[teragram (OP)]

I notice that there are various projects in progress by aircraft manufacturers to build hybrid aircraft.
I refer not to heavier/lighter than air hybrids, but to conventional aircraft powered by petrol(or jetfuel)/electric power trains as in hybrid cars.

As far as I know, the advantage of petrol/electric car power trains is that the engine part can be run at it's optional load/speed conditions, ie., constant speed and near constant load, resulting in the high fuel efficiencies seen in series hybrids.
Also as far as I know, the condition in which an aircraft engine operates is not dissimilar in that the propeller pitch is varied to match requirements whilst the engine runs at a near constant speed, so why is the hybrid going to be much better?
Except of course that the battery in the hybrid can be charged whilst the aircraft is on the ground, allowing a lower fuel requirement during take of and climbing to cruising altitude.
• Or maybe these experiments are to evaluate the use of fully electric aircraft?
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[alancalverd]

Everything about an aeroplane is a compromise between strength and weight, lift and drag, cruise speed versus landing speed....and hybrids offer a few more variables for the designer to play with.

Propellors are a good compromise for slow flight, low altitude, and short sectors, and for very short trips (island hopping, air ambulance..) it turns out that piston engines are more economical in terms of maintenance than turbines. Problem is that you don't want the propellor tips to exceed the speed of sound, so you either need a gearbox (dead weight) or a diesel engine (heavier than gasoline, and needs liquid cooling, but is efficient at a lower speed than a gasoline engine of the same capacity) or a smaller propeller (less efficient because the hub doesn't generate as much thrust as the tip)....or run a combustion engine at its optimum speed and use an electric drive train, as in many railway engines.

One  interesting hybrid uses several small props distributed along the wing. The accelerated airflow improves lift and control at low speeds, and the small electric motors produce less profile drag than two large piston engines: the prime mover is a big diesel in the fuselage, where weight and shape is less of a problem.

Single-engine hybrids can use a battery for takeoff and climb, at say 200 horsepower short term, with cruise power supplied by a somewhat smaller (100 hp continuous) engine than you would need to get off the ground safely. This system is particularly interesting for noise-sensitive missions.

Or you can go the other way: use a combustion engine to climb above the clouds, then solar panels for cruising.

Meanwhile, diesels are getting lighter and gasoline engines are being developed with lightweight gearboxes and improved takeoff power ratings. We live in interesting times.

[evan_au]

Hybrid diesel-electric has been used for a long time in train locomotives and somewhat more recently in shipping.

You can put a diesel engine and generator somewhere that it will help the center of gravity, and then have relatively small electric motors driving the wheels or propellers, without needing a direct mechanical linkage to the motor.

Electric drive has provided a lot of flexibility, such as in azipods:
https://en.wikipedia.org/wiki/Azipod

Maybe some of those tiltrotor concept planes might be more feasible with electric drive?
https://en.wikipedia.org/wiki/Tiltrotor

[syhprum]

I think that the amount of energy that can be stored in liquid fuel will always be greater than the amount that can be stored in a battery of the same weight so I see no prospect of long range aircraft running on battery power.
As Alan has pointed out there are interesting possibilities for small aircraft possibly supplemented by solar power but for the moment I would prefer to travel on liquid fuelled aircraft although these have been known to crash thru inadequate fueling. 

[chris]

Everything about an aeroplane is a compromise between strength and weight, lift and drag, cruise speed versus landing speed....and hybrids offer a few more variables for the designer to play with.

Propellors are a good compromise for slow flight, low altitude, and short sectors, and for very short trips (island hopping, air ambulance..) it turns out that piston engines are more economical in terms of maintenance than turbines. Problem is that you don't want the propellor tips to exceed the speed of sound, so you either need a gearbox (dead weight) or a diesel engine (heavier than gasoline, and needs liquid cooling, but is efficient at a lower speed than a gasoline engine of the same capacity) or a smaller propeller (less efficient because the hub doesn't generate as much thrust as the tip)....or run a combustion engine at its optimum speed and use an electric drive train, as in many railway engines.

One  interesting hybrid uses several small props distributed along the wing. The accelerated airflow improves lift and control at low speeds, and the small electric motors produce less profile drag than two large piston engines: the prime mover is a big diesel in the fuselage, where weight and shape is less of a problem.

Single-engine hybrids can use a battery for takeoff and climb, at say 200 horsepower short term, with cruise power supplied by a somewhat smaller (100 hp continuous) engine than you would need to get off the ground safely. This system is particularly interesting for noise-sensitive missions.

Or you can go the other way: use a combustion engine to climb above the clouds, then solar panels for cruising.

Meanwhile, diesels are getting lighter and gasoline engines are being developed with lightweight gearboxes and improved takeoff power ratings. We live in interesting times.

Thanks @alancalverd - really fascinating post; I'd never thought about any of that properly. Especially the stuff about the constraints on propeller speeds...

[teragram (OP)]

Everything about an aeroplane is a compromise between strength and weight, lift and drag, cruise speed versus landing speed....and hybrids offer a few more variables for the designer to play with.

