# Two cylinder gear-less twin aircraft

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#### Niels

##### Well-Known Member
Electric propulsion of cars have made electric motors and generators very light,efficient and reliable in no time.
Batteries have not improved orders of magnitude since WW1 submarines.
Let try if a single prop/electric motor driven by two single cylinder enginerators can be as reliable / low mass as best small conventional twin aircraft engine system to day.
I propose that a single prop turning 35 rev per second using max 200hp/150kW can be a good comparison point.

#### TFF

##### Well-Known Member
So how are two generators making 4-6kw of power going to make 150kw of power? I’m having to assume you are talking about the Honda type generators.

#### 12notes

##### Well-Known Member
Log Member
You lose efficiency by converting mechanical power to electrical power, then again converting the electrical power back to mechanical power. You'll lose roughly 10% of the power with each conversion, and that's pretty much a best case scenario, resulting in having only 81% of the power you would get by using the engines to directly power the propeller.

#### Niels

##### Well-Known Member
The Pipistrel thing is a fact.
That means mass numbers /weight in kg for components must be available somewhere.
It is a battery- feed electric trainer for pupil and instructer and can do maybe six or seven touch and go on one charge.
For longer single person flights we can remove batteries and hang a generator pods under each wings and have a very reliable propulsion system.Where relative to center of gravity is batteries stored and how much mass?

#### wsimpso1

##### Super Moderator
Staff member
Log Member
In post 1 you propose a scheme with gensets and a motor at 150kW - all of that 150 kW has to come from somewhere and that somewhere is usually the gensets 75kW is a lot - are there single cylinder engine gensets in that range available? Given the usual power conversion efficiencies, you will need more like 170 kW of engines to do that. I know how big 100 hp engines and electrics are - the weight penalty over a simple engine turning a prop will be substantial.

Then you start talking about battery powered flight in post 4 for pattern work and wing pods for the gensets - Sounds more like two airplanes, one with batteries only for flight training near the airport, and another with gensets only for cross country.

Are you looking for folks to contribute information to your idea? Things like specific mass per unit energy and mass per unit power for gensets+fuel and for batteries can be used to figure out how much weight you would be carrying and how long it could run.

I would advise that you make use of the advanced search tool, and look under "hybrid power", "battery power", "energy density", and similar terms. This topic has been talked about several times in the last couple years, and you want to find those threads.

Billski

#### Vigilant1

##### Well-Known Member
I don't think the idea has technical merit due to the conversion losses already mentioned.
Further, using the electric power to turn a single electric motor doesn't make much sense. With electric motors, there are only minor weight savings in using one larger motor vs several smaller ones. So why have multiple generators and then deliberately introduce a single point of failure? Moreover, because propellers are more efficient with more disk area per HP, packaging considerations will normally allow several smaller engines to have better prop efficiency than one larger one.

#### Niels

##### Well-Known Member
The two battery packs must be around 110kg and Ducati make a 135kg 250 horsepower fourcylinder Motorbike.
Best glide ratio is 15 at 33meter per second and MTOW of aircraft 550kg or ca 12 kW.
For a single person long range fligth without batteries we can have two 100 kg pods with generator and fuel.
Some range extending.
Let each pod be able to make 30kw electricity.Let us asume it can be done with 40 kg machinery and that leaves ca 50kg fuel per pod.
Crusing at 84knots or 43 meter per second takes ca40 kW electricity .If electricity can be made with 40% efficiency we get ca 4.7kWh per kg fuel.
We have 100 kg fuel and can thus make470 kWh electricity or fly 12 hours at 84 knots,that is 1000 nautical miles.

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#### 12notes

##### Well-Known Member
Log Member
Your example would be way over gross weight. The useful load of the Pipistrel Alpha Electro is 182 kg, adding 200kg requires a pilot that weighs negative 18kg. Keto isn't going to cut it. To accomodate the increased gross weight plus the reinforcement needed to hang 200kg from the wings would require a complete redesign of the plane. It wouldn't be the same plane anymore.

You've also underestimated the weight of a 30kW generator, and overstated the efficiency. The best piston engines are about 35% efficient. You lose about 10% in the conversion to electricity, leaving 31.5% efficient. You lose 10% again converting it to mechanical, leaving 28.35%, so that would be 3.5kWh per kg of fuel, which would result it 100kg of fuel being enough to travel 8.75 hours, 735 nm. I couldn't find any 30kW generators under 200kg, but they were for houses. I'm sure you could lose much of the weight, but not 80% of it. I found 4 stroke 30kW engines around 40kg, but the generator part isn't going to be lightweight. Two strokes can be found lighter, but that would murder the fuel efficiency, probably cut it in half.

