high rpm engine would be ideal compared to the low rpm typical of aircraft engines. In the 100 to 120 hp range typical light weigh engines would be the Rotax 912ULS or 914, a Subaru RAM, or Hirth 3701. Batteries -The battery needs to be able to supply enough power to clear obstacles during take off and then a few to return to the airport. Lithium ion as used on KillaCyle and Nissan. From A123 website, 100 pounds of batteries gets you 4.5 kW-h of power at 139 kW continuous, which covers the 112 kW (150 hp) engine power requirement. Producing a continuous 112 kW would deplete the 4.5 kW-h rating in 7.5 minutes, without charging by the gasoline engine, but a lower power setting would still allow longer cruise and safe landings.So Karoliina and Kate, how can we get started on a multiengine hybird plane project?Norm

  16. I too am interested in a hybrid design even at a small weight penalty. My particular interest was a low cost multi-engine trainer I posted this on a yahoo forum and also in a letter to Kitplanes before I saw your writeup. Since then I saw the Sonex eflite project that has a nice 45kW electric motor that might work for a multiengine hybrid.Below a letter i wrote to KitPlanes when their editorial said that electric engine airplanes are not ready for use. (edited to fit comment character limit ) in the article, two 87 pound electric engines produced 500 HP; used in Killacycle a drag race all electric motorcycle. Batteries are rated 1000 cycles from the battery maker's website, http://www.a123syst ems.com/ at nominal conditions/A CALL FOR AN ELECTRIC HYBRID MULTI-ENGINE TRAINERThis is a proposal for an electric powered multi-engine trainer aircraft, with a gasoline engine / battery hybrid power source. The energy density of gasoline makes it the preferred source of power. Twenty gallons of gasoline is about 120 pounds and it can provide 660 kW-h of energy. A hybrid aircraft can have advantages in terms of lower operation cost, improved reliability and reduced maintenance costs over traditional multi-engine trainers, to offset the weight penalty.Reliability – Multi-engine aircraft have twice the probability of an engine failure in flight compared to a single engine aircraft, but they may be have better chances of safely landing at an airfield. The loss of either the battery or the gasoline engine during takeoff or flight will depend on ability of the control system to switch to a single source. Proposed design parameters- The electric motors would be capable of 75 hp (56 kW) each for take-off. The total power of 150 hp is consistent with 3 and 4 seat aircraft. The hope is the weight penalty associated with the overall hybrid design will leave the designer with a solid two seat trainer.Ducted Fan- If we elect to keep the cruise speed to less than 100 mph then a ducted fan could be used for each engine. A ducted fan provides good thrust at higher rpm with a smaller overall propeller diameter. The higher rpm is good for electric motor weight efficiency and keeps the tips from going transonic. The ducts give improved thrust for take-off, when compared to a free propeller. The small diameter of the ducted fan means that the two engines can be kept closer to the centerline of the airplane, reducing yaw during engine out simulations. Gasoline Engine – The gasoline engine probably wants to be at least 100 to 120 hp to charge the battery quickly for touch and go type training. A low weight and high rpm engine would be ideal compared to the low rpm typical of aircraft engines. In the 100 to 120 hp range typical light weigh engines would be the Rotax 912ULS or 914, a Subaru RAM, or Hirth 3701. Batteries -The battery needs to be able to supply enough power to clear obstacles during take off and then a few to return to the airport. Lithium ion as used on KillaCyle and Nissan. From A123 website, 100 pounds of batteries gets you 4.5 kW-h of power at 139 kW continuous, which covers the 112 kW (150 hp) engine power requirement. Producing a continuous 112 kW would deplete the 4.5 kW-h rating in 7.5 minutes, without charging by the gasoline engine, but a lower power setting would still allow longer cruise and safe landings.So Karoliina and Kate, how can we get started on a multiengine hybird plane project?Norm

