Hybrid or electric theory?

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Sockmonkey

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I've said this before but it bears repeating.
It's much easier to make hybriding work in ground vehicles than planes because the electric drivetrain is replacing the transmission, clutch/torque converter, and driveshaft. Those things are all heavy, complex, and cause efficiency losses themselves. A series hybrid car can actually have a lower parts count than a standard one.
The bigger the vehicle, the bigger the advantages of hybriding because the mass of your gearbox goes up fast due to having to handle the forces needed to move that big vehicle.
With most planes the prop is right on the crank, and a prop with adjustable pitch does the job that the gearbox does in cars.
For a plane you have to design the airframe from scratch to exploit the advantages of using a hybrid system for it to be worth it.
 

ElectricFlyer

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I know many of you have been waiting for me to sound in on this 🤣
For me - Hybridising aircraft is adding Complexity(ICE) to Simplicity(E). C+E=2C
Ok - I am not a math person😜.
But I wholly believe one is better off without the other when it comes to UL aircraft for most cases. Go ICE if range is your thing. You just wanna go out a few mornings/evenings a week for an 45min then E is the ticket.
Exception to every rule!
The example in the vid was interesting - "use E to take off and ICE to cruise". To me, why put in the Epower for takeoff to complicate things with extra weight from the batteries for "take off power"? Epower for emergency back up cruise maybe if the ICE fails - thus if you dont have the height if the ICE fails to do a turn around back to airstrip or to get to that field you just did not have enough glide to make! Which makes me wonder on the stats for UL crashes -- more due to a control failure or power failure?
Lots if interesting stuff happening non the less in the aviation world.
Anyway - like Kuiil would say - "I have spoken" 😁
Cheers
 

henryk

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Hybridising aircraft is adding Complexity(ICE) to Simplicity(E). C+E=2C
the main UL phactor=Power/ Weight...

circa 20 kW (circa 30 HP) ICE =10 kg,
+ 20 kW STARTER/GENERATOR (5 kg),
+20 (40 max) kW high RPM E-motor (5 kg) + CR Diff.Gear (5 kg with 2 CR Propellers)

+10 kg ( 2 kWh ACCUs)=

=circa 35 kg...+ PETROL !
 

blane.c

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capital district NY
I know many of you have been waiting for me to sound in on this 🤣
For me - Hybridising aircraft is adding Complexity(ICE) to Simplicity(E). C+E=2C
Ok - I am not a math person😜.
But I wholly believe one is better off without the other when it comes to UL aircraft for most cases. Go ICE if range is your thing. You just wanna go out a few mornings/evenings a week for an 45min then E is the ticket.
Exception to every rule!
The example in the vid was interesting - "use E to take off and ICE to cruise". To me, why put in the Epower for takeoff to complicate things with extra weight from the batteries for "take off power"? Epower for emergency back up cruise maybe if the ICE fails - thus if you dont have the height if the ICE fails to do a turn around back to airstrip or to get to that field you just did not have enough glide to make! Which makes me wonder on the stats for UL crashes -- more due to a control failure or power failure?
Lots if interesting stuff happening non the less in the aviation world.
Anyway - like Kuiil would say - "I have spoken" 😁
Cheers
Looking at it differently. If you want electric (because the heart wants what the heart wants) and battery energy density sucks, then hybrid.
 

tspear

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I started a thread a long time ago on series hybrid. Back then, the answer was pretty much the same. For aviation purposes, you will lose efficiency with current aerodynamic choices made. Potentially a change in thrust layout, such as many small props, may change the calculation.

Flip the problem on its head. With the reliability of electric motors, and the life of electric motors, the ability to delivery max power at pretty much any RPM. They are ideal for converting energy to thrust. You can likely eliminate constant speed props, you can place the engine/prop where needed for optimal aerodynamics, structural reasons, or other efficiency requirements. You may also eliminate the running flammable fluid all of the plane. With our planes lasting decades, we have an interest in starting to make these changes and gain as many of the advantages as we can.

With our planes lasting decades, the earlier we we can make the jump the better off we likely will be.

