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Old Jan 07, 2016, 02:19 PM
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I may be wrong but.......

I have two scratch built scale EDFs where to keep both a scale inlet and exhaust I have placed the EDF unit right at the back rather than buried in the fuselage.
I believe this position gives the lowest duct losses for a given length and diameter of duct and provides an exhaust nozzle that is exactly the FSA without using a 'reducing area' thrust tube.
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My big complaint of the thrust tube is the sudden change in area from the FSA to the EDF area which coupled with the turbulence from the bluff end of the motor significantly reduces the speed of the airflow which than has to be speeded up again by the end of the tube. This process adds back pressure to the fan reducing the average exit velocity ie thrust!
The fan at the back has no thrust tube the fan exits directly. Yes there is still the drag of the bluff end of the motor but it has no effect on the fan exit velocity.
So the hypothesis is that the 'fan at the back' has lower total duct losses and a higher exit velocity than a more conventional mid duct arrangement.
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Old Jan 07, 2016, 03:36 PM
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In the bottom pic scenario you better off moving the unit 3-4 inches fore ward and adding tapered thrust tube for optimal performance. But than again it's all up to you wanting either static or dynamic performance.
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Old Jan 07, 2016, 05:01 PM
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What degree of thrust tube reduction is normally used for optimum dynamic performance?

It will depend on the nozzle area chosen but do I suspect what is achieved (with some reduction in the mass flow) is to restore the fan exit velocity. The 'fan at the back' already has the full fan exit velocity with no restriction to the airflow.
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Old Jan 07, 2016, 05:29 PM
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That doesn't mean you will get an optimum performance. Naturally, every power unit is different and it's trial and error by spending some time testing various set ups, and comparing results, it's what I normally do before final installation.
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Old Jan 07, 2016, 05:43 PM
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usually 85% to 90% of the fan shroud size works best ....you also don't want your Fan all the way back and your batt all the way fwd to get the plane to CG .it will cause issues with long heavy wires ,and voltage spikes. you want your center of mass closer to the C/G it will fly a lot better that way . shorter inlets usually work better than a short exhaust. Some fan don't deal with sucking air in through super long ducts . more performance is lost through long inlets than long exhaust.
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Old Jan 07, 2016, 06:14 PM
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Interesting
I would agree there are issues with the weight (and voltage drop) of long wires and possible inertia effects on the plane but I suspect it only becomes significant in tight aerobatics.
Spikes are not an issue if the ESC is kept close to the battery. The motor wires can be very long without damaging the ESC.
An 85% reduction in the thrust tube is for many EDFs actually just reducing the nozzle area to the FSA!
"Some fans don't deal with sucking air in through super long duct" Do we know why that might be?
Surely inlet ducts can be made with a larger area to reduce unit losses without detriment to the EDF's performance whereas the exhaust tube cannot.
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Old Jan 07, 2016, 08:42 PM
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inlets need to be a constant area ...if it gets bigger the air slows down. if smaller it can speed up or get choked off if it is too small. do what you think is right .....but look at all the other successful EDf jets out there ,few if any have the fan all the way in the rear of the plane...
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Old Jan 08, 2016, 05:55 PM
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Quote:
An 85% reduction in the thrust tube is for many EDFs actually just reducing the nozzle area to the FSA!
I'm not sure what your trying to say here.........

But - I think your talking about the fan (shroud) size because you need to shink the exit diameter a bit anyway. This is needed because the FSA (fan swept area) is the area of the shroud minus the area of the motor. Meaning that in order to have a 100% FSA exhaust - you need to shrink the exhaust opening a little bit. Add a little more to that for efflux recovery and usually you end up with an exhaust diameter 85 - 90% smaller than the shroud.

To make the exit the same size as the shroud (or in your case no exit) the air is expanding and your losing thrust.
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Old Jan 08, 2016, 06:40 PM
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The aft placed fan has some advantages and disadvantages. I built a few long ago to test a some theories and was, for the most part, pleasantly surprised. The down side would be the long wiring but it never caused an issue as long as you use oversize cables and a larger speed controller. CG was never a real issue but my model was designed around the fan placement. Thrust angle has to be spot on! With the fan at the very rear of the model, it doesn't take much to screw it up...speaking from experience

Quote:
Originally Posted by Quorneng View Post
AIR SALLY
Interesting
I would agree there are issues with the weight (and voltage drop) of long wires and possible inertia effects on the plane but I suspect it only becomes significant in tight aerobatics.
Spikes are not an issue if the ESC is kept close to the battery. The motor wires can be very long without damaging the ESC.
An 85% reduction in the thrust tube is for many EDFs actually just reducing the nozzle area to the FSA!
"Some fans don't deal with sucking air in through super long duct" Do we know why that might be?
Surely inlet ducts can be made with a larger area to reduce unit losses without detriment to the EDF's performance whereas the exhaust tube cannot.
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Old Jan 09, 2016, 06:12 AM
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I do not see how you can say that with the fan at the back it is allowing the air to expand.
As the motor extends to the end of the shroud and is the same diameter as the hub, the exit is exactly the FSA. The result is it has the same exit velocity as an 85% thrust tube but without the losses that it creates.

I suspect the real question is whether the penalties of placing the fan at the back are worth the benefits in efficiency.
At smaller EDF sizes (where the hub/motor is proportionally bigger in relation to the fan) I suspect it is.
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Old Jan 09, 2016, 10:33 AM
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Quote:
Originally Posted by Quorneng View Post
skyhawk
I do not see how you can say that with the fan at the back it is allowing the air to expand.
As the motor extends to the end of the shroud and is the same diameter as the hub, the exit is exactly the FSA. The result is it has the same exit velocity as an 85% thrust tube but without the losses that it creates.
I don't understand, if you have 100% FSA at the end of the motor/shroud how can you increase the velocity as if it's exiting an 85% FSA cone without actually having an 85% FSA exhaust cone ?

sorry, maybe I'm missing something in your post.
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Old Jan 09, 2016, 11:25 AM
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Quoreng,

For best forward flight performance you should reduce the outlet. The amount is dependent on the fan unit type and the models planned flight profile.
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Old Jan 09, 2016, 02:43 PM
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Are the exhaust tube exits 85% of the FSA or 85% of the shroud diameter?
The ones I have seen are something like 85% of the shroud diameter which for many EDFs gives an exit area about the same as the FSA. If this is the case the 'fan at the back' will actually have a slightly higher mass flow (and thrust) as there are no exhaust tube losses.

Obviously if the area of the exhaust tube exit is less than the FSA the velocity will be higher but at the cost of mass flow and static thrust.
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Old Jan 09, 2016, 03:31 PM
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The shroud diameter is usually in the range of 110% or more of the FSA. The FSA has to be calculated based on the rotor diameter and the motor tube diameter. You then adjust the outlet and inlets based on this value.

I see you that you are using this for your Handley Page HP115 project. The fan unit that you have chosen doesn't like to be restricted too much but leaving it open at the rear usually means it is 110%+ on the exit and approx.120%+ when you maintain a constant intake with a bellmouth of the FSA.

Great build on the model and good luck.
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Old Jan 09, 2016, 03:48 PM
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Quote:
Originally Posted by Quorneng View Post
jumo004
Are the exhaust tube exits 85% of the FSA or 85% of the shroud diameter?
.
Hi Quorneng,
Everything I've read says 85 - 90% of FSA. Rule of thumb for good efflux AND good thrust but if your looking for the 'most' then I think it becomes motor/fan dependent as Kevin Cox said.
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