Jet sound from EDFs

[RonF]

I know there has been a lot of chatter about the 10-12 blade fans and the sound they make. I've done a fair amount of testing and reading on air flow and the function of an EDF unit. I also have an advanced degree in physics and I'm very familiar with particle motion and sound waves. What people seem to be forgetting is that the "vacuum cleaner" sound is mostly coming from the air flow over the sharp edge of the fan shroud and not the motor itself. Just run the motor with no fan and you don't hear any high pitch noise. If you spend some time listening to some of the YouTube videos, you'll hear that some planes sound good and other don't. Sure, the 10-12 blades do dampen the noise and helps smooth out the air flow thus providing that neat jet sound, but I've gotten the same with the 5-blade fans. Take a fan unit which has the flux ring included (this is the rounded piece you can put on the intake of the fan unit). Try the fan both with and without this and you'll hear a big difference.

Case in point. I've put a 80mm fan unit into a plane that was designed for a 90mm fan. This fan unit was from my crashed Habu 32 which did have a high pitched sound when flying. I used my lathe and turned an adapter ring to fit the fan shroud with the outside diameter fitting the 90mm duct of the new plane. This adapter has a rounded inside shape and smooths the air flow between the intake ducts and the fan. The plane sounds more like a turbine than some turbines I've hear and has most people believing it is a turbine until I show them the insides. One other thing I should point out, and most EDF owners already know this, a 10-12 blade fan pulls more current than a 5 blade fan with the same motor and battery. The 10 blade fan creates more thrust but not much extra speed. For me at least, I'd take a little less thrust and gain another 30-60 seconds of flight time with no real loss in speed.

So, if you can get to the fan unit and smooth out the transition between the duct to the fan, you might be surprised to find that much of the vacuum cleaner sounds goes away. It is always a good idea to clean up all of the ducts and edges that the air has to pass by so this is a nice benefit in both duration (less current draw) and a more pleasing sound. I'm sure there will be a few folks that may dispute this or say this has little effect. All I can say to try it first. I have and I have a couple of EDF jets to prove (at least to me) this is a valid point. Hope this helps at least one person, if not in the enjoyment of a better sounding plane, but to learn a little bit by doing some tests themselves. Cheers

[StarHopper]

Re: Jet sound from EDFs

Gold.

Ever taken a look at effects of a 5-blade vs a 5-blade with wider blades?

[RonF]

Re: Jet sound from EDFs

No. It's tough to find specific info on fan units so my observations are based on reading, my long ago schooling, common sense, and playing with different fan units I've aquire over the last 10 years or so. But it would make sense with all else being equal, wider blades mean more thrust. That assumes that the pitch and rpm are similar. This is where doing thrust tests and using a watt meter are needed. My simple thrust test bed is not good enough to deliver high accuracy for such differences.

[RonF]

Re: Jet sound from EDFs

I have a bit more information to add to this topic, mainly in response to an email I got from a fellow member. The question was, "Can I modify an existing edf so it can produce a more jet-like sound?"

This got me thinking and I did some tests with an EDF unit, when mounted in a specific plane, sounded very jet like. The plane is no longer flying so the fan unit is in my parts box. I took it back out and ran it up on my test bench without any extras to see how it sounded. I got a nice, high pitched, sound with almost no sounds from the air flow into or out of the fan unit. This fan has a metal shroud and 5 blades with a nice rounded intake lip on the shroud. There is about 1 cm. of space from the intake's leading edge to the fan blades. So I know the "jet sound" is not coming from the fan unit by itself but the fan is the cause of the high pitch whine one expects from EDF units.

The next part of the test was to take a bit of PVC pipe I had around which was the same size as the inside diameter of the fan. I turned the end of the pipe so the fan unit would fit into it and have a very small exposure of the joint to disturb the air flow. I tried the fan again with this pipe acting as the intake duct. I did get a bit of the jet sound I was looking for but also got a bit more turbulence going into the fan so the high pitched sound also increased a small amount.

What this is telling me, and what I assumed all along, is that the jet sound everyone is wanting is coming from the intake and not from the fan unit itself. If you take a shop vac and put your thumb over the hose while its running, you will hear that distinctive jet sound as the air flows past your thumb. Take away that obstruction and the sound goes away. To get a nice jet sound from an EDF jet, two things need to take place.

