X Prize Clean Aviation: $10 million

[mgb_phys]

For everybody else - there is an excellent and entertaining talk by Sebastian Thrun from Stanford's team.

http://video.google.com/videoplay?docid=8594517128412883394

 

[FredGarvin]

Is there a standard approach for characterizing the noise?

I am not quite sure what you mean by standard approach. There is the area of aeroacoustics in which we look at the noise contributions due to aerodynamic influences. It is a relatively new area but a fair amount is known. In this case, it was mentioned that loading of the blades is a factor, as is relative mach number of each blade and blade tip. The spreding/attenuation over distances is very custom for each application in that the speeds and frequencies are always different. Plus, since this is unshrouded you don't have issues arising from waveguides, i.e. ducts. Things are different when a blade tip is sonic or supersonic. I can only assume they would be sonic, which means they will naturally attenuate/decay with increasing radius from the source.

There is a FAR (I can not remember the number off the top of my head) that governs the testing of noise signatures around airports. It is pretty detailed and measurements are required to simulate an aircraft arriving and departing and at altitudes, of I believe, 400 ft. It has been a while since I have read that FAR. I think it wouldn't be a killer for this application, but it would definitely be a concern.

 

[mheslep]

For everybody else - there is an excellent and entertaining talk by Sebastian Thrun from Stanford's team.

http://video.google.com/videoplay?docid=8594517128412883394

Seen it many times.

Thrun's team setup next to ours in the semi-final, so I had the opportunity to talk him and his team over several days. He's a fantastic engineer, great sense of humor.

 

[mheslep]

I am not quite sure what you mean by standard approach.

I was asking for indulgence in explaining how characterization of prop acoustics - fundamentals. Googling wasn't providing any useful fundamentals. This is a start:

There is the area of aeroacoustics in which we look at the noise contributions due to aerodynamic influences. It is a relatively new area but a fair amount is known. In this case, it was mentioned that loading of the blades is a factor, as is relative mach number of each blade and blade tip. The spreding/attenuation over distances is very custom for each application in that the speeds and frequencies are always different. Plus, since this is unshrouded you don't have issues arising from waveguides, i.e. ducts. Things are different when a blade tip is sonic or supersonic. I can only assume they would be sonic, which means they will naturally attenuate/decay with increasing radius from the source.

Well eventually I wanted to estimate dBl / distance given some prop assumptions. E.g. vehicle developing ~1000 lbs thrust at take off, RPM x, tip speed sonic, etc, add in latest
prop noise reduction techniques, then compare with known noise of 200HP turbine or piston engine.

 

[mgb_phys]

Thrun's team setup next to ours in the semi-final, ... He's a fantastic engineer, great sense of humor.

Rather letting the side down, as a German engineering prof though - stereotype-wise ;-)

 

[mheslep]

Rather letting the side down, as a German engineering prof though - stereotype-wise ;-)

Well he's been in the US for a long time. Then, Einstein apparently had a silly streak.

 

[Brian_C]

Sorry I don't follow. What source of power are you suggesting, if not an electric motor? Edit: Perhaps you are asking: why use an electric motor instead of a traditional gas turbine? Electric motor can be >95% efficient, is quiet, as you mentioned above, and can run on electric charge that may have been generated on the ground and stored in the aircraft.

Post #2, for purposes of this prize. 11 metric tons of battery per 850 mile leg.

I would suggest obtaining a formal education in the subject before making such outlandish claims. That 95% efficiency won't do you much good if it comes with a huge weight penalty. Gas turbine engines are really unparalleled when it comes to thrust/weight ratio and energy density.

 

[mheslep]

I would suggest obtaining a formal education in the subject before making such outlandish claims.

Thanks, I think I have a pretty good one, and a license to use it, though it is limited on the subject of aerodynamics. To be constructive, could you show how those numbers (11 metric tons / 850 miles) are outlandish? I think I've crunched and shown the basic numbers if you backtrack the post.

That 95% efficiency won't do you much good if it comes with a huge weight penalty. Gas turbine engines are really unparalleled when it comes to thrust/weight ratio and energy density.

Gas turbines are not an option for the prize, hence the thread topic "CLEAN AVIATION". Please see the https://www.physicsforums.com/showpost.php?p=2535479&postcount=1":

Superconducting motors operating with almost no losses could achieve power densities of 10–20kW/kg (and perhaps even higher) and torque densities of more than 35Nm/kg (compared to 10Nm/kg for the best conventional motors), the US researchers say.

Edit: maybe gas turbines hold the trophy for thrust/weight; the above challenges power/weight.

Maybe also see another thread for existing HTS propulsion research.
https://www.physicsforums.com/showpost.php?p=2292842&postcount=14

 

[Cyrus]

I took issue with a statement made by many of these articles: namely, that you don't need air intakes because it's electric. This is hog-wash because anyone that has flown an electric airplane knows they get very hot (battery, motor, and electronics). Cooling them is absolutely an issue. I do not buy that they don't need 'air intakes' for this vehicle. There will be significant power losses in the form of $$i^2R$$.

