I buy a 1HP gas engine - Honda has one that weighs 3.5kg. Add another 0.5kg for a full tank of gas. That's 746 Watts for 4kg of weight. I hook it up to a 1HP DC motor, any hobby BLDC rated for 750W or higher will do, at maybe another half kilo. Allocate another 4.5kg of ancillary parts to make up the rest of the quad. So an AUW of 10kg is maybe a good guesstimate...

If you were doing it with a winch, it'd take 10W to lift 1kg at a 1m/s rate. So ignoring all the inefficiencies in converting electric power to mechanical lift, you'd only need 100W to lift a 10kg hybrid gas-electric quad at 1m/s. Assuming the power system is only 20% efficient at converting electrical energy in to mechanical lift, the 1HP gas engine has enough Watts to fly the 10kg weight. If you halve the required rate of climb, the power requirement drops by 2 and the system only has to be 10% efficient.

Back of the napkin math, it seems doable. But you'd have to go big.

Originally Posted by rocketsled666

I buy a 1HP gas engine - Honda has one that weighs 3.5kg. Add another 0.5kg for a full tank of gas. That's 746 Watts for 4kg of weight. I hook it up to a 1HP DC motor, any hobby BLDC rated for 750W or higher will do, at maybe another half kilo. Allocate another 4.5kg of ancillary parts to make up the rest of the quad. So an AUW of 10kg is maybe a good guesstimate...

If you were doing it with a winch, it'd take 10W to lift 1kg at a 1m/s rate. So ignoring all the inefficiencies in converting electric power to mechanical lift, you'd only need 100W to lift a 10kg hybrid gas-electric quad at 1m/s. Assuming the power system is only 20% efficient at converting electrical energy in to mechanical lift, the 1HP gas engine has enough Watts to fly the 10kg weight. If you halve the required rate of climb, the power requirement drops by 2 and the system only has to be 10% efficient.

Back of the napkin math, it seems doable. But you'd have to go big.

Quick search found an OS engine weighing less than 1000g that can produce 2.66HP at 9,000 RPM, which seems to open up the weight budget quite a bit.

After further reflection, I bet a 1/2A glow engine driving a parkflyer motor would make a pretty cool steampunk (nitropunk?) laptop charger

Originally Posted by rocketsled666

...1HP gas engine...full tank of gas...1HP DC motor...

I don't see a weight allowance for a 1kW generator. Did you accidentally omit that?

The only 1 hp Honda engine I found was this one: https://www.electricgeneratordepot.c...w-recoil-start

That does not include a generator, as that engine is sold simply as a "prime mover" for people to hook up to whatever load they need to drive.

I looked up a 1 kW Honda portable generator ( https://www.electricgeneratordepot.c...rter-generator ), and this one weighs 54 lbs with fuel (25 kg, more or less).

Originally Posted by rocketsled666

If you were doing it with a winch, it'd take 10W to lift 1kg at a 1m/s rate. So ignoring all the inefficiencies in converting electric power to mechanical lift, you'd only need 100W to lift a 10kg hybrid gas-electric quad at 1m/s.

It's a good approach to start with a quick order-of-magnitude calculation.

However, in this case, there is a crucial factor that's been omitted. It takes zero power to simply hang a weight from a cable (winch), without raising it or lowering it. But it takes a lot of power to hover a mass stationary in the air - helicopters and drones (and hummingbirds) have to put out a lot of power just to hover stationary. It is not at all the same as hanging motionless from a cable.

Your estimate tells you the additional amount of power needed to go from a hover into a climb; but it entirely omits the enormous baseline power requirement needed to just hover stationary in the first place.

Having stationary from a cable requires no power output; hanging stationary in the air requires lots of power output. Why? Because to hover, you have to accelerate a mass of air downwards, and that requires power. The hovering object may be stationary, but a large mass of air has to be accelerated to a significant velocity, and this requires lots of energy per second, i.e., lots of power.

At this point even an approximate calculation (starting from basic principles like Newton's laws of motion) starts to look quite complicated. We can start with Newton's second law, the famous F = ma, or, for our purposes, F = m dV/dt.

Here F would be the required lift force (i.e the weight of the drone, in order to just barely hover), "m" is the mass of air blown downwards per second (the mass flux), and dV/dt the acceleration imparted to that air.

Since the lift force is the product of "m" and "dV/dt", evidently you can get the required lift force either by accelerating a relatively small mass of air (m) through a large velocity (large dV/dt), or the other way around - a large mass of air, accelerated gently to a relatively low velocity.

The second approach is much more efficient, which is why helicopters have enormous rotors - that huge rotor area helps to grab the largest possible mass of air. That means you have to impart relatively less dV/dt to that air in order to fly. (But anyone who's stood anywhere near a helicopter when it's landing or taking off knows that the downwash below a helicopter is enormous.)

Drones are notoriously inefficient, in part, because they don't have an enormous helicopter-like rotor area. The use of four or more small rotors certainly increases maneuverability and simplifies mechanical construction (no need for the complex collective pitch / cyclic pitch arrangement in the hub of a helicopter rotor, no need for a tail rotor). But that simplification comes at a heavy cost in power required to hover. Drones are inefficient, and the smaller the rotors on the drone, the more inefficient it is.