  17. I too am interested in a hybrid design even at a small weight penalty. My particular interest was a low cost multi-engine trainer I posted this on a yahoo forum and also in a letter to Kitplanes before I saw your writeup. Since then I saw the Sonex eflite project that has a nice 45kW electric motor that might work for a multiengine hybrid.Below a letter I wrote to KitPlanes when their editorial said that electric engine airplanes are not ready for use. (edited to fit character limit ) In the article, two 87 pound electric engines produced 500 HP; used in Killacycle a drag race all electric motorcycle. Batteries from http://www.a123systems.comThis is a proposal for an electric powered multi-engine trainer aircraft, with a gasoline engine / battery hybrid power source. The energy density of gasoline makes it the preferred source of power. Twenty gallons of gasoline is about 120 pounds and it can provide 660 kW-h of energy. A hybrid aircraft can have advantages in terms of lower operation cost, improved reliability and reduced maintenance costs over traditional multi-engine trainers, to offset the weight penalty.Reliability – Multi-engine aircraft have twice the probability of an engine failure in flight compared to a single engine aircraft, but they may be have better chances of safely landing at an airfield. The loss of either the battery or the gasoline engine during takeoff or flight will depend on ability of the control system to switch to a single source. Proposed design parameters- The electric motors would be capable of 75 hp (56 kW) each for take-off. The total power of 150 hp is consistent with 3 and 4 seat aircraft. The hope is the weight penalty associated with the overall hybrid design will leave the designer with a solid two seat trainer.Ducted Fan- If we elect to keep the cruise speed to less than 100 mph then a ducted fan could be used for each engine. A ducted fan provides good thrust at higher rpm with a smaller overall propeller diameter which keeps the tips from going transonic. The ducts give improved thrust for take-off, compared to a free propeller. The small diameter of the ducted fan means that the two engines can be kept closer to the centerline of the airplane, reducing yaw during engine out simulations. Gasoline Engine – 100 to 120 hp to charge the battery quickly for touch and go type training. In the 100 to 120 hp range typical light weigh high rpm engines would be the Rotax 912ULS or 914, a Subaru RAM, or Hirth 3701. Batteries -The battery needs to be able to supply enough power to clear obstacles during take off and then a few to return to the airport. From A123 website, 100 pounds of batteries gets you 4.5 kW-h of power at 139 kW continuous, which covers the 112 kW engine power requirement. Producing a continuous 112 kW would deplete the 4.5 kW-h rating in 7.5 minutes.How can we get started on a multiengine hybird plane project?Norm

  18. My interest is a low cost multi-engine trainer I posted this on a yahoo forum and also in a letter to Kitplanes before I saw your writeup. Since then I saw the Sonex eflite project that has a nice 45kW electric motor that might work for a multiengine hybrid.(edited to fit character limit ) In the article, two 87 pound electric engines produced 500 HP; used in Killacycle a drag race all electric motorcycle. Batteries from http://www.a123systems.comThis is a proposal for an electric powered multi-engine trainer aircraft, with a gasoline engine / battery hybrid power source. The energy density of gasoline makes it the preferred source of power. Twenty gallons of gasoline is about 120 pounds and it can provide 660 kW-h of energy. A hybrid aircraft can have advantages in terms of lower operation cost, improved reliability and reduced maintenance costs over traditional multi-engine trainers, to offset the weight penalty.Reliability – Multi-engine aircraft have twice the probability of an engine failure in flight compared to a single engine aircraft, but they may be have better chances of safely landing at an airfield. The loss of either the battery or the gasoline engine during takeoff or flight will depend on ability of the control system to switch to a single source. Proposed design parameters- The electric motors would be capable of 75 hp (56 kW) each for take-off. The total power of 150 hp is consistent with 3 and 4 seat aircraft. The hope is the weight penalty associated with the overall hybrid design will leave the designer with a solid two seat trainer.Ducted Fan- If we elect to keep the cruise speed to less than 100 mph then a ducted fan could be used for each engine. A ducted fan provides good thrust at higher rpm with a smaller overall propeller diameter which keeps the tips from going transonic. The ducts give improved thrust for take-off, compared to a free propeller. The small diameter of the ducted fan means that the two engines can be kept closer to the centerline of the airplane, reducing yaw during engine out simulations. Gasoline Engine – 100 to 120 hp to charge the battery quickly for touch and go type training. In the 100 to 120 hp range typical light weigh high rpm engines would be the Rotax 912ULS or 914, Subaru RAM, or Hirth 3701. Batteries -The battery needs to be able to supply enough power to clear obstacles during take off and then a few to return to the airport. From A123 website, 100 pounds of batteries gets you 4.5 kW-h of power at 139 kW continuous, which covers the 112 kW engine power requirement. Producing a continuous 112 kW would deplete the 4.5 kW-h rating in 7.5 minutes.Norm

