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Jan 20, 2020, 02:59 PM
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Ran D. St. Clair's Avatar
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Originally Posted by OwlCity
The ball will roll when battery energy density reaches 500 Wh/kg .
I am not sure if 500 is the magic number, but if I remember correctly, 300 Wh/kg cells are physically available now, but rare and expensive, meaning not ready for high volume use. Certainly energy density is the key to unlock this puzzle.
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Jan 21, 2020, 01:42 PM
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Ran D. St. Clair's Avatar
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Originally Posted by PlumbBob
Hello Owl, you're probably right about that. I'm curious, but perhaps you might know what that number is for the new light weight, high kW electric motors with turbine powered generators?
I have been told by someone I trust who has experience with these things... A turbine generator is about the same weight as current batteries for a given power output. Most of the weight is in the generator. After that the weight of the fuel is almost trivial, so the turbine generator has much more range, but not more power for a given weight.
Jan 21, 2020, 04:33 PM
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Originally Posted by Ran D. St. Clair
I have been told by someone I trust who has experience with these things... A turbine generator is about the same weight as current batteries for a given power output. Most of the weight is in the generator. After that the weight of the fuel is almost trivial, so the turbine generator has much more range, but not more power for a given weight.
Having an almost impossibly heavy gas powered generator sitting on my back porch, I suspected the generator itself might be a problem. Perhaps it would be worthwhile to work on light weight high output generators.
Jan 21, 2020, 06:02 PM
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Ran D. St. Clair's Avatar
Quote:
Originally Posted by PlumbBob
Having an almost impossibly heavy gas powered generator sitting on my back porch, I suspected the generator itself might be a problem. Perhaps it would be worthwhile to work on light weight high output generators.
I gather some people are working in that area, and there is some hope, as the higher the RPM and frequency, the smaller the magnetic components can be, but there are still issues with power-heat dissipation. That comes down to a need for extremely high efficiencies, which requires something like high temperature superconductors.

Also, airplanes don't benefit from "hybrid" designs like cars do. With cars there are a lot of short bursts of power required combined with opportunities for regenerative breaking. Airplanes are more like ships. They need steady power for a long time and a lot of it. They don't need much regenerative breaking as they can get most of their potential energy (energy related to altitude) back by doing a gradual descent at low power into their destination. When you combine all that with the efficiency losses associated with energy conversion, it's hard to make a hybrid system that makes sense.
Jan 21, 2020, 10:48 PM
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This is off the topic of air taxis, but if the goal is to build a highly maneuverable light weight and fast propeller driven VTOL military aircraft, would not a single electrical power source centrally mounted be more effective than wing mounted fuel engines? And isn't there also the possibility of regeneration in prop driven aircraft during descent?
Jan 22, 2020, 02:03 AM
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If we're talking VTOL here (and I presume we are...) then lift power is substantially more than cruise power required for on-the-wing flight.

Also, regeneration during descent doesn't make much sense unless you have very limited flight path options. It's much more efficient to use your potential energy in a glide than to try to push it back into a chemical battery via the propellers and motors.
Jan 22, 2020, 01:33 PM
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Ran D. St. Clair's Avatar
[QUOTE=would not a single electrical power source centrally mounted be more effective than wing mounted fuel engines? [/QUOTE]

I doubt it, although the word "effective" is a bit vague. Turboprop engines are relatively light and powerful. They create the power and deliver it to the propeller via an efficient gearbox, which is a bit heavy. Centralized electric power generation begins to get interesting if for some reason you need 10 propellers, not just one or two, but there are lots of inefficiencies involved with all the energy conversions required, plus the generator-alternator is heavy. Then again, energy efficiency isn't the biggest issue for a military aircraft except as it relates to range. If you need to link a bunch of propellers together with spinning shafts then it gets ugly. That is where electric power distribution begins to shine.
Jan 22, 2020, 02:55 PM
An itch?. Scratch build.
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From the efficiency point of view, wouldn't a single large rotor, (helicopter type), be better for the vertical part of flight, and then smaller propellers, (for speed), and the wing used for forward flight.

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Jan 22, 2020, 05:07 PM
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Quote:
Originally Posted by Ran D. St. Clair
I doubt it, although the word "effective" is a bit vague. Turboprop engines are relatively light and powerful. They create the power and deliver it to the propeller via an efficient gearbox, which is a bit heavy. Centralized electric power generation begins to get interesting if for some reason you need 10 propellers, not just one or two, but there are lots of inefficiencies involved with all the energy conversions required, plus the generator-alternator is heavy. Then again, energy efficiency isn't the biggest issue for a military aircraft except as it relates to range. If you need to link a bunch of propellers together with spinning shafts then it gets ugly. That is where electric power distribution begins to shine.
Redundancy doesn't seem to be as much of a concern with single seat military aircraft as it is with multi-passenger commercial aircraft, so 4 propellers would probably suffice for the type of craft I'm thinking about. I was thinking one motor per prop, but perhaps one motor per wing driving both the front and rear props would reduce weight, with hover control achieved thru variable pitch propeller mechanisms. A model, though, would need 4 separate motors.
My original thought concerning centralized power, however, was initiated by reading about new high-output electric airplane motors 4 of which are capable of producing over 1000 hp. I don't know what other motors are available, but the relatively new Seimen's 260 kW motor produces 348 hp. The Pratt & Whitney turboprop engine in the Super Turcano produces 1,196 kW (1,604 hp), so we're in the ballpark.
Jan 23, 2020, 11:47 PM
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Ran D. St. Clair's Avatar
Quote:
Originally Posted by PlumbBob
Redundancy doesn't seem to be as much of a concern with single seat military aircraft as it is with multi-passenger commercial aircraft, so 4 propellers would probably suffice for the type of craft I'm thinking about. I was thinking one motor per prop, but perhaps one motor per wing driving both the front and rear props would reduce weight, with hover control achieved thru variable pitch propeller mechanisms. A model, though, would need 4 separate motors.
My original thought concerning centralized power, however, was initiated by reading about new high-output electric airplane motors 4 of which are capable of producing over 1000 hp. I don't know what other motors are available, but the relatively new Seimen's 260 kW motor produces 348 hp. The Pratt & Whitney turboprop engine in the Super Turcano produces 1,196 kW (1,604 hp), so we're in the ballpark.
I doubt the military would consider a 4 engine hovering aircraft using turboprops without cross shaft coupling. If you lose one of the 4 engines while hovering you crash. That makes the aircraft reliability 1/4 of the reliability of the individual engines. You would need at least 6, preferably 8 or 10 engines before the benefits of redundancy begin to kick in.
Jan 24, 2020, 10:28 AM
An itch?. Scratch build.
eflightray's Avatar
Quote:
Originally Posted by eflightray
From the efficiency point of view, wouldn't a single large rotor, (helicopter type), be better for the vertical part of flight, and then smaller propellers, (for speed), and the wing used for forward flight.

