the main UL phactor=Power/ Weight...Hybridising aircraft is adding Complexity(ICE) to Simplicity(E). C+E=2C
Looking at it differently. If you want electric (because the heart wants what the heart wants) and battery energy density sucks, then hybrid.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"
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.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...
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.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.
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.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.
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.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'.
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.