Jul 13, 2005, 09:22 AM
Registered User
Question

## Efflux velocity from thrust & FSA?

Seeing that EDF's don't compress the air to any significant extent, can you calculate efflux velocity using measured thrust and the swept area of the fan?

Intuitively this makes sense, as you know the surface area the air stream must be passing through, and you know the flow rate...

I suspect this might be more accurate than some of the mathematical "rotor pitch" models I've seen.

(friction losses and turbulence due to the tube area changing past the fan will probably result in the velocity value being lower than reality, but it should be a pretty good estimate)

Ideas?
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 Jul 13, 2005, 12:02 PM Senior Member Don't think you can do this, as the flow rate is dependent on the efflux velocity, and there is some compressability. While the thrust and efflux velocity for any given fan will both be roughly linear with RPM, the constant for efflux will depend on the pitch of the blades. So while you could equate efflux velocity with thrust for a given fan, it's easy to imagine two fans of nearly equal FSA (e.g., the Kyosho fan and the Minifan) with different efflux velocities at the same thrust levels.
 Jul 13, 2005, 12:16 PM Dieselized User Yes you can. All you need is exit area(fan area has nothing to do with it) and thrust. It would give you an average velocity. You should take into account boundary layer and stuff but for a quick idea, exit area is fine. I don't have a simple formula with me now. Later tonight. Maybe Klaus or Eric will beat me to it. Yeah, the rotor pitch stuff is garbage. Greg
Jul 13, 2005, 12:28 PM
Member
Exit area, not FSA!

By measuring input electric parameters, thrust, and Eflux velocity you characterize the efficiency of the “system” and components.

This was a fun exercise with the Kyosho T-33; i.e. stock fan, different impeller/roto (Kress AP-1), and different EDF units (MF480 & HW609).

# Images

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Jul 13, 2005, 01:34 PM
EDF rules... :)
Greg is right, there are compressor calculations that will perform this quite adequately even down to the horsepower needed to make the thrust needed. I do have a calculation for it and am hessitant to give it out due to the fact that it is proprietary to the company I work for. Another soruce of the calcs can be found in the form of seperate calculations and put together to give aproximately the same answers. The last would be Klaus's spreadsheet for his fans, it works pretty darn good.

Cheers,

Eric B.

Quote:
 Originally Posted by gkamysz Yes you can. All you need is exit area(fan area has nothing to do with it) and thrust. It would give you an average velocity. You should take into account boundary layer and stuff but for a quick idea, exit area is fine. I don't have a simple formula with me now. Later tonight. Maybe Klaus or Eric will beat me to it. Yeah, the rotor pitch stuff is garbage. Greg
Jul 13, 2005, 01:43 PM
Registered User
Quote:
 Originally Posted by sguty So while you could equate efflux velocity with thrust for a given fan, it's easy to imagine two fans of nearly equal FSA (e.g., the Kyosho fan and the Minifan) with different efflux velocities at the same thrust levels.
As pointed out by the guys above, this is an error in logic...

If you have the same exit diameter and the same thrust, then by definition, you have the same efflux velocity. (thanks for the area clarification, makes perfect sense now...)

I have proven this and other things with my research into 64mm fans, and the common misconception that any one rotor design is dramatically more efficient than another. (they aren't...)

Certainly, some rotor designs are more suited to higher power levels, but that's more of a materials issue than actual design.

It always kills me when I see comments like: "The Vasa 55 will give you much more thrust than an EDF-55.." and "The MiniFan has a much higher efflux velocity..."

Uh, no!

Assuming that you don't run the EDF-55 to a power level where the blades start to deform, both it and the Vasa 55 will both produce a nearly identical amount of thrust (and therefore efflux velocity) for a identical input power.

With EDF's (and props, really), it's all about power to the shaft.

Taking something like current 75mm fan designs; amps, volts and RPM are simply variables that change with the rotor design and the motor in question. (this is critical: be careful not to interpret an inefficient/inappropriate motor as an "inefficient" fan design)

Given the same shroud ID, the only variable in an EDF rotor output is effective pitch. To generate the same thrust (equivalently, power), higher pitch equates to lower RPM, more torque; lower pitch higher RPM, less torque.

With a given power input (in Watts), contrary to the largely held belief, the efflux velocity has nothing to do with RPM. (I guess it's easy to fall into the flawed logic trap of thinking that: "Fan X spins much faster than Fan Y, therefore it must spit out the air faster...")

An extreme example of this is the 'old' Kress rotors with a hyper-aggressive 7-blade design, as compared to a Kyosho rotor with its nearly-flat 5-bladed design. The Kress rotor will require massive torque to spin it, but it'll move air at nearly the same rate (thrust/velocity) as the Kyosho at an equal power level, and assuming appropriate motor selection. (the Kress fans require so much torque, they really need outrunner motors to work!)

The end result of air moving out of the exit is exactly the same.

FYI, there are "dynamic" considerations, which can make a certain rotor design more efficient in a flight environment, but most EDF aircraft fly well below the speed where these factors become significant.

