Power needed to spin rotors

All_In

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Not sure over 10,000+ logged FW, 260+ ultralights, sailplane, hang-gliders
Sorry for the delayed thank you Chuck et El..
Bingo, I've heard/read you talking about you using those in the past.. U_ROCK!!!

Hughes helicopter blades (269-300) would do it, 7" chord and are easy to cut down to appropriate length.
And another oops: I just reread your post and realized you're looking for a 25' rotor; no cutting necessary!

This is just a proof of concept prototype. I will talk to Denis and Jim about creating exactly what Chuck believes we need. But let's see if she flies and how stable she is, first?
One of the projects for this falls colleges is a tall tail with much more vertical stabilator, even though the torque will be removed from the prerotator an instant before you pull collective.
 

Oskar

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Oskar, Here are my calculated hover powers for MR alone, 15°C, sea level.

On the ground, with no lift: 6,8 kW (Aerodynamic pitch 0 degree)

at 190 kg IGE : 12.5 kW (Pitch 4.25 degrees if no twisted)

Thanks Jean Claude, that corresponds nearly exactly with what I measured. Needing only an additional 1.9kW to lift 40kg more would be fantastic, could I ask what your calculations say for 270kg? At some point things fall off the cliff, and I just want to make sure I don't end up too close to the cliff.
 

Jean Claude

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Oskar,
At 270 kg, IGE : 15.7 kW (Pitch 5.8 degrees)
OGE : 28.0 kW (Pitch 8.9 degrees)
+3 m/s vertical flight: 32.6 kW (Pitch 9.8 degrees)
At 60 mph level and disc A.o.A -6 degree, OGE : 23.3 kW (Pitch 8.5 degrees)
Main rotor alone at sea level, 15°C
 
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grevis

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Jean Claude,
Im another Mosquito owner and found your hover power estimates very interesting, how hard would it be to do the calcs on the other Mosquito heli's at their MTOW's. I expect some other owners would also be interested. If its not too hard the other models are:

Mosquito XE - MTOW 275kg - Rotor diameter 19.5' - Rotor chord 7" - Rotor rpm 540
Mosquito XE285 - MTOW 330kg - Rotor diameter 19.5' - Rotor chord 7" - Rotor rpm 590
Mosquito XET - MTOW 370kg - Rotor diameter 19.5' - Rotor chord 7" - Rotor rpm 590

The rotors are same model as Oskar's air, just larger diameter
Gary
XE285 in NZ
 

Jean Claude

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Gary, My results:
Mosquito XE - MTOW 275kg - Rotor diameter 19.5' x 7" - Rotor rpm 540 Hover OGE, Rotor power = 28.8 kW and 33.5 kW at +3m/s axially
Mosquito XE285 - MTOW 330kg - Rotor diameter 19.5' x 7" - Rotor rpm 590 37.8 kW and 43.4 kW
Mosquito XET - MTOW 370kg - Rotor diameter 19.5' x 7" - Rotor rpm 590 42.8 kW and 49.0 kW

More results?
 
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grevis

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Thats great, It will be interesting now to see what happens as Oskar loads weights on to simulate batteries, see if the power required follows the theoretical numbers. Thanks Jean Claude.
 

Oskar

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Some more test results.

There is something that I didn't mention with the previous results, thought it would be a negligible effect but have since found out that it isn't.
The DC power supply I was using could only deliver 17kW. This was enough power to hover, but not enough to keep rrpm at 100%. In the hover video rrpm was about 90% of nominal speed.
With a new larger DC supply the rotor rpm while hovering can now be kept at 100%, and what I found is that this requires about 10% more power than hovering at 90% rrpm.
When learning to fly helicopters you are told that if you want to stretch an auto you need to lower the rotor rpm by increasing pitch. That ties in with the measurements, a rotor requires less power to keep spinning at lower rpm even though it is supporting the same weight (up to a point of course). The same would be true with gyros, reducing pitch and thus increasing rrpm will require more power to fly.

Have started adding weight as well, with 6kg strapped to the airframe (and close to 100% rrpm) power requirements are just over 20kW.
 

Jean Claude

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Of course. The profile power (air friction on the blades only) increase as Rrpm^3

Thus, the 6.8 kw of my post #20 becomes 9kw

So, 1.1 times 550 Rrpm need extra 2.2 kw for all other load conditions.
 
