Personal Air Vehicle?

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    Banned diarmaid's Avatar
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    Default Personal Air Vehicle?

    Hi,
    Im wondering if anyone can help me here. Is there such a thing as either:

    1) A model jet engine that could lift 100kg vertically off the ground?

    2) A model jet engine that could lift 50kg vertically off the ground and a system to link two of these so that they will open/close throttle in perfect harmony?

    3) A " " " " " " 35kg vertically " " " and a system to link three of these so that they will open/close throttle in perfect harmony?

    Also:

    1) Is there a formula relating to jet thrust and force that can be used to calculate the above for specific engines?

    2) How much fuel is required for...lets say...a 90min flight using a model jet engine (Approximately will do, I just need to figure out roughly how much space 90min worth of fuel for 1 engine will take up) ?

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    Last edited by diarmaid; 08-22-2006 at 04:36 PM.


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    I won't say it can't be done but lifting 100 kg straight up would probably put you into a full scale turbine. Also you would be better with turboprop as your airflow would be spread better and more even. As for fuel load for 90 min. it would be lots. The more power required the more fuel and the model turbines burn about 1/2 lt a min or more.
    The only place I know that may be able to give you a bit of info is the GTBA association which is on the web. I would check there.
    John



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    Banned diarmaid's Avatar
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    Thanks I'll look there but even from what you've just said I think my idea is probably dust. Oh well. L8rs.



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    how high do they have to lift 'it' ?



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    Banned diarmaid's Avatar
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    I think 100 to 150ft would be enough initially for a prototype.



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    I'd look at some sort of rotor craft, or if it doesn't have to be vtol a small fixed wing. Is the 100kg load breathing? at 220lbs load plus fuel you're very close to ultralight specs.

    You can get 'model' jets to 150lbs thrust and above, the AMA limit rc jets to 35lbs thrust, but to use multiple jets to list 100kg you're on diminishing returns when you factor in engine weight, controls, and fuel system and fuel weight. Like a rocket, there comes a point where adding engines and accessories actually decreases the available payload. That ignores problems of controlling multiple motors.

    Also note most jets are rated at lbs thrust in motion, if your vehicle isn't doing 100mph with the appropriate compression benefit then your actual available thrust will be somewhat less.

    interesting question tho'..

    Andrew

    Last edited by fyffe555; 08-19-2006 at 09:22 PM.


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    Banned diarmaid's Avatar
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    Quote Originally Posted by fyffe555
    Is the 100kg load breathing?
    Please explain what you mean by this in more detail. If you mean physically breathing as in 'alive'.....then yes (At a later stage).

    Quote Originally Posted by fyffe555
    The AMA limit rc jets to 35lbs thrust
    Not sure what the limits are over here, if I ever got to the stage where I needed to find out I will. But 'technically' it wouldn't be radio controlled, it'd be controlled electronically by an operator so prob come under general aviation regulations. What dead weight would a 35lb jet lift vertically before getting significant velocity to feed the engine?

    Quote Originally Posted by fyffe555
    But to use multiple jets to lift 100kg you're on diminishing returns when you factor in engine weight, controls, and fuel system and fuel weight.
    Yeah I was worried about that but I dont think that point would be reached with 4 or less small engines....what do you think?

    Quote Originally Posted by fyffe555
    Also note most jets are rated at lbs thrust in motion, if your vehicle isn't doing 100mph with the appropriate compression benefit then your actual available thrust will be somewhat less.
    So how much thrust would I need to lift 100kg dead weight 100ft above ground level before transitioning to horizontal flight? I imagine I'd need a significant reserve of thrust to remain airborne during the transition phase? (I have all the formulas for lbs thrust vs weight vs resistance but Im not going to route them out right now and I can't remember if there are differences for jet vs prop generated....although I dont think so! )

    Going into 3rd year of a 4 year Transportation Engineering Degree course in Sept and Im toying with the idea of designing/building an experimental personal air vehicle for my final thesis/project over the next two years.
    Thanks for your advice Andrew, keep it up! I know that the springtail in the USA ( http://www.trekaero.com/
    http://science.howstuffworks.com/pav2.htm
    http://www.technovelgy.com/ct/Scienc...sp?NewsNum=167 ) is gone beyond the initial development stage, and uses turboprops probably for the reasons mentioned above among others, but Im trying to figure out if the alternative of rc jet engines would be viable. If it is I may be able to get some funding from the university to help with a prototype.
    Thanks.

    Last edited by diarmaid; 08-19-2006 at 06:05 PM.


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    hmmmm...It seems about 20lts of kerosene will provide about 60mins of flight dependant on the various obvious factors. This presents a problem. Does anyone ever use alternative fuel sources for these engines which will allow a more efficient burn and thus either less fuel to be carried and/or longer flight?



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    Does anyone know approximately how much an engine such as this (http://www.heward-microjets.co.uk/en...tairh400.shtml) would cost. Its not physically very large (Although possibly too large ). Im thinking around the 15k mark. Would that be about right?
    Obviously I'd have to build a small scale model over the next year out of my own money before there would be any possibility of getting funding for somthing like this.
    Thanks.



