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Old Mar 26, 2012, 04:07 PM
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turbine generator to power electric motors

hi ,
since i drove in a lexus today, i thought of a hybrid plan / hybrid helicopter... is there a way to use a turbine , maybe a turboprop turbine or a helicopter turbine to get electricity to puwer an electric motor ?

in non model applications one uses a diesel engine to do that but this might be a little heavy , so does anyone have any idea how to do that so i can get arount 8kw of power for about 20- 30 minutes ?

i would gladly appreciate any suggestions

cheers
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Old Mar 27, 2012, 01:06 AM
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Hybrid cars gain their efficiency by generating electricity while braking (energy usually wasted by converting it to heat) and consuming little energy while stopped (no engine idling), neither which would apply to a plane or helicopter.

Model turbines are notoriously inefficient when it comes to performance versus fuel consumption.

The most efficient way to get 8kw for 20-30 minutes would be with a large, noisy, high vibration gas engine.

You do not mention what you want to achieve. A large electric motor & large lipo batteries may do the job. - John.
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Old Mar 27, 2012, 09:37 AM
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Yep.... what he said.

RC turbines are the dirtiest, lowest efficacy turbines produced. My Falcon uses a 64 oz fuel tank (yes, 1/2 gallon) and I get about a 5.5 minute flight on it. If I would have mounted a 120 glow engine on it... I could fly for the same time on about 8 oz of fuel. (it just wouldn't be as cool)

Remember, every time you change power, you loose.

So... burning fuel in a "Trust" engine, to turn a power wheel. = loss

Turing a gear box = loss

turning the generator = loss

regulating the electricity = loss


So... to get 8kw if electricity would require (as a guess) 30 kw of fuel.



Now... going back to your diesel electric comment. Back in the day, the train CO's tried turbines... but they all wound up using Diesels. (Way better efficacy, but heavy) Also, the diesel electric works well since they are trying to move so much weigh. The electric motors can run at low RPM's, where if it was direct... they would roast a clutch.


As an FYI... Jag is trying a turbine for battery charging, and it's not truly a "Hybrid". But... I don't even think they got their engine to run, since they are trying a small, multi-stage compressor.
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Old Mar 27, 2012, 03:17 PM
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i thought about a turbine because of the jag project ^^
what about an engine like this one ? http://www.der-schweighofer.at/artik...r_und_zuendung

it has 12kw

the main thing is , how do i get electricity from my fuel engine , can i use a normal brushless motor ? are there any how tos ?
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Old Sep 15, 2012, 07:04 AM
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I'd look at automotive alternator circuitry as a primer.
Brushless motors are similar, and will require diode bridges and so forth.
The whole business is likely to create a fair amount of electrical "noise",
due to the rpm and switching speeds.
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Old Sep 19, 2012, 01:47 PM
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the issue is scale sizes

Gasoline engine are most optimum fuel effientcy when the piston chamber is betwen 250cc and 350cc explaining why 1000cc engines are often three cyclinders and 1400 engines are 4 cylinders

Less than 600BHP piston engines are very effient but over 600BHP steam and turbines start to become more efiencent
Below 10 BHP steam starts to become more efficent and turbine becomes awful bad effecientcy

Hybrids below 100 BHP tend to less effecient


Ship engines 25,000 BHP for tankers using big pistons and Bunker oil are very effeffecient at 50% effecientcy rates

Electric power gas turbine genrators with local hot water in Holland has acchieved peak 87% effecientcy

The only reason the electric tends to work well below 20cc engines is the bad effecientcy of the small engines and the mismatched props due to high prop RPM issues

I did look the numbers one time of idea to use combination of High RPM 15cc DF 3.5BHP 24,000 RPM engine to drive generator that would drive several electric EDF motors and also the same 15cc engine to make light weight genertor for recharging feild batteries
For EDF use it was marginal at best that time as the Lipos were in that time were not as good as today and so since then new lipos make that idea less interesting
As a generator prices of 750 watt genertors have dropped so much to like $100 a pop would just as cheap to buy several and leave it on sites I need it on .

The problem to get 3.5BHP from a 15cc engine (700 watt genertor need about 3bhp and 50cc gasoline )is that the fuel you need to use is high power fuels like 20% nitro fuels which has a high oxegen content.The weight saving from engine is lost in genertor project as in one hour of that fuel will be a 6 quarts 5000cc not so light as the gasoline genetor that would usea quart for the same time period 1000cc .Also the high nitro is costing like $30 an hour versus the gasoline version less than $2 an hour


What exact project you have in mind exactly 12kw is big number

My info is that you can use a Brushkless engine to generate electic power but its probably much easier with old style DC engine as I dont know how exactly one would tranfer the power to the buffer battery .Using buffer battery that can run the engines for short periods like less than one minute makes the electronics more easy to do . The DC generator send power to the DC battery liFo type .The electric engine take the power from the battery . Everything would be max limits with short flights less than 10 minutes better idea