First, one must eliminate as much turbulence from the air entering the fan shroud as possible. That is what I described in my first post here. If you can cover the area between the fan shroud and the intake ducts so that there is no sharp edges, you can take out much of the noise generated by the fan blades hitting turbulent air and take out much of the whine that this produces.

The second thing that needs to happen, and this is much harder, is to have the intakes be of such a size as to constrict the air flow enough to create that nice "whoosh" sound that is loud enough to mask the whining sound. The constriction needs to be small enough to create the low pitched sounds but not so constricted as to hinder the fan unit's ability to move air.

What I think is happening with the 10-12 blade fans that creates more of a jet sound is this:
First, the addition of more blades increases the thrust which is directly related to the amount of air that is being ingested. More air through the same openings equates to a constriction and yields the whoosh sounds.
Second, more blades reduces the space between blades which reduces the amount of turbulence they have to move through. This reduces the whining noise created by the fan unit. It does not eliminate it but it does seem to reduce the volume of this part of the sound signature.

Those EDF jets that seem to have a better jet sound than others have these two things working in their favor. They have enough air flow into the ducts to cause the whoosh. They also have a nice fit of the fan unit into the plane as to minimize the whining sounds. Together, more whoosh and less whine means a nicer jet sound. Its all relative and seems to be a crap shoot when dealing with foam jets. Some got it, and some don't. Even with the same plane, some sound better than others.

I wish I could provide a detailed process to make any EDF jet sound more jet-like but I can't. There are just too many variables: the length and shape of the intake ducts, the fit of the fan unit to the airframe, what brand of fan unit, the rpm the fan is turning, the length and shape of the exhaust tube, and some I don't know of. Play around with what you got and be safe. I would rather have a vacuum cleaner I can fly than a great sounding jet I can't.

I hope this clarifies my comments and how I determined the results. Without a wind tunnel and test equipment, I can only guess as to what is truly happening. Using what I observe and what I know from experience and book learning, I seem to have a reasonable handle on this. If anyone has better data or additional information they can provide, I welcome it. I know a lot of things, as do many other members here, but that does not mean that what I think I know is actually correct.

Cheers.

[Sovrin]

Re: Jet sound from EDFs

Amazing info RonF.....puts a whole new perspective on EDF units, Great work

[killickb]

Re: Jet sound from EDFs

Great info Ron! Now when you have a spare hour or so (??) it would be great to have a prioritized listing of what to look at first, second, third etc, on a new foamy model, as to those items that could be modified or reworked (with zero or minimal $ outlay) to improve sound while maintaining standard performance -- is that a possible listing to develop?

I say "foamy model" because there probably isn't much you can do to say a Violet or other high end glass EDF model to change and/or improve.

I have some of Don's Wicked fan/motor upgrades that while high performing are just to high pitched to fly at our field anymore and we are in the middle of nowhere! My Freewing models are certainly the best in terms of woosh without performance degradation.

[dahawk]

RE: Jet sound from EDFs

Alpha,

Great explanation. The only thing I didn't see in this discussion was " balancing" . A well balanced fan will perform much smoother than a poorly balanced fan. I've found that a lot of the stock fans that come with these edf's are poorly balanced. Have a friend with a Vortex dynamic balancer and Man, does that make a world of difference in performance.

My humble 2 cents worth...

-Dave

[trying2fly]

RE: Jet sound from EDFs

I have a bit more information to add to this topic, mainly in response to an email I got from a fellow member. The question was, "Can I modify an existing edf so it can produce a more jet-like sound?"

This got me thinking and I did some tests with an EDF unit, when mounted in a specific plane, sounded very jet like. The plane is no longer flying so the fan unit is in my parts box. I took it back out and ran it up on my test bench without any extras to see how it sounded. I got a nice, high pitched, sound with almost no sounds from the air flow into or out of the fan unit. This fan has a metal shroud and 5 blades with a nice rounded intake lip on the shroud. There is about 1 cm. of space from the intake's leading edge to the fan blades. So I know the "jet sound" is not coming from the fan unit by itself but the fan is the cause of the high pitch whine one expects from EDF units.