Or you could just order the gasoline engine Pipistrel Alpha and add a 16.5 gallon (45kg) ferry tank and get the same range 2 hours quicker at 108kts, plus still have a useful load of 137kg without changing the gross weight. The generator idea, while it can work in electric cars, just isn't practical in electric planes.

#### pictsidhe

##### Well-Known Member
Lots of assumptions. Actual numbers are needed to see if any aircraft idea is viable.

#### John.Roo

##### Well-Known Member
Your example would be way over gross weight. The useful load of the Pipistrel Alpha Electro is 182 kg, adding 200kg requires a pilot that weighs negative 18kg. Keto isn't going to cut it. To accomodate the increased gross weight plus the reinforcement needed to hang 200kg from the wings would require a complete redesign of the plane. It wouldn't be the same plane anymore.

You've also underestimated the weight of a 30kW generator, and overstated the efficiency. The best piston engines are about 35% efficient. You lose about 10% in the conversion to electricity, leaving 31.5% efficient. You lose 10% again converting it to mechanical, leaving 28.35%, so that would be 3.5kWh per kg of fuel, which would result it 100kg of fuel being enough to travel 8.75 hours, 735 nm. I couldn't find any 30kW generators under 200kg, but they were for houses. I'm sure you could lose much of the weight, but not 80% of it. I found 4 stroke 30kW engines around 40kg, but the generator part isn't going to be lightweight. Two strokes can be found lighter, but that would murder the fuel efficiency, probably cut it in half.

Or you could just order the gasoline engine Pipistrel Alpha and add a 16.5 gallon (45kg) ferry tank and get the same range 2 hours quicker at 108kts, plus still have a useful load of 137kg without changing the gross weight. The generator idea, while it can work in electric cars, just isn't practical in electric planes.
Hello!
I have to agree with what 12notes wrote... I was also looking for combustion generator with output of 15 kW and there are no light solutions on the market. The idea of extra fuel tank will save you money and a lot of development work....

However if you are really interested in hybrid solution, you can try this one:
With aerodynamically efficient airplane like Alpha Trainer you can achieve 60 minutes of only electric flight with +-50 kg batery + "normal or increased" range with "normal or increased" fuel tanks + more power for takeoff.
Even this solution will request some investment and development work, but is based on serial airplane with Rotax engine and with permit to fly

Martin

##### Well-Known Member
Hello!
I have to agree with what 12notes wrote... I was also looking for combustion generator with output of 15 kW and there are no light solutions on the market. The idea of extra fuel tank will save you money and a lot of development work....

However if you are really interested in hybrid solution, you can try this one:
With aerodynamically efficient airplane like Alpha Trainer you can achieve 60 minutes of only electric flight with +-50 kg batery + "normal or increased" range with "normal or increased" fuel tanks + more power for takeoff.
Even this solution will request some investment and development work, but is based on serial airplane with Rotax engine and with permit to fly

Martin
That sounds a lot like the Aixro/Engiro combination. They've been selling Wankel/generator packages of up to 30 kW for a few years now.

They also have a 15 kW one. 70 lbs for the complete drive:

Serial hybrids (IC=>generator=>electric motor) only makes sense if you can cruise on very low power as compared to what you'd need/want for take/off.

#### 12notes

##### Well-Known Member
Log Member
A much lighter solution would be to put a HKS 700E behind the electric motor (42 kW continuous, 55kg) and have it turn the electric motor/propeller directly. Add another 45kg of fuel for 6.6 hours (550nm) of cruise range, and you have 75% of the range of your proposed system for half the weight (100kg), a much simpler system, enough load left for a lightweight pilot (82kg), and no structural changes are required. You could even use the engine to regenerate battery power through the motor during descents.

Note that this does not include the weight of the mounting, wiring/plumbing and engine fluids, but neither does the original example.