The only possible real benifit that might come from this combo is gasoline engine fails there is enough juice with one minute battery power to get back down.
For hovering planes like VTOL that could save planes
On this power scale 12Kw 15bhp you only contender is gasoline motor 80cc min with tuned pipe going flat out something putting out close to 15kw or 20BHP (might require converting to run on methanol to get the extra 20% extra power that methanol can give )

Finding a small generator DC 15 to 18KW dc motor that can supply 12kw power with the losses isnt easy . Last one I saw was big ish nearly as big as 50cc engine

Alternitivly there is some 3 phase brushless motors that give consume 20KW and and can probably produce 15 KW in generaton mode that will be lighter than DC

However then the problem is what to do with three phase .If your always running full tilt one can attach it I assume to the motor and it might track the generator phase for phase .If not the rough running would probably burn everthing up it real suck and see stuff . Throttling up and down probaly will not work well from Big Back EMF issues as the two sytems have to many differnt requirements

Converting the three phase power to DC to go into battery bufffer supply and then let battery supply the brushless engine with its ESC means the speed of gasoline engine and eletric engine can be differnt

There are some ESC that claim that when the prop aircrat is downlinig the sprinning prop will regenerate the power into battery .I havent seen any that were remotely like the 20KW neeeded but mayby several small ones ganged in parrell might work . Its possible that many ESC can do this but they dont advertise this ability as for most modeling it not really needed . A few emails to 20KW ESC maker could sort that

The only packs that I know that can take this charging discharging rate flow without getting blown up would be the 2200 Dewalt drill types Life types and their weight ratios are not so good .The newer Lipos have got to 8C rates recharging last I looked which might be enough not sure again

My suspission is that its probably gonna turn out that a larger gasoline engine with bigger AC generator such as 80Kw will work better driving 4 of these 12KW EDF fans due to the better effiency of larger gasoline engines of probabaly 2 cyclinder 400 cc engine with tuned pipes

If its done to save on battery costs to make 12KW I dont think it will pan out .If its done to exteend flight time of 12KW fan it might help a bit .However the next years batch of new more power density batteries will probably match the combination fpr flight duration

Type of model and flight requrement speed VTOL STOL FPV filming scale sports economics size EDF fan specs and type all help to figure the numbers

Ralf
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Old Sep 19, 2012, 03:34 PM
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Hi Ralf
this was just an idea to get better flight times with a multicopter. i saw one model which had an extra engine which created lift and 4 electric motors to do the steering and stabilization.

do you have any idea which escs regenerate power ? i definitely need to change escs in my twinstar since there are two different escs in at the moment^^

cheers
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Old Sep 22, 2012, 09:15 PM
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Regeneration isn't free - you'll be slowing the airplane down by using a windmilling prop to generate current. Unless you manage to get that plane to ride thermals or ridge lift, you'll get better times out of a folding or feathered prop than you will by trying to regenerate power.

If you want to loft a combustion engine, your best bet would be to design and build a gear/belt system that turns a set of variable pitch rotors. Think along the lines of helicopter tail drive systems, although you'd have to scale up a bit since tail rotors wouldn't give much lift.
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Old Sep 23, 2012, 03:56 PM
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Originally Posted by M4d_D View Post
Hi Ralf
this was just an idea to get better flight times with a multicopter. snip
As a rough rule fuel energy density with liquid fuels will alway s be better than electric on the larger scale
On the smaller scale because the fuel engines have so bad effiency there can situations where electric can be better than liguid fuel not because electric is so good but the small IC engines are just so bad

In all hover flight like helicopters they can extend the flying time with forward fligth of about 20MPH .The rotors act like wings .A model helicopter with flying time of 20 minutes in forward flight will have probably only have got a flying time of 8 minutes if the helicopter is is kept in hover mode. The helicopter requures full power to hover but only 30% power to maintain slow forward flight .If the helicopter wants to do fast flight then the flying time would drop again to eight minutes as it requires full power to fly fast .
Multi copters on average have bad flying times as they often have props and not rotor blades so they get no power reduction benifits to fly in forward flight

Any methanol or nitro engine will on average also be very bad for fuel.However gasoline can be effiencent after 20cc and get more effienct until 250cc

A good 50cc model aircraft gasoline engine supllying the majorty of lift for a Multi copter could work well where some small engines electric only supplied stabilty . Trying to generate power to drive the electric engines would probably be too heavy so simply adding on the batteries for the electric motors would work well I figure

To get effective power from gasoline there are effeincies to think about.There is the propelers effeincy which gets better the larger it is and heli rotos are better again .The gas engines effeincy which is different to the propeller so a geared solution works better .In hover mode the best is always the heli blades solutions .If going fast in flight is required then the heli blades are less effient than EDF solutions .
Too reach a size where one could have the gasoline engine create electricity to drive the electric stabilty rotors would in my guess reguire at least 250CC to carry all the weight of the complex gear . A 50cc gasline engine would already struggle to supply the lift with the extra electric engines with batteries and rotors attached and most of that is due to small scale issues where such small rotors cant grip the air molicules as well as large rotors can .

