Certification of Vertical Take Off Vehicle

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Pilot-34

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Apr 7, 2020
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875
I really think we look at this backwards
In the end you are not seeking a better rotor you are seeking lift .
It seems like a propeller wing combination is the most efficient way to get lift.
Isn’t that what the Black Fly shows ?
 

Dusan

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Sep 15, 2014
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Canada
I really think we look at this backwards
In the end you are not seeking a better rotor you are seeking lift .
It seems like a propeller wing combination is the most efficient way to get lift.
Isn’t that what the Black Fly shows ?
The propeller/wing combination is not really the best. The most efficient flying is exploiting the large L/D of wings. A good airfoil provides L/D of about 100 driving total aircraft L/D of 20 or more. That means for each 20lbs weight you need only 1lbs thrust force to fly straight and level. The best way to make that thrust is to use - nothing, as birds do - flap the existing wings, impart the least momentum on as much air as possible.

The propeller is a compromise even for conventional air-planes: at slow flight and takeoff you need a large propeller to impart momentum on a large air mass, otherwise the loses are high; at high speed you want a small propeller, the air-mass going through the propeller is high anyway and you want to minimise torque and blade friction, so to minimise power.

A VTOL aircraft exacerbates the rotor compromise - it needs to provide lift to account for the whole weight of the aircraft, or 20 times more than cruise. A conventional rotor is too small for effective hover and too large for good cruise performance. Also sizing for a personal aircraft, the best is using a single rotor (as in single rotor helicopter); having multiple rotors drives the Reynolds number too low for efficient hovering.
 

Yellowhammer

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Feb 21, 2020
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Born In Alabama, reside: Louisiana (unfortunately)
Folks -
One of the young ladies who came up through my EAA Chapter, and who used to hang out at the maintenance shop here on the weekends, has gone to a major university to pursue Aeronautical Engineering. I'm super proud of her because she liked being "hands on" as she felt it would help her design equipment that people could actually "work on" versus being beautifully engineered but not practical....

As an early project she and her team are working on building a vehicle that will weigh about 210 pounds when completed, and is a prototype for a man carrying vehicle, but won't carry humans as it is a concept at this time.

She asked me how they would go about registering such a vehicle and that's where I got stuck. My first thought was that it would be an ultralight, but since it's not carrying people that means it would probably fall into the "drone" category. That means it would need to be registered with the FAA as a drone since it is over 55 pounds. Is that all we are talking about here, or perhaps I'm missing something obvious?

Thanks for any insight.

Wayne

Register as a U.A.V. I would also talk to the local FISDO office to be sure. Hope she succeeds in her goals and would love to see what she has been working on.

Yellowhammer
 

Turd Ferguson

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Upper midwest in a house
That means it would need to be registered with the FAA as a drone since it is over 55 pounds. Is that all we are talking about here, or perhaps I'm missing something obvious?

Thanks for any insight.
Any Small UAS operated under Part 107 has to be registered with the FAA if it weighs more than .55 lbs. (that's decimal five-five)

A UAV weighing more than 55 lbs would be outside Small UAS category (Part 107) and there are several pathways to operate a vehicle of that type. It may required getting a Special Airworthiness Certificate similar to a homebuilt. It's not going to be as simple as operating a Small UAS.
 

Pilot-34

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Joined
Apr 7, 2020
Messages
875
The propeller/wing combination is not really the best. The most efficient flying is exploiting the large L/D of wings. A good airfoil provides L/D of about 100 driving total aircraft L/D of 20 or more. That means for each 20lbs weight you need only 1lbs thrust force to fly straight and level. The best way to make that thrust is to use - nothing, as birds do - flap the existing wings, impart the least momentum on as much air as possible.

The propeller is a compromise even for conventional air-planes: at slow flight and takeoff you need a large propeller to impart momentum on a large air mass, otherwise the loses are high; at high speed you want a small propeller, the air-mass going through the propeller is high anyway and you want to minimise torque and blade friction, so to minimise power.

A VTOL aircraft exacerbates the rotor compromise - it needs to provide lift to account for the whole weight of the aircraft, or 20 times more than cruise. A conventional rotor is too small for effective hover and too large for good cruise performance. Also sizing for a personal aircraft, the best is using a single rotor (as in single rotor helicopter); having multiple rotors drives the Reynolds number too low for efficient hovering.
I have heard that about wanting a large prop for efficiency at low speeds.
But why not do like the Black Fly? Several or a multitude of props accelerating the even larger airflow exactly where you need it ?
 

Dusan

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Sep 15, 2014
Messages
133
Location
Canada
I have heard that about wanting a large prop for efficiency at low speeds.
But why not do like the Black Fly? Several or a multitude of props accelerating the even larger airflow exactly where you need it ?
Several props have an aerodynamic penalty in reduced Reynolds number compared to a single larger rotor/prop. Some are arguing that the benefit of multi-rotors is reduced mechanical complexity, redundancy, etc, so it's a good solution. I'm inclined to differ, since the aerodynamics is the most important to things that fly, and reduced aerodynamic performance is having larger effects down the line on total aircraft performance, such as range, power required etc.
 

Dusan

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Sep 15, 2014
Messages
133
Location
Canada
I discovered this video on you-tube today, it demonstrates that electric helicopters are definitely better performers, the builder claims "21kW in the hover, 17 above ETL" Compare that to Ehang 184 multi-copter: "Power required to hover 47 kW", from https://www.mdpi.com/2226-4310/6/3/26/pdf
 

Pilot-34

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Apr 7, 2020
Messages
875
Several props have an aerodynamic penalty in reduced Reynolds number compared to a single larger rotor/prop. Some are arguing that the benefit of multi-rotors is reduced mechanical complexity, redundancy, etc, so it's a good solution. I'm inclined to differ, since the aerodynamics is the most important to things that fly, and reduced aerodynamic performance is having larger effects down the line on total aircraft performance, such as range, power required etc.
When you’re going to extreme lengths to shorten the landing zone or perhaps even make it completely vertical I’m not sure that other area of aerodynamics matter a lot
 
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