Power needed to spin rotors

Martin W.

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Video plus links


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XXavier

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Xavier,

You’re correct with your tail rotor thrust calculations. There could, however, be a debate regarding the thrust vs power relationship you mention.

(...)
(...)


You mean this?:

Captura de pantalla 2020-08-29 a las 21.29.16.png

I got it from Wikipedia's entry 'disk loading'. https://en.wikipedia.org/wiki/Disk_loading

It's the (theoretical) power required by a helicopter to hover...

Of course, in the real world, the propeller has always an efficiency of less than 100%. In the case of an stationary prop, perhaps it's around 50%...

So, the calculated 9,2 kW may turn out to be 18 or so...
 
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Oskar

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Oskar, when we discussed electric tail rotors back in mid-May (discussing the news that Bell was experimenting with such), you were dubious because you could not find a suitable motor for same.
What motor(s) did you use on the tail?
You're doing GREAT, thanks for providing the updates.
Brian
Brian,

When I started off designing the electric helicopter a few years ago all the calculations were based on power. I assumed a peak system power of 30kW of which about 10% (~3kW) would be used by the tail. For an electric tail helicopter that would require a 27kW main motor and 3kW tail motor.

High performance motors up to a few hundred Watts are freely available, these are commonly used for RC planes, helicopters and drones. From 20kW and up they are again freely available for all kinds of electric vehicles and flying machines. But between 1 and 20kW there is not much choice. I could not find a motor with suitable power, speed and weight specifications, thus the reason for initially using the existing tail rotor system on the Mosquito.

The first flying tests confirmed that the tail rotor was in fact using around 3kW, an electric conversion at that power level just wasn’t feasible.

It was only when I started looking at thrust (and not power) that I noticed that (on paper at least) it should be possible to achieve the required thrust using much less than 3kW. Drones with an AUW of 15kg were starting to become freely available, and that was exactly what I needed. The design of the etail on the Mosquito is based on a Tarot T960 drone, and many of the parts I used come from this drone. The T960 has 6 motors, I added a seventh (just in case) but have already found that it is possible to hover with only 4 motors. They are designed to run with either 17 or 18 inch props, to reduce the power requirements I’m running 18 and 20 inch props.

So far the electric tail is performing even better than I had envisioned. Some of the “fly by wire” aspects are quite intimidating, but we have to remember that cars and many aeroplanes have been doing this for a while already.

I’ll start a new topic on electric tail rotors, it’s quite a fascinating field.
 

wolfy

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Nice auto's mate. The thing I never liked about the AIR is not having skids, running landings (or take off's) are out.
We are not always able to do perfect non running landings when not expecting/prepared for a real auto.

wolfy
 

Oskar

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I've now done accurate measurements on the electric tail rotor, with 4 motors the power required is 1.1kW in the hover. When the three missing motors are added that should drop to between 800 and 900W, which is about one third of what the mechanical tail rotor was using. The battery I'm using to power the etail is rated at 490Wh, which means it should last about 30 minutes. From measurements so far the main battery should last about 15 minutes which works out very nicely, the tail rotor battery should never go flat before the main rotor battery.

While waiting for the main batteries to arrive I'm making up a new drive shaft so that I can remove the gearbox which is no longer needed. Also need to get the driveshaft out of the boom at some point, but that's a tricky job so will just leave it there for now.
 

wolfy

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Very interesting what you are doing, going with an electric (providing it's electrically reliable) tail simplifies a complicated machine.
I have never actually seen an AIR in the flesh, while you naturally have done some flying in one. Are the pads on the gear actually shaped like a ski? Do you think you could do a running landing? Does it worry you not having skids?

wolfy
 

Smack

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I've now done accurate measurements on the electric tail rotor, with 4 motors the power required is 1.1kW in the hover.
Oscar, what is the distance from the rotor axis to the tail rotor axis?
I'm wondering what kind of cancelling torque it provides.
Brian
 

Oskar

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Oscar, what is the distance from the rotor axis to the tail rotor axis?
I'm wondering what kind of cancelling torque it provides.
Brian

Brian,

The original tail rotor was centered 3.25m behind the mast, for the etail it's around 3.5m. The main motor is programmed for 90Nm maximum torque, with the secondary reduction that works out to just over 400Nm on the main rotor. Tail thrust thus needs to be 116N, or around 12kg maximum.

When hovering the main rotor power is 18 to 19kW which works out to a tail thrust of around 11kg. At 1.1kW that's 10g/W, which is good in the drone world. I could let the motors work a bit harder still but that reduces the efficiency.
 

