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  1. #13
    Banned diarmaid's Avatar
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    Default piston/pistons

    Firstly, thankyou for the detailed responses.

    This is at an initial stage (Where I can still decide to ditch it and do something else without major loss!) so there are many ideas swirling around in my head. Initially I was thinking a strap-on system, similar to the trek aero, however if it was using jet engines on a controllable mount at the rear, in theory it would have a lower profile than the trek aero prop system.

    However, it could also be a contained vehicle system which would give the advantages of being larger so allowing more fuel/larger engine etc However this has the obvious disadvantages of weight etc, and essentially you end up building a fixed wing for efficiency, and losing the flexibility of a 'pack' type setup.

    Of course I could build a cross breed type setup of these two types. A 'pack' with small (rotatable possibly) wings and a safety glass enclosure.

    fyffe your info has given me much food for thought and I'll have to keep researching with it in mind. That GTBA site was great by the way.

    epineh, your idea sounds intriguing. Im not sure if Im picturing the same thing as you but if so it certainly sounds plausible, using numerous pistons and rods to change the direction of the power transmission. This is already done in slightly simpler form with certain road vehicle engine layouts. However, Im forseeing a major weight problem as your going to require a heavy metal chamber to get efficient transfer of the jet thrust into the initial piston(s). And then more similar chambers for the remaining pistons, although these could be lighter weight......on my god....so many thoughts about this......I'm going to have to draw it up..........I'll be back!

    PS: The Dublin Institute of Technology owns the rights to any ideas contained in thesis by their contracted students. Sorry epinah!

    Edit: I put the "(s)" in above beside pistons....inventive fires raging!



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    DIT can own the rights... so long as I get the beer

    Might have not have explained properly, 2 combustion chambers (think four rectangular pockets, one in each prop blade times two blades times two props) sealed by the crossover of the prop blades, ignition forcing the rotational motion, once blades past each other, chamber exposed until they meet again... and so on.

    The problem is you need to have an expanding chamber for the gases to do the work, didn't get past that one.

    Never said it would work, just a concept.

    Russell.



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    darmaid,

    Again, interesting idea - I like this stuff, gets you thinking. The real problems, oops sorry Challenge here probably isn't obtaining the lift required but being able to control and direct it in a strap in system. What ever The RocketMan says the thrust direction or aggregate thrust has to pass through the centre of mass. Without that you'll just spin in place. Doing so with a single small jet will be difficult or require surgery.... Hot jet wash might be painfull and noisy too..

    Russels idea is interesting too.. How about using something like a scaled down WWI era gnome rotary where the prop was attached to the crankcase and cylinders? Crankshaft stays fixed. Attach the lifting fan blades to the cylinders....

    Or how about using Four Model helicopter rotor heads/engine assemblies.. they're getting big now, even turbine powered. Chain the collectives together and it will act as one stable rotor disk.

    let us know how it progresses..

    Andrew



  4. #16
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    I see no direct answer Diarmades questions, so here goes:

    1/ No
    2/ No
    3/ No

    1/ The formular is Thrust = Force! One hundred Newtons of thrust is a force of 100 N. It acts in the direction you point the tailpipe.
    2/ The J66 model turbine consumes about 250ml of fuel per minute at rated thrust (70N about 7 KG). If you throttle it to half power, its consumption only reduces to 170ml/minute (efficiency drops steeply). (Reference Schreckling MTB 31).

    The specific consumption of small engines will rarely pass around 3ml/N per min at rated thrust. Turbines scale down extremely poorly. Whereas a GE 90 is approaching the propulsive efficiency of the most efficient diesel aero engines, air refuses to scale down so the efficiency sucks as turbines get smaller. Doesn't really matter that a model engine producing 70N of thrust is chewing through a litre of fuel in three or four minutes flight, but when you try to make a 1000N turbojet and are burning 3L/min (compared to an O-320 lycoming turn a fixed pitch prop in a Cessna 172 burning 0.5 L/Min to produce about 2000N of thrust,) you start to see why small jets are sexy, but impractical.