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    breathing; I was asking if the 220lbs/100kg was a living breathing pilot. If it is you get into all sorts of caa requirements of course but more importantly (!) the fuel load would then have to be accounted as extra payload and then you are into Ultralight territory.

    Bell did a jet powered pack around the time of the more sexy James Bond rocket pack, it was faster and could fly for 20 minutes or more and used a single stage jet engine, no idea of teh specs but google might help.

    Dead weight verses jet thrust; It Depends...! mostly on how the engine is rated. There is a nasty equation to calculate the lift due to thrust of a jet but it requires information that isn't readily available for small jets like volumes, speeds, temperatures, eflux shapes and the like. It's pretty much irrelevant anyway since you 're going to need actual static thrust numbers. If an engine is rated as commercial engines are then 35lbs 'Rated maximum continuous thrust', which is a static measure, means if you turned it skyward it should move up to 35lbs weight. The rating for model and smaller engines isn't formal however and some rate 'estimated' which with commpression due to influs speed can be 20% or more higher than static, some rate 35lbs because that's the AMA limit.

    For a thesis I'd definitely specify a static thrust test as part of the project, either yours or certified from the vendor and note the operational characteristics, thrust at different spool speeds etc. Note there's all sorts of funny effects on a static or nearly so jet engine operating close to ground, from ground effect, exhaust recycling in proximity to ground to compressor stall problems all of which will affect thrust. You'd need to account for that in the left calc as well as control and safety margin.

    To be honest a single stage jet probably isn't the best fit, noisy and thirsty and you appear to need to move large volumes of air relatively slowly, not small volumes fast which is what a single stage jet does. Maybe look at a 2 stage jet such as a bypass turbofan or turboprop.

    Or use IC, Moller skycar has eight wankel engines (wankel - oh no!!) turning four ducted fans to the same effect, not realy flying now but the army/vendors are using the same engine in UAV's like the Sikorsky Cypher. You could do it with something other than jets unless jets are too sexy to avoid. I'd even suggest protyping with electric motors on a tether to test the thing.. That's how Bell started doing helicopters after all.

    The trek aero proposal does much the same as suggested, shifting high volume, low speed air with ducted rotors to provide higher static thrust. The trick is to balance the available thrust with a useful payload, which in english means will it go up and if so how much fuel do I have left. Modern jets use high bypass fans both for efficiency and static thrust at takeoff.

    How much thrust to move 100kg to 100ft Vertical then transition? Depends on the constraints, we've already said that the static thrust of an engine would have to be verified. If it's a back yard project then you could get by with less than a thesis expected to comply with some current flight airworthyness regs or reasonable safty expectations.

    There's a bigger issue in that transition to and from flight requires a surplus of power to maintain altitude while propelling the vehicle forward, regardless of whether lift is subsequently available in horizontal flight. An advantage in single stage jets is that as flight speed increases so does their efficiency to some degree.

    You might want to cover something a little larger than a bunch of small turbines. As you add engines they get proportionally less efficient, even assuming any of the practicalities can be met. If you can get the weight into the sweet spot then thats fine but for the thesis try comparing to smaller commercial engines like the Rolls Allison 250 turboshaft's are rated at shaft horsepower ~320 and ~135lbs dry, used in small helicopters. For pure thrust Williams has a 1000lbs thrust bypass engine at 300lbs dry. Way to big and expensive possibly, but if you're carrying any kind of fual load witha warm body then your all up weight requirement gets big very fast.

    As an aside have a look at the harrier and where the exhausts are located compared to its CoG. It carries its weight mostly on the front 'cold' exhausts. The engine is split with a high bypass fan powering the front exhausts, with the rear hot exhausts powered by the remainder of the bypass and the turbine exhaust.

    Is this a vehicle or a strap on?

    Last edited by fyffe555; 08-20-2006 at 12:00 PM.


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    Diarmaid, maybe you could pursue IC type engines and use some sort of ducted/enclosed fan setup. Myself I toyed with an idea using a hybrid sort of engine (my own idea of course !).

    Imagine two counter rotating two bladed props, positioned axially inline. Now towards the center is a combustion chamber of sorts that is closed when the blades cross each other, fuel is injected and ignited pretty much simultaneously and this forces the blades to spin... and so on.

    My idea was to use high pitch fan blades, using low rev's blah blah blah...

    Now before the flaming begins, I couldn't work out how to get a resultant axial force, instead of just one bang that resulted in scrap metal. I got as far as a port system kinda like a two stroke but there had to be a third part (I figured) to be in the center of the two "pistons" to make them travel away from each other, creating rotational motion.

    The advantage of this theory was pure rotational movement, like a jet but with the torque of an IC engine, allowing relatively slow rotational speed and lots of efficient power.

    Now that you all think I am crazy, let me just say that if some one uses this idea and builds an engine that:

    1 : Works.

    and:

    2 : Makes them incredibly rich...

    Then I expect at least one (1) carton of beer as credit for the idea.



    Russell.



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    AMT Olympus turbines 150lbs of thrust and they are pretty expensive.



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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



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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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    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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    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



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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.



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