Can a Vertical Lift Engine Hover With 30 Pounds?

[bob012345]

I'd trying to design a vertical lift engine to lift about 30 pounds in the air about one foot off the ground with minimum power. The device has to hover. The engine should itself weight 5 pounds or less and have a footprint no larger than one square foot. By minimum power, I mean something like 1-3 KW. I'm not asking for specific ideas (I've got about a dozen) but just if folks think it's going to be possible or not and why. Thanks.

 

[berkeman]

The device has to hover.

What does that mean? You mean like a drone? 1 foot off the ground will not give you much ground effect assist, so if you make 1 foot with a drone, you can make a lot more too.

If you mean like a drone or other aircraft, look up what it takes for a drone to lift 30 pounds. I'm sure they exist, and their engine size and power are probably about what you will need.

If you can include a 1-foot long air skirt, then you can make it more like a tall hovercraft, and the power requirements will go down some. You will still have stability and control issues, but hopefully it won't take much extra power to maintain position and stability.

 

[berkeman]

I did a quick google search for Drone Payload Versus Engine Size, and got good hits. Here is one with a drone listed that can lift 20kg:

https://www.dronethusiast.com/heavy-lift-drones/

 

[bob012345]

What does that mean? You mean like a drone? 1 foot off the ground will not give you much ground effect assist, so if you make 1 foot with a drone, you an make a lot more too.

If you mean like a drone or other aircraft, look up what it takes for a drone to lift 30 pounds. I'm sure they exist, and their engine size and power are probably about what you will need.

If you can include a 1-foot long air skirt, then you can make it more like a tall hovercraft, and the power requirements will go down some. You will still have stability and control issues, but hopefully it won't take much extra power to maintain position and stability.

Thanks. I trying to avoid simply making a drone type quadcopter lifting device. They take a huge amount of power because they are using brute force lifting. I've investigated hovercraft technology but prefer a skirtless design. I'm hoping to minimize power by using various effects. I've seen jet engine designs but they are super complex and expensive.

I'm looking at skirts though.

 

[berkeman]

Looks like this 8-rotor drone (8 x 300W = 2.4kW) has about a 20 pound payload capability: https://freeflysystems.com/alta-8/specs

Without a skirt, I'm not sure you will get much more efficiency than a multi-rotor drone. The 1 foot max height would seem to lend itself to the skirt feature to reduce the power needed...

 

[bob012345]

I did a quick google search for Drone Payload Versus Engine Size, and got good hits. Here is one with a drone listed that can lift 20kg:

https://www.dronethusiast.com/heavy-lift-drones/

Thanks. I'm thinking of a lift engine that has a small footprint of around one square foot.

Looks like this 8-rotor drone (8 x 300W = 2.4kW) has about a 20 pound payload capability: https://freeflysystems.com/alta-8/specs

Without a skirt, I'm not sure you will get much more efficiency than a multi-rotor drone. The 1 foot max height would seem to lend itself to the skirt feature to reduce the power needed...

Thanks for the link! Yes, I think it's either going to be hovercraft technology based or another effect besides pure brute force lift with a battery powered prop. The small footprint and low power is so the engine concept can be chained to lift more weight. I'm also looking at other aerodynamic effects such as blown wings, Coanda and Magnus effects.

 

[cjl]

Why do you prefer a skirtless design? A hovercraft-style vehicle with a skirt will take by far the least power (assuming you aren't able to physically hook to something and lift that way). Beyond that, a multi-rotor style is probably your best bet. For minimum power, you want the largest possible rotor surface area - small, high speed things like ducted fans (or jet engines) are horribly inefficient for producing static thrust.

 

[bob012345]

Why do you prefer a skirtless design? A hovercraft-style vehicle with a skirt will take by far the least power (assuming you aren't able to physically hook to something and lift that way). Beyond that, a multi-rotor style is probably your best bet. For minimum power, you want the largest possible rotor surface area - small, high speed things like ducted fans (or jet engines) are horribly inefficient for producing static thrust.

Frankly, because it's cooler to be visibly floating even though with a skirt you really are floating. But I can also design the skirt to fit in with my concepts of 'cool' too. I'm also exploring new concepts for more efficient 'momentum curtains' formed with air.

 

[russ_watters]

Please note that the smaller footprint you are looking for will make this device substantially less efficient than a drone with, for example, 8 one sq ft propellers.

Also, you can't do it with a single rotor and be stable.

I don't think the sum of your constraints is possible.

