

Jul 14, 2011, 09:22 PM  

Fuzz,
The static thrust is the main variable needed for the calculation. With my F4 and 1075 watts, my spread sheet converges at 59 mph. But from what I have seen of your duct building techniques the static thrust delivered by your system will be higher than that of mine, all other factors being equal. By measuring the static, all the variables caused by duct inefficiency, fan manufacturer, blade count, motor current, rpm, etc., etc. go away. Dan, The nominal power is the current measured by the watt meter times the battery voltage. But the watt meter doesn't know what motor you have so it does not account for Io (I subzero) the idle current. In the case of my motor, Io is 1.7 amps. So the absorbed power is based upon the measured current less Io. 
Jul 15, 2011, 01:59 AM  

Having no way of measuring the thrust, Larry(my scale goes only to 3#) I can say it's close to 11, in the Savage F4. I'm sure the smooth ducting of my glass intakes has a big effect on performance. I will say this, it's much faster than any number noted so far.
Possibly a doppler on the maiden:
But this is not a good rep. I'm taking it out tomorrow, possibly a better vid will be taken. Fuzz 

Jul 15, 2011, 09:02 AM  

Fuzz,
I agree with you about the ducting making a big difference. And from the video, it seems faster than what I'm predicting. But until some one radars one of these F4's, the top speed remains an opinion at best. As well as my value used for the drag coefficient. I'm not saying your opinion is incorrect, I'm just saying opinions are not data. For all I know you might have a 100 mph plane on your hands. 
Jul 15, 2011, 02:31 PM  

It's faster than my Cutlass(the one I crashed last week). But this is only a perspective view. The Cutlass did 93mph at 75% power. The F4 will beat that. But we are comparing 550W to 1075W and 29oz to 4#. But, 100mph is too fast for me. Scale flying is more my style. I like to see it as it cruises by.. The only time I use full power, is when I use it for the flyby into the vertical and immelmann or splits out. Have not tried those with the F4, yet.
Fuzz 
Jul 16, 2011, 08:43 AM  

I have heard a lot about efflux but I never understood what impact it has on flying. Now I have been given a great opportunity to look at the efflux question in detail based on the data supplied by Darin and Dan.
We have Two F4's, built from the same kit by two capable builders. The only apparent difference between the two F4's is their power systems. I'm assuming the same frontal area and the same drag coefficient as both are covered with ironon. Using the efflux velocity I calculated the dynamic pressure developed by a stream of air moving at that speed. Making the assumption the air outlets of both F4's are the same (~3" in diameter or 85% of 90 mm) I calculated what force would be exerted on a 3" diameter disc. I then compared the exerted force to the static thrust. In Darin's F4 the ratio of exerted force is equal to 32% of the static. In Dan's case, the ratio is 54%. I was hoping this ratio would be a lot closer but no joy there. I then compared my calculated Vmax to the efflux. In Darin's case the Vmax is 68% of the efflux while Dan's is only 59%. About the only conclusion drawn from this is Vmax will be less than Efflux. Even if I reduce the drag coefficient to 0.035 the predicted Vmax still is less than efflux velocity. But we already knew this. One other comparison is the ratio between static thrust and power. In Darin's F4, every watt developed .052 oz of thrust; Dan's number was .022 oz/watt while mine is a paltry .017 oz/watt. What this means is anybody's guess. I also compared a number of other parameters of these two F4's, but found nothing that provided a definitive relation between efflux and performance. 
Jul 16, 2011, 10:27 AM  

it really has everything to do with performance, stumax has a great calc that you can figure out the eflux from the thrust, and size of tail pipe.
for instance, in my testing with the midi fan, at 2.5kw, you see around 8 lbs of thrust, and 200mph eflux with a 2.8" tail pipe, at 2kw, 6.5 lbs that hes seeing installed sounds very accurate, especially for his altitude. but his eflux number should be about 175180mph. his eflux meter could be giving a false reading, or its beeing read in the wrong spot of the tail pipe. givin the eflux of 175mph, the model should easily do 120mph, 2kw is quite a bit of power for a model this size, and for the thrust, the eflux number should be much higher, thats why i think the eflux number has to be wrong in his case. 120mph eflux, even installed is what you would see at about 9001000W. 
Jul 16, 2011, 11:52 AM  

Corsair nut,
I can agree with you when you state there is an equation that allows the determination of efflux velocity from the thrust and tail pipe diameter. The classic definition of thrust is mass flow times the change in velocity. From the mass flow and ambient air density, the flow volume, in cu.ft./sec, is easily calculated. Then using the area of the pipe outlet, the efflux velocity can be determined. But, based on the data given, this was not the case. Regarding Darin's F4 and the Vmax my spread sheet calculates; if I change the drag coefficient to 0.05 (might be possible) the velocity converges at 114 mph. You indicate "especially for his altitude." My spread sheet is based on an air density of .075 lbm/ft^3 (sea level). Since drag is linear in air density, the speed of Darin's F4 will be higher than my spread sheet predicts because the calculated drag will be less. Regarding the location of efflux measurement. Air flowing through a circular duct has a very definite velocity profile, provided the flow is "well developed". A flow is considered to be well developed when a settling length of duct is ten times the diameter (L/D=10) is upstream of the flow measuring device. The highest velocity is in the center and the velocity decreases towards the wall. The few times I had to accurately measure the velocity of air in a circular duct I used a static/pitot tube device attached to a differential manometer. Readings were taken at five (5) radii and the values averaged. From the average differential pressure, the velocity could be determined and the mass flow calculated In the small L/D value in our models, the flow has not had a chance to settle so I'm not sure what is being measured and I can see how it would be easy to get different velocities. Accurate measurement of air flow and velocity, even in a lab, is a difficult task. 
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