Gas Engine Quad, Ideas and Solutions...

I had an idea about building a gas powered quad.  It would be powered by some of the new small 4-stroke engines like the honda gx-35.

I know what you're thinking... gas engines don't respond fast enough.  But, everyone bases this on using the throttle and carb to control the power output.  My idea is that in addition to the throttle you could use a microprocessor controlled CDI ignition to control the power.

With a microprocessor ignition you could immediately cut or reduce power output by skipping a spark for super quick power reduction and you could also retard the spark for a smoother, but still quick reduction.

At say 3000 RPM (typical) you could reduce power within 1/3000 of a sec by skipping a spark, or every other spark for a quarter sec or something of that nature.  The throttle could also participate, but the spark control would be immediate.

I'm curious how a gas quad like this would perform.  A Honda GX-35 puts out about 1.5 hp, and many sources say it easily turns a 20x8 prop at around 3000 RPM.  

Would that be too large for practical use in typical applications?  Each engine would be about 1.5 hp @ around 8 pounds (very lightly modified).  That would be 6 hp @ ~32 pounds.  I've also read that when the flywheel/magneto (using CDI) and clutch are stripped they can come in under 6 lbs.  That would be ~6 hp @ 24 lbs.

I'd like to know if this is new territory or if someone has already tried this and what their results are.  I haven't been able to find anything of the sort, so maybe it's a truly new idea.

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  • Why not include a couple of extra propless electric worker motors whose only purpose is to recharge the battery. They could operate by storing energy into an on board capacitor (like a manual flash camera). That could create the needed power to maintain the charge in the battery for longer flight. They would draw power from the battery just as the prop motors do, but their main purpose is only for recharging the battery. I think of my rechargeable crank operated flashlight. The flashlight does not need to be turned off to recharge, and is easily charged while still running. The same could happen here as well. 

    This may not solve the issue of energy reclamation from the prop motors. Reading some posts, a gas engine/electric hybrid would create too much weight and become less practical. Bigger props to lift the weight would need more energy unless this could be miniaturized on an RC scale. Then you would still have the issue of on board fuel storage. Solar power would be a nice back up system to what I mentioned above for overcast days.

    Just my thoughts

  • I'm working on getting a CDI design going with the sophisticated capabilities discussed above...

    A two-stroke engine running at 6000 RPM should provide control opportunities at 100hz.  A 4-stroke would only have 50hz, but given an air-frame that was designed to be naturally stable (lower CG) it might also work.

  • Moderator

    Somebody has to be the Nervous Nellie, it may as well be me this time!

    Be careful with the 20 inch blades, so you don't end up like this guy... we certainly don't want to lose you!

  • Ok,

    I just had the idea of using 4 separate gas powered helicopters attached to a central core, then you could use the individual collective servos to control the pitch of each of the 4 rotors.  Obviously this would be more expensive than using a single central gas engine, but it is a thought none the less. 

    With a special cnc gear box mounted to a central engine, you could then use the helicopter tail boom idea with servos to control each rotor's pitch. 

  • I've always wanted to do this.  How i imagined getting around the throttle lag was to have a central engine running 4 of these:  Except instead of 3 servos running the rotor independently, the 3 servos would only move the rotor up and down adding or removing trust, for each of the 4 props/rotors.

    Just a thought.

  • How NOT to do it  :--

                                       -)An engine with alternator and then electric motor because efficiency would be so terrible. The more the devices between the source of energy and the destination of energy, the more the losses because all the devices would all individually take away bits of efficiency leading to much lower use-able energy by the electric motor which doesn't even have a 100% efficiency on it's own.

    How to DO it :-

                                    -) 2-stroke (perfect power to weight ratio), with disadvantages too though (vibration, pollution & noise).

                                    -)Direct drive system from the engine to the rotor(reduces loss) with variable pitch.

                                    -)Since the whole system would be much heavier than conventional electric qauds, it will be less sensitive to gust/other sources of instability due to big inertia so the speed of the digital servo will be adequate.