Propellors are a good compromise for slow flight, low altitude, and short sectors, and for very short trips (island hopping, air ambulance..) it turns out that piston engines are more economical in terms of maintenance than turbines. Problem is that you don't want the propellor tips to exceed the speed of sound, so you either need a gearbox (dead weight) or a diesel engine (heavier than gasoline, and needs liquid cooling, but is efficient at a lower speed than a gasoline engine of the same capacity) or a smaller propeller (less efficient because the hub doesn't generate as much thrust as the tip)....or run a combustion engine at its optimum speed and use an electric drive train, as in many railway engines.

One  interesting hybrid uses several small props distributed along the wing. The accelerated airflow improves lift and control at low speeds, and the small electric motors produce less profile drag than two large piston engines: the prime mover is a big diesel in the fuselage, where weight and shape is less of a problem.

Single-engine hybrids can use a battery for takeoff and climb, at say 200 horsepower short term, with cruise power supplied by a somewhat smaller (100 hp continuous) engine than you would need to get off the ground safely. This system is particularly interesting for noise-sensitive missions.

Or you can go the other way: use a combustion engine to climb above the clouds, then solar panels for cruising.

Meanwhile, diesels are getting lighter and gasoline engines are being developed with lightweight gearboxes and improved takeoff power ratings. We live in interesting times.

Thanks for your comments, I bow to your superior knowledge.
Interesting that diesel engines are no longer a complete no-no for aircraft, as you say accommodating them in the aircraft fuselage is a distinct advantage. Also, a diesel is more efficient, plus its fuel contains more energy (although is heavier) than petrol or kerosene.

[alancalverd]

I think that the amount of energy that can be stored in liquid fuel will always be greater than the amount that can be stored in a battery of the same weight so I see no prospect of long range aircraft running on battery power.

The difference between energy and power explains the present state of electric and hybrid cars. You can deliver a huge amount of power for a short time from a battery, so the acceleration from rest (or takeoff) can be impressive, but range is limited thereafter by the energy density of the system. The calculation for a small aeroplane is relatively straightforward as, unlike a car, cruise power is around 75% of takeoff and climb power, so you might begin with a 100 kW gasoline engine and add a 20 MJ battery (100V 55Ah) and a 30 kW (peak) electric motor to give you 10 minutes' climb to 10,000 ft. The question is whether this weighs less than a 130 kW engine, and the answer is surprisingly close to "yes" if you can charge the battery on the ground, because the extra deadweight is offset by the extra liquid fuel you don't need for takeoff.

[alancalverd]

Interesting that diesel engines are no longer a complete no-no for aircraft

"Pure aviation" diesels were used in airships and some early transport aeroplanes but were overtaken by gasoline and largely forgotten since the 1940s. Interest was rekindled by (a) the development of mass-produced small diesels for cars and (b) the fact that diesels run on JETA (kerosene) which is a lot cheaper, safer, widely available and more "robust" than AVGAS, which has a very tight specification.

Unfortunately government idiocy is now hampering the development of road diesels and the aviation market is too small to respond by itself.

[alancalverd]

Interesting that diesel engines are no longer a complete no-no for aircraft

"Pure aviation" diesels were used in airships and some early transport aeroplanes but were overtaken by gasoline and largely forgotten since the 1940s. Interest was rekindled by (a) the development of mass-produced small diesels for cars and (b) the fact that diesels run on JETA (kerosene) which is a lot cheaper, safer, widely available and more "robust" than AVGAS, which has a very tight specification.

Unfortunately government idiocy is now hampering the development of road diesels and the aviation market is struggling to respond by itself.

[wolfekeeper]

Road diesels need to die, now, they're simply too polluting.

[alancalverd]

Depends which politician you believe. I prefer to look at the data.

[Professor Mega-Mind]

The A. Calvert writer is correct .  Best efficiency would be the lowest fuel & maintenance costs .  Fuel costs fuel to lift , so do batteries & electric motors .  Electricity from the grid costs .  Coal and batteries for electric-propulsion pollute , plus expensive batteries die quickly .  The most efficient drivetrains overall are still fuel-burners , but they emit more local noise & air pollution .  An extremely light hybrid system might reduce these , but at high cost , and loss of practicality . 
Fast hybrid airships might work , but aren't a going concern yet .  Near-term , aircraft will burn fuel .  They will do it ever cleaner & quieter , ever more efficiently , and perhaps with alternative  fuels , but they will do the do .
Alright , gotta fly !     P.M. 
 

[evan_au]

I see that NASA is looking at a test light aircraft with many propellers.

There are some interesting comparisons in the following article about the advantages & disadvantages of electric motors - for example, the high power to weight ratio of an electric motor means you can put the propeller out on the wingtips (where it can counteract wingtip vortices), which is not mechanically practical for a petrol or diesel engine (too much weight far out on the wing).
https://spectrum.ieee.org/aerospace/aviation/nasa-remakes-the-airplane-wing-with-electric-propulsion-and-a-whole-lot-of-propellers

[Professor Mega-Mind]

To Even A.A.  ,
There be more than electricity outside the box !  For example ; hi-pressure turbine exhaust ducted to free-turbine mechanisms .  A good example is helicopters , another many turboprops .  In theory you could mount such on the tips of the F-35s wings , using the roll post  air tubes for pressure air .  Bet it would weigh less than electric too .  Cuts out the middle-man , don't it ?  OK , time to invest in pneumatic planes , yeehaw !        P.M.