 

[bob012345]

Please note that the smaller footprint you are looking for will make this device substantially less efficient than a drone with, for example, 8 one sq ft propellers.

Also, you can't do it with a single rotor and be stable.

I don't think the sum of your constraints is possible.

This is why I'm trying to get away from brute force propeller driven lift devices. Also, I plan on using multiple lift engines together if it works for stability and higher loads.

 

[russ_watters]

This is why I'm trying to get away from brute force propeller driven lift devices. Also, I plan on using multiple lift engines together if it works for stability and higher loads.

What, then, do you mean by "lift engine"? If it isn't a motor driven propeller I don't know what you mean.

 

[CWatters]

Hovercraft are only efficient when the gap around the skirt is small compared to say the area/footprint. A 1sqf hovercraft with a 4 sqf gap can't really be called a hovercraft.

 

[CWatters]

Think about the effective "wing" loading... If you have a 1sqf "wing" or swept area and a 35lb minimum weight you have a 35lb/sqf effective wing loading. I think only something like a gas turbine might get there.

 

[berkeman]

Hovercraft are only efficient when the gap around the skirt is small compared to say the area/footprint. A 1sqf hovercraft with a 4 sqf gap can't really be called a hovercraft.

I'm picturing a 1 foot tall skirt. Is that what you are picturing? I know it's a bit weird, but it will help to confine the downstream air...

 

[CWatters]

I assumed the lower edge of the craft had to be 1ft off the ground. Perhaps to clear obstacles?

If that's wrong how small can the gap be?

 

[bob012345]

What, then, do you mean by "lift engine"? If it isn't a motor driven propeller I don't know what you mean.

It could be a motor driven propeller but it could be something else also. For example, it could be some sort of compact small footprint hovercraft device or an aerodynamic device such as a 'blown wing' where the airflow is self generated as opposed to using forward motion. It could be magnetic lift if it weren't for having to have a special conductive surface to react against. It's just a means of providing a certain lift in a specified footprint and power usage. It may end up having to be either a hovercraft device or a propeller device if I can't get anything more clever to work.

 

[bob012345]

Think about the effective "wing" loading... If you have a 1sqf "wing" or swept area and a 35lb minimum weight you have a 35lb/sqf effective wing loading. I think only something like a gas turbine might get there.

Yes, it might have to be such a power source.

 

[bob012345]

Hovercraft are only efficient when the gap around the skirt is small compared to say the area/footprint. A 1sqf hovercraft with a 4 sqf gap can't really be called a hovercraft.

It wouldn't strictly be a hovercraft but rather a hovercraft-type lift engine applied to a different kind of device if that's the way I have to go. I do know it's not going to be just slapping a few common elements together and presto, it works! It's going to take a lot of engineering. It may end up being something combining several concepts together.

 

[cjl]

Think about the effective "wing" loading... If you have a 1sqf "wing" or swept area and a 35lb minimum weight you have a 35lb/sqf effective wing loading. I think only something like a gas turbine might get there.

Nah, you could definitely do that with an electric motor and propeller, but the power requirements will be very high. Ballparking the math here...

Mass flow rate will be the product of air density, area, and velocity through the prop. Thrust force is the product of mass flow and velocity. Therefore, F = ρav². Rearranging this and solving for the discharge velocity to achieve 35lbf through 1 square foot, you get 37 m/s (or about 83 mph). This is high, but definitely doable with a propeller. The power in this jet will be one half the product of the mass flow rate and the exhaust velocity (this is just the rate of kinetic energy addition to the jet), which for this example gives 2.88 kW. Propellers are not perfectly efficient though (80% or so is probably achievable with one optimized for this case), so you'd actually need about 3.5-4kW of shaft power to achieve this level of thrust. It's not impossible, but it will be loud, and it will have an 83mph jet of air coming out the bottom at all times (which may or may not be a problem, depending on the intended application).

To show how much this is impacted by the disc area, a 1 square meter rotor area gives 11.3m/s (25mph) discharge, with a jet power of 878 watts and a required motor power of a bit over a kilowatt.

 

[bob012345]

Nah, you could definitely do that with an electric motor and propeller, but the power requirements will be very high. Ballparking the math here...