Also, how is this thing 'stealth' with big spinning rotors?

 

[mheslep]

I took issue with a statement made by many of these articles: namely, that you don't need air intakes because it's electric. This is hog-wash because anyone that has flown an electric airplane knows they get very hot (battery, motor, and electronics).

You mean RC airplanes? That's the 'because'?

Cooling them is absolutely an issue. I absolutely do not buy that they don't need 'air intakes' for this vehicle. There will be significant power losses in the form of $$iR^2$$.

I think the point here is that the air required for combustion, on the order of liters per second per HP*, completely dwarfs any air flow required for dissipating the couple hundred watts from this one man aircraft. Given the stated battery load (45kg) for Moore's VTOL, the [STRIKE]maximum[/STRIKE] continuous power is probably ~11kW (15HP). Though a small intake would suffice, no air intake at all is required dissipate 5% of that power as heat, just air flow over sufficient dissipative surface. I've built enclosed electronics boxes that conduct/convect away 500W from a two cubic foot box.

*quick math for gasoline combustion at 30HP requires 1 liter O2/sec, 5 liters air/sec, STP

Also, how is this thing 'stealth' with big spinning rotors?

How does one calculate the dBls / distance from these rotors?

 

[Cyrus]

How does the fact that a UAV is RC scale, or large scale, change the fact that there *will* be power losses by the engines that pose a thermal problem?

Please explain why you think this is not the case.

Also, what calculation did you do to get this 15HP number from? I don't believe this for one second.

Also, I did not say 'quiet' I said 'stealth': as in, radar cross signature.

 

[Cyrus]

Let's do a simple calculation using the momentum theory equation of an *ideal* rotor. A real rotor will inevitably require more power:

$$P = \frac{T^{3/2}}{\sqrt{2 \rho A}}$$

Assuming:

$$\rho = 0.002378$$ slug ft^-3
$$A = 28.27$$ Assuming a generous 6' rotor diameter
$$T = 300lbs$$ (Assume 600lb GTOW)

That gives a power of about 25.7HP, very close to the claim on SIAM article of 60HP total (30HP each)!

 

[mheslep]

How does the fact that a UAV is RC scale, or large scale, change the fact that there *will* be power losses by the engines that pose a thermal problem?

Please explain why you think this is not the case.

Because the efficiency of electric motors and battery packs varies considerably. Larger motors can hit 95-98% efficient, but it is difficult to do that at small scale. Also I wouldn't imagine there is much room or effort made for thermal dissipation made on an RC, at least not the couple small ones I've toyed with. Small motor/battery mounted on poor heat conductors - plastic/composite - so yeah they'll get hot.

Also, what calculation did you do to get this 15HP number from? I don't believe this for one second.

SIAM said Moore's using a 45 kg battery load. Li Ion puts out http://en.wikipedia.org/wiki/Lithium-ion_battery" continuous (above I said maximum and corrected that to continuous) Edit: maximum battery discharge rate is typically be 10x (250W) for very short periods, i.e. peak up to 150HP. The motor would undoubtedly be the limiting factor - SIAM suggests it is 60HP.

Also, I did not say 'quiet' I said 'stealth': as in, radar cross signature.

Wasn't the 'stealth' reference in SIAM was on thermal signature? Obviously electric will be low compared to combustion.

Yes:

n addition, since electric motors are so efficient, they also generate far less heat. This not only gives them a lower thermal signature for military stealth, but means they don't need anywhere near the same amount of cooling air flowing over them that internal combustion engines do,...

 

[mheslep]

Let's do a simple calculation using the momentum theory equation of an *ideal* rotor. A real rotor will inevitably require more power:

$$P = \frac{T^{3/2}}{\sqrt{2 \rho A}}$$

Assuming:

$$\rho = 0.002378$$ slug ft^-3
$$A = 28.27$$ Assuming a generous 6' rotor diameter
$$T = 300lbs$$ (Assume 600lb GTOW)

That gives a power of about 25.7HP, very close to the claim on SIAM article of 60HP total (30HP each)!

Thanks. How does one estimate the acoustic output of such a prop?

 

[Cyrus]

Thanks. How does one estimate the acoustic output of such a prop?

Want a PhD? Hehehe, honestly, I think its all empirical. The equations look like the NS equations: no closed form solutions.

 

[mheslep]

For illustration:

NEMA electric motor efficiency requirements
Power
(hp) Minimum Nominal Efficiency
1 - 4 78.8
5 - 9 84.0
10 - 19 85.5
20 - 49 88.5
50 - 99 90.2
100 - 124 91.7
> 125 92.4

I suspect the typical RC motor is 0.1 to 0.01 HP. You get the idea.
http://www.engineeringtoolbox.com/electrical-motor-efficiency-d_655.html

 

[mheslep]

Want a PhD? Hehehe, honestly, I think its all empirical. The equations look like the NS equations: no closed form solutions.