  19. Part 1 of comment My interest is a low cost multi-engine trainer I posted this on a yahoo forum and also in a letter to Kitplanes before I saw your writeup. Since then I saw the Sonex eflite project that has a nice 45kW electric motor that might work for a multiengine hybrid.(edited to fit character limit ) In the article, two 87 pound electric engines produced 500 HP; used in Killacycle a drag race all electric motorcycle. Batteries from http://www.a123systems.comThis is a proposal for an electric powered multi-engine trainer aircraft, with a gasoline engine / battery hybrid power source. The energy density of gasoline makes it the preferred source of power. Twenty gallons of gasoline is about 120 pounds and it can provide 660 kW-h of energy. A hybrid aircraft can have advantages in terms of lower operation cost, improved reliability and reduced maintenance costs over traditional multi-engine trainers, to offset the weight penalty.Reliability – Multi-engine aircraft have twice the probability of an engine failure in flight compared to a single engine aircraft, but they may be have better chances of safely landing at an airfield. The loss of either the battery or the gasoline engine during takeoff or flight will depend on ability of the control system to switch to a single source. Norm 1 of 2

  20. Part 2 of 2Proposed design parameters- The electric motors would be capable of 75 hp (56 kW) each for take-off. The total power of 150 hp is consistent with 3 and 4 seat aircraft. The hope is the weight penalty associated with the overall hybrid design will leave the designer with a solid two seat trainer.Ducted Fan- If we elect to keep the cruise speed to less than 100 mph then a ducted fan could be used for each engine. A ducted fan provides good thrust at higher rpm with a smaller overall propeller diameter which keeps the tips from going transonic. The ducts give improved thrust for take-off, compared to a free propeller. The small diameter of the ducted fan means that the two engines can be kept closer to the centerline of the airplane, reducing yaw during engine out simulations. Gasoline Engine – 100 to 120 hp to charge the battery quickly for touch and go type training. In the 100 to 120 hp range typical light weigh high rpm engines would be the Rotax 912ULS or 914, Subaru RAM, or Hirth 3701. Batteries -The battery needs to be able to supply enough power to clear obstacles during take off and then a few to return to the airport. From A123 website, 100 pounds of batteries gets you 4.5 kW-h of power at 139 kW continuous, which covers the 112 kW engine power requirement. Producing a continuous 112 kW would deplete the 4.5 kW-h rating in 7.5 minutes.Norm

  21. Norm, sorry that I did not approve your comment earlier, I did not notice it and I have been busy with other things (see my latest post). Thanks for your contribution!

  22. About the ducted fans:- The speed under 100 mph is not so impressive for a twin, not even for a hybrid nor pure electric aircraft. I am currently flying Diamond DA40-180 and its cruise performance is about minimum useful for traveling anywhere (it cruises with economy setting 130 kts TAS). I have a benchmark for the twin concept: DynAero Twin R:http://www.hamradio-friedrichshafen.com/aero-en/press/novelties-worldwide.php?lid=23&sMode=detail2 x Rotax 912ULS180 kts cruise speedPayload > 500 kg2000 km rangeThe weight penalties from batteries, extra motors etc. might be too unfavorable to match this benchmark though, since this particular plane gets its performance from the very low empty weight. Of course it could be possible to match it with low IAS but high TAS at high altitude with a heavier plane by optimizing the airframe and combustion engine for that in mind.