Mass flow rate will be the product of air density, area, and velocity through the prop. Thrust force is the product of mass flow and velocity. Therefore, F = ρav². Rearranging this and solving for the discharge velocity to achieve 35lbf through 1 square foot, you get 37 m/s (or about 83 mph). This is high, but definitely doable with a propeller. The power in this jet will be one half the product of the mass flow rate and the exhaust velocity (this is just the rate of kinetic energy addition to the jet), which for this example gives 2.88 kW. Propellers are not perfectly efficient though (80% or so is probably achievable with one optimized for this case), so you'd actually need about 3.5-4kW of shaft power to achieve this level of thrust. It's not impossible, but it will be loud, and it will have an 83mph jet of air coming out the bottom at all times (which may or may not be a problem, depending on the intended application).

To show how much this is impacted by the disc area, a 1 square meter rotor area gives 11.3m/s (25mph) discharge, with a jet power of 878 watts and a required motor power of a bit over a kilowatt.

Thanks. Is this a hovercraft with a skirt or a direct lift propeller you are calculating?

 

[cjl]

That's no skirt. A skirt dramatically improves the situation because it allows most of the lift to come from a slight pressure increase rather than purely from fluid momentum.

 

[bob012345]

That's no skirt. A skirt dramatically improves the situation because it allows most of the lift to come from a slight pressure increase rather than purely from fluid momentum.

Thanks. That's a relief since I could live with a skirt. I've got design options to make a skirt more 'cool' and less obtrusive or even less visible. I would prefer optimizing the 'momentum curtain' concept though.

 

[cjl]

What exactly is your "momentum curtain" concept?

 

[CWatters]

As I recall full size helicopter manage about 5kg per kW? So I would have thought it was a tall ask to manage 35kg with a few kW using a helicopter type arrangement Perhaps I have that wrong?

Edit: oops, it's been pointed out that should be 35 lbs not kg.

 

[cjl]

Helicopter lifting efficiency depends strongly on rotor loading - power per pound goes up pretty dramatically if you try to carry more per square foot of rotor area. For an extreme example, a human powered helicopter has been made that was able to lift itself (briefly), and a high level athlete can only manage a bit over a kilowatt peak. Note though that the earlier calculations were 35 pounds, not 35kg - 35kg will substantially increase power requirements, especially if rotor size doesn't increase (power will go up linearly with mass if rotor size is increased to maintain rotor loading, but power will go up faster than mass if rotor size is kept fixed).

 

[bob012345]

What exactly is your "momentum curtain" concept?

That is the key invention by British engineer Christopher Cockerell that first enabled the hovercraft to hover and it consisted if forcing air down around the periphery in a thin, high velocity jet. The jet helped contain the air inside and under the craft from escaping as fast as it would have. The first hovercrafts visibly hovered around 9 inches off the ground. Later, the skirt concept made hovercraft much more practical allowing greater hovering heights and thus obstacle clearance. The height of the air cushions then became on the order of a meter or more.

https://en.m.wikipedia.org/wiki/Momentum_curtain

 

[bob012345]

As an aside, an extremely remote possibility (and dangerous) would be a gasoline powered pulse jet concept. I'm thinking compressed fuel and air combusting against a simple pusher plate. Sort of like the buzz bomb concept from WW2. Probably completely impractical but still fun to think about. Franky Zapata uses four small jet engines to power his 'Flyboard' but that uses 1000hp and consumes about a gallon per minute of jet fuel.

https://www.theverge.com/2016/4/15/...acing-jet-powered-flying-hoverboard-interview

 

[bob012345]

I assumed the lower edge of the craft had to be 1ft off the ground. Perhaps to clear obstacles?

If that's wrong how small can the gap be?

I could live with 6 inches. Obstacles aren't a big concern as this would be more recreational rather than utilitarian. Just fly over relatively smooth surfaces. My issue is I have so many approaches, I just don't know where to start as far as testing concepts.

 

[berkeman]

My issue is I have so many approaches, I just don't know where to start as far as testing concepts.

If you do go with the 1 foot tall skirt option, what are your thoughts on how to control stability and how to vector sideways?

 

[bob012345]

If you do go with the 1 foot tall skirt option, what are your thoughts on how to control stability and how to vector sideways?

I would place three to six lift engines around my device depending on load for stability. Hopefully, a more passive system would work but if not I could adapt an Arduino based micro controller to vary the pressure in the individual lift engines for stability. As for sideways vectoring, I thought varying the power might provide asymmetrical lift a portion of which could provide lateral force as well as having the engines at a slight tilt to begin with. I could also 'pump' the individual lift engines in a controlled sequence to provide a sort of walking motion since walking is a kind of controlled falling motion.