Replies to This Discussion

Permalink Reply by Jake Stew on May 18, 2012 at 1:17am

Thanks for the links Brad.  Very informative, although my eyes started to glaze over after a bit.

The Pounds paper (PDF) seemed to say that slew limiting in the ESC contributes a lot to the control response lags.  Perhaps 50hz would be enough if there weren't other factors, like the slew limit, adding to the lag.  I may still give it a try to see if it's workable.  I'm sure you realize by now that I'm more of an experimentalist than a calculator.  From what I can tell it's within the realm of possibility that the general idea could be made to work.

In any case I'll hopefully be getting some more small 4-strokes this summer and I want to get a custom CDI ignition design working well for plane use anyways.  Can you suggest the proper prop to use with 1.5 and 2 HP engines that like to run around 6500 RPM?

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On another note, I've seen you mention that a single rotor design is more efficient.  Is there any reason there are so many quads out there and few, if anybody, seem to be flying 3D plane style?

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Permalink Reply by Andrew Rabbitt on May 18, 2012 at 1:49am

I would say there is more than enough system response in a gas engine, even with just manipulating the throttle, to make a quad work.

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Permalink Reply by Brad Hughey on May 18, 2012 at 8:01am

@Andrew: I would tend to agree, but I have no data to support such a position.  To my knowledge, no one has ever made a direct-drive gas-engine multicopter fly with autonomous control - at any scale.

Virtually all auto and cycle industry reviews I've read tend to note the primary advantage of electric drive is the "right now" torque compared to their gas counterparts (think Lotus Elise v. Tesla Roadster as an example).  There is no denying that electric motors have faster response, but how fast is fast enough?  That, of course, depends on the airframe and how the mass is distributed. Heck, there are so many variables that one will notice most of the banter in these very forums surrounds PID settings and other control logic issues.

The electric advantage in this area lies in the fact that most of the mass is in the battery pack, and that can be placed very nearly at the precise center of total mass.  While the power-to-weight ratio of gas engines is clearly superior, the designer is faced with either the prospects of having most of the mass (and inertia) out the periphery where it is least desirable, or putting some sort of tail-rotor-like drive transmission mechanism in place which would allow the mass of the engines to be moved closer to the center.  The "successful" gas multicopters I've seen all have a central engine (except some of the turbine tilt-rotor Osprey-like incarnations) but they all still use variable pitch for control.

In short, the benefits of electric direct drive are:

• Much lower mechanical complexity and concomitant increases in reliability
• Much faster dynamic response
• The ability to concentrate the airframe mass near the center.

So, while one might be able, eventually, to make a gas multicopter work, the only advantage I can see is having a longer duration flight time due to the energy density.  However, the costs in complexity are potentially huge.

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Permalink Reply by Brad Hughey on May 18, 2012 at 7:28am

At first glance, the 2 HP (~1.5 KW) and 6500 RPM appear mutually exclusive in the 20" diameter range.  A gear reduction is in order.  Think about how larger BLDC motors are usually in the 200 to 600 KV (RPM/volt) range.  Once you get beyond the ~16" range, the section chord lengths (with mechanically-practical solidity ratios) start to drive the Re's past 150K where turning faster is not to your advantage (actual drag becomes more influential than Cd).

By single rotor, do you mean single blade?  Theoretically they might be, but the vibration and aerodynamic (with the focus on dynamic) force issues have never, to my knowledge, been successfully addressed in a practical application.

By 3D you mean inverted?  That's hard to do with a fixed-pitch multicopter, but I've seen at least one variable-pitch quad that might have a go at it.

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Permalink Reply by Jake Stew on May 18, 2012 at 7:07pm

By 3D I mean the type of plane that can fly straight up, basically can take off from their tail.  Youtube is filled with planes hovering around hanging from their props.  They seem almost equal to most quads in terms of maneuverability.  I would think you could find plenty of areas for efficiency gains if you remove the requirement for high speed forward flight with a lifting wing.

The next advancement in quads just might be an "uno".

I'm still quite a ways off from getting my 4-stroke flying.  All I really have to go off of at the moment is the posts in various forums suggesting a 20x8 prop will run at 3000 RPM in fixed wing designs that are currently flying, and the manufacturer specs of 1.5 or 2 hp @ 6500 RPM.  I've got two 20x8 wood props at the moment, but I'm very curious about running at different RPMs and finding that magic compromise between all the factors that might make things work.  There's the control issue (more RPM means faster potential response), the power issue (manu. specs. HP at 6500), and the thrust efficiency issue (optimal prop/RPM).

I'd happily trust your calculations/experience over my random stabs and iterative solutions.  My current timetable is just to have some data by the middle to end of summer, so there's certainly no hurry.  Hopefully this is enough mystery and potential to get a few subconscious wheels turning.

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Permalink Reply by Tian on May 18, 2012 at 7:26pm

Hi Brad,

I am not sure how it is controlled. No descriptions were given. It was said that at the current stage, it only hovers. It was tested to hover with stability for around 10 sec.

