HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Nov 02, 2010, 10:23 AM
The blade numbers go up to 11
stumax's Avatar
Sydney, Australia
Joined Aug 2002
4,385 Posts
Discussion
Here's a neat efflux & thrust calculator I made

Here's a little program I wrote to help us verify or calculate efflux velocity based on a thrust measurement, or thrust based on an efflux velocity measurement. It's based on simple momentum theory and assumes that the velocity is constant across the whole exhaust. It makes no allowance for boundary layer thickness, but you can allow for it by adjusting the exhaust diameter. It's very good for checking to see if the efflux velocity we've just read from our Eagle Tree probe is correct or way off the mark. It's probably full of bugs as it's 2am over here, and I haven't written PC based code in ages but here it is to play with.

Edit: added new version with ordered tabbing.

Edit2: New version added with air temperature and altitude inputs. Not fully debugged but seems to be working OK.

Edit3: fixed a bug in air density calc

Edit4: fixed a bug in the altitude in ft calc.

Stu.
stumax is offline Find More Posts by stumax
Last edited by stumax; Nov 16, 2010 at 04:51 PM.
Reply With Quote
Sign up now
to remove ads between posts
Old Nov 02, 2010, 10:39 AM
My project: FAIREY DELTA 1
Erik v. Schaik's Avatar
Uden Volkel, Netherlands
Joined Dec 2003
4,976 Posts
Nice program Stu!
I only like to see km/h instead of m/s

greets!
Erik
Erik v. Schaik is offline Find More Posts by Erik v. Schaik
Reply With Quote
Old Nov 02, 2010, 10:39 AM
Registered User
Oshkosh, WI
Joined Nov 2002
2,478 Posts
Nice calculation aid, stu! I always thought measuring thrust & exhaust area are easy, but efflux measurement needed more skill as it is not uniform across plus may have swirling. Also redundant, once you have the other two. Hope folks use the simple aid you've made. Thanks!

Phil Lin
PhilLin is offline Find More Posts by PhilLin
Reply With Quote
Old Nov 02, 2010, 11:40 AM
efflux RC's Avatar
United States, CA, Santa Clarita
Joined Aug 2009
8,767 Posts
J was right Stu. Helpful you are, indeed.
Thanks for the tool.
Gary
efflux RC is offline Find More Posts by efflux RC
Site Sponsor
Latest blog entry: Updated blog Jan 2014
Reply With Quote
Old Nov 02, 2010, 01:12 PM
Registered User
Doug Bateman's Avatar
Swarthmore, PA 19081
Joined Dec 2004
2,839 Posts
Thanks Stu.
Doug Bateman is offline Find More Posts by Doug Bateman
Reply With Quote
Old Nov 02, 2010, 01:55 PM
Renegade Fun Forever!!
rvincent's Avatar
Fontana, California, United States
Joined Jul 2004
27,499 Posts
Very nice Stu!! Does it matter what fan it is? Or is it simply thrust through a diameter and thats all? In otherwords will a Dynamax and Ramtec show the same efflux with the same exit and same thrust?
rvincent is online now Find More Posts by rvincent
Reply With Quote
Old Nov 02, 2010, 01:59 PM
Renegade Fun Forever!!
rvincent's Avatar
Fontana, California, United States
Joined Jul 2004
27,499 Posts
Here's another question?

My A-4 has a 3.380 exit with 15.6lbs thrust. I measure 236 mph efflux and your calculator says 245.2. Is this because of the very long exit duct?
rvincent is online now Find More Posts by rvincent
Reply With Quote
Old Nov 02, 2010, 02:11 PM
Dieselized User
gkamysz's Avatar
Chicagoland
Joined Feb 2000
7,387 Posts
Quote:
Originally Posted by rvincent View Post
Very nice Stu!! Does it matter what fan it is? Or is it simply thrust through a diameter and thats all? In otherwords will a Dynamax and Ramtec show the same efflux with the same exit and same thrust?
This is the toughest concept for people to understand. It doesn't matter what kind of fan is moving the air.

Stu, if you really want to create some controversy discussion, just add a cell to calculate power static thrust power, a cell for a wattmeter reading, and output a cell for motor/fan efficiency. We'd have endless fun reading threads.

