Talk-Polywell.org

A DGI phantom 2 has a power to weight ratio (watts to grams) lower than a Cesna 182. However as we should all know, the Cesna can't exactly point its nose to the sky and hover like a helicopter. I suppose then the more important metric than power is thurst. thoughts?

The gentlemen at NASA can explain it far better than I can
http://www.grc.nasa.gov/WWW/k-12/Virtua ... fwrat.html

My bro has a DGI phantom. I want to try attaching weights to it until it can just barely hover to estimate about how much thrust it really has.

I designed, built, flew and mostly crashed RC aircraft for years. It's great fun. If you look online, you'll find several different kinds of thrust balances you can use to get real thrust figures for these aircraft, with the craft attached to the motor, prop, fan, etc. You don't need to attach weights. That would be the hard way to do it.

Thanks GTI

I was thinking, could there be a relationship to scale and thrust via the same power? Maybe it could explain how insects are able to fly for as long as they do with chemical power. Either that or insect energy is simply far superior to batteries. Might be some of both.

Viscosity becomes more important with decreasing scale. Micro=Molasses.

DeltaV wrote:Viscosity becomes more important with decreasing scale. Micro=Molasses.

Cool! Also those props on rc machines seem to go a heck of a lot faster than their normal scale counterparts. Is that right?

I bought a quadcopter that fits within 3 fingers of your hands for 20 bucks. I'll be getting it sometime early next year. I think those things have only a couple hundred miliamp hours of current. Technology is marching fast...

I remember the days with the rc cars that went only forward and turned when put in reverse

ohiovr wrote:

DeltaV wrote:Viscosity becomes more important with decreasing scale. Micro=Molasses.

Cool! Also those props on rc machines seem to go a heck of a lot faster than their normal scale counterparts. Is that right?

I suspect that RC have higher RPMs. The same RPM on a big machine would likely overstress the blades.

There is also a size-related speed perception effect. A C-5 and a Piper Cub can fly in formation (mind those wingtip vortices...), yet the C-5 gives the impression of going slower because it takes longer to cover its own length.

At smaller scales, I think the more dominant viscosity effects give props more bite. Think of bacteria with simple spinning helices, driven by electric rotary (!) motors.

If batteries approached one fifth the usable energy density of liquid fuels think of the fun we could have

ohiovr wrote:I bought a quadcopter that fits within 3 fingers of your hands for 20 bucks. I'll be getting it sometime early next year. I think those things have only a couple hundred miliamp hours of current. Technology is marching fast...

I remember the days with the rc cars that went only forward and turned when put in reverse

I remember those days as well, not to mention the "rc" cars that had a cable from teh controller to the car.

I picked up one of those small quadcopters as well last week. Its pretty interesting, though difficult to control because of how responsive it is. Very sensitive on maneuvering. I find it humorous that it has a 3.7mWh lipo battery that takes about 10 minutes to charge, but the four prop motors drain it in 5 to 10 minutes of continuous use.

ohiovr wrote:Thanks GTI

I was thinking, could there be a relationship to scale and thrust via the same power? Maybe it could explain how insects are able to fly for as long as they do with chemical power. Either that or insect energy is simply far superior to batteries. Might be some of both.

Yes, smaller usually has higher thrust to weight. It's not just that mass goes with the cube of radii, but the relative strength of materials as compared to their mass is inverse linear and several other scale issues. Also with insects, there is the issue of O2 content. The primary limiting factor for how large insects can grow is the % O2 in the air. Presently they don't get nearly as large as they did when we had more free O2. In the past, some bugs were the size of Basset Hounds and could fly. There are fossils of dragonflies with 30" wingspans, for example. Today we don't see anything that large and the largest bugs can only barely fly for very short duration.

ohiovr wrote:I was thinking, could there be a relationship to scale and thrust via the same power?

Scale is difficult to account for. There is a clean relation between thrust, velocity, and power, neglecting efficiency:
Power = Force(dot)Velocity.
also
Power = ((mass/time)*Velocity^2)/2
and so forth... Which boils down to pushing a lot of mass at a slow velocity for low power, or little mass at high velocity for high power. A low mass/area flier can use less power to fly.

A fun experiment would be to see how many tiny RC copters it takes to pick up a human being.

choff wrote:A fun experiment would be to see how many tiny RC copters it takes to pick up a human being.

that's be a 10 million view hit on youtube at least!