Ball park for me anyproject with gas engine less than 20CC probably wont work well and cheaper and more easy to stick to electric and carry more battery packs or even look to make umbilical cords

There are many second hand heli with a 50 gasoline engine that can with extended tanks hower long time periods .Not sure how well the video quality is with the vibration though

In steady hover mode hard to see what benifirÁts can come from mixxing gasoline and eletric in hybrids .In model planes with superior carrry cappasity with large wing there might exist some benifits for STOL or Hover landing for short power pulse demands

hope that helps

Ralf
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Old Sep 23, 2012, 04:18 PM
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Ralf:

You're generally right conceptually...but a bit off in the details.

Best translational lift is at approx. 20KIAS, as you suggest.

However:
- A helicopter doesn't need full power to hover, otherwise it wouldn't be able to climb vertically out of ground effect. Given that most helis can climb vertically from sea level to 5 thousand meters or so, this is axiomatically incorrect. The last helicopter I'm aware of that was that limited was the Korean war era EVAC helicopters. With a full load of wounded, they would have to pop up a couple of feet and then start moving to get enough lift to climb any more. On the other hand, a helicopter can fly slow forward at a higher altitude than it can hover (or fly fast forward, as translational lift is gone by 30 KIAS or so).

- multirotors will in fact exhibit a similar benefit from translational lift. This is why they balloon in wind.
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Old Sep 26, 2012, 02:28 AM
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Originally Posted by treehog View Post
In all hover flight like helicopters they can extend the flying time with forward fligth of about 20MPH .The rotors act like wings .A model helicopter with flying time of 20 minutes in forward flight will have probably only have got a flying time of 8 minutes if the helicopter is is kept in hover mode. The helicopter requures full power to hover but only 30% power to maintain slow forward flight .If the helicopter wants to do fast flight then the flying time would drop again to eight minutes as it requires full power to fly fast .
Definitely not true for RC, the current endurance record for RC helis (a couple hours IIRC) was done pretty much entirely in hover.

Power differences (logged) are only really 10-15% IIRC
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Old Sep 26, 2012, 03:35 AM
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Definitely not true for RC, the current endurance record for RC helis (a couple hours IIRC) was done pretty much entirely in hover.

Power differences (logged) are only really 10-15% IIRC
more like 1 hour but it was a flying lipo and it just hovered lol also used 800mm blades and a VERY low head speed
was thread on helifreak a month or so ago
http://www.helifreak.com/showthread.php?t=432329

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Old Sep 30, 2012, 06:23 AM
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I got thinking a bit on what your looking for with multi rotor and endurance hover
For example a three bladed or four bladed rotor unit can also spin the rotors in a circle .
As one commenter earlier said the multi rotors will gain effeincy from forward movements through the air .
If the multi rotor were to stay stationary and using extra large extensions on the arms to rotor heads and the multi rotor were to spin itself the rotors would experience forward movement without going any where .
There might be issues from turbulance as each rotor crossed into the rotor turbulance of previous rotor .It could help sort of suck and see really

I am doing another thread in LTUP on gasoline and deisel electric generators for the idea to come of the electric grid .

http://www.rcgroups.com/forums/showthread.php?t=1742226

Running the numbers the example I used is large 160Kw genertor versus small genertor
The big generator 6000cc approx engine uses at least 5 times less fuel to make each kilowatt than the smaller 100cc gasoline engine making 1000 watts

The numbers for power generation with gasoile engines do not stack up for flight as the commercail units are way too heavy
Even trying to make a unit using modeling parts hard to see how the weight could come down to suitable flying weights .

Each year the Lipos get more capisty and seach year flying times will increase for all lipo driven models

Certianly the Lipo helis have come a long way in few years

Basically that because the small model engines of less than 50CC throw away most of the fuel due to huge innefiencies in the small scale

Thanks guys for the inputs on heli effeintcy made a few minor mistakes that needed clearing up

Hope that all helps
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Old Sep 30, 2012, 01:19 PM
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I got thinking a bit on what your looking for with multi rotor and endurance hover
For example a three bladed or four bladed rotor unit can also spin the rotors in a circle .
As one commenter earlier said the multi rotors will gain effeincy from forward movements through the air .
If the multi rotor were to stay stationary and using extra large extensions on the arms to rotor heads and the multi rotor were to spin itself the rotors would experience forward movement without going any where .
Nope.

The benefit of translational lift isn't the rotor discs acting like a wing, it's keeping the rotor disc in clean air. A hovering helicopter / multirotor tends to reciruclate/churn a lot of the "same air" through the disc. By moving forward the turbulent air is left behind.

Having a multirotor that had its propulsion units orbiting wouldn't help.
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Old Oct 02, 2012, 09:42 PM
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The benefit of translational lift isn't the rotor discs acting like a wing,
A helis rotor disc IS a wing, the blades are airfoils. Multicopters aren't so it would not surprise me if there was minimal efficiency improvement from moving.
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