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How heavy can it be??

Old 09-23-2017, 03:32 AM
  #1  
schafe
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Default How heavy can it be??

I have the same old question. I am scratch building an EDF jet with 2 motors. The plans I am working with indicate the total wing area will be 169.5 sq in - that is both wing sections. Using my watt meter one motor puts out 250 watts X 2 = 500 total watts.

I cant seem to find an online calculator that will give me the max weight. Maybe I am trying to make this too complicated. . . .

I don't really care about aerobatics, so 100 watts/pound seems like a good target. That means 5 pounds or less right?? How does wing area fit into this mess??

Thanks
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Old 09-23-2017, 07:12 AM
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I am making a guess that it would be more of a fighter type, but I may be totally wrong.

If you want good performance, keep it as light as possible.

5 lbs would probably not fly with 500 watts EDF.

I have a 21" wingspan F-16 that weighs 18oz. with battery ready to fly.
I have read 487 watts on my wattmeter

Oh man does it fly!!!

EDF's don't have a lot of start out acceleration like a propeller plane does, so you must keep them light or have a long take off runway.

I can hand launch mine with no problem.

More information on what you are trying to build will let us help more. EDF's can be used even in gliders,
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Old 09-23-2017, 08:17 AM
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Maybe those can help ?

Nigel
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Old 09-23-2017, 12:44 PM
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quorneng
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schafe
The speed a plane must fly at depends on its wing area and for any specific air frame the power to fly depends directly on its weight and roughly on the square of it speed. So the lighter and slower the better!
So you certainly can fly an EDF at 100W/lb but unlikely with a 5lb plane with a wing area of only 1.18 sq ft. It gives a wing loading of 68oz/sqft.

I have several EDFs that fly nicely on less than 100W/lb but they all have a wing loading of no more than 6 of/sqft. That's just 1/10 of what you are proposing.
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Old 09-23-2017, 10:03 PM
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What I am attempting is a cartoon scale EA-6B Prowler that I am printing with a 3D printer. I have no idea at this time what it will weigh. I would just like to know the "it'll never fly" weight. Note: I have doubts that it will fly even if it is light enough.
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Old 09-23-2017, 10:43 PM
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quorneng
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Given the relative inefficiency of EDFs (note two smaller ones are even less efficient than one big one) my gut feel for 500W twin motor power would be a maximum of 2 lbs with the wing area you have suggested.
I would very much doubt this would be achievable with a 'printed' air frame as they are known to 'heavy' when compared to conventional structures and even more so against foam.
But I could be wrong!
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Old 09-24-2017, 12:14 AM
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There is a Co that offers printed RC planes. They can be seen on youtube flying. I actually bought and printed one, but lost motivation before I got it flying.

Balsa is a much better material for planes - it is way stronger and lighter. It is much easier to make the complex shapes with a printer and faster too. I'm just playing. Stay tuned for an exciting video of my new plane shaped RC car.

If I could just figure out how to inject some type of foam into a mold I printed then I would have something.
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Old 09-24-2017, 01:29 AM
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Originally Posted by schafe View Post
I have the same old question. I am scratch building an EDF jet with 2 motors. The plans I am working with indicate the total wing area will be 169.5 sq in - that is both wing sections. Using my watt meter one motor puts out 250 watts X 2 = 500 total watts.

I cant seem to find an online calculator that will give me the max weight. Maybe I am trying to make this too complicated. . . .

I don't really care about aerobatics, so 100 watts/pound seems like a good target. That means 5 pounds or less right?? How does wing area fit into this mess??

Thanks
Don't fly EDF's anymore, but the one I did have took 200 Watts per pound to fly well. At 169 square inches, and 2.5 pounds, that winds up at 34 ounces per square foot of wing area.

That 34 ounces per square foot of wing is on the high side for a small model airplane. Some of my models have wing loadings on this order, but they have 1400 square inches of wing.
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Old 09-25-2017, 06:29 AM
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I originally tested the fan units with 3S power. Turns out they can run off of 4S. I tried a fan with 4S and it had almost twice the watt output. I am shocked at the increase in power. So now it looks like there will be 900-1000 watts available.
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Old 09-25-2017, 12:33 PM
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Originally Posted by schafe View Post
So now it looks like there will be 900-1000 watts available.
Which is all well and good but that doesn't help with the wing loading. This is a small model, to keep the wing loading within reasonable limits you need to be under 1.5lb flying weight. Even at 1.5lb it's very much in the 'lead sled' bracket so will have high stall speed and probably very tricky flying characteristics. More power doesn't really help, in fact it probably hinders because the heavier 4s batteries give you more of a weight problem.

Seems to me that with a 1000W power system you should make the plane bigger to give you the wing area it needs to actually fly.
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Old 09-26-2017, 01:36 AM
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schafe
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Since it is a cartoon scale plane, I could make the wings bigger without adding much weight. I am almost done printing the parts as shown above. Most of the weight will come from the electronics.
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Old 09-28-2017, 03:54 PM
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Originally Posted by schafe View Post
Since it is a cartoon scale plane, I could make the wings bigger without adding much weight. I am almost done printing the parts as shown above. Most of the weight will come from the electronics.
Realistically unless you can make the wings MUCH bigger you would still be looking at a sub 2lb model. If the electronics make achieving that weight impossible then you have the wrong electronic package for a model of this size.
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Old 09-30-2017, 03:17 AM
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Why does everything have to be so hard???

I added up all of the electronic parts I know for sure along with the landing gear. That comes up to 16.16oz. I did not weigh the push rods, but they can't add very much. The sq ft of the wing is 1.177.

I averaged the weights of 1800mah 3S batteries - 5.19oz X 2 = 10.38oz + 16.16oz = 26.54oz. divided by 1.177 = 22.55oz/sq ft. That is without the airframe which I still don't have a total weight for.

What I have printed so far weighs 12.55oz. My guess is there will be about that much more, so around 25oz in the airframe. 25 + 26.54 = 51.54oz divided by 1.177 = 43.79 oz/sq ft.

IT'S TOO HEAVY WILBUR!!! It'll never fly Orville.

Stay tuned for the exciting video of a plane shaped car careening off of the end of the runway. . .

There went a billion hours.

Last edited by schafe; 09-30-2017 at 03:40 AM.
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Old 10-01-2017, 01:35 AM
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quorneng
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schafe
Building a successful EDF is not easy. The hard bit is knowing how to keep the weight down to achieve a practical wing loading.

This plane has a single 55 mm EDF, weighs 21.5 ounces with an 1800 mAh 3s and has a wing area of 2.5 sqft yet it doesn't even need all of the 220W available to fly really nicely.
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Old 10-02-2017, 02:11 AM
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schafe
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I guess I will redo the wing and maybe only use 1 battery.
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Old 10-02-2017, 02:18 AM
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Its going to end up looking like the U-2 spyplane.
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Old 02-28-2018, 07:28 PM
  #17  
Flite-Metal
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Default 1.623 Times The Trust

1.623 times the trust (in ounces) = prototypical flight weight.

This is the age old formula from recip ducted fans. Prototypical flight,
not endless vertical... This flying on the wing like a real aircraft...not
a rocket.

The point of doing (making) something because you are obsessed by
it is understandable; we all have been there...However most of us
acquire cumulative knowledge of the practical versus a "Gotsta".
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