However, the using just batteries does not hold sufficient power to make the plane truly useful. Since the plane will last for decades, eventually the battery tech will get there where it can be the sole source. Therefore, from a design standpoint, we would want to make what is effectively the power source as a replaceable component. Next, we would want to look to other higher volume industries when possible, but we then run into a reliability problem (perception might be the bigger issue than the reality). So from my perspective, using a cheap commercial genset with enough battery power for roughly thirty minutes is likely the optimal solution. By using mass produced genset, you lower costs, by having 30 or more minutes of battery as a backup to the genset, you can make the argument to the FAA genset does not need to meet current PMA or design requirements.

Tim
 

rv7charlie

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While the idea is attractive, 'cheap genset' means 'really heavy' genset. My 12KW not-cheap home backup power genset uses a Generac 992cc Vtwin that's typically rated at around 32 HP continuous, but the genset is only yielding about 16 HP worth of electricity (they derate the engines a lot, for reliability). It weighs 470 lbs; roughly the same as a 260 HP a/c engine.

The next issue is that aircraft are *always* designed around the powerplant (if they're successful). If you design an airframe with 'distributed propulsion' using electric motors, and design into it a 'generator/fuel bay' with current tech, then the idea could work, for particular missions. As storage tech improves, the mission list could likely expand a bit.
 

Dan Thomas

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Flip the problem on its head. With the reliability of electric motors, and the life of electric motors, the ability to delivery max power at pretty much any RPM. They are ideal for converting energy to thrust. You can likely eliminate constant speed props...
I've mentioned before that eliminating constant-speed props isn't an option for any airplane where a wide speed range is desired. Propellers, whether they're driven by an ICE or an electric motor, have a narrow useful range of RPMs. For example, run your 172's engine at idle (650 RPM) and it probably won't even taxi. Double that to 1300 and it will taxi really fast. Add only 1000 RPM to get 2300 and it takes off and climbs. But try to increase that RPM to, say, 3300, by repitching that 76" prop, and you don't gain anything since the tips are now supersonic and the engine's power is consumed mostly by drag. If we use a smaller prop to get the tip speeds down, we lose propeller efficiency and takeoff and climb suffer.

The angle of attack (AoA) of a propeller's blades decrease as forward speed increases. As the AoA gets down to around 2°, the lift falls of so that forward speed stops increasing. If our propeller is near it's max useful RPM, as it will be, the only way to get more speed is to take that fixed-pitch prop off and repitch it to a higher pitch, but now we lose takeoff and climb performance since the engine can't rev up to its max output HP. Or we can install the variable-pitch prop, normally seen as a governor-controlled constant-speed prop, and vary the blade angle to whatever AoA works best in any flight regime.

A fixed-pitch prop is a bit like a car that has only one gear, with the ratio midway between low gear and high gear in the normal transmission. Cheaper to build, but poor acceleration and poor cruise speed. A lousy compromise. Air's low viscosity allows the prop to get away with it, but performance is still severely limited.

As far as the electric motor delivering max power at any RPM? Nope. Power (horsepower) is a function of torque times RPM. Low RPM cannot deliver max power, just max torque, and HP will be low. Physics and math are really destructive to wishful thinking.
 
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Dusan

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As far as the electric motor delivering max power at any RPM? Nope. Power (horsepower) is a function of torque times RPM. Low RPM cannot deliver max power, just max torque, and HP will be low.
There is a range of RPM electric motors can deliver constant power, good motor designs can get close to max power. By using different controls algorithm you get 'field weakening' enabling the electric motor to increase speed by reducing torque. The motor can be designed for low RPM, close to max power and torque, and by control techniques it's able to provide constant power on a large speed range by reducing torque. Power=torque x RPM still holds. This is why electric cars don't need gearboxes. Same reasoning for diesel electric locomotives. I think for diesel electric locomotives (and large ships) 'hybrid' is a misnomer, as only the transmission is electric. As much as counter-intuitive as it sounds, it's much cheaper and lighter to have a generator, controls, wires and electric motors to drive the wheels (propellers), a mechanical gearbox and clutch would be of gargantuan proportions and not even fit.

