Is this scheme for a single-fan electric airship with a rocket-like geometry workable?

Question

I am currently designing a vertical take-off and landing vehicle that uses a Tesla powertrain connected to an electric ducted fan. Please comment on if my thought process is sound on my feasibility analysis.

My initial mission goal is extremely modest to grease the wheels of my feasibility analysis and have a realistic utility in society (and for people in my sport, recreational/experimental skydiving).

The vehicle simply needs to lift itself and a 100kg payload (coincidentally the approximate mass of a skydiver with his parachute) to the altitude of 1 mile.

I think I need to calculate my total delta_v, but because I don't need a high speed (like an orbital escape velocity or something like that) I am not positive how to go about getting it without first understanding the maximum thrust from my system.

My current plan for determining thrust from the tesla-system: 1) identify maximum output of tesla battery and motor (lets say Model S P100D) 2) calculate maximum propeller size for #1 based on algorithms available in electric drone aircraft websites/forums 3) calculate potential thrust produced by #2 4) use #3 to determine if liftoff is possible carrying at least #1

My initial calculations are not intended to include all outside forces; I will incorporate these if it is in fact, theoretically feasible at all.

Based on information above as well as the weights of the battery, motor, and an estimate of a basic rocket-shaped frame, I can figure out if a basic propeller will work for this, or if I will need to incorporate an electric turbojet, which would blow up my budget for this.

It is also feasible I could have a combination of several motors / batteries attached to multiple propellers with something like a rocket sitting in the middle of an oversized quadcopter. Again, budget goes up considerably.

Technically it will be something in between both, likely classified as a helicopter. I guess I don't totally understand your question. The flight mission is to deliver a skydiver to altitude so he can jump out, then the vehicle regenerates energy on decent to prepare for another flight. Rocket-geometry is very aerodynamic, and the physics of a rocket-shaped vehicle taking off and landing reliably have now been shown even in extremely large models (eg. SpaceX Falcon 9). — Dr-Brando, Nov 07 '17 at 06:26
yeah an electric helicopter or airplane does all you said better and cheaper so why not? also since your velocity would be slower than a bicycle when who cares about aerodynamic? — user3528438, Nov 07 '17 at 15:13
@user3528438. I am not sure I know what helicopter or airplane you are talking about that is cheaper. What i'm proposing is simply a small aluminum cylinder with a salvage Tesla powertrain attached to a rotor. Helicopters are typically quite more expensive, and there aren't a plethora of electric helicopters i know of, especially of the inexpensive type. I reiterate that the above craft would likely be classified as a helicopter. The airplane has many more requirements, airfield not the least of them, cost of wings and body are not cheap. Please link me to the cheap electric helicopter. — Dr-Brando, Nov 10 '17 at 06:12
@user3528438 also, I think you fail to understand the question. the question is not "what is better, my new experimental theoretical rocket-shaped helicopter, or existing aircraft?" which would be an easy answer as you've suggested. The question is about the theoretical thought process on determining if it is even physically possible. I'm not trying to buy the cheapest thing on the market. This is a thought exercise in aeronautical design. Thanks. — Dr-Brando, Nov 10 '17 at 06:15
Thank you very much @Sanchises. I agree Delta-v not typically useful for atmospheric flight. Can you please opine on what metrics would be best for determining some design metrics when the desired goal is to launch the aircraft (and it's payload, a skydiver) straight up 1 mile. As for the regenerative breaking, I don't anticipate reclaiming much energy, but perhaps enough to help facilitate and stabilize a powered vertical landing. Any constructive input is appreciated! — Dr-Brando, Dec 10 '17 at 01:44

Koyovis · Answer 1 · 2017-12-07T09:02:33.810

2

The best propeller driven vertical take-off and landing craft would be a helicopter, actually. Yes a duct around a propeller increases thrust, but upsizing a propeller creates way more thrust from the available power.

Propeller thrust has scaling effects with size due to different Reynold numbers, data does not scale up well. Best way forward from where you are now would be to look at the weight, rotor dimensions and installed power of existing designs for small helicopters such as the R-22. Or better yet, design an electrical version of the Hiller Flying Platform Platform.

edited Dec 07 '17 at 09:02

answered Nov 07 '17 at 06:24

Koyovis

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Great point! I am pretty much proposing a Hiller Flying Platform shielded in a rocket-body (to protect from the elements and normalize aerodynamic forces) and adding basic self-balancing flight-controller dynamics to negate the need for a pilot. Given that the Hiller Platform works (worked) with 2x 44HP motors, I am pretty confident the 700+HP Tesla dual-motor could produce the necessary lift to even carry it's heavy 1,200lb battery-pack (of which really only a fraction would be required to bring the craft to 1 mi. altitude... – Dr-Brando Nov 07 '17 at 06:33

score 2 · Accepted Answer · answered Jan 06 '18 at 17:59

First, you need thrust-to-weight¹ ratio more than 1, so the device can maintain flight. You should be able to find data about how much thrust (lift in case of rotors; it's the same thing) they can produce and how much power from the engine it takes for some existing propellers and rotors.

The efficiency, that is thrust-to-power ratio, generally increases with the disc area. That is why helicopters have large rotors—they need less power to produce the desired lift.

So you'll also need enough area. You can chose a rotor like a normal helicopter, or you can choose multiple propellers in a style similar to

octacopter carrying a human (via this answer)

or

volocopter (from this company)

You should be able to get (static) thrust and efficiency for some standard propellers to use in your estimates.

Keep in mind, that the maximum thrust decreases with air density and you'll need the thrust up to your desired altitude. At 5000 ft, the density is ~86% of that at sea level, further subject to weather as density also decreases with temperature.

Now when you have a set of propellers that can produce enough thrust with your available power, you'll still need some excess power. This excess power will roughly indicate your maximum climb rate: just divide the excess power by weight².

Note that the excess power will decrease with altitude as the efficiency will decline with density.

And last you'll need to make sure you have enough energy stored to maintain full power for the time it will take you to climb to that altitude.

¹ Weight is a force acting on a body due to gravity, i.e. mass times gravity constant, so it can be directly compared to other forces (thrust).

² Since weight is force, dividing power by it yields (vertical) velocity.

Note regarding delta_v. That is a characteristic quantity for rockets, where the limiting factor is the momentum they can impart to the exhaust gasses that are generated from the fuel only. And since momentum is mass times velocity, dividing the momentum by mass gives a useful characteristic value, the $\Delta V$, with dimension of velocity.

However you have propellers and they impart the momentum to surrounding air, which you have unlimited supply of. So the momentum is not limiting for you, the energy that you lose with it is. You minimize the energy by using a lot of the air, i.e. increasing the mass of the reactive mass, not its velocity. Rockets can't increase the mass, as the fuel is all they have, so they must maximize the exhaust velocity instead and are intentionally inefficient in the sense of the thrust-to-power ratio mentioned above.

Wow; Thank you a ton Jan. You've articulated that so incredibly well and boiled down a complicated concept into a simple thought process. I can't thank you enough. I think I have enough design constraints and component parameters to put together some realistic estimates. Thanks again. — Dr-Brando, Jan 08 '18 at 10:26

Is this scheme for a single-fan electric airship with a rocket-like geometry workable?

2 Answers2