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Not military related, but a VTOL something along the lines of this UAV in the video.

The rotor is doing the lift and hover, plus a motor and wings for forward flight, with the rotor stopped.


Hybrid RotorWing aircraft completes first in-flight transition (1 min 4 sec)



Just another 'different' example.

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Jan 24, 2020, 11:13 AM
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Ran D. St. Clair's Avatar
Quote:
Originally Posted by eflightray
Not military related, but a VTOL something along the lines of this UAV in the video.

The rotor is doing the lift and hover, plus a motor and wings for forward flight, with the rotor stopped.


https://www.youtube.com/watch?v=yYvCuY3269s


Just another 'different' example.

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Wow! I give them a lot of credit for making a difficult concept work.

There is a lot of stuff going on in that video. It is only showing the transition from forward flight to hover. There may be other videos showing the transition from hover to forward flight, i don't know.

First they are flying in forward flight.
Then they slow down and stall the aircraft so it is falling almost straight down, but fuselage level.
Then they spin up the rotor, but when doing so they can't hold yaw, so it rotates about 90 degrees.
They never showed it in a hover, but once the rotor is spinning the hard part is done.

Obviously this is not a technique that could be applied to passenger aircraft. The falling straight down part would be far too scary.

I am guessing at the inbound transition sequence, but it probably goes like this.

Climb to a substantial altitude on the rotor.
Drop almost straight down while the rotor is stopped
Once the rotor is stopped, drop the nose a bit and push into forward flight.

Both outbound and inbound probably lose a fair amount of altitude in the process, but it could be worth it if they spend a lot of time either hovering or flying on the wing, with few transitions. It would be challenging to make it very low drag for a good L/D in forward flight. There are just so many practical problems to deal with for this concept.
Jan 24, 2020, 11:35 AM
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Ran D. St. Clair's Avatar
Quote:
Originally Posted by eflightray
From the efficiency point of view, wouldn't a single large rotor, (helicopter type), be better for the vertical part of flight, and then smaller propellers, (for speed), and the wing used for forward flight.

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I have done some testing comparing the efficiency (technically it is the FOM or Figure of Merit) of a single rotor vs. multiple smaller rotors. The single large rotor does win, but only by a small margin, and the margin gets very small as the multiple rotors are spaced farther apart. The bigger and more important issue is disk loading, or for a given weight, disk area. If the single large rotor and the multiple smaller rotors have the same disk area then their FOM is nearly the same.

Meanwhile, there are many advantages to multiple small rotors including redundancy for safety and reliability, and much lower torque which leads to smaller direct drive motors with no need for a heavy gear box which is a single point of failure. Small rotors can also be much more easily stopped horizontally, or feathered and stopped vertically for low drag. If they can be feathered then they are almost certainly variable pitch (collective pitch only) but with an array of them that amounts to the equivalent of cyclic pitch in a single larger rotor, but without the swash plate and all that. Multiple smaller rotors are also much easier to package in many possible configurations, and we have seen several. The single large rotor really needs to be treated as a rotor, not a big propeller. That means it needs lead lang hinges, flapping hinges, or at least some degree of compliance. Small rotors are just propellers, with either fixed pitch or variable pitch, and the forces on them are considerably less.

All that being said, single large rotor concepts are not out of the running. A conventional helicopter, possibly electric, is a viable candidate for some missions, mostly short range and low speed. The VTOL concepts do better at longer ranges because their L/D is so much better. The poor L/D for a helicopter is less of an issue when they are liquid fuel powered, but it really hurts range with electric. Some of the Autogiro concepts like the Carter Copter look technically promising (to me) but many people think they are a non-starter due to the fact that they can't really hover. They are more a form of STOL, which I have always said is a fine solution in many cases. Where this all ends up may have less to do with the technical issues and more to do with who gets funding and establishes the standard. That is why we are seeing alliances with companies like Honda and Boeing.
Jan 24, 2020, 11:58 AM
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As is likely obvious to you guys, I am not an engineer and hope my enthusiasm, which I shall try to restrain, for the subject compensates for my ignorance of it. Thanks.


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