I agree with you that multicopters do need fast response times, but I feel that it is very dependent on the mass of the copter. When you have a copter with a large mass, it means that it has way much more inertia, therefore, it is less prone to rapid changes in stability.

After reading part of the thesis, I agree that it is very difficult to build a heavy multicopter (Most of the quads/hex/octos seen in the hobby world are built so they weigh as less as possible). I also agree that for fast response, large rotors with high inertia will be harder to control purely through speed control. However, as I had stated before, I feel that when the size gets to a certain magnitude, the large inertia of the entire system will make the small movements negligible. I have seen the effect of large mass system with underwater vehicles (ROVs), as the larger ROVs will be less affected by waves, water movements, and etc. However, since I have never tested this theory with multirotors in the air, please do not take any of my statements for granted.

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ModeratorPermalink Reply by Dean Wynton on May 18, 2012 at 2:08am

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..

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Permalink Reply by Veikko Vierola on May 18, 2012 at 2:39am

How about using one gas engine that runs all the variable propellers with belt or chain?

http://www.diydrones.com/photo/oboe-quad-one-belt-one-engine-quad-c...

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ModeratorPermalink Reply by Dean Wynton on May 18, 2012 at 8:37am

That would work for sure.. In fact someone has already done a similar thing in Australia

A bit larger scale but the idea is there and it works..

This is supercool

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Permalink Reply by Brad Hughey on May 18, 2012 at 9:09am

Yes, it's been done - that's exactly what De Bothezat did in 1922, and many others have tried to improve on the idea since.

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

The Australian "hover bike" is not the first of its ilk either.  In so far as a manned flying anything goes, the golden question one must always ask is, "what happens when something breaks?"

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ModeratorPermalink Reply by Dean Wynton on May 18, 2012 at 3:49pm

Unless you have lots of money to play with the weight of a generator plus an engine, plus electric motors no longer makes it a viable option, I believe if you have around £2500 you could buy a very small lightweight alternator that might make it effiecient enough to generate enough energy to make it worth while?

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Permalink Reply by Jake Stew on May 18, 2012 at 6:36pm

   A small alternator won't generate enough power to run electric motors.  You need a relatively large and heavy generator.  Then you need bigger electric motors to lift that large generator.  Then you need a bigger gas motor to drive that generator fast and hard enough to power those large electric motors.  By then you've added so much weight you need a heavier frame... and the cycle repeats.  

   That's the problem with a hybrid design IMHO.  If you could afford a fancy turbine engine with an ultra-efficient generator it could probably work, but you'd end up with a super complicated, ultra expensive system with high maintenance and running costs which defeats the entire purpose.

   I'm trying to explore the possibility of using the new micro 4-stroke gasoline engines (35-50cc) that have recently come out by Honda, Echo, Stihl, and the cheap clones that have also come out as a result of the trend towards replacing 2-strokes with 4-strokes.  These clone engines are cheap, as low as $60 each (shipping still sucks).

   4-strokes are far superior in many respects.  Much quieter, lower pitch exaust, more efficient, less pollution, less vibration, much more reliable, and produce more torque at lower RPMs.  For these reasons I believe they are potential candidates for the next generation of gas airplanes and hopefully some sort of VTOL aircraft.

   Just like with MEMS gyros and accelerometers when a tech suddenly becomes lots better at a fraction of the cost it can result in really cool, and potentially world changing, applications being developed.  Look at autopilots and quads.  What once was shitty and super expensive is now very slick and cheap.  I'd like to be part of that sort of revolution.

   So the engines are what they are.  Everything must be designed around them as they are the "new tech" that is ripe to be pimped.  If you hit up alibaba.com or look at the honda gx-35 specs you can see what is now available off-the-shelf.  These engines can be stripped down by several pounds by removing the flywheel/magneto (replace with CDI ignition), and removing the clutch and pull starter (replace with prop starting).  What you end up with isn't something that's some amazing revolution in terms of power-to-weight, but it IS solidly in the realm of what you can fly with. And it has all the earlier mentioned advantages of 4-stroke power.  And it's CHEAP!  The cheapness factor can't be underestimated.  It allows people with modest resources to do a lot of experimenting.  It also encourages the first working designs to be widely replicated, which creates a snowball effect of rapid advances.

   I think it's something worth exploring at least.  A couple years ago these engines weren't available.  Now that they are we need to see how far they can go.  People are already flying with them in fixed wings.  The main thing is that they sound way better and are more acceptable, and most importantly are many time smore reliable than 2-strokes.

   I've got 7+ 2-stroke engines laying around, which I paid nothing for, just for parts and experimentation.  If I wanted to create the loudest, most polluting, highest vibration, least efficient, most dangerous, and least reliable quad in existence I would try to cobble something together.  But I can barely keep my weedeater running long enough to do my yard, let alone trust all the time and money involved in trying to put them in the air.  I think 4-strokes are the answer to all those problems, and are ripe to be experimented with.

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