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Oskar

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Because power is such a strong function of rotor speed it was necessary to first do a bit of work on motor speed control.

The speed controller (governor in heli talk) works beautifully now. It operates like a Robbie governor, up to 80% speed the motor torque is controlled by the throttle, but then the governor kicks in and takes the motor speed up to 2450rpm (rrpm = 542rpm). As long as the throttle is kept wide open the speed doesn't change, to disengage the governor the throttle needs to be closed.

All power measurements from now on will be taken at a governed rrpm of 542rpm.

AUW of 190kg: Motor power = 20.7kW
AUW of 212kg: Motor power = 22.0kW
AUW of 224kg: Motor power = 23.6kW

Off to the beach to get 15kg more sand ;)

The big question is how much of this power is being eaten up by the tail rotor and tail rotor drive. Might have the answer to that question in a month or two.
 

grevis

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I guess the Air rotor hub is exactly the same as the XE/XE285/XET, just with shorter blades, which should mean it can operate at 590 rpm like the XE285 and XET? Might be useful to test if its not too hard, as it may mean a lot better tail authority when at the higher MTOW.
Gary
 

Oskar

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I guess the Air rotor hub is exactly the same as the XE/XE285/XET, just with shorter blades, which should mean it can operate at 590 rpm like the XE285 and XET? Might be useful to test if its not too hard, as it may mean a lot better tail authority when at the higher MTOW.
Gary
Gary,
As I've measured and Jean Claude has confirmed, rotor power requirements increase rapidly as rrpm goes up. When flying on batteries the biggest limitation is available power, which implies trying to keep the rrpm as low as possible. At some point tail authority will become an issue, but so far with 34kg of extra weight to simulate battery weight the tail is still very responsive. I'm hoping that the optimum rrpm will end up being less than 540 and not more.
 

Oskar

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I was curious to find out how much power the tail rotor uses. An easy way to measure that is to disconnect the mechanical tail rotor drive and replace the tail rotor drive with it's own independently powered motor. More than one motor makes matters even easier, as we're now talking standard off the shelf drone parts.

So here's the first flight with e-tail. Not all the motors had arrived yet, the maths said tail rotor authority would be marginal but that proved not to be the case.


Some positive features:

With the original 2 stroke motor there was so much vibration that the tail rotor drive shaft vibration never played a role. With the electric drive the tail drive shaft vibration becomes very obvious, I spent about a day trying to straighten the drive shaft to within 0.1mm which improved but never completely eliminated the vibration. Now the vibration is gone for good!!

Main motor power @190kg AUW is now 17.9kW compared to 20.7kW, the tail rotor was using nearly 3kW of power! Still have to measure the etail power accurately, but it will be less than half that.

Weight of the etail including battery is about the same as the mechanical tail rotor. So we end up with 3kW of free power for no weight penalty.

Because the tail rotor thrust is higher than the tail boom, the heli no longer lifts right skid first. It was an unusual feeling at the first lift-off, but could very well get used to that.
 

XXavier

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I'm not sure if the calculation below is right, but there are people in the RWF who may correct it if necessary...

Let's imagine we have a helicopter with tail rotor. The power supplied to the main rotor, that spins at 350RPM = 37 rad/s is 100kW = 100.000W

Let's sat that the horizontal distance from the axis of the main rotor to the axis of the tail rotor is 5m

The torque induced by the main rotor is 100.000/37 = 2730 N-m

in order to compensate that torque with a horizontal distance tail rotor axis-main rotr axis of 5m, we need a force of:

2730/5 = 546 N

There's a formula, based upon momentum theory, that relates power P, rotor swept area A and thrust T:

P^2 = (T^3)/2·rho·A

If the tail rotor has a diameter of 1 m => A = 0,78

Inserting values, P = 9210 W

That's 9% of the main rotor power, roughly...
 
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Smack

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Oskar, have you calculated how much power will be needed once the helicopter is out of 'ground effect' ?
GREAT work, please keep the updates coming.
Brian
 

Oskar

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Oskar, have you calculated how much power will be needed once the helicopter is out of 'ground effect' ?
GREAT work, please keep the updates coming.
Brian
Hi Brian,

Jean Claude has done that for various AUW (see posts 20 and 23). So far the measurement I've done correspond closely to his model, so I trust his numbers.

