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.

Views: 5919

Reply to This

Replies to This Discussion

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.

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).

Louis 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.

http://en.wikipedia.org/wiki/Louis_Breguet

http://en.wikipedia.org/wiki/Paul_Cornu

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.

http://www.aviastar.org/helicopters_eng/oemichen.phpchen

http://en.wikipedia.org/wiki/De_Bothezat_helicopter

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.

Thanks for the excellent response and background Brad.  Good catch on the 4-stroke issue.

I guess I was going too much on what I saw on the RC forums.  A similar engine I'm looking at (GX-35 clone) has specs of 1kW @ 6500 RPM.  I assume it could run 7-9K also if needed, but at 6500 RPM that would give control opportunity at 3250 times/min = 54 Hz =  18.5 millisec.

Would that be more reasonable?  If not are you aware of any other simple control schemes used successfully?  I recall seeing a setup with a control surface under the rotor, but can't seem to find any details.

i have seen petrol motors used in a variable pitch setup.

 

A small nitro motor spins 4 tail rotors from a standard heli, standard torque tube tail booms, the lift is controlled by a servo changing the pitch rather than the motor changing speed.

 

there is a couple of youtube vids of it, I would give you a link but the internet spped where i am is about 12kb/s... ahh the memories of dial up speeds

Why not use a small gas engine to charge the battery instead of the complexity of multiple gas engines. If you insist on gas. Then you may want to consider a gas engine to power a hydraulic pump and control the flow of the hydraulic fluid to each "motor". I think that would allow for a more consistent control of each prop and likely weigh less than for engines... But hey those ideas are PFA...

We already had a discussion in another thread about the hybrid idea.  I personally don't think it's practical, or even a good idea.  There just isn't a whole lot of power to squander on a complex system and energy conversions.

.

I didn't really want to go there again... but you actually gave me a really good idea!  What if we used a small brushless generator on each motor.  Instead of trying to generate a lot of power the generators just freewheel most of the time.  Then when thrust needs to be reduced the generator controller kicks in, thus braking the engine.

It would work exactly the same as regenerative braking.  It would provide the needed fast power control AND generate electricity to keep the batteries charged.

Maybe the magneto could even be rewired as a generator to save cost and size.

Brad, any idea on some of the numbers like how much braking is required in terms of how much generated power would produce the necessary effect?

I have previously toyed with the idea that an ESC could use a Lenz's Law braking technique to improve control response, and the concept has been recently discussed in these forums...somewhere.  However, the focus is on mitigating rotational inertia, and it's doubtful that much energy recovery could be achieved.

Don't forget that the total ship control response must necessarily include the airframe axes' inertial moments, and placing all that engine weight out on the periphery will certainly make any control system work harder.

Another great point about the airframe inertia.  

Controlling the motors (gas or electric) with fancier electrical control schemes can only lead to interesting things I think.

I'm thinking that the easiest sort of gas motor VTOL might just be a sort of 3D plane style control scheme.  I just saw that Javanese black sphere video which got me back on that mode of thinking.  Do you know of any good references on this sort of control or any good examples of various schemes using control surfaces like this? 

Hi, I don't know if you guys have seen this but here:
http://boingboing.net/2011/09/21/china-gentleman-builds-homemade-fl...
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 thread to which Mr. Stew refers is this one:

http://www.diydrones.com/forum/topics/confusion-over-engine-sizes-c...

Feel free to draw your own conclusions.

@Tian: Exactly how is it controlled?  In the link provided, I see some tether ropes on the perimeter, but nowhere has there been any discussion of a control means (this was first posted here about a year ago).  Not to diminish the inventor's creativity or mechanical acumen, but otherwise all he's proven is that parallel thrust vectors add, something we pretty much take for granted anyway.

Your assertion that multicopter motors don't require fast response times is contrary to the conventions expressed by many proven control experts in these forums.  I'd like to share two academic papers written by Paul Pounds, now a professor at Yale, who sums up the issues involved with some rather elegant mathematics.  Are there any specific points with which you disagree?  (I don't agree with his undue concerns surrounding disymmetry of lift, but that's a different conversation.)

The first covers control, and specifically, why 50 Hz is not fast enough.  The second, Paul's PhD thesis, is just a good all-around primer on the design of multicopters.

Attachments:

@Tian: Exactly how is it controlled?  In the link provided, I see some tether ropes on the perimeter, but nowhere has there been any discussion of a control means (this was first posted here about a year ago).  Not to diminish the inventor's creativity or mechanical acumen, but otherwise all he's proven is that parallel thrust vectors add, something we pretty much take for granted anyway.

Your assertion that multicopter motors don't require fast response times is contrary to the conventions expressed by many proven control experts in these forums.  I'd like to share two academic papers written by Paul Pounds, now a professor at Yale, who sums up the issues involved with some rather elegant mathematics.  Are there any specific points with which you disagree?  (I don't agree with his undue concerns surrounding disymmetry of lift, but that's a different conversation.)

The first covers control, and specifically, why 50 Hz is not fast enough.  The second, Paul's PhD thesis, is just a good all-around primer on the design of multicopters.

(the thesis is too big to upload - here's the link)

http://www.eng.yale.edu/pep5/P_Pounds_Thesis_2008.pdf

Attachments:

Reply to Discussion

RSS

© 2019   Created by Chris Anderson.   Powered by

Badges  |  Report an Issue  |  Terms of Service