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Understanding Power/Props - Ecalc

srfnmnk

Senior Member
#1
Hey guys,

I have found this awesome website (ecalc) but am struggling to fully understand it. Could a few of you help me out please?

I have attached an image of an example The motor I am trying to compare it to is the motor I am currently using and the prop I am using. Everything is working fine but ecalc is saying it has problems. Thoughts?

Here is the Motor
Prop is 8x6 &/or 9x4.5

Below is what ecalc is saying
Questions are
1. How the heck did we get to 30A pull? My 18A ESC would melt
2. Why does it think it will stall out on the ground? Clearly this doesn't happen...but...even stranger, the static thrust is almost 3 times that of the weight of the plane...what's the problem here

Thanks.
Untitled.jpg
 

joshuabardwell

Senior Member
Mentor
#3
The "stall" in this case is the propeller being stalled, not the wing. A propeller is actually an airfoil, and it generates lift just like a wing does. Just like a wing, if the angle of attack on a prop is too high and the airspeed is too low, the airflow will detach from the airfoil and the propeller will stall. With a propeller, angle of attack is related to the pitch of the prop and airspeed is related to the RPMs and prop size, as well as forward speed. Long story short, if you have too much pitch for your motor, the prop will stall at low speeds. It will not generate as much thrust as it could. This typically happens if you are trying to prop up your plane to increase the top speed. With a fixed-pitch prop, a good guideline is that your top speed (at wide-open-throttle) should be about 3x your stall speed. A prop that can get you above about 3x your stall speed at WOT will probably stall at close to your stall speed. This will result in poor low-speed performance, since right as your wing is getting ready to stall, your prop will stall too and you will suddenly lose thrust. However, a little bit of prop stalling is okay because eCalc calculates prop stall based on the plane being stationary. When the plane is actually moving forward, the prop stall speed will be a little higher.
 

joshuabardwell

Senior Member
Mentor
#6
Yeah, the only thing is that the airspeed has nothing to do with it.
I once read that a prop stalling when the plane is stationary is not too big a deal, because as soon as the plane picks up speed, the situation will improve. I don't know... I can see the argument that the only airspeed that matters is the airspeed parallel to the airfoil of the prop. In that case, the plane's forward airspeed is irrelevant, since that vector has zero magnitude parallel to the prop. But the air moving perpendicular to the prop certainly has some effect on the air moving parallel to the prop, so I assumed some non-intuitive interaction was occurring.

If the plane was at increased alpha, such as on climbout, then the oncoming air would have a non-zero velocity magnitude parallel to the prop, in which case it seems like it would act as a tail-wind on the up-swinging side of the prop and a head-wind on the down-swinging side of the prop. I have no idea what the net effect of that would be.
 

srfnmnk

Senior Member
#7
So this is the setup I flew...does that mean I am probably experiencing some prop stall? If so, What prop should I use with this motor? I think this motor is larger than what FT recommends but the prop is the same...sooo...I'm confused on how to make this equal out? Make the prop smaller?
 

joshuabardwell

Senior Member
Mentor
#9
So this is the setup I flew...does that mean I am probably experiencing some prop stall? If so, What prop should I use with this motor? I think this motor is larger than what FT recommends but the prop is the same...sooo...I'm confused on how to make this equal out? Make the prop smaller?
Try reducing the pitch from 8x6 to 8x4.5 and see what eCalc says. If you don't notice any bad tendencies close to stall speed, and you are happy with the thrust and top speed, then keep flying. But put an amp meter on there to make sure you're not over-driving your motor and ESC. 30+ amps through an 18 amp motor is a good way to let the magic smoke out.
 

joshuabardwell

Senior Member
Mentor
#10
BTW, the Turnigy 28-26 1350 kv, which your motor is supposed to be similar to, shows 14 amps with an 8x3.8 prop on 3S, producing 0.73 kg of thrust. The Park 370 also recommends an 8x3.8. So it sounds like you should be around an 8x3.8 to bring your current draw down under what your 18A ESC can handle.

A little amp-meter like this one is a good investment, to keep you from burning out motors and ESCs.
 
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joshuabardwell

Senior Member
Mentor
#12
Hi guys, I wasn't aware of the ability of a prop-stall! So I googled it and found this awesome explanation:
http://www.stefanv.com/rcstuff/qf200203.html
Good link! I think this image shows why airplane speed factors into the equation.

t-figure2.gif

But if I am interpreting the image correctly, it indicates that increased forward speed increases the angle of attack, which would mean that a prop would stall more at higher speed, which seems counter-intuitive.
 
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stay-fun

Helicopter addict
#13
But if I am interpreting the image correctly, it indicates that increased forward speed increases the angle of attack, which would mean that a prop would stall more at higher speed, which seems counter-intuitive.
Uh to me it seems exactly opposite (so intuitive). Rotate the image 90 degrees to the right (clockwise) and see if you get it. I think in my case it is a typical incident of 'understanding it, but not yet well enough to explain it'.

It's like a regular plane wing, with forward motion, while there is airflow going from above to below. As in, if the air would stand still, a plane wing that would gain altitude.

Umm maybe there's someone else who does a better job at explaining :rolleyes:
 

joshuabardwell

Senior Member
Mentor
#15
I see. For some reason, I was imagining the "incoming air" vector getting shorter as the incoming airflow increased, as if the incoming airflow was compressing it like a spring. As soon as I sat down to draw out the vector diagram, I immediately realized my error.

So, then, bottom line is we agree that airspeed DOES affect whether a prop stalls or not. A prop that stalls when the plane is stationary may not stall when the plane is moving because the relative airflow will have a shallower angle of attack when the incoming air is moving faster.