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Dr. Slack

Sat May 10 2014, 03:58PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Patrick wrote ...

look here... tri-copter with wings.

pic

That looks really cool, but at what cost?

Rotating the forward motors costs a control channel, the servo and rotator weight, and the fact that the prop pitch, size and power is a compromise between low pitch high power for hover and high pitch low power for forward flight.

What if he used two big low pitch fixed counter rotating lift props ahead of the wings, with the ESC programmed to stop them in the low drag fore-aft position, and a single fixed high pitch prop in the nose for forward flight? The servo channel becomes the forward channel, the weight of the rotator becomes the weight of the forward prop, so no significant change in weight or control channels. However, each prop is now more efficient as better matched to its job, and the mechanics are simpler with no rotator.

Now, additionally, what if control were completely split with forward flight motor and control surfaces on one radio, and vertical services on the other radio? Result total parallel get you home survivability so can land vertically, or like a plane, or glide in on receiver batteries only, if there's a malfunction with one set of controls. The cost at the vehicle being the weight of a second receiver and control battery. Is it worth the weight for a much more survivable plane?

Patrick

Sat May 10 2014, 04:47PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

you make compelling points Dr. Slack. based on good engineering philosophy, rarely taught in college couses.

and ash small, id rather not have the swash plate at all, might as well go to a full heli then.
i want collective equivlent only, cyclic is uselessly complicated. (unless this tail rotor is what you menat) ->

ok then let me do some more thinking, and ty to get the machine flying today.

Ash Small

Sun May 11 2014, 04:57PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

cyclic is uselessly complicated.

I don't agree. It can be accomplished easily with four servo's. The only other thing required over 'normal' variable pitch is a spherical bearing, which isn't subjected to rotational motion (EDIT: there is an element of rotational motion, depending on 'cyclical' stuff, but I'm sure this can be dealt with) and so can therefore be quite simple. You could either adapt something like a self-aligning bearing or get something machined out of nylon 66, or similar. I'd get this machined with some form of spline on the inside so it will slide on the prop shaft thus serving the variable pitch function at the same time. The only difference between the two systems is one extra spherical bearing and three servos, but they can each be much smaller than the single one required for VPP as they share the load.

EDIT: You still need a couple of taper-roller bearings, or something similar, (along with the blade carriers and pushrods) but you need all this for VPP anyway, unless I'm missing something.

EDIT: You need a bit of extra software too, but software doesn't weigh much

Chip Fixes

Mon May 12 2014, 05:38AM

Registered Member #3781 Joined: Sat Mar 26 2011, 02:25AM
Location:
Posts: 701

Just saw this on Hack A Day today:

Although, besides being able to hover for 19 hours, it really doesn't seem that novel.

BigBad

Tue May 13 2014, 06:55PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

I think maybe VPPs are particularly good for variable mass vehicles.

For battery powered vehicles, you just optimise the prop for one speed (the one needed to hover), and you can deal with the differences in thrust by varying the speed somewhat, hopefully not running it too much off the optimum speed.

Ash Small

Tue May 13 2014, 07:08PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

BigBad wrote ...

I think maybe VPPs are particularly good for variable mass vehicles.

For battery powered vehicles, you just optimise the prop for one speed (the one needed to hover), and you can deal with the differences in thrust by varying the speed somewhat, hopefully not running it too much off the optimum speed.

i agree, but once you start considering IC engines I think you have to consider VPP and swashplate, as swashplate doesn't contribute much mass compared to VPP (and is hardly any more complicated to implement). I'm assuming that a large percentage of 'take off' weight will be fuel, and most of the fuel will be consumed during flight, I think this is where the advantages of VPP really lie. Also, I think the torque characteristics of IC compared to electric would also favour this approach. I can't really see much advantage of using VPP with electric motors if the payload is reasonably constant.

EDIT: Although I can see advantages of using swashplate with electric, as it means you only need one motor, assuming the tail rotor is driven by gears, and one large prop is more efficient than several smaller ones.

EDIT: I think you'd only need VPP for the tail rotor. I can't see any advantage of using a swashplate there.

Patrick

Wed May 14 2014, 12:47AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i shoud have said this sooner, but the real problem with a traditional heli swashplate is that when you have a prop strike, youll bend all theose precision pieces. And if i want a commercial product to eventually come of this, ill need reliability and repairabilty.

Steve Conner

Wed May 14 2014, 09:30AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hence see Dr. Slack's earlier comments about complexity.

My favourite design right now is the RCExplorer tricopter. The frame is a couple of softwood battens from the hardware store, and the motors are held on with cable ties so they will come loose in a crash before the shaft bends. I expect the average crash would cost about $7 and take 15 minutes to fix.

BigBad

Wed May 14 2014, 02:19PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

If we're talking mechanical simplicity I quite like the sycamore seed type design where you spin the whole vehicle as a wing and then control it exclusively on cyclic and thrust. Maybe use an airjet at the tip and a compressor in the central hub. You could probably synchronise the rotation with a magnetic compass.

If you put a camera on it, you get 360 degree vision for free.

Patrick

Wed May 14 2014, 03:19PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

BigBad wrote ...

If we're talking mechanical simplicity I quite like the sycamore seed type design where you spin the whole vehicle as a wing and then control it exclusively on cyclic and thrust. Maybe use an airjet at the tip and a compressor in the central hub. You could probably synchronise the rotation with a magnetic compass.

If you put a camera on it, you get 360 degree vision for free.

i saw the embry riddel team demonstrate this at our academic competition in north dakota, it was really cool.

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