    The next point is containment. The 0.6g blade on a Kamps J66 exerts around 1250N of centrifugal force on it's disc at 76000RPM (Source Kamps-Modellstrahltriebwerke). That 0.6g mass is exerting the same force as a 127Kg stationary mass! Luckily when the model engines suffer structural failures, the low masses are still containable even in the thin containment rings provided. As you increase the size of turbomachinery, the energy stored in the rotors increases rapidly.

    In certified engines, containment quickly becomes impractical as the structures get thick and heavy fast. The engine manufactures use safe life engineered solutions, which require expensive process control, expensive testing and verification of the engineering models and assumptions, and expensive monitoring of the fleet Temp/speed profiles. When they get it wrong, people die.

    A 1000N (roughly 100KG) thrust engine is not going to lift a 100KG payload vertically. To lift a 100KG payload you need thrust greater than Payload +vehicle weight+ fuel weight + excess thrust to accelerate.

    The Williams FJ33 is in the 5000-7000N thrust range, with a basic dry mass of about 150Kg. It is conceiveable that you could build a vehicle for a short duration flight which could have a 100 KG payload, but it would have to be an extremely light vehicle (too light to give your expensive engine any sort of crashworthiness). Williams grew out of their cruise missile businees. There missile engines have outstanding power to weight, bought at the expense of durability - The high Turbine entry temps consume the turbine, but it is only a one mission design.

    For best propulsive efficiency you want to accelerate the mass airflow required to just faster than the vehicle speed (F = MV^2).

    That is why a 150HP helicopter can easily lift two adults and enough fuel for an hours flight (large rotor is accelerating a very large airmass to a relatively low velocity). Although turbojets are sexy, they are a solution to a difference problem. They really come into their won when the vehicle is going so fast that it gets differcult to accelerate the surrounding airmass any faster with a prop.

    Prop efficiency peaks at about 300 Knots, Turbojets at about 1100 Knots, and Rocket engines at about 4300 knots (source Kermode - Mechanics of flight). The military is willing to accept hideous fuel consumption and plenty of other compromises for the JSF, or Harrier to perform a mission which can otherwise not be achieved. If the propulsion engineers had their way, the mission would have been flown slower with a helicopter

    Regards,
    Mark


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    Banned diarmaid's Avatar
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    Thanks for all the info. I have one more question for now though. 'IF' I could use a small jet engine and manage to convert its thrust into propeller rotary motion is it a viable option, as the engine if geared correctly for the transfer of power would not be pushed to very high limits and would consume a minimum of fuel, and I think there would be an increase in efficiency from the jet output to the propeller thrust? (Although then we're probably better not using the jet to power the props)

    I think using one or more model heli engines may be a viable proposition, but then Im doing nothing new and Im basically copying the aero trek which Im not interested in doing.

    Thanks. L8rs.



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    Banned diarmaid's Avatar
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    FYI: The ALTAIR H400 TURBOFAN engine referenced above in post no. 9 does not yet have a fixed price but anna responded very efficiently to my query and its expected to be around the $50,000US mark. (I was out by a measly 35k ! )



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    Diarmaid-
    Looking to do something like this???
    http://video.google.com/videoplay?do...19303200331106

    NEATman



  8. #20
    Banned diarmaid's Avatar
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    Not at my own computer at the moment and this stupid Apple Mac wont let me see it but will look at the video ASAP. Thanks.



  9. #21
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    Using a mciro gas turbine as a Turboshaft or turbo prop is again very very cool, but extremely inefficient. The power extraction turbine and gear box are reducing your system efficiency. This is balanced against the improved propulsive efficiency of the large diameter helicopter rotor, meaning you can now lift the engine, it's fuel and it's payload vertically. In a model helicopter, the gain in coolness offsets the terrible efficiency of having to land every three minutes to refuel. This is a niche market.

    You really can't get around the fact that turbo machinery doesn't become competitive for flight until you get to continous power of about 400-600HP. By that power output, your recips are getting so heavy, that the mission weight of a the turbine plus fuel is less than the weight of the heavy but efficient recip plus fuel.