  23. The concept is great, and the fuel savings look very promising as well, but I just wanted to step in on the part where you mentioned recharging the batteries with the diesel engine off during descent/slow down.The problem with that is that the aerodynamic setup for a wind generator (i.e to collect energy from moving air) is completely different from the aerodynamic setup for a propeller generating the air flow.Unless you can actually find a way to change the airfoil when you're doing this, you're going to end up with a very inefficient generator, even if it will be a very powerful airbrake. The drag it'll cause is pretty powerful.Just something to keep in mind if you wanted to run some simulations.

  24. Why not do a push/pull concept or coaxial twin? You could have the diesel direct drive one prop while charging a battery which powers the electric. The electric would only be used on take off and as a backup when the diesel fails. Now you have full redundancy without 2 engines and 2 electric motors. The best of both.

  25. I may post this as a separate article also because otherwise it possibly does not get read by that many:Brent: A valid point. However, there is a little incompatibility here that I don't see how to overcome:- the diesel engine operates at medium rpm which requires reduction drive- the electric motor can designed to be direct drive and low rpm without need for reduction unitHaving series hybrid there is weight penalty of two brushless DC motors and the engine and the battery, but no other systems. The engine runs the brushless DC motor without reduction gear and the motor that is used as generator can be designed to operate at the rpm the engine operates. The other motor which drives the prop can be made to operate at low rpm.-> this sytem has NO:- weight penalty of reduction gear unit- reliability penalty of reduction gear unit- need for propeller clutch and the associated reliability penalty and weight penalty- need for drive shaft to achieve aerodynamic cowling shapeYou already listed the most of the pros for the diesel direct drive. I list the cons:The diesel direct drive cons:- would not work without clutch, the power pulses would make the prop come off in flight if it did not fail on ground testing already- does not get necessary power to weight ratio from the engine because of the need to run it at low rpm because of the prop requires low rpm- weight penalty of the additional gear reduction unit- reliability penalty of the additional gear reduction unit- weight penalty of the clutch- reliability penalty of the clutch (in Thielert engines they have failed now and then, especially in the original design, the latest engine models might have addressed this issue but I am not sure)- added complexity- aerodynamic cowling shape may require drive shaft, and reliable drive shaft has been proven to be hard to design and manufacture such way that it would be 100% reliable- the diesel engine is harder for the prop than a electric motor because of power pulses (even with clutch) and more expensive propeller is needed than would be needed with the electric motor alone.There is however a case what has not been talked about for your case: – planetary gear system for driving the electric motor and the diesel engine at the same time – Toyota Prius hybrid synergy drive thing. That is about bullet proof and single point of failure will not stop the prop, one motor is enough to continue driving the prop. – This of course has associated weight penalty. On Toyota Prius it does not matter, but on aircraft it does matter.Case for push-pull:- To avoid drive shaft, the diesel engine would need to be the front engine. – case for achieving any kind of laminar flow to the fuselage would be pretty much lost- inefficiency problems on the rear prop because of the front prop. I have not quantified this on the other hand, apparently nobody is able to answer how much is the penalty, it is not even exact in literature.