Oskar
 

Martin W.

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Oskar ... I completely enjoy following your experiments , thank you for sharing with us.

Reminds me of Arthur Young who invented the Bell 47 helicopter .... he would hover the prototype with the tail-boom tethered to a scale which provided the information for tail rotor thrust requirements . He also said he could determine actual horsepower of the Franklin engine using that method .... much to the chagrin of the Franklin factory who had the habit of overstating their engine horsepower.

Your tethered electric Mosquito would be an excellent low cost primary trainer for someone wanting to fly helicopters . 10 hours on a (lower cost) rig like that a person could then transition to an actual helicopter much easier.

Years ago a company with a sport helicopter dealership mounted a machine on a wheeled platform on scissor arms for initial hover practice ... they were called hangar36 or something ... have not heard from them lately .... your electric version would be more practical and lower cost.

Good luck with your inventing and testing .
.
 

Oskar

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Oskar ... I completely enjoy following your experiments , thank you for sharing with us.

Reminds me of Arthur Young who invented the Bell 47 helicopter .... he would hover the prototype with the tail-boom tethered to a scale which provided the information for tail rotor thrust requirements . He also said he could determine actual horsepower of the Franklin engine using that method .... much to the chagrin of the Franklin factory who had the habit of overstating their engine horsepower.

Your tethered electric Mosquito would be an excellent low cost primary trainer for someone wanting to fly helicopters . 10 hours on a (lower cost) rig like that a person could then transition to an actual helicopter much easier.

Years ago a company with a sport helicopter dealership mounted a machine on a wheeled platform on scissor arms for initial hover practice ... they were called hangar36 or something ... have not heard from them lately .... your electric version would be more practical and lower cost.

Good luck with your inventing and testing .
.
Martin,

Thanks for the kind words.

What Arthur Young achieved in his time is truly amazing, with the technology available today it’s so much easier to measure power.

The tethered electric Mosquito behaves just like the real thing in the hover, it would definitely be a good tool for learning to hover. As it’s set up now you just have to be very accurate and not wander off or yaw. The spot I use is far from ideal, it’s not flat and the slope increases as you move away from the building. The closest wall is less than 1m (3 feet) away from rotor blade tips, and there’s a pole that would be even closer to the tail rotor if the heli were to turn 60 degrees. Some would call it a “confined space”.

Oskar
 

Oskar

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I'm not sure if the calculation below is right, but there are people in the RWF who may correct it if necessary...

Let's imagine we have a helicopter with tail rotor. The power supplied to the main rotor, that spins at 350RPM = 37 rad/s is 100kW = 100.000W

Let's sat that the horizontal distance from the axis of the main rotor to the axis of the tail rotor is 5m

The torque induced by the main rotor is 100.000/37 = 2730 N-m

in order to compensate that torque with a horizontal distance tail rotor axis-main rotr axis of 5m, we need a force of:

2730/5 = 546 N

There's a formula, based upon momentum theory, that relates power P, rotor swept area A and thrust T:

P^2 = (T^3)/2·rho·A

If the tail rotor has a diameter of 1 m => A = 0,78

Inserting values, P = 9210 W

That's 9% of the main rotor power, roughly...
Xavier,

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

Advances in drone technology over the past few years have been absolutely amazing. What I really like is how much effort has gone into quantifying the thrust/power relationship for drone motors, any reputable motor manufacturer will have thrust vs power tables for different motors and props. The unit of measurement that the drone community has standardised on is grams per Watt (g/W). At low thrust the best motor/prop combinations approach 20g/W, with that number dropping as thrust increases. At maximum thrust the ratio is in the order of 5g/W. For optimal flight times drones typically operate in the 8 to 10g/W region.

The tail rotor thrust of the electric Mosquito at 190kg AUW is about 9kg, while the power consumed was measured as 2.8kW. That gives a thrust to power ratio of about 3.2g/W. With the new e-tail I’m aiming to achieve at least three times that. This will result in a power saving of around 2kW (nearly 10% of total power) which is huge for an electric helicopter.

Maybe there’s a good reason why electric tail rotors aren’t more common, they seem to be a perfect solution, especially for hybrid helicopters. Comments anyone?

Oskar
 

Martin W.

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Bell Helicopter is experimenting with an electric tail rotor.

.
 

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Smack

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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
 
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