We've heard this before, that a variable pitch propeller acts as a variable gearbox, this is only partially true. For cruise you need a smaller prop, high pitch. For takeoff, climb and acceleration you need a larger diameter prop, lower pitch. This is because thrust is proportional to air mass flow rate through the propeller and at slow speed you need a larger diameter to compensate for the slow speed driving a lower mass flow. The variable pitch is better than a fixed pitch, as pitch is variable, but the variable pitch prop is still a compromise, as it's diameter is fixed. A fixed pitch prop can be used efficiently only if the speed range of the aircraft is low, the cruise speed low enough so the prop performs acceptable at cruise without too much of performance penalty for takeoff and climb.

All this is regardless of what is turning the prop, an ICE or electric motor. Sure, the piston ICE also delivers it's max power in a limited RPM range, and here the electric motor may outperform it, but the performance gain is too small to envision a hybrid only for this.

Actually hybrid does not make sense for aviation same as for cars. The ICE has a large performance penalty when operated at light loads, for cars this is the norm most of the time, close to max power only while hard accelerating. Combined with the need to decelerate and stop often, it makes sense not to waste that kinetic energy and operate the engine close to max efficiency using a hybrid system. ICE for aviation are almost always used close to full power, and the opportunity to extract kinetic energy is small.
 

rv7charlie

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We've heard this before, that a variable pitch propeller acts as a variable gearbox, this is only partially true. For cruise you need a smaller prop, high pitch. For takeoff, climb and acceleration you need a larger diameter prop, lower pitch. This is because thrust is proportional to air mass flow rate through the propeller and at slow speed you need a larger diameter to compensate for the slow speed driving a lower mass flow.
This is true, but given the practical limits in prop diameter for any a/c designs and their landing gear (that we'd be considering here), the reality is that the largest diameter prop you can safely fit is the right one, whether it's fixed or variable pitch.

I'm sure that the engineers can show you the math, but practical, empirical evidence shows that the biggest prop you can fit to a typical 2-4 place plane will lose almost nothing in efficiency up to around 200 mph compared to any smaller diameter prop fitted to the same airframe & engine. The difference in efficiency on the low end, on the other hand, can be quite dramatic. Just going from 68" to 72" on an RV-x makes a very noticeable difference in takeoff/climb, with no penalty in cruise. Going to 76" (only possible to do safely on the -7 & -8) makes an even bigger difference. I once owned an RV4 that had a 72" dia wood prop. It had awesome takeoff performance, and easily made Van's 'book' numbers for cruise and top speed; something that numerous owner-built versions don't achieve. It's worth noting that the 'traditional' diameter for fixed pitch props on RVs has been 68". But Hartzell C/S props with blades set up for RV speeds are available now, from Van's, in 74" diameter.

'Distributed propulsion' with multiple props does have the potential to alter the above 'rule of thumb'.
 

patrickrio

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This is true, but given the practical limits in prop diameter for any a/c designs and their landing gear (that we'd be considering here), the reality is that the largest diameter prop you can safely fit is the right one, whether it's fixed or variable pitch.

I'm sure that the engineers can show you the math, but practical, empirical evidence shows that the biggest prop you can fit to a typical 2-4 place plane will lose almost nothing in efficiency up to around 200 mph compared to any smaller diameter prop fitted to the same airframe & engine. The difference in efficiency on the low end, on the other hand, can be quite dramatic. Just going from 68" to 72" on an RV-x makes a very noticeable difference in takeoff/climb, with no penalty in cruise. Going to 76" (only possible to do safely on the -7 & -8) makes an even bigger difference. I once owned an RV4 that had a 72" dia wood prop. It had awesome takeoff performance, and easily made Van's 'book' numbers for cruise and top speed; something that numerous owner-built versions don't achieve. It's worth noting that the 'traditional' diameter for fixed pitch props on RVs has been 68". But Hartzell C/S props with blades set up for RV speeds are available now, from Van's, in 74" diameter.

