This prop stall business...how serious is it in actual use? I find myself needing to go to square props (e.g. APC-E 12x12, 14x14) in the calculators to achieve decent (80mph +) pitch speed at low amps for the scale warbirds. Also, how closely does pitch speed correlate to actual flight speed, realizing that variables such as available thrust and airframe drag come into play.

Finally, P-Calc (brantuas) seems to deliver massive thrust numbers. How accurate is this program?

Pitch speed on the bench and actual flight speed have no direct correlation; I have some models that won't break 1/2 pitch speed in a vertical dive, and some models that defy hobby grade physics by traveling in steady level flight at 10-15mph faster than pitch speed on the bench.

Bottom line... relying on equations to accurately model drag of a real model aircraft is just not practical, and unless you can do that (CFD/CAD?), then bench pitch speed numbers are only good for rough comparisons (ie. 'if that prop has the same thrust and more pitch speed, it should be faster'). Trial and error testing, and referring to archived data is way a more efficient means of arriving at the best prop choice.

I haven't checked out P-Calc, but Drive Calculator numbers are always very close to the thrust I measure on my stand. Lot's of folks with thrust stands have verified DCalc's accuracy. There are some prop constants that aren't modeled as well as others, but overall DCalc tends to be on the money for what I do.

Cheers,
Kev

Thanks Kev. Time for me to crawl out of the cave and buy some props to try out

A little better choice for getting rough estimate of static thrust capabilities of various props and at various RPM's is to look them up in the FlyBrushless.com prop database. That will give you some real static thrust numbers that were taken by nice guys that also have some credibility here.

Some don't like to use or even look at static thrust numbers, I consider them to be better than knowing nothing at all. And a good startor intermediate step in the process of finding a good prop.

Jack

Originally Posted by TTRotary

This prop stall business...how serious is it in actual use? I find myself needing to go to square props (e.g. APC-E 12x12, 14x14) in the calculators to achieve decent (80mph +) pitch speed at low amps for the scale warbirds. Also, how closely does pitch speed correlate to actual flight speed, realizing that variables such as available thrust and airframe drag come into play.

Finally, P-Calc (brantuas) seems to deliver massive thrust numbers. How accurate is this program?

With a square prop your acceleration is probably pretty poor.

I would look at the whole power system and select the right battery voltage, motor, ESC & prop combo to reach the desired speed.

Meaning if you needed to throttle up to get out of a bind (e.g. you start to lose control of a low hover and need to fly out of it?)

Originally Posted by TTRotary

Meaning if you needed to throttle up to get out of a bind (e.g. you start to lose control of a low hover and need to fly out of it?)

Well, for my style of flying yes ... but I was assuming you were sport flying it.

I really meant just accelerating for take off, or throttling up for a missed runway pass etc. The higher the pitch, the longer rollout you will need before you reach flight speed on takeoff or the harder you will have to hit the throttle on a go around.

I don't think I could even hover a plane with a 12x12 prop. I was trying to hover a 10 pound plane with a 70" wing span and a APC 20x15 prop. At full throttle it would just barely hover. I primarily used that prop for high speed precision acrobatics. It was also sucking down over 3000 watts at full throttle which would kill the battery pretty quick. I wouldn't hesitate to use it for sport flying though. Even at a 4:3 ratio of diameter to pitch it was pretty soft on takeoff and needed quite a bit of throttle and additional runway. At a 1:1 ratio, I imagine it is an even softer rollout.

I see...I think. Should I think of a coarse prop as a high gear...low torque high speed?

Originally Posted by TTRotary

I see...I think. Should I think of a coarse prop as a high gear...low torque high speed?

Exactly!

Hi,

Here is the link to my calc (excel sheet):

http://g.rouby.free.fr/tetacalc.php

and the link to my site, where I'll try to write some tutorials, if needed... and if you want to post some comments:

http://aerotrash.over-blog.com/pages...r-6330077.html

My calc works fine with any excel version, and can be used with open office (but without macro).

The purpose of this calc is more like power system optimization, so it might be uneasy to use but can be very accurate.

The electric motor model is similar to Drivecalc's model, but it can also work with a simpler model (like eCalc's model).

The propeller's performance is computed from wind tunnel data (mainly from UIUC website).

- simple aircraft aerodynamics,

- 160 motors database, with accurate motor model, or...
- generic motor model,
- simple motor model (from Kv, Io and R),

- 100 + propellers database,
- in-flight thrust and power, efficiency...

- etc, etc...

Please try it ! nothing commercial about this, just sharing and hoping someone has idea to improve it.

Guillaume

the issue I have with ecalc is that even if I put in the req setup by the motor manufacture it always comes back that either watts to high or amps to high.
Example is rimfire 50cc 22 x 8 prop on 12S 30/45C batterys. You get this.
Motor @ Maximum
Current: 161.14 A
Voltage: 40.39 V
Revolutions*: 8707 rpm
electric Power: 6508.9 W
mech. Power: 5976.2 W
Efficiency: 91.8 %
est. Temperature: 74 °C
165 °F

Max surge current for motor is 135, max cont. is 110. Your setup pulls 160+. So you get a warning. Simple to understand that isn't it?

Either e-calc is not calculating right, due to incorrect parameter input, or manf is lying about capabilities of the motor.

Originally Posted by Mike Dubovsky

Max surge current for motor is 135, max cont. is 110. Your setup pulls 160+. So you get a warning. Simple to understand that isn't it?

Either e-calc is not calculating right, due to incorrect parameter input, or manf is lying about capabilities of the motor.

I take the stats from the ESC and they are know were close to these. This is why I stopped going by these, this is not the first plane this has happened to.

Did a quick input on the PropEngine calculator. Results where a little higher than I observe with my wattmeter. Used my Radian as an example. Have a 3S 30C pack on it and an 11X8 prop. Wattmeter reads about 24 peak. Tool predicts 27. Close enough for estimation purposes.

E-calc, work best if allowed to feed the value of Rm, and eliminate the automatic and useless value given by the manufacturer.