Help Don!
Calling on the expert!

.. just as the title reads.. im building a kamdax mig15 into an Electric im using a hobby people F16 fan and housing.. i want to make a proper exit tube for it need to know what to measure and how to do the math to get the most out of this little bugger!

thanks in advance

I always us 80% of the fan sweep area (FSA). The fan sweep area is the doughnut shaped ring that the fan blades push air through. You need to measure the diameters on the fan you are using. The calculate the exit diameter is easy the simplest way to do is like this:


De= sqrt(.80) * sqrt(D*D - d*d)
De= diameter for 80% FSA
D= rotor diameter
d= diameter of rotor spinner/motor (depends on fan used)
.80 = 80% FSA

I will post a photo and do an example.

Don

hey don thanks alot!... that will be awesome i wanna get this figured out so i can get the most out of this little guy

Are you using the stock HP 3 blade fan/rotor?

Don

yes don i will be

OK here we go.

De= sqrt(.80) * sqrt(D*D - d*d)
De= diameter for 80% FSA
D= rotor diameter
d= diameter of rotor spinner/motor (depends on fan used)
.80 = 80% FSA

Measuring the fan we get:
D= 56 mm
d= 24 mm

De= sqrt(.80) * sqrt(56*56 - 24*24)
De= .8944 * sqrt(3136-576)
De= .8944 * sqrt(2560)
De= .8944 * 50.596
De= 45.25mm = 1.78 "

awesome don.. thanks!

Here's a related question, does the exit have to be round? I know its probably the easiest to form, but could the duct exit be oval or some other shape without sacrificing too much performance?

I have used slightly oval at times but round I believe is the best as it distributes the airflow evenly. The EDF F117 I had had long rectangular slots and it worked great but they also had a nice gradual transition from round to rectangle to reduce losses as much as possible.
Don

What would be my ideal exit diameter for max speed with a 69mm fan with a 24mm center??

Hi Don;
I am doing up a pair of HP F16's with your wicked fan and 5400 motor. I have calculated the exit diameter of the thrust tube based on the above at 1.844 inches.

My question is about the thrust tube length. Doesn't the length from the fan to the exit somehow come into play here? I am asking as I have moved the fan back in the fuse as I am planning on using a 204g battery.

Thanx for the assist!
Scott

Here is a spreadsheet that I wrote back in December and modified today. It will calculate FSA when you input the fan shroud inner diameter and the fan hub diameter. It will output FSA from 100% down to 70% in 5% increments. It will also give you the exhaust diameter for 100% FSA down to 70% FSA in 5% increments. It will give you the diameter for both a single exhaust outlet or a bifurcated exhaust (two outlets from one fan). Input in the green cells only. The calculator works for both English and Metric units but you must input both diameters in the same units.

Example: if you put mm in for both diameters then area would be mm squared and exhaust diameters would be in mm.

http://www.rcgroups.com/forums/showthread.php?t=1160865

Steve

Originally Posted by scotware

Hi Don;
I am doing up a pair of HP F16's with your wicked fan and 5400 motor. I have calculated the exit diameter of the thrust tube based on the above at 1.844 inches.

My question is about the thrust tube length. Doesn't the length from the fan to the exit somehow come into play here? I am asking as I have moved the fan back in the fuse as I am planning on using a 204g battery.

Thanx for the assist!
Scott

I use 1.8" on the 60mm fan. I try to keep the thrust tube no less than 5-6" long on the 60mm fan.

Originally Posted by SteveC68

Here is a spreadsheet that I wrote back in December and modified today. It will calculate FSA when you input the fan shroud inner diameter and the fan hub diameter. It will output FSA from 100% down to 70% in 5% increments. It will also give you the exhaust diameter for 100% FSA down to 70% FSA in 5% increments. It will give you the diameter for both a single exhaust outlet or a bifurcated exhaust (two outlets from one fan). Input in the green cells only. The calculator works for both English and Metric units but you must input both diameters in the same units.

Example: if you put mm in for both diameters then area would be mm squared and exhaust diameters would be in mm.

http://www.rcgroups.com/forums/showthread.php?t=1160865

Steve

Thanks for posting that!!!

Hey everybody I've been looking around for someways to make a thrust tube for my T45 to see if i could get any performance increase. After looking around on rcgroups and some videos i found a great idea make it out of a paper cup. So I did I got one from subway, first i was just going to cut it the right size fan not really worrying about the exit diameter. Then I did some research on thrust tubs and found a hole bunch of info on FSA and the percentages that it needs to be for making and effective thrust tube. On my research hunt for thrust tubes i did find most of the info needed but it wasnt all in one place so I decided to make a video. The video is over how to find your fsa and the right percentage it needs to be also on me installing it into my Sapac T45.

I'm about 90% sure that all the info and math in the video is right, but if im wrong can someone please tell me and I will fix it. Don't want to tell someone the wrong thing.

well enjoy

How to make a cheap and easy thrust tube for your edf jets (4 min 19 sec)

j3tman,