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Old Jul 14, 2011, 05:46 PM
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Larry Dudeck's Avatar
Williamsville, NY, USA
Joined Sep 2000
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Dan,

My spread sheet converges at 64.5 mph, based upon a frontal area of 119.4 sq.inch (my F4 has a frontal area of 126 sq.inch, the difference due to the three tanks hanging from the bottom), 92 oz AUW, 44 oz. of static thrust and a Cd=0.225 The value of the Cd is the big unknown but I'm guessing it is 10% lower than that of my F4 due to the iron-on covering.

The Cd plays a major role in all of this. For example, if the Cd was 0.15, the program would converge at 75 mph. If the actual Cd was 0.3, the program converges at 58 mph.

Once I get my F4 flying I will mount a "how fast" pitot/static device and measure the maximum speed. Then I can re-run the spread sheet and vary the Cd until it converges.

Can you be more precise in your power of 900 watts? Is that nominal or absorbed? My F4 with 900 watts absorbed would have an indicated top speed of 55.5 mph. The extra speed of yours is due to reduced frontal area and lower Cd.

I'm not sure where efflux velocity fits in any of this, other than the assumption higher efflux yields higher velocity. But I do not know how to quantify that. Over the years I have heard the term tossed about but I really have no feel for it. I can not relate efflux velocity to static thrust and it is thrust that is pushing us along so I choose to ignore the efflux. Perhaps I will connect my "how fast" and measure the efflux my F4 generates.

Once I sanitize my spread sheet and make it a bit more user friendly I will e-mail it to you. The spread sheet is totally transparent to power systems and air frames and will work with any model you may want to try it with.

Thanks for the data.
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Old Jul 14, 2011, 05:50 PM
Big gov never Works
St. Martin's Avatar
Skunk Water, Rhode Island
Joined Jul 2002
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Interesting, Larry. Unfortunately, I cannot supply the thrust or efflux data.

5 cells, 4#,7oz, 1075W. But I only have 3 flights on it. And still have not flown it full power yet. Heck..it took me two years to fly the 70mm RBC Panther full power.

Fuzz
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Old Jul 14, 2011, 07:05 PM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
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Quote:
Originally Posted by Larry Dudeck View Post
Can you be more precise in your power of 900 watts? Is that nominal or absorbed? My F4 with 900 watts absorbed would have an indicated top speed of 55.5 mph. The extra speed of yours is due to reduced frontal area and lower Cd.

I'm not sure where efflux velocity fits in any of this, other than the assumption higher efflux yields higher velocity. But I do not know how to quantify that. Over the years I have heard the term tossed about but I really have no feel for it. I can not relate efflux velocity to static thrust and it is thrust that is pushing us along so I choose to ignore the efflux. Perhaps I will connect my "how fast" and measure the efflux my F4 generates.
I'm not sure how to tell the difference between nominal and absorbed power.

With the fan installed, I connect the Whattmeter between the ESC and battery pack and run the fan up to full power and the meter reads ~900w.

To me, efflux is the same as propeller pitch. The lower the efflux, the slower the model since the model cannot fly faster than the air it's pushing out the tail pipe. The higher the efflux, the higher the speed.

My F-4 is kind of a special case. I'm using the FlyFly fan which has a higher pitch blade than the Midi fan. So, even though my fan is only pulling 900w and 2-3/4 lbs. of static thrust, the higher pitch of the fan allows it to push the air out at a higher velocity.

Dan
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Old Jul 14, 2011, 09:22 PM
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Larry Dudeck's Avatar
Williamsville, NY, USA
Joined Sep 2000
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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.
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Old Jul 15, 2011, 01:59 AM
Big gov never Works
St. Martin's Avatar
Skunk Water, Rhode Island
Joined Jul 2002
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Having no way of measuring the thrust, Larry(my scale goes only to 3#) I can say it's close to 1-1, 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:
Steves F-4 Phantom.wmv (2 min 17 sec)


But this is not a good rep. I'm taking it out tomorrow, possibly a better vid will be taken.

Fuzz
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Old Jul 15, 2011, 09:02 AM
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Larry Dudeck's Avatar
Williamsville, NY, USA
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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.
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Old Jul 15, 2011, 02:31 PM
Big gov never Works
St. Martin's Avatar
Skunk Water, Rhode Island
Joined Jul 2002
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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 fly-by into the vertical and immelmann or split-s out. Have not tried those with the F4, yet.

Fuzz
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Old Jul 15, 2011, 04:43 PM
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Daren's Avatar
Los Alamos, NM, USofA
Joined Nov 2000
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Larry,

Here's the info on my F-4:

Cells: 8
Weight: 6.4 lbs.
Watts: 2000
Static: 6.5 lbs.

Efflux: 130 mph

I have not had it radared or flown it too much at top speed, but it's pretty speedy when I do!

Daren
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Old Jul 15, 2011, 06:04 PM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
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Quote:
Originally Posted by Daren View Post
Weight: 6.4 lbs.
Static: 6.5 lbs.


I gotta change my power system out. That's all there is to it.
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Old Jul 15, 2011, 06:08 PM
DELTAS RULE
corsair nut's Avatar
tehachapi, CA
Joined Jan 2006
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even for installed, eflux seems awful low, but thrust seems pretty close to where it should be
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Old Jul 15, 2011, 09:15 PM
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Larry Dudeck's Avatar
Williamsville, NY, USA
Joined Sep 2000
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Darin,

Assuming the same frontal area as Dan's F4 and 6.5 lbs static, with a Cd of 0.225 (iron-on covering), my spread sheet converges at 89 mph.

If the Cd was 0.15 (not impossible), convergence occurs at an even 100 mph. This is level flight, of course.

Thanks for the data.
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Old Jul 15, 2011, 09:18 PM
DELTAS RULE
corsair nut's Avatar
tehachapi, CA
Joined Jan 2006
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with 2kw, should do a solid 115-20. somethings wrong with his eflux#
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Old Jul 16, 2011, 08:43 AM
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Larry Dudeck's Avatar
Williamsville, NY, USA
Joined Sep 2000
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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 iron-on.

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.
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Old Jul 16, 2011, 10:27 AM
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corsair nut's Avatar
tehachapi, CA
Joined Jan 2006
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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 175-180mph. 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 900-1000W.
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Old Jul 16, 2011, 11:52 AM
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Larry Dudeck's Avatar
Williamsville, NY, USA
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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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