    Reliablity, and cheaper, lower quality fuel are further reasons why gas turbines have come to dominate from about this power range and above.

    Below there, you can not (with current materials and process technology) beat a intermittant combustion engine.

    A gas turbine is a continuous combustion device, so as it gets smaller, the combustion chamber has to shrink. The CC volume reduces at the cube of linear dimensions, while the surface area is only reducing at the square. As you ratio of surface area to volume increases with reducing size, your heat losses through conduction increase (minus efficiency) and your skin friction losses increase (minus efficiency). In the turbine and comprossor, your tip clearances reach a practical minimum, but you are using small blades, to the relative tip losses increase as well.

    An intermittant combustion engine is also affected by these effects, but to a lessor degree, as you can reduce the number of cylinders, and still keep the CC volume resonably high. The Cylinder is sealed, so your tip losses aren't an issue, meaning you can keep pressure ratios up even as size reduces. As efficiency remains high even down to very small engines, while the weight drops dramatically, your small recip plus fuel load, can be far lighter than your small gas turbine plus fuel load.

    There was a NASA study in the late 70's which idendified the Wankel as the most promising technology for low power GA aicraft. It's combination of high power to mass, high power to frontal area, soft failure modes, and low vibration are all favourable for A/C. It is a shame that the bottom fell out for the GA market just as Curtiss Wright etc were prototyping new wankels.

    The power level where gas turbines become the most attractive solution will creep down as breakthroughs occur in blade coatings and small turbomachinery design, and trickle down.

    A real revolution can probably only occur if monolithic ceramic rotating assemblies become feasible. The hotter you can run your turbine inlet temps, the higher your efficiency. At present, once you get below the about 10000 HP, your turbine blades are too small to support internal blade cooling like the bigger engine have. Therefore your material becomes the limiting factor for efficiency (a big commercial engine can run turbine inlet temps above the melting point of the blade material, as the blade is internally cooled, shrouding in a cooling film of cooler air, and has a ceramic coating).

    Metals are already pretty much at the physical limits today, to make a signicant jump in temp, ceramics are the holy grail (this has been know since the war - RR, GE and P&W have investing millions in trying to reach the goal of a ceramic which can survive the heat cycles of repeated starts without shattering as they cool).

    The laws of physics shall be strictly inforced!

    Regards,
    Mark


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    Quote Originally Posted by diarmaid
    its expected to be around the $50,000US mark. (I was out by a measly 35k ! )
    At that price you should buy two !!!

    Funny vid Neatman, nothing like strapping a couple of hot water bottles full of fuel to your belly, a model gas turbine to each foot then jumping from a hot air balloon !!!
    It put a smile on my face.

    Russell.



  11. #23
    Banned diarmaid's Avatar
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    Hi. Cool video. I, like epineh, was slightly concerned about the location of the fuel bottles. However, I thought they were stuffed slightly lower than his belly.....bit disturbing whatever way you look at it!

    I would like to do something like he did in the video. I just have two 'simple' extra requirements.

    1) I need to be able to fly like that without the requirement of getting to 3000ft first. Maybe adding larger more solid flying surfaces is an idea. Im willing to sacrifice the vertical takeoff capability for a short takeoff run. Maybe 100ft. Possibly by strapping wheels to knees and elbows and taking off in a hunched position on the ground....(Just an idea) Possibly using a small catapult to get to flying speed instead of a takeoff run(I know kinda dangerous and funky but if it worked then it wouldn't be dangerous because you wouldnt come crashing to the ground!)

    2) I need to be able to strap a fuel tank to the persons back holding enough for.....60mins flight....even 30mins would be a starting point....Sounds like the easy part....

    Yeah the wankel rotary is pretty cool. I'll have to consider it as an option and look into it more. We studied it last year and I did a short paper on it.



  12. #24
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    Diarmaid, you are working on a Darwin award!

    Regards,
    Mark


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