Greg
gkamysz is offline Find More Posts by gkamysz
Reply With Quote
Old Nov 02, 2010, 02:25 PM
Renegade Fun Forever!!
rvincent's Avatar
Fontana, California, United States
Joined Jul 2004
27,499 Posts
No Greg I do understand it quite well Its the same as the hydraulics I work with all day. The air could care less what is moving it through the tube. 15lbs pressure (thrust) being pushed through an oriface (exit) will be the same Gallons per minute (efflux) no matter what type pump is moving it, air or hydraulic.

Now the amount of power to make 15lbs of thrust (r pressure) will vary with the fan (or pump) used.
rvincent is online now Find More Posts by rvincent
Reply With Quote
Old Nov 02, 2010, 02:31 PM
Renegade Fun Forever!!
rvincent's Avatar
Fontana, California, United States
Joined Jul 2004
27,499 Posts
I just want to hear Stu explain if the run length of the exit will affect the results and why? Will a more efficient fan maintain better thrust with longer intakes? Exits?
rvincent is online now Find More Posts by rvincent
Reply With Quote
Old Nov 02, 2010, 02:33 PM
Dieselized User
gkamysz's Avatar
Chicagoland
Joined Feb 2000
7,387 Posts
You're answering your own questions.
gkamysz is offline Find More Posts by gkamysz
Reply With Quote
Old Nov 02, 2010, 02:51 PM
DELTAS RULE
corsair nut's Avatar
tehachapi, CA
Joined Jan 2006
22,444 Posts
that could really just be a factor of density alt. if you were in perfect air, it very well could make 245. i know on my sabre, im making over 230mph eflux, installed with a 75% inlet, at 4,000 asl....this is a calculator, and its going to give you a close estimate, but wont be exact. and yeah, i think long pipes effect thrust/eflux....but im no expert...id rather hear from somebody like stu.



Quote:
Originally Posted by rvincent View Post
Here's another question?

My A-4 has a 3.380 exit with 15.6lbs thrust. I measure 236 mph efflux and your calculator says 245.2. Is this because of the very long exit duct?
corsair nut is offline Find More Posts by corsair nut
Reply With Quote
Old Nov 02, 2010, 03:23 PM
Renegade Fun Forever!!
rvincent's Avatar
Fontana, California, United States
Joined Jul 2004
27,499 Posts
Not really Brent. I mean if I take a set up and test it with a 12" exit and a 4" intake and I get 15.6lbs. Then I take the same set up and addd a 24" exit what will the effect be? Now take the same set up and change the 4" intake with a 16" intake and what will the effect be? Would one fan work better in these situations than another? Does the type of fan or air pump make a difference?

We all know that 15lbs and a 3.380 exit will make 245 mph efflux. But since air is compressable, will a longer exit make a difference "IF" you still have 15lbs of thrust?
rvincent is online now Find More Posts by rvincent
Reply With Quote
Old Nov 02, 2010, 03:37 PM
Fly allot, Crash allot, next?
United States, CA, Corona
Joined Feb 2006
7,242 Posts
The longer the run, the less the thrust.

The calculation can only be used as an estimate unless all the comparisons have the same overhead.

Bottom line, if you are in the ballpark, your plane might fly. Too many variables to make it exact.

Just like any other program there are several the preceed it. The EDF on line calculator, just like Motocalc for prop planes.
SCALEFAN is offline Find More Posts by SCALEFAN
Reply With Quote
Old Nov 02, 2010, 03:48 PM
Renegade Fun Forever!!
rvincent's Avatar
Fontana, California, United States
Joined Jul 2004
27,499 Posts
Well Al not really The equation is law, just like amps x volts is watts. Yes the longer pipe will reduce thrust and therefore efflux, but if I STILL have 15lbs after extending the pipe will the efflux drop? In my A-4 as an example it did. I still have 15.6lbs thrust, but my actual efflux is lower than the calculator. Is it due to compressability? waiting for Stu to answer
rvincent is online now Find More Posts by rvincent
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Cool Here's a neat trick moody07747 Batteries and Chargers 11 Sep 16, 2006 11:38 PM
Question Efflux velocity from thrust & FSA? meteor Electric Ducted Fan Jet Talk 20 Jul 14, 2005 11:37 AM
Thrust or Efflux: what flies a jet best? meteor Electric Ducted Fan Jet Talk 23 Apr 30, 2004 06:16 PM
Some thoughts on efflux vs thrust. Hopperfly22 Electric Ducted Fan Jet Talk 57 Sep 16, 2003 10:30 AM
Thrust vs Efflux velocity curves stumax Electric Ducted Fan Jet Talk 6 Jul 28, 2003 05:47 PM