Nov 02, 2010, 11:23 AM
The blade numbers go up to 11
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.
Last edited by stumax; Nov 16, 2010 at 05:51 PM.
 Nov 02, 2010, 11:39 AM My project: FAIREY DELTA 1 Nice program Stu! I only like to see km/h instead of m/s greets! Erik
 Nov 02, 2010, 11:39 AM Registered User 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
 Nov 02, 2010, 12:40 PM J was right Stu. Helpful you are, indeed. Thanks for the tool. Gary Latest blog entry: Updated blog Oct 2014
 Nov 02, 2010, 02:12 PM Registered User Thanks Stu.
 Nov 02, 2010, 02:55 PM Renegade Fun Forever!! 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?
 Nov 02, 2010, 02:59 PM Renegade Fun Forever!! 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?
Nov 02, 2010, 03:11 PM
Dieselized User
Quote:
 Originally Posted by rvincent 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
 Nov 02, 2010, 03:25 PM Renegade Fun Forever!! 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.
 Nov 02, 2010, 03:31 PM Renegade Fun Forever!! 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?
 Nov 02, 2010, 03:33 PM Dieselized User You're answering your own questions.
Nov 02, 2010, 03:51 PM
fly like an EAGLE ;)
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 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?
 Nov 02, 2010, 04:23 PM Renegade Fun Forever!! 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?
 Nov 02, 2010, 04:37 PM Done it all 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.
 Nov 02, 2010, 04:48 PM Renegade Fun Forever!! 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