The next part of the test was to take a bit of PVC pipe I had around which was the same size as the inside diameter of the fan. I turned the end of the pipe so the fan unit would fit into it and have a very small exposure of the joint to disturb the air flow. I tried the fan again with this pipe acting as the intake duct. I did get a bit of the jet sound I was looking for but also got a bit more turbulence going into the fan so the high pitched sound also increased a small amount.

What this is telling me, and what I assumed all along, is that the jet sound everyone is wanting is coming from the intake and not from the fan unit itself. If you take a shop vac and put your thumb over the hose while its running, you will hear that distinctive jet sound as the air flows past your thumb. Take away that obstruction and the sound goes away. To get a nice jet sound from an EDF jet, two things need to take place.

First, one must eliminate as much turbulence from the air entering the fan shroud as possible. That is what I described in my first post here. If you can cover the area between the fan shroud and the intake ducts so that there is no sharp edges, you can take out much of the noise generated by the fan blades hitting turbulent air and take out much of the whine that this produces.

The second thing that needs to happen, and this is much harder, is to have the intakes be of such a size as to constrict the air flow enough to create that nice "whoosh" sound that is loud enough to mask the whining sound. The constriction needs to be small enough to create the low pitched sounds but not so constricted as to hinder the fan unit's ability to move air.

What I think is happening with the 10-12 blade fans that creates more of a jet sound is this:
First, the addition of more blades increases the thrust which is directly related to the amount of air that is being ingested. More air through the same openings equates to a constriction and yields the whoosh sounds.
Second, more blades reduces the space between blades which reduces the amount of turbulence they have to move through. This reduces the whining noise created by the fan unit. It does not eliminate it but it does seem to reduce the volume of this part of the sound signature.

Those EDF jets that seem to have a better jet sound than others have these two things working in their favor. They have enough air flow into the ducts to cause the whoosh. They also have a nice fit of the fan unit into the plane as to minimize the whining sounds. Together, more whoosh and less whine means a nicer jet sound. Its all relative and seems to be a crap shoot when dealing with foam jets. Some got it, and some don't. Even with the same plane, some sound better than others.

I wish I could provide a detailed process to make any EDF jet sound more jet-like but I can't. There are just too many variables: the length and shape of the intake ducts, the fit of the fan unit to the airframe, what brand of fan unit, the rpm the fan is turning, the length and shape of the exhaust tube, and some I don't know of. Play around with what you got and be safe. I would rather have a vacuum cleaner I can fly than a great sounding jet I can't.

I hope this clarifies my comments and how I determined the results. Without a wind tunnel and test equipment, I can only guess as to what is truly happening. Using what I observe and what I know from experience and book learning, I seem to have a reasonable handle on this. If anyone has better data or additional information they can provide, I welcome it. I know a lot of things, as do many other members here, but that does not mean that what I think I know is actually correct.

Cheers.

[Bandiferous93]

So in this sense, we realize that part of that "whoosh" or "rush" we like to hear, is actually a sign of inefficiency. Ironic, huh?

Just like "blade farts" when flying helis. Sounds cool, but inefficient.

[xviper]

Realizing that this thread started almost 5 years ago, EDF tech has changed tremendously and some of the things said back then no longer apply. EDF planes today come right out of the box already sounding like a "real" jet and some people even install Nerf whistles to get that extra bit of "realism". Blade count, blade pitch, motor speed, fan size, all contribute to giving a desired sound and whether or not efficiency is affected, quite frankly, nobody really cares. Today's EDFs sound great and propel the plane like a scalded cat, generally as fast as most people want and for some, maybe even faster than they can handle. They even come with "upgrade" versions of the same plane, with more punch, more speed, more grunt, a different sound. So, if efficiency is hampered, I don't see many people complaining about it.

[Alpha]

Holy thread-resurrection, Batman!

You bring up a really good point, X-viper: Most people don't care and/or don't detect enough of an efficiency loss to demand high efficiency three blade fans anymore. Nine blades is a fair compromise these days, and twelve is often chosen, anyway. As you astutely pointed out also, the overall user experience of modern EDFs has been elevated by improvements on many different facts of design. Ducts, wings, power systems, battery space, dollar-per-smile, etc. We infuse our newer understandings of acoustics into each new release to offset what the math says about strict output efficiency.