Turbine thrust question
 

I have noticed on some of the posts that some kits are saying 12-13LBS dry weight takes off and lands in 100 ft.
OK i flew ducted fan years ago and using a top of the line fan unit and if we were at 13 lbs we had good performance not great and that is with a 1to 1 power to weight ratio.
Now i saw one add saying a 13 lb airframe with a 12lb turbine spectacular performance.
Are they rated different than a ducted fan then?
Just wondering how can a 12 lb turbine push a 18-19 lb jet (WET) with any type of real performance.
Thanks..........................JOHN

RE: Turbine thrust question
 

first off, I doubt you really ever got more than 10lbs of thrust from a ducted fan, no matter what the manufacturer says.

secondly, the turbine has a tailpipe velocity on the order of 400mph while the fan had maybe 200mph.
the exceleration and performance is very noticeable. I could never fly my AV8R at 17lbs ready to fly on my field with a fan, would never have happened, but my 13lbs turbine yanks it off and straight up.

RE: Turbine thrust question
 

What happens to a ducted fan jet in la mens terms is it looses thrust with speed, if your fan is moving air out the tail cone at 170 mph and the jet is going 120 mph your thrust to speed is equliing out causing loss of thrust. I am not sure of the spec but the volosity is around 500 mph exiting a micro turbine, so you have to be going well over the speed we fly at to start to equlizing thrust to speed ratio. I would like to learn myself the math or formulas of what the true specs are on this subject.

RE: Turbine thrust question
 

Shok sent you a pm..............JOHN

RE: Turbine thrust question
 

For a simple and graphic explanation, draw a graph, with thrust on the vertical scale, and airspeed on the horizontal scale. For static, mark a point up the axis of the vertical scale at the point which corresponds to the thrust - say 10lbs. for both cases. Now, along the horizontal scale, mark a point which corresponds to the efflux velocity, let's say 250mph for the DF (probably OK for a Viojet with a small tailpipe) and 500mph for the turbine (just guessing here, but good enough for the explanation). Now, join the points on the vertical scale with the points on the horizontal scale (which will now become the aircraft's flying speed). As you follow each line you can see approx how much thrust the DF or turbine will have at each flying speed. Notice how much more thrust the turbine has at 100mph? 150mph? This is why turbine planes have such awesome performance - they still have half their thrust at 250mph, whereas a DF has none. Of course, this is a simple analogy, and doesn't take into account any ram effects on the inlets, turbine cooling in flight etc, but it's good at explaining the performance difference.

RE: Turbine thrust question
 

as i see it, with a ducted fan, you do NOT have full power on take off, most of the time the aircraft does not come on the pipe until it picks up speed in the air, where as the turbine has full power from the very beginning.