Although this is full-scale, a lot of info here is very relevant: http://www89.pair.com/techinfo/MassFlow/ductbook.htm

This is a subject with a lot of myth, misinformation and misunderstanding stubbornly floating around out there!

Fun!
Last edited by meteor; Jul 13, 2005 at 02:21 PM. Reason: edits
 Jul 13, 2005, 02:19 PM Dieselized User Yes, that's basically what is comes down to. I do not agree with this: "Certainly, some rotor designs are more suited to higher power levels, but that's more of a materials issue than actual design." Actual design IS were efficiency is gained. Materials has to do with the intended price and power range. Yes, a lot of people are ingnorant of physics. Ducted fan stuff is not easy to grasp for many. Greg
Jul 13, 2005, 02:34 PM
Registered User
Quote:
 Originally Posted by gkamysz ...Actual design IS were efficiency is gained. Materials has to do with the intended price and power range.

Agreed, but I think that vast majority of EDF'ers out there assume that the magnitude of "efficiency" gains are far higher than what the reality is.

We've been through this before, and many EDF designers have agreed that differences of 2%-5% are considered "substantial", from their point of view.

But, for the everyday modeler this difference is negligible.

You'd be far better off spending money on better batteries or motors (in particular), than a supposedly more "efficient" fan unit.

Without a doubt, for those who push extraordinary power (say 500W+ in a 75mm fan), material quality will come to the forefront, and the money is well spent.

But, these people are a small minority of all EDF'ers out there...

It's all about focusing on what is relevant to the bigger picture, not getting mired down in trivialities.
Last edited by meteor; Jul 13, 2005 at 02:39 PM. Reason: sp.
 Jul 13, 2005, 03:03 PM Dieselized User I look at things from the design side. Trivialities are important. Greg
 Jul 13, 2005, 03:03 PM Registered User Meteor If you want to know efflux velocity, why not measure it, with a wind meter?
Jul 13, 2005, 03:14 PM
Registered User
Quote:
 Originally Posted by peterangus If you want to know efflux velocity, why not measure it, with a wind meter?
That would work fine, but there's a lot of good published data out there that list thrust, but there's no mention of efflux velocity, or it's just a crudely derived number.

Generally speaking, thrust is what I call a "multi-use" parameter, like power. (in watts).

Measurements like volts, amps and RPM are useful, but they in themselves don't tell you a whole lot, other than the motor's operational parameters.

If you know the thrust, you can reasonably accurately derive other parameters, like efflux velocity.
 Jul 13, 2005, 08:04 PM EDF rules... :) This is good discussion on performance parameters and how they are defined. The fan itself operates by the volume of air it displaces for every rpm of the rotor, the pitch does come into play because the rotor moves air directly in relationship to it, The airfoil of the rotor blade becomes important only for the amount of drag it can overcome not its lift coeficient, so just like any engine it is the displacement of the fan that governs performance, to a point. It is modified by the outlet diameter of the outlet tube as this can effect the efflux velocity quite a bit both to squelch or enhance depending on how it is used and this is even modified by the the fan itself depending on how it handles compression after the rotor. The motor like you have stated is just adapted to fit the needs of the fan for the performance level we seek. Cheers, Eric B.
 Jul 13, 2005, 08:31 PM Senior Member [QUOTE=meteor]As pointed out by the guys above, this is an error in logic... Actually, no, Brian, if you re-read what I said, I was still talking about your initial post regarding FSA. I agree with Greg etal that if the exhaust area is equal, equal thrust implies equal efflux velocity. But, if you're talking FSA alone, there's no direct correlation. Heck you could rig two identical fans with different exhaust duct areas--say one at 90% FSA and the other at, say 60%. If you pump equal power into the 60% FSA exhaust and the 90% FSA unit, you'd have a higher velocity from the more constricted unit than from the 90% FSA one, because if you're moving the same mass of air through a smaller aperture, it has to move faster, right? Last edited by sguty; Jul 13, 2005 at 08:40 PM.
 Jul 13, 2005, 09:47 PM Registered User Sorry Steve, musta mis-read the context... But, after this interesting side-bar, does anyone have the math to figure out the efflux velocity from exit nozzle and thrust? Thanks.
Jul 14, 2005, 06:03 AM
Micro Jet Pilot

## Inlet area

I have read the ducted fan design design article mentioned above. Now I am confused. I build a L-39 Albatros scale model with 38cm wingspan. This results in inlets only 30mm in diameter. I am using a microfan (50mm). Inlets seem to be way too small but they are scale. What should I do?

Found out that a cheater hole increses thrust but that hole would be on the bottom side in the wing. I fear it would suck my plane down when powered up. My question is, if the inlet becomes more efficient at high cruise speed since the air is pushed into the duct. (maybe the effect of cheater hole is gone at high speed).

Indeed the duct in front of the fan is a diffusor, meaning that the cross section increases downstream (even more than fan diameter). What can I do to fix this problem. Or might this be a design efficient at high speeds??

BTW: Outlet is 35mm
Last edited by Chris T; Jul 14, 2005 at 08:53 AM.

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