#### Niels

##### Well-Known Member
The scheme from 12 notes sounds worth a thought and will have to be judged on safety/MTBF.
My present best candidate is a single electric motor up front with two sets of windings and magnets and two wing mounted outboard enginerators that will look dramatic if on fire but not be really dangerous before plane is on ground.
The present pipistrel electromotor is 60kW at 40 rps and around 20kg
It is not unreasonable to expect two 30kW generator at say 100 rps to be 20 kg as well .
That is 10 kg each.
If we remove the gearbox from a rotax 912 and cut motor in half we have around 20 kg in each that can make 30 kW.
The electrified machinery will be two times (20 plus 10) for enginerators and 20 for the prop motor or ca 80 kg
where the old fashioned Rotax 912 model also uses 80 kg for same purpose.
Is it a safer system?

#### 12notes

##### Well-Known Member
Log Member
You can't just cut the weight in half for engines. There's a significant portion of the weight that does not change when you remove two cylinders.

There is someone who built a 1/2 Rotax 912, it makes 36.7kW and weighs 39kg dry. It will weigh more with the necessary fluids:

The same applies to electric motors/generators, unless you found a specific 30kW generator that weighs 10kg, you're just dreaming. Also, you need to couple them, regulate the power, and wire them, which adds significant weight.

Just one of your generator pods without fuel will weigh more than the single engine behind the motor, and be less reliable. You have potential failures in the generator, regulator and wiring that don't exist with the direct drive, and failure modes that could stop the propeller from turning while the engines are still running.

#### John.Roo

##### Well-Known Member
You can't just cut the weight in half for engines. There's a significant portion of the weight that does not change when you remove two cylinders.

There is someone who built a 1/2 Rotax 912, it makes 36.7kW and weighs 39kg dry. It will weigh more with the necessary fluids:

The same applies to electric motors/generators, unless you found a specific 30kW generator that weighs 10kg, you're just dreaming. Also, you need to couple them, regulate the power, and wire them, which adds significant weight.

Just one of your generator pods without fuel will weigh more than the single engine behind the motor, and be less reliable. You have potential failures in the generator, regulator and wiring that don't exist with the direct drive, and failure modes that could stop the propeller from turning while the engines are still running.
Exactly - I wouldn't write it better

Engiro / Aixro - Wankel engine based generator.
This has been tested by Stuttgart Akaflieg on e-Genius. Not working.

Aviation world is still waiting for 30 kW generator with weight of 10 kg....
I mean real one, not just project on paper

#### Niels

##### Well-Known Member
Emrax has some motor and generator options with this power-to-weight ratio:
Thank You very much for drawings and test reports.
It is my personal feeling that batteries are not the solution for transport but electric drives are.
I will dream much more realistic about my scheme with the power,mass and efficiencies in Your source
My scheme is a single centerline electric motor and fixed pitch prop.
This prop - motor can have two sets of windings and magnets.
Electricity shall come from at least two sources in my system.
They need not be identical.
A potential low-mass system is described some-where here

Phantasies

I will do a little more scheming from Your cataloque numbers and be back.
The number we really need ,to lift level of discussion ,is Mean Time Between Failure from Rotax 912.
The lead consuming dinosaurs are not relevant in future.
Is there somewhere one can get this number? Rotax surely not.

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#### Niels

##### Well-Known Member
The Emrax 188 is a 30kW/40 horsepower electric machine and weigh 7.3 kg.
Continous at 110 rounds per second.
Two two stroke cylinders of 225 ccm will do.
Most of crankshaft is already there and each cylinder will be less than 2 kg.A crankdisc,counterweight,conrods and pistons will be less than 5 kg.Some crankcase ekstra on generator can be made max 5 kg.
Total 7.3 plus 4 plus 5 plus 5 say 25 kg to make 30kW electricity

#### Niels

##### Well-Known Member
Instead of making 30 kW using two cylinders and one generator ,we can use one cylinder two pistons and two phaselocked generators as I demonstrated around 1994.
There is no specific electric machine in the Emrax range for 15kW but an Emrax 188, where all axial dimensions are halved, will be a good first guess.
Say 4 kg converts 15 kW at 100 rounds per second.It will be easier to keep cool so it is a 20kW electric machine really.

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#### Niels

##### Well-Known Member
A cylinder of half a litre with two opposing pistons will make 40 horsepower at 100 rps if mean effective pressure is 7.5 bar.That is roughly a mixture of 3/4 fresh gas and 1/4 exhaust.If the strokes are 60mm we will need a cylinder inner diameter of ca 75mm.Pure outboard boat engine thermal load country.