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Old Mar 11, 2014, 12:43 PM
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powdermnky007's Avatar
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Ecalc Airframe Drag Losses?

Ecalc Airframe Drag Losses?

I recently built a flitetest versawing and used ecalc to pick out a motor/prop for it. One is a tmotor 2212 and another is a tmotor 2216. The 2212 had a little higher static thrust than the bigger 2216. It also had a higher thrust-weight ratio because the motor weighs less and has a little lower resistance.

I built the versawing using the tmotor 2212 and apc 8x6 prop just like I used in the calc and am happy with the performance. However I'm now wondering it ecalc takes the aircraft drag in to account. (I know it can't be exact because airframes can be so different). When I'm throttling up the plane is steadily accelerating and then maybe the last 30% of the throttle stick the plane has pretty much maxed out in top speed and stops accelerating at the same rate.

Is it that the wind drag is just too much and the 2212 can't keep up? Even though the calcs show the 2212 as the winner, maybe in real life the larger 2216 would have had the extra oomph to keep accelerating linearly with the last 30% of the throttle stick.

What do you think? I can attach a screenshot of the calcs if that would help.
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Old Mar 11, 2014, 02:01 PM
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Bruce Abbott's Avatar
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E-Calc does not have any input for airframe parameters, so it cannot take drag into account.

Airframe drag is proportional to airspeed squared, but wing drag also increases at higher angles of attack. Since a higher angle of attack is required to maintain lift at low airspeed, there is increased drag at both low and high airspeeds, with minimum drag at some cruising speed. Combined with prop thrust reducing as airspeed increases, the result is that you seem to reach a high speed 'wall' where increasing motor power has little effect.

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Originally Posted by powdermnky007 View Post
I can attach a screenshot of the calcs if that would help.
Yes, that would help (I can't find a T-motor 2212 in E-Calc!).
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Old Mar 11, 2014, 02:14 PM
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I attached the two calcs to this post. Last week they changed ecalc to a paid program. If you use the free version it randomly hides list selections. So if you pick tmotor it will only show you 30 out of 50 motors.

Please let me know what you think about these two calcs.
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Old Mar 12, 2014, 06:18 AM
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The second E-Calc has custom motor parameters. What motor is it?
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Old Mar 12, 2014, 07:57 AM
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That is the Tmotor AS2216. Same KV just a larger motor. I was jumping back and forth between the two motors changing lots of stuff. I guess at some point it was just easier to type in numbers than find it in the list. It should be the exact specs as choosing that motor out of the list.

So in my scenario simply fitting a larger motor of the same kv using the same prop would not increase top speed much? I'm thinking about it like a car. Say you top out at 4000 RPM in 5th gear only doing 100 mph because the engine doesn't have enough power to overcome the drag. Put a bigger motor in the car and leave everything else the same. Now the bigger motor has enough oomph to turn 6000RPM and you top out around 130 mph. Make sense?
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Old Mar 12, 2014, 03:15 PM
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If the specs are to be believed then the larger motor is actually less powerful, as its resistance is higher. Unfortunately the AS2216 appears to be out of production (Tigermotor have removed it from their website) and I can't find any real-world test data. However I strongly suspect that the specs are wrong.

If the 33% longer stator enables a similar reduction in resistance, the 2216 should be about 0.059 Ohms, almost identical to the AT2216-1250. So I chose that motor and duplicated all your other settings.

First thing to note is that the efficiency figures are too high. This is quite normal for calc programs that use a simple DC motor model, as DC winding resistance doesn't take into account other factors that affect impedance (eg. inductance). However this doesn't matter if we are only interested in comparing the relative performance of two different motors.

According to E-calc the 2216 draws a little more power and is more efficient, so it produces higher rpm and greater static thrust. However the pitch speed is lower. I don't understand how this can be true when rpm is higher - I suspect a bug in E-calc.

In reality I would expect to see 1 or 2 mph extra pitch speed with the larger motor, which would be barely noticeable. More importantly it should run cooler and so be able to maintain higher power output and better efficiency with a larger prop.