Martin Hepperle's website includes the basic propeller equations (which have also been used by fixed-wing RC modellers for decades) to estimate static thrust and power requirements of a propeller; "static" meaning the propeller is not moving through the air, which is the condition it would experience in a stationary hover. It should be possible to use those equations to estimate the power needed to get enough static thrust to just lift a drone off the ground; then you can add an extra percentage of the drone's weight to allow for upwards acceleration, i.e. if thrust is 120% of weight, that should allow for one-fifth-gee upwards acceleration.

Here are the equations: http://www.mh-aerotools.de/airfoils/propuls3.htm

The following RCG thread shows the equations in simplified form as used by RC modellers (posts #2 and #3 are particularly useful): https://www.rcgroups.com/forums/show...op-calculators


-Flieslikeabeagle

Originally Posted by Piece

After further reflection, I bet a 1/2A glow engine driving a parkflyer motor would make a pretty cool steampunk (nitropunk?) laptop charger

And a remarkably quiet one, too. I bet it would be welcome at any coffee shop in the world.

-Flieslikeabeagle

Originally Posted by John61CT

so direct ICE propulsion is supremely practical.

For your information in context of a multi-copter with engine/generator...
1. Electrical systems have efficiency of more than 85%!
2. Gasoline engine is no more than 40% efficient but of course gasoline itself has very high energy density!
3. Speed control of rotors: simple and efficient if done using BLDC motor speed control but very, very complicated with prop pitch and swatch plate along with mechanical losses and wear and tear!
In short direct ICE propulsion is practically inferior!!!

Originally Posted by FCC

1. Electrical systems have efficiency of more than 85%!

Some do, but most in the sizes used for drones do not. You also have to account for I^2 R losses in the ESC, wiring, and battery, which are not negligible.

Then there is the compounded efficiency of using both a motor and a generator. If you manage 75% efficiency from each one individually, that's still a combined efficiency of only 56% for both of them. At that point you have thrown away nearly half the mechanical energy from the engine.

Originally Posted by FCC

2. Gasoline engine is no more than 40% efficient

You're dreaming, dude. Automotive engineers struggle to reach 30% efficiency after optimizing every detail and computer-controlling the fuel injection and cam timing. A small cheap engine of the sort you're considering will almost certainly be less than 20% efficient, and my guess is that it will be a lot less efficient than even that.

I note you have left out another major factor, namely propeller efficiency, which is usually below 60% for small propellers used in RC models, and can often be below 50%.

But: since you already know all the answers, why are you even bothering to ask questions? You certainly are not bothering to pay any attention to the replies, and I don't think you can tell the difference between an informed opinion and a wild guess, so all this is just wasting our time.

Go ahead, build your hybrid drone, and show us how it flies, and that will prove how we were all wrong, and too ignorant to realize it.

Good luck!

-Flieslikeabeagle

I don't see a weight allowance for a 1kW generator. Did you accidentally omit that?

No. You just didn't look hard enough.

Any DC motor is also a DC generator. Power in and Power out are the same in terms of what the motor can handle. So any hobby BLDC capable of consuming 745W is by definition also a BLDC capable of generating 745W.

There are plenty of guys (and women, too, probably) flying large Helis with on-board generators to power the radio. Big helis can draw a lot of Amps just running the servos. So if you wanted to fly one with on-board battery power, you need a big battery that adds a lot of weight. Instead, they use a small battery with enough juice to auto-rotate a landing if the gas engine quits, and they spin a BLDC that drives a rectifier and a boost/buck regulator, and that produces the power the radio and servos use in flight.

The use of BLDC motors as in-flight power generators is a tried and proven technology. I budgeted a half Kilo for this as a generator.

Originally Posted by flieslikeabeagle

since you already know all the answers, why are you even bothering to ask questions? You certainly are not bothering to pay any attention to the replies, and I don't think you can tell the difference between an informed opinion and a wild guess, so all this is just wasting our time

I am looking for a engine/generator solution and NOT fooling around!!!
If you can't help then walk away, not wasting YOUR time!

Figure out the proportion of power used for that radio, compared to the total used by the helicopter while in flight.

Sticking a little wind-powered gennie out the window of a car driving down the highway does not appear to slow the car down at all

but does not mean a wind-powered genset cojld power the propulsion of the whole vehicle

True, but if you couple a slightly bigger generator to the engine, you've got a Prius

Originally Posted by Piece

After further reflection, I bet a 1/2A glow engine driving a parkflyer motor would make a pretty cool steampunk (nitropunk?) laptop charger

Already done. Sort of ...
translate...www.powerditto.de/elocox.html, construction, more pictures

Now that is cool.

Originally Posted by Ron van Sommeren

Already done. Sort of ... [/IMG]

That is nice... how much average power you got for both?

Built by Dieter Gerblinger, both pictures are from the same engine/motor.
It is a rewound&remagneted CD-ROM motor power, say 50watt or thereabouts???
The Graupner Topsy plane in my earlier link weighs 260gr.


Vriendelijke groeten, en wees voorzichtig, Ron
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