  26. I may post this as a separate article also because otherwise it possibly does not get read by that many:Brent: A valid point. However, there is a little incompatibility here that I don't see how to overcome:- the diesel engine operates at medium rpm which requires reduction drive- the electric motor can designed to be direct drive and low rpm without need for reduction unitHaving series hybrid there is weight penalty of two brushless DC motors and the engine and the battery, but no other systems. The engine runs the brushless DC motor without reduction gear and the motor that is used as generator can be designed to operate at the rpm the engine operates. The other motor which drives the prop can be made to operate at low rpm.-> this sytem has NO:- weight penalty of reduction gear unit- reliability penalty of reduction gear unit- need for propeller clutch and the associated reliability penalty and weight penalty- need for drive shaft to achieve aerodynamic cowling shapeYou already listed the most of the pros for the diesel direct drive. I list the cons:The diesel direct drive cons:- would not work without clutch, the power pulses would make the prop come off in flight if it did not fail on ground testing already- does not get necessary power to weight ratio from the engine because of the need to run it at low rpm because of the prop requires low rpm- weight penalty of the additional gear reduction unit- reliability penalty of the additional gear reduction unit- weight penalty of the clutch- reliability penalty of the clutch (in Thielert engines they have failed now and then, especially in the original design, the latest engine models might have addressed this issue but I am not sure)- added complexity- aerodynamic cowling shape may require drive shaft, and reliable drive shaft has been proven to be hard to design and manufacture such way that it would be 100% reliable- the diesel engine is harder for the prop than a electric motor because of power pulses (even with clutch) and more expensive propeller is needed than would be needed with the electric motor alone.There is however a case what has not been talked about for your case: – planetary gear system for driving the electric motor and the diesel engine at the same time – Toyota Prius hybrid synergy drive thing. That is about bullet proof and single point of failure will not stop the prop, one motor is enough to continue driving the prop. – This of course has associated weight penalty. On Toyota Prius it does not matter, but on aircraft it does matter.Case for push-pull:- To avoid drive shaft, the diesel engine would need to be the front engine. – case for achieving any kind of laminar flow to the fuselage would be pretty much lost- inefficiency problems on the rear prop because of the front prop. I have not quantified this on the other hand, apparently nobody is able to answer how much is the penalty, it is not even exact in literature.

  27. I may post this as a separate article also because otherwise it possibly does not get read by that many:Brent: A valid point. However, there is a little incompatibility here that I don't see how to overcome:- the diesel engine operates at medium rpm which requires reduction drive- the electric motor can designed to be direct drive and low rpm without need for reduction unitHaving series hybrid there is weight penalty of two brushless DC motors and the engine and the battery, but no other systems. The engine runs the brushless DC motor without reduction gear and the motor that is used as generator can be designed to operate at the rpm the engine operates. The other motor which drives the prop can be made to operate at low rpm.-> this sytem has NO:- weight penalty of reduction gear unit- reliability penalty of reduction gear unit- need for propeller clutch and the associated reliability penalty and weight penalty- need for drive shaft to achieve aerodynamic cowling shapeYou already listed the most of the pros for the diesel direct drive. I list the cons:The diesel direct drive cons:- would not work without clutch, the power pulses would make the prop come off in flight if it did not fail on ground testing already- does not get necessary power to weight ratio from the engine because of the need to run it at low rpm because of the prop requires low rpm- weight penalty of the additional gear reduction unit- reliability penalty of the additional gear reduction unit- weight penalty of the clutch- reliability penalty of the clutch (in Thielert engines they have failed now and then, especially in the original design, the latest engine models might have addressed this issue but I am not sure)- added complexity- aerodynamic cowling shape may require drive shaft, and reliable drive shaft has been proven to be hard to design and manufacture such way that it would be 100% reliable- the diesel engine is harder for the prop than a electric motor because of power pulses (even with clutch) and more expensive propeller is needed than would be needed with the electric motor alone.There is however a case what has not been talked about for your case: – planetary gear system for driving the electric motor and the diesel engine at the same time – Toyota Prius hybrid synergy drive thing. That is about bullet proof and single point of failure will not stop the prop, one motor is enough to continue driving the prop. – This of course has associated weight penalty. On Toyota Prius it does not matter, but on aircraft it does matter.Case for push-pull:- To avoid drive shaft, the diesel engine would need to be the front engine. – case for achieving any kind of laminar flow to the fuselage would be pretty much lost- inefficiency problems on the rear prop because of the front prop. I have not quantified this on the other hand, apparently nobody is able to answer how much is the penalty, it is not even exact in literature.

  28. Brent: I may post this as a separate article also because otherwise it possibly does not get read by that many.

  29. Siemens, Diamond and EADS made this kind of (serial hybrid electric drive train) prototype plane: Siemens hybrid electric aircraft debuts in Paris

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