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i need another opinion on energy of flight...

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Pinky's Brain

Tue Nov 23 2010, 07:32AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

You're thinking clearly, but you are not taking into account asymmetrical drag.

Assume you apply the thrust in bursts, and rather than hover you oscillate up and down. In between the bursts you horizontally extend an aerofoil slowing your decent to less than 9.8 m/s ... you end up needing less than 9.8 m/s average thrust to maintain altitude.

Patrick

Tue Nov 23 2010, 07:50AM

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

Pinky's Brain wrote ...

You're thinking clearly.....

thats a relief, the voices in my head can scare me sometimes.

Ruben

Tue Nov 23 2010, 12:55PM

Registered Member #3263 Joined: Sat Oct 02 2010, 04:43AM
Location: Sydney, Australia
Posts: 34

from wiki -> power (watts) = thrust (newtons) x speed (metres/second)

if stationary hover is equal to a speed of 9.8m/s, and i have a 1Kg sphere of lead, or styrofoam,
then 9.8N = 1Kg so 9.8N * 9.8m/s = 96 watts for each second of desired hover.

or am i not thinking clearly...

I respectfully disagree. I think you may be confusing your units:

In order to 'hover' your object requires zero net acceleration, i.e:

F=ma=0

this requires a thrust (directed downwards) to produce a force directed upwards of 9.8 N (assuming a 1 kg craft). note that we're talking about force not velocity.

The equation on wiki:

P=Tv

refers to the propulsive power of a jet/fan. note that:

If the speed is zero, then the propulsive power is zero

Because we're considering a hover, this equation is irrelevant and misleading.

I'm not super familiar with how lifters work, but I assume that a certain amount of electrical energy is required to produce a given thrust (N) (which may also be dependent on the crafts velocity???). I would use this as a starting point for your problem.

remember that the 'power' required, will depend on the way you produce it (eg a ''jet" with an intake duct, a rocket, etc);

this may help:

klugesmith

Tue Nov 23 2010, 05:34PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

As others have said, the power to sustain a hovercraft depends on air leakage. It theoretically approaches zero with a leakless skirt. (on the side, ever calculate the draft of a hovercraft over water? Clearly the pressure depresses the water surface, so the stationary hovering craft displaces a water weight equal to its own weight).

As for the lifter hovering high in the air (i.e. outside of ground effect):
It's supported by the reaction of a jet of air continuously accelerated downward.
The net power of a jet engine at zero speed is, of course, zero.
But you need power to accelerate the air, and that depends on the tradeoff between mass flow and exhaust velocity or specific impulse.

To get 9.8 N of thrust from an engine with specific impulse of 100 seconds,
you eject 0.01 kg/s at 980 m/s. That takes at least 4800 watts.
These are reasonable values for model rocket engines.

For 9.8 N of thrust with specific impulse of 10 seconds,
you are accelerating 0.1 kg/s of air to 98 m/s. That uses 480 watts.
I think that would fit a high performance model helicopter.

If you could blow 1 kg/s of air downward at 9.8 m/s, the reaction force
would support your 1 kgf weight, with only 48 watts (plus losses).
Ultralight helicopter contest winner?

So you see, for low speeds it helps to have low gears.
[edit] There may be room for advancement in "lifters" involving ducts and diffusers
to optimize the entrainment of ambient air by electrostatically-driven jets.

Patrick

Tue Nov 23 2010, 09:11PM

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

Klugesmith wrote ...

As others have said, the power to sustain a hovercraft depends on air leakage. It theoretically approaches zero with a leakless skirt. (on the side, ever calculate the draft of a hovercraft over water? Clearly the pressure depresses the water surface, so the stationary hovering craft displaces a water weight equal to its own weight).

this explanation makes more sense for a hovercraft.

Klugesmith wrote ...

As for the lifter hovering high in the air (i.e. outside of ground effect):
It's supported by the reaction of a jet of air continuously accelerated downward.
The net power of a jet engine at zero speed is, of course, zero.
But you need power to accelerate the air, and that depends on the tradeoff between mass flow and exhaust velocity or specific impulse.

yes thats what i thought, i need to specify my exhaust v and mass ejected, as per Isp, only then can power be calculated in a useful way.

Klugesmith wrote ...

It's supported by the reaction of a jet of air continuously accelerated downward.
The net power of a jet engine at zero speed is, of course, zero.

yes even though the net force is zero, from zero movement, that zero movement requires energy expended to establish the slipstream, and not violate the law of conservation of E.
(ideal slipstream E) = (mass of air stream)* (acceleration of that airstream mass.)