Oskar

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Very interesting what you are doing, going with an electric (providing it's electrically reliable) tail simplifies a complicated machine.
I have never actually seen an AIR in the flesh, while you naturally have done some flying in one. Are the pads on the gear actually shaped like a ski? Do you think you could do a running landing? Does it worry you not having skids?

wolfy
Wolfy,

The Air has very high rotor inertia, as long as you stay outside the dead man's zone and don't get caught going downwind low level you can always land at zero ground speed.

I had an XE as well but preferred the Air.
 

okkman

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Oskar, Amazing job mate, you are doing exactly what I have been dreaming of doing with my Helicycle for about 2 years. One comment on the tail rotor, I believe you could increase the efficiency by ducting your rotors. Max thrust per watt being the goal it may be worth the effort.

Thank you for pioneering this!

Eric
N424KA
 

Oskar

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Oskar, Amazing job mate, you are doing exactly what I have been dreaming of doing with my Helicycle for about 2 years. One comment on the tail rotor, I believe you could increase the efficiency by ducting your rotors. Max thrust per watt being the goal it may be worth the effort.

Thank you for pioneering this!

Eric
N424KA
Thanks Eric,

Ducting is definitely another way to go. I decided against it for the following reasons:
1) Designing and building a duct is difficult, it's definitely above my pay grade.
2) Ducts need space, I decided to rather fill that space with larger props. At the moment there is less than 10mm prop clearance, squeezing a duct in between would require smaller props.

The mechanical tail rotor was using nearly 15% of total power, the electric tail about 5% of total power. The first 10% reduction was easy to achieve, from now on every 1% improvement will be difficult. What I'll do first is test it as it is to see if there are any issues in forward flight.

Oskar
 

okkman

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Agreed on all points. I'll be anxiously awaiting updates on your progress, this is very exciting to see.

Best,

Eric
 

Oskar

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Here’s what’s been happening over the past few weeks:

The plumbing is now finished and the water cooling pump automatically turns on when either the drive or motor temperature reaches 30C. Without cooling the drive and motor would get to 60C after 1 minute 30 seconds, now after 3 minutes 30 seconds the drive is at 50C and the motor at 60C.

A new drive shaft between the motor and the main rotor reduction now replaces two driveshafts and the front tail rotor gearbox. No more gearboxes!! The tail rotor driveshaft is still inside the tail boom, it’s a messy job to get it out so I’ll just leave it there for now.

The 12V system now runs on it’s own battery. It’s a small 3300mAh battery which will last about an hour.

Half the main batteries are mounted but not wired up yet, they are the blue blocks at the end of the rear landing struts. In the video below additional weight of 21.5kg was added to simulate the rest of the batteries. Since total main battery weight is 40kg, AUW in this test was slightly more than what it would be with all batteries mounted.


It was a bit windy (check the trees in the background) so the power measurements were not stable. Main rotor power was between 21 and 22kW, with tail rotor power between 1.2 and 1.5kW. The wind was from the right which is worst case for the tail rotor. The tail rotor motors had been derated by more than 50% (easily done in software) and I was sitting very close to the control limit. Will have to reduce the derating a bit.

Getting very close to the first fully battery powered flight!
 

Oskar

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Rookie mistake!!

When testing on a slope always measure the slope before starting out. I had assumed the slope to be about 3 degrees, and very quickly got used to what I “thought” was level. But the hang test showed that my “level” was actually 3 degrees tail heavy. Going back to the slope I found that it was between 5 and 6 degrees, the main batteries will have to move forward a bit more than envisioned.

On the paperwork side New Zealand will soon have it’s first registered electric helicopter ZK-IAB. There’s not much work left now to get the flight permit under way, and after that working out a flight test procedure with CAA.
 

All_In

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...

On the paperwork side New Zealand will soon have it’s first registered electric helicopter ZK-IAB. There’s not much work left now to get the flight permit under way, and after that working out a flight test procedure with CAA.
Wow, how cool is that!!!!
 

Oskar

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Do CAA have to come visit you again, or can it just be checked by a LAME
Gary,

Someone from CAA might come in for a nosey, but their visit is usually just to check that all the paperwork is in order. Since the helicopter was registered before and I have all the relevant logbooks they might skip their visit.

What definitely has to be done is a RAANZ (or SAC) modification approval signed by either a LAME or IA. I might struggle to find a LAME wanting to touch this, but in NZ many IAs are used to home builds so that won’t be a problem.
 
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