Yes silly question but I thought some rough bounds could be put on the maximum noise. For instance, if 60HP is converted completely to audible sound (of course its not) how loud is that, assuming no resonance or amplification.

 

[Cyrus]

Yes silly question but I thought some rough bounds could be put on the maximum noise. For instance, if 60HP is converted completely to audible sound (of course its not) how loud is that, assuming no resonance or amplification.

I'll ask someone I know about a relationship to noise. I think its something like the log of the disk loading.

 

[Cyrus]

For illustration:

NEMA electric motor efficiency requirements
Power
(hp) Minimum Nominal Efficiency
1 - 4 78.8
5 - 9 84.0
10 - 19 85.5
20 - 49 88.5
50 - 99 90.2
100 - 124 91.7
> 125 92.4

I suspect the typical RC motor is 0.1 to 0.01 HP. You get the idea.
http://www.engineeringtoolbox.com/electrical-motor-efficiency-d_655.html

On takeoff, my motor would draw almost ~750W. The speed controller was rated at 100A max.

 

[mheslep]

On takeoff, my motor would draw almost ~750W. The speed controller was rated at 100A max.

What's rated power of the motor? That's a nice thing about electrics - they can peak well above rated (5x at least).

 

[Cyrus]

http://www.allerc.com/z40all.htm

~1100W

 

[mheslep]

http://www.allerc.com/z40all.htm

~1100W

I don't see efficiency or output power, only input, but its still impressive. 3kW/kg (input) continuous in 1.9" OD case. That compares to, say, 4kW/kg (output) in the much larger Tesla AC induction motor.

 

[FredGarvin]

You asked for it. I scanned this in from one of my references. The mB notation refers to the mth harmonic/mode and B is the number of blades. The rest should be somewhat decipherable.

 

[Cyrus]

I don't see efficiency or output power, only input, but its still impressive. 3kW/kg (input) continuous in 1.9" OD case. That compares to, say, 4kW/kg (output) in the much larger Tesla AC induction motor.

Boy let me tell you, those guys get HOT.

 

[FredGarvin]

So do the lithium batteries most of them use for power.

 

[mheslep]

You asked for it. I scanned this in from one of my references. The mB notation refers to the mth harmonic/mode and B is the number of blades. The rest should be somewhat decipherable.

Thanks Fred!

 

[FredGarvin]

BTW...This all stems from the basic wave equation. I sat through many a proofs showing this stuff and it is amazing to see just how much can be pulled from such a seemingly unimposing equation. You may want to look up Ffowcs Williams for a reference.

 

[Cyrus]

Bessel function, wave equations? Eeeeuccccwkkkk!

Quick, something more pleasing to the ear: Laplace transforms, Bode Diagrams, Nyquist plots, PSD, Autocorrelations...ahhhhhhhhhhh. I can breathe again, whew.

 

[Cyrus]

I'll tell you what, if you guys want to formulate a list of questions, I can email them to Dr. Moore and report back his answer. I'd prefer to keep the list short, so think of good ones.

 

[FredGarvin]

Quick, something more pleasing to the ear: Laplace transforms, Bode Diagrams, Nyquist plots, PSD, Autocorrelations...

:zzz: :zzz::zzz: huh...wha?...hmmm...:zzz: :zzz::zzz:

 

[mheslep]

So do the lithium batteries most of them use for power.

A good Li Ion is ~95% efficient on discharge at room temperature (unlike lead acid), so Cyrus's RC battery would shed 55W of heat on take off, probably 5W cruise. In a small package w/ no thermal path, yes I imagine that gets very hot. If so, RC'ers might consider measures to cool them, as running a Li Ion hot will degrade its lifetime.

 

[FredGarvin]

All of the ones I have used and seen have no external means for cooling other than some form of exposure to the outside air.

 

[mheslep]

You asked for it. I scanned this in from one of my references. The mB notation refers to the mth harmonic/mode and B is the number of blades. The rest should be somewhat decipherable.

Think I have it all except for theta, the 'retarded radiation angle'? I would expect the SPL is not isotropic, so maybe this is addresses the acoustic pattern?

 

[mheslep]

I'll tell you what, if you guys want to formulate a list of questions, I can email them to Dr. Moore and report back his answer. I'd prefer to keep the list short, so think of good ones.

On the subject of prop/fan noise, I expect that his answer would concur w/ the article which he probably informed:

At up to 95 percent efficiency, electric motors are far more efficient than internal combustion engines, which only rate some 18 to 23 percent. This means electric aircraft are much quieter than regular planes—at some 150 meters, it is as loud as 50 decibels, or roughly the volume of a conversation, making it roughly 10 times quieter than current low-noise helicopters.

You gents suggested otherwise upthread, so first I want to run the numbers from Fred's Propeller and Fan Noise reference; then we can ask a more informed question.

 

[FredGarvin]

That is not a proper statement. It should read that the ENGINE noise is lower, not the overall aircraft noise. You can not change the acoustic performance of a prop by changing the thing that drives it. The prop noise will still be there.