To get more airspeed you need to increase prop pitch (to increase pitch speed) and diameter (to increase thrust). The 2216 motor should handle this a bit better than the 2212, but if you are happy with your current setup then it's probably not worth changing motors.
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Old Mar 12, 2014, 10:32 PM
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That makes sense. To take advantage of the bigger motor I would need a bigger prop. One reason for the variances in our results is that screenshot is a few months old. They've probably slightly tweaked some formulas in the mean time.

I don't fly around at full throttle all the time but like to make wide open passes. I won't be changing motors. I just use ecalc a lot to make sure I'm making educated choices and love to learn. Since my combo hits a wall, would it be better for me to prop down some? I'm currently using an 8x6. Would an 8x5 be better? Maybe give the motor a little easier job.
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Old Mar 13, 2014, 03:30 PM
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Quote:
Originally Posted by powdermnky007 View Post
Would an 8x5 be better?
Perhaps, or perhaps not! An 8x5 should put less load on the motor at full throttle, but would be slower and require more throttle to cruise. I would try a few different props and see which one works best - eg. 7x6, 7x7, 8x5, 8x6, 8x8.
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Old Mar 13, 2014, 03:51 PM
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I think I will. Next time I order, I will experiment Thank you very much for your help. I do have one more question though. Estimating model speed from ecalc. I have been looking at pitch speed to guess, but I now believe that the model will be flying a good deal slower than the prop pitch speed due to prop slippage and wind resistance.

On my calcs above it shows my pitch speed to be 65 mph. Now I'm totally guessing, but is it reasonable to assume my plane is probably going around 45 mph? I am planning on doing a doppler sound test to see later. Just trying to get a basic understanding here.
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Old Mar 14, 2014, 06:19 AM
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Originally Posted by powdermnky007 View Post
I have been looking at pitch speed to guess, but I now believe that the model will be flying a good deal slower than the prop pitch speed due to prop slippage and wind resistance.
Yes. Prop thrust drops to zero at just above pitch speed. Flying speed stabilizes when thrust equals drag, which is usually considerably lower than pitch speed.

The plot below is a wind tunnel test showing the relationship between prop thrust and airspeed. In this example thrust drops to zero at an advance ratio of about 0.74 (0.7 is the 'pitch speed' advance ratio of a 10x7 prop).

Quote:
On my calcs above it shows my pitch speed to be 65 mph. Now I'm totally guessing, but is it reasonable to assume my plane is probably going around 45 mph?
Yes. According to Motocalc it should max out at 46mph.

Quote:
I am planning on doing a doppler sound test to see later. Just trying to get a basic understanding here.
Good idea!
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Old Mar 14, 2014, 08:17 AM
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That makes perfect sense! Thank you so much for taking the time to help me with this!
Honestly, I when I built the plane I was expecting it to go the pitch speed.... The first flight I knew something was up. I'm happy with the speed and power it has and I'm planning on keeping the same motor. I will probably step down to an 8x5 and see how it handles. It bothers me that the throttle 'hits a wall'. I already have some 8x4 props. Maybe I'll do some doppler tests on 8x6, 8x5, and then 8x4 and see what I get.

I'm thinking that since the 8x6 hits a wall about 2/3 of the way into the throttle. Maybe the lower pitch props will not hit the wall and will continue pulling until the very end and may reach the same exact speed. Sound plausible?
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Old Mar 16, 2014, 07:09 AM
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Some people use high-diameter/low-pitch (think 12x4) that limits their speed as a benefit/feature instead of a bug. I guess 3D flyers care more for hovering and low speed performance and do not bother to go fast.
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Old Mar 27, 2014, 08:45 AM
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In case anyone else finds this thread, I redid my throttle range calibration on a guess and that fixed it. Now there is no 'wall'. My plane accelerates until the end of the throttle stick is reached. So that motor/prop/battery combo was perfect for the Flitetest Versa Wing! However I still learned much in this thread and am glad I posted it!
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