  • Moderator

    Start small, and work up...

    The best components I can think of using for a small model and not something you could sit on would be 2 stroke nitro engines as they are very responsive lightweight and relatively cheap, and for further control variable pitch..

  • Hi, I don't know if you guys have seen this but here:
    It is this Chinese guy that built his own man sized octocopter that can carry one person. It is powered by 8 motorcycle engines and the props are from an airplane junk yard (I read this off a Chinese blog). The cost is surprisingly cheap because materials such as aluminum and steel bars can be easily bought and fabricated in China.

    The point of this is that I think due to the large mass of the copter, it does not require any precise IMU unit and fast response motors to help balance the whole thing. The same concept, I believe, can be applied to your quadcopter design, since it weighs 24lbs at least. You might still need an IMU unit, but as I had said, the motors probably don't need to be extremely fast responsive. Just make sure that your design keeps most of the heavy parts such as fuel and etc in the center and try to make everything as balanced as possible.

    Correct me if anything I said is wrong, because I'm still learning about multi-rotor designs.

  • The math is a bit "off" for control response in your supposition.  At 3000 rotations per Minute on a four-stroke, your opportunities for change occur only every-other rotation.  So that's 1500 divided by 60 to a get minimum possible control interval of 40 milliseconds.  By most accounts (from seasoned control engineers who populate these forums), the consensus seems to be that 20 mS (a 50 Hz update rate) is too long , hence the desire to increase the ESC refresh rate to > 200 Hz, for an interval of 5 mS or less.

    The idea for the multicopter has been around for a very, very long time - since before the idea of electric flight was anything but a preposterous notion.  It has been tried on numerous occasions.  The first known historical expression of the idea was in 1904 by Russian aviation pioneering legend Nikolai Zhukovsky, who believed that a multicopter could be made to fly more efficiently than a single-rotor ship.  Here's a drawing of the first known rendition of the concept, put forth by an ardent student of Zhukovsky's, Boris Yur'ev (a helicopter engineering legend in his own right), in a Russian patent dated 1924.  There is no evidence that he actually built one.  (don't have a link for this, having gleaned it from Vertiflite magazine's fall 2008 paper issue).

    3692418700?profile=originalLouis Breguet was the first person to ever to lift off the ground vertically in a heavier-than-air craft in 1907, but his invention had no real means of control, and so was only a proof-of-concept experiment.  It was Paul Cornu who first flew with some semblance of control means, using a dual-rotor configuration later that same year.  Control of the craft outside of a hover in ground effect proved elusive, however, and he abandoned the project.

    While De Bothezat often gets the credit for the first successful controllable VTOL aircraft flight in history, that honor might belong to a Frenchman named Étienne Oehmichen, who first flew his rather oddly configured multicopter in 1921.  I say might because he did cheat just a tad, augmenting his first flight with a bag of hydrogen. De Bothezat lifted off in his "Flying Octopus" a year or so before Oehmichen managed to fly without the hydrogen, but the latter's craft is worth mentioning because it was ultimately far more basically airworthy than the Octopus.  Both ships used variable-pitch blades for control, and relied upon a single engine for power.

    When you look at these aircraft in contrast with today's designs, it's amazing that they flew at all.

    The idea of a gas-powered multicopter has been around for a very long time.  I can find no examples of a multi-engine gas-powered multicopters because the control math simply doesn't work.  There is just no way of making it respond fast enough without resorting to variable-pitch blades, and then you've almost created four single-rotor helicopters hooked together.

  • I forgot to mention... with the flywheel and clutch removed there shouldn't be all that much inertial energy in the system.  That is to say there's not a lot of weight spinning around to resist quick changes in power output.  The piston is essentially vibration and you'd just have the crankshaft (mostly balanced by the piston) and prop inertia to deal with.  That doesn't seem much worse than an electric motor.

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