Klugesmith wrote ...

To get 9.8 N of thrust from an engine with specific impulse of 100 seconds,
you eject 0.01 kg/s at 980 m/s. That takes at least 4800 watts.
These are reasonable values for model rocket engines.

yes. as an example i agree.

Klugesmith wrote ...

For 9.8 N of thrust with specific impulse of 10 seconds,
you are accelerating 0.1 kg/s of air to 98 m/s. That uses 480 watts.
This would be unremarkable in a model helicopter.

do you mean typical helicopters would be in this approximate range?

Klugesmith wrote ...

If you could blow 1 kg/s of air downward at 9.8 m/s, the reaction force
would support your 1 kgf weight, with only 48 watts (plus losses).
Ultralight helicopter contest winner?

yes i see my calculations depend on conditions, for a small lifter 1Kg of air per second would be hard to entrain. so this would tend to push me towards the helicopter operating conditions. (less mass, but accelerated faster, thus needing more E)

Klugesmith wrote ...

So you see, for low speeds it helps to have low gears.
[edit] There may be room for advancement in "lifters" involving ducts and diffusers
to optimize the entrainment of ambient air by electrostatically-driven jets.

yes the entrainment and inducement of air mass-flow, will be important for any useful advancement in lifter science.

once, again Klugesmith saves me...

2Spoons

Tue Nov 23 2010, 10:04PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Simply put:
force= mass x delta V (momentum equation)
energy= 0.5 x mass x (delta V)^2 (kinetic energy)

So it is easy to see why creating thrust by moving a lot of air slowly is more efficient (in terms of Newtons per watt) than by moving a little air fast.

Patrick

Tue Nov 23 2010, 10:37PM

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

2Spoons wrote ...

Simply put:
force= mass x delta V (momentum equation)
energy= 0.5 x mass x (delta V)^2 (kinetic energy)

So it is easy to see why creating thrust by moving a lot of air slowly is more efficient (in terms of Newtons per watt) than by moving a little air fast.

yes, yes, yes, thats the thought i had, when i started this thread, but, needed some removal of confusion.

BigBad

Sun Nov 28 2010, 12:57PM

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

That's right. That's why gliders need a lot less energy to stay up than a harrier jump jet hovering. A glider's long wings throws a large amount of air down slowly, whereas harriers throw a small amount of air down very fast. A harrier can empty its tanks of highly energy-rich fuel in a few minutes, whereas a glider can almost rely purely on its potential energy. Helicopters, with their long rotors are somewhere in between.

If you use permanent magnets you can reduce the weight further, although *static* magnets can't lift you off in a stable way (unless you stick to the magnet).

Ash Small

Mon Nov 29 2010, 05:33AM

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

Technically speaking, I don't think a hovercraft is the ideal example to use here, as it is the plenum chamber that enables a hovercraft to be efficient. Very little energy would actually be required to lift a hovercraft an infintesimally short distance above a table.

Helicopters and harriers operate differently to hovercraft. The plenum chamber operates in a (sort of) similar way to 'wing in ground effect' compared to normal fixed wing flight.

I don't know if this is relevant to this project, as not a lot of information has been provided.

The original experiment that Chris Cockerell did to demonstrate the principle using two coffee cans, a vacuum cleaner set to blow, and some kitchen scales demonstrates this. The force generated when placing one coffee tin inside the other to create the plenum was considerably greater than the force generated using only one tin.

From wikipedia:

"The idea of the modern hovercraft is most often associated with Sir Christopher Cockerell. Cockerell came across the key concept in his design when studying the ring of airflow when high-pressure air was blown into the annular area between two concentric tin cans, one coffee and the other from cat food. This produced an annular ring of airflow, as expected, but he noticed an unexpected benefit as well; the sheet of fast moving air presented a sort of physical barrier to the air on either side of it. This effect, which he called the "momentum curtain", could be used to trap high-pressure air in the area inside the curtain, providing lift based on pressure, not airflow. In theory, only a small amount of active airflow would be needed to create lift, much less than a design that relied only on the momentum of the air to provide lift, like Frost's Avrocar or a helicopter. In terms of power, a hovercraft would only need between one quarter to half of the power required by a helicopter."

Patrick

Mon Nov 29 2010, 06:24AM

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

yes ash small, a rocket, heli, and jet would be better examples, and forget that "infinite distence table" remark i made. i meant for that to make things easier, it does not.

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