'Distributed propulsion' with multiple props does have the potential to alter the above 'rule of thumb'.
I would be very interested in seeing a graph showing maximum efficiency propeller diameter plotted against air speed. I assume that the most efficient diameter must get huge at airspeeds around 25mph/40kph.... look at the diameter of modern windmills. I am guessing that such a plot is VERY hard to do though, as rpm at max efficiency must also change for different speeds. Air pressure is surely an issue too. Such a complicated calculation but it would be really nice to better understand all the various maximization criteria and how they interrelate.
 

rv7charlie

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The genius Vance Jaqua had some interesting data related to that on his website prior to his death back in 2006, but unfortunately, all the wonderful info on his site is no longer available. One graph that he had showed that a significantly smaller (meaning higher rpm, direct drive engine like a V6/V8) *variable pitch* 3 blade prop could *approach* the low speed thrust of a 2 blade fixed pitch prop designed for the 'standard' 2700 rpm a/c engine. But notice that it compares apples & kumquats; if you simply went to a controllable 2 blade 2700 rpm prop, you're back to the larger diameter winning.

While coming up with an all-defining graph would be pretty tough, you can at least get a feel for what works best by looking at what has come before. The really fast piston powered a/c all use relatively large diameter props. The Questair Venture specs a 68" dia prop (likely limited by short gear legs) @ 2500 rpm, for a 250 kt (287 mph) top speed. The prop that Hartzell specs for the Glassair III is 80" in diameter, and the Lyc that drives it is a 2700 rpm engine. Since this is almost 50% faster than anything most of us will own/build (except maybe Toobuilder; about 30% for him), it seems safe to say, use the largest diameter you can safely fit on the airframe.
 

tspear

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@Dan Thomas ,

I am NOT an engineer. I am a computer geek :)
I have read, and @Dusan explained it better than I can about constant power electric motors. They are the ones I am used to dealing with when I was programing building control systems, and some proof of concept electric vehicle designs.
In terms of the prop, the effective speed range of most GA aircraft is much smaller than you alluded too. Think about in a SR22; your rotation is 70 KIAS, climb is normally 110-120, cruise is roughly 150 KIAS. Blade angle below 70 KIAS is only about helping the engine deliver max power.
So the speed only doubles, and maybe my thinking is incorrect, but I know from the VAV systems I used to control in buildings, we used pretty linear speed controls on the engines to control airflow speeds. And we worked a much wider range than doubling.

Tim
 

rv7charlie

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I thought an SR22 was supposed to be a high performance a/c. ;-) This is a different world. A typical RV stalls at less than 55 mph and can cruise over 200.

But one big reason for controllable props on higher performance a/c is to get max available power (full rpm) for climb. Climb rate is based on 'excess power', or how much power is available in excess of that required for level flight, and how much weight must be lifted. A clean, relatively light weight a/c like an RV may only need 60-80 HP to maintain level flight, so even with a highly pitched cruise prop, there's plenty of excess power available even with rpm limited to around 2300-2400 (<75%) on climbout. Apply the same HP to a relatively heavy 4 seat Cessna or similar that might need 100+ HP to maintain level flight, and that 'extra' 25% of total HP might move from the 'fun' category to 'required'. The variable pitch prop (pitched for climb) doesn't just get more efficient at lower speeds, it allows the engine to develop full power.

Prop loads are extremely non-linear with rpm; basically an exponential load curve.
 

Dan Thomas

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@Dan Thomas ,
Blade angle below 70 KIAS is only about helping the engine deliver max power.
So the speed only doubles, and maybe my thinking is incorrect, but I know from the VAV systems I used to control in
To get max power at or below 70 kts, the blade pitch has to be low enough that the engine can spin up to redline RPM. A fixed-pitch prop can't do that. If we make a fixed-pitch prop with a low enough pitch to achieve that, we end up going over redline as soon as we start levelling off and have to reduce the power, and our max speed ends up being really low, maybe only 85 or 90 kts because of that.

From a textbook on the subject (I lost the link):

The blade angle is also an excellent method of adjusting the AOA of the propeller. On constant-speed propellers, the blade angle must be adjusted to provide the most efficient AOA at all engine and airplane speeds. Lift versus drag curves, which are drawn for propellers as well as wings, indicate that the most efficient AOA is a small one varying from 2° to 4° positive. The actual blade angle necessary to maintain this small AOA varies with the forward speed of the airplane. This is due to a change in the relative wind direction, which varies with aircraft speed.
 
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